Ensure lone handlers are handled correctly

Author: andreiborza

packages/core/src/tracing/langchain/utils.ts

@@ -552,10 +552,7 @@ function isCallbackManager(value: unknown): value is {
 }
 
 function isSentryHandler(handler: unknown): boolean {
-  return (
-    typeof handler === 'object' &&
-    (handler as Record<string, unknown>)?.name === 'SentryCallbackHandler'
-  );
+  return typeof handler === 'object' && (handler as Record<string, unknown>)?.name === 'SentryCallbackHandler';
 }
 
 function containsSentryHandler(handlers: unknown[]): boolean {
@@ -571,17 +568,21 @@ export function _INTERNAL_mergeLangChainCallbackHandler(existing: unknown, sentr
     return [sentryHandler];
   }
 
-  if (Array.isArray(existing) && !containsSentryHandler(existing)) {
-    return [...existing, sentryHandler];
-  }
-
-  if (isCallbackManager(existing) && !containsSentryHandler(existing.handlers ?? [])) {
+  // CallbackManager: copy and register the handler so inheritable chains are preserved.
+  if (isCallbackManager(existing)) {
+    if (containsSentryHandler(existing.handlers ?? [])) {
+      return existing;
+    }
     const copied = existing.copy() as {
       addHandler: (handler: unknown, inherit?: boolean) => void;
     };
     copied.addHandler(sentryHandler, true);
     return copied;
   }
 
-  return existing;
+  const handlers = Array.isArray(existing) ? existing : [existing];
+  if (containsSentryHandler(handlers)) {
+    return existing;
+  }
+  return [...handlers, sentryHandler];
 }

packages/core/test/lib/tracing/langchain-utils.test.ts

@@ -325,8 +325,13 @@ describe('_INTERNAL_mergeLangChainCallbackHandler', () => {
     expect(manager.addHandler).not.toHaveBeenCalled();
   });
 
-  it('returns the value unchanged when it is neither an array nor a CallbackManager', () => {
+  it('wraps a lone callback object into an array with the sentry handler', () => {
     const opaque = { name: 'NotAManager' };
-    expect(_INTERNAL_mergeLangChainCallbackHandler(opaque, sentryHandler)).toBe(opaque);
+    expect(_INTERNAL_mergeLangChainCallbackHandler(opaque, sentryHandler)).toStrictEqual([opaque, sentryHandler]);
+  });
+
+  it('returns unchanged when the lone callback object is already a sentry handler', () => {
+    const existing = { name: 'SentryCallbackHandler' };
+    expect(_INTERNAL_mergeLangChainCallbackHandler(existing, sentryHandler)).toBe(existing);
   });
 });