Phasing

The agent-testing surface shipped in three compatibility-preserving phases.

v1: OpenAI-compatible model boundary

OpenAI-compatible chat-completions proxy (non-streaming), app: agent process driver, cassette in trace v1 (additive header + step artifacts), assert_tool_call grammar, trajectory diff on re-record.

v2: Anthropic, streaming, and HTTP targets

Landed: the Anthropic Messages API (/v1/messages) and streaming replay for both dialects. A request with stream: true is served the recorded turn as a synthetic SSE stream in the client's own dialect (OpenAI chat-completion chunks, or Anthropic message_start / content_block_* / message_delta / message_stop events), so every existing cassette serves a streaming client with no re-record and no schema change. Chunk boundaries are synthesized rather than recorded (they carry no test signal, and recording them would break turn matching); the assembled turn is still what matches, and stream is transport, never part of the comparison. Both wire protocols normalize into one neutral cassette, tagged per turn (protocol, defaulting to openai so v1 traces are byte-unchanged); a turn recorded in one dialect and replayed in another diverges on that first. To keep record and replay symmetric, the record path forwards non-streaming to the upstream and synthesizes the same stream back to the agent. Also landed: http-target agents (drive an already-running service via agent.url instead of spawning a process; see "Driving a running service" above). v2 is complete.

v3: MCP tool boundary

MCP servers are a second mockable boundary for systems whose tools are external MCP processes rather than internal functions. Landed (v3.1): the stdio transport, with per-tool result mocks and per-server strict-positional lanes in the trace (see "Mocking MCP tool servers" above). Landed (v3.2): the streamable-HTTP transport (url:/port:), an in-process loopback listener that forwards to the real server at record (reading application/json or text/event-stream answers) and replays the lane as single JSON bodies with zero network. The trace shape is unchanged, so a lane is transport-blind: one recorded through stdio replays through an HTTP-declared server and vice versa. Landed (v3.3): server-initiated NOTIFICATIONS and the standalone server-push SSE stream. A notification is recorded (inline in a POST's SSE body, or off the bridged GET stream) into its lane with an after anchor, and replayed over the GET stream flowproof now serves (a second concurrent GET is a 409); anchors are recorded and replayed but never matched, so the verdict is unchanged. The remaining v3.4 slice is server-initiated REQUESTS (sampling, elicitation, roots-list), which need answer correlation: on both transports a request mid-record fails by name rather than corrupt a lane, and a JSON-RPC batch is a 400.