diff --git a/desktop/src/lib/auth-guard.test.ts b/desktop/src/lib/auth-guard.test.ts
index e148adc51..568cb9d0e 100644
--- a/desktop/src/lib/auth-guard.test.ts
+++ b/desktop/src/lib/auth-guard.test.ts
@@ -37,10 +37,32 @@ describe("installAuthGuard CSRF wiring", () => {
     const lookalike = spy.mock.calls.at(-1)?.[1] as RequestInit;
     expect(new Headers(lookalike.headers).get("X-CSRF-Token")).toBeNull();
 
-    // Request-object input for a same-origin mutating call: the wrapper rebuilds
-    // the Request with the header attached (first arg is the Request).
+    // Request-object input for a same-origin mutating call: the wrapper merges
+    // the CSRF token into effectiveInit so native fetch respects any
+    // init-provided method/headers and preserves the original body stream.
     await window.fetch(new Request(`${window.location.origin}/api/x`, { method: "POST" }));
-    const reqArg = spy.mock.calls.at(-1)?.[0] as Request;
-    expect(reqArg.headers.get("X-CSRF-Token")).toBe("tok123");
+    const reqInit = spy.mock.calls.at(-1)?.[1] as RequestInit;
+    expect(new Headers(reqInit.headers).get("X-CSRF-Token")).toBe("tok123");
+
+    // Request headers AND init headers together: `init?.headers || input.headers`
+    // discarded the Request's own headers whenever init carried any, so an
+    // Authorization set on the Request went missing on every mutating call that
+    // also passed init.headers. Both sources must survive, init winning ties.
+    // (These assertions live in this block deliberately: installAuthGuard has a
+    // module-level `installed` guard, so a second install in a new it() is a
+    // no-op and window.fetch would be the bare spy.)
+    await window.fetch(
+      new Request(`${window.location.origin}/api/y`, {
+        method: "POST",
+        headers: { Authorization: "Bearer abc", "X-From-Request": "1" },
+      }),
+      { headers: { "X-From-Init": "2" } },
+    );
+    const merged = new Headers((spy.mock.calls.at(-1)?.[1] as RequestInit).headers);
+    expect(merged.get("Authorization")).toBe("Bearer abc");
+    expect(merged.get("X-From-Request")).toBe("1");
+    expect(merged.get("X-From-Init")).toBe("2");
+    expect(merged.get("X-CSRF-Token")).toBe("tok123");
   });
+
 });
diff --git a/desktop/src/lib/auth-guard.ts b/desktop/src/lib/auth-guard.ts
index 7cd121ac9..c1f59cd52 100644
--- a/desktop/src/lib/auth-guard.ts
+++ b/desktop/src/lib/auth-guard.ts
@@ -65,7 +65,7 @@ export function installAuthGuard(): void {
     // the backend's router-wide verify_csrf gate is satisfied at every call
     // site, not only the handful that wrap withCsrf() by hand. Covers both
     // string/URL inputs (via withCsrf on init) and Request-object inputs (by
-    // rebuilding the Request with the header). Gated to same-origin so the token
+    // merging the CSRF token into effectiveInit). Gated to same-origin so the token
     // never leaks off-site; never overwrites an X-CSRF-Token a caller already set.
     let effectiveInput: RequestInfo | URL = input;
     let effectiveInit = init;
@@ -73,21 +73,28 @@ export function installAuthGuard(): void {
       if (isSameOrigin(input.toString())) effectiveInit = withCsrf(init);
     } else if (typeof Request !== "undefined" && input instanceof Request) {
       try {
-        const method = (input.method || "GET").toUpperCase();
-        if (
-          CSRF_MUTATING.has(method) &&
-          isSameOrigin(input.url) &&
-          !input.headers.has("X-CSRF-Token")
-        ) {
+        const method = (init?.method || input.method || "GET").toUpperCase();
+        if (CSRF_MUTATING.has(method) && isSameOrigin(input.url)) {
           const token = getCsrfToken();
           if (token) {
+            // Merge BOTH sources rather than picking one. `init?.headers ||
+            // input.headers` silently discarded the Request's own headers
+            // whenever init also carried headers, so
+            //   fetch(new Request(url, {headers: {Authorization}}), {method, headers})
+            // lost the Authorization header entirely. init wins on conflict,
+            // matching how fetch itself resolves the two.
             const headers = new Headers(input.headers);
-            headers.set("X-CSRF-Token", token);
-            effectiveInput = new Request(input, { headers });
+            if (init?.headers) {
+              new Headers(init.headers).forEach((v, k) => headers.set(k, v));
+            }
+            if (!headers.has("X-CSRF-Token")) {
+              headers.set("X-CSRF-Token", token);
+              effectiveInit = { ...init, headers };
+            }
           }
         }
       } catch {
-        effectiveInput = input;
+        // Fallback to original input/init on error
       }
     }
     const response = await originalFetch(effectiveInput, effectiveInit);