A builder that writes /app/… paths builds the app instead of being refused

Builders driven by our own loop (an API model such as Ollama) get file tools whose paths are
relative to the project. Models often write the project's folders from the root (/app/src/…), which
was refused as outside the project, so they could neither list, read nor write app/ and went round
in circles while each check found nothing. A path from the root whose first folder the project has
is now read inside the project (/etc/… and folders it doesn't have are still refused), and a write
outside app/ says so in its result, so a model that leaves out app/ learns where the app goes.

Refs #24

crates/engine/src/tools/build_tools.rs

@@ -194,14 +194,15 @@ fn workspace(ctx: &Ctx<'_>) -> Result<Workspace, ToolError> {
 }
 
 /// `rel` inside the project folder, or an error (no escaping with `..` or an absolute path).
+/// Models often write the project's folders from the root (`/app/src/main.rs`): a path like that
+/// whose first folder the project has is read inside the project, where it was meant (#24).
 fn in_project(ws: &Workspace, rel: &str) -> Result<PathBuf, ToolError> {
-    match resolve(&ws.root, rel) {
-        Some(p) if inside(&ws.root, &p) => Ok(p),
-        _ => {
-            Err(ToolError::invalid(format!("{rel} is outside the project."))
-                .with_remediation("Use a path relative to the project folder, like app/src/main.rs."))
-        }
-    }
+    let from_root = rel.trim_start_matches(['/', '\\']);
+    let names_a_folder = from_root != rel
+        && from_root.split(['/', '\\']).next().is_some_and(|first| !first.is_empty() && (ws.root.join(first).is_dir() || ws.root.join(first) == ws.app_dir()));
+    [Some(rel), names_a_folder.then_some(from_root)].into_iter().flatten().find_map(|r| resolve(&ws.root, r).filter(|p| inside(&ws.root, p))).ok_or_else(|| {
+        ToolError::invalid(format!("{rel} is outside the project.")).with_remediation("Use a path relative to the project folder, like app/src/main.rs.")
+    })
 }
 
 fn shown(ws: &Workspace, p: &Path) -> String {
@@ -284,7 +285,12 @@ fn build_file_write(ctx: &mut Ctx<'_>, params: Value) -> Result<ToolOutput, Tool
     let ws = workspace(ctx)?;
     let path = in_project(&ws, &p.path)?;
     write_checked(ctx, &path, &p.content)?;
-    Ok(ToolOutput::Value(json!({"written": shown(&ws, &path), "bytes": p.content.len()})))
+    let mut out = json!({"written": shown(&ws, &path), "bytes": p.content.len()});
+    // A model that leaves out `app/` would otherwise build beside the app, where no check looks.
+    if !inside(&ws.app_dir(), &path) {
+        out["note"] = json!("This file is outside app/: the app's own files go in app/ (paths like app/package.json).");
+    }
+    Ok(ToolOutput::Value(out))
 }
 
 fn build_file_edit(ctx: &mut Ctx<'_>, params: Value) -> Result<ToolOutput, ToolError> {
@@ -379,6 +385,23 @@ mod tests {
         assert!(call(&e, "build_file_write", json!({"path": "../outside.txt", "content": "x"})).is_err());
     }
 
+    /// #24: a model that writes `/app/…` builds the app, and one that leaves out `app/` is told.
+    #[test]
+    fn paths_from_the_root_name_the_project_folders() {
+        let (_d, ws, e) = building();
+        let r = call(&e, "build_file_write", json!({"path": "/app/package.json", "content": "{}"})).unwrap();
+        assert_eq!((r["written"].as_str(), r.get("note")), (Some("app/package.json"), None));
+        assert!(ws.app_dir().join("package.json").is_file());
+        assert_eq!(call(&e, "build_file_read", json!({"path": "\\app\\package.json"})).unwrap()["text"], "{}\n");
+        assert!(call(&e, "build_files_list", json!({"path": "/app"})).unwrap()["entries"].to_string().contains("app/package.json"));
+        // A folder the project doesn't have stays outside it.
+        assert!(call(&e, "build_file_read", json!({"path": "/etc/passwd"})).is_err());
+        assert!(call(&e, "build_file_write", json!({"path": "/nowhere/x.txt", "content": "x"})).is_err());
+        assert!(call(&e, "build_file_write", json!({"path": "/../outside.txt", "content": "x"})).is_err());
+        let stray = call(&e, "build_file_write", json!({"path": "index.html", "content": "<p>hi</p>"})).unwrap();
+        assert!(stray["note"].as_str().unwrap().contains("outside app/"), "{stray}");
+    }
+
     #[test]
     fn writes_outside_app_and_dev_servers_follow_the_policy() {
         let (_d, ws, e) = building();