H3 Harness SDK for Python

Python SDK for building H3-compliant agent harnesses.

Install

pip install h3-harness-sdk

Install fallback (source / git)

If a release isn't published to PyPI yet (or you want the latest unreleased changes), install directly from the repository:

# From git
pip install git+https://github.com/get-h3/sdk-python.git

# Editable source install (development)
git clone https://github.com/get-h3/sdk-python.git
cd sdk-python
pip install -e .

From a clean system (no pip preinstalled — e.g. a minimal Ubuntu container): bootstrap a venv first, then install into it:

python3 -m venv .venv
.venv/bin/pip install -e ".[dev]"
# or the one-command equivalent: make install

make install creates .venv (via python3 -m venv), upgrades pip inside it, and installs the package with dev extras — no system pip required.

Quickstart

from datetime import datetime, timezone

from h3_harness import (
    BaseHarness,
    Decision,
    DecisionType,
    End,
    TextResponse,
    create_router,
)
from fastapi import FastAPI


class MyHarness(BaseHarness):
    def __init__(self):
        # Track sessions so cancel/session lookups 404 on unknown ids
        # (battery: test_5_9b cancel_unknown_session, test_5_10 session_not_found).
        self._sessions: dict[str, dict] = {}

    async def on_process(self, req):
        # Echo conversation history from context (battery: history preserved).
        history = list(req.context.history)
        # Streaming: "do not finish" in message -> unfinished text.
        streaming = "do not finish" in req.message.content
        finished = not streaming
        self._sessions[req.session_id] = {
            "started_at": datetime.now(timezone.utc).isoformat(),
            "turn_count": (
                self._sessions.get(req.session_id, {}).get("turn_count", 0) + 1
            ),
        }
        return Decision(
            decision=DecisionType.TEXT,
            text=TextResponse(
                content=f"Echo: {req.message.content}",
                finished=finished,
            ),
            history=history,
        )

    async def on_result(self, req):
        return Decision(decision=DecisionType.END, end=End(reason="task_complete"))

    def get_session_info(self, session_id: str) -> dict | None:
        return self._sessions.get(session_id)

    async def on_session_terminate(self, session_id: str) -> None:
        # DELETE /v1/sessions/{id} -> forget the session so a later GET 404s.
        self._sessions.pop(session_id, None)


app = FastAPI()
app.include_router(create_router(MyHarness()))
# Run with: uvicorn my_harness:app --port 9191

Trying it out: with the server running, send a minimal request. The payload must include identity, context.config, and context.session_state (plus message and session_id) or the router rejects it with a 422:

curl -X POST http://localhost:9191/v1/process \
  -H 'Content-Type: application/json' \
  -d '{
    "session_id": "sess-001",
    "identity": {"chat_id": "chat-1", "platform": "cli"},
    "message": {"content": "Hello, harness!"},
    "context": {"config": {}, "session_state": {}, "history": []}
  }'

→ {"decision":"text","decision_id":"...","history":[],"text":{"content":"Echo: Hello, harness!","finished":true}}

Health is at /v1/health (not /health): curl http://localhost:9191/v1/health.

Session lifecycle

DELETE /v1/sessions/{id} invokes on_session_terminate(session_id) on the harness (unknown ids 404 at the router before the harness is involved). The base implementation is a no-op, so without an override a terminated session stays fully retrievable: the DELETE returns {"terminated": true}, but a later GET /v1/sessions/{id} still returns 200 with the full session. The fix is a 3-line cleanup — drop the session from your tracking dict, exactly as in the quickstart above:

async def on_session_terminate(self, session_id: str) -> None:
    # DELETE /v1/sessions/{id} -> forget the session so a later GET 404s.
    self._sessions.pop(session_id, None)

Keep this override in any harness that tracks sessions — it keeps harness state consistent with what the wire promises (a terminated session 404s on later GET, like any unknown session).

Testbed

import asyncio

from h3_harness.testbed import MockHermes
from h3_harness.examples.echo import EchoHarness


async def main() -> None:
    mock = MockHermes(EchoHarness())
    decision = await mock.send_message("Hello!")
    assert decision.text.content == "Echo: Hello!"


if __name__ == "__main__":
    asyncio.run(main())

Examples

• echo.py — Echo harness that mirrors user messages
• minimal.py — Minimal harness with health endpoint, uvicorn runner
• langchain_agent.py — LangChain integration: LLM call with text response

Each example exposes a module-level app and a python -m runner. Either works:

# uvicorn against the module-level app (any port)
uvicorn h3_harness.examples.echo:app --port 9191
uvicorn h3_harness.examples.minimal:app --port 8000

# or the built-in runner (echo.py accepts an optional port argument)
python -m h3_harness.examples.echo 9191
python -m h3_harness.examples.minimal

langchain_agent.py additionally requires pip install langchain langchain-openai.

Passing the battery (h3-test compliance)

The gate for any H3 harness is the test battery (test_battery.py from get-h3/shim — 45 tests across 6 categories). Run it against any running harness endpoint:

# The shim is not yet published to PyPI — install from source (get-h3/shim)
pip install git+https://github.com/get-h3/shim
h3-test --endpoint http://localhost:9191   # exit 0 = compliant

The Quickstart harness above implements all four conventions and is fully battery-compliant (45/45). If you modify it, keep the conventions intact — a naive harness that drops them scores 42/45. The four conventions the battery checks (beyond "return a Decision") are:

1. Echo context.history in every Decision returned from on_process. The battery sends a session with prior history and asserts it flows back through the response (test_2_8_process_preserves_history). Pass it through explicitly:

   history = list(req.context.history)
   return Decision(..., history=history)
   

   This applies to decisions returned from on_process — the ProcessRequest carries the context field. on_result receives a ResultRequest, which has no context field (only decision_id/result/session_id); a decision returned from on_result simply omits history. Echoing req.context.history there raises AttributeError.
2. Never issue llm_call when context.models is empty. The battery sends context.models: [] and FAILS any harness that returns an llm_call decision (test_5_8_no_models_available — "hallucinated model"). Only return LLM_CALL when the request actually lists models.
3. Return text.finished=false for "do not finish" prompts. The battery sends "Just start a thought, do not finish it yet." and asserts the response has text.finished == False (test_2_4_process_text_finished_false). Detect streaming/unfinished intent and set finished accordingly.
4. 404 unknown sessions. The battery cancels a nonexistent session (test_5_9b cancel_unknown_session) and GETs one (test_5_10 session_not_found) and asserts a 404. Track sessions in the harness (get_session_info returning None for unknown ids) — the router turns that into the 404.

The canonical battery-ready template is echo.py — it implements all four conventions and scores 45/45. Use it as the starting point for your own harness.

Error handling

Exceptions raised inside your harness's on_process / on_result are caught by the router and masked as a successful HTTP 200 end decision:

{"decision": "end", "end": {"reason": "error", "summary": "<exception text>"}}

This masking is the current contract — the spec (get-h3/h3 → specs/04-SDK-Libraries.md) is silent on handler exceptions, and the behavior is locked in by tests/test_handler_crash.py. From the shim's point of view the session simply ends: it sees a normal end and stops, so the session dies silently. If you need to distinguish a crash from a real completion, validate the decision — the end.reason == "error" marker (and the server-side on_process failed / on_result failed log lines from logger.exception) is the only signal.

Real HTTP 500s are reserved for non-handler failures and are not disturbed by the masking: exceptions from on_cancel / on_session_terminate surface as HTTP 500 ({"detail": ...}), and any exception that escapes the router entirely is caught by the logging middleware's catch-all, which returns an ErrorResponse with ErrorCode.INTERNAL_ERROR. Handler-crash coverage in the shim battery (get-h3/shim → test_battery.py) belongs to that repo — it is out of scope for this SDK.

Result payloads

result["tool_calls"] is a LIST. When a result carries tool calls (OpenAI-style LLM APIs), req.result["tool_calls"] is a list[dict], one entry per call — never a single dict:

tool_calls = req.result.get("tool_calls") or []
if not isinstance(tool_calls, list):
    tool_calls = [tool_calls]  # defensive: accept a lone dict too
for call in tool_calls:
    print(call["name"])  # each entry is a plain dict

Dict-style access on the list (req.result["tool_calls"].get("name")) raises AttributeError: 'list' object has no attribute 'get' — which, under the error contract above, surfaces as a silent HTTP 200 end/error and kills the session with no signal.

req.result itself is a plain dict — read fields with .get(), never attribute style (req.result.type raises AttributeError). The canonical shape is ResultPayload; see docs/api/protocol.md.

GET /v1/sessions/{id} reports session status: "active" (default) or "completed". Harnesses that track lifecycle return a status key from get_session_info ("completed" once the loop reaches end) and the router passes it through.

Development

make install   # create venv + install deps
make build     # build wheel (and sdist) into dist/
make test      # run tests
make lint      # ruff check
make fmt       # ruff format

Running tests: use the project venv — make install then .venv/bin/pytest (147 tests). Bare pytest on an ambient interpreter may fail to import h3_harness; pytest.ini's pythonpath = src covers collection from the source tree without an install, but the project venv is the supported path.

Reference

• Spec: get-h3/h3 — specs/04-SDK-Libraries.md
• Protocol: get-h3/protocol
• API reference: docs/api/index.md