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/Cargo.toml b/Cargo.toml
index b52b6ead0..8116cb38a 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -69,4 +69,4 @@ opt-level = 2
 [profile.dev.package.regex-automata]
 opt-level = 2
 [profile.dev.package.regex-syntax]
-opt-level = 2
+opt-level = 2
\ No newline at end of file
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..88b60cb7e
--- /dev/null
+++ b/lib/crates/fabro-beastie/src/dummy.rs
@@ -0,0 +1,16 @@
+use tracing::debug;
+
+pub(crate) struct DummySleepInhibitor;
+
+impl DummySleepInhibitor {
+    pub(crate) fn acquire() -> Option<Self> {
+        debug!("Sleep inhibitor: using dummy backend (no-op)");
+        Some(DummySleepInhibitor)
+    }
+}
+
+impl Drop for DummySleepInhibitor {
+    fn drop(&mut self) {
+        debug!("Sleep inhibitor: dummy backend 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..df20f916e
--- /dev/null
+++ b/lib/crates/fabro-beastie/src/iokit_bindings.rs
@@ -0,0 +1,36 @@
+#![allow(non_upper_case_globals, dead_code)]
+
+use core_foundation::base::TCFType;
+use core_foundation::string::CFString;
+
+// IOKit power management assertion types
+pub type IOPMAssertionID = u32;
+pub const kIOPMAssertionIDInvalid: IOPMAssertionID = 0;
+
+// IOReturn type
+pub type IOReturn = i32;
+pub const kIOReturnSuccess: IOReturn = 0;
+
+extern "C" {
+    pub fn IOPMAssertionCreateWithName(
+        assertion_type: core_foundation::string::CFStringRef,
+        assertion_level: u32,
+        reason_for_activity: core_foundation::string::CFStringRef,
+        assertion_id: *mut IOPMAssertionID,
+    ) -> IOReturn;
+
+    pub fn IOPMAssertionRelease(assertion_id: IOPMAssertionID) -> IOReturn;
+}
+
+// Assertion level
+pub const kIOPMAssertionLevelOn: u32 = 255;
+
+/// Create the CFString for "PreventUserIdleSystemSleep".
+pub fn prevent_idle_sleep_type() -> CFString {
+    CFString::new("PreventUserIdleSystemSleep")
+}
+
+/// Create a CFString reason.
+pub fn assertion_reason() -> CFString {
+    CFString::new("Fabro workflow running")
+}
diff --git a/lib/crates/fabro-beastie/src/lib.rs b/lib/crates/fabro-beastie/src/lib.rs
new file mode 100644
index 000000000..70bc6a758
--- /dev/null
+++ b/lib/crates/fabro-beastie/src/lib.rs
@@ -0,0 +1,90 @@
+//! Cross-platform idle sleep prevention (No Sleep Till Brooklyn).
+//!
+//! Call [`guard(true)`] to acquire a sleep inhibitor that prevents the system
+//! from idle-sleeping while Fabro is working. The guard is released on drop.
+
+#[cfg(target_os = "macos")]
+mod iokit_bindings;
+#[cfg(target_os = "macos")]
+mod macos;
+
+#[cfg(target_os = "linux")]
+mod linux;
+
+mod dummy;
+
+use tracing::debug;
+
+/// RAII guard that prevents idle system sleep while held.
+pub struct SleepInhibitorGuard {
+    _inner: InnerGuard,
+}
+
+// Fields are held for their Drop implementations, not read directly.
+#[allow(dead_code)]
+enum InnerGuard {
+    #[cfg(target_os = "macos")]
+    MacOS(macos::MacOSSleepInhibitor),
+    #[cfg(target_os = "linux")]
+    Linux(linux::LinuxSleepInhibitor),
+    Dummy(dummy::DummySleepInhibitor),
+}
+
+/// Acquire a sleep inhibitor guard.
+///
+/// If `enabled` is `false`, returns `None` immediately.
+/// If `enabled` is `true`, attempts to acquire a platform-specific sleep
+/// inhibitor. Falls back to a dummy (no-op) backend if the platform backend
+/// is unavailable.
+pub fn guard(enabled: bool) -> Option<SleepInhibitorGuard> {
+    if !enabled {
+        debug!("Sleep inhibitor: disabled by configuration");
+        return None;
+    }
+
+    #[cfg(target_os = "macos")]
+    {
+        if let Some(inner) = macos::MacOSSleepInhibitor::acquire() {
+            return Some(SleepInhibitorGuard {
+                _inner: InnerGuard::MacOS(inner),
+            });
+        }
+    }
+
+    #[cfg(target_os = "linux")]
+    {
+        if let Some(inner) = linux::LinuxSleepInhibitor::acquire() {
+            return Some(SleepInhibitorGuard {
+                _inner: InnerGuard::Linux(inner),
+            });
+        }
+    }
+
+    // Fallback to dummy
+    dummy::DummySleepInhibitor::acquire().map(|inner| SleepInhibitorGuard {
+        _inner: InnerGuard::Dummy(inner),
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn guard_enabled_returns_some() {
+        let g = guard(true);
+        assert!(g.is_some(), "guard(true) should return Some");
+    }
+
+    #[test]
+    fn guard_disabled_returns_none() {
+        let g = guard(false);
+        assert!(g.is_none(), "guard(false) should return None");
+    }
+
+    #[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..fe56f20d7
--- /dev/null
+++ b/lib/crates/fabro-beastie/src/linux.rs
@@ -0,0 +1,124 @@
+use std::process::{Child, Command};
+use tracing::{debug, warn};
+
+pub(crate) struct LinuxSleepInhibitor {
+    child: Child,
+}
+
+impl LinuxSleepInhibitor {
+    pub(crate) fn acquire() -> Option<Self> {
+        // Try systemd-inhibit first, then gnome-session-inhibit as fallback
+        if let Some(inhibitor) = Self::try_systemd_inhibit() {
+            return Some(inhibitor);
+        }
+        if let Some(inhibitor) = Self::try_gnome_inhibit() {
+            return Some(inhibitor);
+        }
+        warn!("Sleep inhibitor: no supported inhibitor found on this system");
+        None
+    }
+
+    fn try_systemd_inhibit() -> Option<Self> {
+        let result = Command::new("systemd-inhibit")
+            .args([
+                "--what=idle",
+                "--mode=block",
+                "--who=fabro",
+                "--reason=Fabro workflow running",
+                "sleep",
+                "infinity",
+            ])
+            .stdin(std::process::Stdio::null())
+            .stdout(std::process::Stdio::null())
+            .stderr(std::process::Stdio::null())
+            .spawn();
+        match result {
+            Ok(mut child) => {
+                // Set PR_SET_PDEATHSIG so the child is killed if the parent dies
+                #[cfg(target_os = "linux")]
+                {
+                    use std::os::unix::process::CommandExt;
+                    // The child is already spawned, but we can set pdeathsig via /proc
+                    // Actually, PR_SET_PDEATHSIG must be set from within the child process.
+                    // For a pre-spawned child, we rely on explicit Drop cleanup.
+                    // The safer approach is to use pre_exec, so let's re-spawn.
+                    let _ = child.kill();
+                    let _ = child.wait();
+
+                    let result = unsafe {
+                        Command::new("systemd-inhibit")
+                            .args([
+                                "--what=idle",
+                                "--mode=block",
+                                "--who=fabro",
+                                "--reason=Fabro workflow running",
+                                "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 result {
+                        Ok(child) => {
+                            debug!("Sleep inhibitor: acquired via systemd-inhibit");
+                            Some(Self { child })
+                        }
+                        Err(e) => {
+                            warn!("Sleep inhibitor: failed to respawn systemd-inhibit: {e}");
+                            None
+                        }
+                    }
+                }
+
+                #[cfg(not(target_os = "linux"))]
+                {
+                    debug!("Sleep inhibitor: acquired via systemd-inhibit");
+                    Some(Self { child })
+                }
+            }
+            Err(e) => {
+                debug!("Sleep inhibitor: systemd-inhibit not available: {e}");
+                None
+            }
+        }
+    }
+
+    fn try_gnome_inhibit() -> Option<Self> {
+        let result = Command::new("gnome-session-inhibit")
+            .args([
+                "--inhibit=idle",
+                "--reason",
+                "Fabro workflow running",
+                "sleep",
+                "infinity",
+            ])
+            .stdin(std::process::Stdio::null())
+            .stdout(std::process::Stdio::null())
+            .stderr(std::process::Stdio::null())
+            .spawn();
+        match result {
+            Ok(child) => {
+                debug!("Sleep inhibitor: acquired via gnome-session-inhibit");
+                Some(Self { child })
+            }
+            Err(e) => {
+                debug!("Sleep inhibitor: gnome-session-inhibit not available: {e}");
+                None
+            }
+        }
+    }
+}
+
+impl Drop for LinuxSleepInhibitor {
+    fn drop(&mut self) {
+        debug!("Sleep inhibitor: releasing (killing inhibitor child process)");
+        let _ = self.child.kill();
+        let _ = self.child.wait();
+    }
+}
diff --git a/lib/crates/fabro-beastie/src/macos.rs b/lib/crates/fabro-beastie/src/macos.rs
new file mode 100644
index 000000000..38a0fcf26
--- /dev/null
+++ b/lib/crates/fabro-beastie/src/macos.rs
@@ -0,0 +1,55 @@
+use core_foundation::base::TCFType;
+use tracing::{debug, warn};
+
+use crate::iokit_bindings::*;
+
+pub(crate) struct MacOSSleepInhibitor {
+    assertion_id: IOPMAssertionID,
+}
+
+impl MacOSSleepInhibitor {
+    pub(crate) fn acquire() -> Option<Self> {
+        let assertion_type = prevent_idle_sleep_type();
+        let reason = assertion_reason();
+        let mut assertion_id: IOPMAssertionID = kIOPMAssertionIDInvalid;
+
+        let result = unsafe {
+            IOPMAssertionCreateWithName(
+                assertion_type.as_concrete_TypeRef(),
+                kIOPMAssertionLevelOn,
+                reason.as_concrete_TypeRef(),
+                &mut assertion_id,
+            )
+        };
+
+        if result == kIOReturnSuccess {
+            debug!(
+                assertion_id,
+                "Sleep inhibitor: acquired IOKit power assertion"
+            );
+            Some(Self { assertion_id })
+        } else {
+            warn!(
+                result,
+                "Sleep inhibitor: failed to create IOKit power assertion"
+            );
+            None
+        }
+    }
+}
+
+impl Drop for MacOSSleepInhibitor {
+    fn drop(&mut self) {
+        debug!(
+            assertion_id = self.assertion_id,
+            "Sleep inhibitor: releasing IOKit power assertion"
+        );
+        let result = unsafe { IOPMAssertionRelease(self.assertion_id) };
+        if result != kIOReturnSuccess {
+            warn!(
+                result,
+                "Sleep inhibitor: failed to release IOKit power assertion"
+            );
+        }
+    }
+}
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..04bd71a45 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()),
@@ -604,6 +606,7 @@ async fn main_inner() -> (String, Result<()>) {
                     styles,
                     github_app,
                     git_author,
+                    cli_config.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..2c8a8e0a9 100644
--- a/lib/crates/fabro-workflows/src/cli/run.rs
+++ b/lib/crates/fabro-workflows/src/cli/run.rs
@@ -294,7 +294,12 @@ 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;