workspace-server/internal/handlers/mcp.go

@@ -192,6 +192,32 @@ var mcpAllTools = []mcpTool{
 					"type":        "string",
 					"description": "The message to send to the user",
 				},
+				"attachments": map[string]interface{}{
+					"type":        "array",
+					"description": "Optional files to render as canvas chat attachments. Each item must include uri and name; mimeType and size are optional.",
+					"items": map[string]interface{}{
+						"type": "object",
+						"properties": map[string]interface{}{
+							"uri": map[string]interface{}{
+								"type":        "string",
+								"description": "Workspace attachment URI, usually workspace:/absolute/path",
+							},
+							"name": map[string]interface{}{
+								"type":        "string",
+								"description": "Display filename",
+							},
+							"mimeType": map[string]interface{}{
+								"type":        "string",
+								"description": "Optional MIME type",
+							},
+							"size": map[string]interface{}{
+								"type":        "number",
+								"description": "Optional file size in bytes",
+							},
+						},
+						"required": []string{"uri", "name"},
+					},
+				},
 			},
 			"required": []string{"message"},
 		},

workspace-server/internal/handlers/mcp_test.go

@@ -937,6 +937,75 @@ func TestMCPHandler_SendMessageToUser_PersistsToActivityLog(t *testing.T) {
 	}
 }
 
+func TestMCPHandler_SendMessageToUser_WithAttachments_PersistsFileParts(t *testing.T) {
+	t.Setenv("MOLECULE_MCP_ALLOW_SEND_MESSAGE", "true")
+	h, mock := newMCPHandler(t)
+
+	mock.ExpectQuery("SELECT name, talk_to_user_enabled FROM workspaces").
+		WithArgs("ws-mcp-attach").
+		WillReturnRows(sqlmock.NewRows([]string{"name", "talk_to_user_enabled"}).AddRow("Hermes Agent", true))
+
+	mock.ExpectExec(`INSERT INTO activity_logs.*'a2a_receive'.*'notify'`).
+		WithArgs(
+			"ws-mcp-attach",
+			sqlmock.AnyArg(),
+			jsonMatcher{
+				desc: "MCP send_message_to_user response_body has result + file parts",
+				predicate: func(parsed map[string]any) bool {
+					if parsed["result"] != "see attached" {
+						return false
+					}
+					parts, ok := parsed["parts"].([]any)
+					if !ok || len(parts) != 1 {
+						return false
+					}
+					part, ok := parts[0].(map[string]any)
+					if !ok || part["kind"] != "file" {
+						return false
+					}
+					file, ok := part["file"].(map[string]any)
+					return ok &&
+						file["uri"] == "workspace:/workspace/org_chart_v2.png" &&
+						file["name"] == "org_chart_v2.png" &&
+						file["mimeType"] == "image/png" &&
+						file["size"] == float64(12345)
+				},
+			},
+		).
+		WillReturnResult(sqlmock.NewResult(1, 1))
+
+	w := mcpPost(t, h, "ws-mcp-attach", map[string]interface{}{
+		"jsonrpc": "2.0",
+		"id":      102,
+		"method":  "tools/call",
+		"params": map[string]interface{}{
+			"name": "send_message_to_user",
+			"arguments": map[string]interface{}{
+				"message": "see attached",
+				"attachments": []map[string]interface{}{
+					{
+						"uri":      "workspace:/workspace/org_chart_v2.png",
+						"name":     "org_chart_v2.png",
+						"mimeType": "image/png",
+						"size":     12345,
+					},
+				},
+			},
+		},
+	})
+
+	var resp mcpResponse
+	if err := json.Unmarshal(w.Body.Bytes(), &resp); err != nil {
+		t.Fatalf("response was not valid JSON-RPC: %v\nbody=%s", err, w.Body.String())
+	}
+	if resp.Error != nil {
+		t.Errorf("unexpected JSON-RPC error: %+v", resp.Error)
+	}
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Errorf("MCP attachment response_body drift: %v", err)
+	}
+}
+
 // ─────────────────────────────────────────────────────────────────────────────
 // Parse error
 // ─────────────────────────────────────────────────────────────────────────────

workspace-server/internal/handlers/mcp_tools.go

@@ -348,12 +348,12 @@ func (h *MCPHandler) toolSendMessageToUser(ctx context.Context, workspaceID stri
 	// activity.go:Notify is what produced the reno-stars data-loss
 	// regression; both paths now route through the same writer.
 	//
-	// MCP send_message_to_user does not currently surface attachments
-	// (the tool args don't accept them); pass nil. If a future tool
-	// schema adds an attachments arg, build []AgentMessageAttachment
-	// and pass through.
+	attachments, err := parseAgentMessageAttachments(args["attachments"])
+	if err != nil {
+		return "", err
+	}
 	writer := NewAgentMessageWriter(h.database, h.broadcaster)
-	if err := writer.Send(ctx, workspaceID, message, nil); err != nil {
+	if err := writer.Send(ctx, workspaceID, message, attachments); err != nil {
 		if errors.Is(err, ErrWorkspaceNotFound) {
 			return "", fmt.Errorf("workspace not found")
 		}
@@ -362,6 +362,56 @@ func (h *MCPHandler) toolSendMessageToUser(ctx context.Context, workspaceID stri
 	return "Message sent.", nil
 }
 
+func parseAgentMessageAttachments(raw interface{}) ([]AgentMessageAttachment, error) {
+	if raw == nil {
+		return nil, nil
+	}
+	items, ok := raw.([]interface{})
+	if !ok {
+		return nil, fmt.Errorf("attachments must be an array")
+	}
+	if len(items) == 0 {
+		return nil, nil
+	}
+	attachments := make([]AgentMessageAttachment, 0, len(items))
+	for i, item := range items {
+		m, ok := item.(map[string]interface{})
+		if !ok {
+			return nil, fmt.Errorf("attachment[%d]: must be an object", i)
+		}
+		uri, _ := m["uri"].(string)
+		name, _ := m["name"].(string)
+		if uri == "" || name == "" {
+			return nil, fmt.Errorf("attachment[%d]: uri and name are required", i)
+		}
+		att := AgentMessageAttachment{
+			URI:  uri,
+			Name: name,
+		}
+		if mimeType, ok := m["mimeType"].(string); ok {
+			att.MimeType = mimeType
+		}
+		if size, ok := numericInt64(m["size"]); ok {
+			att.Size = size
+		}
+		attachments = append(attachments, att)
+	}
+	return attachments, nil
+}
+
+func numericInt64(raw interface{}) (int64, bool) {
+	switch v := raw.(type) {
+	case int:
+		return int64(v), true
+	case int64:
+		return v, true
+	case float64:
+		return int64(v), true
+	default:
+		return 0, false
+	}
+}
+
 func (h *MCPHandler) toolCommitMemory(ctx context.Context, workspaceID string, args map[string]interface{}) (string, error) {
 	// Issue #1733 — v2 memory plugin is now the only path. The legacy
 	// SQL fallback on `agent_memories` is gone; an unconfigured plugin