fix(openai-compatible): inject JSON schema instruction when structuredOutputs disabled#12608
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giulio-leone wants to merge 2 commits intovercel:mainfrom
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fix(openai-compatible): inject JSON schema instruction when structuredOutputs disabled#12608giulio-leone wants to merge 2 commits intovercel:mainfrom
giulio-leone wants to merge 2 commits intovercel:mainfrom
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…dOutputs disabled (vercel#12491) When the OpenAI-compatible provider has structuredOutputs disabled (the default), it falls back from json_schema to json_object response format, which silently drops the schema. The model is told to produce JSON but has no knowledge of what schema to produce, leading to validation failures. This fix injects the JSON schema as a system instruction into the prompt messages (using injectJsonInstructionIntoMessages from provider-utils) when the provider cannot enforce the schema via response_format. This gives the model explicit guidance about the expected output structure, matching the reliability of providers that support structured outputs natively. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Fixes #12491
Problem
generateText + Output.object() had ~15% failure rate vs generateObject's 100% reliability with OpenAI-compatible providers.
Root Cause
When supportsStructuredOutputs is false (default for openai-compatible), the provider sends response_format: json_object but silently drops the schema - giving the model no knowledge of what JSON structure to produce.
Fix
In openai-compatible-chat-language-model.ts, when falling back to json_object mode, inject the JSON schema as a system instruction using injectJsonInstructionIntoMessages from @ai-sdk/provider-utils. This matches patterns used by other providers (e.g., Mistral).