3tears-langgraph

Three-tier LangGraph checkpoint saver: L1 (SQLite) -> L2 (NATS KV) -> L3 (PostgreSQL).

L1 and L2 are optional cache layers that degrade gracefully on failure. L3 (PostgreSQL) is the source of truth, reached through the AsyncQueryExecutor protocol so the same saver serves trusted services (direct asyncpg pool) and sandboxed agents (NATS L3 proxy).

L3 failures reach the caller, with one channel-scoped carve-out: aput_writes is called from LangGraph's executor teardown, where raising kills a turn that has already answered, so a failed crash-recovery write is logged and degraded. Writes on LangGraph's control channels (__interrupt__, __resume__, __error__, __scheduled__) still raise: losing one changes what the run does, and a lost __interrupt__ would silently skip a human-approval gate. A control-channel write also overwrites an earlier write on the same channel, matching the reference saver, so a second Command(resume=...) replaces the first rather than being dropped.

Installation

pip install 3tears-langgraph

Usage

from threetears.langgraph import (
    AsyncpgPoolAdapter,
    ThreeTierCheckpointSaver,
)

# Trusted service with direct asyncpg.Pool: wrap once
saver = ThreeTierCheckpointSaver(executor=AsyncpgPoolAdapter(pool))

# Sandboxed agent: NatsProxyL3Backend already implements
# AsyncQueryExecutor, pass it straight through
saver = ThreeTierCheckpointSaver(executor=nats_l3_backend)

graph = builder.compile(checkpointer=saver)

Middleware

The package ships platform-level AgentMiddleware for langchain.agents.create_agent — the framework-aligned successor to the old hand-rolled AgentNodeHook / ToolNodeHook protocols. Consumer-specific policy lives in each consumer as its own middleware; only the reusable platform seams live here:

  • PromptCachingMiddleware (wrap_model_call) — annotates a leading bare-string system message with Anthropic cache_control={"type": "ephemeral"} when the model supports it, then normalizes cache-hit/creation counters onto usage_metadata["cache_usage"]. Non-Anthropic adapters degrade silently to bare-string system messages.
  • ToolResultOffloadMiddleware (wrap_tool_call) — when a ToolResultOffloader is injected on config["configurable"] and a tool result exceeds offload_threshold_chars, stores the full content out-of-band and shows the model "<summary>\n\n[ctx:<handle>]" (the structured artifact is preserved). Opt-in: no offloader ⇒ byte-for-byte no-op.
  • ObjectCatalogMiddleware (wrap_tool_call) — when a tool returns an ObjectHandle in its result artifact and an ObjectCataloger is injected, persists a catalog record under the verified call identity. Soft-fail side-effect: a catalog error never breaks the tool result.
from langchain.agents import create_agent

from threetears.langgraph import (
    ObjectCatalogMiddleware,
    PromptCachingMiddleware,
    ToolResultOffloadMiddleware,
)

agent = create_agent(
    model=chat_anthropic,
    tools=tools,
    middleware=[
        PromptCachingMiddleware(),
        ToolResultOffloadMiddleware(),
        ObjectCatalogMiddleware(),
    ],
)
# after a run, PromptCachingMiddleware has stamped:
# message.usage_metadata["cache_usage"]
# == {"cache_read_input_tokens": ..., "cache_creation_input_tokens": ..., "cached_tokens": ...}

The offload / catalog contracts (ToolResultOffloader, ObjectCataloger) are pure structural Protocols exported from threetears.langgraph.offload / threetears.langgraph.catalog; a consumer injects a concrete implementation on config["configurable"] (e.g. tool_result_offloader, object_cataloger) without the package taking any dependency on the consumer's context store.

See 3tears/docs/prompt-caching.md for the full caching contract, summarization interaction, downstream wiring checklist, and a worked example.

Streaming

The package ships StreamingResponse, a transport-agnostic primitive that owns the lifecycle of one streaming response: start -> any number of emit_token / emit_tool_call_* -> mutually-exclusive end (success) or error (failure) terminal. run_graph(compiled_graph, state, config) consumes a LangGraph astream_events(version="v2") loop with the start/end ordering managed; on graph exception it fires error(code="AGENT_FAILED", ...) and re-raises so the caller still sees the failure on the synchronous path.

The wire vocabulary is fixed: StreamStartEvent / StreamTokenEvent / StreamEndEvent / StreamErrorEvent / ToolCallStartEvent / ToolCallEndEvent / ToolCallProgressEvent, dispatched via the StreamEvent discriminated union and the parse_stream_event(payload) adapter. The transport seam is the StreamTransport Protocol -- one method, async def publish(self, payload: bytes) -> None. Any wire (NATS subject, websocket, chunked HTTP body) satisfies it.

from threetears.langgraph import StreamingResponse, StreamTransport

class WebSocketStreamTransport:
    """example transport for a websocket consumer."""
    def __init__(self, ws): self._ws = ws
    async def publish(self, payload: bytes) -> None:
        await self._ws.send_bytes(payload)

stream = StreamingResponse(
    transport=WebSocketStreamTransport(ws),
    correlation_id=correlation_id,
    conversation_id=conversation_id,
    start_time_monotonic=request_start,
)
final_state = await stream.run_graph(compiled_graph, state, config)

A reference adapter can bind the primitive to a per-correlation-id stream subject via nc.publish_raw. Tool-call observation envelopes flow through ToolCallProgressHook reading the active StreamingResponse from config["configurable"]["streaming_response"].

Project links

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  • Released:
Newer release available (0.41.4)

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Credits

Author: pace

GitHub Statistics

Data verified by PyPI on Aug 18, 2026
The GitHub source repository was provided by the project maintainers and verified by PyPI at the time of upload. Stars, forks, and open issues/PRs are derived from that repository and have not been independently verified.

License expression

MIT
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Requires

Python >=3.14

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