LangChain Integration

Strathon evaluates every LangChain tool call against your policies before it executes: a matched block or throttle stops the call at the callback boundary. Chains, agents, tools, and LLM calls run through the same BaseCallbackHandler used for LangGraph, which you attach to each invocation.

Enforcement scope: the LangChain callback surface is synchronous. block and throttle enforce (the tool never runs); steer is recorded but the original tool still runs; require_approval fails closed (the call is blocked and recorded) because a sync callback cannot pause for a human decision. For steer substitution or interactive approval, use enforce_steer (tool-invoke wrapping). Full picture in the approval matrix.

Installation

bash

pip install "strathon[langchain]"

Setup

python

from strathon import Client
from strathon.instrumentation.langchain import instrument

client = Client(
    api_key="stra_...",
    endpoint="http://localhost:4318",
)

# instrument() returns a LangChain callback handler. Strathon enforces and
# traces through it, so attach it to every chain or agent invocation.
handler = instrument(client)

result = chain.invoke(
    {"input": "..."},
    config={"callbacks": [handler]},
)

What Gets Captured

Chain runs: start, end, error
LLM calls: model, tokens, latency, prompt/completion
Tool calls: tool name, arguments, return value
Retriever calls: query, document count, sources

Each chain invocation creates a trace with nested spans.

Example Policy

Block tool calls to unapproved external APIs:

cel

attrs["gen_ai.tool.name"] == "requests_get"
  && !attrs["strathon.tool.args"].contains("api.internal.com")

Throttle retrieval calls to prevent runaway RAG loops:

cel

attrs["gen_ai.tool.name"] == "vector_search"

With action throttle and a rate limit, Strathon caps the call frequency.

Notes

Shares the handler with LangGraph; one handler covers both.
The handler must be passed on every invocation via config={"callbacks": [handler]}. LangChain has no global callback registry, so an unattached handler does nothing: no spans, no enforcement.
Requires langchain-core>=1.5.1 (installed by the langchain extra); works with LangChain 0.3+ and LangChain Community packages.