diff --git a/crates/engine/src/cmd/layout/tests.rs b/crates/engine/src/cmd/layout/tests.rs
index 9bb73f5fc..2cb9b0c2e 100644
--- a/crates/engine/src/cmd/layout/tests.rs
+++ b/crates/engine/src/cmd/layout/tests.rs
@@ -303,3 +303,36 @@ fn mspace_edits_model_through_viewport() {
     s.execute("layout.set", &json!({"name": "Model"})).unwrap();
     assert!(s.state().unwrap().mspace.is_none());
 }
+
+/// The path's extension picks the plot format as EXPORT does, in any case; one that names no
+/// format (or none) is refused instead of writing a PDF under that name, and a device that writes
+/// another format is refused.
+#[cfg(not(target_arch = "wasm32"))]
+#[test]
+fn plot_path_extension_picks_the_format_like_export() {
+    super::super::file::set_io(super::super::file::IoHooks {
+        read: |_, _| Err("no reader in tests".into()),
+        write: |_, _| Err("no writer in tests".into()),
+        plot: Some(fake_plot),
+    });
+    if io().and_then(|h| h.plot).is_none() {
+        return; // another test installed hooks without a plotter first
+    }
+    let mut s = session_with_model();
+    let dir = std::env::temp_dir().join(format!("cadcraft-plot-ext-{}", std::process::id()));
+    std::fs::create_dir_all(&dir).unwrap();
+    let path = |n: &str| dir.join(n).to_string_lossy().to_string();
+    let upper = s.execute("plot", &json!({"path": path("X.PNG")})).unwrap();
+    assert_eq!(upper["format"], "png", "an upper-case extension still picks png");
+    let e = s.execute("plot", &json!({"path": path("x.txt")})).unwrap_err().to_string();
+    assert!(e.contains("png") && e.contains("svg") && e.contains("pdf"), "{e}");
+    assert!(!dir.join("x.txt").exists(), "nothing written under an unknown extension");
+    let e = s.execute("plot", &json!({"path": path("noext")})).unwrap_err().to_string();
+    assert!(e.contains("png") && e.contains("svg") && e.contains("pdf"), "{e}");
+    assert!(!dir.join("noext").exists());
+    let e = s.execute("plot", &json!({"path": path("y.png"), "device": "CADCraft PDF"})).unwrap_err().to_string();
+    assert!(e.contains("writes pdf") && e.contains("png"), "{e}");
+    // An explicit output still decides for an odd name.
+    assert_eq!(s.execute("plot", &json!({"path": path("z.txt"), "output": "pdf"})).unwrap()["format"], "pdf");
+    std::fs::remove_dir_all(&dir).ok();
+}
diff --git a/crates/engine/src/cmd/plotters.rs b/crates/engine/src/cmd/plotters.rs
index 521b67cf9..d9aed574f 100644
--- a/crates/engine/src/cmd/plotters.rs
+++ b/crates/engine/src/cmd/plotters.rs
@@ -296,6 +296,19 @@ pub(crate) fn plot_options(s: &Session, p: &Value, cmd: &str) -> Result<Value> {
     // The path's extension names the format, like EXPORT, unless a device is given; a given
     // device that writes another format is refused. EXPORTPDF always writes PDF.
     let by_path = if cmd == "exportpdf" { None } else { str_param(p, "path").and_then(path_output) };
+    // A path whose extension names no format (or has none) would hold a PDF under that name:
+    // refuse it, as EXPORT does, unless `output` or a device says what to write.
+    if cmd == "plot"
+        && by_path.is_none()
+        && p.get("output").is_none()
+        && !given.is_some_and(|n| !n.is_empty())
+        && let Some(path) = str_param(p, "path")
+    {
+        return Err(bad(
+            cmd,
+            format!("can't tell the format of `{path}`: give a path ending in .png, .svg or .pdf, or set `output` to png, svg or pdf"),
+        ));
+    }
     if let (Some(want), Some(dev), true) = (by_path, &dev, given.is_some_and(|n| !n.is_empty()))
         && dev.output != want
     {
diff --git a/crates/engine/tests/plot_format.rs b/crates/engine/tests/plot_format.rs
index 1f4509c8c..48a4306bb 100644
--- a/crates/engine/tests/plot_format.rs
+++ b/crates/engine/tests/plot_format.rs
@@ -36,11 +36,12 @@ fn the_extension_picks_png_svg_or_pdf() {
         assert_eq!(r["format"], format, "{name}: the reply names the format");
         assert_eq!(magic(&out), head, "{name}: the file holds that format");
     }
-    // No path, or an extension that names no format: PDF as before.
+    // No path: PDF data as before. An extension that names no format is refused, like EXPORT.
     assert_eq!(s.execute("plot", &json!({ "layout": "Model" })).unwrap()["format"], "pdf");
     let other = dir.join("e.plt");
-    assert_eq!(s.execute("plot", &json!({ "path": other.to_string_lossy(), "layout": "Model" })).unwrap()["format"], "pdf");
-    assert_eq!(magic(&other), b"%PDF-");
+    let e = s.execute("plot", &json!({ "path": other.to_string_lossy(), "layout": "Model" })).unwrap_err().to_string();
+    assert!(e.contains("png") && e.contains("svg") && e.contains("pdf"), "{e}");
+    assert!(!other.exists(), "nothing is written under a name that is not a format");
     // EXPORTPDF always writes PDF.
     let pdf = dir.join("f.png");
     s.execute("exportpdf", &json!({ "path": pdf.to_string_lossy(), "layout": "Model" })).unwrap();