fix(builtin): strip preamble before frontmatter + stronger prompt

- Add stripPreamble() to handle LLM output with text before ---
- Strengthen system prompt: CRITICAL instruction for --- at position 0
- Fixes frontmatter parsing failures on first output turn

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

@@ -17,6 +17,24 @@ import { initSessionDir } from "./session.js";
 
 const log = createLogger({ sink: { kind: "stderr" } });
 
+const FRONTMATTER_FENCE = "---";
+
+/**
+ * Strip any text before the first `---` fence.
+ * LLMs sometimes emit preamble text before the frontmatter block.
+ */
+function stripPreamble(text: string): string {
+  if (text.startsWith(FRONTMATTER_FENCE)) {
+    return text;
+  }
+  const idx = text.indexOf(`\n${FRONTMATTER_FENCE}\n`);
+  if (idx !== -1) {
+    log("6GWRP3QX", `stripped ${idx + 1} chars of preamble before frontmatter`);
+    return text.slice(idx + 1);
+  }
+  return text;
+}
+
 type SessionRecord = {
   sessionId: string;
   model: string;
@@ -74,7 +92,7 @@ async function runBuiltinWithMessages(
     session.startedAtMs,
   );
 
-  return { output: loopResult.finalText, detailHash, sessionId: session.sessionId };
+  return { output: stripPreamble(loopResult.finalText), detailHash, sessionId: session.sessionId };
 }
 
 async function runBuiltin(ctx: AgentContext): Promise<AgentRunResult> {

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

@@ -66,7 +66,9 @@ export function buildBuiltinMessages(ctx: AgentContext): ChatMessage[] {
     "You have tools available (read_file, write_file, run_command). " +
       "Use them to complete your task — read files, run commands, make changes as needed. " +
       "When you are done, output your final response with the YAML frontmatter block as specified above. " +
-      "Do NOT output the frontmatter until you have completed all necessary work.",
+      "Do NOT output the frontmatter until you have completed all necessary work. " +
+      "CRITICAL: Your final output MUST start with the `---` fence on the very first line — " +
+      "no preamble text, no explanation before it. The parser requires `---` at position 0.",
   );
 
   const messages: ChatMessage[] = [{ role: "system", content: systemParts.join("\n") }];