prefactor-langchain 0.2.3

LangChain integration for Prefactor observability

prefactor-langchain

LangChain integration for Prefactor observability. This package provides automatic tracing for LangChain agents using LangChain-specific span types.

Installation

pip install prefactor-langchain

Usage

Factory pattern (quickest setup)

from prefactor_langchain import PrefactorMiddleware

middleware = PrefactorMiddleware.from_config(
    api_url="https://api.prefactor.ai",
    api_token="your-api-token",
    agent_id="my-agent",
    agent_name="My Agent",  # optional
)

# Use with LangChain's create_agent()
# Your agent will automatically create spans for:
# - Agent execution (langchain:agent)
# - LLM calls (langchain:llm)
# - Tool executions (langchain:tool)

result = agent.invoke({"messages": [...]})

# Middleware owns both client and instance; close when done
await middleware.close()

Pre-configured client

Pass a client you created yourself when you need full control over its configuration or when you want to share a client across multiple middlewares.

from prefactor_core import PrefactorCoreClient, PrefactorCoreConfig
from prefactor_http.config import HttpClientConfig
from prefactor_langchain import PrefactorMiddleware

http_config = HttpClientConfig(api_url="https://api.prefactor.ai", api_token="your-api-token")
config = PrefactorCoreConfig(http_config=http_config)
client = PrefactorCoreClient(config)
await client.initialize()

middleware = PrefactorMiddleware(
    client=client,
    agent_id="my-agent",
    agent_name="My Agent",
)

result = agent.invoke({"messages": [...]})

# You own the client; close both separately
await middleware.close()  # closes the agent instance only
await client.close()

Pre-configured instance (spans outside the agent)

Pass an AgentInstanceHandle you created yourself when you also need to instrument code that lives outside the LangChain agent — for example, pre-processing steps, post-processing, or any custom business logic that should appear as siblings of the agent spans in the same trace.

from prefactor_core import PrefactorCoreClient, PrefactorCoreConfig
from prefactor_http.config import HttpClientConfig
from prefactor_langchain import PrefactorMiddleware

http_config = HttpClientConfig(api_url="https://api.prefactor.ai", api_token="your-api-token")
config = PrefactorCoreConfig(http_config=http_config)
client = PrefactorCoreClient(config)
await client.initialize()

instance = await client.create_agent_instance(agent_id="my-agent")
await instance.start()

# Share the instance with the middleware
middleware = PrefactorMiddleware(instance=instance)

# Instrument your own code using the same instance
async with instance.span("custom:preprocessing") as ctx:
    ctx.set_payload({"step": "preprocess", "status": "ok"})

# Run your agent — the middleware traces it automatically under the same instance
result = agent.invoke({"messages": [...]})

async with instance.span("custom:postprocessing") as ctx:
    ctx.set_payload({"step": "postprocess", "result": str(result)})

# You own the instance and client; clean them up yourself
await instance.finish()
await client.close()

Span Types

This package creates LangChain-specific spans with the langchain:* namespace:

• langchain:agent - Agent executions and chain runs
• langchain:llm - LLM calls with model metadata (name, provider, token usage)
• langchain:tool - Tool executions including retrievers

Each span payload includes:

• Timing information (start_time, end_time)
• Inputs and outputs
• Error information with stack traces
• LangChain-specific metadata

Trace correlation (span_id, parent_span_id, trace_id) is handled automatically by the prefactor-core client.

Features

• Automatic LLM call tracing - Captures model name, provider, token usage, temperature
• Tool execution tracing - Records tool name, arguments, execution time
• Agent/chain tracing - Tracks agent lifecycle and message history
• Token usage capture - Automatically extracts prompt/completion/total tokens
• Error tracking - Captures error type, message, and stack traces
• Automatic parent-child relationships - Uses SpanContextStack for hierarchy
• Bring your own instance - Share a single AgentInstanceHandle between the middleware and your own instrumentation

Architecture

This package follows the LangChain Adapter Redesign principles:

1. Package Isolation: LangChain-specific span types and schemas live in this package
2. Opaque Payloads: Span data is sent as payload to prefactor-core
3. Type Namespacing: Uses langchain:agent, langchain:llm, langchain:tool prefixes
4. Uses prefactor-core: All span/instance management via the prefactor-core client

The middleware:

1. Accepts a PrefactorCoreClient, or a pre-created AgentInstanceHandle, or creates its own client via from_config()
2. Registers or borrows an agent instance
3. Creates spans with LangChain-specific payloads
4. Leverages SpanContextStack for automatic parent detection

Development

Run tests:

pytest tests/

License

MIT