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LangChain Python ships OTel via LangSmith. Register a TracerProvider pointing at Agnost before importing LangChain, enable LangSmith’s OTel mode, then pass session_id / user_id per call.

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1. Install

Already have LangSmith with the OTel extra and an OTLP exporter installed? Skip. No setup yet? 
pip install "langsmith[otel]" langchain \
            opentelemetry-sdk opentelemetry-exporter-otlp-proto-http

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2. Wire OTel + LangSmith pointing at Agnost

The TracerProvider must be registered before importing LangChain — LangSmith detects an existing provider on import and uses it. Already have an OTel TracerProvider? Append Agnost as an additional span processor and switch LangSmith into OTel mode: 
import os
from opentelemetry import trace
from opentelemetry.sdk.trace.export import BatchSpanProcessor
from opentelemetry.exporter.otlp.proto.http.trace_exporter import OTLPSpanExporter

trace.get_tracer_provider().add_span_processor(
    BatchSpanProcessor(
        OTLPSpanExporter(
            endpoint="https://otel.agnost.ai/v1/traces",
            headers={"X-Agnost-Org-ID": os.environ["AGNOST_ORG_ID"]},
        )
    )
)

os.environ["LANGSMITH_OTEL_ENABLED"] = "true"
os.environ["LANGSMITH_TRACING"] = "true"
os.environ["LANGSMITH_OTEL_ONLY"] = "true"  # don't ship to LangSmith's hosted backend
No OTel yet? Full setup: 
import os
from opentelemetry import trace
from opentelemetry.sdk.trace import TracerProvider
from opentelemetry.sdk.trace.export import BatchSpanProcessor
from opentelemetry.exporter.otlp.proto.http.trace_exporter import OTLPSpanExporter

provider = TracerProvider()
provider.add_span_processor(
    BatchSpanProcessor(
        OTLPSpanExporter(
            endpoint="https://otel.agnost.ai/v1/traces",
            headers={"X-Agnost-Org-ID": os.environ["AGNOST_ORG_ID"]},
        )
    )
)
trace.set_tracer_provider(provider)

os.environ["LANGSMITH_OTEL_ENABLED"] = "true"
os.environ["LANGSMITH_TRACING"] = "true"
os.environ["LANGSMITH_OTEL_ONLY"] = "true"

# Import LangChain AFTER the provider is registered
from langchain_openai import ChatOpenAI
LANGSMITH_OTEL_ONLY=true stops LangSmith from also shipping to its hosted dashboard — no LangSmith API key needed.

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3. Pass user_id / session_id per call

result = agent.invoke(
    {"messages": [{"role": "user", "content": "Hello"}]},
    config={
        "metadata": {
            "session_id": "conv-abc123",
            "user_id": "user-42",
        },
    },
)
LangChain prefixes any key in config.metadata with langsmith.metadata.* — Agnost reads langsmith.metadata.session_id and langsmith.metadata.user_id natively for user / session grouping.

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References