fix(builtin): split prompt into system/user messages

System message = agent identity (role prompt + output format instruction)
User message = moderator speech (task + edge prompt + history)

This reflects the workflow's core model: moderator speaks to agent
via the graph's edge prompt. Previously all content was in a single
system message with no user message, causing Claude API 400 errors.

- buildBuiltinPrompt now returns { system, user } instead of string
- agent.ts sends system + user as separate messages
- Tests updated accordingly

packages/workflow-agent-builtin/__tests__/prompt.test.ts

@@ -26,22 +26,30 @@ function minimalContext(overrides: Partial<AgentContext> = {}): AgentContext {
     start: { workflow: "wf-hash", prompt: "Fix the bug" },
     steps: [],
     outputFormatInstruction: "---\nstatus: done\n---",
+    edgePrompt: "Implement the fix described in the plan.",
+    isFirstVisit: true,
     ...overrides,
   };
 }
 
 describe("buildBuiltinPrompt", () => {
-  test("includes output format, task, and role goal", () => {
-    const prompt = buildBuiltinPrompt(minimalContext());
-    expect(prompt).toContain("status: done");
-    expect(prompt).toContain("## Goal");
-    expect(prompt).toContain("Ship the fix");
-    expect(prompt).toContain("## Task");
-    expect(prompt).toContain("Fix the bug");
+  test("system includes output format and role goal", () => {
+    const { system } = buildBuiltinPrompt(minimalContext());
+    expect(system).toContain("status: done");
+    expect(system).toContain("## Goal");
+    expect(system).toContain("Ship the fix");
   });
 
-  test("includes history when steps exist", () => {
-    const prompt = buildBuiltinPrompt(
+  test("user includes task and edge prompt", () => {
+    const { user } = buildBuiltinPrompt(minimalContext());
+    expect(user).toContain("## Task");
+    expect(user).toContain("Fix the bug");
+    expect(user).toContain("## Current Step Instruction");
+    expect(user).toContain("Implement the fix");
+  });
+
+  test("user includes history when steps exist", () => {
+    const { user } = buildBuiltinPrompt(
       minimalContext({
         steps: [
           {
@@ -53,7 +61,7 @@ describe("buildBuiltinPrompt", () => {
         ],
       }),
     );
-    expect(prompt).toContain("## Previous Steps");
-    expect(prompt).toContain("planner");
+    expect(user).toContain("## Previous Steps");
+    expect(user).toContain("planner");
   });
 });

packages/workflow-agent-builtin/src/agent.ts

@@ -69,8 +69,11 @@ async function runBuiltin(ctx: AgentContext): Promise<AgentRunResult> {
   const provider = resolveModel(config, config.defaultModel);
 
   const sessionId = generateUlid(Date.now());
-  const systemPrompt = buildBuiltinPrompt(ctx);
-  const messages: ChatMessage[] = [{ role: "system", content: systemPrompt }];
+  const promptParts = buildBuiltinPrompt(ctx);
+  const messages: ChatMessage[] = [
+    { role: "system", content: promptParts.system },
+    { role: "user", content: promptParts.user },
+  ];
 
   const session: BuiltinSessionState = {
     sessionId,

packages/workflow-agent-builtin/src/prompt.ts

@@ -19,18 +19,32 @@ function buildHistorySummary(steps: AgentContext["steps"]): string {
   return lines.join("\n");
 }
 
-/** Assemble output format, role prompt, task, and history (aligned with buildHermesPrompt). */
-export function buildBuiltinPrompt(ctx: AgentContext): string {
+export type BuiltinPromptParts = {
+  system: string;
+  user: string;
+};
+
+/** Assemble system prompt (role + format) and user prompt (task + edge + history). */
+export function buildBuiltinPrompt(ctx: AgentContext): BuiltinPromptParts {
   const roleDef = ctx.workflow.roles[ctx.role];
   const rolePrompt = roleDef !== undefined ? buildRolePrompt(roleDef) : "";
-  const parts: string[] = [];
+  const systemParts: string[] = [];
   if (ctx.outputFormatInstruction !== "") {
-    parts.push(ctx.outputFormatInstruction, "");
+    systemParts.push(ctx.outputFormatInstruction, "");
+  }
+  systemParts.push(rolePrompt);
+
+  const userParts: string[] = ["## Task", ctx.start.prompt];
+  if (ctx.edgePrompt !== "") {
+    userParts.push("", "## Current Step Instruction", ctx.edgePrompt);
   }
-  parts.push(rolePrompt, "", "## Task", ctx.start.prompt);
   const historyBlock = buildHistorySummary(ctx.steps);
   if (historyBlock !== "") {
-    parts.push("", historyBlock);
+    userParts.push("", historyBlock);
   }
-  return parts.join("\n");
+
+  return {
+    system: systemParts.join("\n"),
+    user: userParts.join("\n"),
+  };
 }