Coverage

Every v1 method name, on both sides. Beyond the stable core, all eight areas the spec still marks unstable are implemented: session fork, LLM providers, plan operations, next edit suggestions, end-of-turn token usage, tool call names, auth methods, and MCP over ACP.

Extension

Both mechanisms the spec defines work:

• _meta is a field on every message that carries it in the spec, read and written untouched
• _-prefixed methods reach ext_request / ext_notification on either role, which decline by default like every other optional method

Unknown shapes

Update kinds, content blocks, tool call content, plan payloads and enum values this revision does not know arrive in an Other variant and round-trip whole, rather than failing the message they came in. A newer peer does not break you.

Not there yet

The leniency upstream applies field by field, where a malformed optional field falls back to its default and a bad array item is skipped rather than failing the message around it. Unknown shapes are handled — that is what the Other variants are for — but malformed ones are still an error.