packages/client/src/links/internals/httpUtils.ts (1)

209-216: Optional: Skip empty-body Content-Type and normalize headers in fetchHTTPResponse

Verified that the only explicit “content-type” header is set in fetchHTTPResponse (packages/client/src/links/internals/httpUtils.ts lines 209–216), and that all JSON requests (via jsonHttpRequester) and batch streams funnel through this function. Because getBody returns undefined for queries (and any mutation with input === undefined), we can safely skip setting content-type whenever the body is empty to avoid needless CORS preflights. Also, user-provided headers from resolvedHeaders are merged as-is, so normalizing keys to lowercase prevents duplicates like Content-Type vs content-type.

Files to update:

• packages/client/src/links/internals/httpUtils.ts

Proposed diff at ~lines 205–215:

@@ export async function fetchHTTPResponse(opts: HTTPRequestOptions) {
-  const method = opts.methodOverride ?? METHOD[opts.type];
+  const method = opts.methodOverride ?? METHOD[opts.type];
+  const hasBody = body != null;
@@
-  const headers = {
-    ...(opts.contentTypeHeader && method !== 'GET'
-      ? { 'content-type': opts.contentTypeHeader }
-      : {}),
-    ...(opts.trpcAcceptHeader
-      ? { 'trpc-accept': opts.trpcAcceptHeader }
-      : undefined),
-    ...resolvedHeaders,
-  };
+  const headers = {
+    // Only set Content-Type when not GET and there’s a body
+    ...(opts.contentTypeHeader && method !== 'GET' && hasBody
+      ? { 'content-type': opts.contentTypeHeader }
+      : {}),
+    // Always include tRPC accept header if provided
+    ...(opts.trpcAcceptHeader
+      ? { 'trpc-accept': opts.trpcAcceptHeader }
+      : {}),
+    // Normalize user headers to lowercase keys to avoid duplicates
+    ...Object.fromEntries(
+      Object.entries(resolvedHeaders).map(([k, v]) => [k.toLowerCase(), v]),
+    ),
+  };

You may also consider guarding trpc-accept the same way (e.g. && method !== 'GET') if you want to avoid preflights when streaming GET batches.

packages/tests/server/contentTypeHeaders.test.ts (3)

15-41: Strengthen assertions: also assert no body and that input is in the URL for GET.

Both checks help catch regressions (e.g., accidentally sending a body or dropping URL-encoded input).

   const [url, options] = mockFetch.mock.calls[0]!;
 
   expect(options.method).toBe('GET');
 
   expect(options.headers).not.toHaveProperty('content-type');
+  // No payload with GET
+  expect(options.body).toBeUndefined();
+  // Input should be URL-encoded on GET
+  expect(String(url)).toContain('input=');
 
   await close();

---

43-69: Add body assertion to ensure POST carries JSON payload.

Verifies we’re actually sending a JSON string when posting.

   const [url, options] = mockFetch.mock.calls[0]!;
 
   expect(options.method).toBe('POST');
 
   expect(options.headers).toHaveProperty('content-type', 'application/json');
+  expect(typeof options.body).toBe('string');
+  expect(options.body).toBe(JSON.stringify('test'));

---

71-105: Method override test: also assert no URL-encoded input and body presence.

Confirms the query was truly POSTed (payload in body, not query string).

   const [url, options] = mockFetch.mock.calls[0]!;
 
   expect(options.method).toBe('POST');
 
   expect(options.headers).toHaveProperty('content-type', 'application/json');
+  expect(String(url)).not.toContain('input=');
+  expect(options.body).toBe(JSON.stringify('test'));

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