diff --git a/internal/cli/run.go b/internal/cli/run.go
index 5ff832bd6..fd7d544d4 100644
--- a/internal/cli/run.go
+++ b/internal/cli/run.go
@@ -592,7 +592,7 @@ func runAgent(agentName, fullsendDir, outputBase, targetRepo, fullsendBinary str
 			break
 		}
 
-		printer.StepFail("Validation failed: " + strings.TrimSpace(string(valOut)))
+		printer.StepFail("Validation failed: " + validationFailMessage(valOut, valErr))
 		if iteration < maxIterations {
 			printer.StepInfo(fmt.Sprintf("Will retry (%d iterations remaining)", maxIterations-iteration))
 		}
@@ -936,6 +936,16 @@ func bootstrapEnv(sandboxName, repoDir string, h *harness.Harness) error {
 	return nil
 }
 
+// validationFailMessage returns a human-readable message for a validation
+// script failure. When the script produces output, that output is used;
+// otherwise it falls back to the exec error string (e.g. ENOENT / EACCES).
+func validationFailMessage(output []byte, execErr error) string {
+	if msg := strings.TrimSpace(string(output)); msg != "" {
+		return msg
+	}
+	return execErr.Error()
+}
+
 // envToList converts a map of env vars to a sorted list of KEY=VALUE strings.
 func envToList(env map[string]string) []string {
 	keys := make([]string, 0, len(env))
diff --git a/internal/cli/run_test.go b/internal/cli/run_test.go
index ca05647f5..ae6f83270 100644
--- a/internal/cli/run_test.go
+++ b/internal/cli/run_test.go
@@ -915,3 +915,23 @@ func TestDownloadReleaseBinary_ChecksumMatch(t *testing.T) {
 	require.NoError(t, err)
 	assert.Equal(t, "good binary", string(data))
 }
+
+func TestValidationFailMessage_UsesOutputWhenPresent(t *testing.T) {
+	msg := validationFailMessage([]byte("check failed: lint errors"), fmt.Errorf("exit status 1"))
+	assert.Equal(t, "check failed: lint errors", msg)
+}
+
+func TestValidationFailMessage_FallsBackToError(t *testing.T) {
+	msg := validationFailMessage([]byte(""), fmt.Errorf("exec: \"missing-script\": executable file not found in $PATH"))
+	assert.Equal(t, "exec: \"missing-script\": executable file not found in $PATH", msg)
+}
+
+func TestValidationFailMessage_FallsBackWhenWhitespaceOnly(t *testing.T) {
+	msg := validationFailMessage([]byte("  \n\t  "), fmt.Errorf("exit status 127"))
+	assert.Equal(t, "exit status 127", msg)
+}
+
+func TestValidationFailMessage_TrimsOutput(t *testing.T) {
+	msg := validationFailMessage([]byte("  some output\n"), fmt.Errorf("exit status 1"))
+	assert.Equal(t, "some output", msg)
+}