Schema-Free Output

Problem

The model varies its punctuation, capitalisation, field names, and ordering in ways the parser was not written for: smart quotes instead of straight quotes, a missing comma, a 'sure, here is the answer' preamble the parser tried to skip but did not. The downstream code fails in non-obvious ways, corrupts state, or silently misinterprets the result. Post-mortems then blame the model for being flaky when the real bug is in the parser, and the team chases evals that were never going to fix a parsing problem.

Solution

Don't. Use structured-output (JSON Schema, Pydantic, function calling). See structured-output, tool-use.

When to use

• Cite this entry when downstream code regex-parses prose the model happened to emit.
• You are already here if format drift in model output silently corrupts state.
• Use structured-output (JSON Schema, Pydantic, function calling); if the provider lacks it, validate strictly post-parse and retry.