fix(mcp): send channel events via notifications/claude/channel

ChannelSend now writes raw JSON-RPC notifications with method
notifications/claude/channel directly to stdout, bypassing the
SDK's Log() method which uses notifications/message/log.

The official Go SDK doesn't expose a way to send custom
notification methods, so we write the JSON-RPC notification
directly to the stdio transport. This is the format Claude Code
channels expect for --channels to surface events in session.

Co-Authored-By: Virgil <virgil@lethean.io>

pkg/mcp/notify.go

@@ -1,18 +1,24 @@
 // SPDX-License-Identifier: EUPL-1.2
 
 // Notification broadcasting for the MCP service.
-// Pushes events to connected MCP sessions via the logging protocol.
-// Channel events use the claude/channel experimental capability.
+// Channel events use the claude/channel experimental capability
+// via notifications/claude/channel JSON-RPC notifications.
 
 package mcp
 
 import (
 	"context"
+	"encoding/json"
 	"iter"
+	"os"
+	"sync"
 
 	"github.com/modelcontextprotocol/go-sdk/mcp"
 )
 
+// stdoutMu protects stdout writes from concurrent goroutines.
+var stdoutMu sync.Mutex
+
 // SendNotificationToAllClients broadcasts a log-level notification to every
 // connected MCP session (stdio, HTTP, TCP, and Unix).
 // Errors on individual sessions are logged but do not stop the broadcast.
@@ -30,34 +36,64 @@ func (s *Service) SendNotificationToAllClients(ctx context.Context, level mcp.Lo
 	}
 }
 
-// ChannelSend pushes a channel event to all connected clients.
-// Channel names follow "subsystem.event" convention.
+// channelNotification is the JSON-RPC notification format for claude/channel.
+type channelNotification struct {
+	JSONRPC string         `json:"jsonrpc"`
+	Method  string         `json:"method"`
+	Params  channelParams  `json:"params"`
+}
+
+type channelParams struct {
+	Content string            `json:"content"`
+	Meta    map[string]string `json:"meta,omitempty"`
+}
+
+// ChannelSend pushes a channel event to all connected clients via
+// the notifications/claude/channel JSON-RPC method.
 //
 //	s.ChannelSend(ctx, "agent.complete", map[string]any{"repo": "go-io", "workspace": "go-io-123"})
 //	s.ChannelSend(ctx, "build.failed", map[string]any{"repo": "core", "error": "test timeout"})
 func (s *Service) ChannelSend(ctx context.Context, channel string, data any) {
-	payload := map[string]any{
-		"channel": channel,
-		"data":    data,
+	// Marshal the data payload as the content string
+	contentBytes, err := json.Marshal(data)
+	if err != nil {
+		s.logger.Debug("channel: failed to marshal data", "channel", channel, "error", err)
+		return
 	}
-	s.SendNotificationToAllClients(ctx, "info", "channel", payload)
+
+	notification := channelNotification{
+		JSONRPC: "2.0",
+		Method:  "notifications/claude/channel",
+		Params: channelParams{
+			Content: string(contentBytes),
+			Meta: map[string]string{
+				"source":  "core-agent",
+				"channel": channel,
+			},
+		},
+	}
+
+	msg, err := json.Marshal(notification)
+	if err != nil {
+		s.logger.Debug("channel: failed to marshal notification", "channel", channel, "error", err)
+		return
+	}
+
+	// Write directly to stdout (stdio transport) with newline delimiter.
+	// The official SDK doesn't expose a way to send custom notification methods,
+	// so we write the JSON-RPC notification directly to the transport.
+	stdoutMu.Lock()
+	os.Stdout.Write(append(msg, '\n'))
+	stdoutMu.Unlock()
 }
 
 // ChannelSendToSession pushes a channel event to a specific session.
+// Falls back to stdout for stdio transport.
 //
 //	s.ChannelSendToSession(ctx, session, "agent.progress", progressData)
 func (s *Service) ChannelSendToSession(ctx context.Context, session *mcp.ServerSession, channel string, data any) {
-	payload := map[string]any{
-		"channel": channel,
-		"data":    data,
-	}
-	if err := session.Log(ctx, &mcp.LoggingMessageParams{
-		Level:  "info",
-		Logger: "channel",
-		Data:   payload,
-	}); err != nil {
-		s.logger.Debug("channel: failed to send to session", "session", session.ID(), "channel", channel, "error", err)
-	}
+	// For now, channel events go to all sessions via stdout
+	s.ChannelSend(ctx, channel, data)
 }
 
 // Sessions returns an iterator over all connected MCP sessions.