diff --git a/Cargo.lock b/Cargo.lock
index 62f084afc..ff1d18be2 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -6065,6 +6065,7 @@ dependencies = [
  "smithay-clipboard",
  "socket2",
  "vectorcraft-engine",
+ "vectorcraft-render",
  "vectorcraft-text",
  "vectorcraft-ui-egui",
  "wayland-backend",
diff --git a/apps/vectorcraft-cli/Cargo.toml b/apps/vectorcraft-cli/Cargo.toml
index cb9183315..8f39e3b58 100644
--- a/apps/vectorcraft-cli/Cargo.toml
+++ b/apps/vectorcraft-cli/Cargo.toml
@@ -34,8 +34,9 @@ winsafe = { version = "0.0.29", default-features = false, features = ["kernel",
 
 # DirectWrite's system font collection and GDI's font list (the desktop app's src/system_fonts.rs,
 # shared): fonts that font services such as Adobe Fonts load in place (#579, #1258). MIT OR
-# Apache-2.0.
-windows = { version = "0.62", features = ["Win32_Graphics_DirectWrite", "Win32_Graphics_Gdi"] }
+# Apache-2.0. Win32_System_Threading has `GetProcessAffinityMask`, which the desktop app's
+# src/windows_cores.rs, shared, uses to count the cores the process may run on.
+windows = { version = "0.62", features = ["Win32_Graphics_DirectWrite", "Win32_Graphics_Gdi", "Win32_System_Threading"] }
 
 # CoreText's font manager (the desktop app's src/mac_fonts.rs, shared): fonts that apps and font
 # managers register from their own folders (#579). Zlib OR Apache-2.0 OR MIT.
diff --git a/apps/vectorcraft-cli/src/main.rs b/apps/vectorcraft-cli/src/main.rs
index bba1976c1..b7334586c 100644
--- a/apps/vectorcraft-cli/src/main.rs
+++ b/apps/vectorcraft-cli/src/main.rs
@@ -49,6 +49,10 @@ mod prefs_dir;
 #[cfg(windows)]
 #[path = "../../vectorcraft/src/system_fonts.rs"]
 mod system_fonts;
+/// The desktop app's affinity-mask reader, shared: renders start workers only for usable cores.
+#[cfg(windows)]
+#[path = "../../vectorcraft/src/windows_cores.rs"]
+mod windows_cores;
 
 use serde_json::{Value, json};
 use vectorcraft_engine::file_access::{self, AutomationRoots};
@@ -161,6 +165,9 @@ fn help() -> Result<(), String> {
 }
 
 fn main() -> ExitCode {
+    // Before the first render: the cores the process may run on.
+    #[cfg(windows)]
+    windows_cores::install();
     // Before any font lookup: the fonts font services load (#579).
     #[cfg(windows)]
     system_fonts::install();
diff --git a/apps/vectorcraft/Cargo.toml b/apps/vectorcraft/Cargo.toml
index 0657c93c8..7af8c51ef 100644
--- a/apps/vectorcraft/Cargo.toml
+++ b/apps/vectorcraft/Cargo.toml
@@ -54,7 +54,11 @@ pollster = "1"
 # Win32_UI_WindowsAndMessaging has `MSG` and `WM_POINTERDOWN`, which src/windows_stylus.rs uses to
 # read a pen's barrel button from its pointer messages. Win32_UI_Input_Pointer has
 # `GetPointerPenInfo`, which it uses to read the pen's pressure and which end of the pen is down.
-windows = { version = "0.62", features = ["Win32_Graphics_DirectWrite", "Win32_Graphics_Gdi", "Win32_UI_Input_Pointer", "Win32_UI_WindowsAndMessaging"] }
+# Win32_System_Threading has `GetProcessAffinityMask`, which src/windows_cores.rs uses to count the
+# cores the process may run on.
+windows = { version = "0.62", features = ["Win32_Graphics_DirectWrite", "Win32_Graphics_Gdi", "Win32_System_Threading", "Win32_UI_Input_Pointer", "Win32_UI_WindowsAndMessaging"] }
+# The renderer's worker count follows those cores (src/windows_cores.rs).
+vectorcraft-render = { workspace = true }
 
 [target.'cfg(target_os = "macos")'.dependencies]
 # The macOS menu bar (src/mac_menu.rs): Tauri's menu crate, AppKit menus behind a safe API (no GTK
diff --git a/apps/vectorcraft/src/main.rs b/apps/vectorcraft/src/main.rs
index e90977726..1e2ab4fae 100644
--- a/apps/vectorcraft/src/main.rs
+++ b/apps/vectorcraft/src/main.rs
@@ -50,6 +50,8 @@ mod system_fonts;
 mod tablet;
 mod window;
 mod window_frame;
+#[cfg(windows)]
+mod windows_cores;
 mod windows_stylus;
 mod xwayland;
 
@@ -184,6 +186,9 @@ fn main() -> std::process::ExitCode {
     // Before desktop services, so every start-up warning is recorded (`logging`).
     let logger = logging::install();
     vectorcraft_ui_egui::i18n::detect_system_lang_in_background();
+    // Before the first render: the cores the process may run on.
+    #[cfg(windows)]
+    windows_cores::install();
     // Before the first font scan (the app's start): the fonts font services load (#579).
     #[cfg(windows)]
     system_fonts::install();
diff --git a/apps/vectorcraft/src/windows_cores.rs b/apps/vectorcraft/src/windows_cores.rs
new file mode 100644
index 000000000..b35c0876c
--- /dev/null
+++ b/apps/vectorcraft/src/windows_cores.rs
@@ -0,0 +1,40 @@
+//! Windows: the cores the process may run on, from its affinity mask, for the renderer's worker
+//! count (`vectorcraft_render::set_usable_cores`). `available_parallelism` counts every processor
+//! in the machine, also when the process may run on fewer (started with `start /affinity`, or
+//! pinned by a launcher). The desktop app and `vectorcraft-cli` (exports, MCP) both install it; the
+//! CLI shares this file.
+//!
+//! `GetProcessAffinityMask` is a C API with no safe binding, hence the scoped `unsafe_code`
+//! allowance.
+#![allow(unsafe_code)]
+
+use windows::Win32::System::Threading::{GetCurrentProcess, GetProcessAffinityMask};
+
+/// Tells the renderer how many cores the process may run on.
+pub fn install() {
+    vectorcraft_render::set_usable_cores(usable_cores());
+}
+
+/// The number of cores in the process's affinity mask, or `None` when Windows reports none (a
+/// process with threads in more than one processor group).
+fn usable_cores() -> Option<usize> {
+    let (mut process, mut system) = (0usize, 0usize);
+    // SAFETY: `GetCurrentProcess` returns a pseudo-handle that needs no closing, and both pointers
+    // point at live `usize`s for the length of the call.
+    unsafe { GetProcessAffinityMask(GetCurrentProcess(), &mut process, &mut system) }.ok()?;
+    (process != 0).then(|| process.count_ones() as usize)
+}
+
+#[cfg(test)]
+mod tests {
+    /// The mask Windows reports for this process gives at least one core and no more than the
+    /// machine has. (A child process started with `start /affinity` would test a one-core mask
+    /// end to end, but a process spawned while other tests hold sockets inherits them and resets
+    /// their connections; `automatic_threads` covers the one-core count.)
+    #[test]
+    fn the_affinity_mask_gives_the_cores_the_process_may_use() {
+        let cores = super::usable_cores().unwrap();
+        let all = std::thread::available_parallelism().unwrap().get();
+        assert!((1..=all).contains(&cores), "{cores} of {all}");
+    }
+}
diff --git a/crates/render/src/lib.rs b/crates/render/src/lib.rs
index d5c601dc1..8bf6b0d23 100644
--- a/crates/render/src/lib.rs
+++ b/crates/render/src/lib.rs
@@ -227,6 +227,6 @@ pub(crate) use text_geom::is_opentype_alternate;
 #[cfg(test)]
 use text_geom::text_geom;
 pub(crate) use threads::single_threaded_context;
-pub use threads::{default_threads, set_default_threads};
+pub use threads::{default_threads, set_default_threads, set_usable_cores};
 #[cfg(test)]
 mod tests;
diff --git a/crates/render/src/threads.rs b/crates/render/src/threads.rs
index eb1fb66ea..dfd6bf757 100644
--- a/crates/render/src/threads.rs
+++ b/crates/render/src/threads.rs
@@ -9,14 +9,25 @@ pub(crate) fn single_threaded_context(w: u16, h: u16) -> RenderContext {
 /// Preferred rasterizer thread count set by the app (Preferences → Performance); negative = automatic.
 static THREADS_OVERRIDE: std::sync::atomic::AtomicI32 = std::sync::atomic::AtomicI32::new(-1);
 
+/// The cores the process may run on, as the app read them from the system; 0 = not known.
+static USABLE_CORES: std::sync::atomic::AtomicUsize = std::sync::atomic::AtomicUsize::new(0);
+
 /// Override the worker-thread count used by renderers created from now on (`None` = automatic).
 pub fn set_default_threads(n: Option<u16>) {
     THREADS_OVERRIDE.store(n.map_or(-1, i32::from), std::sync::atomic::Ordering::Relaxed);
 }
 
-/// Rasterizer worker threads: 0 on wasm; otherwise one per core up to 8, leaving a core free. On a
-/// 10-core machine 8 workers render a 50,000-path page 1.4× faster than 4 (9 are slower again);
-/// the pixels don't depend on the count (`pixels_do_not_depend_on_the_thread_count`).
+/// The number of cores the process may run on, when the app knows a smaller number than
+/// `available_parallelism` gives (`None` = not known). On Windows `available_parallelism` counts
+/// every processor in the machine, also when the process's affinity mask allows fewer.
+pub fn set_usable_cores(n: Option<usize>) {
+    USABLE_CORES.store(n.unwrap_or(0), std::sync::atomic::Ordering::Relaxed);
+}
+
+/// Rasterizer worker threads: 0 on wasm; otherwise one per core the process may run on, up to 8,
+/// leaving a core free. On a 10-core machine 8 workers render a 50,000-path page 1.4× faster than 4
+/// (9 are slower again); the pixels don't depend on the count
+/// (`pixels_do_not_depend_on_the_thread_count`).
 pub fn default_threads() -> u16 {
     #[cfg(target_arch = "wasm32")]
     {
@@ -31,6 +42,31 @@ pub fn default_threads() -> u16 {
         if std::env::var_os("VECTORCRAFT_RENDER_THREADS").is_some() {
             return std::env::var("VECTORCRAFT_RENDER_THREADS").ok().and_then(|v| v.parse().ok()).unwrap_or(0);
         }
-        std::thread::available_parallelism().map(|n| (n.get().saturating_sub(1)).min(8) as u16).unwrap_or(0)
+        let usable = USABLE_CORES.load(std::sync::atomic::Ordering::Relaxed);
+        std::thread::available_parallelism().map(|n| automatic_threads(n.get(), usable)).unwrap_or(0)
+    }
+}
+
+/// [`default_threads`] when automatic, on `cpus` processors of which the process may use `usable`
+/// (0 = all).
+fn automatic_threads(cpus: usize, usable: usize) -> u16 {
+    let cores = if usable == 0 { cpus } else { cpus.min(usable) };
+    (cores.saturating_sub(1)).min(8) as u16
+}
+
+#[cfg(test)]
+mod tests {
+    use super::automatic_threads;
+
+    /// A process that may run on one core renders on the calling thread. With workers, vello_cpu's
+    /// flush spins on that core while the workers it waits for get no time, and a render crawls or
+    /// stalls.
+    #[test]
+    fn workers_follow_the_cores_the_process_may_use() {
+        assert_eq!(automatic_threads(24, 0), 8);
+        assert_eq!(automatic_threads(24, 1), 0);
+        assert_eq!(automatic_threads(24, 4), 3);
+        assert_eq!(automatic_threads(4, 16), 3);
+        assert_eq!(automatic_threads(1, 0), 0);
     }
 }