diff --git a/HANDOFF.md b/HANDOFF.md
index 18df973..e8663a5 100644
--- a/HANDOFF.md
+++ b/HANDOFF.md
@@ -1,4 +1,4 @@
-# HANDOFF — 2026-09-20 (cross-table z-order bug fixed: webcam between sources survives reload; defaults/current split queued as TASK 37)
+# HANDOFF — 2026-09-20 (cross-table z-order bug fixed; REC/ON-AIR pills now radio-exclusive; defaults/current split queued as TASK 37)
 
 ## Branch / Commit State
 
@@ -36,6 +36,18 @@ shape `scene[0-4].layerList.[default|current].elementList`; config-file vs secon
 decided (`TASKS/task-37-defaults-current-split.md` has both + facts). **Process rule established:**
 capture out-of-scope needed work in TASKS.md rather than bolting it onto the in-flight change.
 
+## ✅ Committed — REC/ON-AIR pills are radio-exclusive (2026-09-20)
+
+**Gap found:** the record-OR-live ruling (2026-09-01) was documented but the "pills = radios" UI
+constraint never landed — the two pills were independent toggles, so a creator COULD arm both and
+`StartSession`/`BuildEncoderOptions` would happily dual-encode (RTMP + `-f mp4`) — the exact thing the
+ruling forbids (a concurrent disk write streams from the same sustained ingest Kbps and cripples the
+stream on mid-range chassis; "the VOD is already the copy"). Fix: mutual clear in both pill setters
+(`RecordPillOn`/`OnAirPillOn` in `MainViewModel.Streaming.cs`) — arming one clears the other (silent
+mutual clear, creator's choice). The dual-block ffmpeg machinery stays generic/unreachable by design.
+Test `PillRadioTests` (RealApp + temp DB): REC→ON-AIR clears REC; ON-AIR→REC clears ON-AIR; single
+pill still arms Start. **310/310 pass.**
+
 ## ✅ Committed earlier — layer-list drag-to-reorder persists (2026-09-18, `f91bf87` + below)
 
 `StagedScene.Elements` reorder (RemoveAt/Insert) bypasses SceneGraph's mutation surface, so the drop
diff --git a/ViewModels/MainViewModel.Streaming.cs b/ViewModels/MainViewModel.Streaming.cs
index 589b2f9..543990b 100644
--- a/ViewModels/MainViewModel.Streaming.cs
+++ b/ViewModels/MainViewModel.Streaming.cs
@@ -33,8 +33,10 @@ public partial class MainViewModel : ViewModelBase
     private double _recDotPulse = 1.0;
     private TimeSpan _liveElapsed;
 
-    // ─── Recording (TASK 18): independent REC / ON-AIR pills. The pills are
-    // ─── intent; IsRecording / IsLive are reality. ON-AIR needs a sign-in.
+    // ─── Recording (TASK 18): REC / ON-AIR pills — the pills are intent;
+    // ─── IsRecording / IsLive are reality. ON-AIR needs a sign-in, and the
+    // ─── pills are radio-exclusive (record-OR-live, creator ruling 2026-09-01):
+    // ─── arming one clears the other.
     private bool _recordPillOn;
     private bool _onAirPillOn;
     private bool _isRecording;
@@ -85,6 +87,7 @@ public partial class MainViewModel : ViewModelBase
         set
         {
             if (!SetProperty(ref _recordPillOn, value)) return;
+            if (value && _onAirPillOn) OnAirPillOn = false; // record-OR-live — arming REC drops ON-AIR
             OnPropertyChanged(nameof(ShowPrimaryStartButton));
             OnPropertyChanged(nameof(CanStartSession));
         }
@@ -97,6 +100,7 @@ public partial class MainViewModel : ViewModelBase
         {
             if (value && !CanToggleOnAir) return; // needs a sign-in
             if (!SetProperty(ref _onAirPillOn, value)) return;
+            if (value && _recordPillOn) RecordPillOn = false; // record-OR-live — arming ON-AIR drops REC
             OnPropertyChanged(nameof(ShowPrimaryStartButton));
             OnPropertyChanged(nameof(CanStartSession));
         }
diff --git a/ai.md b/ai.md
index 5786c68..2710fef 100644
--- a/ai.md
+++ b/ai.md
@@ -596,7 +596,8 @@ anullsrc` silence in the TASK 8 audio milestone) + explicit `-map 0:v -map 1:a`
 the engine runs record-only (no RTMP) or stream-only — **stream+record simultaneously is OUT by creator
 ruling (2026-09-01): the VOD is already the copy, and dual-encoding drags mid-range chassis and degrades
 BOTH outputs ("we're not them"). The old "cheap on NVENC" claim was an unverified assumption; the
-UI constraint (pills = radios) lands as a small change, the dual-block machinery stays generic.**
+UI constraint is landed (2026-09-20): the REC/ON-AIR pills are radio-exclusive — arming one clears the
+other — while the dual-block machinery stays generic.**
 `FfmpegEncoder.StartAsync` throws unless at least one output is enabled.
 **Encoder choice is probed from the binary's `-encoders` listing** (`FfmpegEncoderPicker`, pure):
 NVENC → QSV → AMF → OpenH264 fallback, **never libx264** (GPL; see Licensing). `EncoderOptions.VideoEncoder`
@@ -611,13 +612,13 @@ declare intent: **REC pill** (local file, works signed-out) and **ON-AIR pill**
 until `IsConnected`). The pill is intent; `IsRecording`/`IsLive` are reality — the REC status dot only turns
 green when a session is actually recording, the ON-AIR dot when actually live.
 
-- **State model (`MainViewModel`):** pills `RecordPillOn`/`OnAirPillOn` (ON-AIR setter no-ops if
-  `!CanToggleOnAir`), `ShowPrimaryStartButton` = **always the idle face** (`IsOffline && !IsRecording`,
+- **State model (`MainViewModel`):** pills `RecordPillOn`/`OnAirPillOn` (radio-exclusive — arming one
+  clears the other, record-OR-live 2026-09-01; the ON-AIR setter also no-ops if `!CanToggleOnAir`), `ShowPrimaryStartButton` = **always the idle face** (`IsOffline && !IsRecording`,
   2026-09-01 — the old connected/record-only gate blanked the bar after stopping signed-out),
   `ShowEndStreamButton`, `CanStartSession`, `AccountStatusLightToolTip`. Sign-in is a **context-menu
   item on Start** ("Sign in to YouTube" → `SignInCommand`, visible while disconnected) — the standalone
   Sign In button is gone (TASK 30's single-button rule, completed). `StartSession()` routes: ON-AIR on
-  → GoLive dialog then stream (± record); REC on or **nothing armed** → `BeginRecordOnly()` (unarmed
+  → GoLive dialog then stream; REC on or **nothing armed** → `BeginRecordOnly()` (unarmed
   Start lights the REC pill and records — no dead-end no-ops; local recording needs no account).
   `IsEditMode` also requires `!IsRecording` (lock scrubbing while recording).
   **Top bar order (creator spec 2026-09-01):** `[sign light] REC [pill]  [sign light] ON-AIR [pill]`
@@ -640,8 +641,9 @@ green when a session is actually recording, the ON-AIR dot when actually live.
   `OnFramePumpFailed` + `IsRecording` setter made internal (test-only, InternalsVisibleTo).
   Test: `SessionTeardownTests`.
 - `EncoderOptions.StreamEnabled/RecordEnabled/RecordPath` gate outputs; record+stream is one ffmpeg with two
-  output blocks. Rest of the engine (FramePump/encoder) is unchanged — `BuildEncoderOptions` now always
-  returns an options (with flags from the pills + `_activeRecordPath`), so record-only reaches the pump.
+  output blocks (the dual-block machinery is unreachable from the UI since the pills are radio-exclusive).
+  Rest of the engine (FramePump/encoder) is unchanged — `BuildEncoderOptions` always returns an options
+  (flags from the pills + `_activeRecordPath`), so record-only reaches the pump.
 
 ### Live audio capture (TASK 4 ship step 4 — shipped 2026-08-12; game audio bar 2026-08-13; **TASK 8 audio milestone: real stream audio + filters + duck + TRAX, shipped 2026-08-14**, plan in TASKS.md)
 
diff --git a/ytLive.Tests/PillRadioTests.cs b/ytLive.Tests/PillRadioTests.cs
new file mode 100644
index 0000000..86fd044
--- /dev/null
+++ b/ytLive.Tests/PillRadioTests.cs
@@ -0,0 +1,64 @@
+using System;
+using System.IO;
+using Microsoft.Data.Sqlite;
+using Xunit;
+using ytLive.Services;
+using ytLive.ViewModels;
+
+namespace ytLive.Tests;
+
+/// <summary>
+/// Record-OR-live (creator ruling 2026-09-01): the REC and ON-AIR pills are
+/// radio-exclusive — arming one clears the other, so a session can never dual-encode
+/// to both the YouTube ingest and a local file at once (a concurrent disk write
+/// streamed from the same sustained Kbps cripples the stream on mid-range chassis).
+/// The dual-block encoder machinery in FfmpegArgs stays generic; the constraint
+/// lives at the pills.
+/// </summary>
+[Collection("RealApp")]
+public sealed class PillRadioTests
+{
+    private readonly RealAppHost _app;
+
+    public PillRadioTests(RealAppHost app) => _app = app;
+
+    [Fact]
+    public void Arming_One_Output_Pill_Clears_The_Other()
+    {
+        _app.Run(Run);
+    }
+
+    private void Run()
+    {
+        var tempDb = Path.Combine(Path.GetTempPath(), $"ytLlive-pillradio-{Guid.NewGuid():N}.db");
+        MainViewModel.LayoutPathOverride = tempDb;
+        try
+        {
+            using (var schema = new LayoutStore(tempDb)) { }
+            SqliteConnection.ClearAllPools();
+
+            var vm = new MainViewModel();
+            vm.IsConnected = true; // the ON-AIR setter requires a sign-in
+
+            // Arm REC first, then ON-AIR → REC must clear.
+            vm.RecordPillOn = true;
+            vm.OnAirPillOn = true;
+            Assert.False(vm.RecordPillOn, "arming ON-AIR must clear the REC pill");
+            Assert.True(vm.OnAirPillOn);
+
+            // Arm REC while ON-AIR is lit → ON-AIR must clear.
+            vm.RecordPillOn = true;
+            Assert.False(vm.OnAirPillOn, "arming REC must clear the ON-AIR pill");
+            Assert.True(vm.RecordPillOn);
+
+            // A single armed pill still makes the Start button actionable.
+            Assert.True(vm.CanStartSession);
+        }
+        finally
+        {
+            MainViewModel.LayoutPathOverride = null;
+            SqliteConnection.ClearAllPools();
+            try { File.Delete(tempDb); } catch { /* best-effort cleanup */ }
+        }
+    }
+}
\ No newline at end of file