From 7304ba1a02c27403edaa7705f63c7d1f51a5fb7e Mon Sep 17 00:00:00 2001
From: Paddy <89168564+patrickballard@users.noreply.github.com>
Date: Mon, 26 Oct 2026 19:02:03 +0000
Subject: [PATCH] fix(export): return the .mp4 path an MP4 export writes

MP4File always writes to an .mp4 path, but the MP4 exporter returned the
path it was given, so `cap export -o out.gif` reported out.gif while
writing out.mp4. Give the output path the .mp4 extension up front, as
the GIF exporter does for .gif.
---
 crates/export/src/mp4.rs               |  5 +-
 crates/export/tests/mp4_output_path.rs | 83 ++++++++++++++++++++++++++
 2 files changed, 87 insertions(+), 1 deletion(-)
 create mode 100644 crates/export/tests/mp4_output_path.rs

diff --git a/crates/export/src/mp4.rs b/crates/export/src/mp4.rs
index be25ee2e62f..14c8f25d283 100644
--- a/crates/export/src/mp4.rs
+++ b/crates/export/src/mp4.rs
@@ -245,9 +245,12 @@ impl Mp4ExportSettings {
 
     async fn export_inner(
         self,
-        base: ExporterBase,
+        mut base: ExporterBase,
         on_progress: impl FnMut(u32) -> bool + Send + 'static,
     ) -> Result<PathBuf, String> {
+        // `MP4File` writes to an `.mp4` path whatever extension it is given, so
+        // the path returned to the caller has to carry it too.
+        base.output_path.set_extension("mp4");
         info!("Exporting mp4 with settings: {:?}", &self);
         info!("Expected to render {} frames", base.total_frames(self.fps));
 
diff --git a/crates/export/tests/mp4_output_path.rs b/crates/export/tests/mp4_output_path.rs
new file mode 100644
index 00000000000..84be3daa737
--- /dev/null
+++ b/crates/export/tests/mp4_output_path.rs
@@ -0,0 +1,83 @@
+use cap_export::{
+    ExporterBase,
+    mp4::{ExportCompression, Mp4ExportSettings},
+};
+use cap_project::{ProjectConfiguration, XY};
+use std::{fs, path::Path, process::Command};
+
+fn create_project(path: &Path) {
+    fs::create_dir_all(path).unwrap();
+    let video = Command::new("ffmpeg")
+        .args([
+            "-v",
+            "error",
+            "-f",
+            "lavfi",
+            "-i",
+            "testsrc2=size=64x64:rate=10:duration=1",
+            "-c:v",
+            "libx264",
+            "-preset",
+            "ultrafast",
+            "-threads",
+            "1",
+        ])
+        .arg(path.join("display.mp4"))
+        .output()
+        .unwrap();
+    assert!(
+        video.status.success(),
+        "{}",
+        String::from_utf8_lossy(&video.stderr)
+    );
+    fs::write(
+        path.join("recording-meta.json"),
+        serde_json::to_vec(&serde_json::json!({
+            "pretty_name": "MP4 output path",
+            "display": { "path": "display.mp4", "fps": 10 }
+        }))
+        .unwrap(),
+    )
+    .unwrap();
+}
+
+async fn export(project: &Path, output: &Path) -> Result<std::path::PathBuf, String> {
+    let base = Box::pin(
+        ExporterBase::builder(project.to_path_buf())
+            .with_config(ProjectConfiguration::default())
+            .with_output_path(output.to_path_buf())
+            .build(),
+    )
+    .await
+    .unwrap();
+    Box::pin(
+        Mp4ExportSettings {
+            fps: 10,
+            resolution_base: XY::new(64, 64),
+            compression: ExportCompression::Maximum,
+            custom_bpp: None,
+            force_ffmpeg_decoder: false,
+            optimize_filesize: false,
+        }
+        .export(base, |_| true),
+    )
+    .await
+}
+
+#[tokio::test]
+#[ignore = "requires ffmpeg and a wgpu adapter"]
+async fn mp4_export_returns_the_path_it_wrote() {
+    let directory = tempfile::tempdir().unwrap();
+    let project = directory.path().join("project.cap");
+    create_project(&project);
+
+    for requested in ["export.gif", "export.webm", "export"] {
+        let written = export(&project, &directory.path().join(requested))
+            .await
+            .unwrap();
+        assert_eq!(written, directory.path().join("export.mp4"), "{requested}");
+        assert!(written.is_file(), "{requested}");
+        assert!(!directory.path().join(requested).exists(), "{requested}");
+        fs::remove_file(&written).unwrap();
+    }
+}