fix: add guard for default conversation id (#1245)

src/headless.ts

@@ -114,6 +114,7 @@ import type {
   StreamEvent,
   SystemInitMessage,
 } from "./types/protocol";
+import { debugLog } from "./utils/debug";
 import {
   markMilestone,
   measureSinceMilestone,
@@ -526,7 +527,10 @@ export async function handleHeadlessCommand(
   // Validate shared mutual-exclusion rules for startup flags.
   try {
     validateFlagConflicts({
-      guard: specifiedConversationId && specifiedConversationId !== "default",
+      guard:
+        specifiedConversationId &&
+        specifiedConversationId !== "default" &&
+        !specifiedConversationId.startsWith("agent-"),
       checks: [
         {
           when: specifiedAgentId,
@@ -730,8 +734,16 @@ export async function handleHeadlessCommand(
   // Priority 0: --conversation derives agent from conversation ID.
   // "default" is a virtual agent-scoped conversation (not a retrievable conv-*).
   // It requires --agent and should not hit conversations.retrieve().
-  if (specifiedConversationId && specifiedConversationId !== "default") {
+  if (
+    specifiedConversationId &&
+    specifiedConversationId !== "default" &&
+    !specifiedConversationId.startsWith("agent-")
+  ) {
     try {
+      debugLog(
+        "conversations",
+        `retrieve(${specifiedConversationId}) [headless conv→agent lookup]`,
+      );
       const conversation = await client.conversations.retrieve(
         specifiedConversationId,
       );
@@ -1007,6 +1019,10 @@ export async function handleHeadlessCommand(
     } else {
       // User specified an explicit conversation to resume - validate it exists
       try {
+        debugLog(
+          "conversations",
+          `retrieve(${specifiedConversationId}) [headless --conv validate]`,
+        );
         await client.conversations.retrieve(specifiedConversationId);
         conversationId = specifiedConversationId;
       } catch {
@@ -1030,6 +1046,10 @@ export async function handleHeadlessCommand(
       } else {
         // Verify the conversation still exists
         try {
+          debugLog(
+            "conversations",
+            `retrieve(${lastSession.conversationId}) [headless lastSession resume]`,
+          );
           await client.conversations.retrieve(lastSession.conversationId);
           conversationId = lastSession.conversationId;
         } catch {