@@ -103,7 +103,7 @@ func createGitHubClients(cfg github.MCPServerConfig, apiHost utils.APIHostResolv
 		if cfg.RepoAccessTTL != nil {
 			opts = append(opts, lockdown.WithTTL(*cfg.RepoAccessTTL))
 		}
-		repoAccessCache = lockdown.GetInstance(gqlClient, restClient, opts...)
+		repoAccessCache = lockdown.NewRepoAccessCache(gqlClient, restClient, opts...)
 	}
 
 	return &githubClients{
 
@@ -399,7 +399,7 @@ func (d *RequestDeps) GetRepoAccessCache(ctx context.Context) (*lockdown.RepoAcc
 	}
 
 	// Create repo access cache
-	instance := lockdown.GetInstance(gqlClient, restClient, d.RepoAccessOpts...)
+	instance := lockdown.NewRepoAccessCache(gqlClient, restClient, d.RepoAccessOpts...)
 	return instance, nil
 }
 
 
@@ -70,16 +70,15 @@ func (rt *repoAccessMockTransport) RoundTrip(req *http.Request) (*http.Response,
 		value = repoAccessValue{isPrivate: false}
 	}
 
-	responseBody, err := json.Marshal(map[string]any{
-		"data": map[string]any{
-			"viewer": map[string]any{
-				"login": "test-viewer",
-			},
-			"repository": map[string]any{
-				"isPrivate": value.isPrivate,
-			},
-		},
-	})
+	data := map[string]any{}
+	if strings.Contains(payload.Query, "viewer") {
+		data["viewer"] = map[string]any{"login": "test-viewer"}
+	}
+	if strings.Contains(payload.Query, "repository") {
+		data["repository"] = map[string]any{"isPrivate": value.isPrivate}
+	}
+
+	responseBody, err := json.Marshal(map[string]any{"data": data})
 	if err != nil {
 		return nil, err
 	}
 
@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"log/slog"
+	"maps"
 	"strings"
 	"sync"
 	"time"
@@ -15,39 +16,36 @@ import (
 
 // RepoAccessCache caches repository metadata related to lockdown checks so that
 // multiple tools can reuse the same access information safely across goroutines.
+// In HTTP mode each request must construct its own instance so viewer-scoped
+// lookups run under the requesting user's credentials.
 type RepoAccessCache struct {
 	client           *githubv4.Client
 	restClient       *github.Client
-	mu               sync.Mutex
 	cache            *cache2go.CacheTable
 	ttl              time.Duration
 	logger           *slog.Logger
 	trustedBotLogins map[string]struct{}
+
+	viewerMu    sync.Mutex
+	viewerLogin string
 }
 
 type repoAccessCacheEntry struct {
-	isPrivate   bool
-	knownUsers  map[string]bool // normalized login -> has push access
-	viewerLogin string
+	isPrivate  bool
+	knownUsers map[string]bool // normalized login -> has push access
 }
 
 // RepoAccessInfo captures repository metadata needed for lockdown decisions.
 type RepoAccessInfo struct {
 	IsPrivate     bool
 	HasPushAccess bool
-	ViewerLogin   string
 }
 
 const (
 	defaultRepoAccessTTL      = 20 * time.Minute
 	defaultRepoAccessCacheKey = "repo-access-cache"
 )
 
-var (
-	instance   *RepoAccessCache
-	instanceMu sync.Mutex
-)
-
 // RepoAccessOption configures RepoAccessCache at construction time.
 type RepoAccessOption func(*RepoAccessCache)
 
@@ -66,8 +64,8 @@ func WithLogger(logger *slog.Logger) RepoAccessOption {
 	}
 }
 
-// WithCacheName overrides the cache table name used for storing entries. This option is intended for tests
-// that need isolated cache instances.
+// WithCacheName overrides the cache table name used for storing entries.
+// Use this to isolate cache entries between tenants or in tests.
 func WithCacheName(name string) RepoAccessOption {
 	return func(c *RepoAccessCache) {
 		if name != "" {
@@ -76,25 +74,8 @@ func WithCacheName(name string) RepoAccessOption {
 	}
 }
 
-// GetInstance returns the singleton instance of RepoAccessCache.
-// It initializes the instance on first call with the provided client and options.
-// Subsequent calls ignore the client and options parameters and return the existing instance.
-// This is the preferred way to access the cache in production code.
-func GetInstance(client *githubv4.Client, restClient *github.Client, opts ...RepoAccessOption) *RepoAccessCache {
-	instanceMu.Lock()
-	defer instanceMu.Unlock()
-	if instance == nil {
-		instance = newRepoAccessCache(client, restClient, opts...)
-	}
-	return instance
-}
-
-// NewRepoAccessCache creates a standalone cache instance, used for tests.
+// NewRepoAccessCache creates a RepoAccessCache bound to the supplied clients.
 func NewRepoAccessCache(client *githubv4.Client, restClient *github.Client, opts ...RepoAccessOption) *RepoAccessCache {
-	return newRepoAccessCache(client, restClient, opts...)
-}
-
-func newRepoAccessCache(client *githubv4.Client, restClient *github.Client, opts ...RepoAccessOption) *RepoAccessCache {
 	c := &RepoAccessCache{
 		client:     client,
 		restClient: restClient,
@@ -113,13 +94,6 @@ func newRepoAccessCache(client *githubv4.Client, restClient *github.Client, opts
 	return c
 }
 
-// SetLogger updates the logger used for cache diagnostics.
-func (c *RepoAccessCache) SetLogger(logger *slog.Logger) {
-	c.mu.Lock()
-	c.logger = logger
-	c.mu.Unlock()
-}
-
 // CacheStats summarizes cache activity counters.
 type CacheStats struct {
 	Hits      int64
@@ -150,10 +124,55 @@ func (c *RepoAccessCache) IsSafeContent(ctx context.Context, username, owner, re
 	c.logDebug(ctx, fmt.Sprintf("evaluated repo access for user %s to %s/%s for content filtering, result: hasPushAccess=%t, isPrivate=%t",
 		username, owner, repo, repoInfo.HasPushAccess, repoInfo.IsPrivate))
 
-	if repoInfo.IsPrivate || repoInfo.ViewerLogin == strings.ToLower(username) {
+	if repoInfo.IsPrivate {
+		return true, nil
+	}
+	if repoInfo.HasPushAccess {
 		return true, nil
 	}
-	return repoInfo.HasPushAccess, nil
+
+	viewerLogin, err := c.viewerLoginFor(ctx)
+	if err != nil {
+		return false, err
+	}
+	return viewerLogin == strings.ToLower(username), nil
+}
+
+func (c *RepoAccessCache) viewerLoginFor(ctx context.Context) (string, error) {
+	c.viewerMu.Lock()
+	defer c.viewerMu.Unlock()
+	if c.viewerLogin != "" {
+		return c.viewerLogin, nil
+	}
+	if c.client == nil {
+		return "", fmt.Errorf("nil GraphQL client")
+	}
+	var query struct {
+		Viewer struct {
+			Login githubv4.String
+		}
+	}
+	if err := c.client.Query(ctx, &query, nil); err != nil {
+		return "", fmt.Errorf("failed to query viewer login: %w", err)
+	}
+	login := strings.ToLower(string(query.Viewer.Login))
+	if login == "" {
+		return "", fmt.Errorf("viewer login returned empty")
+	}
+	c.viewerLogin = login
+	return c.viewerLogin, nil
+}
+
+// setViewerLogin seeds the cached viewer login from a piggy-backed query response.
+func (c *RepoAccessCache) setViewerLogin(login string) {
+	if login == "" {
+		return
+	}
+	c.viewerMu.Lock()
+	defer c.viewerMu.Unlock()
+	if c.viewerLogin == "" {
+		c.viewerLogin = strings.ToLower(login)
+	}
 }
 
 func (c *RepoAccessCache) getRepoAccessInfo(ctx context.Context, username, owner, repo string) (RepoAccessInfo, error) {
@@ -163,19 +182,16 @@ func (c *RepoAccessCache) getRepoAccessInfo(ctx context.Context, username, owner
 
 	key := cacheKey(owner, repo)
 	userKey := strings.ToLower(username)
-	c.mu.Lock()
-	defer c.mu.Unlock()
 
-	// Try to get entry from cache - this will keep the item alive if it exists
-	cacheItem, err := c.cache.Value(key)
-	if err == nil {
+	// Entries are immutable once added: the cache table is shared across instances,
+	// so we publish a fresh entry with a cloned knownUsers map on every miss.
+	if cacheItem, err := c.cache.Value(key); err == nil {
 		entry := cacheItem.Data().(*repoAccessCacheEntry)
 		if cachedHasPush, known := entry.knownUsers[userKey]; known {
 			c.logDebug(ctx, fmt.Sprintf("repo access cache hit for user %s to %s/%s", username, owner, repo))
 			return RepoAccessInfo{
 				IsPrivate:     entry.isPrivate,
 				HasPushAccess: cachedHasPush,
-				ViewerLogin:   entry.viewerLogin,
 			}, nil
 		}
 
@@ -186,41 +202,48 @@ func (c *RepoAccessCache) getRepoAccessInfo(ctx context.Context, username, owner
 			return RepoAccessInfo{}, pushErr
 		}
 
-		entry.knownUsers[userKey] = hasPush
-		c.cache.Add(key, c.ttl, entry)
+		users := make(map[string]bool, len(entry.knownUsers)+1)
+		maps.Copy(users, entry.knownUsers)
+		users[userKey] = hasPush
+		c.cache.Add(key, c.ttl, &repoAccessCacheEntry{
+			isPrivate:  entry.isPrivate,
+			knownUsers: users,
+		})
 
 		return RepoAccessInfo{
 			IsPrivate:     entry.isPrivate,
 			HasPushAccess: hasPush,
-			ViewerLogin:   entry.viewerLogin,
 		}, nil
 	}
 
 	c.logDebug(ctx, fmt.Sprintf("repo access cache miss for user %s to %s/%s", username, owner, repo))
 
-	info, queryErr := c.queryRepoAccessInfo(ctx, username, owner, repo)
+	isPrivate, viewerLogin, queryErr := c.queryRepoAccessInfo(ctx, owner, repo)
 	if queryErr != nil {
 		return RepoAccessInfo{}, queryErr
 	}
+	c.setViewerLogin(viewerLogin)
 
-	// Create new entry
-	entry := &repoAccessCacheEntry{
-		knownUsers:  map[string]bool{userKey: info.HasPushAccess},
-		isPrivate:   info.IsPrivate,
-		viewerLogin: info.ViewerLogin,
+	hasPush, pushErr := c.checkPushAccess(ctx, username, owner, repo)
+	if pushErr != nil {
+		return RepoAccessInfo{}, pushErr
 	}
-	c.cache.Add(key, c.ttl, entry)
+
+	c.cache.Add(key, c.ttl, &repoAccessCacheEntry{
+		knownUsers: map[string]bool{userKey: hasPush},
+		isPrivate:  isPrivate,
+	})
 
 	return RepoAccessInfo{
-		IsPrivate:     entry.isPrivate,
-		HasPushAccess: entry.knownUsers[userKey],
-		ViewerLogin:   entry.viewerLogin,
+		IsPrivate:     isPrivate,
+		HasPushAccess: hasPush,
 	}, nil
 }
 
-func (c *RepoAccessCache) queryRepoAccessInfo(ctx context.Context, username, owner, repo string) (RepoAccessInfo, error) {
+// queryRepoAccessInfo fetches repository visibility and the viewer login in a single GraphQL round-trip.
+func (c *RepoAccessCache) queryRepoAccessInfo(ctx context.Context, owner, repo string) (bool, string, error) {
 	if c.client == nil {
-		return RepoAccessInfo{}, fmt.Errorf("nil GraphQL client")
+		return false, "", fmt.Errorf("nil GraphQL client")
 	}
 
 	var query struct {
@@ -238,22 +261,12 @@ func (c *RepoAccessCache) queryRepoAccessInfo(ctx context.Context, username, own
 	}
 
 	if err := c.client.Query(ctx, &query, variables); err != nil {
-		return RepoAccessInfo{}, fmt.Errorf("failed to query repository metadata: %w", err)
-	}
-
-	hasPush, err := c.checkPushAccess(ctx, username, owner, repo)
-	if err != nil {
-		return RepoAccessInfo{}, err
+		return false, "", fmt.Errorf("failed to query repository metadata: %w", err)
 	}
 
-	c.logDebug(ctx, fmt.Sprintf("queried repo access info for user %s to %s/%s: isPrivate=%t, hasPushAccess=%t, viewerLogin=%s",
-		username, owner, repo, bool(query.Repository.IsPrivate), hasPush, query.Viewer.Login))
+	c.logDebug(ctx, fmt.Sprintf("queried repo access info for %s/%s: isPrivate=%t", owner, repo, bool(query.Repository.IsPrivate)))
 
-	return RepoAccessInfo{
-		IsPrivate:     bool(query.Repository.IsPrivate),
-		HasPushAccess: hasPush,
-		ViewerLogin:   string(query.Viewer.Login),
-	}, nil
+	return bool(query.Repository.IsPrivate), string(query.Viewer.Login), nil
 }
 
 // checkPushAccess checks if the user has push access to the repository via the REST permission endpoint.
Original file line number	Diff line number	Diff line change
`@@ -103,7 +103,7 @@ func createGitHubClients(cfg github.MCPServerConfig, apiHost utils.APIHostResolv`
`103`	`103`	`if cfg.RepoAccessTTL != nil {`
`104`	`104`	`opts = append(opts, lockdown.WithTTL(*cfg.RepoAccessTTL))`
`105`	`105`	`}`
`106`		`- repoAccessCache = lockdown.GetInstance(gqlClient, restClient, opts...)`
	`106`	`+ repoAccessCache = lockdown.NewRepoAccessCache(gqlClient, restClient, opts...)`
`107`	`107`	`}`
`108`	`108`
`109`	`109`	`return &githubClients{`
Original file line number	Diff line number	Diff line change
`@@ -399,7 +399,7 @@ func (d RequestDeps) GetRepoAccessCache(ctx context.Context) (lockdown.RepoAcc`
`399`	`399`	`}`
`400`	`400`
`401`	`401`	`// Create repo access cache`
`402`		`- instance := lockdown.GetInstance(gqlClient, restClient, d.RepoAccessOpts...)`
	`402`	`+ instance := lockdown.NewRepoAccessCache(gqlClient, restClient, d.RepoAccessOpts...)`
`403`	`403`	`return instance, nil`
`404`	`404`	`}`
`405`	`405`