fix(node): Preserve CallbackManager handlers in LangChain instrumentation

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+- fix(node): Preserve `CallbackManager` children when augmenting LangChain callbacks
 - "You miss 100 percent of the chances you don't take. — Wayne Gretzky" — Michael Scott
 
 ## 10.53.1

packages/node/src/integrations/tracing/langchain/instrumentation.ts

@@ -28,7 +28,48 @@ interface PatchedLangChainExports {
 }
 
 /**
- * Augments a callback handler list with Sentry's handler if not already present
+ * Duck-types a LangChain `CallbackManager` instance. We can't `instanceof`
+ * check because `@langchain/core` may be bundled, deduped, or absent from
+ * our module graph; checking the shape avoids that coupling.
+ */
+function isCallbackManager(value: unknown): value is {
+  addHandler: (handler: unknown, inherit?: boolean) => void;
+  copy: () => unknown;
+  handlers?: unknown[];
+} {
+  if (!value || typeof value !== 'object') {
+    return false;
+  }
+  const candidate = value as { addHandler?: unknown; copy?: unknown };
+  return typeof candidate.addHandler === 'function' && typeof candidate.copy === 'function';
+}
+
+/**
+ * Exported for testing.
+ * @internal
+ */
+export const _INTERNAL_augmentCallbackHandlers = augmentCallbackHandlers;
+
+/**
+ * Augments a callback handler list with Sentry's handler if not already present.
+ *
+ * `options.callbacks` may be one of three shapes (per LangChain's RunnableConfig):
+ *   - `undefined`           → no callbacks configured
+ *   - `BaseCallbackHandler[]` → list of handler instances
+ *   - `CallbackManager`     → a manager that already holds (potentially
+ *                             inheritable) child handlers
+ *
+ * The `CallbackManager` case is the load-bearing one: when LangGraph sets up
+ * a run with `streamMode: ['messages']`, it puts a `StreamMessagesHandler`
+ * onto a `CallbackManager` and passes that manager through `options.callbacks`.
+ * If we naively wrap the manager into `[manager, sentryHandler]`, LangChain
+ * downstream treats the whole manager as a single opaque handler — its
+ * inheritable children (`StreamMessagesHandler`, the tracer, etc.) are never
+ * unpacked, and per-token streaming events silently disappear.
+ *
+ * Instead, when we receive a `CallbackManager`, we copy it (so we don't
+ * mutate the caller's manager across invocations) and register Sentry's
+ * handler as an inheritable child via `.addHandler()`.
  */
 function augmentCallbackHandlers(handlers: unknown, sentryHandler: unknown): unknown {
   // Handle null/undefined - return array with just our handler
@@ -46,9 +87,20 @@ function augmentCallbackHandlers(handlers: unknown, sentryHandler: unknown): unk
     return [...handlers, sentryHandler];
   }
 
-  // If it's a single handler object, convert to array
-  if (typeof handlers === 'object') {
-    return [handlers, sentryHandler];
+  // CallbackManager: register our handler as an inheritable child on a copy
+  // so we preserve any handlers already attached (notably LangGraph's
+  // StreamMessagesHandler used by `streamMode: ['messages']`).
+  if (isCallbackManager(handlers)) {
+    const copied = handlers.copy() as {
+      addHandler: (handler: unknown, inherit?: boolean) => void;
+      handlers?: unknown[];
+    };
+    // Avoid double-registering if the caller already added us.
+    const existing = copied.handlers ?? [];
+    if (!existing.includes(sentryHandler)) {
+      copied.addHandler(sentryHandler, true);
+    }
+    return copied;
   }
 
   // Unknown type - return original

packages/node/test/integrations/tracing/langchain.test.ts

@@ -0,0 +1,87 @@
+import { describe, expect, test, vi } from 'vitest';
+import { _INTERNAL_augmentCallbackHandlers } from '../../../src/integrations/tracing/langchain/instrumentation';
+
+const sentryHandler = { name: 'SentryCallbackHandler' };
+
+/**
+ * Minimal `CallbackManager` stand-in. We only need the duck-typed shape
+ * (`addHandler` + `copy`) for the production code to recognize this as a
+ * `CallbackManager` rather than fall through to the "unknown" branch.
+ */
+function makeFakeCallbackManager(existingHandlers: unknown[] = []) {
+  const manager = {
+    handlers: [...existingHandlers],
+    addHandler: vi.fn(function (this: any, handler: unknown, _inherit?: boolean) {
+      this.handlers.push(handler);
+    }),
+    copy: vi.fn(function (this: any) {
+      return makeFakeCallbackManager(this.handlers);
+    }),
+  };
+  return manager;
+}
+
+describe('augmentCallbackHandlers', () => {
+  test('wraps Sentry handler in an array when no callbacks are configured', () => {
+    const result = _INTERNAL_augmentCallbackHandlers(undefined, sentryHandler);
+    expect(result).toEqual([sentryHandler]);
+  });
+
+  test('appends Sentry handler when callbacks is already an array', () => {
+    const other = { name: 'OtherHandler' };
+    const result = _INTERNAL_augmentCallbackHandlers([other], sentryHandler);
+    expect(result).toEqual([other, sentryHandler]);
+  });
+
+  test('is idempotent when Sentry handler is already in the array', () => {
+    const result = _INTERNAL_augmentCallbackHandlers([sentryHandler], sentryHandler);
+    expect(result).toEqual([sentryHandler]);
+  });
+
+  test('preserves inheritable handlers when callbacks is a CallbackManager', () => {
+    // Reproduces the LangGraph `streamMode: ['messages']` setup: a
+    // CallbackManager carrying a StreamMessagesHandler is passed via
+    // options.callbacks. Without this fix, the manager would be wrapped as
+    // `[manager, sentryHandler]`, dropping all its inheritable children.
+    const streamMessagesHandler = {
+      name: 'StreamMessagesHandler',
+      lc_prefer_streaming: true,
+    };
+    const manager = makeFakeCallbackManager([streamMessagesHandler]);
+
+    const result = _INTERNAL_augmentCallbackHandlers(manager, sentryHandler) as {
+      handlers: unknown[];
+    };
+
+    // The result is a manager (object), not a wrapping array.
+    expect(Array.isArray(result)).toBe(false);
+    // The original child handler is still there alongside Sentry's.
+    expect(result.handlers).toEqual([streamMessagesHandler, sentryHandler]);
+  });
+
+  test('copies the manager rather than mutating the caller-supplied one', () => {
+    // If we mutated the original manager, repeated invocations would
+    // accumulate Sentry handlers (and tracers from prior runs would leak
+    // into subsequent unrelated runs).
+    const manager = makeFakeCallbackManager([]);
+    _INTERNAL_augmentCallbackHandlers(manager, sentryHandler);
+    expect(manager.copy).toHaveBeenCalledTimes(1);
+    expect(manager.handlers).toEqual([]);
+  });
+
+  test('does not double-register Sentry handler when copy already contains it', () => {
+    const manager = makeFakeCallbackManager([sentryHandler]);
+    const result = _INTERNAL_augmentCallbackHandlers(manager, sentryHandler) as {
+      handlers: unknown[];
+      addHandler: ReturnType<typeof vi.fn>;
+    };
+    expect(result.handlers).toEqual([sentryHandler]);
+    expect(result.addHandler).not.toHaveBeenCalled();
+  });
+
+  test('returns the value unchanged when it is neither an array nor a CallbackManager', () => {
+    const opaque = { name: 'NotAManager' };
+    const result = _INTERNAL_augmentCallbackHandlers(opaque, sentryHandler);
+    expect(result).toBe(opaque);
+  });
+});