MCP SDK 1.17.4 StreamableHttp auto-detection breaks SSE-based MCP server connections in agent framework

[raycoderhk]

Discovered a break for examples/simple-prompt-agent
where pnpm dev gives error: Failed to create URL MCP client for "todo-list"

Behavior: Agent generates text responses instead of using MCP tools
Error pattern: StreamableHTTPEdgeClientTransport trying to POST to /sse

🔍 Key Evidence from Terminal Logs:

1. MCP SDK Version: @modelcontextprotocol/sdk@1.17.4 (line 1021 from earlier logs)
2. Transport Auto-Detection Issue: SDK chooses StreamableHTTPEdgeClientTransport for HTTP URLs
3. Protocol Mismatch: StreamableHttp POSTs to /sse, but SSE servers expect GET /sse then POST /sse/message
4. Inspector Works: MCP Inspector successfully uses SSE protocol with same server
5. Agent Framework Fails: Cannot connect using URL-based configuration

🔧 Affected Components:

• File: packages/agent/src/client/mcp-client-manager.ts
• Method: connectUrl() - relies on external SDK transport auto-detection
• Issue: No way to force SSE transport for HTTP URLs
• Impact: All SSE-based MCP servers cannot be used with agent framework

Detailed error in terminal:

[0]     _name: 'agent-toolbox',
[0]     _version: '1.0.0',
[0]     mcpConnections: {},
[0]     _callbackUrls: [],
[0]     enhancedConnections: Map(0) {}
[0]   },
[0]   mcpConfig: { mcpServers: { 'todo-list': [Object] } }
[0] }
[0] 🔧 Using static MCP configuration (imported mcp.json)
[0] 🔍 Found 0 Fetcher service bindings to test: 
[0] 🔧 Toolbox Service: Initializing with 1 configured servers and 0 Fetcher bindings to test
[0] 🔗 Initializing URL MCP client for "todo-list" at http://localhost:8788/sse
[0] ▲ [WARNING] No authProvider provided in the transport options. This client will only support unauthenticated remote MCP Servers
[0] 
[0] 
[0] ✘ [ERROR] ❌ Failed to create URL MCP client for "todo-list": Error: Network connection lost.
[0] 
[0]       at async StreamableHTTPEdgeClientTransport.send (file:///Users/raymond/Documents/GitHub/typescript-agent-framework/node_modules/.pnpm/@modelcontextprotocol+sdk@1.17.4/node_modules/@modelcontextprotocol/sdk/src/client/streamableHttp.ts:430:24) {
[0]     remote: true,
[0]     retryable: true
[0]   }
[0] 
[0] 
[0] 
[0] 🎯 Toolbox Service Initialization Summary:
[0] 📈 Total servers/bindings processed: 1
[0] ✅ Successfully connected: 0
[0] ❌ Failed to connect: 1
[0] ⏭️  Skipped: 0
[0] 
[0] 🔗 Active connections: 0
[0] 🛠️  Total tools available: 0 (0 unique)
[0] 
[0] 🚀 Toolbox Service ready! Configured + Auto-discovery enabled.
[0] 
[0] ℹ️  No tools detected from MCP servers```