refactor: migrate memory middleware to use SystemMessage (#178)

* refactor(deepagents): migrate memory middleware to use SystemMessage

Update memory middleware to use SystemMessage class instead of string-based systemPrompt. This change aligns the middleware with the updated request interface and provides better type safety.

- Replace systemPrompt string with SystemMessage object in memory.ts
- Use SystemMessage.concat() method for combining memory section with existing system message
- Update all test cases to use SystemMessage constructor and .text property for assertions

* add changeset

---------

Co-authored-by: Christian Bromann <git@bromann.dev>

Author: JadenKim-dev

.changeset/stale-pugs-hammer.md

@@ -0,0 +1,5 @@
+---
+"deepagents": patch
+---
+
+refactor: migrate memory middleware to use SystemMessage

libs/deepagents/src/middleware/memory.test.ts

@@ -152,7 +152,7 @@ describe("createMemoryMiddleware", () => {
 
       const mockHandler = vi.fn().mockReturnValue({ response: "ok" });
       const request = {
-        systemPrompt: "Base prompt",
+        systemMessage: new SystemMessage("Base prompt"),
         state: {
           memoryContents: {
             "~/.deepagents/AGENTS.md": "User memory content",
@@ -165,13 +165,23 @@ describe("createMemoryMiddleware", () => {
 
       expect(mockHandler).toHaveBeenCalled();
       const modifiedRequest = mockHandler.mock.calls[0][0];
-      expect(modifiedRequest.systemPrompt).toContain("<agent_memory>");
-      expect(modifiedRequest.systemPrompt).toContain("</agent_memory>");
-      expect(modifiedRequest.systemPrompt).toContain("<memory_guidelines>");
-      expect(modifiedRequest.systemPrompt).toContain("User memory content");
-      expect(modifiedRequest.systemPrompt).toContain("Project memory content");
-      expect(modifiedRequest.systemPrompt).toContain("~/.deepagents/AGENTS.md");
-      expect(modifiedRequest.systemPrompt).toContain("./.deepagents/AGENTS.md");
+      expect(modifiedRequest.systemMessage.text).toContain("<agent_memory>");
+      expect(modifiedRequest.systemMessage.text).toContain("</agent_memory>");
+      expect(modifiedRequest.systemMessage.text).toContain(
+        "<memory_guidelines>",
+      );
+      expect(modifiedRequest.systemMessage.text).toContain(
+        "User memory content",
+      );
+      expect(modifiedRequest.systemMessage.text).toContain(
+        "Project memory content",
+      );
+      expect(modifiedRequest.systemMessage.text).toContain(
+        "~/.deepagents/AGENTS.md",
+      );
+      expect(modifiedRequest.systemMessage.text).toContain(
+        "./.deepagents/AGENTS.md",
+      );
     });
 
     it("should show (No memory loaded) when no content", () => {
@@ -182,14 +192,16 @@ describe("createMemoryMiddleware", () => {
 
       const mockHandler = vi.fn().mockReturnValue({ response: "ok" });
       const request = {
-        systemPrompt: "Base prompt",
+        systemMessage: new SystemMessage("Base prompt"),
         state: { memoryContents: {} },
       };
 
       middleware.wrapModelCall!(request as any, mockHandler);
 
       const modifiedRequest = mockHandler.mock.calls[0][0];
-      expect(modifiedRequest.systemPrompt).toContain("(No memory loaded)");
+      expect(modifiedRequest.systemMessage.text).toContain(
+        "(No memory loaded)",
+      );
     });
 
     it("should prepend memory section to existing system prompt", () => {
@@ -200,16 +212,17 @@ describe("createMemoryMiddleware", () => {
 
       const mockHandler = vi.fn().mockReturnValue({ response: "ok" });
       const request = {
-        systemPrompt: "Original system prompt content",
+        systemMessage: new SystemMessage("Original system prompt content"),
         state: { memoryContents: {} },
       };
 
       middleware.wrapModelCall!(request as any, mockHandler);
 
       const modifiedRequest = mockHandler.mock.calls[0][0];
       // Memory section should come before the original prompt
-      const memoryIndex = modifiedRequest.systemPrompt.indexOf("Agent Memory");
-      const originalIndex = modifiedRequest.systemPrompt.indexOf(
+      const memoryIndex =
+        modifiedRequest.systemMessage.text.indexOf("Agent Memory");
+      const originalIndex = modifiedRequest.systemMessage.text.indexOf(
         "Original system prompt content",
       );
       expect(memoryIndex).toBeLessThan(originalIndex);
@@ -223,14 +236,16 @@ describe("createMemoryMiddleware", () => {
 
       const mockHandler = vi.fn().mockReturnValue({ response: "ok" });
       const request = {
-        systemPrompt: "Base prompt",
+        systemMessage: new SystemMessage("Base prompt"),
         state: {},
       };
 
       middleware.wrapModelCall!(request as any, mockHandler);
 
       const modifiedRequest = mockHandler.mock.calls[0][0];
-      expect(modifiedRequest.systemPrompt).toContain("(No memory loaded)");
+      expect(modifiedRequest.systemMessage.text).toContain(
+        "(No memory loaded)",
+      );
     });
   });
 
@@ -255,16 +270,20 @@ describe("createMemoryMiddleware", () => {
       // Step 2: Use loaded memory in wrapModelCall
       const mockHandler = vi.fn().mockReturnValue({ response: "ok" });
       const request: any = {
-        systemPrompt: "You are a helpful assistant.",
+        systemMessage: new SystemMessage("You are a helpful assistant."),
         state: stateUpdate,
       };
 
       middleware.wrapModelCall!(request, mockHandler);
 
       const modifiedRequest = mockHandler.mock.calls[0][0];
-      expect(modifiedRequest.systemPrompt).toContain("I prefer TypeScript");
-      expect(modifiedRequest.systemPrompt).toContain("This is a React project");
-      expect(modifiedRequest.systemPrompt).toContain(
+      expect(modifiedRequest.systemMessage.text).toContain(
+        "I prefer TypeScript",
+      );
+      expect(modifiedRequest.systemMessage.text).toContain(
+        "This is a React project",
+      );
+      expect(modifiedRequest.systemMessage.text).toContain(
         "You are a helpful assistant",
       );
     });

libs/deepagents/src/middleware/memory.ts

@@ -51,6 +51,7 @@
 import { z } from "zod";
 import {
   createMiddleware,
+  SystemMessage,
   /**
    * required for type inference
    */
@@ -310,13 +311,14 @@ export function createMemoryMiddleware(options: MemoryMiddlewareOptions) {
         formattedContents,
       );
 
-      // Prepend memory section to system prompt
-      const currentSystemPrompt = request.systemPrompt || "";
-      const newSystemPrompt = currentSystemPrompt
-        ? `${memorySection}\n\n${currentSystemPrompt}`
-        : memorySection;
+      // Concat memory section to system prompt
+      const memoryMessage = new SystemMessage(memorySection);
+      const newSystemMessage = memoryMessage.concat(request.systemMessage);
 
-      return handler({ ...request, systemPrompt: newSystemPrompt });
+      return handler({
+        ...request,
+        systemMessage: newSystemMessage,
+      });
     },
   });
 }