diff --git a/cmd/gateway.go b/cmd/gateway.go
index 5a3cac7e..f69b144f 100644
--- a/cmd/gateway.go
+++ b/cmd/gateway.go
@@ -497,11 +497,14 @@ func runGateway() {
 	// Channel manager
 	channelMgr := channels.NewManager(msgBus)
 
-	// Wire channel sender on message tool (now that channelMgr exists)
+	// Wire channel sender + tenant checker on message tool (now that channelMgr exists)
 	if t, ok := toolsReg.Get("message"); ok {
 		if cs, ok := t.(tools.ChannelSenderAware); ok {
 			cs.SetChannelSender(channelMgr.SendToChannel)
 		}
+		if tc, ok := t.(tools.ChannelTenantCheckerAware); ok {
+			tc.SetChannelTenantChecker(channelMgr.ChannelTenantID)
+		}
 	}
 	// Wire group member lister on list_group_members tool
 	if t, ok := toolsReg.Get("list_group_members"); ok {
diff --git a/internal/channels/manager.go b/internal/channels/manager.go
index 33846bd8..dfc47b91 100644
--- a/internal/channels/manager.go
+++ b/internal/channels/manager.go
@@ -6,6 +6,8 @@ import (
 	"log/slog"
 	"sync"
 
+	"github.com/google/uuid"
+
 	"github.com/nextlevelbuilder/goclaw/internal/bus"
 	"github.com/nextlevelbuilder/goclaw/internal/store"
 )
@@ -191,6 +193,22 @@ func (m *Manager) ChannelTypeForName(name string) string {
 	return ""
 }
 
+// ChannelTenantID returns the tenant UUID for a channel instance.
+// Zero UUID means legacy/config-based channel (no tenant scope).
+// Returns (tenantID, exists).
+func (m *Manager) ChannelTenantID(channelName string) (uuid.UUID, bool) {
+	m.mu.RLock()
+	ch, ok := m.channels[channelName]
+	m.mu.RUnlock()
+	if !ok {
+		return uuid.Nil, false
+	}
+	if tc, ok := ch.(interface{ TenantID() uuid.UUID }); ok {
+		return tc.TenantID(), true
+	}
+	return uuid.Nil, true // legacy channel without tenant scope
+}
+
 // ListGroupMembers delegates to the channel's GroupMemberProvider if available.
 func (m *Manager) ListGroupMembers(ctx context.Context, channelName, chatID string) ([]GroupMember, error) {
 	m.mu.RLock()
diff --git a/internal/tools/message.go b/internal/tools/message.go
index 51d5bf74..9bd2c899 100644
--- a/internal/tools/message.go
+++ b/internal/tools/message.go
@@ -4,28 +4,38 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"log/slog"
 	"os"
 	"path/filepath"
+	"regexp"
 	"strings"
 
+	"github.com/google/uuid"
+
 	"github.com/nextlevelbuilder/goclaw/internal/bus"
 	"github.com/nextlevelbuilder/goclaw/internal/store"
 )
 
+// embeddedMediaPattern matches "MEDIA:" followed by a non-whitespace path.
+// Duplicated from agent.mediaPathPattern to avoid tools→agent import cycle.
+var embeddedMediaPattern = regexp.MustCompile(`MEDIA:\S+`)
+
 // MessageTool allows the agent to proactively send messages to channels.
 type MessageTool struct {
-	workspace string
-	restrict  bool
-	sender    ChannelSender
-	msgBus    *bus.MessageBus
+	workspace     string
+	restrict      bool
+	sender        ChannelSender
+	msgBus        *bus.MessageBus
+	tenantChecker ChannelTenantChecker
 }
 
 func NewMessageTool(workspace string, restrict bool) *MessageTool {
 	return &MessageTool{workspace: workspace, restrict: restrict}
 }
 
-func (t *MessageTool) SetChannelSender(s ChannelSender) { t.sender = s }
-func (t *MessageTool) SetMessageBus(b *bus.MessageBus)  { t.msgBus = b }
+func (t *MessageTool) SetChannelSender(s ChannelSender)              { t.sender = s }
+func (t *MessageTool) SetMessageBus(b *bus.MessageBus)               { t.msgBus = b }
+func (t *MessageTool) SetChannelTenantChecker(c ChannelTenantChecker) { t.tenantChecker = c }
 
 func (t *MessageTool) Name() string { return "message" }
 func (t *MessageTool) Description() string {
@@ -85,11 +95,35 @@ func (t *MessageTool) Execute(ctx context.Context, args map[string]any) *Result
 		return ErrorResult("target chat ID is required (no current chat in context)")
 	}
 
+	// Tenant isolation: validate channel belongs to current tenant.
+	if err := t.validateChannelTenant(ctx, channel, target); err != nil {
+		return err
+	}
+
 	// Handle MEDIA: prefix — send file as attachment instead of text.
 	if filePath, ok := t.resolveMediaPath(ctx, message); ok {
 		return t.sendMedia(ctx, channel, target, filePath)
 	}
 
+	// Extract embedded MEDIA: paths from multi-line messages.
+	// LLMs may include MEDIA: in conversational text rather than as a standalone prefix.
+	message, embeddedMedia := t.extractEmbeddedMedia(ctx, message)
+
+	// If we found embedded media and bus is available, prefer bus path (supports media attachments).
+	if len(embeddedMedia) > 0 && t.msgBus != nil {
+		outMsg := bus.OutboundMessage{
+			Channel: channel,
+			ChatID:  target,
+			Content: message,
+			Media:   embeddedMedia,
+		}
+		if isGroupContext(ctx) {
+			outMsg.Metadata = map[string]string{"group_id": target}
+		}
+		t.msgBus.PublishOutbound(outMsg)
+		return SilentResult(fmt.Sprintf(`{"status":"sent","channel":"%s","target":"%s"}`, channel, target))
+	}
+
 	// Prefer direct channel sender for immediate delivery.
 	// For group chats, fall through to message bus which supports metadata.
 	if t.sender != nil && !isGroupContext(ctx) {
@@ -126,6 +160,33 @@ func (t *MessageTool) Execute(ctx context.Context, args map[string]any) *Result
 	return ErrorResult("no channel sender or message bus available")
 }
 
+// validateChannelTenant checks the target channel belongs to the current tenant.
+// Returns an error Result if the send should be blocked, nil if allowed.
+func (t *MessageTool) validateChannelTenant(ctx context.Context, channel, target string) *Result {
+	if t.tenantChecker == nil {
+		return nil
+	}
+	chTenant, chExists := t.tenantChecker(channel)
+	if !chExists {
+		return ErrorResult(fmt.Sprintf("channel %q not found", channel))
+	}
+	// Allow: legacy/config-based channels (zero tenant) or master tenant context (system ops).
+	if chTenant == uuid.Nil {
+		return nil
+	}
+	ctxTenant := store.TenantIDFromContext(ctx)
+	if ctxTenant == uuid.Nil {
+		return nil // master tenant / system context
+	}
+	if chTenant != ctxTenant {
+		slog.Warn("security.cross_tenant_send_blocked",
+			"channel", channel, "target", target,
+			"ctx_tenant", ctxTenant, "ch_tenant", chTenant)
+		return ErrorResult("channel not accessible from this tenant")
+	}
+	return nil
+}
+
 // sendMedia sends a file as a media attachment via the outbound message bus.
 func (t *MessageTool) sendMedia(ctx context.Context, channel, target, filePath string) *Result {
 	if _, err := os.Stat(filePath); err != nil {
@@ -144,7 +205,7 @@ func (t *MessageTool) sendMedia(ctx context.Context, channel, target, filePath s
 	t.msgBus.PublishOutbound(bus.OutboundMessage{
 		Channel:  channel,
 		ChatID:   target,
-		Media:    []bus.MediaAttachment{{URL: filePath}},
+		Media:    []bus.MediaAttachment{{URL: filePath, ContentType: mimeFromPath(filePath)}},
 		Metadata: meta,
 	})
 	out, _ := json.Marshal(map[string]string{
@@ -156,6 +217,85 @@ func (t *MessageTool) sendMedia(ctx context.Context, channel, target, filePath s
 	return SilentResult(string(out))
 }
 
+// extractEmbeddedMedia scans a multi-line message for embedded MEDIA: path references.
+// Returns cleaned text (MEDIA: lines removed) and resolved media attachments.
+// Prevents raw MEDIA: paths from leaking to channels when LLMs embed them
+// in conversational text instead of using a standalone MEDIA: prefix.
+func (t *MessageTool) extractEmbeddedMedia(ctx context.Context, message string) (string, []bus.MediaAttachment) {
+	if !strings.Contains(message, "MEDIA:") {
+		return message, nil
+	}
+
+	lines := strings.Split(message, "\n")
+	var cleaned []string
+	var media []bus.MediaAttachment
+
+	for _, line := range lines {
+		trimmed := strings.TrimSpace(line)
+		// Skip [[audio_as_voice]] tags (TTS voice messages).
+		if strings.HasPrefix(trimmed, "[[audio_as_voice]]") {
+			continue
+		}
+		// Find all MEDIA: tokens on this line.
+		matches := embeddedMediaPattern.FindAllString(trimmed, -1)
+		if len(matches) == 0 {
+			cleaned = append(cleaned, line)
+			continue
+		}
+		// Extract each MEDIA: path and resolve via security-checked path resolution.
+		for _, raw := range matches {
+			if resolved, ok := t.resolveMediaPath(ctx, raw); ok {
+				media = append(media, bus.MediaAttachment{
+					URL:         resolved,
+					ContentType: mimeFromPath(resolved),
+				})
+			}
+		}
+		// Strip MEDIA: tokens from line, keep surrounding text.
+		remainder := strings.TrimSpace(embeddedMediaPattern.ReplaceAllString(line, ""))
+		if remainder != "" {
+			cleaned = append(cleaned, remainder)
+		}
+	}
+
+	return strings.TrimSpace(strings.Join(cleaned, "\n")), media
+}
+
+// mimeFromPath returns a MIME type based on file extension.
+// Duplicated from agent.mimeFromExt to avoid tools→agent import cycle.
+func mimeFromPath(path string) string {
+	switch strings.ToLower(filepath.Ext(path)) {
+	case ".png":
+		return "image/png"
+	case ".jpg", ".jpeg":
+		return "image/jpeg"
+	case ".gif":
+		return "image/gif"
+	case ".webp":
+		return "image/webp"
+	case ".mp4":
+		return "video/mp4"
+	case ".ogg", ".opus":
+		return "audio/ogg"
+	case ".mp3":
+		return "audio/mpeg"
+	case ".wav":
+		return "audio/wav"
+	case ".pdf":
+		return "application/pdf"
+	case ".doc":
+		return "application/msword"
+	case ".docx":
+		return "application/vnd.openxmlformats-officedocument.wordprocessingml.document"
+	case ".xls":
+		return "application/vnd.ms-excel"
+	case ".xlsx":
+		return "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
+	default:
+		return "application/octet-stream"
+	}
+}
+
 // isGroupContext returns true if the current context indicates a group conversation.
 func isGroupContext(ctx context.Context) bool {
 	userID := store.UserIDFromContext(ctx)
diff --git a/internal/tools/message_test.go b/internal/tools/message_test.go
index a7c79b78..61800e72 100644
--- a/internal/tools/message_test.go
+++ b/internal/tools/message_test.go
@@ -5,6 +5,10 @@ import (
 	"os"
 	"path/filepath"
 	"testing"
+
+	"github.com/google/uuid"
+
+	"github.com/nextlevelbuilder/goclaw/internal/store"
 )
 
 // outsidePath returns an absolute path that is guaranteed to be outside the
@@ -144,3 +148,214 @@ func TestIsInTempDir(t *testing.T) {
 		})
 	}
 }
+
+func TestExtractEmbeddedMedia(t *testing.T) {
+	tmpDir := os.TempDir()
+
+	workspace := t.TempDir()
+	workspaceCanonical, _ := filepath.EvalSymlinks(workspace)
+
+	// Create test files in workspace.
+	docsDir := filepath.Join(workspaceCanonical, "docs")
+	os.MkdirAll(docsDir, 0o755)
+	reportFile := filepath.Join(docsDir, "report.docx")
+	os.WriteFile(reportFile, []byte("test"), 0o644)
+	reportCanonical, _ := filepath.EvalSymlinks(reportFile)
+
+	tool := NewMessageTool(workspaceCanonical, true)
+	ctx := context.Background()
+
+	t.Run("no MEDIA: in message", func(t *testing.T) {
+		msg := "Hello, here is your report!"
+		cleaned, media := tool.extractEmbeddedMedia(ctx, msg)
+		if cleaned != msg {
+			t.Errorf("expected unchanged message, got %q", cleaned)
+		}
+		if len(media) != 0 {
+			t.Errorf("expected no media, got %d", len(media))
+		}
+	})
+
+	t.Run("embedded MEDIA: in multi-line message", func(t *testing.T) {
+		msg := "Here is the file:\nMEDIA:" + reportCanonical + "\nPlease download!"
+		cleaned, media := tool.extractEmbeddedMedia(ctx, msg)
+
+		if cleaned != "Here is the file:\nPlease download!" {
+			t.Errorf("unexpected cleaned text: %q", cleaned)
+		}
+		if len(media) != 1 {
+			t.Fatalf("expected 1 media, got %d", len(media))
+		}
+		if media[0].URL != reportCanonical {
+			t.Errorf("media URL = %q, want %q", media[0].URL, reportCanonical)
+		}
+		wantMime := "application/vnd.openxmlformats-officedocument.wordprocessingml.document"
+		if media[0].ContentType != wantMime {
+			t.Errorf("content type = %q, want %q", media[0].ContentType, wantMime)
+		}
+	})
+
+	t.Run("MEDIA: mid-sentence keeps surrounding text", func(t *testing.T) {
+		msg := "Here is your report MEDIA:" + reportCanonical + " please review"
+		cleaned, media := tool.extractEmbeddedMedia(ctx, msg)
+
+		if cleaned != "Here is your report  please review" {
+			t.Errorf("surrounding text lost: %q", cleaned)
+		}
+		if len(media) != 1 {
+			t.Fatalf("expected 1 media, got %d", len(media))
+		}
+	})
+
+	t.Run("multiple MEDIA: on same line", func(t *testing.T) {
+		img := filepath.Join(tmpDir, "photo.png")
+		msg := "MEDIA:" + reportCanonical + " MEDIA:" + img
+		cleaned, media := tool.extractEmbeddedMedia(ctx, msg)
+
+		if cleaned != "" {
+			t.Errorf("expected empty cleaned text, got %q", cleaned)
+		}
+		if len(media) != 2 {
+			t.Fatalf("expected 2 media from same line, got %d", len(media))
+		}
+	})
+
+	t.Run("MEDIA: path outside workspace is stripped but no attachment", func(t *testing.T) {
+		msg := "File:\nMEDIA:" + outsidePath(workspaceCanonical, "etc/passwd") + "\nDone"
+		cleaned, media := tool.extractEmbeddedMedia(ctx, msg)
+
+		if cleaned != "File:\nDone" {
+			t.Errorf("MEDIA: line not stripped: %q", cleaned)
+		}
+		if len(media) != 0 {
+			t.Errorf("expected no media for outside-workspace path, got %d", len(media))
+		}
+	})
+
+	t.Run("message with only MEDIA: lines", func(t *testing.T) {
+		msg := "MEDIA:" + reportCanonical
+		cleaned, media := tool.extractEmbeddedMedia(ctx, msg)
+
+		if cleaned != "" {
+			t.Errorf("expected empty cleaned text, got %q", cleaned)
+		}
+		if len(media) != 1 {
+			t.Fatalf("expected 1 media, got %d", len(media))
+		}
+	})
+
+	t.Run("audio_as_voice tag stripped", func(t *testing.T) {
+		msg := "[[audio_as_voice]]\nMEDIA:" + filepath.Join(tmpDir, "voice.ogg") + "\nExtra text"
+		cleaned, media := tool.extractEmbeddedMedia(ctx, msg)
+
+		if cleaned != "Extra text" {
+			t.Errorf("unexpected cleaned text: %q", cleaned)
+		}
+		if len(media) != 1 {
+			t.Fatalf("expected 1 media, got %d", len(media))
+		}
+	})
+
+	t.Run("multiple MEDIA: paths", func(t *testing.T) {
+		img := filepath.Join(tmpDir, "photo.png")
+		msg := "Files:\nMEDIA:" + reportCanonical + "\nMEDIA:" + img + "\nEnjoy!"
+		cleaned, media := tool.extractEmbeddedMedia(ctx, msg)
+
+		if cleaned != "Files:\nEnjoy!" {
+			t.Errorf("unexpected cleaned text: %q", cleaned)
+		}
+		if len(media) != 2 {
+			t.Fatalf("expected 2 media, got %d", len(media))
+		}
+	})
+}
+
+func TestMimeFromPath(t *testing.T) {
+	tests := []struct {
+		path string
+		want string
+	}{
+		{"/tmp/file.png", "image/png"},
+		{"/tmp/file.jpg", "image/jpeg"},
+		{"/tmp/file.mp4", "video/mp4"},
+		{"/tmp/file.ogg", "audio/ogg"},
+		{"/tmp/file.pdf", "application/pdf"},
+		{"/tmp/file.doc", "application/msword"},
+		{"/tmp/file.docx", "application/vnd.openxmlformats-officedocument.wordprocessingml.document"},
+		{"/tmp/file.xls", "application/vnd.ms-excel"},
+		{"/tmp/file.xlsx", "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"},
+		{"/tmp/file.unknown", "application/octet-stream"},
+	}
+	for _, tt := range tests {
+		t.Run(filepath.Base(tt.path), func(t *testing.T) {
+			if got := mimeFromPath(tt.path); got != tt.want {
+				t.Errorf("mimeFromPath(%q) = %q, want %q", tt.path, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestValidateChannelTenant(t *testing.T) {
+	tenantA := uuid.MustParse("0193a5b0-7000-7000-8000-000000000001")
+	tenantB := uuid.MustParse("019d1135-7087-7aa9-a2c7-cdaf7af851b1")
+
+	tool := NewMessageTool("", true)
+
+	t.Run("no checker configured allows all", func(t *testing.T) {
+		ctx := store.WithTenantID(context.Background(), tenantA)
+		if err := tool.validateChannelTenant(ctx, "telegram", "123"); err != nil {
+			t.Errorf("expected nil, got error: %s", err.ForLLM)
+		}
+	})
+
+	// Wire a mock checker.
+	channels := map[string]uuid.UUID{
+		"telegram":       tenantA,
+		"tenant-b-tg":   tenantB,
+	}
+	tool.SetChannelTenantChecker(func(name string) (uuid.UUID, bool) {
+		tid, ok := channels[name]
+		return tid, ok
+	})
+
+	t.Run("same tenant allows", func(t *testing.T) {
+		ctx := store.WithTenantID(context.Background(), tenantA)
+		if err := tool.validateChannelTenant(ctx, "telegram", "123"); err != nil {
+			t.Errorf("expected nil for same tenant, got: %s", err.ForLLM)
+		}
+	})
+
+	t.Run("cross tenant blocks", func(t *testing.T) {
+		ctx := store.WithTenantID(context.Background(), tenantA)
+		err := tool.validateChannelTenant(ctx, "tenant-b-tg", "456")
+		if err == nil {
+			t.Fatal("expected error for cross-tenant send, got nil")
+		}
+		if !err.IsError {
+			t.Error("expected IsError=true")
+		}
+	})
+
+	t.Run("channel not found blocks", func(t *testing.T) {
+		ctx := store.WithTenantID(context.Background(), tenantA)
+		err := tool.validateChannelTenant(ctx, "nonexistent", "789")
+		if err == nil {
+			t.Fatal("expected error for missing channel, got nil")
+		}
+	})
+
+	t.Run("nil channel tenant allows (legacy)", func(t *testing.T) {
+		channels["legacy-ch"] = uuid.Nil
+		ctx := store.WithTenantID(context.Background(), tenantA)
+		if err := tool.validateChannelTenant(ctx, "legacy-ch", "123"); err != nil {
+			t.Errorf("expected nil for legacy channel, got: %s", err.ForLLM)
+		}
+	})
+
+	t.Run("nil context tenant allows (master/system)", func(t *testing.T) {
+		ctx := context.Background() // no tenant in context
+		if err := tool.validateChannelTenant(ctx, "tenant-b-tg", "456"); err != nil {
+			t.Errorf("expected nil for master context, got: %s", err.ForLLM)
+		}
+	})
+}
diff --git a/internal/tools/types.go b/internal/tools/types.go
index 7c2615d3..88c08894 100644
--- a/internal/tools/types.go
+++ b/internal/tools/types.go
@@ -3,6 +3,8 @@ package tools
 import (
 	"context"
 
+	"github.com/google/uuid"
+
 	"github.com/nextlevelbuilder/goclaw/internal/bus"
 	"github.com/nextlevelbuilder/goclaw/internal/providers"
 	"github.com/nextlevelbuilder/goclaw/internal/store"
@@ -99,6 +101,16 @@ type ChannelSenderAware interface {
 	SetChannelSender(ChannelSender)
 }
 
+// ChannelTenantChecker returns the tenant UUID for a channel instance.
+// Used by the message tool to prevent cross-tenant sends.
+// Returns (tenantID, exists). Zero tenantID means legacy/config-based channel.
+type ChannelTenantChecker func(channelName string) (tenantID uuid.UUID, exists bool)
+
+// ChannelTenantCheckerAware tools can receive a channel tenant checker.
+type ChannelTenantCheckerAware interface {
+	SetChannelTenantChecker(ChannelTenantChecker)
+}
+
 // ChannelAware is optionally implemented by tools that only work on specific channel types.
 // Tools implementing this are filtered out when the current channel type doesn't match.
 type ChannelAware interface {