diff --git a/lib/tui/stream.mjs b/lib/tui/stream.mjs
index 69eba45..797f6dc 100644
--- a/lib/tui/stream.mjs
+++ b/lib/tui/stream.mjs
@@ -173,6 +173,9 @@ export function parseDeltaChunk(text, consumed = 0) {
 //    fails the turn loudly (SSE error frame, no cache, counted on /health). Fail-loud is
 //    the correct posture — a proxy that silently serves text the transcript disagrees with
 //    is the exact class of bug ALIGNMENT.md exists to prevent.
+// Sentinel for "no message seen yet". Deliberately not null/undefined — see the constructor.
+const NO_MESSAGE_YET = Symbol("no-message-yet");
+
 export class TuiDeltaAssembler {
   constructor({ holdbackChars = DEFAULT_HOLDBACK_CHARS, detectError = detectTuiUpstreamError } = {}) {
     this.holdbackChars = holdbackChars;
@@ -180,7 +183,12 @@ export class TuiDeltaAssembler {
     this.emitted = "";   // bytes ALREADY written to the client — unretractable
     this.pending = "";   // held back, not yet written
     this.released = false;
-    this.messageId = null;
+    // NOT null: a payload may legitimately carry message_id === null, and if the sentinel were
+    // also null the FIRST such payload would compare equal to it, register no boundary, and
+    // leave `messages` at 0 — which used to disarm the restartedAfterEmit guard below entirely.
+    // A unique object is === to nothing a JSON payload can produce, so the first fire ALWAYS
+    // registers as message 1, whatever its message_id is (or isn't).
+    this.messageId = NO_MESSAGE_YET;
     this.deltas = 0;     // hook fires seen
     this.messages = 0;   // distinct message_ids seen
     this.restartedAfterEmit = false;
@@ -198,7 +206,12 @@ export class TuiDeltaAssembler {
       this.messages++;
       if (this.emitted === "") {
         this.pending = "";        // safe: the transcript will drop this message too
-      } else if (this.messages > 1) {
+      } else {
+        // A boundary while bytes are ALREADY out is unrecoverable, full stop — the count of
+        // messages seen so far is irrelevant. The old `else if (this.messages > 1)` guard was
+        // the sole reason a null-message_id first payload could disarm F1: it left `messages`
+        // at 0, so the real boundary evaluated 1 > 1 === false and never armed. The invariant
+        // is "a boundary occurred while emitted !== ''", and that is exactly what this says.
         this.restartedAfterEmit = true; // unrecoverable — finalize() will refuse the turn
       }
     }
diff --git a/test-features.mjs b/test-features.mjs
index 8384166..17140ae 100644
--- a/test-features.mjs
+++ b/test-features.mjs
@@ -3598,6 +3598,26 @@ test("F1: an auth banner rendered as a LATER message is never forwarded to the c
   assert.equal(a.finalize(BANNER).ok, false, "and the turn is refused, not served");
 });
 
+test("F1: a first payload with message_id:null cannot disarm the guard", () => {
+  // The residual bypass the reviewer found by probing. `this.messageId` used to be initialized
+  // to null, so a first payload carrying message_id:null compared EQUAL to it → no boundary
+  // registered → `messages` stayed 0 → when the REAL boundary arrived, `messages > 1` evaluated
+  // 1 > 1 === false → restartedAfterEmit never armed → the released branch forwarded the banner.
+  // The whole F1 guard was disarmed by a single null field. parseDeltaChunk does not validate
+  // message_id, so such a payload does reach push().
+  const a = new TuiDeltaAssembler({ holdbackChars: 100 });
+  const narration = "I'll check that file for you and then report back with what I find inside it.";
+  a.push({ hook_event_name: "MessageDisplay", message_id: null, delta: narration + narration });
+  assert.notEqual(a.emitted, "", "precondition: the narration released to the client");
+  assert.equal(a.messages, 1, "a null message_id is still a MESSAGE — it must register as one");
+
+  const BANNER = "Please run /login · API Error: 401 Invalid authentication credentials";
+  const out = a.push({ hook_event_name: "MessageDisplay", message_id: "m2", delta: BANNER });
+  assert.equal(out, null, "the banner must not be forwarded — the guard must arm regardless");
+  assert.equal(a.restartedAfterEmit, true);
+  assert.ok(!a.emitted.includes("401"));
+});
+
 test("F1: whitespace cannot buy a release — the holdback screens TRIMMED length", () => {
   // detectTuiUpstreamError() TRIMS before applying its <=100-char rule, so gating release on
   // the UNTRIMMED pending.length let 101 spaces trim to "" → the detector has nothing to