diff --git a/frontend/src/api/client.test.ts b/frontend/src/api/client.test.ts
new file mode 100644
index 00000000..bf9bcfbb
--- /dev/null
+++ b/frontend/src/api/client.test.ts
@@ -0,0 +1,27 @@
+import { http, HttpResponse } from "msw";
+import { server } from "../test/msw";
+import { apiFetch } from "./client";
+
+test("body-less POST omits content-type (avoids Fastify empty-body 400)", async () => {
+  let contentType: string | null = "unset";
+  server.use(
+    http.post("http://localhost:8787/sessions/s1/resume", ({ request }) => {
+      contentType = request.headers.get("content-type");
+      return HttpResponse.json({ id: "s1" });
+    }),
+  );
+  await apiFetch("/sessions/s1/resume", { method: "POST" });
+  expect(contentType).toBeNull();
+});
+
+test("POST with a body sends application/json content-type", async () => {
+  let contentType: string | null = null;
+  server.use(
+    http.post("http://localhost:8787/sessions/s1/steer", ({ request }) => {
+      contentType = request.headers.get("content-type");
+      return new HttpResponse(null, { status: 204 });
+    }),
+  );
+  await apiFetch("/sessions/s1/steer", { method: "POST", body: JSON.stringify({ text: "hi" }) });
+  expect(contentType).toContain("application/json");
+});
diff --git a/frontend/src/api/client.ts b/frontend/src/api/client.ts
index 04fe194e..58d8c175 100644
--- a/frontend/src/api/client.ts
+++ b/frontend/src/api/client.ts
@@ -1,10 +1,16 @@
 const BASE = import.meta.env.VITE_BACKEND_URL ?? "http://localhost:8787";
 
 export async function apiFetch<T>(path: string, init?: RequestInit): Promise<T> {
-  const res = await fetch(`${BASE}${path}`, {
-    headers: { "content-type": "application/json" },
-    ...init,
-  });
+  // Only declare a JSON content-type when we actually send a body. Body-less
+  // POSTs (resume, close) would otherwise make Fastify reject the empty body
+  // with FST_ERR_CTP_EMPTY_JSON_BODY (400).
+  const headers: Record<string, string> = {
+    ...(init?.headers as Record<string, string> | undefined),
+  };
+  if (init?.body != null && !("content-type" in headers) && !("Content-Type" in headers)) {
+    headers["content-type"] = "application/json";
+  }
+  const res = await fetch(`${BASE}${path}`, { ...init, headers });
   if (!res.ok) throw new Error(`${res.status} ${await res.text().catch(() => "")}`);
   return res.status === 204 ? (undefined as T) : ((await res.json()) as T);
 }