diff --git a/packages/core/src/shared-exports.ts b/packages/core/src/shared-exports.ts
index dcc8c268f509..6edcb3224d81 100644
--- a/packages/core/src/shared-exports.ts
+++ b/packages/core/src/shared-exports.ts
@@ -173,6 +173,7 @@ export { createLangChainCallbackHandler, instrumentLangChainEmbeddings } from '.
 export { LANGCHAIN_INTEGRATION_NAME } from './tracing/langchain/constants';
 export type { LangChainOptions, LangChainIntegration } from './tracing/langchain/types';
 export { instrumentStateGraphCompile, instrumentCreateReactAgent, instrumentLangGraph } from './tracing/langgraph';
+export { mergeSentryCallback } from './tracing/langgraph/utils';
 export { LANGGRAPH_INTEGRATION_NAME } from './tracing/langgraph/constants';
 export type { LangGraphOptions, LangGraphIntegration, CompiledGraph } from './tracing/langgraph/types';
 export type { OpenAiClient, OpenAiOptions, InstrumentedMethod } from './tracing/openai/types';
diff --git a/packages/core/src/tracing/langgraph/utils.ts b/packages/core/src/tracing/langgraph/utils.ts
index 8770cbbd629b..0fadded3c82d 100644
--- a/packages/core/src/tracing/langgraph/utils.ts
+++ b/packages/core/src/tracing/langgraph/utils.ts
@@ -335,7 +335,41 @@ export function setResponseAttributes(span: Span, inputMessages: LangChainMessag
   }
 }
 
-/** Merge `sentryHandler` into a langchain `callbacks` value (`BaseCallbackHandler[]` or `BaseCallbackManager`). */
+/**
+ * Detects a LangChain `CallbackManager` (or subclass) without depending on `instanceof`.
+ * `@langchain/core` is frequently bundled or deduped, so the imported constructor doesn't
+ * necessarily match the one at the user's call site. We walk the prototype chain looking
+ * for the class name, then confirm the shape — the constructor-name check rules out
+ * unrelated objects that happen to expose `addHandler`/`copy`.
+ */
+function isCallbackManager(value: unknown): value is {
+  addHandler: (handler: unknown, inherit?: boolean) => void;
+  copy: () => unknown;
+  handlers?: unknown[];
+} {
+  if (!value || typeof value !== 'object') {
+    return false;
+  }
+
+  let proto: object | null = Object.getPrototypeOf(value);
+  while (proto) {
+    if ((proto as { constructor?: { name?: string } }).constructor?.name === 'CallbackManager') {
+      const candidate = value as { addHandler?: unknown; copy?: unknown };
+      return typeof candidate.addHandler === 'function' && typeof candidate.copy === 'function';
+    }
+    proto = Object.getPrototypeOf(proto);
+  }
+  return false;
+}
+
+/**
+ * Merge `sentryHandler` into a langchain `callbacks` value (undefined, `BaseCallbackHandler[]`, or `BaseCallbackManager`).
+ *
+ * Wrapping a `CallbackManager` into `[manager, sentryHandler]` would make LangChain treat the whole manager
+ * as one opaque handler and drop its inheritable children — notably LangGraph's `StreamMessagesHandler`,
+ * which silently breaks per-token streaming. We register on a `.copy()` (so caller state stays clean across
+ * runs) and add ourselves as inheritable so `getChild()` propagates us into nested calls.
+ */
 export function mergeSentryCallback(existing: unknown, sentryHandler: unknown): unknown {
   if (!existing) {
     return [sentryHandler];
@@ -348,12 +382,15 @@ export function mergeSentryCallback(existing: unknown, sentryHandler: unknown):
     return [...existing, sentryHandler];
   }
 
-  const manager = existing as { addHandler?: (h: unknown) => void; handlers?: unknown[] };
-  if (typeof manager.addHandler === 'function') {
-    const alreadyAdded = Array.isArray(manager.handlers) && manager.handlers.includes(sentryHandler);
-    if (!alreadyAdded) {
-      manager.addHandler(sentryHandler);
+  if (isCallbackManager(existing)) {
+    const copied = existing.copy() as {
+      addHandler: (handler: unknown, inherit?: boolean) => void;
+      handlers?: unknown[];
+    };
+    if (!copied.handlers?.includes(sentryHandler)) {
+      copied.addHandler(sentryHandler, true);
     }
+    return copied;
   }
 
   return existing;
diff --git a/packages/core/test/lib/utils/langgraph-utils.test.ts b/packages/core/test/lib/utils/langgraph-utils.test.ts
index 829317518622..f666a4d6693c 100644
--- a/packages/core/test/lib/utils/langgraph-utils.test.ts
+++ b/packages/core/test/lib/utils/langgraph-utils.test.ts
@@ -48,6 +48,34 @@ describe('extractAgentNameFromParams', () => {
 describe('mergeSentryCallback', () => {
   const sentryHandler = { _sentry: true };
 
+  /**
+   * Minimal `CallbackManager` stand-in. Mirrors `@langchain/core`'s real
+   * semantics: `addHandler(_, inherit)` pushes to both `handlers` and
+   * `inheritableHandlers` when `inherit !== false`, and `copy()` returns
+   * a fresh manager carrying the same handlers — so we don't accidentally
+   * test against a degenerate shape that bypasses `addHandler`.
+   */
+  function makeFakeCallbackManager(existingHandlers: unknown[] = [], existingInheritableHandlers?: unknown[]) {
+    // Use a class so `Object.getPrototypeOf(instance).constructor.name === 'CallbackManager'`,
+    // which is how the production detector identifies a real LangChain CallbackManager.
+    class CallbackManager {
+      public handlers: unknown[];
+      public inheritableHandlers: unknown[];
+      public addHandler = vi.fn((handler: unknown, inherit?: boolean) => {
+        this.handlers.push(handler);
+        if (inherit !== false) {
+          this.inheritableHandlers.push(handler);
+        }
+      });
+      public copy = vi.fn(() => makeFakeCallbackManager(this.handlers, this.inheritableHandlers));
+      constructor(initialHandlers: unknown[], initialInheritableHandlers: unknown[]) {
+        this.handlers = [...initialHandlers];
+        this.inheritableHandlers = [...initialInheritableHandlers];
+      }
+    }
+    return new CallbackManager(existingHandlers, existingInheritableHandlers ?? existingHandlers);
+  }
+
   it('returns a fresh array when no existing callbacks are present', () => {
     expect(mergeSentryCallback(undefined, sentryHandler)).toStrictEqual([sentryHandler]);
     expect(mergeSentryCallback(null, sentryHandler)).toStrictEqual([sentryHandler]);
@@ -65,19 +93,81 @@ describe('mergeSentryCallback', () => {
     expect(mergeSentryCallback(existing, sentryHandler)).toBe(existing);
   });
 
-  it('calls addHandler on a CallbackManager-like object', () => {
-    const addHandler = vi.fn();
-    const manager = { addHandler, handlers: [] as unknown[] };
-    const result = mergeSentryCallback(manager, sentryHandler);
-    expect(result).toBe(manager);
-    expect(addHandler).toHaveBeenCalledWith(sentryHandler);
-    expect(addHandler).toHaveBeenCalledTimes(1);
+  it('preserves inheritable handlers when callbacks is a CallbackManager', () => {
+    // Reproduces the LangGraph `streamMode: ['messages']` setup: a
+    // CallbackManager carrying a StreamMessagesHandler is passed via
+    // options.callbacks. Wrapping it as `[manager, sentryHandler]` would
+    // drop the manager's inheritable children — instead we register
+    // Sentry on a copy and keep the existing handler chain intact.
+    const streamMessagesHandler = {
+      name: 'StreamMessagesHandler',
+      lc_prefer_streaming: true,
+    };
+    const manager = makeFakeCallbackManager([streamMessagesHandler]);
+    const result = mergeSentryCallback(manager, sentryHandler) as {
+      handlers: unknown[];
+    };
+    expect(Array.isArray(result)).toBe(false);
+    expect(result.handlers).toEqual([streamMessagesHandler, sentryHandler]);
+  });
+
+  it('copies the manager and registers Sentry as an inheritable handler', () => {
+    // Two adjacent contracts: we operate on a copy (so repeat invocations
+    // don't accumulate handlers on the caller), and we pass `inherit=true`
+    // so LangChain's `getChild()` propagates Sentry into nested calls.
+    const manager = makeFakeCallbackManager([]);
+    const result = mergeSentryCallback(manager, sentryHandler) as {
+      addHandler: ReturnType<typeof vi.fn>;
+      inheritableHandlers: unknown[];
+    };
+    expect(manager.copy).toHaveBeenCalledTimes(1);
+    expect(manager.handlers).toEqual([]);
+    expect(result.addHandler).toHaveBeenCalledWith(sentryHandler, true);
+    expect(result.inheritableHandlers).toEqual([sentryHandler]);
+  });
+
+  it('does not double-register when the copied manager already contains the handler', () => {
+    const manager = makeFakeCallbackManager([sentryHandler]);
+    const result = mergeSentryCallback(manager, sentryHandler) as {
+      handlers: unknown[];
+      addHandler: ReturnType<typeof vi.fn>;
+    };
+    expect(result.handlers).toEqual([sentryHandler]);
+    expect(result.addHandler).not.toHaveBeenCalled();
+  });
+
+  it('returns the value unchanged when it is neither an array nor a CallbackManager', () => {
+    const opaque = { name: 'NotAManager' };
+    expect(mergeSentryCallback(opaque, sentryHandler)).toBe(opaque);
+  });
+
+  it('does not treat a coincidentally duck-typed object as a CallbackManager', () => {
+    // A plain object that happens to expose `addHandler`/`copy` shouldn't be
+    // mistaken for a real LangChain CallbackManager — the constructor-name
+    // check guards against false positives.
+    const lookalike = { addHandler: vi.fn(), copy: vi.fn(), handlers: [] };
+    expect(mergeSentryCallback(lookalike, sentryHandler)).toBe(lookalike);
+    expect(lookalike.addHandler).not.toHaveBeenCalled();
+    expect(lookalike.copy).not.toHaveBeenCalled();
   });
 
-  it('does not re-add when the manager already has the sentry handler', () => {
-    const addHandler = vi.fn();
-    const manager = { addHandler, handlers: [sentryHandler] };
-    mergeSentryCallback(manager, sentryHandler);
-    expect(addHandler).not.toHaveBeenCalled();
+  it('recognizes subclasses of CallbackManager via the prototype walk', () => {
+    class CallbackManager {
+      public handlers: unknown[] = [];
+      public inheritableHandlers: unknown[] = [];
+      public addHandler = vi.fn((handler: unknown, inherit?: boolean) => {
+        this.handlers.push(handler);
+        if (inherit !== false) {
+          this.inheritableHandlers.push(handler);
+        }
+      });
+      public copy = vi.fn(() => new CallbackManager());
+    }
+    class CustomCallbackManager extends CallbackManager {}
+    const subclass = new CustomCallbackManager();
+    const result = mergeSentryCallback(subclass, sentryHandler) as {
+      addHandler: ReturnType<typeof vi.fn>;
+    };
+    expect(result.addHandler).toHaveBeenCalledWith(sentryHandler, true);
   });
 });
diff --git a/packages/node/src/integrations/tracing/langchain/instrumentation.ts b/packages/node/src/integrations/tracing/langchain/instrumentation.ts
index fb3e80b48583..8c4c594a1375 100644
--- a/packages/node/src/integrations/tracing/langchain/instrumentation.ts
+++ b/packages/node/src/integrations/tracing/langchain/instrumentation.ts
@@ -12,6 +12,7 @@ import {
   createLangChainCallbackHandler,
   GOOGLE_GENAI_INTEGRATION_NAME,
   instrumentLangChainEmbeddings,
+  mergeSentryCallback,
   OPENAI_INTEGRATION_NAME,
   SDK_VERSION,
 } from '@sentry/core';
@@ -27,34 +28,6 @@ interface PatchedLangChainExports {
   [key: string]: unknown;
 }
 
-/**
- * Augments a callback handler list with Sentry's handler if not already present
- */
-function augmentCallbackHandlers(handlers: unknown, sentryHandler: unknown): unknown {
-  // Handle null/undefined - return array with just our handler
-  if (!handlers) {
-    return [sentryHandler];
-  }
-
-  // If handlers is already an array
-  if (Array.isArray(handlers)) {
-    // Check if our handler is already in the list
-    if (handlers.includes(sentryHandler)) {
-      return handlers;
-    }
-    // Add our handler to the list
-    return [...handlers, sentryHandler];
-  }
-
-  // If it's a single handler object, convert to array
-  if (typeof handlers === 'object') {
-    return [handlers, sentryHandler];
-  }
-
-  // Unknown type - return original
-  return handlers;
-}
-
 /**
  * Wraps Runnable methods (invoke, stream, batch) to inject Sentry callbacks at request time
  * Uses a Proxy to intercept method calls and augment the options.callbacks
@@ -82,9 +55,7 @@ function wrapRunnableMethod(
       }
 
       // Inject our callback handler into options.callbacks (request time callbacks)
-      const existingCallbacks = options.callbacks;
-      const augmentedCallbacks = augmentCallbackHandlers(existingCallbacks, sentryHandler);
-      options.callbacks = augmentedCallbacks;
+      options.callbacks = mergeSentryCallback(options.callbacks, sentryHandler);
 
       // Call original method with augmented options
       return Reflect.apply(target, thisArg, args);