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Preview. Two kinds of limit apply: how often you may call, and what the transport will accept at all.

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Rate limits

Requests are counted against several dimensions at once, and the first one to run out refuses the call. The defaults, per 60-second window:
The ordering is the policy: a single end user may burst, a workspace may burst harder, and a credential that fans a whole customer base into one key hardest. These are the same limits /api/v2/ runs on — an MCP client is not throttled separately, it is throttled by the same counter.
A refusal comes back as a tool error carrying rate_limited and a retry hint. Unlike an empty wallet, retrying does help here — wait out the window. The windows are fixed rather than sliding, so a caller can send up to two windows’ worth across a boundary. That is a known and deliberate trade: a sliding window needs a read-modify-write the cache API cannot make atomic, and the cost of the fixed one is something you notice in a graph rather than an outage.

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Transport constraints

These are the ones that look like faults in a test client and are not. Every one of them is what the Streamable HTTP transport permits or requires — the 2025-06-18 spec for a handshake-era client, and 2026-07-28 for a modern one, which turned three of these from a server’s choice into the rule.

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Which revisions are accepted

Four, in two eras: the current 2026-07-28, which has no handshake, and the handshake revisions 2025-06-18, 2025-03-26 and 2024-11-05. The list the server actually answers on is mcpserver/protocol.py: SUPPORTED_PROTOCOL_VERSIONS, and server/discover reads it out over the wire so a client never has to trust this page for it. Which era a request is served in is decided by the MCP-Protocol-Version header alone, so a client working today keeps working untouched — an absent header still means 2025-03-26. Protocol revisions is the whole story: what a modern client must send, what server/discover answers, and the three error codes this revision added. A client built for a revision we do not speak is still not locked out. The initialize handshake answers in the newest handshake revision we speak rather than refusing over a string, so such a client negotiates down and carries on — because a handshake that failed on an unrecognised version string would make the client list a whitelist by accident. The 400 is reserved for a client that keeps asserting an unsupported revision in the header after that, and it names what we do speak.
No Mcp-Session-Id is ever issued, in either era. Sessions were a MAY in the handshake era and 2026-07-28 removed them from the protocol altogether, so there was nothing to unwind here: a stateless server removes an entire class of expiry bugs — there is no session to go stale between a tool listing and a tool call. If your client is waiting for one, it will not arrive, and it does not need it.

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Response bodies

Requests get Content-Type: application/json and a single JSON object. There is no SSE on this endpoint in Preview: streaming a tool result buys nothing when the result is a JSON object, and it costs every client an extra parsing path.

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What is deliberately not here yet

Tools first. The server advertises tools capability and nothing else.
Conformant to omit, and it earns nothing while every result is a JSON object.
See above. Not planned, and 2026-07-28 removed them from the spec.
Deliberately absent from SUPPORTED_PROTOCOL_VERSIONS, and the omission is the decision rather than a gap. Nothing in the installed base speaks it, and it added surface — tasks, URL-mode elicitation — that this server does not implement. Claiming it would be advertising a revision we would then have to refuse features from. A client that asks for it is answered -32022 with the four we do speak, which is one retry rather than a guess. See Protocol revisions.
This server asks nothing of a client. clientCapabilities is required on every modern request and is read only to enforce its presence — which is why -32021 (MissingRequiredClientCapability) is a code this server names and never emits.
Preview means the tool set may change. Names and schemas can move between releases. Call tools/list at connection time rather than hard-coding a tool name in an unattended integration, and expect this page’s guarantees about the transport to be steadier than the ones about the tools.