# Function call validation

**Source:** https://promtable.com/glossary/function-call-validation

> Function call validation is the server-side check that ensures an LLM-emitted function call has a valid name, schema-compliant arguments, and acceptable value ranges — before the tool actually runs.

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Function call validation is the server-side check that ensures an LLM-emitted function call has a valid name, schema-compliant arguments, and acceptable value ranges — before the tool actually runs.

Strict mode catches schema violations at decode time, but models still emit function calls with bad business logic — out-of-range values, references to nonexistent IDs, action requests on resources the user doesn't own. Function call validation runs after decode and before execution: schema check (probably auto), value range check, authorisation check, business-rule check. Failed validation either rejects (model retries) or asks the user for clarification. Production agentic systems in 2026 treat validation as the difference between a working agent and one that does damage when it gets confused.

## When to use

- Any production agent with destructive tools.
- Multi-tenant systems where tool access must be scoped per user.

## Common mistakes

- Validating only the schema, not the business logic.
- No clear error message back to the model — model can't recover without knowing why validation failed.

## Related terms

- [function-calling](https://promtable.com/glossary/function-calling)
- [output-guard](https://promtable.com/glossary/output-guard)
- [response-validation](https://promtable.com/glossary/response-validation)
- [guardrails](https://promtable.com/glossary/guardrails)

*Last updated: 2026-06-01*
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Original page: https://promtable.com/glossary/function-call-validation
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