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fix(langchain): load LangChainTracer synchronously to avoid span loss #169

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fix(langchain): load LangChainTracer synchronously to avoid span loss #169

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38 changes: 11 additions & 27 deletions src/genai/instrumentations/langchain/langchainTraceInstrumentor.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,7 @@ import {
InstrumentationModuleDefinition,
isWrapped,
} from "@opentelemetry/instrumentation";
import type { LangChainTracer } from "./tracer.js";
import { LangChainTracer } from "./tracer.js";

type CallbackManagerModuleType = typeof CallbackManagerModule;
type LangChainTracerCtor = new (tracer: Tracer) => LangChainTracer;
Expand All @@ -21,10 +21,14 @@ class LangChainTraceInstrumentorImpl extends InstrumentationBase<Instrumentation
private _hasBeenEnabled = false;
private _isPatched = false;
protected otelTracer: Tracer;
/** Lazy-loaded to avoid eagerly importing @langchain/core/tracers/base
* (which transitively loads @langchain/core/callbacks/manager) before
* the instrumentation hooks for that module have been registered. */
private _tracerCtor: LangChainTracerCtor | undefined;
// Statically bound at module load. A previous lazy `import("./tracer.js")`
// resolved on a later microtask, so any `_configureSync` call that landed
// in that window (typically the first compiled-graph `invoke` after distro
// startup) silently fell through with no tracer attached, dropping the
// outer `invoke_agent LangGraph` wrapper span and fragmenting the trace.
// A static import is safe: by the time `patch()` runs, the callbacks
// manager module is already loaded (it is the `module` argument).
private _tracerCtor: LangChainTracerCtor = LangChainTracer;

private constructor() {
if (LangChainTraceInstrumentorImpl._instance !== null) {
Expand Down Expand Up @@ -91,35 +95,15 @@ class LangChainTraceInstrumentorImpl extends InstrumentationBase<Instrumentation
return module;
}

// Now that the hooks are registered and @langchain/core/callbacks/manager
// is loaded, it is safe to load tracer.js (and its transitive
// @langchain/core/tracers/base dependency). The promise resolves in the
// next microtask — well before any user code invokes a LangChain
// chain/agent/tool (which is what triggers _configureSync).
void import("./tracer.js").then(
(m) => {
this._tracerCtor = m.LangChainTracer;
},
(err) => {
diag.error(
`[LangChainTraceInstrumentor] Failed to load LangChainTracer: ${err instanceof Error ? err.message : String(err)}`,
);
},
);

// eslint-disable-next-line @typescript-eslint/no-this-alias
const instrumentor = this;
this._wrap(CallbackManager, "_configureSync", (original) => {
return function (
this: CallbackManagerModuleType,
...args: Parameters<(typeof CallbackManager)["_configureSync"]>
) {
if (instrumentor._tracerCtor) {
args[0] = addTracerToHandlers(instrumentor.otelTracer, args[0], instrumentor._tracerCtor);
diag.debug("[LangChainTraceInstrumentor] _configureSync wrapped to add LangChainTracer");
} else {
diag.debug("[LangChainTraceInstrumentor] LangChainTracer not yet loaded, skipping");
}
args[0] = addTracerToHandlers(instrumentor.otelTracer, args[0], instrumentor._tracerCtor);
diag.debug("[LangChainTraceInstrumentor] _configureSync wrapped to add LangChainTracer");
return original.apply(this, args);
};
});
Expand Down
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