diff --git a/AGENTS.md b/AGENTS.md
index 79aced0..c2750ec 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -152,6 +152,13 @@ Established 2026-08-25, and it decides more than it looks like:
   `"""\n-----BEGIN CERTIFICATE-----` costs Android's `CertificateFactory`
   its preamble sniff, so it tries DER instead and fails at runtime with
   `ASN.1 ... DECODE_ERROR` — nowhere near the code that produced it.
+- **ZXing only looks for a dark code on a light ground.** The enrollment
+  QR is block characters in the terminal's foreground colour, so a
+  dark-themed terminal renders it as a negative and the in-app scanner
+  silently never matches — while the phone's own camera app, which tries
+  both, does. The scanner asks for `Intents.Scan.MIXED_SCAN`, which
+  alternates normal and inverted frames; keep it that way rather than
+  making the server dictate the colours.
 - **AGP 9 refuses `Provider`s in the source-set API**: generated sources go
   through `androidComponents.onVariants { it.sources.java?.addGenerated
   SourceDirectory(task, Task::outputDir) }`, which also carries the task
diff --git a/app/androidApp/src/main/kotlin/com/example/aiapp/SettingsScreen.kt b/app/androidApp/src/main/kotlin/com/example/aiapp/SettingsScreen.kt
index 0c0b4b1..af406b9 100644
--- a/app/androidApp/src/main/kotlin/com/example/aiapp/SettingsScreen.kt
+++ b/app/androidApp/src/main/kotlin/com/example/aiapp/SettingsScreen.kt
@@ -24,6 +24,7 @@ import androidx.compose.ui.Modifier
 import androidx.compose.ui.platform.LocalContext
 import androidx.compose.ui.unit.dp
 import androidx.activity.compose.rememberLauncherForActivityResult
+import com.google.zxing.client.android.Intents
 import com.journeyapps.barcodescanner.ScanContract
 import com.journeyapps.barcodescanner.ScanIntentResult
 import com.journeyapps.barcodescanner.ScanOptions
@@ -82,7 +83,22 @@ fun SettingsScreen(
         Spacer(Modifier.height(16.dp))
 
         OutlinedButton(
-            onClick = { scanLauncher.launch(ScanOptions().setDesiredBarcodeFormats(ScanOptions.QR_CODE)) },
+            onClick = {
+                scanLauncher.launch(
+                    ScanOptions()
+                        .setDesiredBarcodeFormats(ScanOptions.QR_CODE)
+                        // Decode both polarities: ZXing otherwise looks only for a
+                        // dark code on a light ground, and ai-server's QR is block
+                        // characters in the terminal's foreground colour -- so on a
+                        // dark-themed terminal it comes out as a photographic
+                        // negative that the scanner silently never matches. Which
+                        // way round it renders is the terminal's business, not
+                        // something this app should depend on. MIXED_SCAN alternates
+                        // normal and inverted frames, so it costs half the frame
+                        // rate at each polarity and nothing else.
+                        .addExtra(Intents.Scan.SCAN_TYPE, Intents.Scan.MIXED_SCAN)
+                )
+            },
             modifier = Modifier.fillMaxWidth(),
         ) { Text("Scan QR code") }
         Spacer(Modifier.height(16.dp))