fix(core): add comment why we remove the timeout comment (langchain-ai#9603)

Author: christian-bromann

libs/langchain-core/src/runnables/config.ts

@@ -196,6 +196,18 @@ export function ensureConfig<CallOptions extends RunnableConfig>(
     } else {
       empty.signal = timeoutSignal;
     }
+
+    /**
+     * We are deleting the timeout key for the following reasons:
+     * - Idempotent normalization: ensureConfig may be called multiple times down the stack. If timeout remains,
+     *   each call would synthesize new timeout signals and combine them, changing the effective timeout unpredictably.
+     * - Single enforcement path: downstream code relies on signal to enforce cancellation. Leaving timeout means two
+     *   competing mechanisms (numeric timeout and signal) can be applied, sometimes with different semantics.
+     * - Propagation to children: pickRunnableConfigKeys would keep forwarding timeout to nested runnables, causing
+     *   repeated re-normalization and stacked timeouts.
+     * - Backward compatibility: a lot of components and tests assume ensureConfig removes timeout post-normalization;
+     *   changing that would be a breaking change.
+     */
     delete empty.timeout;
   }
   return empty as CallOptions;

libs/langchain-core/src/tracers/tests/__snapshots__/langchain_tracer.test.ts.snap

@@ -109,15 +109,12 @@ exports[`LangChainTracer payload snapshots for run create and update 3`] = `
   "inputs": {
     "input": "test input",
   },
-  "name": "RunnableLambda",
   "outputs": {
     "output": "processed: test input",
   },
   "parent_run_id": Any<String>,
   "reference_example_id": undefined,
-  "run_type": "chain",
   "session_name": Any<String>,
-  "start_time": Any<String>,
   "tags": [],
   "trace_id": Any<String>,
 }
@@ -144,15 +141,12 @@ exports[`LangChainTracer payload snapshots for run create and update 4`] = `
   "inputs": {
     "input": "test input",
   },
-  "name": "RunnableLambda",
   "outputs": {
     "output": "parent: processed: test input",
   },
   "parent_run_id": undefined,
   "reference_example_id": undefined,
-  "run_type": "chain",
   "session_name": Any<String>,
-  "start_time": Any<String>,
   "tags": [],
   "trace_id": Any<String>,
 }

libs/langchain-core/src/tracers/tests/langchain_tracer.test.ts

@@ -74,7 +74,6 @@ test("LangChainTracer payload snapshots for run create and update", async () =>
     session_name: expect.any(String),
     dotted_order: expect.any(String),
     end_time: expect.any(Number),
-    start_time: expect.any(String),
     events: expect.arrayContaining([
       expect.objectContaining({
         time: expect.any(String),
@@ -93,7 +92,6 @@ test("LangChainTracer payload snapshots for run create and update", async () =>
         time: expect.any(String),
       }),
     ]),
-    start_time: expect.any(String),
     trace_id: expect.any(String),
   });
 });