diff --git a/.changeset/stale-spiders-sleep.md b/.changeset/stale-spiders-sleep.md
new file mode 100644
index 000000000..95894ba2e
--- /dev/null
+++ b/.changeset/stale-spiders-sleep.md
@@ -0,0 +1,5 @@
+---
+"@hey-api/codegen-core": patch
+---
+
+**planner**: language-aware declaration sharing check
diff --git a/packages/codegen-core/src/__tests__/project.test.ts b/packages/codegen-core/src/__tests__/project.test.ts
index 0d9d3c6be..b7dc6c11f 100644
--- a/packages/codegen-core/src/__tests__/project.test.ts
+++ b/packages/codegen-core/src/__tests__/project.test.ts
@@ -1,4 +1,6 @@
+import { canDeclarationsShareIdentifier } from '../project/namespace';
 import { Project } from '../project/project';
+import type { SymbolKind } from '../symbols/types';
 
 // Mock Planner so we control what files appear in project.files
 vi.mock('../planner/planner', () => {
@@ -54,3 +56,44 @@ describe('Project', () => {
     });
   });
 });
+
+describe('canDeclarationsShareIdentifier', () => {
+  const kinds: ReadonlyArray<SymbolKind> = [
+    'class',
+    'enum',
+    'function',
+    'interface',
+    'namespace',
+    'type',
+    'var',
+  ];
+
+  it('matches TypeScript declaration merging matrix', () => {
+    const allowed = new Set([
+      'interface|interface',
+      'class|interface',
+      'class|namespace',
+      'enum|namespace',
+      'function|namespace',
+      'namespace|namespace',
+      'function|type',
+      'type|var',
+    ]);
+
+    for (const a of kinds) {
+      for (const b of kinds) {
+        expect(canDeclarationsShareIdentifier('typescript', a, b)).toBe(
+          allowed.has([a, b].sort().join('|')),
+        );
+      }
+    }
+  });
+
+  it('returns false for Python declaration pairs', () => {
+    for (const a of kinds) {
+      for (const b of kinds) {
+        expect(canDeclarationsShareIdentifier('python', a, b)).toBe(false);
+      }
+    }
+  });
+});
diff --git a/packages/codegen-core/src/planner/planner.ts b/packages/codegen-core/src/planner/planner.ts
index 452ad9508..b99badc7c 100644
--- a/packages/codegen-core/src/planner/planner.ts
+++ b/packages/codegen-core/src/planner/planner.ts
@@ -4,7 +4,7 @@ import type { ExportModule, ImportModule } from '../bindings';
 import type { IProjectRenderMeta } from '../extensions';
 import type { File } from '../files/file';
 import type { INode } from '../nodes/node';
-import { canShareName } from '../project/namespace';
+import { canDeclarationsShareIdentifier } from '../project/namespace';
 import type { IProject } from '../project/types';
 import { fromRef } from '../refs/refs';
 import type { RenderContext } from '../renderer';
@@ -417,13 +417,13 @@ export class Planner {
 
     const localNames = ctx.localNames(scope);
     while (true) {
+      const language = node?.language || symbol.node?.language || file.language;
+
       const kinds = [...(localNames.get(finalName) ?? [])];
 
-      // TODO: adjust canShareName for language
-      const ok = kinds.every((kind) => canShareName(symbol.kind, kind));
+      const ok = kinds.every((kind) => canDeclarationsShareIdentifier(language, symbol.kind, kind));
       if (ok) break;
 
-      const language = node?.language || symbol.node?.language || file.language;
       const resolver =
         (language ? this.project.nameConflictResolvers[language] : undefined) ??
         this.project.defaultNameConflictResolver;
diff --git a/packages/codegen-core/src/project/namespace.ts b/packages/codegen-core/src/project/namespace.ts
index 209b8979f..a38c8cee3 100644
--- a/packages/codegen-core/src/project/namespace.ts
+++ b/packages/codegen-core/src/project/namespace.ts
@@ -1,6 +1,7 @@
+import type { Language } from '../languages/types';
 import type { SymbolKind } from '../symbols/types';
 
-const kindRank: Record<SymbolKind, number> = {
+const typescriptMergeKindRank: Record<SymbolKind, number> = {
   class: 3,
   enum: 4,
   function: 5,
@@ -14,10 +15,10 @@ const kindRank: Record<SymbolKind, number> = {
  * Returns true if two declarations of given kinds
  * are allowed to share the same identifier in TypeScript.
  */
-export function canShareName(a: SymbolKind, b: SymbolKind): boolean {
+function canTypeScriptDeclarationsShareIdentifier(a: SymbolKind, b: SymbolKind): boolean {
   // sort based on TypeScript merge precedence so `a` is always the weaker merge candidate
   // ensures that asymmetric merges like `type + var` are correctly handled
-  if (kindRank[a] > kindRank[b]) {
+  if (typescriptMergeKindRank[a] > typescriptMergeKindRank[b]) {
     [a, b] = [b, a];
   }
 
@@ -33,3 +34,15 @@ export function canShareName(a: SymbolKind, b: SymbolKind): boolean {
       return false;
   }
 }
+
+export function canDeclarationsShareIdentifier(
+  language: Language | undefined,
+  a: SymbolKind,
+  b: SymbolKind,
+): boolean {
+  if (language === 'typescript') {
+    return canTypeScriptDeclarationsShareIdentifier(a, b);
+  }
+
+  return false;
+}