Skip worktree diffs for read-only languages

crates/prek/src/hooks/mod.rs

@@ -102,8 +102,8 @@ fn fast_path_hook(hook: &Hook) -> Option<PreCommitHooks> {
 pub(crate) fn requires_diff_tracking(hook: &Hook) -> bool {
     match hook.repo() {
         Repo::Meta { .. } | Repo::Builtin { .. } => false,
-        Repo::Remote { .. } => fast_path_hook(hook).is_none(),
-        Repo::Local { .. } => true,
+        Repo::Remote { .. } => fast_path_hook(hook).is_none() && hook.language.can_modify_files(),
+        Repo::Local { .. } => hook.language.can_modify_files(),
     }
 }
 

crates/prek/src/languages/mod.rs

@@ -128,6 +128,10 @@ impl Language {
         }
     }
 
+    pub(crate) fn can_modify_files(self) -> bool {
+        !matches!(self, Self::Fail | Self::Pygrep)
+    }
+
     pub(crate) fn supports_install_env(self) -> bool {
         match self {
             Self::Bun
@@ -351,7 +355,11 @@ impl Language {
             Repo::Remote { .. } | Repo::Local { .. } => Box::pin(async move {
                 let (exit_status, output) =
                     self.backend().run(store, hook, filenames, reporter).await?;
-                Ok(HookOutput::unknown(exit_status, output))
+                if self.can_modify_files() {
+                    Ok(HookOutput::unknown(exit_status, output))
+                } else {
+                    Ok(HookOutput::unchanged(exit_status, output))
+                }
             }),
         };
 

crates/prek/tests/skipped_hooks.rs

@@ -838,6 +838,61 @@ fn read_only_builtin_hook_does_not_run_diff_detection() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn read_only_languages_do_not_run_diff_detection() -> Result<()> {
+    let context = TestContext::new();
+    context.init_project();
+
+    let cwd = context.work_dir();
+    context.write_pre_commit_config(indoc::indoc! {r"
+        repos:
+          - repo: local
+            hooks:
+              - id: fail
+                name: fail
+                language: fail
+                entry: expected failure
+                files: \.txt$
+              - id: pygrep
+                name: pygrep
+                language: pygrep
+                entry: not-present
+                files: \.txt$
+    "});
+
+    cwd.child("file.txt").write_str("original\n")?;
+    context.git_add(".");
+
+    let output = context
+        .run()
+        .arg("--all-files")
+        .env("RUST_LOG", "prek::git=trace")
+        .output()?;
+
+    assert!(!output.status.success(), "the fail hook should fail");
+
+    let stdout = String::from_utf8_lossy(&output.stdout);
+    assert!(stdout.contains("fail") && stdout.contains("Failed"));
+    assert!(stdout.contains("pygrep") && stdout.contains("Passed"));
+    assert!(!stdout.contains("files were modified by this hook"));
+
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    assert_eq!(
+        stderr.matches("has_worktree_diff").count(),
+        0,
+        "Read-only languages should not require a worktree diff check.\n\
+         Trace output:\n{stderr}"
+    );
+    assert_eq!(
+        stderr.matches("diff_worktree").count(),
+        0,
+        "Read-only languages should not require a full worktree diff.\n\
+         Trace output:\n{stderr}"
+    );
+
+    Ok(())
+}
+
 #[test]
 fn modifying_builtin_invalidates_baseline_for_later_external_hook() -> Result<()> {
     let context = TestContext::new();