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

@@ -13,10 +13,28 @@ import { storeBuiltinDetail } from "./detail.js";
 import type { ChatMessage } from "./llm/index.js";
 import { BUILTIN_CONTINUE_MAX_TURNS, BUILTIN_MAX_TURNS, runBuiltinLoop } from "./loop.js";
 import { buildBuiltinMessages } from "./prompt.js";
-import { initSessionDir, removeSession } from "./session.js";
+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;
@@ -62,7 +80,6 @@ async function runBuiltinWithMessages(
 
   if (loopResult.turnCount === 0) {
     log("5RWTK9NB", "no turns produced, returning empty output");
-    await removeSession(storageRoot, session.sessionId);
     return { output: "", detailHash: "", sessionId: session.sessionId };
   }
 
@@ -75,10 +92,7 @@ async function runBuiltinWithMessages(
     session.startedAtMs,
   );
 
-  // Clean up session jsonl
-  await removeSession(storageRoot, session.sessionId);
-
-  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

@@ -59,6 +59,18 @@ export function buildBuiltinMessages(ctx: AgentContext): ChatMessage[] {
   }
   systemParts.push(rolePrompt);
 
+  systemParts.push(
+    "",
+    "## Workflow",
+    "",
+    "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. " +
+      "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") }];
 
   const roleVisitIndices: number[] = [];

packages/workflow-agent-claude-code/src/claude-code.ts

@@ -1,5 +1,4 @@
 import { spawn } from "node:child_process";
-import { mkdirSync, writeFileSync } from "node:fs";
 import type { Store } from "@uncaged/json-cas";
 import {
   type AgentContext,
@@ -118,17 +117,7 @@ function spawnClaudeResume(
   ]);
 }
 
-const NDJSON_DUMP_DIR = "/tmp/uwf-ndjson-dump";
-
 async function processClaudeOutput(stdout: string, store: Store): Promise<AgentRunResult> {
-  // Debug dump: save raw NDJSON for issue #439 investigation
-  try {
-    mkdirSync(NDJSON_DUMP_DIR, { recursive: true });
-    writeFileSync(`${NDJSON_DUMP_DIR}/${Date.now()}.ndjson`, stdout);
-  } catch {
-    // ignore dump failures
-  }
-
   const parsed = parseClaudeCodeStreamOutput(stdout);
 
   if (parsed !== null) {