diff --git a/ui/src/lib/__tests__/api-client.test.ts b/ui/src/lib/__tests__/api-client.test.ts
index 6cc24c4..076a13c 100644
--- a/ui/src/lib/__tests__/api-client.test.ts
+++ b/ui/src/lib/__tests__/api-client.test.ts
@@ -43,6 +43,77 @@ describe("apiFetch", () => {
     spy.mockRestore();
   });
 
+  it("does NOT default Content-Type for FormData (browser must set the multipart boundary)", async () => {
+    const spy = vi.spyOn(globalThis, "fetch").mockResolvedValue(
+      new Response("{}", { status: 200, headers: { "content-type": "application/json" } }),
+    );
+    try {
+      const fd = new FormData();
+      fd.append("files", new Blob(["hello"], { type: "text/plain" }), "hello.txt");
+      await apiFetch("/api/upload", { method: "POST", body: fd });
+      const init = (spy.mock.calls[0][1] ?? {}) as RequestInit;
+      const hdrs = new Headers(init.headers);
+      expect(hdrs.has("Content-Type")).toBe(false);
+      // The exact same FormData instance must reach fetch — passing through
+      // the spread/clone path or being re-serialized would also break uploads.
+      expect(init.body).toBe(fd);
+    } finally {
+      spy.mockRestore();
+    }
+  });
+
+  it("does NOT default Content-Type for Blob, URLSearchParams, ArrayBuffer, or typed arrays", async () => {
+    const spy = vi.spyOn(globalThis, "fetch").mockResolvedValue(new HttpResponse(null, { status: 204 }));
+    try {
+      const bodies: BodyInit[] = [
+        new Blob(["x"], { type: "application/octet-stream" }),
+        new URLSearchParams({ a: "1" }),
+        new ArrayBuffer(4),
+        new Uint8Array([1, 2, 3]),
+      ];
+      for (const body of bodies) {
+        await apiFetch("/api/raw", { method: "POST", body });
+      }
+      for (let i = 0; i < bodies.length; i++) {
+        const init = (spy.mock.calls[i][1] ?? {}) as RequestInit;
+        const hdrs = new Headers(init.headers);
+        expect(hdrs.has("Content-Type")).toBe(false);
+      }
+    } finally {
+      spy.mockRestore();
+    }
+  });
+
+  it("defaults Content-Type to application/json for string bodies", async () => {
+    const spy = vi.spyOn(globalThis, "fetch").mockResolvedValue(
+      new Response("{}", { status: 200, headers: { "content-type": "application/json" } }),
+    );
+    try {
+      await apiFetch("/api/json", { method: "POST", body: JSON.stringify({ x: 1 }) });
+      const hdrs = new Headers(((spy.mock.calls[0][1] ?? {}) as RequestInit).headers);
+      expect(hdrs.get("Content-Type")).toBe("application/json");
+    } finally {
+      spy.mockRestore();
+    }
+  });
+
+  it("respects a caller-provided Content-Type and never overrides it", async () => {
+    const spy = vi.spyOn(globalThis, "fetch").mockResolvedValue(
+      new Response("{}", { status: 200, headers: { "content-type": "application/json" } }),
+    );
+    try {
+      await apiFetch("/api/csv", {
+        method: "POST",
+        body: "a,b,c",
+        headers: { "Content-Type": "text/csv" },
+      });
+      const hdrs = new Headers(((spy.mock.calls[0][1] ?? {}) as RequestInit).headers);
+      expect(hdrs.get("Content-Type")).toBe("text/csv");
+    } finally {
+      spy.mockRestore();
+    }
+  });
+
   it("does not set Authorization header on data-path fetch even when a key exists in a meta tag", async () => {
     const meta = document.createElement("meta");
     meta.setAttribute("name", "docsiq-api-key");
diff --git a/ui/src/lib/api-client.ts b/ui/src/lib/api-client.ts
index e3e551e..fb4b18c 100644
--- a/ui/src/lib/api-client.ts
+++ b/ui/src/lib/api-client.ts
@@ -45,13 +45,27 @@ export class ApiErrorResponse extends Error {
   }
 }
 
+// FormData/Blob/URLSearchParams/streams/buffers carry their own framing —
+// the browser sets Content-Type (with the multipart boundary, etc.) when
+// fetch builds the request. Defaulting to application/json here would clobber
+// that boundary and produce an unparseable body.
+function isBrowserManagedBody(body: BodyInit): boolean {
+  if (typeof FormData !== "undefined" && body instanceof FormData) return true;
+  if (typeof Blob !== "undefined" && body instanceof Blob) return true;
+  if (typeof URLSearchParams !== "undefined" && body instanceof URLSearchParams) return true;
+  if (typeof ReadableStream !== "undefined" && body instanceof ReadableStream) return true;
+  if (body instanceof ArrayBuffer) return true;
+  if (ArrayBuffer.isView(body)) return true;
+  return false;
+}
+
 export async function apiFetch<T>(
   path: string,
   init: RequestInit = {},
 ): Promise<T> {
   if (sessionReady) await sessionReady;
   const headers = new Headers(init.headers);
-  if (init.body && !headers.has("Content-Type")) {
+  if (init.body && !headers.has("Content-Type") && !isBrowserManagedBody(init.body)) {
     headers.set("Content-Type", "application/json");
   }
   const res = await fetch(path, { ...init, headers, credentials: "include" });