core/content supplies the closed block vocabulary. A codec preserves block order and returns typed errors for a block it cannot represent.

Blocks

Neutral block	OpenAI Chat	Responses	Anthropic	Gemini	Bedrock Converse
`TextBlock`	string or `text` part	`output_text` or `input_text`	`text`	text part	`text`
`ImageBlock`	`image_url`	`input_image` URL/data URI	`image` URL/base64	`inlineData` or `fileData`	inline bytes only
`DocumentBlock`	`file` part with a base64 data URI	`input_file` part	`document` block with a title	`inlineData` part plus a text part	document bytes/text
`AudioBlock`	`input_audio` part	unsupported; the input union has no audio member	`UnsupportedAudioError`	`inlineData` part	Converse audio block
`RefusalBlock`	assistant `refusal` member	`refusal` part on the response direction; request replay is `UnsupportedBlockError`	`UnsupportedRefusalError`	unsupported	unsupported
`ThinkingBlock`	omitted on request	reasoning item	`thinking`	thought text part	`reasoningContent` text
`ToolUseBlock`	assistant `tool_calls`	`function_call` item	`tool_use`	`functionCall`	`toolUse`
`ToolResultBlock`	text-only tool message	text-only `function_call_output`	`tool_result`	`functionResponse`	`toolResult`

Images sourced from bytes are base64 encoded by JSON dialects that require a URL or base64 field. Bedrock rejects URL images and requires one of its supported inline formats. Tool and result inputs are JSON objects; empty input is normalized to {} where the native API requires an object.

Unsupported

Unsupported content is not silently discarded. For example, an OpenAI Chat tool result containing an image returns *openaiapi.UnsupportedBlockError, while Bedrock returns *bedrockconverse.UnsupportedBlockError with a reason. Use errors.As instead of matching error strings.

var blockErr *anthropicapi.UnsupportedBlockError
if errors.As(err, &blockErr) {
	log.Printf("choose a representable block: %s", blockErr.Block)
}

Anthropic splits the representation boundary across several typed errors. UnsupportedBlockError is the fallback for a block the dialect does not model at all, while audio yields UnsupportedAudioError, a refusal UnsupportedRefusalError, an unrepresentable document UnsupportedDocumentError, and an out-of-enum image media type UnsupportedImageMediaTypeError.

Source and proof

Run go test ./codec/....