diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/angular-query-experimental/axios/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/angular-query-experimental/axios/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/angular-query-experimental/axios/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/angular-query-experimental/axios/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/angular-query-experimental/axios/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/angular-query-experimental/axios/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/angular-query-experimental/axios/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/angular-query-experimental/axios/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/angular-query-experimental/axios/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/angular-query-experimental/axios/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/angular-query-experimental/axios/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/angular-query-experimental/axios/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/react-query/axios/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/react-query/axios/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/react-query/axios/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/react-query/axios/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/react-query/axios/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/react-query/axios/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/react-query/axios/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/react-query/axios/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/react-query/axios/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/react-query/axios/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/react-query/axios/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/react-query/axios/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/solid-query/axios/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/solid-query/axios/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/solid-query/axios/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/solid-query/axios/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/solid-query/axios/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/solid-query/axios/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/solid-query/axios/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/solid-query/axios/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/solid-query/axios/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/solid-query/axios/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/solid-query/axios/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/solid-query/axios/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/svelte-query/axios/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/svelte-query/axios/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/svelte-query/axios/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/svelte-query/axios/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/svelte-query/axios/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/svelte-query/axios/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/svelte-query/axios/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/svelte-query/axios/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/svelte-query/axios/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/svelte-query/axios/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/svelte-query/axios/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/svelte-query/axios/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/vue-query/axios/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/vue-query/axios/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/vue-query/axios/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/vue-query/axios/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/vue-query/axios/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/vue-query/axios/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/vue-query/axios/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/vue-query/axios/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/vue-query/axios/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/vue-query/axios/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/vue-query/axios/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/2.0.x/plugins/@tanstack/vue-query/axios/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/content-types/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/content-types/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/content-types/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/content-types/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/content-types/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/content-types/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/content-types/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/content-types/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/content-types/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/content-types/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/content-types/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/content-types/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/parameter-explode-false-axios/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/parameter-explode-false-axios/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/parameter-explode-false-axios/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/parameter-explode-false-axios/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/parameter-explode-false-axios/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/parameter-explode-false-axios/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/parameter-explode-false-axios/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/parameter-explode-false-axios/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/parameter-explode-false-axios/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/parameter-explode-false-axios/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/parameter-explode-false-axios/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/parameter-explode-false-axios/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/angular-query-experimental/axios/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/angular-query-experimental/axios/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/angular-query-experimental/axios/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/angular-query-experimental/axios/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/angular-query-experimental/axios/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/angular-query-experimental/axios/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/angular-query-experimental/axios/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/angular-query-experimental/axios/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/angular-query-experimental/axios/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/angular-query-experimental/axios/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/angular-query-experimental/axios/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/angular-query-experimental/axios/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/react-query/axios/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/react-query/axios/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/react-query/axios/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/react-query/axios/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/react-query/axios/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/react-query/axios/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/react-query/axios/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/react-query/axios/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/react-query/axios/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/react-query/axios/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/react-query/axios/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/react-query/axios/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/solid-query/axios/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/solid-query/axios/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/solid-query/axios/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/solid-query/axios/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/solid-query/axios/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/solid-query/axios/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/solid-query/axios/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/solid-query/axios/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/solid-query/axios/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/solid-query/axios/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/solid-query/axios/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/solid-query/axios/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/svelte-query/axios/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/svelte-query/axios/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/svelte-query/axios/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/svelte-query/axios/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/svelte-query/axios/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/svelte-query/axios/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/svelte-query/axios/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/svelte-query/axios/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/svelte-query/axios/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/svelte-query/axios/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/svelte-query/axios/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/svelte-query/axios/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/vue-query/axios/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/vue-query/axios/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/vue-query/axios/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/vue-query/axios/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/vue-query/axios/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/vue-query/axios/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/vue-query/axios/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/vue-query/axios/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/vue-query/axios/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/vue-query/axios/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/vue-query/axios/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.0.x/plugins/@tanstack/vue-query/axios/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-false/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-false/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-false/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-false/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-false/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-false/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-false/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-false/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-false/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-false/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-false/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-false/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-number/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-number/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-number/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-number/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-number/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-number/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-number/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-number/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-number/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-number/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-number/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-number/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-strict/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-strict/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-strict/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-strict/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-strict/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-strict/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-strict/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-strict/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-strict/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-strict/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-strict/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-strict/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-string/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-string/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-string/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-string/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-string/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-string/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-string/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-string/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-string/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-string/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-string/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/base-url-string/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/clean-false/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/clean-false/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/clean-false/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/clean-false/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/clean-false/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/clean-false/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/clean-false/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/clean-false/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/clean-false/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/clean-false/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/clean-false/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/clean-false/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/default/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/default/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/default/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/default/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/default/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/default/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/default/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/default/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/default/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/default/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/default/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/default/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/sdk-client-optional/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/sdk-client-optional/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/sdk-client-optional/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/sdk-client-optional/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/sdk-client-optional/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/sdk-client-optional/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/sdk-client-optional/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/sdk-client-optional/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/sdk-client-optional/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/sdk-client-optional/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/sdk-client-optional/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/sdk-client-optional/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/sdk-client-required/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/sdk-client-required/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/sdk-client-required/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/sdk-client-required/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/sdk-client-required/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/sdk-client-required/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/sdk-client-required/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/sdk-client-required/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/sdk-client-required/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/sdk-client-required/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/sdk-client-required/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/sdk-client-required/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/tsconfig-nodenext-sdk/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/tsconfig-nodenext-sdk/client/client.gen.ts
index 17fc6d73c..f0000c3c1 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/tsconfig-nodenext-sdk/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/tsconfig-nodenext-sdk/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/tsconfig-nodenext-sdk/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/tsconfig-nodenext-sdk/client/types.gen.ts
index 7ad021531..451c1c494 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/tsconfig-nodenext-sdk/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/tsconfig-nodenext-sdk/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/tsconfig-nodenext-sdk/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/tsconfig-nodenext-sdk/client/utils.gen.ts
index 48179a69f..f9e85dd04 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/tsconfig-nodenext-sdk/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/clients/@hey-api/client-axios/tsconfig-nodenext-sdk/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/content-types/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/content-types/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/content-types/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/content-types/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/content-types/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/content-types/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/content-types/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/content-types/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/content-types/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/content-types/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/content-types/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/content-types/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/parameter-explode-false-axios/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/parameter-explode-false-axios/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/parameter-explode-false-axios/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/parameter-explode-false-axios/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/parameter-explode-false-axios/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/parameter-explode-false-axios/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/parameter-explode-false-axios/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/parameter-explode-false-axios/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/parameter-explode-false-axios/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/parameter-explode-false-axios/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/parameter-explode-false-axios/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/parameter-explode-false-axios/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/angular-query-experimental/axios/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/angular-query-experimental/axios/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/angular-query-experimental/axios/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/angular-query-experimental/axios/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/angular-query-experimental/axios/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/angular-query-experimental/axios/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/angular-query-experimental/axios/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/angular-query-experimental/axios/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/angular-query-experimental/axios/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/angular-query-experimental/axios/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/angular-query-experimental/axios/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/angular-query-experimental/axios/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/react-query/axios/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/react-query/axios/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/react-query/axios/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/react-query/axios/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/react-query/axios/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/react-query/axios/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/react-query/axios/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/react-query/axios/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/react-query/axios/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/react-query/axios/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/react-query/axios/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/react-query/axios/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/solid-query/axios/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/solid-query/axios/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/solid-query/axios/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/solid-query/axios/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/solid-query/axios/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/solid-query/axios/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/solid-query/axios/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/solid-query/axios/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/solid-query/axios/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/solid-query/axios/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/solid-query/axios/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/solid-query/axios/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/svelte-query/axios/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/svelte-query/axios/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/svelte-query/axios/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/svelte-query/axios/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/svelte-query/axios/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/svelte-query/axios/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/svelte-query/axios/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/svelte-query/axios/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/svelte-query/axios/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/svelte-query/axios/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/svelte-query/axios/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/svelte-query/axios/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/vue-query/axios/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/vue-query/axios/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/vue-query/axios/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/vue-query/axios/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/vue-query/axios/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/vue-query/axios/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/vue-query/axios/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/vue-query/axios/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/vue-query/axios/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/vue-query/axios/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/vue-query/axios/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/plugins/@tanstack/vue-query/axios/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/sse-axios/client/client.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/sse-axios/client/client.gen.ts
index c8a60c6fa..f81a9e786 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/sse-axios/client/client.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/sse-axios/client/client.gen.ts
@@ -80,7 +80,7 @@ export const createClient = (config: Config = {}): Client => {
       const { auth, ...optsWithoutAuth } = opts;
       const response = await _axios({
         ...optsWithoutAuth,
-        baseURL: opts.baseURL as string,
+        baseURL: '', // the baseURL is already included in `url`
         data: getValidRequestBody(opts),
         headers: opts.headers as RawAxiosRequestHeaders,
         // let `paramsSerializer()` handle query params if it exists
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/sse-axios/client/types.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/sse-axios/client/types.gen.ts
index 21789fe0d..d59239b9a 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/sse-axios/client/types.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/sse-axios/client/types.gen.ts
@@ -151,7 +151,7 @@ type BuildUrlFn = <
     url: string;
   },
 >(
-  options: Pick<TData, 'url'> & Omit<Options<TData>, 'axios'>,
+  options: Pick<TData, 'url'> & Options<TData>,
 ) => string;
 
 export type Client = CoreClient<
diff --git a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/sse-axios/client/utils.gen.ts b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/sse-axios/client/utils.gen.ts
index 37f0f49bf..19d93d19e 100644
--- a/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/sse-axios/client/utils.gen.ts
+++ b/packages/openapi-ts-tests/main/test/__snapshots__/3.1.x/sse-axios/client/utils.gen.ts
@@ -125,9 +125,16 @@ export const setAuthParams = async ({
   }
 };
 
-export const buildUrl: Client['buildUrl'] = (options) =>
-  getUrl({
-    baseUrl: options.baseURL as string,
+export const buildUrl: Client['buildUrl'] = (options) => {
+  const instanceBaseUrl = options.axios?.defaults?.baseURL;
+
+  const baseUrl =
+    !!options.baseURL && typeof options.baseURL === 'string'
+      ? options.baseURL
+      : instanceBaseUrl;
+
+  return getUrl({
+    baseUrl: baseUrl as string,
     path: options.path,
     // let `paramsSerializer()` handle query params if it exists
     query: !options.paramsSerializer ? options.query : undefined,
@@ -137,6 +144,7 @@ export const buildUrl: Client['buildUrl'] = (options) =>
         : createQuerySerializer(options.querySerializer),
     url: options.url,
   });
+};
 
 export const mergeConfigs = (a: Config, b: Config): Config => {
   const config = { ...a, ...b };