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mcp/server: add initial capability server option (#182)

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This CL adds an option for servers to advertise support for features
that have not been added yet. This includes Prompts, Tools, and
Resources as these could be dynamically added after initialization.

Fixes: #135.
This commit is contained in:
Sam Thanawalla 2025-07-29 05:20:08 -04:00 • committed by GitHub
commit 2b6f7b5185
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GPG key ID: B5690EEEBB952194
2 changed files with 29 additions and 3 deletions
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16
mcp/server.go
View file

@ -69,6 +69,15 @@ type ServerOptions struct {
SubscribeHandler func(context.Context, *SubscribeParams) error
// Function called when a client session unsubscribes from a resource.
UnsubscribeHandler func(context.Context, *UnsubscribeParams) error
// If true, advertises the prompts capability during initialization,
// even if no prompts have been registered.
HasPrompts bool
// If true, advertises the resources capability during initialization,
// even if no resources have been registered.
HasResources bool
// If true, advertises the tools capability during initialization,
// even if no tools have been registered.
HasTools bool
}
// NewServer creates a new MCP server. The resulting server has no features:
@ -229,16 +238,17 @@ func (s *Server) capabilities() *serverCapabilities {
defer s.mu.Unlock()
caps := &serverCapabilities{
// TODO(samthanawalla): check for completionHandler before advertising capability.
Completions: &completionCapabilities{},
Logging: &loggingCapabilities{},
}
if s.tools.len() > 0 {
if s.opts.HasTools || s.tools.len() > 0 {
caps.Tools = &toolCapabilities{ListChanged: true}
}
if s.prompts.len() > 0 {
if s.opts.HasPrompts || s.prompts.len() > 0 {
caps.Prompts = &promptCapabilities{ListChanged: true}
}
if s.resources.len() > 0 || s.resourceTemplates.len() > 0 {
if s.opts.HasResources || s.resources.len() > 0 || s.resourceTemplates.len() > 0 {
caps.Resources = &resourceCapabilities{ListChanged: true}
if s.opts.SubscribeHandler != nil {
caps.Resources.Subscribe = true

16
mcp/server_test.go
View file

@ -333,6 +333,22 @@ func TestServerCapabilities(t *testing.T) {
Tools: &toolCapabilities{ListChanged: true},
},
},
{
name: "With initial capabilities",
configureServer: func(s *Server) {},
serverOpts: ServerOptions{
HasPrompts: true,
HasResources: true,
HasTools: true,
},
wantCapabilities: &serverCapabilities{
Completions: &completionCapabilities{},
Logging: &loggingCapabilities{},
Prompts: &promptCapabilities{ListChanged: true},
Resources: &resourceCapabilities{ListChanged: true},
Tools: &toolCapabilities{ListChanged: true},
},
},
}
for _, tc := range testCases {