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fix: support json_object response format for Anthropic models (BRA-3896) #374

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Merged
mklgallegos merged 1 commit into from
Jan 16, 2026
Merged

fix: support json_object response format for Anthropic models (BRA-3896) #374

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30 changes: 30 additions & 0 deletions packages/proxy/src/providers/anthropic.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -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" },
});
});
10 changes: 8 additions & 2 deletions packages/proxy/src/proxy.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -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")
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@knjiang knjiang Jan 15, 2026

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[nit]: code might be more defensive if we match up-front for the data.type. currently, if someone adds another parsed.data.type, it defaults to type: "object".

i think this is fine since we are migrating away though.

) {
isStructuredOutput = true;
if (params.tools || params.tool_choice) {
throw new ProxyBadRequestError(
Expand All @@ -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" },
},
];

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