variants: redirect inner server sending methods to fix dropping notifications in stateless mode (#7)

Registers a sending middleware on each inner server that intercepts all
outgoing messages and routes them through the front session using a
session-swapped request wrapper. Replaces the previous proxy client
handler approach which dropped log messages in stateless mode.

## Motivation and Context
Tool handlers in variant servers use req.Session (the inner session) to
send notifications. Previously, log messages were silently dropped in
stateless mode because the inner session is not aware of the front
session.
This change injects the sendingMethodHandler from the frontSession into
the context and adds a sending middleware to redirect all sending
requests to the frontSession handler directly.
A few considerations in this implementation:
* Having frontSession in the context allows notifications to be sent to
the correct frontSession, when the same backend session could be used by
multiple front sessions.
* This solution will work for synchronous notifications that happen
within the same context (progress, log, sampling, elicitation) but not
working for asynchronous notifications (tool list changed, resource
updated etc), because asynchronous notifications are sent with a new
context.
* All the operations before the sending method will still happen in the
backend. Most notable impact is in stateless mode, requests from one
client could change the log level for other clients. For this reason, I
default the logging level to debug for now.
* This implementation relies on the MCP Go SDK sendingMethodHandler
taking a generic `Request` not one that tied to specific function. Based
on the current SDK implementation this is unlikely to change.
* The same generic method forwarding mechanism could work for receiving
methods too, but with some caveat. That can be addressed in separate PR.

## How Has This Been Tested?
TestNotificationDelivery — verifies progress and log messages arrive at
the front clients with multiple clients making concurrent request.

New tests added for context preservation that was added in
#6.
Context preservation is required to make the notification forwarding
work anyway. Since we use the context to carry frontSession handler to
the backend session.

All existing tests pass

## Breaking Changes
None. Tool handlers continue to use req.Session normally.

## Types of changes
Bug fix (non-breaking change which fixes an issue)

Author: StevenRChen

go/sdk/examples/server/research/README.md

@@ -2,7 +2,9 @@
 
 A variant-aware MCP server that manages context budget by providing the same research tools (`search_papers`, `get_paper`, `summarize`) at different verbosity levels.
 
-**Pattern demonstrated:** Context budget management with description verbosity control.
+Each tool sends progress notifications and log messages as it works, simulating a deep-research workflow where the client can observe each step (e.g. "Searching arXiv", "Resolving references", "Generating summary").
+
+**Patterns demonstrated:** Context budget management with description verbosity control, notification streaming.
 
 ## Variants
 

go/sdk/examples/server/research/main.go

@@ -2,7 +2,12 @@
 // at different verbosity levels. Demonstrates how variants can optimize tool
 // descriptions for different context window sizes and use cases.
 //
-// Capability demonstrated: Context budget management, description verbosity.
+// Each tool sends progress notifications and log messages as it works,
+// simulating a deep-research workflow where the client can observe each
+// step (e.g. "Searching arXiv", "Resolving references").
+//
+// Capability demonstrated: Context budget management, description verbosity,
+// notification streaming.
 //
 // Variants:
 //   - deep-research: Verbose multi-paragraph descriptions with usage examples
@@ -31,7 +36,9 @@ import (
 func main() {
 	// Deep-research variant: verbose descriptions with usage examples
 	// for thorough research workflows with large context windows.
-	deepServer := mcp.NewServer(&mcp.Implementation{Name: "research-assistant", Version: "v1.0.0"}, nil)
+	deepServer := mcp.NewServer(&mcp.Implementation{Name: "research-assistant", Version: "v1.0.0"}, &mcp.ServerOptions{
+		Capabilities: &mcp.ServerCapabilities{Logging: &mcp.LoggingCapabilities{}},
+	})
 	mcp.AddTool(deepServer, &mcp.Tool{
 		Name: "search_papers",
 		Description: "Search academic papers across multiple databases including arXiv, Semantic Scholar, and PubMed. " +
@@ -63,7 +70,9 @@ func main() {
 
 	// Quick-lookup variant: concise descriptions for fast Q&A sessions
 	// where context budget is limited.
-	quickServer := mcp.NewServer(&mcp.Implementation{Name: "research-assistant", Version: "v1.0.0"}, nil)
+	quickServer := mcp.NewServer(&mcp.Implementation{Name: "research-assistant", Version: "v1.0.0"}, &mcp.ServerOptions{
+		Capabilities: &mcp.ServerCapabilities{Logging: &mcp.LoggingCapabilities{}},
+	})
 	mcp.AddTool(quickServer, &mcp.Tool{
 		Name:        "search_papers",
 		Description: "Search academic papers by query, with optional year and field filters.",
@@ -79,7 +88,9 @@ func main() {
 
 	// Synthesis variant: balanced descriptions for report generation
 	// workflows that need moderate detail.
-	synthesisServer := mcp.NewServer(&mcp.Implementation{Name: "research-assistant", Version: "v1.0.0"}, nil)
+	synthesisServer := mcp.NewServer(&mcp.Implementation{Name: "research-assistant", Version: "v1.0.0"}, &mcp.ServerOptions{
+		Capabilities: &mcp.ServerCapabilities{Logging: &mcp.LoggingCapabilities{}},
+	})
 	mcp.AddTool(synthesisServer, &mcp.Tool{
 		Name: "search_papers",
 		Description: "Search academic papers across arXiv, Semantic Scholar, and PubMed. " +

go/sdk/examples/server/research/tools.go

@@ -2,6 +2,8 @@ package main
 
 import (
 	"context"
+	"fmt"
+	"time"
 
 	"github.com/modelcontextprotocol/go-sdk/mcp"
 )
@@ -27,7 +29,33 @@ type SearchPapersOutput struct {
 	TotalFound int     `json:"totalFound"`
 }
 
-func searchPapers(_ context.Context, _ *mcp.CallToolRequest, in SearchPapersInput) (*mcp.CallToolResult, SearchPapersOutput, error) {
+var databases = []string{"arXiv", "Semantic Scholar", "PubMed"}
+
+func searchPapers(ctx context.Context, req *mcp.CallToolRequest, in SearchPapersInput) (*mcp.CallToolResult, SearchPapersOutput, error) {
+	token := req.Params.GetProgressToken()
+	total := len(databases)
+
+	for i, db := range databases {
+		req.Session.Log(ctx, &mcp.LoggingMessageParams{
+			Level:  "info",
+			Logger: "search_papers",
+			Data:   fmt.Sprintf("Searching %s for %q", db, in.Query),
+		})
+		req.Session.NotifyProgress(ctx, &mcp.ProgressNotificationParams{
+			ProgressToken: token,
+			Progress:      float64(i + 1),
+			Total:         float64(total),
+			Message:       fmt.Sprintf("Searching %s (%d/%d)", db, i+1, total),
+		})
+		time.Sleep(100 * time.Millisecond)
+	}
+
+	req.Session.Log(ctx, &mcp.LoggingMessageParams{
+		Level:  "info",
+		Logger: "search_papers",
+		Data:   fmt.Sprintf("Found 1247 results for %q, returning top 2", in.Query),
+	})
+
 	return nil, SearchPapersOutput{
 		Papers: []Paper{
 			{
@@ -71,7 +99,25 @@ type GetPaperOutput struct {
 	Paper PaperDetail `json:"paper"`
 }
 
-func getPaper(_ context.Context, _ *mcp.CallToolRequest, in GetPaperInput) (*mcp.CallToolResult, GetPaperOutput, error) {
+func getPaper(ctx context.Context, req *mcp.CallToolRequest, in GetPaperInput) (*mcp.CallToolResult, GetPaperOutput, error) {
+	token := req.Params.GetProgressToken()
+	steps := []string{"Fetching metadata", "Resolving references", "Loading citations"}
+
+	for i, step := range steps {
+		req.Session.Log(ctx, &mcp.LoggingMessageParams{
+			Level:  "info",
+			Logger: "get_paper",
+			Data:   fmt.Sprintf("%s for %s", step, in.ID),
+		})
+		req.Session.NotifyProgress(ctx, &mcp.ProgressNotificationParams{
+			ProgressToken: token,
+			Progress:      float64(i + 1),
+			Total:         float64(len(steps)),
+			Message:       fmt.Sprintf("%s (%d/%d)", step, i+1, len(steps)),
+		})
+		time.Sleep(80 * time.Millisecond)
+	}
+
 	return nil, GetPaperOutput{
 		Paper: PaperDetail{
 			ID:         in.ID,
@@ -98,11 +144,30 @@ type SummarizeOutput struct {
 	Style   string `json:"style"`
 }
 
-func summarize(_ context.Context, _ *mcp.CallToolRequest, in SummarizeInput) (*mcp.CallToolResult, SummarizeOutput, error) {
+func summarize(ctx context.Context, req *mcp.CallToolRequest, in SummarizeInput) (*mcp.CallToolResult, SummarizeOutput, error) {
 	style := in.Style
 	if style == "" {
 		style = "abstract"
 	}
+
+	token := req.Params.GetProgressToken()
+	steps := []string{"Analyzing text", "Generating summary"}
+
+	for i, step := range steps {
+		req.Session.Log(ctx, &mcp.LoggingMessageParams{
+			Level:  "info",
+			Logger: "summarize",
+			Data:   fmt.Sprintf("%s (%s style)", step, style),
+		})
+		req.Session.NotifyProgress(ctx, &mcp.ProgressNotificationParams{
+			ProgressToken: token,
+			Progress:      float64(i + 1),
+			Total:         float64(len(steps)),
+			Message:       fmt.Sprintf("%s (%d/%d)", step, i+1, len(steps)),
+		})
+		time.Sleep(80 * time.Millisecond)
+	}
+
 	return nil, SummarizeOutput{
 		Summary: "This paper presents key findings in the field, demonstrating significant improvements over prior work through novel methodology and comprehensive evaluation across multiple benchmarks.",
 		Style:   style,

go/sdk/variants/backend.go

@@ -6,7 +6,6 @@ package variants
 
 import (
 	"context"
-	"fmt"
 
 	"github.com/modelcontextprotocol/go-sdk/mcp"
 )
@@ -31,80 +30,118 @@ type backend interface {
 // inMemoryBackend connects to a co-located *mcp.Server via in-memory
 // transports.
 //
-// Limitation: list-changed and resource-updated notifications from
-// inner servers are silently dropped. The mcp.ServerSession API only
-// exposes NotifyProgress and Log; there is no public method to send
-// tool/resource/prompt list-changed or resource-updated notifications
-// to the front client. This means that if an inner server dynamically
-// adds or removes tools/resources/prompts at runtime, the front
-// client will NOT be notified.
+// The sending redirect middleware intercepts all outgoing messages from
+// the inner server and forwards them through the front session. This
+// covers notifications (progress, log) and server-to-client requests
+// (Elicit, CreateMessage, ListRoots) during request handling.
 //
+// Limitation: async notifications (e.g. tool/resource/prompt
+// list-changed) triggered outside request handling use
+// context.Background() and lack the front session in the context, so
+// the middleware falls through and these notifications are dropped.
 // In practice this is acceptable because inner servers are typically
-// statically configured (tools registered at startup). If dynamic
-// capability changes are needed, this will require the Go MCP SDK to
-// expose generic notification sending on ServerSession.
+// statically configured (tools registered at startup).
 type inMemoryBackend struct {
-	server *mcp.Server
+	variantID        string
+	server           *mcp.Server
+	mcpMethodHandler mcp.MethodHandler
 }
 
-// captureMCPMethodHandler captures and returns a reference to the inner
+// sessionSwappedRequest wraps an existing mcp.Request but returns a
+// different session from GetSession(). The embedded mcp.Request satisfies
+// the unexported isRequest() method required by the interface.
+type sessionSwappedRequest struct {
+	mcp.Request
+	session mcp.Session
+}
+
+func (r *sessionSwappedRequest) GetSession() mcp.Session { return r.session }
+
+// sendingRedirectMiddleware returns a sending middleware that intercepts all
+// outgoing messages from the inner server (notifications and server-to-client
+// requests) and redirects them through the front server's sending handler with
+// the front session swapped in. By swapping the session we route messages
+// through the front connection to the real client.
+//
+// The front session (per-request) is read from context; the sending handler
+// (constant) is read from the Server struct.
+func sendingRedirectMiddleware(variantID string, vs *Server) mcp.Middleware {
+	return func(next mcp.MethodHandler) mcp.MethodHandler {
+		return func(ctx context.Context, method string, req mcp.Request) (mcp.Result, error) {
+			frontSession, _ := ctx.Value(frontSessionKeyType{}).(*mcp.ServerSession)
+			if frontSession == nil || vs.frontSendingHandler == nil {
+				return next(ctx, method, req)
+			}
+			return vs.frontSendingHandler(ctx, method, &sessionSwappedRequest{
+				Request: req,
+				session: frontSession,
+			})
+		}
+	}
+}
+
+// newInMemoryBackend creates an inMemoryBackend, registers the sending
+// redirect middleware on the inner server, and captures the inner
+// server's handler chain for direct dispatch.
+func newInMemoryBackend(server *mcp.Server, variantID string, vs *Server) *inMemoryBackend {
+	server.AddSendingMiddleware(sendingRedirectMiddleware(variantID, vs))
+	return &inMemoryBackend{
+		variantID:        variantID,
+		server:           server,
+		mcpMethodHandler: captureReceivingMethodHandler(server),
+	}
+}
+
+// captureReceivingMethodHandler captures and returns a reference to the inner
 // server's handler chain. This is a workaround using AddReceivingMiddleware
 // to gain a reference to mcp.Server.receivingMethodHandler_, since the SDK
 // does not expose a public accessor for it. This can be replaced once the
 // SDK exposes a public accessor for the receiving handler chain.
-func captureMCPMethodHandler(server *mcp.Server) (mcp.MethodHandler, error) {
+func captureReceivingMethodHandler(server *mcp.Server) mcp.MethodHandler {
 	var handler mcp.MethodHandler
-
 	// The middleware is identity (returns next unmodified), so the handler
 	// chain is unchanged, no extra hop introduced even if called multiple times.
 	server.AddReceivingMiddleware(func(next mcp.MethodHandler) mcp.MethodHandler {
 		handler = next
 		return next
 	})
 
-	if handler == nil {
-		return nil, fmt.Errorf("failed to capture backend MCP method handler")
-	}
-	return handler, nil
+	return handler
 }
 
 // connect creates an in-memory transport pair and connects the inner server.
-// Requests bypass the transport via serverHandler to preserve context values.
-// The transport is kept alive only for notification forwarding (progress,
-// logging) from the inner server to the front client.
+// Requests bypass the transport via mcpMethodHandler to preserve context
+// values. Notifications are redirected by the sending middleware registered
+// in newInMemoryBackend; the proxy client is kept only for the initialize
+// handshake and to set the inner session's log level.
 func (b *inMemoryBackend) connect(ctx context.Context, variant ServerVariant, frontSession *mcp.ServerSession) (*innerConnection, error) {
-	mcpMethodHandler, err := captureMCPMethodHandler(b.server)
-	if err != nil {
-		return nil, err
-	}
-
 	serverTransport, clientSideTransport := mcp.NewInMemoryTransports()
 
 	serverSession, err := b.server.Connect(ctx, serverTransport, nil)
 	if err != nil {
 		return nil, err
 	}
 
+	// The proxy client completes the initialize handshake and sets the
+	// inner session's log level. It must remain open for the lifetime of
+	// the serverSession — closing it tears down the in-memory transport,
+	// causing the serverSession to shut down. The nop handlers advertise
+	// capabilities so the inner ServerSession doesn't short-circuit
+	// methods like Elicit with "client does not support X". They are
+	// never called because the sending redirect middleware intercepts
+	// all outgoing messages before they reach the transport.
+	nopElicit := func(context.Context, *mcp.ElicitRequest) (*mcp.ElicitResult, error) {
+		return nil, nil
+	}
+	nopSampling := func(context.Context, *mcp.CreateMessageRequest) (*mcp.CreateMessageResult, error) {
+		return nil, nil
+	}
 	client := mcp.NewClient(&mcp.Implementation{
 		Name:    "variant-proxy-client",
 		Version: "1.0.0",
 	}, &mcp.ClientOptions{
-		ProgressNotificationHandler: func(ctx context.Context, req *mcp.ProgressNotificationClientRequest) {
-			if frontSession != nil {
-				injectVariantMeta(req.Params, variant.ID)
-				_ = frontSession.NotifyProgress(ctx, req.Params)
-			}
-		},
-		LoggingMessageHandler: func(ctx context.Context, req *mcp.LoggingMessageRequest) {
-			if frontSession != nil {
-				injectVariantMeta(req.Params, variant.ID)
-				_ = frontSession.Log(ctx, req.Params)
-			}
-		},
-		ToolListChangedHandler:     func(context.Context, *mcp.ToolListChangedRequest) {},
-		ResourceListChangedHandler: func(context.Context, *mcp.ResourceListChangedRequest) {},
-		PromptListChangedHandler:   func(context.Context, *mcp.PromptListChangedRequest) {},
-		ResourceUpdatedHandler:     func(context.Context, *mcp.ResourceUpdatedNotificationRequest) {},
+		ElicitationHandler:   nopElicit,
+		CreateMessageHandler: nopSampling,
 	})
 
 	clientSession, err := client.Connect(ctx, clientSideTransport, nil)
@@ -113,11 +150,19 @@ func (b *inMemoryBackend) connect(ctx context.Context, variant ServerVariant, fr
 		return nil, err
 	}
 
+	// Set the inner session's log level to the lowest so that
+	// ServerSession.Log() does not short-circuit before reaching the
+	// sending middleware. The actual log-level filtering is performed by
+	// the front-facing session when the middleware redirects the
+	// notification. Errors are ignored: if the inner server does not
+	// advertise the Logging capability the call simply fails harmlessly.
+	_ = clientSession.SetLoggingLevel(ctx, &mcp.SetLoggingLevelParams{Level: "debug"})
+
 	return &innerConnection{
 		backendSession: &backendSession{
-			variantID:        variant.ID,
+			variantID:        b.variantID,
 			serverSession:    serverSession,
-			mcpMethodHandler: mcpMethodHandler,
+			mcpMethodHandler: b.mcpMethodHandler,
 		},
 		cleanupFn: func() {
 			clientSession.Close()