ChainlessChain Agent SDK for Python

chainlesschain-agent-sdk is the zero-runtime-dependency Python client for the same Agent Protocol v1 used by @chainlesschain/agent-sdk. It starts one cc agent subprocess, frames its NDJSON stream safely, exposes frozen typed events, and performs approval, question, and MCP elicitation round trips.

Version 0.2.6 is the current release candidate for PyPI. Python 3.10 or newer is required. It carries forward the generated 37-variant Agent stream payload union and strict canonical-event validator from the unpublished 0.2.5 candidate, while correcting install guidance to the latest public CLI. Runtime APIs and lossless unknown-event delivery are unchanged.

Install

Install the SDK and a compatible cc CLI:

python -m pip install "chainlesschain-agent-sdk==0.2.6"
npm install --global "chainlesschain@0.166.7"

The Python distribution has no runtime dependencies. The CLI is installed separately because AgentSession controls it as a subprocess:

python -c "import chainlesschain_agent_sdk as sdk; print(sdk.__version__)"
cc --version

Basic session

import asyncio

from chainlesschain_agent_sdk import (
    AgentSession,
    AgentSessionOptions,
    ElicitationResponse,
    ResultEvent,
    UnknownAgentEvent,
)


async def main() -> None:
    session = AgentSession(
        AgentSessionOptions(
            cwd=".",
            session_id="ci-fix-1042",  # declare new sessions that must be resumable
            permission_mode="acceptEdits",
        ),
        on_approval=lambda request: (
            {"kind": "acceptOnce"}
            if request.tool == "run_shell"
            else {"kind": "decline", "reason": "Tool is not allow-listed"}
        ),
        on_question=lambda request: None,  # cancel in non-interactive hosts
        on_elicitation=lambda request: ElicitationResponse("decline"),
    )
    await session.start()
    await session.send("Run the focused tests and fix failures.")

    async for event in session:
        # Every wire object is yielded. A newer CLI type is never discarded.
        if isinstance(event, UnknownAgentEvent):
            print("unknown event preserved:", event.to_dict())
        elif isinstance(event, ResultEvent):
            print(event.subtype, event.result)
            await session.end()

    await session.wait()


asyncio.run(main())

Approval callbacks may still return booleans for source compatibility. Direct respond_approval(id, bool) calls retain the legacy boolean wire; structured decisions echo the request binding and can express scoped turn/session grants.

SystemInitEvent.session_id is the authoritative live ID. Persist it and resume later with AgentSessionOptions(resume=that_id). Anonymous stream sessions are not persisted by CLI design; use session_id when creating a session that must be resumable. resume takes precedence over session_id, matching the TypeScript SDK.

Event and callback guarantees

• The generated AgentStreamEventPayload is the authoritative closed wire union. KNOWN_EVENT_CLASSES is discovered from the open runtime class hierarchy and exists only for ergonomic dataclass dispatch; it no longer mirrors or redefines the schema discriminator inventory.
• Every event retains its original object in the read-only raw mapping; to_dict() returns a deep mutable copy, including unknown additive fields.
• Unknown outer type values are delivered as UnknownAgentEvent through both on_event and async iteration.
• CC_AGENT_STREAM_EVENT_TYPES and validate_agent_stream_event expose the canonical known discriminator inventory without changing that lossless unknown-event behavior.
• AgentStreamEventPayload, CanonicalAgentStreamEvent, and validate_canonical_agent_stream_event expose the generated strict contract for hosts that must reject a malformed payload with a known discriminator.
• NDJSON decoding carries split lines and split UTF-8 code points across chunks, accepts CRLF, and flushes a final line without a newline.
• Approval callback errors answer approve:false (fail closed). Question callbacks automatically echo the opaque runtime binding on both normal and null answers, so stale/cross-turn answers remain fail closed. MCP elicitation accepts only an explicit ElicitationResponse("accept", content) (or equivalent mapping); all other outcomes cancel. URL requests expose metadata.url / metadata.url_host; the host must show the full HTTPS URL and obtain explicit consent before opening it. URL completion and non-interactive fallback arrive as typed ElicitationCompleteEvent / ElicitationDeferredEvent objects.
• stderr is diagnostics only. It is available through on_stderr and is never parsed as protocol data.

Callbacks may be synchronous functions or coroutines. Keep them bounded: the CLI also applies its own interaction timeouts.

CI consumer

examples/ci_gate.py is an executable consumer with an explicit handler for every current event class. It journals every raw event before dispatch and preserves unknown events in the same artifact:

python examples/ci_gate.py \
  --prompt "Run unit tests and fix only the failing implementation" \
  --provider openai \
  --session-id "ci-${GITHUB_RUN_ID}" \
  --output agent-events.ndjson

The script denies approvals unless a tool is explicitly repeated with --approve-tool. examples/github-actions.yml is a manual-dispatch GitHub Actions template with read-only repository permissions and a short-lived event artifact. Raw events can contain prompts, tool output, and model responses, so do not publish that artifact publicly.

The same script has a hermetic replay mode used by this repository's CI:

python examples/ci_gate.py \
  --replay ../agent-sdk/__fixtures__/protocol/*.ndjson \
  --output protocol-events.ndjson

Conformance and tests

Python tests read the canonical fixtures in packages/agent-sdk/__fixtures__/protocol/ directly; there is no copied Python fixture set to drift from TypeScript and Java consumers.

cd packages/agent-sdk-python
PYTHONPATH=src python -m unittest discover -s tests -v

The language-neutral contract remains packages/agent-sdk/docs/PROTOCOL.md.