The codecs expose the same neutral client contract, but their wire contracts are not interchangeable.

Request shape

Format	Conversation shape	Images	Tools	Structured output
OpenAI Chat	`messages`	URL/data URL	function tools	`response_format.json_schema`
OpenAI Responses	`input` items	`input_image`	function items	`text.format.json_schema`
Anthropic	`messages` plus top-level `system`	URL/base64	`input_schema` tools	`output_config.format`
Gemini	`contents` parts plus `systemInstruction`	inline/file data	grouped declarations	`responseJsonSchema`
Bedrock Converse	tagged content union	inline bytes	`toolConfig.toolSpec`	`outputConfig.textFormat`

OpenAI and Anthropic put stream in JSON. Gemini and Bedrock do not; streaming is a route or operation concern.

Caching and usage

Only behavior implemented by these codecs is documented here:

Format	Request-side cache hint	Response usage evidence
OpenAI Chat	none emitted	`cached_tokens`, optional `cache_write_tokens`
OpenAI Responses	none emitted; `store` is always false	`input_tokens_details.cached_tokens` and `cache_write_tokens`
Anthropic	opt-in `PromptCaching` emits up to two `cache_control: ephemeral` breakpoints	separate read and creation counts
Gemini	no request cache hint	`cachedContentTokenCount`
Bedrock	no cache marker in the request DTO	`cacheReadInputTokens` and `cacheWriteInputTokens`

An API may perform other server-side caching, but the codecs do not infer or promise it beyond these fields and tests.

Run go test ./codec/....