diff --git a/pkg/service/soundtouchweb/browser_compatibility_test.go b/pkg/service/soundtouchweb/browser_compatibility_test.go
index ac1c3253..54c4a5a5 100644
--- a/pkg/service/soundtouchweb/browser_compatibility_test.go
+++ b/pkg/service/soundtouchweb/browser_compatibility_test.go
@@ -370,6 +370,10 @@ func TestSourceSelectionUsesOneWriteAndAbsoluteReadbacks(t *testing.T) {
 
 			w.Header().Set("Content-Type", "application/json")
 			if body.Source == "SPOTIFY" {
+				// 500 is how a failed Client.SelectSource surfaces. It does not
+				// prove the speaker ignored the command, so the readbacks below
+				// must still run.
+				w.WriteHeader(http.StatusInternalServerError)
 				_ = json.NewEncoder(w).Encode(webtypes.APIResponse{Success: false, Error: "source rejected"})
 				return
 			}
@@ -406,6 +410,7 @@ func TestSourceSelectionUsesOneWriteAndAbsoluteReadbacks(t *testing.T) {
 		chromedp.Poll(`document.querySelector('.source-command-status').textContent === 'Source selection unverified'`, nil),
 		chromedp.Click(`.source-btn:nth-child(3)`, chromedp.ByQuery),
 		chromedp.Poll(`document.querySelector('.source-btn:nth-child(3)').classList.contains('unverified')`, nil),
+		chromedp.Poll(`document.querySelector('.source-command-status').textContent === 'Source selection unverified: source rejected'`, nil),
 	); err != nil {
 		t.Fatalf("exercise source commands: %v", err)
 	}
@@ -553,6 +558,55 @@ func TestSourceSelectionReadbacksDoNotWaitForSlowWriteResponse(t *testing.T) {
 	}
 }
 
+// TestSourceSelectionDefinitiveRefusalFailsImmediately: a 4xx is produced
+// before AfterTouch ever calls the speaker, so the command provably never went
+// out. There is nothing for the readbacks to confirm; report it at once, with
+// the server's reason, instead of polling for the full readback window.
+func TestSourceSelectionDefinitiveRefusalFailsImmediately(t *testing.T) {
+	var mu sync.Mutex
+	reads := 0
+	server := newPlayerFixtureServer(t, sourceFixtureScript, func(r chi.Router) {
+		r.Post("/api/control/devices/speaker/action/source", func(w http.ResponseWriter, _ *http.Request) {
+			w.Header().Set("Content-Type", "application/json")
+			w.WriteHeader(http.StatusNotFound)
+			_, _ = w.Write([]byte(`{"success":false,"error":"Device not found"}`))
+		})
+		r.Get("/api/control/devices/speaker", func(w http.ResponseWriter, _ *http.Request) {
+			mu.Lock()
+			reads++
+			mu.Unlock()
+			w.Header().Set("Content-Type", "application/json")
+			_, _ = w.Write([]byte(`{"success":true,"data":{"status":{"revision":2,"nowPlayingRevision":2,"nowPlaying":{"Source":"STANDBY","SourceAccount":""}}}}`))
+		})
+	})
+
+	ctx := newHeadlessChromeContext(t)
+	var statusText string
+	if err := chromedp.Run(ctx,
+		chromedp.Navigate(server.URL+"/fixture"),
+		chromedp.WaitVisible(`.source-btn`, chromedp.ByQuery),
+		chromedp.Click(`.source-btn:nth-child(1)`, chromedp.ByQuery),
+		chromedp.Poll(`document.querySelector('.source-btn').classList.contains('failed')`, nil),
+		chromedp.Text(`.source-command-status`, &statusText, chromedp.ByQuery),
+		// Well past every readback deadline in this fixture (100/250/500ms), so
+		// a zero read count means the failure came from the write itself.
+		chromedp.Sleep(700*time.Millisecond),
+	); err != nil {
+		t.Fatalf("exercise definitively refused source write: %v", err)
+	}
+
+	if !strings.Contains(statusText, "Source selection failed") ||
+		!strings.Contains(statusText, "Device not found") {
+		t.Errorf("status = %q, want the failure and the server's reason", statusText)
+	}
+
+	mu.Lock()
+	defer mu.Unlock()
+	if reads != 0 {
+		t.Errorf("readbacks after a definitive refusal = %d, want 0", reads)
+	}
+}
+
 func TestSourceSelectionLaterFirmwareErrorOverridesProvisionalConfirmation(t *testing.T) {
 	var mu sync.Mutex
 	reads := 0
@@ -586,7 +640,7 @@ func TestSourceSelectionLaterFirmwareErrorOverridesProvisionalConfirmation(t *te
 		chromedp.Click(`.source-btn:nth-child(1)`, chromedp.ByQuery),
 		chromedp.Poll(`document.querySelector('.source-command-status').textContent === 'Source selected, confirming'`, nil),
 		chromedp.Evaluate(`document.querySelector('.source-btn:nth-child(1)').classList.contains('provisional-confirmed')`, &provisional),
-		chromedp.Poll(`document.querySelector('.source-command-status').textContent === 'Source selection failed'`, nil),
+		chromedp.Poll(`document.querySelector('.source-command-status').textContent === 'Source selection failed: INVALID_SOURCE'`, nil),
 	); err != nil {
 		t.Fatalf("exercise provisional source rejection: %v", err)
 	}
@@ -658,7 +712,7 @@ func TestSourceSelectionTreatsFirmwareErrorSourceAsFailed(t *testing.T) {
 		chromedp.Navigate(server.URL+"/fixture"),
 		chromedp.WaitVisible(`.source-btn`, chromedp.ByQuery),
 		chromedp.Click(`.source-btn:nth-child(1)`, chromedp.ByQuery),
-		chromedp.Poll(`document.querySelector('.source-command-status').textContent === 'Source selection failed'`, nil),
+		chromedp.Poll(`document.querySelector('.source-command-status').textContent === 'Source selection failed: INVALID_SOURCE'`, nil),
 	); err != nil {
 		t.Fatalf("exercise firmware source rejection: %v", err)
 	}
diff --git a/pkg/service/soundtouchweb/frontend_test/api.test.mjs b/pkg/service/soundtouchweb/frontend_test/api.test.mjs
index 99b43a37..1ed3a8ba 100644
--- a/pkg/service/soundtouchweb/frontend_test/api.test.mjs
+++ b/pkg/service/soundtouchweb/frontend_test/api.test.mjs
@@ -23,22 +23,44 @@ test('selectSource posts the exact source and account body', async () => {
     assert.deepEqual(JSON.parse(requests[1].options.body), { source: 'PRODUCT', account: '' });
 });
 
-test('source selection rejects non-2xx responses', async () => {
-    globalThis.fetch = async () => new Response('{"success":false,"error":"offline"}', {
-        status: 503,
+// A 4xx is produced before any speaker call (unknown device, unparseable
+// body, empty source), so the command provably never went out.
+test('source selection reports a 4xx as definitive', async () => {
+    globalThis.fetch = async () => new Response('{"success":false,"error":"Device not found"}', {
+        status: 404,
         headers: { 'Content-Type': 'application/json' },
     });
 
-    await assert.rejects(api.selectSource('speaker', 'AUX', 'AUX1'), /offline/);
+    await assert.rejects(api.selectSource('speaker', 'AUX', 'AUX1'), error => {
+        assert.match(error.message, /Device not found/);
+        assert.equal(error.definitive, true);
+        return true;
+    });
 });
 
-test('source selection rejects application failures on 2xx responses', async () => {
-    globalThis.fetch = async () => new Response('{"success":false,"error":"rejected"}', {
-        status: 200,
+// A 5xx is how a failed Client.SelectSource surfaces, and a request that timed
+// out after the speaker already switched is indistinguishable from one it never
+// received. The caller must keep verifying by readback.
+test('source selection reports a 5xx as non-definitive', async () => {
+    globalThis.fetch = async () => new Response('{"success":false,"error":"speaker unreachable"}', {
+        status: 500,
         headers: { 'Content-Type': 'application/json' },
     });
 
-    await assert.rejects(api.selectSource('speaker', 'AUX', 'AUX1'), /rejected/);
+    await assert.rejects(api.selectSource('speaker', 'AUX', 'AUX1'), error => {
+        assert.match(error.message, /speaker unreachable/);
+        assert.equal(error.definitive, false);
+        return true;
+    });
+});
+
+test('source selection reports an unreadable body as non-definitive on 5xx', async () => {
+    globalThis.fetch = async () => new Response('gateway timeout', { status: 504 });
+
+    await assert.rejects(api.selectSource('speaker', 'AUX', 'AUX1'), error => {
+        assert.equal(error.definitive, false);
+        return true;
+    });
 });
 
 test('legacy API consumers retain response-level error handling', async () => {
diff --git a/pkg/service/soundtouchweb/static/js/api.js b/pkg/service/soundtouchweb/static/js/api.js
index 747b075c..59a8bd82 100644
--- a/pkg/service/soundtouchweb/static/js/api.js
+++ b/pkg/service/soundtouchweb/static/js/api.js
@@ -5,16 +5,33 @@ async function req(url, opts = {}) {
     return r.json();
 }
 
+// checkedReq turns a failed request into a thrown Error, tagging whether the
+// failure is DEFINITIVE, meaning proof the command never reached the speaker.
+//
+// Only 4xx qualifies. Every 4xx on the control endpoints is produced before
+// any speaker call (missing/unknown device, unparseable body, empty source,
+// unknown action), so nothing was sent onward. A 5xx is NOT proof of anything:
+// handleSourceControl reports a failed Client.SelectSource through
+// sendControlResponse, which maps any speaker-call error to 500, and a request
+// that timed out after the speaker already switched looks exactly like one it
+// never received. Transport errors are ambiguous for the same reason.
+//
+// Callers that verify by readback must keep verifying unless the failure is
+// definitive.
 async function checkedReq(url, opts = {}) {
     const r = await fetch(url, opts);
+    const definitive = r.status >= 400 && r.status < 500;
     let response;
     try {
         response = await r.json();
     } catch (_) {
-        throw new Error(`Request failed (${r.status})`);
+        throw Object.assign(new Error(`Request failed (${r.status})`), { definitive });
     }
     if (!r.ok || response?.success === false) {
-        throw new Error(response?.error || `Request failed (${r.status})`);
+        throw Object.assign(
+            new Error(response?.error || `Request failed (${r.status})`),
+            { definitive },
+        );
     }
     return response;
 }
diff --git a/pkg/service/soundtouchweb/static/js/components/Sources.js b/pkg/service/soundtouchweb/static/js/components/Sources.js
index e75d269f..015a6133 100644
--- a/pkg/service/soundtouchweb/static/js/components/Sources.js
+++ b/pkg/service/soundtouchweb/static/js/components/Sources.js
@@ -190,7 +190,23 @@ export function Sources({
         try {
             await api.selectSource(deviceId, target.source, target.account);
         } catch (error) {
-            if (commandRef.current.active === active) active.writeError = error;
+            if (commandRef.current.active !== active) return;
+            // A definitive refusal (4xx) means the speaker never saw the
+            // command, so there is nothing for the readbacks to confirm and
+            // reporting it now beats waiting out the readback window. Anything
+            // else stays pending: a 5xx or a transport error does not tell us
+            // whether the speaker acted, so we keep verifying and carry the
+            // reason into whatever outcome the readbacks reach.
+            if (!error?.definitive) {
+                active.writeError = error;
+
+                return;
+            }
+            clearReadbacks();
+            commandRef.current.active = null;
+            setCommand(previous => previous?.generation === generation
+                ? { ...previous, outcome: 'failed', error: error?.message }
+                : previous);
         }
     }
 
@@ -209,6 +225,12 @@ export function Sources({
         failed: 'Source selection failed',
     };
 
+    function commandMessage(cmd) {
+        if (!cmd) return '';
+        const text = outcomeText[cmd.outcome];
+        return cmd.error ? `${text}: ${cmd.error}` : text;
+    }
+
     return html`
         <div class="sources-section">
             <h3 class="section-title">Sources</h3>
@@ -225,7 +247,7 @@ export function Sources({
                             class="source-btn ${isActive ? 'active' : ''} ${src.IsLocal ? 'local' : ''} ${outcome}"
                             onClick=${() => select(src)}
                             disabled=${sourcesStale}
-                            title=${availabilityMessage || (outcome ? outcomeText[outcome] : src.Source)}
+                            title=${availabilityMessage || (outcome ? commandMessage(command) : src.Source)}
                             aria-describedby=${availabilityId}
                             aria-busy=${outcome === 'pending' || outcome === 'provisional-confirmed' ? 'true' : null}
                         >
@@ -241,7 +263,7 @@ export function Sources({
                 role="status"
                 aria-live="polite"
             >
-                ${availabilityMessage || (command ? outcomeText[command.outcome] : '')}
+                ${availabilityMessage || commandMessage(command)}
             </div>
         </div>
     `;