diff --git a/server/packages/sandbox-agent/src/browser_runtime.rs b/server/packages/sandbox-agent/src/browser_runtime.rs
new file mode 100644
index 0000000..cec6a88
--- /dev/null
+++ b/server/packages/sandbox-agent/src/browser_runtime.rs
@@ -0,0 +1,982 @@
+use std::collections::HashMap;
+use std::collections::VecDeque;
+use std::fs::{self, OpenOptions};
+use std::path::{Path, PathBuf};
+use std::sync::Arc;
+use std::time::Duration;
+
+use tokio::sync::Mutex;
+
+use crate::browser_cdp::CdpClient;
+use crate::browser_errors::BrowserProblem;
+use crate::browser_install::{
+    browser_platform_support_message, detect_missing_browser_dependencies,
+};
+use crate::browser_types::{
+    BrowserConsoleMessage, BrowserNetworkRequest, BrowserStartRequest, BrowserState,
+    BrowserStatusResponse,
+};
+use crate::desktop_install::find_binary;
+use crate::desktop_runtime::DesktopRuntime;
+use crate::desktop_streaming::DesktopStreamingManager;
+use crate::desktop_types::{DesktopErrorInfo, DesktopProcessInfo, DesktopResolution};
+use crate::process_runtime::{
+    ProcessOwner, ProcessRuntime, ProcessStartSpec, ProcessStatus, RestartPolicy,
+};
+
+const DEFAULT_WIDTH: u32 = 1440;
+const DEFAULT_HEIGHT: u32 = 900;
+const DEFAULT_DPI: u32 = 96;
+const DEFAULT_DISPLAY_NUM: i32 = 98;
+const MAX_DISPLAY_PROBE: i32 = 10;
+const STARTUP_TIMEOUT: Duration = Duration::from_secs(15);
+const CDP_POLL_TIMEOUT: Duration = Duration::from_secs(15);
+const CDP_PORT: u16 = 9222;
+const MAX_CONSOLE_MESSAGES: usize = 1000;
+const MAX_NETWORK_REQUESTS: usize = 1000;
+
+#[derive(Debug, Clone)]
+pub struct BrowserRuntime {
+    config: BrowserRuntimeConfig,
+    process_runtime: Arc<ProcessRuntime>,
+    desktop_runtime: Arc<DesktopRuntime>,
+    streaming_manager: DesktopStreamingManager,
+    inner: Arc<Mutex<BrowserRuntimeStateData>>,
+}
+
+#[derive(Debug, Clone)]
+pub struct BrowserRuntimeConfig {
+    state_dir: PathBuf,
+    display_num: i32,
+    assume_linux_for_tests: bool,
+}
+
+impl Default for BrowserRuntimeConfig {
+    fn default() -> Self {
+        Self {
+            state_dir: default_state_dir(),
+            display_num: DEFAULT_DISPLAY_NUM,
+            assume_linux_for_tests: false,
+        }
+    }
+}
+
+struct BrowserRuntimeStateData {
+    state: BrowserState,
+    display_num: i32,
+    display: Option<String>,
+    resolution: Option<DesktopResolution>,
+    started_at: Option<String>,
+    last_error: Option<DesktopErrorInfo>,
+    missing_dependencies: Vec<String>,
+    install_command: Option<String>,
+    runtime_log_path: PathBuf,
+    environment: HashMap<String, String>,
+    xvfb: Option<ManagedBrowserProcess>,
+    chromium: Option<ManagedBrowserProcess>,
+    cdp_client: Option<CdpClient>,
+    context_id: Option<String>,
+    streaming_config: Option<crate::desktop_streaming::StreamingConfig>,
+    recording_fps: Option<u32>,
+    console_messages: VecDeque<BrowserConsoleMessage>,
+    network_requests: VecDeque<BrowserNetworkRequest>,
+}
+
+impl std::fmt::Debug for BrowserRuntimeStateData {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("BrowserRuntimeStateData")
+            .field("state", &self.state)
+            .field("display", &self.display)
+            .field("resolution", &self.resolution)
+            .field("started_at", &self.started_at)
+            .finish_non_exhaustive()
+    }
+}
+
+#[derive(Debug)]
+struct ManagedBrowserProcess {
+    name: &'static str,
+    process_id: String,
+    pid: Option<u32>,
+    running: bool,
+}
+
+impl BrowserRuntime {
+    pub fn new(process_runtime: Arc<ProcessRuntime>, desktop_runtime: Arc<DesktopRuntime>) -> Self {
+        Self::with_config(
+            process_runtime,
+            desktop_runtime,
+            BrowserRuntimeConfig::default(),
+        )
+    }
+
+    pub fn with_config(
+        process_runtime: Arc<ProcessRuntime>,
+        desktop_runtime: Arc<DesktopRuntime>,
+        config: BrowserRuntimeConfig,
+    ) -> Self {
+        let runtime_log_path = config.state_dir.join("browser-runtime.log");
+        Self {
+            streaming_manager: DesktopStreamingManager::new(process_runtime.clone()),
+            process_runtime,
+            desktop_runtime,
+            inner: Arc::new(Mutex::new(BrowserRuntimeStateData {
+                state: BrowserState::Inactive,
+                display_num: config.display_num,
+                display: None,
+                resolution: None,
+                started_at: None,
+                last_error: None,
+                missing_dependencies: Vec::new(),
+                install_command: None,
+                runtime_log_path,
+                environment: HashMap::new(),
+                xvfb: None,
+                chromium: None,
+                cdp_client: None,
+                context_id: None,
+                streaming_config: None,
+                recording_fps: None,
+                console_messages: VecDeque::new(),
+                network_requests: VecDeque::new(),
+            })),
+            config,
+        }
+    }
+
+    // -----------------------------------------------------------------------
+    // Public API
+    // -----------------------------------------------------------------------
+
+    pub async fn status(&self) -> BrowserStatusResponse {
+        let mut state = self.inner.lock().await;
+        self.refresh_status_locked(&mut state).await;
+        let mut response = self.snapshot_locked(&state);
+        drop(state);
+        self.append_neko_process(&mut response).await;
+        response
+    }
+
+    pub async fn start(
+        &self,
+        request: BrowserStartRequest,
+    ) -> Result<BrowserStatusResponse, BrowserProblem> {
+        // Check mutual exclusivity with desktop runtime
+        let desktop_status = self.desktop_runtime.status().await;
+        if desktop_status.state == crate::desktop_types::DesktopState::Active {
+            return Err(BrowserProblem::desktop_conflict());
+        }
+
+        let mut state = self.inner.lock().await;
+
+        if !self.platform_supported() {
+            let problem = BrowserProblem::start_failed(browser_platform_support_message());
+            self.record_problem_locked(&mut state, &problem);
+            state.state = BrowserState::Failed;
+            return Err(problem);
+        }
+
+        if matches!(state.state, BrowserState::Starting | BrowserState::Stopping) {
+            return Err(BrowserProblem::start_failed(
+                "Browser runtime is busy transitioning state",
+            ));
+        }
+
+        self.refresh_status_locked(&mut state).await;
+        if state.state == BrowserState::Active {
+            let mut response = self.snapshot_locked(&state);
+            drop(state);
+            self.append_neko_process(&mut response).await;
+            return Ok(response);
+        }
+
+        if !state.missing_dependencies.is_empty() {
+            return Err(BrowserProblem::install_required(format!(
+                "Missing browser dependencies: {}. Run: sandbox-agent install browser --yes",
+                state.missing_dependencies.join(", ")
+            )));
+        }
+
+        self.ensure_state_dir()
+            .map_err(|err| BrowserProblem::start_failed(err))?;
+        self.write_runtime_log_locked(&state, "starting browser runtime");
+
+        let width = request.width.unwrap_or(DEFAULT_WIDTH);
+        let height = request.height.unwrap_or(DEFAULT_HEIGHT);
+        let dpi = request.dpi.unwrap_or(DEFAULT_DPI);
+        if width == 0 || height == 0 {
+            return Err(BrowserProblem::start_failed(
+                "Browser width and height must be greater than 0",
+            ));
+        }
+
+        let headless = request.headless.unwrap_or(false);
+
+        // Store streaming/recording config
+        state.streaming_config = if request.stream_video_codec.is_some()
+            || request.stream_audio_codec.is_some()
+            || request.stream_frame_rate.is_some()
+            || request.webrtc_port_range.is_some()
+        {
+            Some(crate::desktop_streaming::StreamingConfig {
+                video_codec: request
+                    .stream_video_codec
+                    .unwrap_or_else(|| "vp8".to_string()),
+                audio_codec: request
+                    .stream_audio_codec
+                    .unwrap_or_else(|| "opus".to_string()),
+                frame_rate: request.stream_frame_rate.unwrap_or(30).clamp(1, 60),
+                webrtc_port_range: request
+                    .webrtc_port_range
+                    .unwrap_or_else(|| "59050-59070".to_string()),
+            })
+        } else {
+            None
+        };
+        state.recording_fps = request.recording_fps.map(|fps| fps.clamp(1, 60));
+        state.context_id = request.context_id.clone();
+
+        // Choose display and set up environment
+        let display_num = if headless {
+            // Headless doesn't need Xvfb but we still pick a display_num for consistency
+            self.config.display_num
+        } else {
+            self.choose_display_num()?
+        };
+        let display = format!(":{display_num}");
+        let resolution = DesktopResolution {
+            width,
+            height,
+            dpi: Some(dpi),
+        };
+        let environment = self.base_environment(&display)?;
+
+        state.state = BrowserState::Starting;
+        state.display_num = display_num;
+        state.display = Some(display.clone());
+        state.resolution = Some(resolution.clone());
+        state.started_at = None;
+        state.last_error = None;
+        state.environment = environment;
+        state.install_command = None;
+        state.console_messages.clear();
+        state.network_requests.clear();
+
+        // Start Xvfb (unless headless)
+        if !headless {
+            if let Err(problem) = self.start_xvfb_locked(&mut state, &resolution).await {
+                return Err(self.fail_start_locked(&mut state, problem).await);
+            }
+            if let Err(problem) = self.wait_for_socket(display_num).await {
+                return Err(self.fail_start_locked(&mut state, problem).await);
+            }
+        }
+
+        // Start Chromium
+        if let Err(problem) = self
+            .start_chromium_locked(&mut state, &resolution, headless, request.url.as_deref())
+            .await
+        {
+            return Err(self.fail_start_locked(&mut state, problem).await);
+        }
+
+        // Wait for CDP to become ready
+        if let Err(problem) = self.wait_for_cdp().await {
+            return Err(self.fail_start_locked(&mut state, problem).await);
+        }
+
+        // Connect CDP client
+        match CdpClient::connect().await {
+            Ok(client) => {
+                state.cdp_client = Some(client);
+            }
+            Err(problem) => {
+                return Err(self.fail_start_locked(&mut state, problem).await);
+            }
+        }
+
+        // Optionally start Neko for streaming
+        if !headless {
+            if let Some(streaming_config) = state.streaming_config.clone() {
+                let display_ref = state.display.clone().unwrap_or_default();
+                let resolution_ref = state.resolution.clone().unwrap_or(DesktopResolution {
+                    width,
+                    height,
+                    dpi: Some(dpi),
+                });
+                let env_ref = state.environment.clone();
+                drop(state);
+                let _ = self
+                    .streaming_manager
+                    .start(
+                        &display_ref,
+                        resolution_ref,
+                        &env_ref,
+                        Some(streaming_config),
+                        None,
+                    )
+                    .await;
+                state = self.inner.lock().await;
+            }
+        }
+
+        state.state = BrowserState::Active;
+        state.started_at = Some(chrono::Utc::now().to_rfc3339());
+        state.last_error = None;
+        self.write_runtime_log_locked(
+            &state,
+            &format!(
+                "browser runtime active on {} ({}x{}, dpi {})",
+                display, width, height, dpi
+            ),
+        );
+
+        let mut response = self.snapshot_locked(&state);
+        drop(state);
+        self.append_neko_process(&mut response).await;
+        Ok(response)
+    }
+
+    pub async fn stop(&self) -> Result<BrowserStatusResponse, BrowserProblem> {
+        let mut state = self.inner.lock().await;
+        if matches!(state.state, BrowserState::Starting | BrowserState::Stopping) {
+            return Err(BrowserProblem::start_failed(
+                "Browser runtime is busy transitioning state",
+            ));
+        }
+
+        state.state = BrowserState::Stopping;
+        self.write_runtime_log_locked(&state, "stopping browser runtime");
+
+        // Close CDP client
+        if let Some(cdp_client) = state.cdp_client.take() {
+            cdp_client.close().await;
+        }
+
+        // Stop streaming
+        let _ = self.streaming_manager.stop().await;
+
+        // Stop Chromium
+        self.stop_chromium_locked(&mut state).await;
+
+        // Stop Xvfb
+        self.stop_xvfb_locked(&mut state).await;
+
+        state.state = BrowserState::Inactive;
+        state.display = None;
+        state.resolution = None;
+        state.started_at = None;
+        state.last_error = None;
+        state.context_id = None;
+        state.missing_dependencies = self.detect_missing_dependencies();
+        state.install_command = self.install_command_for(&state.missing_dependencies);
+        state.environment.clear();
+        state.streaming_config = None;
+        state.recording_fps = None;
+        state.console_messages.clear();
+        state.network_requests.clear();
+
+        let mut response = self.snapshot_locked(&state);
+        drop(state);
+        self.append_neko_process(&mut response).await;
+        Ok(response)
+    }
+
+    pub async fn shutdown(&self) {
+        let _ = self.stop().await;
+    }
+
+    /// Get a reference to the CDP client, if connected.
+    pub async fn cdp_client(&self) -> Result<CdpClient, BrowserProblem> {
+        let state = self.inner.lock().await;
+        if state.state != BrowserState::Active {
+            return Err(BrowserProblem::not_active());
+        }
+        // We cannot return a reference out of the Mutex, so we need to use
+        // the send method directly. For now, return an error if not connected.
+        // Callers should use `with_cdp` instead.
+        Err(BrowserProblem::cdp_error(
+            "Use with_cdp() to execute CDP commands",
+        ))
+    }
+
+    /// Execute a closure with the CDP client while holding the state lock.
+    pub async fn with_cdp<F, Fut, T>(&self, f: F) -> Result<T, BrowserProblem>
+    where
+        F: FnOnce(&CdpClient) -> Fut,
+        Fut: std::future::Future<Output = Result<T, BrowserProblem>>,
+    {
+        let state = self.inner.lock().await;
+        if state.state != BrowserState::Active {
+            return Err(BrowserProblem::not_active());
+        }
+        let cdp = state
+            .cdp_client
+            .as_ref()
+            .ok_or_else(|| BrowserProblem::cdp_error("CDP client is not connected"))?;
+        f(cdp).await
+    }
+
+    /// Get the streaming manager for WebRTC signaling.
+    pub fn streaming_manager(&self) -> &DesktopStreamingManager {
+        &self.streaming_manager
+    }
+
+    /// Push a console message into the ring buffer.
+    pub async fn push_console_message(&self, message: BrowserConsoleMessage) {
+        let mut state = self.inner.lock().await;
+        if state.console_messages.len() >= MAX_CONSOLE_MESSAGES {
+            state.console_messages.pop_front();
+        }
+        state.console_messages.push_back(message);
+    }
+
+    /// Push a network request into the ring buffer.
+    pub async fn push_network_request(&self, request: BrowserNetworkRequest) {
+        let mut state = self.inner.lock().await;
+        if state.network_requests.len() >= MAX_NETWORK_REQUESTS {
+            state.network_requests.pop_front();
+        }
+        state.network_requests.push_back(request);
+    }
+
+    /// Get console messages, optionally filtered by level.
+    pub async fn console_messages(
+        &self,
+        level: Option<&str>,
+        limit: Option<u32>,
+    ) -> Vec<BrowserConsoleMessage> {
+        let state = self.inner.lock().await;
+        let limit = limit.unwrap_or(100) as usize;
+        state
+            .console_messages
+            .iter()
+            .filter(|msg| level.map_or(true, |l| msg.level == l))
+            .rev()
+            .take(limit)
+            .cloned()
+            .collect::<Vec<_>>()
+            .into_iter()
+            .rev()
+            .collect()
+    }
+
+    /// Get network requests, optionally filtered by URL pattern.
+    pub async fn network_requests(
+        &self,
+        url_pattern: Option<&str>,
+        limit: Option<u32>,
+    ) -> Vec<BrowserNetworkRequest> {
+        let state = self.inner.lock().await;
+        let limit = limit.unwrap_or(100) as usize;
+        state
+            .network_requests
+            .iter()
+            .filter(|req| url_pattern.map_or(true, |pattern| req.url.contains(pattern)))
+            .rev()
+            .take(limit)
+            .cloned()
+            .collect::<Vec<_>>()
+            .into_iter()
+            .rev()
+            .collect()
+    }
+
+    // -----------------------------------------------------------------------
+    // Internal: state management
+    // -----------------------------------------------------------------------
+
+    async fn refresh_status_locked(&self, state: &mut BrowserRuntimeStateData) {
+        let missing_dependencies = if self.platform_supported() {
+            self.detect_missing_dependencies()
+        } else {
+            Vec::new()
+        };
+        state.missing_dependencies = missing_dependencies.clone();
+        state.install_command = self.install_command_for(&missing_dependencies);
+
+        if !self.platform_supported() {
+            state.state = BrowserState::Failed;
+            state.last_error = Some(
+                BrowserProblem::start_failed(browser_platform_support_message()).to_error_info(),
+            );
+            return;
+        }
+
+        if !missing_dependencies.is_empty() {
+            state.state = BrowserState::InstallRequired;
+            state.last_error = Some(
+                BrowserProblem::install_required(format!(
+                    "Missing: {}",
+                    missing_dependencies.join(", ")
+                ))
+                .to_error_info(),
+            );
+            return;
+        }
+
+        if matches!(
+            state.state,
+            BrowserState::Inactive | BrowserState::Starting | BrowserState::Stopping
+        ) {
+            if state.state == BrowserState::Inactive {
+                state.last_error = None;
+            }
+            return;
+        }
+
+        if state.state == BrowserState::Failed
+            && state.display.is_none()
+            && state.xvfb.is_none()
+            && state.chromium.is_none()
+        {
+            return;
+        }
+
+        // Check Xvfb is running (if we started one)
+        if let Some(ref xvfb) = state.xvfb {
+            if let Ok(snapshot) = self.process_runtime.snapshot(&xvfb.process_id).await {
+                if snapshot.status != ProcessStatus::Running {
+                    let problem = BrowserProblem::start_failed("Xvfb process exited unexpectedly");
+                    self.record_problem_locked(state, &problem);
+                    state.state = BrowserState::Failed;
+                    return;
+                }
+            }
+        }
+
+        // Check Chromium is running
+        if let Some(ref chromium) = state.chromium {
+            if let Ok(snapshot) = self.process_runtime.snapshot(&chromium.process_id).await {
+                if snapshot.status != ProcessStatus::Running {
+                    let problem =
+                        BrowserProblem::start_failed("Chromium process exited unexpectedly");
+                    self.record_problem_locked(state, &problem);
+                    state.state = BrowserState::Failed;
+                    return;
+                }
+            }
+        }
+    }
+
+    fn snapshot_locked(&self, state: &BrowserRuntimeStateData) -> BrowserStatusResponse {
+        BrowserStatusResponse {
+            state: state.state,
+            display: state.display.clone(),
+            resolution: state.resolution.clone(),
+            started_at: state.started_at.clone(),
+            cdp_url: if state.state == BrowserState::Active {
+                Some(format!("ws://127.0.0.1:{CDP_PORT}/devtools/browser"))
+            } else {
+                None
+            },
+            url: None,
+            missing_dependencies: state.missing_dependencies.clone(),
+            install_command: state.install_command.clone(),
+            processes: self.processes_locked(state),
+            last_error: state.last_error.clone(),
+        }
+    }
+
+    fn processes_locked(&self, state: &BrowserRuntimeStateData) -> Vec<DesktopProcessInfo> {
+        let mut processes = Vec::new();
+        if let Some(ref process) = state.xvfb {
+            processes.push(DesktopProcessInfo {
+                name: process.name.to_string(),
+                pid: process.pid,
+                running: process.running,
+                log_path: None,
+            });
+        }
+        if let Some(ref process) = state.chromium {
+            processes.push(DesktopProcessInfo {
+                name: process.name.to_string(),
+                pid: process.pid,
+                running: process.running,
+                log_path: None,
+            });
+        }
+        processes
+    }
+
+    async fn append_neko_process(&self, response: &mut BrowserStatusResponse) {
+        if let Some(neko_info) = self.streaming_manager.process_info().await {
+            response.processes.push(neko_info);
+        }
+    }
+
+    fn record_problem_locked(&self, state: &mut BrowserRuntimeStateData, problem: &BrowserProblem) {
+        state.last_error = Some(problem.to_error_info());
+        self.write_runtime_log_locked(
+            state,
+            &format!("{}: {}", problem.code(), problem.to_error_info().message),
+        );
+    }
+
+    // -----------------------------------------------------------------------
+    // Internal: subprocess management
+    // -----------------------------------------------------------------------
+
+    async fn start_xvfb_locked(
+        &self,
+        state: &mut BrowserRuntimeStateData,
+        resolution: &DesktopResolution,
+    ) -> Result<(), BrowserProblem> {
+        let Some(display) = state.display.clone() else {
+            return Err(BrowserProblem::start_failed(
+                "Display was not configured before starting Xvfb",
+            ));
+        };
+        let args = vec![
+            display,
+            "-screen".to_string(),
+            "0".to_string(),
+            format!("{}x{}x24", resolution.width, resolution.height),
+            "-dpi".to_string(),
+            resolution.dpi.unwrap_or(DEFAULT_DPI).to_string(),
+            "-nolisten".to_string(),
+            "tcp".to_string(),
+        ];
+        let snapshot = self
+            .process_runtime
+            .start_process(ProcessStartSpec {
+                command: "Xvfb".to_string(),
+                args,
+                cwd: None,
+                env: state.environment.clone(),
+                tty: false,
+                interactive: false,
+                owner: ProcessOwner::Desktop,
+                restart_policy: Some(RestartPolicy::Always),
+            })
+            .await
+            .map_err(|err| BrowserProblem::start_failed(format!("failed to start Xvfb: {err}")))?;
+        state.xvfb = Some(ManagedBrowserProcess {
+            name: "Xvfb",
+            process_id: snapshot.id,
+            pid: snapshot.pid,
+            running: snapshot.status == ProcessStatus::Running,
+        });
+        Ok(())
+    }
+
+    async fn start_chromium_locked(
+        &self,
+        state: &mut BrowserRuntimeStateData,
+        resolution: &DesktopResolution,
+        headless: bool,
+        initial_url: Option<&str>,
+    ) -> Result<(), BrowserProblem> {
+        let chromium_binary = find_chromium_binary().ok_or_else(|| {
+            BrowserProblem::install_required(
+                "Chromium binary not found. Run: sandbox-agent install browser --yes",
+            )
+        })?;
+
+        let mut args = vec![
+            "--no-sandbox".to_string(),
+            "--disable-gpu".to_string(),
+            "--disable-dev-shm-usage".to_string(),
+            "--disable-software-rasterizer".to_string(),
+            format!("--remote-debugging-port={CDP_PORT}"),
+            "--remote-debugging-address=127.0.0.1".to_string(),
+            format!("--window-size={},{}", resolution.width, resolution.height),
+            "--no-first-run".to_string(),
+            "--no-default-browser-check".to_string(),
+        ];
+
+        if headless {
+            args.push("--headless=new".to_string());
+        }
+
+        // Set user-data-dir for persistent contexts
+        if let Some(ref context_id) = state.context_id {
+            let context_dir = self
+                .config
+                .state_dir
+                .join("browser-contexts")
+                .join(context_id);
+            args.push(format!("--user-data-dir={}", context_dir.display()));
+        }
+
+        // Initial URL
+        let url = initial_url.unwrap_or("about:blank");
+        args.push(url.to_string());
+
+        let snapshot = self
+            .process_runtime
+            .start_process(ProcessStartSpec {
+                command: chromium_binary.to_string_lossy().to_string(),
+                args,
+                cwd: None,
+                env: state.environment.clone(),
+                tty: false,
+                interactive: false,
+                owner: ProcessOwner::Desktop,
+                restart_policy: Some(RestartPolicy::Always),
+            })
+            .await
+            .map_err(|err| {
+                BrowserProblem::start_failed(format!("failed to start Chromium: {err}"))
+            })?;
+        state.chromium = Some(ManagedBrowserProcess {
+            name: "chromium",
+            process_id: snapshot.id,
+            pid: snapshot.pid,
+            running: snapshot.status == ProcessStatus::Running,
+        });
+        Ok(())
+    }
+
+    async fn stop_xvfb_locked(&self, state: &mut BrowserRuntimeStateData) {
+        if let Some(process) = state.xvfb.take() {
+            self.write_runtime_log_locked(state, "stopping Xvfb");
+            let _ = self
+                .process_runtime
+                .stop_process(&process.process_id, Some(2_000))
+                .await;
+            if self
+                .process_runtime
+                .snapshot(&process.process_id)
+                .await
+                .ok()
+                .is_some_and(|snapshot| snapshot.status == ProcessStatus::Running)
+            {
+                let _ = self
+                    .process_runtime
+                    .kill_process(&process.process_id, Some(1_000))
+                    .await;
+            }
+        }
+    }
+
+    async fn stop_chromium_locked(&self, state: &mut BrowserRuntimeStateData) {
+        if let Some(process) = state.chromium.take() {
+            self.write_runtime_log_locked(state, "stopping Chromium");
+            let _ = self
+                .process_runtime
+                .stop_process(&process.process_id, Some(2_000))
+                .await;
+            if self
+                .process_runtime
+                .snapshot(&process.process_id)
+                .await
+                .ok()
+                .is_some_and(|snapshot| snapshot.status == ProcessStatus::Running)
+            {
+                let _ = self
+                    .process_runtime
+                    .kill_process(&process.process_id, Some(1_000))
+                    .await;
+            }
+        }
+    }
+
+    async fn fail_start_locked(
+        &self,
+        state: &mut BrowserRuntimeStateData,
+        problem: BrowserProblem,
+    ) -> BrowserProblem {
+        self.record_problem_locked(state, &problem);
+        self.write_runtime_log_locked(state, "browser runtime startup failed; cleaning up");
+
+        // Close CDP client if any
+        if let Some(cdp) = state.cdp_client.take() {
+            cdp.close().await;
+        }
+
+        self.stop_chromium_locked(state).await;
+        self.stop_xvfb_locked(state).await;
+
+        state.state = BrowserState::Failed;
+        state.display = None;
+        state.resolution = None;
+        state.started_at = None;
+        state.environment.clear();
+        problem
+    }
+
+    // -----------------------------------------------------------------------
+    // Internal: helpers
+    // -----------------------------------------------------------------------
+
+    async fn wait_for_socket(&self, display_num: i32) -> Result<(), BrowserProblem> {
+        let socket = socket_path(display_num);
+        let parent = socket
+            .parent()
+            .map(Path::to_path_buf)
+            .unwrap_or_else(|| PathBuf::from("/tmp/.X11-unix"));
+        let _ = fs::create_dir_all(parent);
+
+        let start = tokio::time::Instant::now();
+        while start.elapsed() < STARTUP_TIMEOUT {
+            if socket.exists() {
+                return Ok(());
+            }
+            tokio::time::sleep(Duration::from_millis(100)).await;
+        }
+
+        Err(BrowserProblem::timeout(format!(
+            "timed out waiting for X socket {}",
+            socket.display()
+        )))
+    }
+
+    async fn wait_for_cdp(&self) -> Result<(), BrowserProblem> {
+        let url = format!("http://127.0.0.1:{CDP_PORT}/json/version");
+        let client = reqwest::Client::new();
+        let start = tokio::time::Instant::now();
+
+        while start.elapsed() < CDP_POLL_TIMEOUT {
+            match client.get(&url).send().await {
+                Ok(resp) if resp.status().is_success() => return Ok(()),
+                _ => {}
+            }
+            tokio::time::sleep(Duration::from_millis(200)).await;
+        }
+
+        Err(BrowserProblem::timeout(format!(
+            "CDP endpoint at {url} did not become ready within {}s",
+            CDP_POLL_TIMEOUT.as_secs()
+        )))
+    }
+
+    fn choose_display_num(&self) -> Result<i32, BrowserProblem> {
+        let start = self.config.display_num;
+        if start <= 0 {
+            return Err(BrowserProblem::start_failed("displayNum must be > 0"));
+        }
+        for offset in 0..MAX_DISPLAY_PROBE {
+            let candidate = start + offset;
+            if !socket_path(candidate).exists() {
+                return Ok(candidate);
+            }
+        }
+        Err(BrowserProblem::start_failed(format!(
+            "unable to find an available X display starting at :{start}"
+        )))
+    }
+
+    fn base_environment(&self, display: &str) -> Result<HashMap<String, String>, BrowserProblem> {
+        let mut environment = HashMap::new();
+        environment.insert("DISPLAY".to_string(), display.to_string());
+        environment.insert(
+            "HOME".to_string(),
+            self.config
+                .state_dir
+                .join("home")
+                .to_string_lossy()
+                .to_string(),
+        );
+        environment.insert(
+            "USER".to_string(),
+            std::env::var("USER").unwrap_or_else(|_| "sandbox-agent".to_string()),
+        );
+        environment.insert(
+            "PATH".to_string(),
+            std::env::var("PATH").unwrap_or_default(),
+        );
+        fs::create_dir_all(self.config.state_dir.join("home")).map_err(|err| {
+            BrowserProblem::start_failed(format!("failed to create browser home: {err}"))
+        })?;
+        Ok(environment)
+    }
+
+    fn detect_missing_dependencies(&self) -> Vec<String> {
+        detect_missing_browser_dependencies()
+    }
+
+    fn install_command_for(&self, missing_dependencies: &[String]) -> Option<String> {
+        if !self.platform_supported() || missing_dependencies.is_empty() {
+            None
+        } else {
+            Some("sandbox-agent install browser --yes".to_string())
+        }
+    }
+
+    fn platform_supported(&self) -> bool {
+        cfg!(target_os = "linux") || self.config.assume_linux_for_tests
+    }
+
+    fn ensure_state_dir(&self) -> Result<(), String> {
+        fs::create_dir_all(&self.config.state_dir).map_err(|err| {
+            format!(
+                "failed to create browser state dir {}: {err}",
+                self.config.state_dir.display()
+            )
+        })
+    }
+
+    fn write_runtime_log_locked(&self, state: &BrowserRuntimeStateData, message: &str) {
+        if let Some(parent) = state.runtime_log_path.parent() {
+            let _ = fs::create_dir_all(parent);
+        }
+        let line = format!("{} {}\n", chrono::Utc::now().to_rfc3339(), message);
+        let _ = OpenOptions::new()
+            .create(true)
+            .append(true)
+            .open(&state.runtime_log_path)
+            .and_then(|mut file| std::io::Write::write_all(&mut file, line.as_bytes()));
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Free functions
+// ---------------------------------------------------------------------------
+
+fn default_state_dir() -> PathBuf {
+    if let Ok(value) = std::env::var("XDG_STATE_HOME") {
+        return PathBuf::from(value).join("sandbox-agent").join("browser");
+    }
+    if let Some(home) = dirs::home_dir() {
+        return home
+            .join(".local")
+            .join("state")
+            .join("sandbox-agent")
+            .join("browser");
+    }
+    PathBuf::from("/tmp/sandbox-agent/browser")
+}
+
+fn socket_path(display_num: i32) -> PathBuf {
+    PathBuf::from(format!("/tmp/.X11-unix/X{display_num}"))
+}
+
+fn find_chromium_binary() -> Option<PathBuf> {
+    find_binary("chromium")
+        .or_else(|| find_binary("chromium-browser"))
+        .or_else(|| find_binary("google-chrome"))
+        .or_else(|| find_binary("google-chrome-stable"))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn default_config_uses_display_98() {
+        let config = BrowserRuntimeConfig::default();
+        assert_eq!(config.display_num, DEFAULT_DISPLAY_NUM);
+    }
+
+    #[test]
+    fn find_chromium_binary_returns_some_on_path() {
+        // This test is environment-dependent; just ensure no panic
+        let _ = find_chromium_binary();
+    }
+
+    #[test]
+    fn socket_path_matches_expected_format() {
+        let path = socket_path(98);
+        assert_eq!(path, PathBuf::from("/tmp/.X11-unix/X98"));
+    }
+
+    #[test]
+    fn install_command_for_empty_deps_is_none() {
+        let rt = BrowserRuntime::new(
+            Arc::new(ProcessRuntime::new()),
+            Arc::new(DesktopRuntime::new(Arc::new(ProcessRuntime::new()))),
+        );
+        assert_eq!(rt.install_command_for(&[]), None);
+    }
+}
diff --git a/server/packages/sandbox-agent/src/lib.rs b/server/packages/sandbox-agent/src/lib.rs
index f7d4da9..d095c5c 100644
--- a/server/packages/sandbox-agent/src/lib.rs
+++ b/server/packages/sandbox-agent/src/lib.rs
@@ -4,6 +4,7 @@ mod acp_proxy_runtime;
 mod browser_cdp;
 mod browser_errors;
 mod browser_install;
+mod browser_runtime;
 pub mod browser_types;
 pub mod cli;
 pub mod daemon;
diff --git a/server/packages/sandbox-agent/src/router.rs b/server/packages/sandbox-agent/src/router.rs
index 195a5cd..7b19772 100644
--- a/server/packages/sandbox-agent/src/router.rs
+++ b/server/packages/sandbox-agent/src/router.rs
@@ -37,6 +37,7 @@ use tracing::Span;
 use utoipa::{IntoParams, Modify, OpenApi, ToSchema};
 
 use crate::acp_proxy_runtime::{AcpProxyRuntime, ProxyPostOutcome};
+use crate::browser_runtime::BrowserRuntime;
 use crate::desktop_errors::DesktopProblem;
 use crate::desktop_runtime::DesktopRuntime;
 use crate::desktop_types::*;
@@ -92,6 +93,7 @@ pub struct AppState {
     opencode_server_manager: Arc<OpenCodeServerManager>,
     process_runtime: Arc<ProcessRuntime>,
     desktop_runtime: Arc<DesktopRuntime>,
+    browser_runtime: Arc<BrowserRuntime>,
     pub(crate) branding: BrandingMode,
     version_cache: Mutex<HashMap<AgentId, CachedAgentVersion>>,
 }
@@ -117,6 +119,10 @@ impl AppState {
         ));
         let process_runtime = Arc::new(ProcessRuntime::new());
         let desktop_runtime = Arc::new(DesktopRuntime::new(process_runtime.clone()));
+        let browser_runtime = Arc::new(BrowserRuntime::new(
+            process_runtime.clone(),
+            desktop_runtime.clone(),
+        ));
         Self {
             auth,
             agent_manager,
@@ -124,6 +130,7 @@ impl AppState {
             opencode_server_manager,
             process_runtime,
             desktop_runtime,
+            browser_runtime,
             branding,
             version_cache: Mutex::new(HashMap::new()),
         }
@@ -149,6 +156,10 @@ impl AppState {
         self.desktop_runtime.clone()
     }
 
+    pub(crate) fn browser_runtime(&self) -> Arc<BrowserRuntime> {
+        self.browser_runtime.clone()
+    }
+
     pub(crate) fn purge_version_cache(&self, agent: AgentId) {
         self.version_cache.lock().unwrap().remove(&agent);
     }