modelcontextprotocol · arimu1 · Aug 1, 2026
diff --git a/mcp/protocol.go b/mcp/protocol.go
@@ -1192,7 +1192,14 @@ func (c Cacheable) GetTTLMs() int { return c.TTLMs }
 func (c Cacheable) GetCacheScope() string { return c.CacheScope }
 
 // setDefaultCacheableValues sets the default values for the cacheable fields.
-func (c *Cacheable) setDefaultCacheableValues() {
+// If defaults is non-nil, its TTLMs and CacheScope are copied onto c.
+// Otherwise CacheScope is set to "public" (TTLMs stays 0 unless already set).
+func (c *Cacheable) setDefaultCacheableValues(defaults *Cacheable) {
+	if defaults != nil {
+		c.TTLMs = defaults.TTLMs
+		c.CacheScope = defaults.CacheScope
+		return
+	}
 	c.CacheScope = "public"
 }
 

diff --git a/mcp/server.go b/mcp/server.go
@@ -169,6 +169,15 @@ type ServerOptions struct {
 	// GetSessionID is not consulted when [StreamableHTTPOptions.Stateless] is
 	// true, since stateless servers do not maintain sessions.
 	GetSessionID func() string
+
+	// DefaultCacheable, if non-nil, supplies the [Cacheable] values stamped on
+	// SDK-generated results (server/discover, list methods, and resources/read
+	// after the handler returns). If nil, those results use the historical
+	// defaults: CacheScope "public" and TTLMs 0.
+	//
+	// Receiving middleware can still overwrite Cacheable on a per-result basis
+	// after the SDK stamps these values.
+	DefaultCacheable *Cacheable
 }
 
 // NewServer creates a new MCP server. The resulting server has no features:
@@ -851,7 +860,7 @@ func (s *Server) listPrompts(_ context.Context, req *ListPromptsRequest) (*ListP
 	if err != nil {
 		return nil, err
 	}
-	res.setDefaultCacheableValues()
+	res.setDefaultCacheableValues(s.opts.DefaultCacheable)
 	return res, nil
 }
 
@@ -905,7 +914,7 @@ func (s *Server) discover(_ context.Context, req *ServerRequest[*DiscoverParams]
 		Capabilities:      s.capabilities(),
 		Instructions:      s.opts.Instructions,
 	}
-	res.setDefaultCacheableValues()
+	res.setDefaultCacheableValues(s.opts.DefaultCacheable)
 	return res, nil
 }
 
@@ -941,7 +950,7 @@ func (s *Server) listTools(_ context.Context, req *ListToolsRequest) (*ListTools
 	if err != nil {
 		return nil, err
 	}
-	res.setDefaultCacheableValues()
+	res.setDefaultCacheableValues(s.opts.DefaultCacheable)
 	return res, nil
 }
 
@@ -989,7 +998,7 @@ func (s *Server) listResources(_ context.Context, req *ListResourcesRequest) (*L
 	if err != nil {
 		return nil, err
 	}
-	res.setDefaultCacheableValues()
+	res.setDefaultCacheableValues(s.opts.DefaultCacheable)
 	return res, nil
 }
 
@@ -1009,7 +1018,7 @@ func (s *Server) listResourceTemplates(_ context.Context, req *ListResourceTempl
 	if err != nil {
 		return nil, err
 	}
-	res.setDefaultCacheableValues()
+	res.setDefaultCacheableValues(s.opts.DefaultCacheable)
 	return res, nil
 }
 
@@ -1030,7 +1039,7 @@ func (s *Server) readResource(ctx context.Context, req *ReadResourceRequest) (*R
 	if err := handleMultiRoundTripResult(req.Session, s.opts.Logger, res); err != nil {
 		return nil, err
 	}
-	res.setDefaultCacheableValues()
+	res.setDefaultCacheableValues(s.opts.DefaultCacheable)
 	if res.resultType == resultTypeInputRequired {
 		return res, nil
 	}

diff --git a/mcp/server_test.go b/mcp/server_test.go
@@ -1691,3 +1691,105 @@ func TestServerSession_RejectsServerInitiated(t *testing.T) {
 		}
 	}
 }
+
+func TestServerDefaultCacheable(t *testing.T) {
+	ctx := context.Background()
+
+	for _, tc := range []struct {
+		name     string
+		defaults *Cacheable
+		want     Cacheable
+	}{
+		{
+			name: "historical defaults",
+			want: Cacheable{TTLMs: 0, CacheScope: "public"},
+		},
+		{
+			name:     "private with TTL",
+			defaults: &Cacheable{TTLMs: 60_000, CacheScope: "private"},
+			want:     Cacheable{TTLMs: 60_000, CacheScope: "private"},
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			server := NewServer(testImpl, &ServerOptions{DefaultCacheable: tc.defaults})
+			AddTool(server, &Tool{Name: "t", Description: "d"},
+				func(context.Context, *CallToolRequest, struct{}) (*CallToolResult, any, error) {
+					return &CallToolResult{Content: []Content{&TextContent{Text: "ok"}}}, nil, nil
+				})
+			server.AddPrompt(&Prompt{Name: "p"}, func(context.Context, *GetPromptRequest) (*GetPromptResult, error) {
+				return &GetPromptResult{}, nil
+			})
+			server.AddResource(&Resource{URI: "test://r", Name: "r"}, func(context.Context, *ReadResourceRequest) (*ReadResourceResult, error) {
+				return &ReadResourceResult{Contents: []*ResourceContents{{URI: "test://r", Text: "x"}}}, nil
+			})
+			server.AddResourceTemplate(&ResourceTemplate{URITemplate: "test://{id}", Name: "rt"},
+				func(context.Context, *ReadResourceRequest) (*ReadResourceResult, error) {
+					return &ReadResourceResult{Contents: []*ResourceContents{{Text: "x"}}}, nil
+				})
+
+			ct, st := NewInMemoryTransports()
+			if _, err := server.Connect(ctx, st, nil); err != nil {
+				t.Fatal(err)
+			}
+			cs, err := NewClient(testImpl, nil).Connect(ctx, ct, nil)
+			if err != nil {
+				t.Fatal(err)
+			}
+			t.Cleanup(func() { _ = cs.Close() })
+
+			check := func(t *testing.T, label string, got Cacheable) {
+				t.Helper()
+				if got != tc.want {
+					t.Errorf("%s Cacheable = %+v, want %+v", label, got, tc.want)
+				}
+			}
+
+			tools, err := cs.ListTools(ctx, nil)
+			if err != nil {
+				t.Fatal(err)
+			}
+			check(t, "ListTools", tools.Cacheable)
+
+			prompts, err := cs.ListPrompts(ctx, nil)
+			if err != nil {
+				t.Fatal(err)
+			}
+			check(t, "ListPrompts", prompts.Cacheable)
+
+			resources, err := cs.ListResources(ctx, nil)
+			if err != nil {
+				t.Fatal(err)
+			}
+			check(t, "ListResources", resources.Cacheable)
+
+			templates, err := cs.ListResourceTemplates(ctx, nil)
+			if err != nil {
+				t.Fatal(err)
+			}
+			check(t, "ListResourceTemplates", templates.Cacheable)
+
+			read, err := cs.ReadResource(ctx, &ReadResourceParams{URI: "test://r"})
+			if err != nil {
+				t.Fatal(err)
+			}
+			check(t, "ReadResource", read.Cacheable)
+		})
+	}
+}
+
+func TestSetDefaultCacheableValues(t *testing.T) {
+	t.Run("nil defaults", func(t *testing.T) {
+		c := Cacheable{TTLMs: 42}
+		c.setDefaultCacheableValues(nil)
+		if c.TTLMs != 42 || c.CacheScope != "public" {
+			t.Fatalf("got %+v, want TTLMs preserved and CacheScope public", c)
+		}
+	})
+	t.Run("explicit defaults", func(t *testing.T) {
+		c := Cacheable{TTLMs: 1, CacheScope: "public"}
+		c.setDefaultCacheableValues(&Cacheable{TTLMs: 60_000, CacheScope: "private"})
+		if c.TTLMs != 60_000 || c.CacheScope != "private" {
+			t.Fatalf("got %+v, want private/60000", c)
+		}
+	})
+}