diff --git a/orchestrator.py b/orchestrator.py
index cb10c25..60d0497 100644
--- a/orchestrator.py
+++ b/orchestrator.py
@@ -607,6 +607,32 @@ RULE 7: No hardcoding. RULE 8: AI finds 1px first. RULE 9: LLM classifies, code
 RULE 10: Don't uncritically accept. RULE 11: Checkpoint. RULE 12: Full paths. RULE 13: Anchor sync.
 PZ-1: AI=0 normal. PZ-2: 1turn=1step. PZ-3: No speculative. PZ-4: No silent shrink.
 
+=== COMMENT FORMAT (P5b 2026-05-20 — STRICT, OVERRIDES ALL STAGE-SPECIFIC BODY RULES) ===
+The FIRST non-empty line of EVERY Gitea comment MUST start with one of:
+  [Claude #N] <stage description>
+  [Codex #N] <stage description>
+
+This rule applies to ALL stages (Stage 1 ~ Stage 6) and ALL issue types
+(regular, execution-issue, audit-only). No prefix, no decoration, no banner,
+no audit anchor before the agent header. Examples:
+
+CORRECT:
+  [Codex #3] Stage 2 simulation-plan review — IMP-24
+
+  📌 Verification table
+  ...
+
+WRONG (orchestrator detect_agent will fail; stage cannot advance):
+  📌 **[Claude #3] Stage 2 ...**
+  ## [Codex #3] Stage 2 ...
+  === IMPLEMENTATION_UNITS ===     (header missing entirely)
+  Audit anchor: ...                (preface before header)
+
+This first-line-strict rule OVERRIDES any stage-specific "body MUST contain
+ONLY" rule (e.g., COMPACT_PLAN_RULE). Those body rules apply AFTER the
+mandatory first-line agent header. Decorations / banners / anchors go on
+line 2 or later.
+
 === CONSENSUS + REWIND (2026-05-16 lock) ===
 Final line of every Codex review comment MUST be exactly one of:
   FINAL_CONSENSUS: YES
@@ -901,10 +927,14 @@ def _check_dormant_triggers():
 COMPACT_PLAN_RULE = """
 
 COMPACT PLAN REQUIREMENTS (strict):
+- The FIRST non-empty line of your comment MUST be the agent header
+  ([Claude #N] ... or [Codex #N] ...). This is enforced by RULES (P5b 2026-05-20)
+  and OVERRIDES the "body" constraints below. The Stage 2 compact body begins
+  AFTER the first-line agent header — NOT on line 1.
 - Total Stage 2 plan body MUST be ≤ 5,000 chars (4,000 chars target).
 - NO code snippets in this comment. Code goes in Stage 3 (code-edit), not Stage 2 plan.
   References to file:line locations are fine. Inline code blocks are forbidden.
-- The Stage 2 plan body MUST contain ONLY:
+- After the first-line agent header, the Stage 2 plan body MUST contain ONLY:
     a) === IMPLEMENTATION_UNITS === YAML block (units with id/summary/files/tests/estimate_lines)
     b) Brief per-unit rationale (≤ 3 lines per unit, no full code)
     c) Out-of-scope notes
@@ -1328,20 +1358,40 @@ def run_stage(n, title, body, sid):
         last = comments[-1]["body"]
         is_codex = detect_agent(last) == "codex"
         if not is_codex:
-            log("  Codex 응답 미감지 — continuing")
-            # P5 (2026-05-20) — audit-mode 에서 detect_agent None 의 흔한 원인 =
-            # agent 가 audit anchor / preface 를 첫 줄에 박아서 P0-1 strict 가 못 찾음.
-            # 자동 supplement 로 format 교정 요청 → 무한 루프 자동 break.
-            if _audit_mode(title):
+            log(f"  Codex 응답 미감지 — first line: {last.lstrip().splitlines()[0][:80]!r}" if last and last.strip() else "  Codex 응답 미감지 — empty body")
+            # P5b (2026-05-20) — detect_agent None 시 supplement 가드.
+            # 범위 변경: audit-only 제한 해제 — 모든 issue 에서 작동 (#24 같은 일반 이슈 silent loop fix).
+            # Throttle: 현재 stage 안에 이미 N (=2) 회 supplement 가 누적되면 stop + user-action-required.
+            #   직전 N supplement 가 박혀도 LLM 이 또 위반하면 4 번째 round 부터는 hard stop.
+            SUPP_MAX = 2
+            SUPP_MARKER = "⚠️ **[Orchestrator]** Agent header missing"
+            stage_cmts = comments[start_cnt:]
+            supp_count = sum(1 for c in stage_cmts if (c.get("body") or "").lstrip().startswith(SUPP_MARKER))
+            if supp_count >= SUPP_MAX:
+                log(f"⛔ Agent header supplement {supp_count}/{SUPP_MAX} reached — STOP (user action required)")
                 try: gitea(f"issues/{n}/comments", "POST", {"body":
-                    "⚠️ **[Orchestrator]** Codex 응답 미감지 — `detect_agent` 가 첫 줄에서 "
-                    "`[Codex #N]` 또는 `[Claude #N]` 패턴을 찾지 못함.\n\n"
-                    "AUDIT-ONLY mode 의 흔한 원인: `Audit anchor:` 같은 preface 가 첫 줄에 있음.\n\n"
-                    "다음 round 부터 모든 comment 의 **FIRST non-empty line 은 반드시**:\n"
-                    "  `[Codex #N] <stage description>` 또는 `[Claude #N] <stage description>`\n"
-                    "Audit anchor / banner / preface 는 line 2 이후 에만. 안 그러면 orchestrator 가 "
-                    "stage 진행 못 함 (P0-1 first-line strict)."})
+                    f"⛔ **[Orchestrator]** STOP — Stage `{sid}` cannot advance.\n\n"
+                    f"`detect_agent` failed {supp_count}+ times in this stage. The LLM is not honoring "
+                    f"the first-line agent header contract despite supplements.\n\n"
+                    "**Action required (human)**: review last few comments, ensure FIRST non-empty line is "
+                    "`[Claude #N]` or `[Codex #N]`, then restart `python -u .\\orchestrator.py --issue {n}`.\n\n"
+                    "Orchestrator run is exiting this issue to prevent further token waste."})
                 except: pass
+                return False  # exit run_stage → run_issue treats as external close → moves on
+            try: gitea(f"issues/{n}/comments", "POST", {"body":
+                f"{SUPP_MARKER} — orchestrator `detect_agent` could not find "
+                "`[Claude #N]` or `[Codex #N]` on the first non-empty line.\n\n"
+                "**Comment format contract (P5b 2026-05-20, see RULES)**:\n"
+                "The FIRST non-empty line of EVERY Gitea comment (both Claude and Codex, ALL stages) MUST be:\n"
+                "  `[Claude #N] <stage description>`\n"
+                "  `[Codex #N] <stage description>`\n\n"
+                "No prefix. No decoration. No banner. No audit anchor before the header.\n"
+                "Decorations (`📌`, `##`, `**`, audit anchor, etc.) go on line 2 or later.\n\n"
+                "This rule OVERRIDES any stage-specific 'body MUST contain ONLY' rule (e.g., COMPACT_PLAN_RULE) — "
+                "those body rules apply AFTER the mandatory first-line agent header.\n\n"
+                f"Supplement count for this stage: {supp_count + 1}/{SUPP_MAX}. "
+                f"At {SUPP_MAX}+ violations the orchestrator will hard-stop this issue."})
+            except: pass
             continue
 
         status, target = parse_consensus(last)
diff --git a/tests/orchestrator_unit/test_orchestrator_core.py b/tests/orchestrator_unit/test_orchestrator_core.py
index af88d89..62b4063 100644
--- a/tests/orchestrator_unit/test_orchestrator_core.py
+++ b/tests/orchestrator_unit/test_orchestrator_core.py
@@ -128,6 +128,76 @@ Addressing [Codex #2] findings ...
         )
         assert detect_agent(body_header_first) == "codex"
 
+    # P5b (2026-05-20) — Stage 2 compact-plan first-line conflict regression.
+    # #24 IMP-24 K6: Codex r1~r3 가 첫 줄을 '=== IMPLEMENTATION_UNITS ===' 로 시작 →
+    # detect_agent None → orchestrator silent loop. fix path = comment format strict,
+    # NOT detect_agent 완화 (P0-1 강화 그대로 유지).
+
+    def test_implementation_units_first_line_breaks_detection(self):
+        """=== IMPLEMENTATION_UNITS === 가 첫 줄이면 detect_agent None (P0-1 strict 정상 동작)."""
+        body = (
+            "=== IMPLEMENTATION_UNITS ===\n"
+            "- id: u1\n"
+            "  summary: ...\n"
+            "  files:\n"
+            "    - docs/architecture/PHASE-Q-AUDIT.md\n"
+            "  tests:\n"
+            "    - pytest -q tests\n"
+            "  estimate_lines: 1\n"
+            "\n"
+            "FINAL_CONSENSUS: YES\n"
+        )
+        assert detect_agent(body) is None, (
+            "=== IMPLEMENTATION_UNITS === as first line MUST cause detect_agent None "
+            "(P0-1 strict). Fix path: enforce agent header first-line in prompt, not relax detect_agent."
+        )
+
+    def test_compact_plan_with_header_first_works(self):
+        """올바른 Stage 2 compact format: [Codex #N] 첫 줄 → === IMPLEMENTATION_UNITS === 둘째 줄+."""
+        body = (
+            "[Codex #4] Stage 2 simulation-plan review - IMP-24 K6\n"
+            "\n"
+            "=== IMPLEMENTATION_UNITS ===\n"
+            "- id: u1\n"
+            "  summary: ...\n"
+            "  tests:\n"
+            "    - pytest -q tests\n"
+            "\n"
+            "FINAL_CONSENSUS: YES\n"
+        )
+        assert detect_agent(body) == "codex"
+
+    def test_markdown_prefix_breaks_detection(self):
+        """P5b — `## [Codex #N]` 같은 markdown header prefix 도 detect_agent None.
+        (#21 Stage 4 에서 관찰된 latent silent loop 원인.)"""
+        body_hash = "## [Codex #1] Stage 4 test-verify Round #1\n\nVerdict: PASS\n"
+        body_emoji = "📌 **[Claude #1] Stage 2 plan**\n\nbody\n"
+        body_bold = "**[Codex #1] Stage 4**\n\nbody\n"
+        assert detect_agent(body_hash) is None
+        assert detect_agent(body_emoji) is None
+        assert detect_agent(body_bold) is None
+
+
+class TestRulesAndCompactPlanFirstLineContract:
+    """P5b (2026-05-20) — RULES 와 COMPACT_PLAN_RULE 둘 다 first-line agent header
+    rule 을 명시해야 함. wording 검증."""
+
+    def test_rules_has_first_line_strict(self):
+        from orchestrator import RULES
+        # RULES 안에 first-line strict + 모든 stage 적용 명시 있어야 함.
+        assert "FIRST non-empty line" in RULES
+        assert "[Claude #N]" in RULES and "[Codex #N]" in RULES
+        # P5b OVERRIDES 키워드 — body rule 들이 first-line rule 보다 우선하지 않음을 강조
+        assert "OVERRIDES" in RULES or "overrides" in RULES.lower()
+
+    def test_compact_plan_rule_carves_out_first_line(self):
+        from orchestrator import COMPACT_PLAN_RULE
+        # "body" 는 first-line agent header 다음부터 시작한다고 명시
+        assert "FIRST non-empty line" in COMPACT_PLAN_RULE or "first-line agent header" in COMPACT_PLAN_RULE
+        # "after the first-line" 같은 carve-out wording 검증
+        body_lower = COMPACT_PLAN_RULE.lower()
+        assert "after the first" in body_lower or "after the agent header" in body_lower
+
 
 # ─────────────────────────────────────────────────────────────────
 # parse_consensus — YES/NO + rewind_target