feat(langchain/createAgent): Add support for modelSettings to middleware

libs/langchain/src/agents/nodes/AgentNode.ts

@@ -806,6 +806,7 @@ export class AgentNode<
      */
     const modelWithTools = await bindTools(model, allTools, {
       ...options,
+      ...(preparedOptions?.modelSettings ?? {}),
       tool_choice: toolChoice,
     });
 

libs/langchain/src/agents/nodes/types.ts

@@ -54,4 +54,20 @@ export interface ModelRequest<
    * The runtime context containing metadata, signal, writer, interrupt, etc.
    */
   runtime: Runtime<TContext>;
+
+  /**
+   * Additional settings to bind to the model when preparing it for invocation.
+   * These settings are applied via `bindTools()` and can include parameters like
+   * `headers`, `container`, etc. The model is re-bound on each request,
+   * so these settings can vary per invocation.
+   *
+   * @example
+   * ```ts
+   * modelSettings: {
+   *   headers: { "anthropic-beta": "code-execution-2025-08-25" },
+   *   container: "container_abc123"
+   * }
+   * ```
+   */
+  modelSettings?: Record<string, unknown>;
 }

libs/langchain/src/agents/tests/modelSettings.test.ts

@@ -0,0 +1,74 @@
+import { ChatAnthropic } from "@langchain/anthropic";
+import { HumanMessage } from "@langchain/core/messages";
+import { describe, expect, it, vi } from "vitest";
+import { createAgent, createMiddleware } from "../index.js";
+
+describe("modelSettings middleware support", () => {
+  it("should pass modelSettings to real Anthropic model via bindTools", async () => {
+    // Mock the Anthropic client
+    const mockCreate = vi.fn().mockResolvedValue({
+      id: "msg_123",
+      type: "message",
+      role: "assistant",
+      content: [{ type: "text", text: "Response from model" }],
+      model: "claude-sonnet-4-20250514",
+      stop_reason: "end_turn",
+      usage: { input_tokens: 10, output_tokens: 20 },
+    });
+
+    const mockClient = {
+      messages: {
+        create: mockCreate,
+      },
+    };
+
+    // Create real ChatAnthropic with mocked client
+    const model = new ChatAnthropic({
+      model: "claude-sonnet-4-20250514",
+      temperature: 0,
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      createClient: () => mockClient as any,
+    });
+
+    const middleware = createMiddleware({
+      name: "testMiddleware",
+      wrapModelCall: async (request, handler) => {
+        return handler({
+          ...request,
+          modelSettings: {
+            headers: {
+              "anthropic-beta":
+                "code-execution-2025-08-25,files-api-2025-04-14",
+            },
+            container: "container_abc123",
+          },
+        });
+      },
+    });
+
+    const agent = createAgent({
+      model,
+      tools: [],
+      middleware: [middleware] as const,
+    });
+
+    await agent.invoke({
+      messages: [new HumanMessage("Hello, world!")],
+    });
+
+    // Verify the client was called
+    expect(mockCreate).toHaveBeenCalled();
+
+    // Check the actual parameters passed to the Anthropic client
+    const clientCallArgs = mockCreate.mock.calls[0][0];
+    expect(clientCallArgs).toHaveProperty("container", "container_abc123");
+
+    // Check that headers were passed via options (second parameter)
+    const clientOptions = mockCreate.mock.calls[0][1];
+    expect(clientOptions).toHaveProperty("headers");
+    expect(clientOptions.headers).toHaveProperty(
+      "anthropic-beta",
+      "code-execution-2025-08-25,files-api-2025-04-14"
+    );
+  });
+});