diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c5bd929..547beca 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -89,6 +89,37 @@ jobs:
       - name: Test compatibility harness
         run: cargo test -p splice-compat
 
+      # This is deliberately separate from the workspace suite: native Windows
+      # is the authority for ConPTY behavior, not a cross-compiled approximation.
+      - name: ConPTY runtime regression gate (authoritative Windows)
+        if: ${{ !cancelled() }}
+        run: cargo test -p splice-pty --test conpty_smoke
+
       - name: Test
         if: ${{ !cancelled() }}
         run: cargo test --workspace
+
+  ubuntu-runtime:
+    name: Native Ubuntu PTY runtime
+    runs-on: ubuntu-24.04
+    timeout-minutes: 20
+    steps:
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
+        with:
+          ref: ${{ inputs.ref || github.ref }}
+
+      - name: Set up Rust toolchain
+        uses: dtolnay/rust-toolchain@4be7066ada62dd38de10e7b70166bc74ed198c30 # stable
+
+      - name: Install Tauri build prerequisites
+        run: >-
+          sudo apt-get update && sudo apt-get install --yes
+          libwebkit2gtk-4.1-dev libgtk-3-dev libayatana-appindicator3-dev librsvg2-dev
+
+      - name: Run native Ubuntu PTY fixtures
+        run: cargo test -p splice-pty --test platform_runtime_matrix -- --nocapture
+
+      - name: Run platform default-shell and authority integration
+        run: >-
+          cargo test -p splice-pty --test session_contract &&
+          cargo test -p splice-shell-desktop --test platform_authority
diff --git a/crates/splice-pty/tests/platform_runtime_matrix.rs b/crates/splice-pty/tests/platform_runtime_matrix.rs
new file mode 100644
index 0000000..850ee82
--- /dev/null
+++ b/crates/splice-pty/tests/platform_runtime_matrix.rs
@@ -0,0 +1,129 @@
+use std::{
+    fs,
+    path::PathBuf,
+    sync::mpsc,
+    time::{Duration, Instant},
+};
+
+use splice_pty::{PtySession, TerminalSize};
+
+fn size() -> TerminalSize {
+    TerminalSize::new(80, 24).expect("fixture terminal size is valid")
+}
+
+fn supported_ubuntu_runtime() -> bool {
+    let release = fs::read_to_string("/etc/os-release").unwrap_or_default();
+    release.lines().any(|line| line == "ID=ubuntu")
+        && release
+            .lines()
+            .any(|line| matches!(line, "VERSION_ID=\"22.04\"" | "VERSION_ID=\"24.04\""))
+}
+
+fn require_supported_ubuntu_runtime() -> bool {
+    if supported_ubuntu_runtime() {
+        true
+    } else {
+        eprintln!(
+            "SKIP: native Ubuntu runtime fixture requires Ubuntu 22.04 or 24.04; this host is unsupported"
+        );
+        false
+    }
+}
+
+fn receive_until(receiver: &mpsc::Receiver<String>, marker: &str) -> String {
+    let deadline = Instant::now() + Duration::from_secs(5);
+    let mut output = String::new();
+    while Instant::now() < deadline && !output.contains(marker) {
+        if let Ok(chunk) = receiver.recv_timeout(Duration::from_millis(250)) {
+            output.push_str(&chunk);
+        }
+    }
+    output
+}
+
+#[test]
+fn native_ubuntu_runtime_fixture_exercises_shell_utf8_input_resize_and_liveness() {
+    if !require_supported_ubuntu_runtime() {
+        return;
+    }
+
+    let (sender, receiver) = mpsc::channel();
+    let session = PtySession::spawn(
+        "/bin/sh",
+        &[
+            "-c",
+            "printf 'ready utf8=café\\n'; stty size; read value; printf 'input=%s\\n' \"$value\"; stty size",
+        ],
+        size(),
+        move |_, output| {
+            let _ = sender.send(output);
+        },
+        |_| {},
+    )
+    .expect("native Ubuntu default shell starts");
+
+    assert!(session.is_running().expect("liveness is available"));
+    assert_eq!(session.active_process_name().expect("process name"), "sh");
+    assert!(receive_until(&receiver, "24 80").contains("ready utf8=café"));
+
+    session
+        .resize(TerminalSize::new(132, 43).expect("resized fixture terminal size"))
+        .expect("resize succeeds");
+    session.write("fixture-input\n").expect("input succeeds");
+    let output = receive_until(&receiver, "43 132");
+    assert!(output.contains("input=fixture-input"));
+    assert!(output.contains("43 132"));
+    session.close();
+    println!("RECEIPT: native Ubuntu PTY startup, UTF-8, input, resize, and liveness passed");
+}
+
+#[test]
+fn native_ubuntu_runtime_fixture_exercises_interrupt_teardown_and_idempotent_close() {
+    if !require_supported_ubuntu_runtime() {
+        return;
+    }
+
+    let (sender, receiver) = mpsc::channel();
+    let session = PtySession::spawn(
+        "/bin/sh",
+        &[
+            "-c",
+            "trap 'printf interrupted; exit' INT; printf ready; while :; do sleep 1; done",
+        ],
+        size(),
+        move |_, output| {
+            let _ = sender.send(output);
+        },
+        |_| {},
+    )
+    .expect("native Ubuntu default shell starts");
+
+    assert!(receive_until(&receiver, "ready").contains("ready"));
+    session.interrupt().expect("interrupt succeeds");
+    assert!(receive_until(&receiver, "interrupted").contains("interrupted"));
+    session.close();
+    session.close();
+    assert!(!session.is_running().expect("liveness after close"));
+    println!("RECEIPT: native Ubuntu PTY interrupt, teardown, and idempotent close passed");
+}
+
+#[test]
+fn ci_declares_authoritative_windows_and_native_ubuntu_runtime_gates() {
+    let workflow = PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("../../.github/workflows/ci.yml");
+    let workflow = fs::read_to_string(workflow).expect("CI workflow is readable");
+
+    for required_gate in [
+        "ConPTY runtime regression gate (authoritative Windows)",
+        "cargo test -p splice-pty --test conpty_smoke",
+        "Native Ubuntu PTY runtime",
+        "runs-on: ubuntu-24.04",
+        "cargo test -p splice-pty --test platform_runtime_matrix",
+        "cargo test -p splice-pty --test session_contract",
+        "cargo test -p splice-shell-desktop --test platform_authority",
+    ] {
+        assert!(
+            workflow.contains(required_gate),
+            "CI workflow must retain runtime gate: {required_gate}"
+        );
+    }
+}