claude-agent-cassette 0.2.0

Record & replay the claude-agent-sdk wire for deterministic, offline tests.

Claude Agent Cassette

Record & replay the claude-agent-sdk wire for deterministic, offline tests — no API key, no subprocess, no mocks.

Why

Apps built on claude-agent-sdk read a stream of typed messages (assistant turns, tool results, task notifications, control-protocol frames) and drive logic off them. The nasty bugs live at that stream → your-handler seam: the SDK emits a slightly different shape than you expected, and your handler quietly does the wrong thing.

Mocked tests can't catch this — you build the mock, so you only test your understanding of your own mock. A cassette records the real wire once and replays it through the SDK's real parser, so:

• a shape change in the SDK turns your test red instead of shipping to prod;
• tests run with no API cost, no network, no claude subprocess;
• the replayed frames go through the genuine message_parser, not a stand-in.

  PRODUCTION:   real CLI ──raw frames──► SDK parser ──► your code
                                              ▲
  REPLAY:       ReplayTransport ──raw frames──┘   (same parser, same code)

Install

pip install claude-agent-cassette   # (or: uv add claude-agent-cassette)

Replay (the common case — offline, no key)

from claude_agent_cassette import replay, load_cassette

async def test_my_handler():
    async with replay(load_cassette("tests/cassettes/happy_path.jsonl")) as client:
        kinds = [type(m).__name__ async for m in client.receive_messages()]
        assert "ResultMessage" in kinds
        # ...or feed client.receive_messages() into your own dispatcher and
        #    assert on what it produces.

A cassette is a JSONL file of raw inbound stream-json frames — the exact dicts the CLI emits. replay() injects them into a real ClaudeSDKClient and answers the SDK's initialize control handshake for you.

Record (capture a real session)

record_sdk_wire() works with both SDK entry points — the one-shot query() and the interactive ClaudeSDKClient (it patches both transport-construction sites the SDK uses):

from pathlib import Path
from claude_agent_cassette import record_sdk_wire, serialize_tape

# one-shot query()
from claude_agent_sdk import query

with record_sdk_wire() as tape:                  # tees the full duplex wire
    async for _ in query(prompt="...", options=...):
        pass
Path("session.jsonl").write_text(serialize_tape(tape))

# interactive ClaudeSDKClient
from claude_agent_sdk import ClaudeAgentOptions, ClaudeSDKClient

with record_sdk_wire() as tape:
    async with ClaudeSDKClient(options=ClaudeAgentOptions()) as client:
        await client.query("...")
        async for _ in client.receive_messages():
            pass
Path("session.jsonl").write_text(serialize_tape(tape))

record_sdk_wire() captures both directions, including the control plane (control_request/control_response, mcp_message, hook_callback, the handshake), so one recording can feed both conversation replay and control-protocol replay. Derive a conversation cassette with conversation_messages(tape).

Drift detection (gate SDK bumps)

Re-parse a cassette's message frames through the installed SDK's own message_parser. A frame that no longer parses — or whose content blocks the parser silently drops — is flagged. Because it reuses the SDK's own parser, there is no schema to maintain: the judge is the thing being judged.

claude-agent-cassette drift tests/cassettes/      # *.jsonl files, or dirs of them

drift: 5 cassette(s) vs claude-agent-sdk 0.2.87

  ok    happy_path.jsonl
  DRIFT stop_midtask.jsonl — 1 frame(s):
          frame[3] assistant: content_dropped — 1 of 2 content block(s) dropped on parse
  ok    notification.jsonl

5 checked, 1 drifted (1 frame) — re-record the drifted cassettes.

• Exits non-zero on drift — use it to gate an SDK-bump PR in CI.
• Fails closed: if no cassette files are found it exits non-zero (a mispointed path can't pass as a false green); pass --allow-empty to override.
• Three drift signals: parse_error (the parser rejected the frame), unrecognized_type (the message type is gone), content_dropped (a content block silently vanished).
• Scope: catches parse-level drift (rejected/skipped frames) + dropped content blocks. It does not catch additive field-level drift (a still-parsing frame that gained a field) — see ROADMAP.md.

In Python: parse_drift(frames) / check_tape(tape) → list[DriftFinding].

Examples

examples/ has a runnable, no-key demo:

python examples/replay_cassette.py
# AssistantMessage:
# ResultMessage: Hello! How can I help?

It replays the saved examples/cassettes/hello_world.jsonl through a real ClaudeSDKClient. (That cassette is a small, illustrative hand-written sample with realistic wire shapes; real cassettes are recorded — see above.)

API

replay(messages, options=None)	async CM → a connected ClaudeSDKClient over a ReplayTransport
ReplayTransport(messages)	raw frames → real parser (answers the initialize handshake)
RecordingTransport(inner, tape)	passive MITM tee, both directions
record_sdk_wire()	CM that wraps the SDK's transport to capture a query's wire
serialize_tape / load_tape / load_cassette	tape & cassette I/O
read_frames(tape) / conversation_messages(tape)	derive replay views from a tape
parse_drift(frames) / check_tape(tape)	drift findings vs the installed SDK
claude-agent-cassette drift <path…>	CLI drift gate (non-zero on drift / empty)

How it works (the non-obvious bits)

• Replay rides the public Transport ABC (ClaudeSDKClient(transport=...), stable since SDK 0.0.22). It's solid across versions.
• The initialize handshake: connect() writes a control_request with a fresh request_id and blocks until it sees a control_response echoing it. So ReplayTransport reads that id off write() and synthesises the response — otherwise replay hangs.
• Record patches two sites: ClaudeSDKClient does a call-time import of the transport from its source module, while one-shot query() uses the name bound in _internal.client. Patching only one silently misses the other.

Compatibility

Replay uses only the public Transport API. Record and drift reach into claude_agent_sdk._internal (the subprocess transport, control-protocol shape, and message_parser), so they are version-sensitive — this release targets claude-agent-sdk 0.2.x. Pin your SDK and re-verify on bumps. (Drift being version-sensitive is the point: it tells you when a bump broke a cassette.)

Roadmap

See ROADMAP.md. Shipped: conversation replay, recording, Direction-A control replay (ReplayTransport.from_tape), and drift detection. Next up: faithful Direction-B control replay (can_use_tool/hook_callback/mcp_message stubbing) + interrupt lockstep, a pytest plugin with record-on-miss, field-level drift, and a redaction helper.

License

MIT.