diff --git a/extensions/qa-lab/src/providers/mock-openai/server.test.ts b/extensions/qa-lab/src/providers/mock-openai/server.test.ts
index 570aeeaa642..e10229cc2b3 100644
--- a/extensions/qa-lab/src/providers/mock-openai/server.test.ts
+++ b/extensions/qa-lab/src/providers/mock-openai/server.test.ts
@@ -4240,7 +4240,7 @@ describe("qa mock openai server", () => {
     expect(body).not.toContain("HEARTBEAT_OK");
   });
 
-  it("rejects malformed Anthropic /v1/messages JSON with an invalid_request_error", async () => {
+  it("rejects malformed or non-object Anthropic /v1/messages JSON", async () => {
     const server = await startQaMockOpenAiServer({
       host: "127.0.0.1",
       port: 0,
@@ -4249,20 +4249,25 @@ describe("qa mock openai server", () => {
       await server.stop();
     });
 
-    const response = await fetch(`${server.baseUrl}/v1/messages`, {
-      method: "POST",
-      headers: { "content-type": "application/json" },
-      body: '{"model":"claude-opus-4-8","messages":[',
-    });
+    for (const rawBody of ['{"model":"claude-opus-4-8","messages":[', "null", "[]", '"text"']) {
+      const response = await fetch(`${server.baseUrl}/v1/messages`, {
+        method: "POST",
+        headers: { "content-type": "application/json" },
+        body: rawBody,
+      });
 
-    expect(response.status).toBe(400);
-    const body = (await response.json()) as {
-      type: string;
-      error: { type: string; message: string };
-    };
-    expect(body.type).toBe("error");
-    expect(body.error.type).toBe("invalid_request_error");
-    expect(body.error.message).toContain("Malformed JSON body");
+      expect(response.status).toBe(400);
+      const body = (await response.json()) as {
+        type: string;
+        error: { type: string; message: string };
+      };
+      expect(body.type).toBe("error");
+      expect(body.error.type).toBe("invalid_request_error");
+      expect(body.error.message).toContain("Malformed JSON body");
+    }
+
+    const health = await fetch(`${server.baseUrl}/healthz`);
+    expect(health.status).toBe(200);
   });
 
   it("rejects malformed OpenAI-compatible JSON without crashing the mock server", async () => {
diff --git a/extensions/qa-lab/src/providers/mock-openai/server.ts b/extensions/qa-lab/src/providers/mock-openai/server.ts
index d7c142c33bb..0bca5c54835 100644
--- a/extensions/qa-lab/src/providers/mock-openai/server.ts
+++ b/extensions/qa-lab/src/providers/mock-openai/server.ts
@@ -228,7 +228,7 @@ function readBody(req: IncomingMessage): Promise<string> {
   });
 }
 
-function parseOpenAiJsonBody(raw: string): Record<string, unknown> | null {
+function parseJsonObjectBody(raw: string): Record<string, unknown> | null {
   try {
     const parsed = raw ? (JSON.parse(raw) as unknown) : {};
     return parsed && typeof parsed === "object" && !Array.isArray(parsed)
@@ -3438,7 +3438,7 @@ export async function startQaMockOpenAiServer(params?: { host?: string; port?: n
       }
       if (req.method === "POST" && url.pathname === "/v1/images/generations") {
         const raw = await readBody(req);
-        const body = parseOpenAiJsonBody(raw);
+        const body = parseJsonObjectBody(raw);
         if (!body) {
           writeOpenAiMalformedJsonError(res, "OpenAI Images");
           return;
@@ -3466,7 +3466,7 @@ export async function startQaMockOpenAiServer(params?: { host?: string; port?: n
       }
       if (req.method === "POST" && url.pathname === "/v1/embeddings") {
         const raw = await readBody(req);
-        const body = parseOpenAiJsonBody(raw);
+        const body = parseJsonObjectBody(raw);
         if (!body) {
           writeOpenAiMalformedJsonError(res, "OpenAI Embeddings");
           return;
@@ -3492,7 +3492,7 @@ export async function startQaMockOpenAiServer(params?: { host?: string; port?: n
       }
       if (req.method === "POST" && url.pathname === "/v1/responses") {
         const raw = await readBody(req);
-        const body = parseOpenAiJsonBody(raw);
+        const body = parseJsonObjectBody(raw);
         if (!body) {
           writeOpenAiMalformedJsonError(res, "OpenAI Responses");
           return;
@@ -3534,10 +3534,8 @@ export async function startQaMockOpenAiServer(params?: { host?: string; port?: n
       }
       if (req.method === "POST" && url.pathname === "/v1/messages") {
         const raw = await readBody(req);
-        let body: AnthropicMessagesRequest;
-        try {
-          body = raw ? (JSON.parse(raw) as AnthropicMessagesRequest) : {};
-        } catch {
+        const body = parseJsonObjectBody(raw) as AnthropicMessagesRequest | null;
+        if (!body) {
           writeJson(res, 400, {
             type: "error",
             error: {