When to reach for it
- Downstream systems expect structured data.
- Errors must be caught early.
- Automated processing follows.
Output Validation / Schema Enforcement.
Reject malformed model output before it pollutes the next step.
Model responses are validated against schemas, types, or business rules before use, and a failed check triggers a reject-and-re-prompt loop rather than letting the malformed output flow downstream.
In practiceAn order-processing agent validates that every model-generated JSON payload matches its Pydantic schema before inserting it into the database, rejecting and re-prompting on the first malformed response.
Without itWithout output validation, a hallucinated field or mistyped value from the model propagates silently into downstream systems and may corrupt database records or trigger unintended actions.
When it backfires
- Free-form text is the actual goal.
- Schemas inadmissibly restrict the answer.
- Validation is purely syntactic while being blind to business logic.
The tradeoff
Higher reliability is gained against restrictions on expressive freedom.
What it actually does.
Output must conform to a schema before it is used.
Two ways this pattern will hurt you.
Schema passes; business logic fails
The JSON parses and matches the type, but the values violate an unstated rule — a negative quantity, a date in the past, a customer that doesn't exist.
Fix · Layer business validators on top of the schema. Treat syntactic validation as a precondition, not a guarantee.
Re-prompt loops forever on an unsatisfiable schema
The schema demands a field the model cannot produce for this input. Reject-and-re-prompt never converges, burning tokens and latency on a validation that can never pass.
Fix · Cap validation retries. On exhaustion, fall back to a typed error or human review rather than looping; log the schema that never validated so it can be fixed.
Where Output Validation / Schema Enforcement is native.
OpenAI Structured Outputsconstrained decodingNative
Microsoft Agent Frameworkschema-typed responsesNative
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