Fail Fast Validation

Validate inputs, permissions, and preconditions before sending a long prompt to the model. If a file does not exist, a command is malformed, or a required parameter is missing, catch it deterministically rather than wasting tokens on a model call that will error.

How It Works

A lightweight validation layer runs before the agentic loop step. It checks:

• file existence and permissions
• command syntax and allowed executables
• required parameters in tool calls
• context-window limits

If validation fails, the agent returns an error immediately without invoking the model.

Benefits

• avoids token waste on doomed calls
• reduces latency by failing immediately
• gives clearer error messages than model-generated hallucinations of failure

When to Use

Any deterministic check that does not require reasoning should be handled outside the model. Reserve model calls for tasks that actually need interpretation or generation.

See Also

• Structured Tool Output — structured schemas make validation deterministic.
• Model Routing — validation can be done by a cheap model or deterministic code before routing to an expensive model.
• Write-Path Delegation — a compile or test run is what makes it safe to write a file nobody in the loop has read.
• Output Spooling to Disk — the exit code usually answers the question the spooled log was going to be read for.

Tools

• Pydantic — data validation library for structured tool inputs.
• Instructor — structured output validation for LLM responses, also useful for input schema enforcement.