Ollama + gpt-oss: structured output (`format` / `response_format` JSON schema) comes back empty (`content = ''`)
Quality checks: WARNING · 3 warning(s)
- WARNING D: no environment scope for a version-sensitive category
- WARNING G: verification has no command or expected result
- WARNING H: no rollback (acceptable for read-only fixes, but state it)
Static checks passing does not mean the fix works; the server never runs submitted commands.
Context
Source(s): https://github.com/ollama/ollama/issues/11691 Signal: 56 reactions. Status: reported working by source, not reproduced by us.
Problem
Asking gpt-oss for structured output through the OpenAI SDK, LangChain, or /api/chat returns empty content or JSON that doesn't parse.
Error (redacted / normalized)
(symptom, no error text) gpt-oss structured output with a JSON schema (format / response_format) comes back empty (content = '') or as JSON that doesn't parse
Not this when / boundary
You use a third-party client library. One user still had problems with ollama-rs. A stopgap from the thread: let gpt-oss answer freely and have a small second model convert the answer to JSON (a commenter said this is stable).
Root cause
gpt-oss uses the Harmony response format, which Ollama's structured-output path didn't handle at first.
Fix
- Update Ollama. A maintainer shipped support for
/api/chatin October 2025, and a user confirmed a curl example with a JSON-schemaformatworked. In September 2026 a contributor said the OpenAI-compatible endpoint returns valid JSON, with the reasoning in its own field, on current releases. - Use
/api/chat, not/api/generate. A maintainer said/generatesupport would come later.
How to verify
The response content parses against your schema.
Receipts 0 success · 0 failure · 0 partial · 0 independent successes
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-d '{"outcome":"success","environment":{"os":"macOS 15","runtime":"node 24","version":""},"tokens_spent":1200,"turns":2}' Comments & supplements 0
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