diff --git a/Cargo.lock b/Cargo.lock
index b4498b453..67b5269f7 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1218,6 +1218,15 @@ dependencies = [
  "x509-parser",
 ]
 
+[[package]]
+name = "fabro-beastie"
+version = "0.4.0"
+dependencies = [
+ "core-foundation 0.9.4",
+ "libc",
+ "tracing",
+]
+
 [[package]]
 name = "fabro-cli"
 version = "0.4.0"
@@ -1235,6 +1244,7 @@ dependencies = [
  "dotenvy",
  "fabro-agent",
  "fabro-api",
+ "fabro-beastie",
  "fabro-config",
  "fabro-github",
  "fabro-llm",
@@ -1550,6 +1560,7 @@ dependencies = [
  "dirs",
  "dotenvy",
  "fabro-agent",
+ "fabro-beastie",
  "fabro-devcontainer",
  "fabro-exe",
  "fabro-git-storage",
diff --git a/lib/crates/fabro-beastie/Cargo.toml b/lib/crates/fabro-beastie/Cargo.toml
new file mode 100644
index 000000000..0b9c94237
--- /dev/null
+++ b/lib/crates/fabro-beastie/Cargo.toml
@@ -0,0 +1,18 @@
+[package]
+name = "fabro-beastie"
+edition.workspace = true
+version.workspace = true
+license.workspace = true
+description = "Cross-platform idle sleep prevention (No Sleep Till Brooklyn)"
+
+[lib]
+doctest = false
+
+[dependencies]
+tracing.workspace = true
+
+[target.'cfg(target_os = "macos")'.dependencies]
+core-foundation = "0.9"
+
+[target.'cfg(target_os = "linux")'.dependencies]
+libc = "0.2"
diff --git a/lib/crates/fabro-beastie/src/dummy.rs b/lib/crates/fabro-beastie/src/dummy.rs
new file mode 100644
index 000000000..0995b0d8d
--- /dev/null
+++ b/lib/crates/fabro-beastie/src/dummy.rs
@@ -0,0 +1,17 @@
+/// Dummy sleep inhibitor backend (no-op).
+///
+/// Used on platforms without a native sleep inhibitor implementation.
+pub(crate) struct DummyGuard;
+
+impl DummyGuard {
+    pub(crate) fn acquire() -> Option<Self> {
+        tracing::debug!("Sleep inhibitor: dummy backend (no-op)");
+        Some(Self)
+    }
+}
+
+impl Drop for DummyGuard {
+    fn drop(&mut self) {
+        tracing::debug!("Sleep inhibitor: dummy guard released");
+    }
+}
diff --git a/lib/crates/fabro-beastie/src/iokit_bindings.rs b/lib/crates/fabro-beastie/src/iokit_bindings.rs
new file mode 100644
index 000000000..6c1a6263a
--- /dev/null
+++ b/lib/crates/fabro-beastie/src/iokit_bindings.rs
@@ -0,0 +1,19 @@
+//! Raw FFI bindings for IOKit power management on macOS.
+
+use core_foundation::string::CFStringRef;
+
+pub type IOPMAssertionID = u32;
+pub type IOReturn = i32;
+
+pub const kIOPMAssertionLevelOn: u32 = 255;
+
+extern "C" {
+    pub fn IOPMAssertionCreateWithName(
+        assertion_type: CFStringRef,
+        assertion_level: u32,
+        reason_for_activity: CFStringRef,
+        assertion_id: *mut IOPMAssertionID,
+    ) -> IOReturn;
+
+    pub fn IOPMAssertionRelease(assertion_id: IOPMAssertionID) -> IOReturn;
+}
diff --git a/lib/crates/fabro-beastie/src/lib.rs b/lib/crates/fabro-beastie/src/lib.rs
new file mode 100644
index 000000000..ba669fbca
--- /dev/null
+++ b/lib/crates/fabro-beastie/src/lib.rs
@@ -0,0 +1,79 @@
+//! Cross-platform idle sleep prevention.
+//!
+//! Named after Beastie Boys — No Sleep Till Brooklyn.
+//!
+//! Call [`guard(true)`] to acquire an inhibitor that prevents the OS from
+//! entering idle sleep. The guard releases automatically on [`Drop`].
+
+#[cfg_attr(any(target_os = "macos", target_os = "linux"), allow(dead_code))]
+mod dummy;
+
+#[cfg(target_os = "macos")]
+mod iokit_bindings;
+#[cfg(target_os = "macos")]
+mod macos;
+
+#[cfg(target_os = "linux")]
+mod linux;
+
+/// RAII guard that prevents idle system sleep while held.
+pub struct SleepInhibitorGuard {
+    #[cfg(target_os = "macos")]
+    _inner: macos::MacOsGuard,
+    #[cfg(target_os = "linux")]
+    _inner: linux::LinuxGuard,
+    #[cfg(not(any(target_os = "macos", target_os = "linux")))]
+    _inner: dummy::DummyGuard,
+}
+
+/// Acquire a sleep inhibitor guard.
+///
+/// Returns `Some(guard)` if `enabled` is `true` and the platform backend
+/// succeeds. Returns `None` if `enabled` is `false` or the backend fails.
+/// The guard prevents idle system sleep until it is dropped.
+pub fn guard(enabled: bool) -> Option<SleepInhibitorGuard> {
+    if !enabled {
+        return None;
+    }
+    tracing::info!("Acquiring sleep inhibitor");
+
+    #[cfg(target_os = "macos")]
+    {
+        macos::MacOsGuard::acquire().map(|g| SleepInhibitorGuard { _inner: g })
+    }
+    #[cfg(target_os = "linux")]
+    {
+        linux::LinuxGuard::acquire().map(|g| SleepInhibitorGuard { _inner: g })
+    }
+    #[cfg(not(any(target_os = "macos", target_os = "linux")))]
+    {
+        dummy::DummyGuard::acquire().map(|g| SleepInhibitorGuard { _inner: g })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn guard_disabled_returns_none() {
+        assert!(guard(false).is_none());
+    }
+
+    #[test]
+    fn guard_enabled_returns_some() {
+        // On CI/Linux without systemd-inhibit this may return None,
+        // so we only assert it doesn't panic. On macOS it should return Some.
+        let g = guard(true);
+        // Dummy backend always succeeds; real backends may fail in CI.
+        #[cfg(not(any(target_os = "macos", target_os = "linux")))]
+        assert!(g.is_some());
+        drop(g);
+    }
+
+    #[test]
+    fn guard_drop_does_not_panic() {
+        let g = guard(true);
+        drop(g);
+    }
+}
diff --git a/lib/crates/fabro-beastie/src/linux.rs b/lib/crates/fabro-beastie/src/linux.rs
new file mode 100644
index 000000000..00381b0d8
--- /dev/null
+++ b/lib/crates/fabro-beastie/src/linux.rs
@@ -0,0 +1,105 @@
+use std::os::unix::process::CommandExt;
+use std::process::{Child, Command};
+
+/// Linux sleep inhibitor using `systemd-inhibit` (preferred) or
+/// `gnome-session-inhibit` (fallback).
+///
+/// Spawns an inhibitor child process that blocks idle sleep as long as it is
+/// alive. On `Drop`, the child is killed to release the inhibition.
+pub(crate) struct LinuxGuard {
+    child: Child,
+}
+
+impl LinuxGuard {
+    pub(crate) fn acquire() -> Option<Self> {
+        // Try systemd-inhibit first
+        if let Some(guard) = Self::try_systemd_inhibit() {
+            return Some(guard);
+        }
+        // Fallback to gnome-session-inhibit
+        if let Some(guard) = Self::try_gnome_inhibit() {
+            return Some(guard);
+        }
+        tracing::warn!(
+            "Sleep inhibitor: no supported inhibitor found \
+             (tried systemd-inhibit, gnome-session-inhibit)"
+        );
+        None
+    }
+
+    fn try_systemd_inhibit() -> Option<Self> {
+        let child = unsafe {
+            Command::new("systemd-inhibit")
+                .args([
+                    "--what=idle",
+                    "--who=fabro",
+                    "--why=Workflow in progress",
+                    "--mode=block",
+                    "sleep",
+                    "infinity",
+                ])
+                .stdin(std::process::Stdio::null())
+                .stdout(std::process::Stdio::null())
+                .stderr(std::process::Stdio::null())
+                .pre_exec(|| {
+                    // Ensure the child is killed if the parent dies unexpectedly.
+                    libc::prctl(libc::PR_SET_PDEATHSIG, libc::SIGTERM);
+                    Ok(())
+                })
+                .spawn()
+        };
+        match child {
+            Ok(child) => {
+                let pid = child.id();
+                tracing::debug!(pid, "Sleep inhibitor: systemd-inhibit started");
+                Some(Self { child })
+            }
+            Err(e) => {
+                tracing::debug!(%e, "systemd-inhibit not available");
+                None
+            }
+        }
+    }
+
+    fn try_gnome_inhibit() -> Option<Self> {
+        let child = unsafe {
+            Command::new("gnome-session-inhibit")
+                .args([
+                    "--inhibit",
+                    "idle",
+                    "--reason",
+                    "Workflow in progress",
+                    "sleep",
+                    "infinity",
+                ])
+                .stdin(std::process::Stdio::null())
+                .stdout(std::process::Stdio::null())
+                .stderr(std::process::Stdio::null())
+                .pre_exec(|| {
+                    libc::prctl(libc::PR_SET_PDEATHSIG, libc::SIGTERM);
+                    Ok(())
+                })
+                .spawn()
+        };
+        match child {
+            Ok(child) => {
+                let pid = child.id();
+                tracing::debug!(pid, "Sleep inhibitor: gnome-session-inhibit started");
+                Some(Self { child })
+            }
+            Err(e) => {
+                tracing::debug!(%e, "gnome-session-inhibit not available");
+                None
+            }
+        }
+    }
+}
+
+impl Drop for LinuxGuard {
+    fn drop(&mut self) {
+        let pid = self.child.id();
+        let _ = self.child.kill();
+        let _ = self.child.wait();
+        tracing::debug!(pid, "Sleep inhibitor: linux inhibitor child killed");
+    }
+}
diff --git a/lib/crates/fabro-beastie/src/macos.rs b/lib/crates/fabro-beastie/src/macos.rs
new file mode 100644
index 000000000..102159c85
--- /dev/null
+++ b/lib/crates/fabro-beastie/src/macos.rs
@@ -0,0 +1,57 @@
+use super::iokit_bindings::*;
+use core_foundation::base::TCFType;
+use core_foundation::string::CFString;
+
+/// macOS sleep inhibitor using IOKit power assertions.
+pub(crate) struct MacOsGuard {
+    assertion_id: IOPMAssertionID,
+}
+
+impl MacOsGuard {
+    pub(crate) fn acquire() -> Option<Self> {
+        let assertion_type = CFString::from_static_string("PreventUserIdleSystemSleep");
+        let reason = CFString::new("fabro workflow in progress");
+
+        let mut assertion_id: IOPMAssertionID = 0;
+        let result = unsafe {
+            IOPMAssertionCreateWithName(
+                assertion_type.as_concrete_TypeRef(),
+                kIOPMAssertionLevelOn,
+                reason.as_concrete_TypeRef(),
+                &mut assertion_id,
+            )
+        };
+
+        if result != 0 {
+            tracing::warn!(
+                io_return = result,
+                "Failed to create IOKit power assertion for sleep prevention"
+            );
+            return None;
+        }
+
+        tracing::debug!(
+            assertion_id,
+            "Sleep inhibitor: macOS IOKit assertion acquired"
+        );
+        Some(Self { assertion_id })
+    }
+}
+
+impl Drop for MacOsGuard {
+    fn drop(&mut self) {
+        let result = unsafe { IOPMAssertionRelease(self.assertion_id) };
+        if result != 0 {
+            tracing::warn!(
+                io_return = result,
+                assertion_id = self.assertion_id,
+                "Failed to release IOKit power assertion"
+            );
+        } else {
+            tracing::debug!(
+                assertion_id = self.assertion_id,
+                "Sleep inhibitor: macOS IOKit assertion released"
+            );
+        }
+    }
+}
diff --git a/lib/crates/fabro-cli/Cargo.toml b/lib/crates/fabro-cli/Cargo.toml
index 745258e21..2d7a6df73 100644
--- a/lib/crates/fabro-cli/Cargo.toml
+++ b/lib/crates/fabro-cli/Cargo.toml
@@ -13,6 +13,7 @@ path = "src/main.rs"
 default = []
 server = ["dep:fabro-api"]
 exedev = ["fabro-config/exedev", "fabro-workflows/exedev"]
+sleep_inhibitor = ["dep:fabro-beastie", "fabro-workflows/sleep_inhibitor"]
 
 [dependencies]
 fabro-config = { path = "../fabro-config" }
@@ -23,6 +24,7 @@ fabro-agent = { path = "../fabro-agent" }
 fabro-mcp = { path = "../fabro-mcp" }
 fabro-workflows = { path = "../fabro-workflows" }
 fabro-api = { path = "../fabro-api", optional = true }
+fabro-beastie = { path = "../fabro-beastie", optional = true }
 fabro-util = { path = "../fabro-util" }
 clap.workspace = true
 console.workspace = true
@@ -67,4 +69,4 @@ predicates = "3"
 tempfile = "3"
 serde_json.workspace = true
 httpmock = "0.8"
-trycmd = "0.15"
+trycmd = "0.15"
\ No newline at end of file
diff --git a/lib/crates/fabro-cli/src/main.rs b/lib/crates/fabro-cli/src/main.rs
index 2411ead71..a4dcbd601 100644
--- a/lib/crates/fabro-cli/src/main.rs
+++ b/lib/crates/fabro-cli/src/main.rs
@@ -522,6 +522,8 @@ async fn main_inner() -> (String, Result<()>) {
             }
             Command::Exec(mut args) => {
                 let cli_config = cli_config::load_cli_config(None)?;
+                #[cfg(feature = "sleep_inhibitor")]
+                let _sleep_guard = fabro_beastie::guard(cli_config.prevent_idle_sleep);
                 let exec_defaults = cli_config.exec.as_ref();
                 args.apply_cli_defaults(
                     exec_defaults.and_then(|a| a.provider.as_deref()),
@@ -591,6 +593,7 @@ async fn main_inner() -> (String, Result<()>) {
                     Box::leak(Box::new(fabro_util::terminal::Styles::detect_stderr()));
                 let cli_config = cli_config::load_cli_config(None)?;
                 args.verbose = args.verbose || cli_config.verbose;
+                let prevent_idle_sleep = cli_config.prevent_idle_sleep;
                 let github_app = build_github_app_credentials(cli_config.app_id());
 
                 let git_author = fabro_workflows::git::GitAuthor::from_options(
@@ -604,6 +607,7 @@ async fn main_inner() -> (String, Result<()>) {
                     styles,
                     github_app,
                     git_author,
+                    prevent_idle_sleep,
                 )
                 .await?;
             }
diff --git a/lib/crates/fabro-config/src/cli.rs b/lib/crates/fabro-config/src/cli.rs
index 6b5d909c9..ecfd1e954 100644
--- a/lib/crates/fabro-config/src/cli.rs
+++ b/lib/crates/fabro-config/src/cli.rs
@@ -50,6 +50,8 @@ pub struct CliConfig {
     pub exec: Option<ExecDefaults>,
     pub git: Option<CliGitConfig>,
     #[serde(default)]
+    pub prevent_idle_sleep: bool,
+    #[serde(default)]
     pub verbose: bool,
     #[serde(default)]
     pub log: crate::server::LogConfig,
@@ -246,6 +248,18 @@ email = "me@local"
         assert_eq!(config.git, None);
     }
 
+    #[test]
+    fn parse_prevent_idle_sleep_true() {
+        let config: CliConfig = toml::from_str("prevent_idle_sleep = true").unwrap();
+        assert!(config.prevent_idle_sleep);
+    }
+
+    #[test]
+    fn parse_prevent_idle_sleep_defaults_to_false() {
+        let config: CliConfig = toml::from_str("").unwrap();
+        assert!(!config.prevent_idle_sleep);
+    }
+
     #[test]
     fn parse_verbose_true() {
         let config: CliConfig = toml::from_str("verbose = true").unwrap();
diff --git a/lib/crates/fabro-workflows/Cargo.toml b/lib/crates/fabro-workflows/Cargo.toml
index 93f4aea64..8736eee3d 100644
--- a/lib/crates/fabro-workflows/Cargo.toml
+++ b/lib/crates/fabro-workflows/Cargo.toml
@@ -15,6 +15,7 @@ doctest = false
 [features]
 default = []
 exedev = ["dep:fabro-exe"]
+sleep_inhibitor = ["dep:fabro-beastie"]
 
 [dependencies]
 clap.workspace = true
@@ -22,6 +23,7 @@ anyhow.workspace = true
 dotenvy.workspace = true
 fabro-agent = { path = "../fabro-agent" }
 fabro-devcontainer = { path = "../fabro-devcontainer" }
+fabro-beastie = { path = "../fabro-beastie", optional = true }
 fabro-exe = { path = "../fabro-exe", optional = true }
 fabro-ssh = { path = "../fabro-ssh" }
 fabro-mcp = { path = "../fabro-mcp" }
@@ -66,4 +68,4 @@ tokio = { workspace = true, features = ["test-util", "macros"] }
 tempfile = "3"
 dotenvy.workspace = true
 assert_cmd = "2"
-predicates = "3"
+predicates = "3"
\ No newline at end of file
diff --git a/lib/crates/fabro-workflows/src/cli/run.rs b/lib/crates/fabro-workflows/src/cli/run.rs
index 86ad2fabe..9426577b0 100644
--- a/lib/crates/fabro-workflows/src/cli/run.rs
+++ b/lib/crates/fabro-workflows/src/cli/run.rs
@@ -294,7 +294,13 @@ pub async fn run_command(
     styles: &'static Styles,
     github_app: Option<fabro_github::GitHubAppCredentials>,
     git_author: crate::git::GitAuthor,
+    prevent_idle_sleep: bool,
 ) -> anyhow::Result<()> {
+    #[cfg(feature = "sleep_inhibitor")]
+    let _sleep_guard = fabro_beastie::guard(prevent_idle_sleep);
+    #[cfg(not(feature = "sleep_inhibitor"))]
+    let _ = prevent_idle_sleep;
+
     // Handle --run-branch resume: read everything from git metadata
     if let Some(branch) = args.run_branch.clone() {
         return run_from_branch(args, &branch, styles, git_author, run_defaults, github_app).await;