Fiddler LangChain SDK

1.0

1.0.0

March 18, 2026

Initial release of the Fiddler LangChain SDK.

• Added

  • FiddlerLangChainInstrumentor: Auto-instrumentor for LangChain V1 agents. Monkey-patches langchain.agents.create_agent so every subsequent call automatically receives a FiddlerAgentMiddleware. Idempotent — calling instrument() multiple times is safe.

    • instrument(): Enable automatic instrumentation of all create_agent calls.

    • uninstrument(): Restore the original create_agent function. Does not affect already-created agents.

    • is_instrumented_by_opentelemetry: Property indicating whether instrumentation is currently active.

  • FiddlerAgentMiddleware: LangChain V1 AgentMiddleware that instruments agents with Fiddler tracing. Produces a clean, flat trace hierarchy: agent → LLM calls → tool calls, with no noisy Chain wrappers.

    • before_agent(): Opens a root Agent span (TYPE=agent) at the start of each invocation.

    • after_agent(): Closes the root Agent span.

    • wrap_model_call() / awrap_model_call(): Wraps synchronous and asynchronous model calls with LLM spans (TYPE=llm). Captures model name, provider, system prompt, user prompt, full message history, output messages, token usage, LLM context, and tool definitions.

    • wrap_tool_call() / awrap_tool_call(): Wraps synchronous and asynchronous tool calls with Tool spans (TYPE=tool). Captures tool name, input, and output.

  • set_llm_context(): Attach a context string to a BaseLanguageModel or RunnableBinding. The middleware reads this at invocation time and sets gen_ai.llm.context on the LLM span.

  • add_span_attributes(**kwargs): Attach custom metadata to a specific LangChain component (model, tool, or retriever). Emitted as fiddler.span.user.{key} only on the span for that component — scoped, unlike add_session_attributes.

  • set_conversation_id(): Re-exported from fiddler_otel for convenience. Propagates conversation ID to all spans in the current execution context.

  • add_session_attributes(): Re-exported from fiddler_otel. Attaches session-level metadata to all spans in the current context.

  • Full async support: awrap_model_call and awrap_tool_call hooks fully support agent.ainvoke().

  • Error handling: Failing LLM or tool spans are marked StatusCode.ERROR with the exception recorded. The root Agent span is always closed cleanly, preventing dangling open spans. Exceptions are re-raised so normal application error handling is unaffected.

  • Multi-agent single-trace nesting: When a sub-agent is invoked from within a delegation tool, its root Agent span is automatically created as a child of the active tool span — the entire multi-agent flow appears in one trace. Top-level agents (no active parent span) start a new trace as usual.

  • Retriever-as-tool: Retrievers wrapped with @tool are automatically traced as TYPE=tool spans.

  • JSONL local capture: Inherited from fiddler-otel. Use jsonl_capture_enabled=True on FiddlerClient. The FIDDLER_JSONL_FILE environment variable overrides the output file path.

0.1

0.1.1

March 18, 2026

Pre-release.