diff --git a/crates/engine/src/media.rs b/crates/engine/src/media.rs
--- a/crates/engine/src/media.rs
+++ b/crates/engine/src/media.rs
@@ -1645,6 +1645,7 @@ impl crate::Session {
 
     /// Synchronous render (CLI, MCP, tests).
     pub fn render_now(&mut self, id: PhotoId, max_w: usize, max_h: usize) -> Result<Rendered, String> {
+        lightcraft_pipeline::check_output_size(max_w, max_h)?;
         let job = self.render_job(id, max_w, max_h, false, true).ok_or("no such photo")?;
         let r = job.run();
         self.accept(&r);
@@ -1675,6 +1676,7 @@ impl crate::Session {
         space: lightcraft_pipeline::OutputSpace,
         depth: lightcraft_pipeline::OutputDepth,
     ) -> Result<RenderJob, String> {
+        lightcraft_pipeline::check_output_size(max_w, max_h)?;
         let mut job = self.render_job(id, max_w, max_h, false, true).ok_or("no such photo")?;
         // An export renders the full image: never a thumbnail's source developed from a smaller copy of the raw
         // image that the file keeps (Lightroom's merged DNGs)
diff --git a/crates/engine/src/tests_export.rs b/crates/engine/src/tests_export.rs
--- a/crates/engine/src/tests_export.rs
+++ b/crates/engine/src/tests_export.rs
@@ -1203,3 +1203,16 @@ fn same_folder_as_original_refuses_photos_without_a_folder() {
     assert!(e.contains("folder is missing") && e.contains(&gone), "{e}");
     let _ = std::fs::remove_dir_all(&dir);
 }
+
+/// An absurd export size is refused, not a multiply overflow (it panicked before the fix).
+#[test]
+fn huge_export_size_is_an_error() {
+    use lightcraft_pipeline::{OutputDepth, OutputSpace};
+    let mut s = Session::with_demo();
+    let id = s.active().unwrap();
+    let huge = usize::MAX >> 8;
+    assert!(s.render_export(id, huge, huge, OutputSpace::Srgb, OutputDepth::U8).is_err());
+    assert!(s.render_export(id, 70_000, 10, OutputSpace::Srgb, OutputDepth::U8).is_err());
+    assert!(s.render_now(id, huge, huge).is_err());
+    assert!(s.render_now(id, 10, 70_000).is_err());
+}
diff --git a/crates/gpu/src/render.rs b/crates/gpu/src/render.rs
--- a/crates/gpu/src/render.rs
+++ b/crates/gpu/src/render.rs
@@ -599,7 +599,10 @@ pub fn render(
     };
     let plan = lightcraft_pipeline::plan(src, info, s, req);
     let (w, h) = (plan.w, plan.h);
-    let n = w * h;
+    let Some(n) = w.checked_mul(h).filter(|n| n.checked_mul(12).is_some()) else {
+        fail(FailKind::Limit, format!("{w}×{h} output is too large"));
+        return None;
+    };
     let _limit = match fault {
         Some(crate::Fault::Limit(bytes)) => Some(crate::ctx::LimitOverride::new(bytes)),
         _ => None,
diff --git a/crates/pipeline/src/lib.rs b/crates/pipeline/src/lib.rs
--- a/crates/pipeline/src/lib.rs
+++ b/crates/pipeline/src/lib.rs
@@ -173,6 +173,21 @@ impl PixelWindow {
     }
 }
 
+/// The longest output side a render may be asked for (TIFF/PNG/JPEG all fit within it).
+pub const MAX_OUTPUT_SIDE: usize = 65_535;
+/// The most output pixels a render may be asked for (the box it fits into, ≈ 1.07 Gpx).
+pub const MAX_OUTPUT_PIXELS: usize = 1 << 30;
+
+/// Rejects an output box past [`MAX_OUTPUT_SIDE`] or [`MAX_OUTPUT_PIXELS`] before anything is
+/// sized from it.
+pub fn check_output_size(max_w: usize, max_h: usize) -> Result<(), String> {
+    let pixels = max_w.checked_mul(max_h);
+    if max_w > MAX_OUTPUT_SIDE || max_h > MAX_OUTPUT_SIDE || pixels.is_none_or(|n| n > MAX_OUTPUT_PIXELS) {
+        return Err(format!("output size {max_w}×{max_h} is too large (at most {MAX_OUTPUT_SIDE} px per side and {MAX_OUTPUT_PIXELS} pixels)"));
+    }
+    Ok(())
+}
+
 impl RenderRequest {
     pub fn fit(max_w: usize, max_h: usize) -> Self {
         Self {