feature: Add prompt filter and handler middleware support (parity with tools) #750
Description
Problem Statement
Tools support pre-list filtering via WithToolFilter and handler middleware via WithToolHandlerMiddleware. Resources recently gained WithResourceHandlerMiddleware (#569). Prompts have no equivalent for either.
This means servers that need to conditionally show/hide prompts (e.g. based on session, permissions, or feature flags) cannot filter them from prompts/list responses, and have no middleware hook for prompts/get handlers.
Proposed Solution
Add prompt equivalents following the existing tool/resource patterns:
New types:
PromptFilterFunc func(ctx context.Context, prompts []mcp.Prompt) []mcp.PromptPromptHandlerMiddleware func(PromptHandlerFunc) PromptHandlerFunc
New server options:
WithPromptFilter(PromptFilterFunc) ServerOptionWithPromptHandlerMiddleware(PromptHandlerMiddleware) ServerOption
Apply filters in handleListPrompts (before pagination) and the middleware chain in handleGetPrompt (before calling the handler), matching how handleListTools and handleToolCall work today.
Example Usage
s := server.NewMCPServer(
"My Server",
"1.0.0",
server.WithPromptFilter(func(ctx context.Context, prompts []mcp.Prompt) []mcp.Prompt {
session := server.ClientSessionFromContext(ctx)
if session == nil {
return prompts
}
// Filter prompts based on session
var filtered []mcp.Prompt
for _, p := range prompts {
if isPromptAllowed(session, p) {
filtered = append(filtered, p)
}
}
return filtered
}),
server.WithPromptHandlerMiddleware(func(next server.PromptHandlerFunc) server.PromptHandlerFunc {
return func(ctx context.Context, req mcp.GetPromptRequest) (*mcp.GetPromptResult, error) {
log.Printf("prompt requested: %s", req.Params.Name)
return next(ctx, req)
}
}),
)Alternatives/Workarounds Considered
Currently we wrap each prompt handler individually at registration time to gate prompts/get. This works for handler-level checks but cannot filter prompts/list — ineligible prompts are still visible to clients. A server-level filter and middleware would solve both cases cleanly.