Harden configuration validation

internal/config/session_template_test.go

@@ -121,6 +121,24 @@ inputs:
 	}
 }
 
+func TestLoadSessionWithOptionsRequiresExpectedPreviousSession(t *testing.T) {
+	dir := t.TempDir()
+	sessionPath := filepath.Join(dir, "session.yml")
+	sessionYAML := `session_id: 2026-05-03
+campaign: sample-campaign
+inputs:
+  audio_dir: ./audio
+`
+	if err := os.WriteFile(sessionPath, []byte(sessionYAML), 0o644); err != nil {
+		t.Fatalf("write session.yml: %v", err)
+	}
+
+	_, err := LoadSessionWithOptions(sessionPath, SessionLoadOptions{PreviousSessionID: "2026-04-26"})
+	if err == nil || !strings.Contains(err.Error(), "previous_session_id is required") {
+		t.Fatalf("LoadSessionWithOptions() error = %v, want missing previous-session expectation error", err)
+	}
+}
+
 func TestLoadSessionWithOptionsUnknownFieldStillRejected(t *testing.T) {
 	dir := t.TempDir()
 	sessionPath := filepath.Join(dir, "session.yml")
@@ -217,6 +235,17 @@ func TestLoadSessionBytesWithOptionsMismatchFails(t *testing.T) {
 	}
 }
 
+func TestLoadSessionBytesWithOptionsRequiresExpectedPreviousSession(t *testing.T) {
+	_, err := LoadSessionBytesWithOptions(
+		"s3://bucket/session.yml",
+		[]byte("session_id: 2026-05-03\n"),
+		SessionLoadOptions{PreviousSessionID: "2026-04-26"},
+	)
+	if err == nil || !strings.Contains(err.Error(), "previous_session_id is required") {
+		t.Fatalf("LoadSessionBytesWithOptions() error = %v, want missing previous-session expectation error", err)
+	}
+}
+
 func assertConcreteSessionTemplateError(t *testing.T, err error, vars ...string) {
 	t.Helper()
 	if !strings.Contains(err.Error(), "session.yml must be concrete") {