fix(xai): case-insensitive user override for Grok CLI default headers

Reviewer MINOR: a mixed-case user header (X-Grok-Client-Version) previously
coexisted with the lowercase default and merged into an invalid combined
value in Bun Headers. Defaults are now dropped whenever the user overrides
them under any casing.

Author: lidge-jun

src/providers/xai-transport.ts

@@ -35,6 +35,16 @@ function hasHeaderCaseInsensitive(headers: Record<string, string> | undefined, n
   return Object.keys(headers).some(key => key.toLowerCase() === target);
 }
 
+/** Drop default entries the user already overrides under any header-name casing. */
+function withoutUserOverridden(defaults: Readonly<Record<string, string>>, userHeaders: Record<string, string> | undefined): Record<string, string> {
+  if (!userHeaders) return { ...defaults };
+  const out: Record<string, string> = {};
+  for (const [key, value] of Object.entries(defaults)) {
+    if (!hasHeaderCaseInsensitive(userHeaders, key)) out[key] = value;
+  }
+  return out;
+}
+
 export function deriveXaiConvId(promptCacheKey: string): string {
   return createHash("sha256").update(promptCacheKey).digest("hex").slice(0, 32);
 }
@@ -71,7 +81,7 @@ export function resolveProviderTransport(
     ...provider,
     baseUrl: XAI_GROK_CLI_BASE_URL,
     headers: {
-      ...XAI_GROK_CLI_HEADERS,
+      ...withoutUserOverridden(XAI_GROK_CLI_HEADERS, provider.headers),
       ...convIdHeaders,
       ...(provider.headers ?? {}),
     },

tests/xai-transport.test.ts

@@ -141,6 +141,18 @@ describe("xAI prompt-cache conv-id affinity", () => {
     const convIdKeys = Object.keys(mixedResolved.headers ?? {}).filter(k => k.toLowerCase() === XAI_CONV_ID_HEADER);
     expect(convIdKeys).toHaveLength(1);
   });
+
+  test("mixed-case user override of a Grok CLI default header suppresses the default", () => {
+    const custom = provider("oauth");
+    custom.headers = { "X-Grok-Client-Version": "0.2.94" };
+
+    const resolved = resolveProviderTransport("xai", custom);
+    const versionKeys = Object.keys(resolved.headers ?? {}).filter(k => k.toLowerCase() === "x-grok-client-version");
+    expect(versionKeys).toEqual(["X-Grok-Client-Version"]);
+    expect(resolved.headers?.["X-Grok-Client-Version"]).toBe("0.2.94");
+    // Untouched defaults still apply.
+    expect(resolved.headers?.["x-grok-client-identifier"]).toBe("opencodex");
+  });
 });
 
 describe("xAI reasoning_content cache preservation", () => {