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mcp: advertise a capability if and only if added (#93)

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If tools are added to a Server when Run is called, it will send
toolCapabilities to the client. Otherwise it won't. Ditto for prompts
and resources.

For #56.
This commit is contained in:
Jonathan Amsterdam 2025-07-10 15:52:11 -04:00 • committed by GitHub
commit b4febf129b
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1 changed files with 118 additions and 16 deletions
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41
mcp/server.go
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@ -204,6 +204,26 @@ func (s *Server) RemoveResourceTemplates(uriTemplates ...string) {
func() bool { return s.resourceTemplates.remove(uriTemplates...) })
}
func (s *Server) capabilities() *serverCapabilities {
s.mu.Lock()
defer s.mu.Unlock()
caps := &serverCapabilities{
Completions: &completionCapabilities{},
Logging: &loggingCapabilities{},
}
if s.tools.len() > 0 {
caps.Tools = &toolCapabilities{ListChanged: true}
}
if s.prompts.len() > 0 {
caps.Prompts = &promptCapabilities{ListChanged: true}
}
if s.resources.len() > 0 || s.resourceTemplates.len() > 0 {
caps.Resources = &resourceCapabilities{ListChanged: true}
}
return caps
}
func (s *Server) complete(ctx context.Context, ss *ServerSession, params *CompleteParams) (Result, error) {
if s.opts.CompletionHandler == nil {
return nil, jsonrpc2.ErrMethodNotFound
@ -407,6 +427,11 @@ func fileResourceHandler(dir string) ResourceHandler {
//
// Run blocks until the client terminates the connection or the provided
// context is cancelled. If the context is cancelled, Run closes the connection.
//
// If tools have been added to the server before this call, then the server will
// advertise the capability for tools, including the ability to send list-changed notifications.
// If no tools have been added, the server will not have the tool capability.
// The same goes for other features like prompts and resources.
func (s *Server) Run(ctx context.Context, t Transport) error {
ss, err := s.Connect(ctx, t)
if err != nil {
@ -659,20 +684,8 @@ func (ss *ServerSession) initialize(ctx context.Context, params *InitializeParam
// TODO(rfindley): alter behavior when falling back to an older version:
// reject unsupported features.
ProtocolVersion: version,
Capabilities: &serverCapabilities{
Completions: &completionCapabilities{},
Prompts: &promptCapabilities{
ListChanged: true,
},
Tools: &toolCapabilities{
ListChanged: true,
},
Resources: &resourceCapabilities{
ListChanged: true,
},
Logging: &loggingCapabilities{},
},
Instructions: ss.server.opts.Instructions,
Capabilities: ss.server.capabilities(),
Instructions: ss.server.opts.Instructions,
ServerInfo: &implementation{
Name: ss.server.name,
Version: ss.server.version,