diff --git a/packages/proxy/src/providers/anthropic.test.ts b/packages/proxy/src/providers/anthropic.test.ts
index 070bcf61..4df3ee4a 100644
--- a/packages/proxy/src/providers/anthropic.test.ts
+++ b/packages/proxy/src/providers/anthropic.test.ts
@@ -575,3 +575,33 @@ it("should use 128000 max_tokens and add beta header for claude-3-7-sonnet when
     "output-128k-2025-02-19",
   );
 });
+
+it("should translate json_object response format to tool-based structured output", async () => {
+  // This test verifies BRA-3896: json_object response format should be translated
+  // to a tool-based workaround for Anthropic, similar to how json_schema is handled.
+  const { fetch, requests } = createCapturingFetch({ captureOnly: true });
+
+  await callProxyV1<OpenAIChatCompletionCreateParams, OpenAIChatCompletion>({
+    body: {
+      model: "claude-3-haiku-20240307",
+      messages: [{ role: "user", content: "Return JSON" }],
+      response_format: { type: "json_object" },
+      stream: false,
+      max_tokens: 150,
+    },
+    fetch,
+  });
+
+  expect(requests).toHaveLength(1);
+  // Verify the proxy created a tool with generic object schema
+  expect(requests[0].body).toMatchObject({
+    tools: [
+      {
+        name: "json",
+        description: "Output the result in JSON format",
+        input_schema: { type: "object" },
+      },
+    ],
+    tool_choice: { type: "tool", name: "json" },
+  });
+});
diff --git a/packages/proxy/src/proxy.ts b/packages/proxy/src/proxy.ts
index a29a4b9a..247b5322 100644
--- a/packages/proxy/src/proxy.ts
+++ b/packages/proxy/src/proxy.ts
@@ -2597,7 +2597,10 @@ async function fetchAnthropicChatCompletions({
 
   let isStructuredOutput = false;
   const parsed = responseFormatSchema.safeParse(oaiParams.response_format);
-  if (parsed.success && parsed.data.type === "json_schema") {
+  if (
+    parsed.success &&
+    (parsed.data.type === "json_schema" || parsed.data.type === "json_object")
+  ) {
     isStructuredOutput = true;
     if (params.tools || params.tool_choice) {
       throw new ProxyBadRequestError(
@@ -2608,7 +2611,10 @@ async function fetchAnthropicChatCompletions({
       {
         name: "json",
         description: "Output the result in JSON format",
-        input_schema: parsed.data.json_schema.schema,
+        input_schema:
+          parsed.data.type === "json_schema"
+            ? parsed.data.json_schema.schema
+            : { type: "object" },
       },
     ];