diff --git a/cmd/memos/main.go b/cmd/memos/main.go
index 3daaa619..5bc68849 100644
--- a/cmd/memos/main.go
+++ b/cmd/memos/main.go
@@ -193,6 +193,13 @@ func printGreetings(profile *profile.Profile) {
 		fmt.Printf("Server running on unix socket: %s\n", profile.UNIXSock)
 	}
 
+	// Access mode is derived from instance_url: set = public, unset = private.
+	accessMode := "private"
+	if profile.AllowAnonymous() {
+		accessMode = "public"
+	}
+	fmt.Printf("Access mode: %s\n", accessMode)
+
 	fmt.Println()
 	fmt.Printf("Documentation: %s\n", "https://usememos.com")
 	fmt.Printf("Source code: %s\n", "https://github.com/usememos/memos")
diff --git a/internal/profile/profile.go b/internal/profile/profile.go
index 7a78457b..30158436 100644
--- a/internal/profile/profile.go
+++ b/internal/profile/profile.go
@@ -36,6 +36,17 @@ type Profile struct {
 	InstanceURL string
 }
 
+// AllowAnonymous reports whether unauthenticated visitors may access the instance.
+//
+// Anonymous access is enabled only when an InstanceURL is configured. An instance
+// with no InstanceURL set is treated as private: anonymous callers are limited to
+// the auth-bootstrap endpoints (sign-in, share links, etc.) and the web UI redirects
+// them to the sign-in page instead of the public Explore view. Authenticated callers
+// (session, access token, or personal access token) are never affected.
+func (p *Profile) AllowAnonymous() bool {
+	return strings.TrimSpace(p.InstanceURL) != ""
+}
+
 func checkDataDir(dataDir string) (string, error) {
 	// Convert to absolute path if relative path is supplied.
 	if !filepath.IsAbs(dataDir) {
diff --git a/internal/profile/profile_test.go b/internal/profile/profile_test.go
new file mode 100644
index 00000000..4f15b15a
--- /dev/null
+++ b/internal/profile/profile_test.go
@@ -0,0 +1,24 @@
+package profile
+
+import "testing"
+
+func TestAllowAnonymous(t *testing.T) {
+	cases := []struct {
+		name string
+		url  string
+		want bool
+	}{
+		{"empty is private", "", false},
+		{"whitespace only is private", "   ", false},
+		{"configured url is public", "https://memos.example.com", true},
+		{"configured url with padding is public", "  https://memos.example.com  ", true},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			p := &Profile{InstanceURL: c.url}
+			if got := p.AllowAnonymous(); got != c.want {
+				t.Fatalf("AllowAnonymous() with InstanceURL=%q = %v, want %v", c.url, got, c.want)
+			}
+		})
+	}
+}
diff --git a/server/auth/authenticator.go b/server/auth/authenticator.go
index 16e98611..bf0b3b77 100644
--- a/server/auth/authenticator.go
+++ b/server/auth/authenticator.go
@@ -130,39 +130,75 @@ type AuthResult struct {
 	AccessToken string      // Non-empty if authenticated via JWT
 }
 
-// AuthenticateToUser resolves the current request to a *store.User, checking the
-// Authorization header first (access token or PAT), then falling back to the
-// refresh token cookie. Returns (nil, nil) when no credentials are present.
-func (a *Authenticator) AuthenticateToUser(ctx context.Context, authHeader, cookieHeader string) (*store.User, error) {
-	// Try Bearer token first.
-	if authHeader != "" {
-		token := ExtractBearerToken(authHeader)
-		if token != "" {
-			if !strings.HasPrefix(token, PersonalAccessTokenPrefix) {
-				claims, err := a.AuthenticateByAccessTokenV2(token)
-				if err == nil && claims != nil {
-					user, err := a.store.GetUser(ctx, &store.FindUser{ID: &claims.UserID})
-					if err != nil {
-						return nil, err
-					}
-					if user == nil || user.RowStatus == store.Archived {
-						return nil, nil
-					}
-					return user, nil
-				}
-			} else {
-				user, _, err := a.AuthenticateByPAT(ctx, token)
-				if err == nil {
-					return user, nil
-				}
+// bearerAuth is the outcome of successfully validating a Bearer token: the resolved
+// user plus the credential-specific detail needed to build an AuthResult.
+type bearerAuth struct {
+	user   *store.User
+	claims *UserClaims                                                  // set for an Access Token V2
+	pat    *storepb.PersonalAccessTokensUserSetting_PersonalAccessToken // set for a Personal Access Token
+}
+
+// resolveBearer validates a Bearer token — an Access Token V2 or a Personal Access
+// Token — into an active user. It returns:
+//   - (nil, nil)    when the token is absent, malformed, expired, unknown, or resolves
+//     to a missing/archived user (callers may then fall back to other credentials);
+//   - (nil, err)    on an unexpected store error;
+//   - (result, nil) on success.
+//
+// It performs no side effects; callers decide whether to record PAT usage.
+func (a *Authenticator) resolveBearer(ctx context.Context, token string) (*bearerAuth, error) {
+	if token == "" {
+		return nil, nil
+	}
+
+	if !strings.HasPrefix(token, PersonalAccessTokenPrefix) {
+		// Access Token V2 (stateless). An invalid token yields no identity and no
+		// error, so the caller can fall back to other credentials.
+		claims, err := a.AuthenticateByAccessTokenV2(token)
+		if err == nil && claims != nil {
+			user, err := a.store.GetUser(ctx, &store.FindUser{ID: &claims.UserID})
+			if err != nil {
+				return nil, err
+			}
+			if user != nil && user.RowStatus != store.Archived {
+				return &bearerAuth{user: user, claims: claims}, nil
 			}
 		}
+		return nil, nil
+	}
+
+	// Personal Access Token.
+	if user, pat, err := a.AuthenticateByPAT(ctx, token); err == nil && user != nil {
+		return &bearerAuth{user: user, pat: pat}, nil
+	}
+	return nil, nil
+}
+
+// recordPATUsage updates a personal access token's last-used timestamp
+// asynchronously; failures are logged and never affect the request.
+func (a *Authenticator) recordPATUsage(userID int32, tokenID string) {
+	go func() {
+		if err := a.store.UpdatePATLastUsed(context.Background(), userID, tokenID, timestamppb.Now()); err != nil {
+			slog.Warn("failed to update PAT last used time", "error", err, "userID", userID)
+		}
+	}()
+}
+
+// AuthenticateToUser resolves the current request to a *store.User, checking the
+// Authorization header first (access token or PAT), then falling back to the
+// refresh token cookie. Returns (nil, nil) when no valid credentials are present.
+func (a *Authenticator) AuthenticateToUser(ctx context.Context, authHeader, cookieHeader string) (*store.User, error) {
+	bearer, err := a.resolveBearer(ctx, ExtractBearerToken(authHeader))
+	if err != nil {
+		return nil, err
+	}
+	if bearer != nil {
+		return bearer.user, nil
 	}
 
 	// Fallback: refresh token cookie.
 	if cookieHeader != "" {
-		refreshToken := ExtractRefreshTokenFromCookie(cookieHeader)
-		if refreshToken != "" {
+		if refreshToken := ExtractRefreshTokenFromCookie(cookieHeader); refreshToken != "" {
 			user, _, err := a.AuthenticateByRefreshToken(ctx, refreshToken)
 			return user, err
 		}
@@ -171,40 +207,18 @@ func (a *Authenticator) AuthenticateToUser(ctx context.Context, authHeader, cook
 	return nil, nil
 }
 
-// Authenticate tries to authenticate using the provided credentials.
-// Priority: 1. Access Token V2, 2. PAT
-// Returns nil if no valid credentials are provided.
+// Authenticate resolves a Bearer token (Access Token V2 or PAT) into an AuthResult,
+// returning nil when no valid credentials are present. Unlike AuthenticateToUser it
+// ignores the refresh cookie, and it records PAT last-used on success.
 func (a *Authenticator) Authenticate(ctx context.Context, authHeader string) *AuthResult {
 	token := ExtractBearerToken(authHeader)
-
-	// Try Access Token V2 (stateless)
-	if token != "" && !strings.HasPrefix(token, PersonalAccessTokenPrefix) {
-		claims, err := a.AuthenticateByAccessTokenV2(token)
-		if err == nil && claims != nil {
-			user, err := a.store.GetUser(ctx, &store.FindUser{ID: &claims.UserID})
-			if err != nil || user == nil || user.RowStatus == store.Archived {
-				return nil
-			}
-			return &AuthResult{
-				Claims:      claims,
-				AccessToken: token,
-			}
-		}
+	bearer, err := a.resolveBearer(ctx, token)
+	if err != nil || bearer == nil {
+		return nil
 	}
-
-	// Try PAT
-	if token != "" && strings.HasPrefix(token, PersonalAccessTokenPrefix) {
-		user, pat, err := a.AuthenticateByPAT(ctx, token)
-		if err == nil && user != nil {
-			// Update last used (fire-and-forget with logging)
-			go func() {
-				if err := a.store.UpdatePATLastUsed(context.Background(), user.ID, pat.TokenId, timestamppb.Now()); err != nil {
-					slog.Warn("failed to update PAT last used time", "error", err, "userID", user.ID)
-				}
-			}()
-			return &AuthResult{User: user, AccessToken: token}
-		}
+	if bearer.pat != nil {
+		a.recordPATUsage(bearer.user.ID, bearer.pat.TokenId)
+		return &AuthResult{User: bearer.user, AccessToken: token}
 	}
-
-	return nil
+	return &AuthResult{Claims: bearer.claims, AccessToken: token}
 }
diff --git a/server/auth/authenticator_test.go b/server/auth/authenticator_test.go
new file mode 100644
index 00000000..f6a866a8
--- /dev/null
+++ b/server/auth/authenticator_test.go
@@ -0,0 +1,34 @@
+package auth
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// TestAuthenticateNoCredentials covers the store-free paths: absent or malformed
+// credentials must resolve to "unauthenticated" without touching the store.
+// Token-valid paths are exercised by the API/fileserver integration tests.
+func TestAuthenticateNoCredentials(t *testing.T) {
+	ctx := context.Background()
+	a := &Authenticator{secret: "test-secret"} // nil store: these paths never reach it.
+
+	t.Run("Authenticate returns nil without an Authorization header", func(t *testing.T) {
+		assert.Nil(t, a.Authenticate(ctx, ""))
+	})
+	t.Run("Authenticate returns nil for a malformed bearer token", func(t *testing.T) {
+		assert.Nil(t, a.Authenticate(ctx, "Bearer not-a-valid-jwt"))
+	})
+
+	t.Run("AuthenticateToUser returns nil without any credentials", func(t *testing.T) {
+		user, err := a.AuthenticateToUser(ctx, "", "")
+		assert.NoError(t, err)
+		assert.Nil(t, user)
+	})
+	t.Run("AuthenticateToUser returns nil for a malformed bearer and no cookie", func(t *testing.T) {
+		user, err := a.AuthenticateToUser(ctx, "Bearer not-a-valid-jwt", "")
+		assert.NoError(t, err)
+		assert.Nil(t, user)
+	})
+}
diff --git a/server/router/api/v1/acl_config.go b/server/router/api/v1/acl_config.go
index d937ef68..172a74c2 100644
--- a/server/router/api/v1/acl_config.go
+++ b/server/router/api/v1/acl_config.go
@@ -46,3 +46,38 @@ func IsPublicMethod(procedure string) bool {
 	_, ok := PublicMethods[procedure]
 	return ok
 }
+
+// AuthBootstrapMethods is the subset of PublicMethods that stays reachable by
+// anonymous callers even when the instance is private (no InstanceURL configured).
+//
+// It is the minimum required to render the sign-in page, authenticate, and follow
+// share links. Every entry here MUST also exist in PublicMethods. CreateUser is
+// intentionally excluded and handled separately (allowed only during first-run
+// setup, while the instance has no users yet).
+var AuthBootstrapMethods = map[string]struct{}{
+	// Auth Service - sign-in and token refresh.
+	"/memos.api.v1.AuthService/SignIn":       {},
+	"/memos.api.v1.AuthService/RefreshToken": {},
+
+	// Instance Service - needed to render the sign-in page (branding, auth options).
+	"/memos.api.v1.InstanceService/GetInstanceProfile":       {},
+	"/memos.api.v1.InstanceService/GetInstanceSetting":       {},
+	"/memos.api.v1.InstanceService/BatchGetInstanceSettings": {},
+
+	// Identity Provider Service - SSO buttons on the sign-in page.
+	"/memos.api.v1.IdentityProviderService/ListIdentityProviders": {},
+
+	// Memo sharing - share-token access stays public even on a private instance.
+	"/memos.api.v1.MemoService/GetMemoByShare": {},
+}
+
+// createUserProcedure is the CreateUser endpoint. On a private instance it is
+// served to anonymous callers only while no user exists yet (initial admin setup).
+const createUserProcedure = "/memos.api.v1.UserService/CreateUser"
+
+// IsAuthBootstrapMethod reports whether an anonymous request to procedure is one
+// of the fixed endpoints allowed while the instance is private.
+func IsAuthBootstrapMethod(procedure string) bool {
+	_, ok := AuthBootstrapMethods[procedure]
+	return ok
+}
diff --git a/server/router/api/v1/acl_config_test.go b/server/router/api/v1/acl_config_test.go
index f0a484bc..e4ff9967 100644
--- a/server/router/api/v1/acl_config_test.go
+++ b/server/router/api/v1/acl_config_test.go
@@ -88,3 +88,50 @@ func TestUnknownMethodsRequireAuth(t *testing.T) {
 		})
 	}
 }
+
+// TestAuthBootstrapMethodsAreSubsetOfPublic verifies every auth-bootstrap method is
+// also a public method. A bootstrap method that wasn't public would be rejected before
+// the private-instance check runs, breaking sign-in on a private instance.
+func TestAuthBootstrapMethodsAreSubsetOfPublic(t *testing.T) {
+	for method := range AuthBootstrapMethods {
+		t.Run(method, func(t *testing.T) {
+			assert.True(t, IsPublicMethod(method), "auth-bootstrap method %s must also be a public method", method)
+		})
+	}
+}
+
+// TestAuthBootstrapClassification verifies which endpoints remain reachable by
+// anonymous callers on a private instance (no InstanceURL configured).
+func TestAuthBootstrapClassification(t *testing.T) {
+	// Reachable while private: sign-in flow, instance metadata, SSO, share links.
+	bootstrap := []string{
+		"/memos.api.v1.AuthService/SignIn",
+		"/memos.api.v1.AuthService/RefreshToken",
+		"/memos.api.v1.InstanceService/GetInstanceProfile",
+		"/memos.api.v1.InstanceService/GetInstanceSetting",
+		"/memos.api.v1.InstanceService/BatchGetInstanceSettings",
+		"/memos.api.v1.IdentityProviderService/ListIdentityProviders",
+		"/memos.api.v1.MemoService/GetMemoByShare",
+	}
+	for _, method := range bootstrap {
+		t.Run("bootstrap/"+method, func(t *testing.T) {
+			assert.True(t, IsAuthBootstrapMethod(method), "expected %s to be reachable on a private instance", method)
+		})
+	}
+
+	// Public on an open instance, but gated on a private one: browsing and profiles.
+	// CreateUser is gated here too; it is allowed separately only during first-run setup.
+	gatedWhilePrivate := []string{
+		"/memos.api.v1.MemoService/ListMemos",
+		"/memos.api.v1.MemoService/GetMemo",
+		"/memos.api.v1.MemoService/ListMemoComments",
+		"/memos.api.v1.UserService/GetUser",
+		"/memos.api.v1.UserService/ListAllUserStats",
+		"/memos.api.v1.UserService/CreateUser",
+	}
+	for _, method := range gatedWhilePrivate {
+		t.Run("gated/"+method, func(t *testing.T) {
+			assert.False(t, IsAuthBootstrapMethod(method), "expected %s to be gated on a private instance", method)
+		})
+	}
+}
diff --git a/server/router/api/v1/authz.go b/server/router/api/v1/authz.go
new file mode 100644
index 00000000..93cf13bf
--- /dev/null
+++ b/server/router/api/v1/authz.go
@@ -0,0 +1,90 @@
+package v1
+
+import (
+	"context"
+	"errors"
+
+	"github.com/usememos/memos/internal/profile"
+	"github.com/usememos/memos/server/auth"
+	"github.com/usememos/memos/store"
+)
+
+// ErrUnauthenticated is returned by the Authorizer when a request must be rejected
+// for lack of valid credentials. Each transport maps it to its own status code
+// (Connect: CodeUnauthenticated, gRPC-Gateway: HTTP 401).
+var ErrUnauthenticated = errors.New("authentication required")
+
+// Authorizer is the single source of truth for method-level access control.
+//
+// It authenticates a request from its Authorization header and decides whether the
+// (possibly anonymous) caller may reach a given RPC procedure. The Connect
+// interceptor and the gRPC-Gateway middleware share one Authorizer so both
+// transports enforce identical rules.
+//
+// Role-based authorization (admin checks) stays in the service layer; this type
+// governs only authentication and anonymous access.
+type Authorizer struct {
+	authenticator *auth.Authenticator
+	store         *store.Store
+	profile       *profile.Profile
+}
+
+// NewAuthorizer creates an Authorizer backed by the given store, token secret, and
+// instance profile.
+func NewAuthorizer(store *store.Store, secret string, profile *profile.Profile) *Authorizer {
+	return &Authorizer{
+		authenticator: auth.NewAuthenticator(store, secret),
+		store:         store,
+		profile:       profile,
+	}
+}
+
+// Authenticate resolves the caller from the Authorization header, returning nil for
+// an anonymous request. It never enforces policy — pair it with CheckAccess.
+func (a *Authorizer) Authenticate(ctx context.Context, authHeader string) *auth.AuthResult {
+	return a.authenticator.Authenticate(ctx, authHeader)
+}
+
+// CheckAccess enforces method-level access policy for procedure given the
+// authentication result (nil = anonymous). It returns nil when the request is
+// permitted and ErrUnauthenticated otherwise.
+//
+// Policy:
+//   - Authenticated caller (access token or PAT): always permitted here.
+//   - Anonymous + protected method: denied.
+//   - Anonymous + public method, open instance: permitted.
+//   - Anonymous + public method, private instance (no InstanceURL): permitted only
+//     for the auth-bootstrap set, plus CreateUser during first-run setup.
+func (a *Authorizer) CheckAccess(ctx context.Context, procedure string, result *auth.AuthResult) error {
+	if result != nil {
+		return nil
+	}
+	if !IsPublicMethod(procedure) {
+		return ErrUnauthenticated
+	}
+	if a.profile.AllowAnonymous() || a.allowedOnPrivateInstance(ctx, procedure) {
+		return nil
+	}
+	return ErrUnauthenticated
+}
+
+// allowedOnPrivateInstance reports whether an anonymous request to a public
+// procedure is still permitted while the instance is private. It allows the
+// auth-bootstrap set, plus CreateUser while the instance has no users yet
+// (first-run admin setup).
+func (a *Authorizer) allowedOnPrivateInstance(ctx context.Context, procedure string) bool {
+	if IsAuthBootstrapMethod(procedure) {
+		return true
+	}
+	if procedure == createUserProcedure {
+		return a.noUsersExist(ctx)
+	}
+	return false
+}
+
+// noUsersExist reports whether the instance has no users yet (fresh install).
+func (a *Authorizer) noUsersExist(ctx context.Context) bool {
+	limitOne := 1
+	users, err := a.store.ListUsers(ctx, &store.FindUser{Limit: &limitOne})
+	return err == nil && len(users) == 0
+}
diff --git a/server/router/api/v1/authz_test.go b/server/router/api/v1/authz_test.go
new file mode 100644
index 00000000..3e88bd95
--- /dev/null
+++ b/server/router/api/v1/authz_test.go
@@ -0,0 +1,56 @@
+package v1
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/usememos/memos/internal/profile"
+	"github.com/usememos/memos/server/auth"
+)
+
+// TestAuthorizerCheckAccess exercises the method-level access policy matrix.
+//
+// The store-backed first-run CreateUser branch is covered by integration tests;
+// every case here is decided without touching the store, so a nil store is safe.
+func TestAuthorizerCheckAccess(t *testing.T) {
+	ctx := context.Background()
+	authenticated := &auth.AuthResult{AccessToken: "token"}
+
+	openInstance := &Authorizer{profile: &profile.Profile{InstanceURL: "https://memos.example.com"}}
+	privateInstance := &Authorizer{profile: &profile.Profile{InstanceURL: ""}}
+
+	const (
+		protectedMethod = "/memos.api.v1.MemoService/CreateMemo"
+		publicMethod    = "/memos.api.v1.MemoService/ListMemos"
+		bootstrapMethod = "/memos.api.v1.AuthService/SignIn"
+		shareMethod     = "/memos.api.v1.MemoService/GetMemoByShare"
+	)
+
+	cases := []struct {
+		name      string
+		az        *Authorizer
+		procedure string
+		result    *auth.AuthResult
+		wantErr   bool
+	}{
+		{"authenticated reaches protected method", privateInstance, protectedMethod, authenticated, false},
+		{"authenticated reaches public method on private instance", privateInstance, publicMethod, authenticated, false},
+		{"anonymous denied on protected method", openInstance, protectedMethod, nil, true},
+		{"anonymous allowed on public method, open instance", openInstance, publicMethod, nil, false},
+		{"anonymous denied on public method, private instance", privateInstance, publicMethod, nil, true},
+		{"anonymous allowed on bootstrap method, private instance", privateInstance, bootstrapMethod, nil, false},
+		{"anonymous allowed on share access, private instance", privateInstance, shareMethod, nil, false},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			err := c.az.CheckAccess(ctx, c.procedure, c.result)
+			if c.wantErr {
+				assert.ErrorIs(t, err, ErrUnauthenticated)
+			} else {
+				assert.NoError(t, err)
+			}
+		})
+	}
+}
diff --git a/server/router/api/v1/connect_interceptors.go b/server/router/api/v1/connect_interceptors.go
index 84d736e7..36ba68ec 100644
--- a/server/router/api/v1/connect_interceptors.go
+++ b/server/router/api/v1/connect_interceptors.go
@@ -2,7 +2,6 @@ package v1
 
 import (
 	"context"
-	"errors"
 	"fmt"
 	"log/slog"
 	"reflect"
@@ -13,7 +12,6 @@ import (
 	"google.golang.org/grpc/metadata"
 
 	"github.com/usememos/memos/server/auth"
-	"github.com/usememos/memos/store"
 )
 
 // MetadataInterceptor converts Connect HTTP headers to gRPC metadata.
@@ -204,19 +202,16 @@ func (in *RecoveryInterceptor) logPanic(procedure string, panicValue any) {
 	slog.LogAttrs(context.Background(), slog.LevelError, "panic recovered in Connect handler", attrs...)
 }
 
-// AuthInterceptor handles authentication for Connect handlers.
-//
-// It enforces authentication for all endpoints except those listed in PublicMethods.
-// Role-based authorization (admin checks) remains in the service layer.
+// AuthInterceptor enforces authentication and anonymous-access policy for Connect
+// handlers by delegating to the shared Authorizer. Role-based authorization
+// (admin checks) remains in the service layer.
 type AuthInterceptor struct {
-	authenticator *auth.Authenticator
+	authorizer *Authorizer
 }
 
-// NewAuthInterceptor creates a new auth interceptor.
-func NewAuthInterceptor(store *store.Store, secret string) *AuthInterceptor {
-	return &AuthInterceptor{
-		authenticator: auth.NewAuthenticator(store, secret),
-	}
+// NewAuthInterceptor creates a new auth interceptor backed by the shared Authorizer.
+func NewAuthInterceptor(authorizer *Authorizer) *AuthInterceptor {
+	return &AuthInterceptor{authorizer: authorizer}
 }
 
 func (in *AuthInterceptor) WrapUnary(next connect.UnaryFunc) connect.UnaryFunc {
@@ -224,11 +219,9 @@ func (in *AuthInterceptor) WrapUnary(next connect.UnaryFunc) connect.UnaryFunc {
 		header := req.Header()
 		authHeader := header.Get("Authorization")
 
-		result := in.authenticator.Authenticate(ctx, authHeader)
-
-		// Enforce authentication for non-public methods
-		if result == nil && !IsPublicMethod(req.Spec().Procedure) {
-			return nil, connect.NewError(connect.CodeUnauthenticated, errors.New("authentication required"))
+		result := in.authorizer.Authenticate(ctx, authHeader)
+		if err := in.authorizer.CheckAccess(ctx, req.Spec().Procedure, result); err != nil {
+			return nil, connect.NewError(connect.CodeUnauthenticated, err)
 		}
 
 		ctx = auth.ApplyToContext(ctx, result)
diff --git a/server/router/api/v1/test/authz_test.go b/server/router/api/v1/test/authz_test.go
new file mode 100644
index 00000000..65a2ff7c
--- /dev/null
+++ b/server/router/api/v1/test/authz_test.go
@@ -0,0 +1,81 @@
+package test
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"google.golang.org/protobuf/types/known/timestamppb"
+
+	"github.com/usememos/memos/internal/profile"
+	"github.com/usememos/memos/internal/util"
+	storepb "github.com/usememos/memos/proto/gen/store"
+	"github.com/usememos/memos/server/auth"
+	apiv1 "github.com/usememos/memos/server/router/api/v1"
+)
+
+// TestAuthorizerPrivateInstanceFirstRun verifies the private-instance access policy
+// against a real store: anonymous CreateUser is permitted only until the first user
+// exists, bootstrap methods stay open, and other public methods are gated.
+func TestAuthorizerPrivateInstanceFirstRun(t *testing.T) {
+	ctx := context.Background()
+	ts := NewTestService(t)
+	defer ts.Cleanup()
+
+	// InstanceURL empty => private instance.
+	authorizer := apiv1.NewAuthorizer(ts.Store, ts.Secret, &profile.Profile{InstanceURL: ""})
+
+	const (
+		createUser   = "/memos.api.v1.UserService/CreateUser"
+		signIn       = "/memos.api.v1.AuthService/SignIn"
+		listMemos    = "/memos.api.v1.MemoService/ListMemos"
+		getMemoShare = "/memos.api.v1.MemoService/GetMemoByShare"
+	)
+
+	// Anonymous request with no Authorization header resolves to no identity.
+	require.Nil(t, authorizer.Authenticate(ctx, ""))
+
+	// Fresh instance (no users): first-run CreateUser is allowed, and so are the
+	// bootstrap methods; browsing is still gated.
+	require.NoError(t, authorizer.CheckAccess(ctx, createUser, nil), "first-run CreateUser should be allowed")
+	require.NoError(t, authorizer.CheckAccess(ctx, signIn, nil))
+	require.NoError(t, authorizer.CheckAccess(ctx, getMemoShare, nil))
+	require.ErrorIs(t, authorizer.CheckAccess(ctx, listMemos, nil), apiv1.ErrUnauthenticated)
+
+	// Once a user exists, anonymous CreateUser is denied while bootstrap stays open.
+	_, err := ts.CreateHostUser(ctx, "host")
+	require.NoError(t, err)
+	require.ErrorIs(t, authorizer.CheckAccess(ctx, createUser, nil), apiv1.ErrUnauthenticated, "CreateUser must be denied once a user exists")
+	require.NoError(t, authorizer.CheckAccess(ctx, signIn, nil))
+}
+
+// TestAuthorizerAccessTokenAlwaysWorksOnPrivateInstance verifies that a valid
+// Personal Access Token authenticates through the Authorizer even on a private
+// instance, and then passes the access check for an otherwise-gated method.
+func TestAuthorizerAccessTokenAlwaysWorksOnPrivateInstance(t *testing.T) {
+	ctx := context.Background()
+	ts := NewTestService(t)
+	defer ts.Cleanup()
+
+	user, err := ts.CreateRegularUser(ctx, "pat-user")
+	require.NoError(t, err)
+
+	token := auth.GeneratePersonalAccessToken()
+	require.NoError(t, ts.Store.AddUserPersonalAccessToken(ctx, user.ID, &storepb.PersonalAccessTokensUserSetting_PersonalAccessToken{
+		TokenId:     util.GenUUID(),
+		TokenHash:   auth.HashPersonalAccessToken(token),
+		Description: "authz test PAT",
+		CreatedAt:   timestamppb.Now(),
+	}))
+
+	// InstanceURL empty => private instance; the PAT must still authenticate.
+	authorizer := apiv1.NewAuthorizer(ts.Store, ts.Secret, &profile.Profile{InstanceURL: ""})
+
+	result := authorizer.Authenticate(ctx, "Bearer "+token)
+	require.NotNil(t, result)
+	require.NotNil(t, result.User)
+	require.Equal(t, user.ID, result.User.ID)
+
+	// An authenticated caller passes access control even for a gated method.
+	require.NoError(t, authorizer.CheckAccess(ctx, "/memos.api.v1.MemoService/ListMemos", result))
+}
diff --git a/server/router/api/v1/v1.go b/server/router/api/v1/v1.go
index 83c26fd2..378b41e8 100644
--- a/server/router/api/v1/v1.go
+++ b/server/router/api/v1/v1.go
@@ -63,32 +63,29 @@ func NewAPIV1Service(secret string, profile *profile.Profile, store *store.Store
 
 // RegisterGateway registers the gRPC-Gateway and Connect handlers with the given Echo instance.
 func (s *APIV1Service) RegisterGateway(ctx context.Context, echoServer *echo.Echo) error {
-	// Auth middleware for gRPC-Gateway - runs after routing, has access to method name.
-	// Uses the same PublicMethods config as the Connect AuthInterceptor.
-	authenticator := auth.NewAuthenticator(s.Store, s.Secret)
+	// Shared authorizer: one source of truth for authentication and anonymous-access
+	// policy, used by both the gRPC-Gateway middleware and the Connect interceptor.
+	authorizer := NewAuthorizer(s.Store, s.Secret, s.Profile)
 	gatewayAuthMiddleware := func(next runtime.HandlerFunc) runtime.HandlerFunc {
 		return func(w http.ResponseWriter, r *http.Request, pathParams map[string]string) {
 			ctx := r.Context()
 
-			// Get the RPC method name from context (set by grpc-gateway after routing)
+			// The RPC method name is set by grpc-gateway after routing. When it can't be
+			// determined, skip the policy check and let the service layer handle visibility.
 			rpcMethod, ok := runtime.RPCMethod(ctx)
-
-			// Extract credentials from HTTP headers
 			authHeader := r.Header.Get("Authorization")
 
-			result := authenticator.Authenticate(ctx, authHeader)
-
-			// Enforce authentication for non-public methods
-			// If rpcMethod cannot be determined, allow through, service layer will handle visibility checks
-			if result == nil && ok && !IsPublicMethod(rpcMethod) {
-				http.Error(w, `{"code": 16, "message": "authentication required"}`, http.StatusUnauthorized)
-				return
+			result := authorizer.Authenticate(ctx, authHeader)
+			if ok {
+				if err := authorizer.CheckAccess(ctx, rpcMethod, result); err != nil {
+					http.Error(w, `{"code": 16, "message": "authentication required"}`, http.StatusUnauthorized)
+					return
+				}
 			}
 
-			// Apply auth result to context (no-op when result is nil for public endpoints)
+			// Apply the identity to the context (no-op for permitted anonymous requests).
 			if result != nil {
-				ctx = auth.ApplyToContext(ctx, result)
-				r = r.WithContext(ctx)
+				r = r.WithContext(auth.ApplyToContext(ctx, result))
 			}
 
 			next(w, r, pathParams)
@@ -137,7 +134,7 @@ func (s *APIV1Service) RegisterGateway(ctx context.Context, echoServer *echo.Ech
 		NewMetadataInterceptor(), // Convert HTTP headers to gRPC metadata first
 		NewLoggingInterceptor(logStacktraces),
 		NewRecoveryInterceptor(logStacktraces),
-		NewAuthInterceptor(s.Store, s.Secret),
+		NewAuthInterceptor(authorizer),
 	)
 	connectMux := http.NewServeMux()
 	connectHandler := NewConnectServiceHandler(s)
diff --git a/server/router/fileserver/fileserver.go b/server/router/fileserver/fileserver.go
index c3cac654..63c4dc1f 100644
--- a/server/router/fileserver/fileserver.go
+++ b/server/router/fileserver/fileserver.go
@@ -167,6 +167,19 @@ func (s *FileServerService) serveAttachmentFile(c *echo.Context) error {
 // serveUserAvatar serves user avatar images.
 func (s *FileServerService) serveUserAvatar(c *echo.Context) error {
 	ctx := c.Request().Context()
+
+	// On a private instance (no InstanceURL), avatars are not exposed to anonymous
+	// visitors; a valid session, access token, or PAT is required.
+	if !s.Profile.AllowAnonymous() {
+		viewer, err := s.getCurrentUser(ctx, c)
+		if err != nil {
+			return echo.NewHTTPError(http.StatusInternalServerError, "failed to get current user").Wrap(err)
+		}
+		if viewer == nil {
+			return echo.NewHTTPError(http.StatusUnauthorized, "unauthorized access")
+		}
+	}
+
 	identifier := c.Param("identifier")
 
 	user, err := s.getUserByUsername(ctx, identifier)
@@ -658,7 +671,10 @@ func (s *FileServerService) checkAttachmentPermission(ctx context.Context, c *ec
 		return echo.NewHTTPError(http.StatusNotFound, "memo not found")
 	}
 
-	if memo.Visibility == store.Public {
+	// Public-visibility attachments are served to anonymous visitors only when the
+	// instance allows anonymous access. On a private instance (no InstanceURL), the
+	// request must still resolve to an authenticated user or a valid share token below.
+	if memo.Visibility == store.Public && s.Profile.AllowAnonymous() {
 		return nil
 	}
 
diff --git a/server/router/fileserver/fileserver_test.go b/server/router/fileserver/fileserver_test.go
index 3ffe8aac..6826f4d9 100644
--- a/server/router/fileserver/fileserver_test.go
+++ b/server/router/fileserver/fileserver_test.go
@@ -3,6 +3,7 @@ package fileserver
 import (
 	"bytes"
 	"context"
+	"encoding/base64"
 	"encoding/binary"
 	"fmt"
 	"hash/crc32"
@@ -13,14 +14,18 @@ import (
 	"net/http/httptest"
 	"strings"
 	"testing"
+	"time"
 
 	"github.com/labstack/echo/v5"
 	"github.com/stretchr/testify/require"
+	"google.golang.org/protobuf/types/known/timestamppb"
 
 	"github.com/usememos/memos/internal/markdown"
 	"github.com/usememos/memos/internal/profile"
 	"github.com/usememos/memos/internal/testutil"
+	"github.com/usememos/memos/internal/util"
 	apiv1 "github.com/usememos/memos/proto/gen/api/v1"
+	storepb "github.com/usememos/memos/proto/gen/store"
 	"github.com/usememos/memos/server/auth"
 	apiv1service "github.com/usememos/memos/server/router/api/v1"
 	"github.com/usememos/memos/store"
@@ -433,3 +438,164 @@ func newShareAttachmentTestServices(ctx context.Context, t *testing.T) (*apiv1se
 		testStore.Close()
 	}
 }
+
+// makePNGDataURI returns a minimal valid PNG encoded as a data URI, suitable for a
+// user avatar.
+func makePNGDataURI(t *testing.T) string {
+	t.Helper()
+	img := image.NewRGBA(image.Rect(0, 0, 1, 1))
+	img.Set(0, 0, color.RGBA{R: 1, G: 2, B: 3, A: 255})
+	var buf bytes.Buffer
+	require.NoError(t, png.Encode(&buf, img))
+	return "data:image/png;base64," + base64.StdEncoding.EncodeToString(buf.Bytes())
+}
+
+// TestServeAttachmentFile_PrivateInstanceDeniesAnonymous verifies that a public
+// memo's attachment is served to anonymous visitors on an open instance but denied
+// on a private instance (no InstanceURL configured).
+func TestServeAttachmentFile_PrivateInstanceDeniesAnonymous(t *testing.T) {
+	ctx := context.Background()
+	svc, fs, _, cleanup := newShareAttachmentTestServices(ctx, t)
+	defer cleanup()
+
+	creator, err := svc.Store.CreateUser(ctx, &store.User{
+		Username: "private-attachment-owner",
+		Role:     store.RoleUser,
+		Email:    "private-attachment-owner@example.com",
+	})
+	require.NoError(t, err)
+	creatorCtx := context.WithValue(ctx, auth.UserIDContextKey, creator.ID)
+
+	attachment, err := svc.CreateAttachment(creatorCtx, &apiv1.CreateAttachmentRequest{
+		Attachment: &apiv1.Attachment{
+			Filename: "public.txt",
+			Type:     "text/plain",
+			Content:  []byte("public content"),
+		},
+	})
+	require.NoError(t, err)
+
+	_, err = svc.CreateMemo(creatorCtx, &apiv1.CreateMemoRequest{
+		Memo: &apiv1.Memo{
+			Content:     "public memo",
+			Visibility:  apiv1.Visibility_PUBLIC,
+			Attachments: []*apiv1.Attachment{{Name: attachment.Name}},
+		},
+	})
+	require.NoError(t, err)
+
+	e := echo.New()
+	fs.RegisterRoutes(e)
+	url := fmt.Sprintf("/file/%s/%s", attachment.Name, attachment.Filename)
+
+	anonymousGet := func() int {
+		rec := httptest.NewRecorder()
+		e.ServeHTTP(rec, httptest.NewRequest(http.MethodGet, url, nil))
+		return rec.Code
+	}
+
+	// Open instance: anonymous access to a public memo's attachment is allowed.
+	fs.Profile.InstanceURL = "http://localhost:8080"
+	require.Equal(t, http.StatusOK, anonymousGet())
+
+	// Private instance: the same anonymous request is denied.
+	fs.Profile.InstanceURL = ""
+	require.Equal(t, http.StatusUnauthorized, anonymousGet())
+}
+
+// TestServeUserAvatar_PrivateInstanceRequiresAuth verifies that avatars are exposed
+// to anonymous visitors on an open instance but require authentication on a private
+// instance.
+func TestServeUserAvatar_PrivateInstanceRequiresAuth(t *testing.T) {
+	ctx := context.Background()
+	svc, fs, _, cleanup := newShareAttachmentTestServices(ctx, t)
+	defer cleanup()
+
+	_, err := svc.Store.CreateUser(ctx, &store.User{
+		Username:  "avatar-owner",
+		Role:      store.RoleUser,
+		Email:     "avatar-owner@example.com",
+		AvatarURL: makePNGDataURI(t),
+	})
+	require.NoError(t, err)
+
+	e := echo.New()
+	fs.RegisterRoutes(e)
+
+	anonymousGet := func() int {
+		rec := httptest.NewRecorder()
+		e.ServeHTTP(rec, httptest.NewRequest(http.MethodGet, "/file/users/avatar-owner/avatar", nil))
+		return rec.Code
+	}
+
+	// Open instance: anonymous avatar access is allowed.
+	fs.Profile.InstanceURL = "http://localhost:8080"
+	require.Equal(t, http.StatusOK, anonymousGet())
+
+	// Private instance: anonymous avatar access is denied.
+	fs.Profile.InstanceURL = ""
+	require.Equal(t, http.StatusUnauthorized, anonymousGet())
+}
+
+// TestServeAttachmentFile_RefreshCookieAuthenticatesOwner verifies that the file
+// server authenticates a request via the refresh-token cookie (the browser <img>
+// flow) — the AuthenticateToUser cookie fallback — letting the owner fetch their own
+// private memo's attachment without an Authorization header.
+func TestServeAttachmentFile_RefreshCookieAuthenticatesOwner(t *testing.T) {
+	ctx := context.Background()
+	svc, fs, _, cleanup := newShareAttachmentTestServices(ctx, t)
+	defer cleanup()
+
+	owner, err := svc.Store.CreateUser(ctx, &store.User{
+		Username: "cookie-owner",
+		Role:     store.RoleUser,
+		Email:    "cookie-owner@example.com",
+	})
+	require.NoError(t, err)
+	ownerCtx := context.WithValue(ctx, auth.UserIDContextKey, owner.ID)
+
+	attachment, err := svc.CreateAttachment(ownerCtx, &apiv1.CreateAttachmentRequest{
+		Attachment: &apiv1.Attachment{
+			Filename: "secret.txt",
+			Type:     "text/plain",
+			Content:  []byte("secret content"),
+		},
+	})
+	require.NoError(t, err)
+
+	_, err = svc.CreateMemo(ownerCtx, &apiv1.CreateMemoRequest{
+		Memo: &apiv1.Memo{
+			Content:     "private memo",
+			Visibility:  apiv1.Visibility_PRIVATE,
+			Attachments: []*apiv1.Attachment{{Name: attachment.Name}},
+		},
+	})
+	require.NoError(t, err)
+
+	// Mint a valid refresh token for the owner and store its record.
+	tokenID := util.GenUUID()
+	require.NoError(t, svc.Store.AddUserRefreshToken(ctx, owner.ID, &storepb.RefreshTokensUserSetting_RefreshToken{
+		TokenId:   tokenID,
+		ExpiresAt: timestamppb.New(time.Now().Add(auth.RefreshTokenDuration)),
+		CreatedAt: timestamppb.Now(),
+	}))
+	refreshToken, _, err := auth.GenerateRefreshToken(owner.ID, tokenID, []byte(svc.Secret))
+	require.NoError(t, err)
+
+	e := echo.New()
+	fs.RegisterRoutes(e)
+	url := fmt.Sprintf("/file/%s/%s", attachment.Name, attachment.Filename)
+
+	// Without credentials, the private attachment is denied.
+	rec := httptest.NewRecorder()
+	e.ServeHTTP(rec, httptest.NewRequest(http.MethodGet, url, nil))
+	require.Equal(t, http.StatusUnauthorized, rec.Code)
+
+	// With the refresh-token cookie, the owner is authenticated and served.
+	req := httptest.NewRequest(http.MethodGet, url, nil)
+	req.AddCookie(&http.Cookie{Name: auth.RefreshTokenCookieName, Value: refreshToken})
+	rec = httptest.NewRecorder()
+	e.ServeHTTP(rec, req)
+	require.Equal(t, http.StatusOK, rec.Code)
+	require.Equal(t, "secret content", rec.Body.String())
+}
diff --git a/web/src/contexts/AuthContext.tsx b/web/src/contexts/AuthContext.tsx
index f152567f..69ac0f44 100644
--- a/web/src/contexts/AuthContext.tsx
+++ b/web/src/contexts/AuthContext.tsx
@@ -30,6 +30,17 @@ interface AuthContextValue extends AuthState {
 
 const AuthContext = createContext<AuthContextValue | null>(null);
 
+/** Settled auth state for a request with no valid session (init finished, not loading). */
+const UNAUTHENTICATED_STATE: AuthState = {
+  currentUser: undefined,
+  userGeneralSetting: undefined,
+  userWebhooksSetting: undefined,
+  userTagsSetting: undefined,
+  shortcuts: [],
+  isInitialized: true,
+  isLoading: false,
+};
+
 export function AuthProvider({ children }: { children: ReactNode }) {
   const queryClient = useQueryClient();
   const [state, setState] = useState<AuthState>({
@@ -78,15 +89,7 @@ export function AuthProvider({ children }: { children: ReactNode }) {
     // If we still don't have a token after refresh attempt, skip getCurrentUser call
     // to avoid unnecessary network request for unauthenticated users.
     if (!getAccessToken()) {
-      setState({
-        currentUser: undefined,
-        userGeneralSetting: undefined,
-        userWebhooksSetting: undefined,
-        userTagsSetting: undefined,
-        shortcuts: [],
-        isInitialized: true,
-        isLoading: false,
-      });
+      setState(UNAUTHENTICATED_STATE);
       return;
     }
 
@@ -95,15 +98,7 @@ export function AuthProvider({ children }: { children: ReactNode }) {
 
       if (!currentUser) {
         clearAccessToken();
-        setState({
-          currentUser: undefined,
-          userGeneralSetting: undefined,
-          userWebhooksSetting: undefined,
-          userTagsSetting: undefined,
-          shortcuts: [],
-          isInitialized: true,
-          isLoading: false,
-        });
+        setState(UNAUTHENTICATED_STATE);
         return;
       }
 
@@ -122,15 +117,7 @@ export function AuthProvider({ children }: { children: ReactNode }) {
     } catch (error) {
       console.error("Failed to initialize auth:", error);
       clearAccessToken();
-      setState({
-        currentUser: undefined,
-        userGeneralSetting: undefined,
-        userWebhooksSetting: undefined,
-        userTagsSetting: undefined,
-        shortcuts: [],
-        isInitialized: true,
-        isLoading: false,
-      });
+      setState(UNAUTHENTICATED_STATE);
     }
   }, [fetchUserSettings, queryClient]);
 
@@ -141,15 +128,7 @@ export function AuthProvider({ children }: { children: ReactNode }) {
       console.error("[AuthContext] Failed to sign out:", error);
     } finally {
       clearAccessToken();
-      setState({
-        currentUser: undefined,
-        userGeneralSetting: undefined,
-        userWebhooksSetting: undefined,
-        userTagsSetting: undefined,
-        shortcuts: [],
-        isInitialized: true,
-        isLoading: false,
-      });
+      setState(UNAUTHENTICATED_STATE);
       queryClient.clear();
     }
   }, [queryClient]);
diff --git a/web/src/layouts/RootLayout.tsx b/web/src/layouts/RootLayout.tsx
index 02483b0e..25e52ef8 100644
--- a/web/src/layouts/RootLayout.tsx
+++ b/web/src/layouts/RootLayout.tsx
@@ -1,10 +1,12 @@
 import { useEffect, useRef } from "react";
-import { Outlet, useLocation, useSearchParams } from "react-router-dom";
+import { Navigate, Outlet, useLocation, useSearchParams } from "react-router-dom";
 import Navigation from "@/components/Navigation";
 import { useInstance } from "@/contexts/InstanceContext";
 import { useMemoFilterContext } from "@/contexts/MemoFilterContext";
+import useCurrentUser from "@/hooks/useCurrentUser";
 import useMediaQuery from "@/hooks/useMediaQuery";
 import { cn } from "@/lib/utils";
+import { buildAuthRoute, shouldGatePrivateInstance } from "@/utils/auth-redirect";
 import { useTranslate } from "@/utils/i18n";
 
 const MEMOS_DEPLOY_URL = "https://usememos.com/docs/deploy";
@@ -29,6 +31,7 @@ const RootLayout = () => {
   const location = useLocation();
   const [searchParams] = useSearchParams();
   const sm = useMediaQuery("sm");
+  const currentUser = useCurrentUser();
   const { profile } = useInstance();
   const { removeFilter } = useMemoFilterContext();
   const { pathname } = location;
@@ -45,6 +48,14 @@ const RootLayout = () => {
     prevPathnameRef.current = pathname;
   }, [pathname, searchParams, removeFilter]);
 
+  // Private instance (no InstanceURL configured): anonymous visitors may only reach
+  // share links; everything else redirects to the sign-in page, preserving the intended
+  // destination. Public instances keep the open Explore behavior for logged-out users.
+  if (shouldGatePrivateInstance({ isPrivateInstance: !profile.instanceUrl, isAuthenticated: !!currentUser, pathname })) {
+    const redirect = `${pathname}${location.search}${location.hash}`;
+    return <Navigate to={buildAuthRoute({ redirect })} replace />;
+  }
+
   return (
     <div className="w-full min-h-full flex flex-row justify-center items-start sm:pl-16">
       {sm && (
diff --git a/web/src/utils/auth-redirect.ts b/web/src/utils/auth-redirect.ts
index c7f63a0f..9d195342 100644
--- a/web/src/utils/auth-redirect.ts
+++ b/web/src/utils/auth-redirect.ts
@@ -12,6 +12,7 @@ export {
   buildAuthRoute,
   getSafeRedirectPath,
   isPublicRoute,
+  shouldGatePrivateInstance,
 } from "./redirect-safety";
 
 /**
diff --git a/web/src/utils/redirect-safety.ts b/web/src/utils/redirect-safety.ts
index 48fb134f..3a88c417 100644
--- a/web/src/utils/redirect-safety.ts
+++ b/web/src/utils/redirect-safety.ts
@@ -71,3 +71,19 @@ const PUBLIC_ROUTE_PREFIXES = [
 export function isPublicRoute(path: string): boolean {
   return PUBLIC_ROUTE_PREFIXES.some((route) => path.startsWith(route));
 }
+
+/**
+ * Reports whether an anonymous visitor to a private instance should be redirected
+ * to the sign-in page for the given path.
+ *
+ * A private instance (no configured instance URL) hides everything from anonymous
+ * visitors except share-link pages, which stay accessible so public shares keep
+ * working. Authenticated visitors and open instances are never gated.
+ */
+export function shouldGatePrivateInstance(params: { isPrivateInstance: boolean; isAuthenticated: boolean; pathname: string }): boolean {
+  const { isPrivateInstance, isAuthenticated, pathname } = params;
+  if (!isPrivateInstance || isAuthenticated) {
+    return false;
+  }
+  return !pathname.startsWith(`${ROUTES.SHARED_MEMO}/`);
+}
diff --git a/web/tests/redirect-safety.test.ts b/web/tests/redirect-safety.test.ts
index fc1e0a57..a6c803dd 100644
--- a/web/tests/redirect-safety.test.ts
+++ b/web/tests/redirect-safety.test.ts
@@ -1,5 +1,11 @@
 import { describe, expect, it } from "vitest";
-import { AUTH_REDIRECT_PARAM, buildAuthRoute, getSafeRedirectPath, isPublicRoute } from "@/utils/redirect-safety";
+import {
+  AUTH_REDIRECT_PARAM,
+  buildAuthRoute,
+  getSafeRedirectPath,
+  isPublicRoute,
+  shouldGatePrivateInstance,
+} from "@/utils/redirect-safety";
 
 describe("getSafeRedirectPath", () => {
   it("accepts safe same-origin internal paths", () => {
@@ -75,3 +81,24 @@ describe("isPublicRoute", () => {
     expect(isPublicRoute("/archived")).toBe(false);
   });
 });
+
+describe("shouldGatePrivateInstance", () => {
+  it("never gates on an open (public) instance", () => {
+    expect(shouldGatePrivateInstance({ isPrivateInstance: false, isAuthenticated: false, pathname: "/" })).toBe(false);
+    expect(shouldGatePrivateInstance({ isPrivateInstance: false, isAuthenticated: false, pathname: "/explore" })).toBe(false);
+  });
+
+  it("never gates an authenticated visitor", () => {
+    expect(shouldGatePrivateInstance({ isPrivateInstance: true, isAuthenticated: true, pathname: "/explore" })).toBe(false);
+  });
+
+  it("gates anonymous visitors to non-share pages on a private instance", () => {
+    for (const pathname of ["/", "/explore", "/about", "/memos/abc", "/u/steven", "/setting"]) {
+      expect(shouldGatePrivateInstance({ isPrivateInstance: true, isAuthenticated: false, pathname })).toBe(true);
+    }
+  });
+
+  it("keeps share links reachable for anonymous visitors on a private instance", () => {
+    expect(shouldGatePrivateInstance({ isPrivateInstance: true, isAuthenticated: false, pathname: "/memos/shares/token123" })).toBe(false);
+  });
+});