ThinkingChunk carries reasoning text or a signature delta. Providers may emit the signature separately from the reasoning text, so the accumulator must retain the last non-empty signature.

API surface

type ThinkingChunk struct {
	Thinking  string
	Signature string
}

var accumulated streamaccumulator.Thinking
accumulated.Add(&content.ThinkingChunk{Thinking: "check "})
accumulated.Add(&content.ThinkingChunk{Thinking: "facts", Signature: "signed"})
block := accumulated.Block()

Block returns nil before any chunk, then a *content.ThinkingBlock. An empty text delta still marks the accumulator non-empty. The Signature remains empty until a provider sends it, which is valid for an in-progress stream.

if chunk, ok := value.(*content.ThinkingChunk); ok {
	if chunk.Thinking != "" {
		reasoningDisplay.WriteString(chunk.Thinking)
	}
	thinking.Add(chunk)
}

Provider-opaque replay state is a separate ThinkingBlock concern. Do not manufacture a signature or replay state from display text.

Proof

Related: ThinkingBlock, AIMessage.