Capabilities is local gating and informational metadata. It is never serialized onto the provider wire. A custom model starts with every capability off; options explicitly opt features in.

API surface

type Capabilities struct {
	AcceptsImages             bool
	Tools                     bool
	Thinking                  bool
	StructuredOutput          bool
	StructuredOutputWithTools bool
	PromptCaching             bool
}

Capability	Used by
`AcceptsImages`	Request validation when any message contains an image, including nested tool-result blocks
`Tools`	Tool request admission and model catalogue metadata
`Thinking`	Extended-thinking request policy
`StructuredOutput`	`OutputSchema` admission
`StructuredOutputWithTools`	Structured output when ordinary tools are also exposed; implies `Tools` and `StructuredOutput`
`PromptCaching`	Codec opt-in for explicit `cache_control` breakpoints

descriptor := model.CustomModel(
	model.ProviderName("acme"), model.APIFormatAnthropic,
	"https://api.example.test", "reasoner",
	model.WithThinking(), model.WithPromptCaching(),
)
if !descriptor.Caps.Thinking {
	panic("the option was not applied")
}

Model.Validate rejects StructuredOutputWithTools unless both prerequisite booleans are true. It does not decide whether a provider really supports a label; that policy belongs to the integration layer.

Proof

Source: inference/model/capabilities.go, inference/model/model.go
Tests: inference/model/model_test.go verifies fail-safe defaults and each option.