Add Opt-In ttlMs / cacheScope Cache Hints to List and Read Results per SEP-2549

README.md

@@ -1850,6 +1850,40 @@ tools = client.tools # => Array<MCP::Client::Tool> of every tool on the server.
 Use these when you want the complete list; use `list_tools(cursor:)` etc. when you need
 fine-grained iteration (e.g. to stream-process pages without loading everything into memory).
 
+#### List Result Caching (`ttlMs` / `cacheScope`)
+
+Per SEP-2549, list and read results can carry cache hints telling clients how long a result stays fresh (`ttlMs`, max-age semantics in milliseconds;
+`0` means do not cache) and whether shared intermediaries may cache it (`cacheScope`: `"public"` or `"private"`).
+
+Emission is opt-in: pass `ttl_ms:` and/or `cache_scope:` to `MCP::Server.new` and both fields are added to `tools/list`, `prompts/list`, `resources/list`,
+`resources/templates/list`, and `resources/read` results (a missing field is filled with the defaults `ttlMs: 0` / `cacheScope: "public"`).
+When neither is set, responses are serialized exactly as before.
+
+```ruby
+server = MCP::Server.new(
+  name: "my_server",
+  tools: tools,
+  ttl_ms: 60_000,        # results stay fresh for one minute
+  cache_scope: "private", # only the requesting client may cache them
+)
+```
+
+A `resources_read_handler` can override the hints per result by returning a full result hash instead of bare contents:
+
+```ruby
+server.resources_read_handler do |params|
+  { contents: [{ uri: params[:uri], mimeType: "text/plain", text: "..." }], ttlMs: 5_000 }
+end
+```
+
+On the client, the values are surfaced on the paginated result structs as `ttl_ms` and `cache_scope`:
+
+```ruby
+page = client.list_tools
+page.ttl_ms      # => 60000 (nil when the server sent no hint)
+page.cache_scope # => "private"
+```
+
 ### Advanced
 
 #### Custom Methods

lib/mcp/client.rb

@@ -134,7 +134,13 @@ def list_tools(cursor: nil, meta: nil, cancellation: nil)
         )
       end
 
-      ListToolsResult.new(tools: tools, next_cursor: result["nextCursor"], meta: result["_meta"])
+      ListToolsResult.new(
+        tools: tools,
+        next_cursor: result["nextCursor"],
+        meta: result["_meta"],
+        ttl_ms: result["ttlMs"],
+        cache_scope: result["cacheScope"],
+      )
     end
 
     # Returns every tool available on the server. Iterates through all pages automatically
@@ -175,6 +181,8 @@ def list_resources(cursor: nil, meta: nil, cancellation: nil)
         resources: result["resources"] || [],
         next_cursor: result["nextCursor"],
         meta: result["_meta"],
+        ttl_ms: result["ttlMs"],
+        cache_scope: result["cacheScope"],
       )
     end
 
@@ -208,6 +216,8 @@ def list_resource_templates(cursor: nil, meta: nil, cancellation: nil)
         resource_templates: result["resourceTemplates"] || [],
         next_cursor: result["nextCursor"],
         meta: result["_meta"],
+        ttl_ms: result["ttlMs"],
+        cache_scope: result["cacheScope"],
       )
     end
 
@@ -241,6 +251,8 @@ def list_prompts(cursor: nil, meta: nil, cancellation: nil)
         prompts: result["prompts"] || [],
         next_cursor: result["nextCursor"],
         meta: result["_meta"],
+        ttl_ms: result["ttlMs"],
+        cache_scope: result["cacheScope"],
       )
     end
 

lib/mcp/client/paginated_result.rb

@@ -4,10 +4,12 @@ module MCP
   class Client
     # Result objects returned by `list_tools`, `list_prompts`, `list_resources`, and `list_resource_templates`.
     # Each carries the page items, an optional opaque `next_cursor` string for continuing pagination,
-    # and an optional `meta` hash mirroring the MCP `_meta` response field.
-    ListToolsResult = Struct.new(:tools, :next_cursor, :meta, keyword_init: true)
-    ListPromptsResult = Struct.new(:prompts, :next_cursor, :meta, keyword_init: true)
-    ListResourcesResult = Struct.new(:resources, :next_cursor, :meta, keyword_init: true)
-    ListResourceTemplatesResult = Struct.new(:resource_templates, :next_cursor, :meta, keyword_init: true)
+    # an optional `meta` hash mirroring the MCP `_meta` response field, and the optional SEP-2549
+    # cache hints `ttl_ms` (freshness lifetime in milliseconds; 0 means do not cache) and
+    # `cache_scope` (`"public"` or `"private"`) mirroring the `ttlMs`/`cacheScope` response fields.
+    ListToolsResult = Struct.new(:tools, :next_cursor, :meta, :ttl_ms, :cache_scope, keyword_init: true)
+    ListPromptsResult = Struct.new(:prompts, :next_cursor, :meta, :ttl_ms, :cache_scope, keyword_init: true)
+    ListResourcesResult = Struct.new(:resources, :next_cursor, :meta, :ttl_ms, :cache_scope, keyword_init: true)
+    ListResourceTemplatesResult = Struct.new(:resource_templates, :next_cursor, :meta, :ttl_ms, :cache_scope, keyword_init: true)
   end
 end

lib/mcp/server.rb

@@ -102,8 +102,11 @@ class ValidationError < StandardError; end
     include Instrumentation
     include Pagination
 
+    # Allowed values for the SEP-2549 `cacheScope` cache hint.
+    CACHE_SCOPES = ["public", "private"].freeze
+
     attr_accessor :description, :icons, :name, :title, :version, :website_url, :instructions, :tools, :prompts, :resources, :server_context, :configuration, :capabilities, :transport, :logging_message_notification
-    attr_reader :page_size, :client_capabilities
+    attr_reader :page_size, :client_capabilities, :ttl_ms, :cache_scope
 
     def initialize(
       description: nil,
@@ -121,6 +124,8 @@ def initialize(
       configuration: nil,
       capabilities: nil,
       page_size: nil,
+      ttl_ms: nil,
+      cache_scope: nil,
       transport: nil
     )
       @description = description
@@ -138,6 +143,8 @@ def initialize(
       @resource_index = index_resources_by_uri(resources)
       @server_context = server_context
       self.page_size = page_size
+      self.ttl_ms = ttl_ms
+      self.cache_scope = cache_scope
       @configuration = MCP.configuration.merge(configuration)
       @client = nil
 
@@ -232,6 +239,27 @@ def page_size=(page_size)
       @page_size = page_size
     end
 
+    # SEP-2549 cache hint: freshness lifetime in milliseconds for list and read results
+    # (max-age semantics; 0 means do not cache). Emission is opt-in: when both `ttl_ms`
+    # and `cache_scope` are nil, results are serialized exactly as before.
+    def ttl_ms=(ttl_ms)
+      unless ttl_ms.nil? || (ttl_ms.is_a?(Integer) && ttl_ms >= 0)
+        raise ArgumentError, "ttl_ms must be nil or a non-negative integer"
+      end
+
+      @ttl_ms = ttl_ms
+    end
+
+    # SEP-2549 cache hint: whether shared intermediaries may cache the result ("public")
+    # or only the requesting client ("private").
+    def cache_scope=(cache_scope)
+      unless cache_scope.nil? || CACHE_SCOPES.include?(cache_scope)
+        raise ArgumentError, "cache_scope must be nil, \"public\", or \"private\""
+      end
+
+      @cache_scope = cache_scope
+    end
+
     def notify_tools_list_changed
       return unless @transport
 
@@ -472,7 +500,7 @@ def handle_request(request, method, session: nil, related_request_id: nil)
           when Methods::INITIALIZE
             init(params, session: session)
           when Methods::RESOURCES_READ
-            { contents: read_resource_contents(params, session: session, related_request_id: related_request_id, cancellation: cancellation) }
+            build_read_resource_result(read_resource_contents(params, session: session, related_request_id: related_request_id, cancellation: cancellation))
           when Methods::RESOURCES_SUBSCRIBE, Methods::RESOURCES_UNSUBSCRIBE
             dispatch_optional_context_handler(@handlers[method], params, session: session, related_request_id: related_request_id, cancellation: cancellation)
             {}
@@ -611,7 +639,7 @@ def configure_logging_level(request, session: nil)
     def list_tools(request)
       page = paginate(@tools.values, cursor: cursor_from(request), page_size: @page_size, request: request, &:to_h)
 
-      { tools: page[:items], nextCursor: page[:next_cursor] }.compact
+      apply_cache_metadata({ tools: page[:items], nextCursor: page[:next_cursor] }.compact)
     end
 
     def call_tool(request, session: nil, related_request_id: nil, cancellation: nil)
@@ -667,7 +695,7 @@ def call_tool(request, session: nil, related_request_id: nil, cancellation: nil)
     def list_prompts(request)
       page = paginate(@prompts.values, cursor: cursor_from(request), page_size: @page_size, request: request, &:to_h)
 
-      { prompts: page[:items], nextCursor: page[:next_cursor] }.compact
+      apply_cache_metadata({ prompts: page[:items], nextCursor: page[:next_cursor] }.compact)
     end
 
     def get_prompt(request, session: nil, related_request_id: nil, cancellation: nil)
@@ -696,7 +724,7 @@ def get_prompt(request, session: nil, related_request_id: nil, cancellation: nil
     def list_resources(request)
       page = paginate(@resources, cursor: cursor_from(request), page_size: @page_size, request: request, &:to_h)
 
-      { resources: page[:items], nextCursor: page[:next_cursor] }.compact
+      apply_cache_metadata({ resources: page[:items], nextCursor: page[:next_cursor] }.compact)
     end
 
     # Server implementation should set `resources_read_handler` to override no-op default
@@ -708,7 +736,32 @@ def read_resource_no_content(request)
     def list_resource_templates(request)
       page = paginate(@resource_templates, cursor: cursor_from(request), page_size: @page_size, request: request, &:to_h)
 
-      { resourceTemplates: page[:items], nextCursor: page[:next_cursor] }.compact
+      apply_cache_metadata({ resourceTemplates: page[:items], nextCursor: page[:next_cursor] }.compact)
+    end
+
+    # Builds the `resources/read` result. The documented handler contract is "return value becomes `contents`";
+    # a Hash with a `:contents` key is also accepted as a full result so a handler can override the server-level
+    # `ttlMs`/`cacheScope` cache hints per result (SEP-2549).
+    def build_read_resource_result(handler_result)
+      result = if handler_result.is_a?(Hash) && handler_result.key?(:contents)
+        handler_result
+      else
+        { contents: handler_result }
+      end
+
+      apply_cache_metadata(result)
+    end
+
+    # Adds the SEP-2549 cache hints (`ttlMs`, `cacheScope`) to a result. Emission is opt-in: nothing is added
+    # unless the server was configured with `ttl_ms`/`cache_scope` or the result already carries one of the fields, in
+    # which case the missing one is filled with the spec defaults (`ttlMs: 0` = do not cache, `cacheScope: "public"`).
+    # Values already in the result win, enabling per-result overrides.
+    # https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2549
+    def apply_cache_metadata(result)
+      explicit = result.key?(:ttlMs) || result.key?(:cacheScope)
+      return result if @ttl_ms.nil? && @cache_scope.nil? && !explicit
+
+      { ttlMs: @ttl_ms || 0, cacheScope: @cache_scope || "public" }.merge(result)
     end
 
     def complete(params, session: nil, related_request_id: nil, cancellation: nil)

test/mcp/client_test.rb

@@ -883,6 +883,40 @@ def test_list_tools_returns_single_page_with_cursor
       assert_equal("cursor1", result.next_cursor)
     end
 
+    def test_list_results_expose_ttl_ms_and_cache_scope
+      # SEP-2549 cache hints are surfaced on every paginated result struct.
+      [
+        ["tools/list", "tools", :list_tools],
+        ["prompts/list", "prompts", :list_prompts],
+        ["resources/list", "resources", :list_resources],
+        ["resources/templates/list", "resourceTemplates", :list_resource_templates],
+      ].each do |method, items_key, client_method|
+        transport = mock
+        mock_response = {
+          "result" => { items_key => [], "ttlMs" => 5000, "cacheScope" => "private" },
+        }
+        transport.expects(:send_request).with do |args|
+          args.dig(:request, :method) == method
+        end.returns(mock_response).once
+
+        result = Client.new(transport: transport).public_send(client_method)
+
+        assert_equal(5000, result.ttl_ms, "#{method} missing ttl_ms")
+        assert_equal("private", result.cache_scope, "#{method} missing cache_scope")
+      end
+    end
+
+    def test_list_results_have_nil_cache_hints_when_server_omits_them
+      transport = mock
+      mock_response = { "result" => { "tools" => [] } }
+      transport.expects(:send_request).returns(mock_response).once
+
+      result = Client.new(transport: transport).list_tools
+
+      assert_nil(result.ttl_ms)
+      assert_nil(result.cache_scope)
+    end
+
     def test_list_tools_with_cursor_param
       transport = mock
 

test/mcp/server_test.rb

@@ -2936,6 +2936,86 @@ def server_context
       assert_empty response.dig(:result, :content)
     end
 
+    test "list results carry ttlMs and cacheScope when ttl_ms is configured" do
+      # SEP-2549 cache hints. The cacheScope default is "public", matching
+      # the spec default and the Python SDK.
+      server = Server.new(name: "ttl_test", ttl_ms: 5000)
+
+      ["tools/list", "prompts/list", "resources/list", "resources/templates/list"].each_with_index do |method, index|
+        result = server.handle({ jsonrpc: "2.0", method: method, id: index + 1 })[:result]
+
+        assert_equal 5000, result[:ttlMs], "#{method} missing ttlMs"
+        assert_equal "public", result[:cacheScope], "#{method} missing cacheScope"
+      end
+    end
+
+    test "resources/read carries cache hints when cache_scope is configured" do
+      # The ttlMs default is 0 (do not cache), the only universally safe value.
+      server = Server.new(name: "ttl_test", cache_scope: "private")
+
+      result = server.handle({
+        jsonrpc: "2.0",
+        method: "resources/read",
+        id: 1,
+        params: { uri: "file:///x" },
+      })[:result]
+
+      assert_equal 0, result[:ttlMs]
+      assert_equal "private", result[:cacheScope]
+    end
+
+    test "results omit cache hints when ttl_ms and cache_scope are not configured" do
+      # Wire-format regression: opt-in emission keeps default output unchanged.
+      list_result = @server.handle({ jsonrpc: "2.0", method: "tools/list", id: 1 })[:result]
+      read_result = @server.handle({
+        jsonrpc: "2.0",
+        method: "resources/read",
+        id: 2,
+        params: { uri: "file:///x" },
+      })[:result]
+
+      refute list_result.key?(:ttlMs)
+      refute list_result.key?(:cacheScope)
+      refute read_result.key?(:ttlMs)
+      refute read_result.key?(:cacheScope)
+    end
+
+    test "cache hints appear alongside nextCursor when paginating" do
+      tool_a = Tool.define(name: "tool_a", description: "Tool A")
+      tool_b = Tool.define(name: "tool_b", description: "Tool B")
+      server = Server.new(name: "ttl_test", tools: [tool_a, tool_b], page_size: 1, ttl_ms: 1000, cache_scope: "private")
+
+      result = server.handle({ jsonrpc: "2.0", method: "tools/list", id: 1 })[:result]
+
+      assert_not_nil result[:nextCursor]
+      assert_equal 1000, result[:ttlMs]
+      assert_equal "private", result[:cacheScope]
+    end
+
+    test "a resources_read_handler can override the server-level cache hints per result" do
+      server = Server.new(name: "ttl_test", ttl_ms: 5000)
+      server.resources_read_handler do |params|
+        { contents: [{ uri: params[:uri], mimeType: "text/plain", text: "hi" }], ttlMs: 60_000 }
+      end
+
+      result = server.handle({
+        jsonrpc: "2.0",
+        method: "resources/read",
+        id: 1,
+        params: { uri: "file:///x" },
+      })[:result]
+
+      assert_equal 60_000, result[:ttlMs]
+      assert_equal "public", result[:cacheScope]
+      assert_equal [{ uri: "file:///x", mimeType: "text/plain", text: "hi" }], result[:contents]
+    end
+
+    test "ttl_ms and cache_scope writers reject invalid values" do
+      assert_raises(ArgumentError) { Server.new(name: "ttl_test", ttl_ms: -1) }
+      assert_raises(ArgumentError) { Server.new(name: "ttl_test", ttl_ms: 1.5) }
+      assert_raises(ArgumentError) { Server.new(name: "ttl_test", cache_scope: "internal") }
+    end
+
     test "#handle tools/list returns paginated results when page_size is set" do
       tool_a = Tool.define(name: "tool_a", title: "Tool A", description: "Tool A")
       tool_b = Tool.define(name: "tool_b", title: "Tool B", description: "Tool B")