From 03e455761403e1295bc2c1494da7833c55a73890 Mon Sep 17 00:00:00 2001
From: Phil Pluckthun <phil@kitten.sh>
Date: Thu, 5 Feb 2026 08:54:21 +0000
Subject: [PATCH] fix: Prevent overriding `Content-Type` when it's already set
 (#30)

---
 .changeset/nice-masks-buy.md |  5 +++++
 src/__tests__/fetch.test.ts  | 10 ++++++++++
 src/fetch.ts                 |  7 +++++--
 3 files changed, 20 insertions(+), 2 deletions(-)
 create mode 100644 .changeset/nice-masks-buy.md

diff --git a/.changeset/nice-masks-buy.md b/.changeset/nice-masks-buy.md
new file mode 100644
index 0000000..d8a86b6
--- /dev/null
+++ b/.changeset/nice-masks-buy.md
@@ -0,0 +1,5 @@
+---
+'fetch-nodeshim': patch
+---
+
+Fix `Content-Type` being overridden for string inputs when it's already set
diff --git a/src/__tests__/fetch.test.ts b/src/__tests__/fetch.test.ts
index c9fdd29..65fcada 100644
--- a/src/__tests__/fetch.test.ts
+++ b/src/__tests__/fetch.test.ts
@@ -122,6 +122,16 @@ describe(fetch, () => {
       });
     });
 
+    it('should send custom Content-Type with body', async () => {
+      const response = await fetch(new URL('inspect', baseURL), {
+        headers: { 'content-type': 'some/magic' },
+        body: 'test',
+      });
+      expect(await response.json()).toMatchObject({
+        headers: expect.objectContaining({ 'content-type': 'some/magic' }),
+      });
+    });
+
     it('should prefer init headers when Request is passed', async () => {
       const request = new Request(new URL('inspect', baseURL), {
         headers: { 'x-custom-header': 'abc' },
diff --git a/src/fetch.ts b/src/fetch.ts
index 96c5277..3d5c323 100644
--- a/src/fetch.ts
+++ b/src/fetch.ts
@@ -287,9 +287,12 @@ async function _fetch(
 
     outgoing.on('error', destroy);
 
-    if (!requestHeaders.has('Accept')) requestHeaders.set('Accept', '*/*');
-    if (requestBody.contentType)
+    if (!requestHeaders.has('Accept')) {
+      requestHeaders.set('Accept', '*/*');
+    }
+    if (!requestHeaders.has('Content-Type') && requestBody.contentType) {
       requestHeaders.set('Content-Type', requestBody.contentType);
+    }
 
     if (
       requestBody.body == null &&
-- 
2.51.2