diff --git a/src/audio/cpal_impl.rs b/src/audio/cpal_impl.rs
index f73c030..4ae5b7d 100644
--- a/src/audio/cpal_impl.rs
+++ b/src/audio/cpal_impl.rs
@@ -516,6 +516,26 @@ fn find_device_by_name(host: &cpal::Host, output: bool, name: &str) -> Option<De
         .find(|d| d.name().is_ok_and(|n| n == name))
 }
 
+/// Whether the backend can actually open this config range. The workers only build
+/// `F32`/`I16`/`U16` streams ([`build_input`]/[`build_output`] — every other sample
+/// format hits the `other => Err(...)` arm), and a zero-channel range would later be
+/// rejected by [`resolve`]'s geometry check. Filtering both here keeps
+/// [`choose_config`] from *ranking* a range it can't drive ahead of a usable one and
+/// then hard-failing the start instead of trying the next candidate (Codex B5
+/// re-review, P3).
+fn usable_range(r: &cpal::SupportedStreamConfigRange) -> bool {
+    r.channels() > 0 && format_supported(r.sample_format())
+}
+
+/// Sample formats the capture/playback stream builders accept. Pure, so it's
+/// unit-testable independently of the cpal range types.
+fn format_supported(fmt: SampleFormat) -> bool {
+    matches!(
+        fmt,
+        SampleFormat::F32 | SampleFormat::I16 | SampleFormat::U16
+    )
+}
+
 /// Pick a sample rate inside both a device's supported `[r_min, r_max]` span and the
 /// backend's drivable `[MIN_DEVICE_RATE, MAX_DEVICE_RATE]` window, preferring
 /// [`SAMPLE_RATE`] when it's reachable and otherwise the nearest in-window bound.
@@ -558,7 +578,7 @@ fn choose_config(device: &Device, output: bool) -> Result<cpal::SupportedStreamC
     let pick = |channels: Option<u16>| {
         ranges
             .iter()
-            .find(|r| supports_48k(r) && channels.is_none_or(|c| r.channels() == c))
+            .find(|r| usable_range(r) && supports_48k(r) && channels.is_none_or(|c| r.channels() == c))
             .cloned()
     };
 
@@ -581,7 +601,7 @@ fn choose_config(device: &Device, output: bool) -> Result<cpal::SupportedStreamC
     let pick_bounded = |channels: Option<u16>| -> Option<(cpal::SupportedStreamConfigRange, u32)> {
         ranges
             .iter()
-            .filter(|r| channels.is_none_or(|c| r.channels() == c))
+            .filter(|r| usable_range(r) && channels.is_none_or(|c| r.channels() == c))
             .filter_map(|r| {
                 bounded_rate(r.min_sample_rate().0, r.max_sample_rate().0)
                     .map(|rate| (r.clone(), rate))
@@ -1363,6 +1383,27 @@ mod tests {
         );
     }
 
+    #[test]
+    fn format_supported_matches_the_stream_builders() {
+        // Exactly the three the build_input/build_output match arms accept.
+        for f in [SampleFormat::F32, SampleFormat::I16, SampleFormat::U16] {
+            assert!(format_supported(f), "{f:?} should be drivable");
+        }
+        // Everything else cpal can expose must be filtered out before ranking, or a
+        // start could pick it and then hit the `unsupported sample format` arm (P3).
+        for f in [
+            SampleFormat::I8,
+            SampleFormat::U8,
+            SampleFormat::I32,
+            SampleFormat::U32,
+            SampleFormat::I64,
+            SampleFormat::U64,
+            SampleFormat::F64,
+        ] {
+            assert!(!format_supported(f), "{f:?} must not be reported drivable");
+        }
+    }
+
     #[test]
     fn ensure_idle_allows_an_idle_slot() {
         let mut s = SlotState::Idle;