diff --git a/docs/ISSUES.md b/docs/ISSUES.md
index f69864bf..49d2212d 100644
--- a/docs/ISSUES.md
+++ b/docs/ISSUES.md
@@ -121,6 +121,51 @@ than a REJECT-review fix batch; CH5-or-later.
 **Campaign:** `docs/plans/2026-08-09-chat-parity-campaign.md` (Campaign CH,
 CH4 REJECT-review).
 
+## #365 — Headless host cannot move at head: session quarantines on the first advance tick; world never hydrates
+
+**Status:** OPEN — filed 2026-08-10 during the Campaign CH jump-probe
+reproduction. Distinct from #330 (no collision) and #332 (no remote DR):
+this is the LOCAL player.
+
+**Symptom:** a headless session (`jump-probe` policy, real ACE connect,
+`testaccount`/`+Acdream`) reaches `InWorld`, `live: in world — CreateObject
+stream active` prints, but `entityCount` stays **0** for the whole run and
+the local movement controller is never published. The first
+`SetIntent(Jump: true)` → next `AdvanceBeforeNetwork` tick quarantined the
+session.
+
+**Evidence chain (2026-08-10):**
+1. First quarantine: `RuntimeLocalPlayerFrameController.AdvanceBeforeNetwork:93`
+   unconditionally re-assigned `controller.LocalEntityId` — a sealed
+   configuration property — on a still-dormant controller. FIXED in the
+   probing session: a same-value re-assertion is now a no-op (a different
+   id while sealed still throws).
+2. Second quarantine (one layer deeper): the frame controller takes the
+   `ObjectClockDisposition == Suspend` branch and calls
+   `SuspendObjectUpdate` → `EnsurePublishedForRuntimeOperation` throws —
+   `_host.CanAdvancePlayer` is true while the controller is unpublished.
+3. Root condition: the session's world never hydrates (`entities: 0`
+   despite the active CreateObject stream), so dormant local activation
+   never completes and the controller never publishes. K3/K4 ran full
+   connected multi-session gates with movement — this regressed somewhere
+   in the many Runtime/placement/CH landings since 2026-07-27.
+
+**Also observed in the same run:** `[weenie-error] unmapped code=0x051D` —
+an ACE-only id outside retail's 344-case `HandleFailureEvent` switch;
+CH2's silent-toward-player + diagnostics-line fallback handled it as
+designed (no action needed, noted for completeness).
+
+**Repro:** `dotnet run --project src/AcDream.Headless -c Release -- run
+--config <cfg>` with a `jump-probe` policy session against local ACE
+(config shape: version 1, endpoint 127.0.0.1:9000, credential provider
+Environment). `HeadlessDiagnosticWriter.Failure` now emits `errorDetail`
+(full exception) — added during this diagnosis.
+
+**Next step:** bisect the headless hydration path (why zero entities admit
+headless when the graphical host hydrates fine) BEFORE touching
+`CanAdvancePlayer` — the #357 closeout warns that dormant-activation
+classification changes have a strict test matrix.
+
 ## #364 — Three `/help` group topics still partial: HelpStupidChannelHack unresolved
 
 **Status:** OPEN — filed 2026-08-09, Campaign CH user-gate round 2, item 3.
diff --git a/src/AcDream.Headless/Diagnostics/HeadlessDiagnosticWriter.cs b/src/AcDream.Headless/Diagnostics/HeadlessDiagnosticWriter.cs
index 8290a2d2..4efc41f3 100644
--- a/src/AcDream.Headless/Diagnostics/HeadlessDiagnosticWriter.cs
+++ b/src/AcDream.Headless/Diagnostics/HeadlessDiagnosticWriter.cs
@@ -58,6 +58,9 @@ internal sealed class HeadlessDiagnosticWriter
             sessionId,
             phase,
             errorType = error.GetType().FullName,
+            // Full detail: a quarantine that reports only the exception TYPE
+            // costs a whole diagnosis round-trip (jump-probe, 2026-08-10).
+            errorDetail = error.ToString(),
         });
     }
 
diff --git a/src/AcDream.Runtime/Gameplay/PlayerMovementController.cs b/src/AcDream.Runtime/Gameplay/PlayerMovementController.cs
index b18ab208..b4daa174 100644
--- a/src/AcDream.Runtime/Gameplay/PlayerMovementController.cs
+++ b/src/AcDream.Runtime/Gameplay/PlayerMovementController.cs
@@ -381,6 +381,14 @@ public sealed class PlayerMovementController
         get => _localEntityId;
         set
         {
+            // A same-value re-assertion is not a mutation:
+            // RuntimeLocalPlayerFrameController.AdvanceBeforeNetwork
+            // re-asserts the resolved id every advance tick, which on the
+            // headless host reached a still-dormant/sealed controller and
+            // quarantined the whole session (jump-probe repro, 2026-08-10).
+            // A DIFFERENT id while sealed remains the error it always was.
+            if (value == _localEntityId)
+                return;
             EnsureConfigurationMutable();
             _localEntityId = value;
         }