Tagged: protocol
2026-10-20Streamable HTTP vs SSE: moving a remote MCP server to the current transportHow HTTP+SSE (2024-11-05) differs from Streamable HTTP, what 2026-07-28 changed again, how to keep old clients working, and curl checks for the move.2026-10-16MCP server returns 406 Not Acceptable: what it means and how to fix itWhy a Streamable HTTP MCP server answers 406, how to reproduce it with curl, and how to fix it in the client, the server and the proxy in between.2026-10-16How many tools does a remote MCP server expose? The median is sevenOf the 1,903 remote MCP servers whose tool list we could read, the median exposes 7 tools, 40% expose five or fewer, and 28 expose 100 or more.2026-10-13The remote MCP server launch checklistThirteen checks before a remote MCP server goes live, from TLS to a toolset fingerprint per release, each with why it matters and a command to verify it.2026-10-13Tool descriptions that give orders to the agent reading themAn AI agent reads every MCP tool description before it picks a tool. Our probe flags a few patterns in that text; 409 of 26,575 tools matched one.2026-10-10How to test an MCP server before you ship itA layer-by-layer way to test a remote MCP server from the outside — transport, handshake, tools/list, errors, auth and tool descriptions — with curl commands you can copy.2026-09-30Which MCP protocol revision do servers answer with?Every probe starts by agreeing on a protocol revision, and every signed result records the one it got. Across the 1,004 servers whose latest result is signed, 580 ended on 2025-06-18, the revision our fallback offers, and 394 on 2026-07-28. What those two numbers do and do not say.2026-09-30Three in ten public MCP endpoints ask for a key before anything elseOf the 2,282 public MCP endpoints we observe, 694 answer our anonymous handshake with an authentication challenge. We never send credentials, so we have never seen their tools. What we can still observe about them without a key, and what that observation cannot tell you.2026-09-30The one call our probe sometimes holds backTo see how an MCP server refuses an unknown tool, our probe calls a name reserved for a tool that should not exist. A server could define that name, so the probe checks the tool list first and holds the call back when it cannot rule that out: 838 of 2,282 public endpoints on 2026-09-30.2026-09-24We asked 1,144 MCP servers for a made-up toolThe one tool call our probe makes names a made-up tool, to see how the server says no. Until this week we recorded 58% of signed servers as a risk for that check and could not say what they had answered. Now we record seven kinds of answer, and nine in ten of the flagged servers gave the same one.2026-09-14The several ways an MCP server says "no", and which ones we count against itA refusal is not a failure. MCP Checkup distinguishes a well-formed JSON-RPC error, a credential gate with a real challenge, a modern protocol rejection, and a malformed refusal, and records each differently. Here is the taxonomy the probe uses.