# Request validation errors

> Reject invalid feature combinations before a codec or transport performs I/O.

- Path: `Guides > Inference > Errors and Cancellation > Request validation errors`
- Human: https://looprig.com/docs/guides/inference/errors-and-cancellation/request-validation
- Machine index: https://looprig.com/llms.txt

`inference.ValidateRequestFeatures` is the common pre-codec gate. It validates
feature combinations against the model's declared capabilities.

## Rules

The function rejects a transient-message count outside the message slice, a
named tool choice whose name matches no declared tool, required tool choice
with no tools, image blocks when
`Caps.AcceptsImages` is false, invalid output schemas, duplicate tool names,
the reserved structured-output tool name, structured output without
`Caps.StructuredOutput`, and structured output with tools without
`Caps.StructuredOutputWithTools`. It does not validate provider policy or wire
JSON; those checks remain in each codec.

```go
if err := inference.ValidateRequestFeatures(req); err != nil {
	var conflict *inference.StructuredOutputConflictError
	if errors.As(err, &conflict) {
		fmt.Println(conflict.Feature)
	}
	return err
}
```

## Errors

Errors are `InvalidTransientMessagesError`, `StructuredOutputConflictError`,
`ImageInputUnsupportedError`, `StructuredOutputUnsupportedError`,
`StructuredOutputWithToolsUnsupportedError`, and `SchemaValidationError`. Model diagnostics are bounded; schema bytes,
tool payloads, and raw model names beyond the documented bound are not retained.

## Source and proof

- [`client.go`](https://github.com/looprig/inference/blob/v0.12.0/client.go)
- [`structured_errors.go`](https://github.com/looprig/inference/blob/v0.12.0/structured_errors.go)
- [`structured_result.go`](https://github.com/looprig/inference/blob/v0.12.0/structured_result.go)

Run `go test ./...`.