diff --git a/.changeset/fix-content-length-request.md b/.changeset/fix-content-length-request.md
new file mode 100644
index 00000000000..3af8993d6da
--- /dev/null
+++ b/.changeset/fix-content-length-request.md
@@ -0,0 +1,5 @@
+---
+"@effect/platform": patch
+---
+
+fix: stop emitting content-length header in HttpClientRequest.setBody
diff --git a/packages/platform/src/internal/httpClientRequest.ts b/packages/platform/src/internal/httpClientRequest.ts
index 14fb718c526..bafafc5a870 100644
--- a/packages/platform/src/internal/httpClientRequest.ts
+++ b/packages/platform/src/internal/httpClientRequest.ts
@@ -394,11 +394,9 @@ export const setBody = dual<
     if (contentType) {
       headers = Headers.set(headers, "content-type", contentType)
     }
-
-    const contentLength = body.contentLength
-    if (contentLength) {
-      headers = Headers.set(headers, "content-length", contentLength.toString())
-    }
+    // Do not set content-length here — the transport layer (fetch/undici)
+    // computes the correct value from what it actually sends on the wire.
+    // See https://github.com/Effect-TS/effect/issues/6240
   }
   return makeInternal(
     self.method,