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internal/mcp: middleware on the sending side

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Implement sending middleware.

Sending middleware is useful for adding tracing, progress tokens,
and so on.

Change-Id: I0acab2550f0212d549c4adbcf1ebf425a9486df6
Reviewed-on: https://go-review.googlesource.com/c/tools/+/676355
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
This commit is contained in:
Jonathan Amsterdam 2025-05-25 07:56:36 -04:00
commit 0d0c48a468
1 changed files with 274 additions and 121 deletions
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10
design/design.md
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@ -408,7 +408,7 @@ func (*ClientSession) ResourceTemplates(context.Context, *ListResourceTemplatesP
### Middleware
We provide a mechanism to add MCP-level middleware on the both the client and server side, which runs after the request has been parsed but before any normal handling.
We provide a mechanism to add MCP-level middleware on the both the client and server side. Receiving middleware runs after the request has been parsed but before any normal handling. It is analogous to traditional HTTP server middleware. Sending middleware runs after a call to a method but before the request is sent. It is an alternative to transport middleware that exposes MCP types instead of raw JSON-RPC 2.0 messages. It is useful for tracing and setting progress tokens, for example.
```go
// A MethodHandler handles MCP messages.
@ -428,8 +428,10 @@ type Middleware[S Session] func(MethodHandler[S]) MethodHandler[S]
//
// For example, AddMiddleware(m1, m2, m3) augments the server method handler as
// m1(m2(m3(handler))).
func (c *Client) AddMiddleware(middleware ...Middleware[*ClientSession])
func (s *Server) AddMiddleware(middleware ...Middleware[*ServerSession])
func (c *Client) AddSendingMiddleware(middleware ...Middleware[*ClientSession])
func (c *Client) AddReceivingMiddleware(middleware ...Middleware[*ClientSession])
func (s *Server) AddSendingMiddleware(middleware ...Middleware[*ServerSession])
func (s *Server) AddReceivingMiddleware(middleware ...Middleware[*ServerSession])
```
As an example, this code adds server-side logging:
@ -443,7 +445,7 @@ func withLogging(h mcp.MethodHandler[*ServerSession]) mcp.MethodHandler[*ServerS
}
}
server.AddMiddleware(withLogging)
server.AddReceivingMiddleware(withLogging)
```
**Differences from mcp-go**: Version 0.26.0 of mcp-go defines 24 server hooks. Each hook consists of a field in the `Hooks` struct, a `Hooks.Add` method, and a type for the hook function. These are rarely used. The most common is `OnError`, which occurs fewer than ten times in open-source code.