# MCP client cannot connect to modern-only 2026-07-28 Streamable HTTP servers (legacy initialize rejected) **URL:** \ **Category:** Bug Reports\ **Tags:** mcp\ **Created:** [September 21, 2026, 5:10pm UTC](https://forum.cursor.com/t/mcp-client-cannot-connect-to-modern-only-2026-07-28-streamable-http-servers-legacy-initialize-rejected/172536 "2026-09-21T17:10:27Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![tentakle](https://sea3.discourse-cdn.com/cursor1/user_avatar/forum.cursor.com/tentakle/32/121687_2.png) [@tentakle](https://forum.cursor.com/u/tentakle)\ **Post date:** [September 21, 2026, 5:10pm UTC](https://forum.cursor.com/t/mcp-client-cannot-connect-to-modern-only-2026-07-28-streamable-http-servers-legacy-initialize-rejected/172536/1 "2026-09-21T17:10:27Z")
### Where does the bug appear (feature/product)? Cursor IDE ### Describe the Bug Remote MCP servers that require MCP protocol revision 2026-07-28 only (stateless Streamable HTTP, no legacy initialize handshake, no SSE) fail to connect. Cursor’s MCP client still performs the 2025-era handshake (initialize / typically 2025-11-25) and either omits MCP-Protocol-Version or sends a legacy body that modern servers reject. This is a general client-era gap, not a single-server misconfiguration. Example endpoint that cut over on 2026-09-21: [https://asdlc.io/mcp](https://asdlc.io/mcp) (modern-only; verified with @modelcontextprotocol/client@2.0.0 pinned to 2026-07-28). ### Steps to Reproduce 1. Add a remote HTTP MCP server that only accepts MCP 2026-07-28 (no legacy initialize), e.g. in ~/.cursor/mcp.json: { “mcpServers”: { “asdlc”: { “type”: “http”, “url”: “[https://asdlc.io/mcp](https://asdlc.io/mcp)” } } } 1. Restart / reload MCP. 2. Open Output → MCP and observe the connection logs. ### Expected Behavior Cursor’s MCP HTTP client should negotiate the modern era (2026-07-28): send MCP-Protocol-Version, use server/discover / per-request \_meta (no initialize), and successfully list/call tools against modern-only Streamable HTTP servers. Prefer also documenting which protocol revisions the client supports. Actual: 1. Streamable HTTP POST fails with JSON-RPC -32020: MCP-Protocol-Version is required; supported: 2026-07-28 2. Client falls back to SSE → 405 (server is POST-only / no SSE) 3. Connection ends conn=failed; tools never appear ### Operating System Linux ### Version Information Version: 3.21.16 Commit: 8ae78e8eee1e63479c7e0504b664bc0a80c68000 OS: Linux x64 (Fedora) ### For AI issues: which model did you use? N/A — not a model/AI-quality issue; this is the MCP client transport/protocol negotiation layer. ### For AI issues: add Request ID with privacy disabled N/A ### Additional Information Independent curl confirms the same rejection for legacy initialize, and that a modern header + legacy initialize body is rejected as header/body mismatch. Servers correctly implementing 2026-07-28 cannot downgrade. References: - Spec: [Specification - Model Context Protocol](https://modelcontextprotocol.io/specification/2026-07-28/) - Changelog (stateless / no initialize): [Key Changes - Model Context Protocol](https://modelcontextprotocol.io/specification/2026-07-28/changelog) - TS SDK modern pin: [Supporting protocol revision 2026-07-28 | MCP TypeScript SDK](https://ts.sdk.modelcontextprotocol.io/v2/migration/support-2026-07-28) - Example modern-only server cutover: [Modernize MCP transport and verify client interoperability by villetakanen · Pull Request #82 · villetakanen/asdlc-io · GitHub](https://github.com/villetakanen/asdlc-io/pull/82) ### Does this stop you from using Cursor No - Cursor works, but with this issue ---
**Author:** ![Colin](https://sea3.discourse-cdn.com/cursor1/user_avatar/forum.cursor.com/colin/32/90452_2.png) [@Colin](https://forum.cursor.com/u/Colin)\ **Post date:** [September 21, 2026, 6:16pm UTC](https://forum.cursor.com/t/mcp-client-cannot-connect-to-modern-only-2026-07-28-streamable-http-servers-legacy-initialize-rejected/172536/5 "2026-09-21T18:16:07Z")
Hey @tentakle, thanks for the detailed report, and for the curl checks. You’ve read it right: Cursor’s MCP client currently speaks the 2025-11-25 revision and earlier, so it still opens with the initialize handshake and doesn’t yet negotiate 2026-07-28. A server that only accepts 2026-07-28 rejects that handshake, and the SSE fallback you saw is our client misreading the 400 as “Streamable HTTP not supported”. There’s no client-side workaround right now. Adding an MCP-Protocol-Version header in mcp.json won’t help, since a 2026-07-28 header on an initialize body gets rejected as a header/body mismatch (which matches what you saw with curl). The only workaround today is on the server: the 2026-07-28 spec allows a server to be dual-era, answering initialize for older clients while serving the stateless protocol to newer ones (the TypeScript SDK v2 server entry points can serve both eras from one endpoint). If [asdlc.io](http://asdlc.io) can keep that legacy path enabled for now, Cursor will connect. For anyone landing here later: Cursor currently supports MCP protocol revisions 2025-11-25, 2025-06-18, 2025-03-26 and 2024-11-05 for remote servers. ---
**Author:** ![jsphbtst](https://sea3.discourse-cdn.com/cursor1/user_avatar/forum.cursor.com/jsphbtst/32/117139_2.png) [@jsphbtst](https://forum.cursor.com/u/jsphbtst)\ **Post date:** [September 25, 2026, 12:22pm UTC](https://forum.cursor.com/t/mcp-client-cannot-connect-to-modern-only-2026-07-28-streamable-http-servers-legacy-initialize-rejected/172536/10 "2026-09-25T12:22:57Z")
Hey @Colin, thanks for the detailed response. Is there a timeline for when Cursor will support the new MCP 2026-07-28 specification? ---
**Author:** ![Colin](https://sea3.discourse-cdn.com/cursor1/user_avatar/forum.cursor.com/colin/32/90452_2.png) [@Colin](https://forum.cursor.com/u/Colin)\ **Post date:** [September 25, 2026, 1:51pm UTC](https://forum.cursor.com/t/mcp-client-cannot-connect-to-modern-only-2026-07-28-streamable-http-servers-legacy-initialize-rejected/172536/11 "2026-09-25T13:51:21Z")
No timeline at the moment!