fix(core): support input_json_delta aggregation for anthropic tool streams

Author: noishey

libs/langchain-core/src/messages/base.ts

@@ -217,8 +217,7 @@ export abstract class BaseMessage<
   TRole extends MessageType = MessageType,
 >
   extends Serializable
-  implements Message<TStructure, TRole>
-{
+  implements Message<TStructure, TRole> {
   lc_namespace = ["langchain_core", "messages"];
 
   lc_serializable = true;
@@ -566,7 +565,14 @@ function hasMergeableId(value: unknown): value is { id: string | number } {
 }
 
 function getMergeableTypeBase(type: string): string {
-  return type.endsWith("_delta") ? type.slice(0, -"_delta".length) : type;
+  if (type === "input_json_delta" || type === "input_json") {
+    return "tool_use";
+  }
+
+  if (type.endsWith("_delta")) {
+    return type.replace("_delta", "");
+  }
+  return type;
 }
 
 function hasMismatchedMergeableType(left: unknown, right: unknown): boolean {
@@ -740,9 +746,9 @@ export type BaseMessageLike =
    * @deprecated Specifying "type" is deprecated and will be removed in 0.4.0.
    */
   | ({
-      type: MessageType | "user" | "assistant" | "placeholder";
-    } & BaseMessageFields &
-      Record<string, unknown>)
+    type: MessageType | "user" | "assistant" | "placeholder";
+  } & BaseMessageFields &
+    Record<string, unknown>)
   | SerializedConstructor;
 
 /**

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

@@ -1111,7 +1111,7 @@ describe("Tool search beta auto-append", () => {
 
     expect(
       paramsWithoutToolSearch.betas === undefined ||
-        !paramsWithoutToolSearch.betas.includes("advanced-tool-use-2025-11-20")
+      !paramsWithoutToolSearch.betas.includes("advanced-tool-use-2025-11-20")
     ).toBe(true);
   });
 });
@@ -1285,6 +1285,52 @@ describe("Streaming tool call consolidation (input_json_delta handling)", () =>
       input: { prompt: "hello" },
     });
   });
+
+  test("should successfully aggregate tool_use and input_json_delta chunks via .concat()", async () => {
+    // 1. Replicate the exact asymmetric sequence Anthropic emits during an active stream
+    const chunk1 = new AIMessageChunk({
+      content: [
+        {
+          type: "tool_use",
+          index: 0,
+          id: "toolu_01Xyz",
+          name: "my_tool",
+        }
+      ]
+    });
+
+    const chunk2 = new AIMessageChunk({
+      content: [
+        {
+          type: "input_json_delta",
+          index: 0,
+          partial_json: '{"prompt":',
+        }
+      ]
+    });
+
+    const chunk3 = new AIMessageChunk({
+      content: [
+        {
+          type: "input_json_delta",
+          index: 0,
+          partial_json: '"hello"}',
+        }
+      ]
+    });
+
+    // 2. Execute the concatenation (this is what threw OUTPUT_PARSING_FAILURE before your fix)
+    const merged = chunk1.concat(chunk2).concat(chunk3);
+
+    // 3. Assert that types map correctly and accumulate into a single valid tool call structure
+    expect(merged.content[0]).toEqual({
+      type: "tool_use",
+      index: 0,
+      id: "toolu_01Xyz",
+      name: "my_tool",
+      partial_json: '{"prompt":"hello"}'
+    });
+  });
 });
 
 describe("ContentBlock.Multimodal.Image format support", () => {
@@ -1959,7 +2005,7 @@ describe("Opus 4.6", () => {
 
       expect(
         params.betas === undefined ||
-          !params.betas.includes("compact-2026-01-12")
+        !params.betas.includes("compact-2026-01-12")
       ).toBe(true);
     });