errand-langgraph

Run LangGraph graphs as errand jobs: background execution with status polling, human-in-the-loop resume, event streaming, smart retries, and an auto-generated FastAPI router — no Celery, no separate broker.

Status: published on PyPI. Submit/status, the FastAPI router, human-in-the-loop resume, SSE streaming, and smart retries are all implemented and tested (100% coverage) — see CHANGELOG.md for what shipped in each version.

Requires Python 3.11+. interrupt() is broken under Python 3.10 in recent langgraph releases (a real, verified upstream bug, not a guess — RuntimeError: Called get_config outside of a runnable context, reproduced in plain langgraph with no errand-langgraph involved). Since human-in-the-loop is this package's flagship feature, 3.10 isn't supported.

Define your graph's state with typing_extensions.TypedDict, not typing.TypedDict. On Python < 3.12, Pydantic v2 can't introspect a typing.TypedDict (PydanticUserError) — mount_graph's request-body validation silently falls back to an unvalidated dict[str, Any] if you get this wrong (with a warning telling you why). from typing_extensions import TypedDict, NotRequired avoids it entirely and works the same on every supported Python version.

Pass a checkpointer. Without one, retries re-run the whole graph from scratch instead of resuming (GraphRunner warns at construction time), and interrupt detection falls back to a less robust signal (still correct, just more exposed to LangGraph's own internal changes across versions — see runner.py's docstring). HITL and efficient retries both assume one.

Install

pip install errand-langgraph[fastapi]

Why

A LangGraph agent run can take anywhere from a few seconds to several minutes. BackgroundTasks in FastAPI gives you no state and no way to poll progress; Celery gives you a broker and a deploy story you probably don't need for a single machine. errand is the middle ground for background jobs in general — this package specializes it for graphs: submit a run, poll its status, resume it after an interrupt(), stream its events, and get retries that resume from the last checkpoint instead of re-running the whole graph (and re-paying for every LLM call along the way).

Quickstart

from fastapi import FastAPI
from langgraph.checkpoint.memory import InMemorySaver

from errand_langgraph import GraphRunner
from errand_langgraph.fastapi import mount_graph

from your_agent import graph  # an uncompiled StateGraph, or a compiled one

runner = GraphRunner(graph, checkpointer=InMemorySaver())
app = FastAPI(lifespan=runner.lifespan)  # starts/drains the worker pool
mount_graph(app, runner, prefix="/agent")

This mounts six endpoints:

curl -X POST localhost:8000/agent/runs -d '{"messages": [["user", "hi"]]}'
# {"job_id": "...", "thread_id": "..."}

curl localhost:8000/agent/runs/<job_id>
# {"state": "succeeded", "result": {...}, ...}
# or, if a node called interrupt():
# {"state": "interrupted", "interrupt": [{"id": "...", "value": {...}}], ...}

curl -X POST localhost:8000/agent/runs/<job_id>/resume -d 'true'
# {"job_id": "<new job id>", "thread_id": "<same thread>"}

Without FastAPI, drive the same thing directly:

from errand_langgraph import GraphRunner

runner = GraphRunner(graph, checkpointer=InMemorySaver())
await runner.startup()
handle = await runner.submit({"messages": [("user", "hi")]})
status = await runner.status(handle.job_id)
# after an interrupt:
resumed = await runner.resume(handle.job_id, value=True)
await runner.shutdown()

See examples/basic/ for a runnable submit/poll version, examples/hitl/ for the full interrupt/resume cycle, and examples/streaming/ for SSE — all three actually run end to end (server + client, no LLM calls).

Retries are configured with a RetryPolicy:

from errand_langgraph import GraphRunner, RetryPolicy

runner = GraphRunner(
    graph,
    checkpointer=InMemorySaver(),
    retry=RetryPolicy(max_attempts=5, base_delay=1.0, max_delay=30.0),
)

The default policy (3 attempts, exponential backoff with full jitter) retries timeouts, connection errors, and HTTP 429/5xx — duck-typed off a status_code attribute so it works with any HTTP-based provider SDK without importing it. Everything else (state-validation errors, GraphRecursionError, auth errors, a tool's own exceptions) isn't retried by default. With a checkpointer, a retry resumes from the last completed node instead of re-running the graph — pass your own is_retryable predicate for anything more specific:

RetryPolicy(is_retryable=lambda exc: isinstance(exc, MyProviderRateLimitError))

Pass RetryPolicy(max_attempts=1) to disable retrying entirely.

Don't forget startup. Nothing runs submitted work without a running worker pool — wire lifespan=runner.lifespan into FastAPI(...) (as above) or call await runner.startup() yourself before submitting. submit()/ resume() warn if you forget.

Streaming is in-process only. GET /runs/{id}/events reads from an in-memory pubsub inside the worker that's actually running the graph — it only works when the client's request and that worker share this process. Behind a load balancer with multiple worker processes, the SSE request has to land on the specific process that owns the run; this package doesn't route that for you. Polling GET /runs/{id} always works regardless of process topology — reach for streaming only where the single-process constraint is already true (which is most errand-based deployments, but not all).

Roadmap

The full feature set below is implemented, tested, and published. See CHANGELOG.md for what shipped in each version.

• Done: GraphRunner.submit/status, mount_graph with polling.
• Done: human-in-the-loop — interrupt() detection, resume(), POST /runs/{id}/resume, thread state/history endpoints.
• Done: SSE streaming of graph events (in-process only — documented above, not hidden).
• Done: smart retries — error classification, backoff with full jitter, resume from the last checkpoint instead of re-running the graph (and re-paying for every LLM call) from scratch.

Not yet built: cancelling an in-flight run, metrics/observability, and a registry for mounting more than one graph under a single FastAPI app (today it's one mount_graph call per graph).

License

MIT