fix(http): preserve SDK headers when injecting User-Agent

The v1.76.0 User-Agent injection object-spread `fetchOptions.headers`, which
silently drops all headers when the value is a Headers instance. The Immich
SDK passes a Headers object with the merged x-api-key header, so requests
lost their API key and returned 401 (issue #6672). Build a real Headers
instance instead, keeping case-insensitive user-agent handling.

packages/core/src/infrastructure/http/index.ts

@@ -5,4 +5,5 @@ export {
   createCustomCheckServerIdentity,
   createHttpsAgentAsync,
   fetchWithTrustedCertificatesAsync,
+  mergeHeadersWithUserAgent,
 } from "./request";

packages/core/src/infrastructure/http/request.ts

@@ -18,6 +18,14 @@ import packageJson from "../../../../../package.json";
 
 export const getDefaultUserAgent = () => `Homarr/${packageJson.version} (+https://homarr.dev)`;
 
+export const mergeHeadersWithUserAgent = (headers: unknown): Headers => {
+  const mergedHeaders = new Headers(headers as HeadersInit);
+  if (!mergedHeaders.has("user-agent")) {
+    mergedHeaders.set("user-agent", getDefaultUserAgent());
+  }
+  return mergedHeaders;
+};
+
 export const createCustomCheckServerIdentity = (
   trustedHostnames: TrustedCertificateHostname[],
 ): typeof checkServerIdentity => {
@@ -77,30 +85,18 @@ export const fetchWithTrustedCertificatesAsync = async (
       undefined,
       options?.bodyTimeout !== undefined ? { bodyTimeout: options.bodyTimeout } : undefined,
     ));
+  const buildFetchOptions = (fetchOptions: Omit<RequestInit, "dispatcher" | "bodyTimeout" | "timeout">) => ({
+    ...fetchOptions,
+    headers: mergeHeadersWithUserAgent(fetchOptions.headers) as unknown as RequestInit["headers"],
+  });
   if (options?.timeout) {
     const { bodyTimeout: _bodyTimeout, dispatcher: _dispatcher, ...fetchOptions } = options;
     return await withTimeoutAsync(
-      async (signal) =>
-        fetch(url, {
-          ...fetchOptions,
-          headers: {
-            "User-Agent": getDefaultUserAgent(),
-            ...fetchOptions.headers,
-          },
-          signal,
-          dispatcher: agent,
-        }),
+      async (signal) => fetch(url, { ...buildFetchOptions(fetchOptions), signal, dispatcher: agent }),
       options.timeout,
     );
   }
 
   const { bodyTimeout: _bodyTimeout, dispatcher: _dispatcher, ...fetchOptions } = options ?? {};
-  return fetch(url, {
-    ...fetchOptions,
-    headers: {
-      "User-Agent": getDefaultUserAgent(),
-      ...fetchOptions.headers,
-    },
-    dispatcher: agent,
-  });
+  return fetch(url, { ...buildFetchOptions(fetchOptions), dispatcher: agent });
 };

packages/core/src/test/infrastructure/http/request.spec.ts

@@ -0,0 +1,50 @@
+import { describe, expect, test } from "vitest";
+
+import { mergeHeadersWithUserAgent } from "@homarr/core/infrastructure/http";
+
+describe("mergeHeadersWithUserAgent", () => {
+  test("should keep existing headers when given a Headers instance", () => {
+    // Arrange
+    const headers = new Headers({ "x-api-key": "some-api-key", Accept: "application/json" });
+
+    // Act
+    const merged = mergeHeadersWithUserAgent(headers);
+
+    // Assert
+    expect(merged.get("x-api-key")).toBe("some-api-key");
+    expect(merged.get("accept")).toBe("application/json");
+    expect(merged.get("user-agent")).toContain("Homarr/");
+  });
+
+  test("should keep existing headers when given a record", () => {
+    // Arrange
+    const headers = { "x-api-key": "some-api-key", Accept: "application/json" };
+
+    // Act
+    const merged = mergeHeadersWithUserAgent(headers);
+
+    // Assert
+    expect(merged.get("x-api-key")).toBe("some-api-key");
+    expect(merged.get("accept")).toBe("application/json");
+    expect(merged.get("user-agent")).toContain("Homarr/");
+  });
+
+  test("should not override an existing user-agent in any casing", () => {
+    // Arrange
+    const headers = new Headers({ "USER-AGENT": "custom-agent" });
+
+    // Act
+    const merged = mergeHeadersWithUserAgent(headers);
+
+    // Assert
+    expect(merged.get("user-agent")).toBe("custom-agent");
+  });
+
+  test("should fall back to the default user agent when no headers are provided", () => {
+    // Act
+    const merged = mergeHeadersWithUserAgent(undefined);
+
+    // Assert
+    expect(merged.get("user-agent")).toContain("Homarr/");
+  });
+});