diff --git a/backend/app/api/endpoints/session_suggested_fixes.py b/backend/app/api/endpoints/session_suggested_fixes.py
index 0804b013..769cd663 100644
--- a/backend/app/api/endpoints/session_suggested_fixes.py
+++ b/backend/app/api/endpoints/session_suggested_fixes.py
@@ -217,6 +217,68 @@ async def record_decision(
     )
 
 
+# ── Suggested fix: apply (stamp applied_at) ──────────────────────────────
+
+@router.post(
+    "/suggested-fixes/{fix_id}/apply",
+    response_model=SessionSuggestedFixResponse,
+)
+async def apply_suggested_fix(
+    session_id: UUID,
+    fix_id: UUID,
+    current_user: Annotated[User, Depends(get_current_active_user)],
+    db: Annotated[AsyncSession, Depends(get_db)],
+    _: None = Depends(require_engineer_or_admin),
+) -> SessionSuggestedFixResponse:
+    """Stamp applied_at when the engineer clicks Apply in the ProposalBanner.
+
+    This does NOT change status (fix remains 'proposed'). Status only flips
+    when the engineer records an outcome via PATCH /outcome.
+
+    Rules:
+    - Fix must be in 'proposed' status; any other status → 409.
+    - Idempotent: if applied_at is already set, returns 200 with the unchanged row.
+    - Bumps ai_sessions.state_version so resolve/escalate preview generators
+      know the fix has entered the verifying phase.
+    """
+    await _load_session_or_404(db, session_id)
+
+    result = await db.execute(
+        select(SessionSuggestedFix).where(
+            SessionSuggestedFix.id == fix_id,
+            SessionSuggestedFix.session_id == session_id,
+        )
+    )
+    fix = result.scalar_one_or_none()
+    if fix is None:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND, detail="Suggested fix not found"
+        )
+
+    if fix.status != "proposed":
+        raise HTTPException(
+            status_code=status.HTTP_409_CONFLICT,
+            detail=f"Apply is only valid from 'proposed'; fix is already '{fix.status}'",
+        )
+
+    # Idempotent: already stamped → return as-is without bumping state_version again.
+    if fix.applied_at is not None:
+        return SessionSuggestedFixResponse.model_validate(fix)
+
+    fix.applied_at = datetime.now(timezone.utc)
+
+    # Bump state_version so preview generators see the verifying-phase signal.
+    await db.execute(
+        update(AISession)
+        .where(AISession.id == session_id)
+        .values(state_version=AISession.state_version + 1)
+    )
+
+    await db.commit()
+    await db.refresh(fix)
+    return SessionSuggestedFixResponse.model_validate(fix)
+
+
 # ── Suggested fix: outcome ────────────────────────────────────────────────
 
 @router.patch(
diff --git a/backend/tests/test_fix_outcome_endpoint.py b/backend/tests/test_fix_outcome_endpoint.py
index b0969417..89a4fb15 100644
--- a/backend/tests/test_fix_outcome_endpoint.py
+++ b/backend/tests/test_fix_outcome_endpoint.py
@@ -327,3 +327,113 @@ async def test_resolution_note_preview_reflects_outcome_after_patch(
     assert distinct_failure_reason in bundle_b, (
         "Bundle for second preview should include the failure_reason text"
     )
+
+
+# ── Apply endpoint ─────────────────────────────────────────────────────────
+
+@pytest.mark.asyncio
+async def test_apply_stamps_applied_at(
+    client: AsyncClient, test_user, auth_headers, test_db
+):
+    """POST /apply stamps applied_at and bumps state_version."""
+    from uuid import UUID
+    session_id, fix_id = await _make_session_with_fix(test_db, test_user)
+
+    result = await test_db.execute(
+        select(AISession).where(AISession.id == UUID(session_id))
+    )
+    session_obj = result.scalar_one()
+    initial_version = session_obj.state_version
+
+    r = await client.post(
+        f"/api/v1/ai-sessions/{session_id}/suggested-fixes/{fix_id}/apply",
+        headers=auth_headers,
+    )
+    assert r.status_code == 200, r.text
+    body = r.json()
+    assert body["applied_at"] is not None, "applied_at must be set after /apply"
+    assert body["status"] == "proposed", "status must remain 'proposed' after /apply"
+
+    await test_db.refresh(session_obj)
+    assert session_obj.state_version == initial_version + 1, (
+        "/apply must bump state_version so preview cache is invalidated"
+    )
+
+
+@pytest.mark.asyncio
+async def test_apply_is_idempotent(
+    client: AsyncClient, test_user, auth_headers, test_db
+):
+    """Second POST /apply returns 200 with applied_at unchanged (no double-bump)."""
+    from uuid import UUID
+    session_id, fix_id = await _make_session_with_fix(test_db, test_user)
+
+    r1 = await client.post(
+        f"/api/v1/ai-sessions/{session_id}/suggested-fixes/{fix_id}/apply",
+        headers=auth_headers,
+    )
+    assert r1.status_code == 200, r1.text
+    applied_at_first = r1.json()["applied_at"]
+
+    result = await test_db.execute(
+        select(AISession).where(AISession.id == UUID(session_id))
+    )
+    session_obj = result.scalar_one()
+    version_after_first = session_obj.state_version
+
+    r2 = await client.post(
+        f"/api/v1/ai-sessions/{session_id}/suggested-fixes/{fix_id}/apply",
+        headers=auth_headers,
+    )
+    assert r2.status_code == 200, r2.text
+    assert r2.json()["applied_at"] == applied_at_first, (
+        "applied_at must not change on second /apply call"
+    )
+
+    await test_db.refresh(session_obj)
+    assert session_obj.state_version == version_after_first, (
+        "state_version must not be bumped a second time on idempotent /apply"
+    )
+
+
+@pytest.mark.asyncio
+async def test_apply_rejects_non_proposed(
+    client: AsyncClient, test_user, auth_headers, test_db
+):
+    """POST /apply returns 409 when fix status is 'applied_success'."""
+    session_id, fix_id = await _make_session_with_fix(test_db, test_user)
+
+    # Advance the fix to a terminal status via the outcome endpoint.
+    r_outcome = await client.patch(
+        f"/api/v1/ai-sessions/{session_id}/suggested-fixes/{fix_id}/outcome",
+        headers=auth_headers,
+        json={"outcome": "applied_success"},
+    )
+    assert r_outcome.status_code == 200
+
+    r = await client.post(
+        f"/api/v1/ai-sessions/{session_id}/suggested-fixes/{fix_id}/apply",
+        headers=auth_headers,
+    )
+    assert r.status_code == 409, r.text
+
+
+@pytest.mark.asyncio
+async def test_apply_rejects_dismissed(
+    client: AsyncClient, test_user, auth_headers, test_db
+):
+    """POST /apply returns 409 when fix status is 'dismissed'."""
+    session_id, fix_id = await _make_session_with_fix(test_db, test_user)
+
+    r_outcome = await client.patch(
+        f"/api/v1/ai-sessions/{session_id}/suggested-fixes/{fix_id}/outcome",
+        headers=auth_headers,
+        json={"outcome": "dismissed"},
+    )
+    assert r_outcome.status_code == 200
+
+    r = await client.post(
+        f"/api/v1/ai-sessions/{session_id}/suggested-fixes/{fix_id}/apply",
+        headers=auth_headers,
+    )
+    assert r.status_code == 409, r.text
diff --git a/frontend/src/api/sessionSuggestedFixes.ts b/frontend/src/api/sessionSuggestedFixes.ts
index 078d0d87..fbbf654c 100644
--- a/frontend/src/api/sessionSuggestedFixes.ts
+++ b/frontend/src/api/sessionSuggestedFixes.ts
@@ -111,6 +111,19 @@ export const sessionSuggestedFixesApi = {
     return r.data
   },
 
+  /**
+   * Stamp applied_at when the engineer clicks Apply in the ProposalBanner.
+   * Does NOT change status (fix remains 'proposed'). Status flips only on
+   * a subsequent PATCH /outcome. Idempotent if applied_at is already set.
+   * Returns 409 if the fix is no longer in 'proposed' status.
+   */
+  async applyFix(sessionId: string, fixId: string): Promise<SessionSuggestedFix> {
+    const r = await apiClient.post<SessionSuggestedFix>(
+      `/ai-sessions/${sessionId}/suggested-fixes/${fixId}/apply`,
+    )
+    return r.data
+  },
+
   /**
    * Record the outcome of applying a suggested fix. Transition rules:
    * - from `proposed` or `applied_partial`: any outcome is valid (partial is
diff --git a/frontend/src/pages/AssistantChatPage.tsx b/frontend/src/pages/AssistantChatPage.tsx
index 20936fd1..ff7bb2ea 100644
--- a/frontend/src/pages/AssistantChatPage.tsx
+++ b/frontend/src/pages/AssistantChatPage.tsx
@@ -135,20 +135,19 @@ export default function AssistantChatPage() {
   const isNarrow = useMediaQuery('(max-width: 1199px)')
   // Phase 8: ProposalBanner + EscalateInterceptDialog state.
   const [bannerCollapsed, setBannerCollapsed] = useState(false)
-  const [bannerApplied, setBannerApplied] = useState(false)
   const [postApplyMsgCount, setPostApplyMsgCount] = useState(0)
   const [nudgeSilenced, setNudgeSilenced] = useState(false)
   const [escalateIntercept, setEscalateIntercept] = useState<{ fixId: string; fixTitle: string } | null>(null)
-  // Phase 8: compute the current banner mode from activeFix + client-side flags.
-  // bannerApplied is a client-side-only flag that flips on Apply click so we
-  // don't need a server round-trip just to stamp applied_at for mode computation.
+  // Phase 8: compute the current banner mode from activeFix.
+  // applied_at is now persisted on the server (stamped by POST /apply),
+  // so bannerMode is derived entirely from server state — no client-side flag.
   const bannerMode: BannerMode | null = (() => {
     if (!activeFix) return null
     if (activeFix.status === 'dismissed') return null
     if (activeFix.ai_outcome_proposal) return 'ai_confirming'
     if (activeFix.status === 'applied_partial') return 'partial'
     if (activeFix.status === 'applied_success' || activeFix.status === 'applied_failed') return null
-    if (bannerApplied || activeFix.applied_at) {
+    if (activeFix.applied_at) {
       if (postApplyMsgCount >= 3 && !nudgeSilenced) return 'nudge'
       return 'verifying'
     }
@@ -341,7 +340,6 @@ export default function AssistantChatPage() {
     setScriptPanelOpen(false)
     // Phase 8: banner state reset
     setBannerCollapsed(false)
-    setBannerApplied(false)
     setPostApplyMsgCount(0)
     setNudgeSilenced(false)
     setEscalateIntercept(null)
@@ -513,16 +511,23 @@ export default function AssistantChatPage() {
     refreshPreview(activeChatId, kind)
   }, [activeChatId, previewKind, refreshPreview])
 
-  // Phase 8: handleApplyFix — opens the existing script panel (same as clicking
-  // the SuggestedFix card) and stamps the client-side bannerApplied flag so
-  // bannerMode transitions to 'verifying' without a server round-trip.
-  const handleApplyFix = useCallback(() => {
-    if (!activeFix) return
-    setBannerApplied(true)
+  // Phase 8: handleApplyFix — stamps applied_at on the server so Verifying state
+  // survives refresh/reselect/multi-tab, then opens the script panel.
+  const handleApplyFix = useCallback(async () => {
+    if (!activeFix || !activeChatId) return
     setPostApplyMsgCount(0)
     setNudgeSilenced(false)
+    try {
+      const updated = await sessionSuggestedFixesApi.applyFix(activeChatId, activeFix.id)
+      setActiveFix(updated)
+    } catch (err: unknown) {
+      // Non-fatal: the script panel still opens. The banner will stay in
+      // 'proposed' mode until the next refreshActiveFix succeeds, which is
+      // a cosmetic gap only — no data loss.
+      console.error('[AssistantChat] applyFix failed:', err)
+    }
     setScriptPanelOpen(true)
-  }, [activeFix])
+  }, [activeFix, activeChatId])
 
   // Phase 8: record a terminal outcome for the active fix. Updates local state
   // on success. For applied_success also opens the Resolve preview.
@@ -532,7 +537,6 @@ export default function AssistantChatPage() {
       const updated = await sessionSuggestedFixesApi.patchOutcome(activeChatId, activeFix.id, outcome, notes)
       setActiveFix(updated)
       // Reset apply tracking state since we now have a terminal outcome.
-      setBannerApplied(false)
       setPostApplyMsgCount(0)
       setNudgeSilenced(false)
       if (outcome === 'applied_success') {
@@ -586,7 +590,7 @@ export default function AssistantChatPage() {
   const handleEscalateClick = useCallback(() => {
     const inVerifyState =
       activeFix && (
-        ((bannerApplied || !!activeFix.applied_at) && activeFix.status === 'proposed') ||
+        (!!activeFix.applied_at && activeFix.status === 'proposed') ||
         activeFix.status === 'applied_partial'
       )
     if (inVerifyState && activeFix) {
@@ -594,7 +598,7 @@ export default function AssistantChatPage() {
       return
     }
     setShowConclude(true)
-  }, [activeFix, bannerApplied])
+  }, [activeFix])
 
   const handleInterceptChoice = useCallback(async (choice: InterceptChoice) => {
     const stored = escalateIntercept
@@ -614,7 +618,7 @@ export default function AssistantChatPage() {
   // Phase 8: Resolve click — auto-mark applied_success if in verifying state
   // before opening the resolution note preview.
   const handleResolveClick = useCallback(async () => {
-    if (activeFix && (bannerApplied || activeFix.applied_at) && activeFix.status === 'proposed' && activeChatId) {
+    if (activeFix && activeFix.applied_at && activeFix.status === 'proposed' && activeChatId) {
       try {
         const updated = await sessionSuggestedFixesApi.patchOutcome(activeChatId, activeFix.id, 'applied_success')
         setActiveFix(updated)
@@ -623,7 +627,7 @@ export default function AssistantChatPage() {
       }
     }
     setShowConclude(true)
-  }, [activeChatId, activeFix, bannerApplied])
+  }, [activeChatId, activeFix])
 
   const handleClosePreview = () => {
     setPreviewKind(null)
@@ -841,8 +845,7 @@ export default function AssistantChatPage() {
       // Only increments when fix is still in 'proposed' (verifying) state —
       // partial/dismissed/terminal states don't render the nudge, and a
       // partial→verifying transition could inherit an already-saturated counter.
-      if (activeFix && (bannerApplied || activeFix.applied_at) &&
-          activeFix.status === 'proposed') {
+      if (activeFix && activeFix.applied_at && activeFix.status === 'proposed') {
         setPostApplyMsgCount(c => c + 1)
       }
       // Refetch facts + active fix; preview refreshes if open.
@@ -917,8 +920,7 @@ export default function AssistantChatPage() {
       }
       // Phase 8: increment post-apply message counter for nudge logic (mirrors handleSend).
       // Only increments in 'proposed' (verifying) state — same rationale as handleSend.
-      if (activeFix && (bannerApplied || activeFix.applied_at) &&
-          activeFix.status === 'proposed') {
+      if (activeFix && activeFix.applied_at && activeFix.status === 'proposed') {
         setPostApplyMsgCount(c => c + 1)
       }
       // Refetch facts + active fix; answering tasks is the primary trigger.