diff --git a/.changeset/tall-jeans-drop.md b/.changeset/tall-jeans-drop.md
new file mode 100644
index 000000000000..3226dcfb3253
--- /dev/null
+++ b/.changeset/tall-jeans-drop.md
@@ -0,0 +1,5 @@
+---
+"@langchain/core": patch
+---
+
+handle missing parent runs in tracer to prevent LangSmith 400 errors
diff --git a/langchain-core/src/tracers/base.ts b/langchain-core/src/tracers/base.ts
index aa0eb8de27e4..2e2ca465c9d9 100644
--- a/langchain-core/src/tracers/base.ts
+++ b/langchain-core/src/tracers/base.ts
@@ -165,6 +165,11 @@ export abstract class BaseTracer extends BaseCallbackHandler {
         // console.debug(
         //   `Parent run with UUID ${storedRun.parent_run_id} not found.`
         // );
+
+        // Child run with no trace_id and dotted_order causes 400 error on LangSmith.
+        // So we set the parent_run_id to undefined as a workaround.
+        // This run will be shown as isolated run on LangSmith.
+        storedRun.parent_run_id = undefined;
       }
     } else {
       storedRun.trace_id = storedRun.id;