diff --git a/interp/runner_redir.go b/interp/runner_redir.go
index d31ccdf6..15c8e927 100644
--- a/interp/runner_redir.go
+++ b/interp/runner_redir.go
@@ -37,6 +37,19 @@ func isQuotedHdoc(rd *syntax.Redirect) bool {
 	return false
 }
 
+// hdocWordRawSize returns the total byte count of literal parts in a heredoc
+// word. This is used as a fast pre-check before expensive expansion — if the
+// raw literals alone exceed the size limit, the expanded output will too.
+func hdocWordRawSize(w *syntax.Word) int {
+	var n int
+	for _, part := range w.Parts {
+		if lit, ok := part.(*syntax.Lit); ok {
+			n += len(lit.Value)
+		}
+	}
+	return n
+}
+
 // hdocLiteral reconstructs the literal (unexpanded) text of a heredoc body.
 // This is used for quoted delimiters where no expansion should occur.
 func hdocLiteral(word *syntax.Word) string {
@@ -86,6 +99,15 @@ func (r *Runner) hdocReader(ctx context.Context, rd *syntax.Redirect) (*os.File,
 		return r.document(w)
 	}
 	if rd.Op != syntax.DashHdoc {
+		// Fast pre-check: if the raw literal content already exceeds the
+		// limit, reject before the expensive expansion pass. This avoids
+		// timeouts on very large heredocs (e.g. under the race detector).
+		if hdocWordRawSize(rd.Hdoc) > MaxHeredocBytes {
+			pr.Close()
+			pw.Close()
+			r.errf("heredoc: content exceeds maximum size (%d bytes)\n", MaxHeredocBytes)
+			return nil, fmt.Errorf("heredoc: content exceeds maximum size (%d bytes)", MaxHeredocBytes)
+		}
 		hdoc := expandWord(rd.Hdoc)
 		if len(hdoc) > MaxHeredocBytes {
 			pr.Close()
diff --git a/interp/tests/redir_devnull_pentest_test.go b/interp/tests/redir_devnull_pentest_test.go
index c563a9e9..ff9f01db 100644
--- a/interp/tests/redir_devnull_pentest_test.go
+++ b/interp/tests/redir_devnull_pentest_test.go
@@ -23,19 +23,20 @@ import (
 
 func pentestRedirRun(t *testing.T, script, dir string) (string, string, int) {
 	t.Helper()
-	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
-	defer cancel()
-	return pentestRedirRunCtx(ctx, t, script, dir)
-}
-
-func pentestRedirRunCtx(ctx context.Context, t *testing.T, script, dir string) (string, string, int) {
-	t.Helper()
+	// Parse outside the timeout so that slow parsing under the race
+	// detector does not eat into the 5-second execution budget.
 	parser := syntax.NewParser()
 	prog, err := parser.Parse(strings.NewReader(script), "")
 	if err != nil {
-		// Parse errors are expected for some pentest cases
 		return "", err.Error(), 2
 	}
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	return pentestRedirRunProg(ctx, t, prog, dir)
+}
+
+func pentestRedirRunProg(ctx context.Context, t *testing.T, prog *syntax.File, dir string) (string, string, int) {
+	t.Helper()
 
 	var outBuf, errBuf bytes.Buffer
 	opts := []interp.RunnerOption{