fix(core): support reasoning/thinking blocks in StringOutputParser (#10002)

Co-authored-by: Christian Bromann <[email protected]>

Author: aditya-gg04

.changeset/odd-mugs-tease.md

@@ -0,0 +1,5 @@
+---
+"@langchain/core": patch
+---
+
+fix(core): support reasoning/thinking blocks in StringOutputParser

libs/langchain-core/src/output_parsers/string.ts

@@ -73,6 +73,10 @@ export class StringOutputParser extends BaseTransformOutputParser<string> {
           );
         }
         break;
+      case "reasoning":
+      case "thinking":
+      case "redacted_thinking":
+        return "";
       default:
         throw new Error(
           `Cannot coerce "${content.type}" message part into a string.`

libs/langchain-core/src/output_parsers/tests/string.test.ts

@@ -71,3 +71,63 @@ describe("StringOutputParser", () => {
     }).rejects.toThrowError();
   });
 });
+
+test("ignores reasoning blocks and returns only text", async () => {
+  const parser = new StringOutputParser();
+
+  const content: ContentBlock[] = [
+    {
+      type: "reasoning",
+      reasoning: "internal reasoning",
+    },
+    {
+      type: "text",
+      text: "final answer",
+    },
+  ];
+
+  const msg: BaseMessage = new AIMessage({ content });
+  const result = await parser.invoke(msg);
+
+  expect(result).toEqual("final answer");
+});
+
+test("ignores thinking blocks", async () => {
+  const parser = new StringOutputParser();
+
+  const content: ContentBlock[] = [
+    {
+      type: "thinking",
+      thinking: "hidden thoughts",
+    },
+    {
+      type: "text",
+      text: "visible output",
+    },
+  ];
+
+  const msg: BaseMessage = new AIMessage({ content });
+  const result = await parser.invoke(msg);
+
+  expect(result).toEqual("visible output");
+});
+
+test("ignores redacted_thinking blocks", async () => {
+  const parser = new StringOutputParser();
+
+  const content: ContentBlock[] = [
+    {
+      type: "redacted_thinking",
+      redacted_thinking: "redacted",
+    },
+    {
+      type: "text",
+      text: "answer",
+    },
+  ];
+
+  const msg: BaseMessage = new AIMessage({ content });
+  const result = await parser.invoke(msg);
+
+  expect(result).toEqual("answer");
+});