fix(anthropic): fix sonnet-4.5 thinking (#9073)

Author: linkesch

.changeset/nice-memes-judge.md

@@ -0,0 +1,5 @@
+---
+"@langchain/anthropic": patch
+---
+
+fix sonnet-4.5 thinking

libs/langchain-anthropic/src/chat_models.ts

@@ -849,7 +849,7 @@ export class ChatAnthropicMessages<
         throw new Error("topK is not supported when thinking is enabled");
       }
       if (
-        this.model.includes("opus-4-1")
+        this.model.includes("opus-4-1") || this.model.includes("sonnet-4-5")
           ? this.topP !== undefined
           : this.topP !== -1
       ) {

libs/langchain-anthropic/src/tests/chat_models.int.test.ts

@@ -1503,4 +1503,20 @@ describe("Sonnet 4.5", () => {
     );
     expect(response.content.length).toBeGreaterThan(0);
   });
+
+  it("works with streaming and thinking", async () => {
+    const model = new ChatAnthropic({
+      model: "claude-sonnet-4-5-20250929",
+      thinking: {
+        type: "enabled",
+        budget_tokens: 1024,
+      },
+    });
+
+    const response = await model.invoke(
+      "Please respond to this message simply with: Hello"
+    );
+
+    expect(response.content.length).toBeGreaterThan(0);
+  });
 });