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fix(openai-compatible): inject JSON schema instruction when structuredOutputs disabled #12608

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fix(openai-compatible): inject JSON schema instruction when structuredOutputs disabled #12608

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fix(openai-compatible): inject JSON schema instruction when structure…
giulio-leone Feb 14, 2026
91897c3
add changeset for #12491
giulio-leone Feb 14, 2026
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5 changes: 5 additions & 0 deletions .changeset/fix-output-object-json-mode.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
---
'@ai-sdk/openai-compatible': patch
---

fix(openai-compatible): inject JSON schema instruction when structuredOutputs is disabled. Improves reliability of generateText + Output.object() with OpenAI-compatible providers that don't support json_schema response format.
8 changes: 7 additions & 1 deletion packages/openai-compatible/src/chat/openai-compatible-chat-language-model.test.ts
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Expand Up @@ -835,7 +835,7 @@ describe('doGenerate', () => {
`);
});

it('should forward json response format as "json_object" and omit schema when structuredOutputs are disabled', async () => {
it('should forward json response format as "json_object" and inject schema instruction when structuredOutputs are disabled', async () => {
prepareJsonResponse({ content: '{"value":"Spark"}' });

const model = new OpenAICompatibleChatLanguageModel('gpt-4o-2024-08-06', {
Expand All @@ -862,6 +862,12 @@ describe('doGenerate', () => {
expect(await server.calls[0].requestBodyJson).toMatchInlineSnapshot(`
{
"messages": [
{
"content": "JSON schema:
{"type":"object","properties":{"value":{"type":"string"}},"required":["value"],"additionalProperties":false,"$schema":"http://json-schema.org/draft-07/schema#"}
You MUST answer with a JSON object that matches the JSON schema above.",
"role": "system",
},
{
"content": "Hello",
"role": "user",
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20 changes: 16 additions & 4 deletions packages/openai-compatible/src/chat/openai-compatible-chat-language-model.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -18,6 +18,7 @@ import {
createJsonResponseHandler,
FetchFunction,
generateId,
injectJsonInstructionIntoMessages,
isParsableJson,
parseProviderOptions,
ParseResult,
Expand Down Expand Up @@ -162,11 +163,15 @@ export class OpenAICompatibleChatLanguageModel implements LanguageModelV3 {
warnings.push({ type: 'unsupported', feature: 'topK' });
}

if (
// When structured outputs are not supported but a schema is provided,
// inject the JSON schema as a system instruction so the model knows
// what structure to produce (since the schema is dropped from response_format).
const shouldInjectSchemaInstruction =
responseFormat?.type === 'json' &&
responseFormat.schema != null &&
!this.supportsStructuredOutputs
) {
!this.supportsStructuredOutputs;

if (shouldInjectSchemaInstruction) {
warnings.push({
type: 'unsupported',
feature: 'responseFormat',
Expand All @@ -175,6 +180,13 @@ export class OpenAICompatibleChatLanguageModel implements LanguageModelV3 {
});
}

const messagesPrompt = shouldInjectSchemaInstruction
? injectJsonInstructionIntoMessages({
messages: prompt,
schema: responseFormat.schema,
})
: prompt;

const {
tools: openaiTools,
toolChoice: openaiToolChoice,
Expand Down Expand Up @@ -231,7 +243,7 @@ export class OpenAICompatibleChatLanguageModel implements LanguageModelV3 {
verbosity: compatibleOptions.textVerbosity,

// messages:
messages: convertToOpenAICompatibleChatMessages(prompt),
messages: convertToOpenAICompatibleChatMessages(messagesPrompt),

// tools:
tools: openaiTools,
Expand Down