llama.cpp JSON schema not working: check the endpoint and required fields

When llama.cpp returns valid JSON with missing required fields, check the actual endpoint, serialized request body and original schema. A JSON parser can accept an answer that still violates your application contract.

The retained evidence covers two different problems: a Chat API schema envelope sent to native /completion, and a mixed properties/oneOf schema whose required action field was not enforced. Start with the endpoint check below.

Check a schema request locally

Check the schema location for the actual route

Repair 010 sent the required schema inside response_format to native POST /completion. The completed response contained the correct inner facts, id: 17 and text: ALPHA, but omitted the required kingy_record wrapper.

Moving the unchanged schema to native top-level json_schema restored the exact contract:

{"kingy_record":{"id":17,"text":"ALPHA"}}

The debugger offers that envelope change only when llama.cpp and the native /completion route are specified and it finds a supported object schema. It preserves the prompt and other request values. Conflicting schema locations produce no proposed request. The proposal is not executed.

Keep Chat API request formats with their corresponding chat endpoint. Repair 010 does not establish that all response_format requests are ignored, or that moving a schema is appropriate for another runtime.

Inspect the exact request-envelope repair · Server documentation for the tested build.

A converter limitation needs a different correction

Repair 004 used the native schema field already. Its mixed schema produced a completed object with location but no required action. The retained grammar made the action key optional.

For that specific contract, expressing each oneOf alternative as a complete, disjoint object restored the action field. The original application schema stayed the validator. Extra fields and empty action arrays remained permitted where the original contract allowed them.

This was a bounded schema rewrite, not a general normalizer. The browser debugger links to the evidence; it does not rewrite arbitrary oneOf schemas or establish full Draft 7 support. The pinned grammar guide documents mixed-keyword limitations and silently skipped unsupported features.

Inspect repair 004 and its semantic checks · Pinned grammar guide and limitations.

Validate syntax, contract and source values separately

1. Keep the original response and request. A truncated response needs a completion check before schema diagnosis.
2. Parse the output. A single enclosing Markdown fence may be a formatting problem, covered by the bounded fence repair.
3. Validate against the original application schema. Required fields must remain required.
4. Check the values against the original task and source. Correct syntax and schema do not prove factual accuracy.

After changing a request envelope or decoder schema, repeat the unchanged task, require complete termination, and validate the original contract. Keep failed attempts alongside the corrected result.

Open the full Fix Finder debugger · Browse all tested repairs

Guide published October 16, 2026. Repair measurements remain dated October 15–16, 2026.