ValidateRequestFeatures is the shared pre-codec gate for optional request features. It checks combinations that provider codecs should not have to rediscover, while leaving dialect-specific validation to the codec.

Validation order

TransientMessages must fall within the message slice.
ToolChoice must be a value built by ToolAuto, ToolRequired, or ToolNamed; a required choice needs at least one tool, and a named choice must name a declared tool.
If the message thread contains images, Model.Caps.AcceptsImages must be true. The scan descends into ToolResultBlock.Content.
If Output is nil, validation ends successfully here.
The output schema is validated.
Tool names must be unique and must not use StructuredOutputToolName.
The model must advertise structured output, and structured output with tools when tools are present.

err := inference.ValidateRequestFeatures(request)
var unsupported *inference.StructuredOutputUnsupportedError
if errors.As(err, &unsupported) {
	// Present a model-capability message without exposing schema bytes.
	log.Printf("model cannot produce structured output: %s", unsupported.Model)
}

Typed failures

Error	When
`*StructuredOutputConflictError`	Invalid tool choice, duplicate name, or reserved output tool
`*ImageInputUnsupportedError`	Image present but model lacks `AcceptsImages`
`*SchemaValidationError`	`OutputSchema` is malformed or outside the portable subset
`*StructuredOutputUnsupportedError`	Model lacks native structured output
`*StructuredOutputWithToolsUnsupportedError`	Model lacks structured output with ordinary tools

Errors intentionally carry bounded classifications, not raw schema or tool payloads.

Proof

Source: inference/client.go, inference/structured_errors.go
Tests: inference/client_test.go covers ordering, nested images, duplicates, reserved names, and bounded diagnostics.