diff --git a/backend/internal/service/session/service.go b/backend/internal/service/session/service.go
index 32d338897a..c794db99e4 100644
--- a/backend/internal/service/session/service.go
+++ b/backend/internal/service/session/service.go
@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"strings"
 	"time"
+	"unicode/utf8"
 
 	"github.com/aoagents/agent-orchestrator/backend/internal/domain"
 	"github.com/aoagents/agent-orchestrator/backend/internal/httpd/apierr"
@@ -77,6 +78,8 @@ type scmProvider interface {
 	FetchReviewThreads(ctx context.Context, ref ports.SCMPRRef) (ports.SCMReviewObservation, error)
 }
 
+const maxDisplayNameLen = 20
+
 // Service is the controller-facing session service. It delegates command-side
 // session operations to the internal sessionmanager.Manager and owns read-model
 // assembly, including user-facing display status derivation.
@@ -325,6 +328,9 @@ func (s *Service) Rename(ctx context.Context, id domain.SessionID, displayName s
 	if displayName == "" {
 		return apierr.Invalid("DISPLAY_NAME_REQUIRED", "Display name is required", nil)
 	}
+	if utf8.RuneCountInString(displayName) > maxDisplayNameLen {
+		return apierr.Invalid("DISPLAY_NAME_TOO_LONG", "Display name must be 20 characters or fewer", nil)
+	}
 	renamed, err := s.store.RenameSession(ctx, id, displayName, time.Now().UTC())
 	if err != nil {
 		return fmt.Errorf("rename %s: %w", id, err)
diff --git a/backend/internal/service/session/service_test.go b/backend/internal/service/session/service_test.go
index c5b1b1ca64..263705c63b 100644
--- a/backend/internal/service/session/service_test.go
+++ b/backend/internal/service/session/service_test.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"strings"
 	"testing"
 	"time"
 
@@ -164,6 +165,20 @@ func TestSessionRenameUpdatesDisplayName(t *testing.T) {
 	}
 }
 
+func TestSessionRenameRejectsTooLongDisplayName(t *testing.T) {
+	st := newFakeStore()
+	st.sessions["mer-1"] = domain.SessionRecord{ID: "mer-1", ProjectID: "mer", DisplayName: "Original"}
+
+	err := (&Service{store: st}).Rename(context.Background(), "mer-1", strings.Repeat("x", 21))
+	var e *apierr.Error
+	if !errors.As(err, &e) || e.Kind != apierr.KindInvalid || e.Code != "DISPLAY_NAME_TOO_LONG" {
+		t.Fatalf("err = %v, want apierr Invalid DISPLAY_NAME_TOO_LONG", err)
+	}
+	if got := st.sessions["mer-1"].DisplayName; got != "Original" {
+		t.Fatalf("display name = %q, want unchanged after rejected rename", got)
+	}
+}
+
 func TestSessionSetPreviewPersistsURL(t *testing.T) {
 	st := newFakeStore()
 	st.sessions["mer-1"] = domain.SessionRecord{ID: "mer-1", ProjectID: "mer"}
diff --git a/frontend/src/renderer/components/CreateProjectAgentSheet.tsx b/frontend/src/renderer/components/CreateProjectAgentSheet.tsx
index 76f6054ede..a2c52fe730 100644
--- a/frontend/src/renderer/components/CreateProjectAgentSheet.tsx
+++ b/frontend/src/renderer/components/CreateProjectAgentSheet.tsx
@@ -142,4 +142,4 @@ export const RequiredAgentField = memo(function RequiredAgentField({
 			</Select>
 		</div>
 	);
-});
\ No newline at end of file
+});
diff --git a/frontend/src/renderer/components/ProjectSettingsForm.test.tsx b/frontend/src/renderer/components/ProjectSettingsForm.test.tsx
index 26776054dc..27ecb900ee 100644
--- a/frontend/src/renderer/components/ProjectSettingsForm.test.tsx
+++ b/frontend/src/renderer/components/ProjectSettingsForm.test.tsx
@@ -210,4 +210,4 @@ describe("ProjectSettingsForm", () => {
 		});
 		expect(await screen.findByText("Saved.")).toBeInTheDocument();
 	});
-});
\ No newline at end of file
+});
diff --git a/frontend/src/renderer/components/ProjectSettingsForm.tsx b/frontend/src/renderer/components/ProjectSettingsForm.tsx
index ea51b7fd4d..5c504e0d60 100644
--- a/frontend/src/renderer/components/ProjectSettingsForm.tsx
+++ b/frontend/src/renderer/components/ProjectSettingsForm.tsx
@@ -300,4 +300,4 @@ function CenteredNote({ children }: { children: React.ReactNode }) {
 // rather than an empty {} the daemon would persist.
 function blankToUndefined<T extends object>(obj: T): T | undefined {
 	return Object.values(obj).some((v) => v !== undefined) ? obj : undefined;
-}
\ No newline at end of file
+}
diff --git a/frontend/src/renderer/lib/agent-options.ts b/frontend/src/renderer/lib/agent-options.ts
index 7cdc56eeee..39ade7eaec 100644
--- a/frontend/src/renderer/lib/agent-options.ts
+++ b/frontend/src/renderer/lib/agent-options.ts
@@ -26,4 +26,4 @@ export const AGENT_OPTIONS = [
 
 // The agent new projects use by default, and the fallback for worker/orchestrator
 // role fields that have no explicit configuration. Users can change it per project.
-export const DEFAULT_PROJECT_AGENT: (typeof AGENT_OPTIONS)[number] = "claude-code";
\ No newline at end of file
+export const DEFAULT_PROJECT_AGENT: (typeof AGENT_OPTIONS)[number] = "claude-code";