MCP Endpoint Limits

MCP endpoints are in beta.

MCP endpoints are deliberately small. Every request and response is bounded, so one agent can’t exhaust your deployment, and every refusal has a stable code that clients can act on.

Limits

Limit	Value
Tools per endpoint	1 to 32
Tool name	A letter followed by letters, digits or underscores; 64 characters max
Tool description	4,096 characters
Request body	64 KiB
Argument nesting depth	32
JSON-RPC request `id`	A number, or a string of at most 256 bytes
`request_key` and `grant_id` in `_meta`	1 to 256 characters
Tool result	144 KiB, counting both the text and structured copies
HTTP response	160 KiB
Operation result retention	14 days

A query result that exceeds the tool result limit fails with result_too_large. Return a smaller projection or paginate instead. An operation whose result is too large still succeeds: the tool returns accepted_result_omitted with the invocation ID, and the client can recover the full result later. After 14 days an operation’s result expires and recovery returns OperationResultExpired. The request key still refuses to run the operation again. For hosts using bijection/mcp/node, a request journal record is at most 256 KiB, a call ID at most 2,048 characters, and each discovery, recovery or status attempt has a 10-second deadline.

Supported methods

The endpoint accepts only HTTP POST and answers with fixed JSON responses. It handles these MCP methods:

initialize and notifications/initialized
server/discover
ping
tools/list
tools/call

Any other method, including resources, prompts and subscriptions, is refused with 400 unsupported_method. The endpoint doesn’t stream responses.

Errors

HTTP errors

When the endpoint refuses a request, it responds with a JSON body of the form { "error": "<code>" } and Cache-Control: no-store.

Status	Code	Cause
400	`missing_request`	The request has no body.
400	`invalid_request`	The body isn’t a JSON object.
400	`unsupported_method`	The MCP method isn’t supported.
400	`invalid_request_id`	The JSON-RPC `id` is too long or of the wrong type.
400	`invalid_request_metadata`	The tool call’s request metadata is malformed, or is sent both in `_meta` and in the arguments.
401	`authentication_required`	The request has no authenticated identity. See Authentication.
403	`origin_refused`	The `Origin` header doesn’t match the endpoint’s resource.
403	`access_refused`	The caller has no valid grant.
403	`request_refused`	The grant stopped being valid while the request ran, or the request failed; the response was withheld.
405	`method_not_allowed`	The request wasn’t a `POST`.
409	`publication_changed`	The URL’s revision isn’t the current one. Get the new endpoint URL.
413	`request_too_large`	The body is larger than 64 KiB.
429	`capacity_exhausted`	Your admission mutation refused the request. Retry after the `Retry-After` header’s seconds.
500	`result_too_large`	The complete response is larger than 160 KiB.

Tool errors

A tool call that is admitted but fails returns a normal MCP result with isError: true and structuredContent of the form { "error": "<code>" }. Messages thrown by your own functions are never passed to the agent.

Code	Cause
`tool_unavailable`	The tool isn’t published, or isn’t in the caller’s grant.
`request_scope_mismatch`	An operation call didn’t carry the caller’s grant `id`.
`tool_refused`	An operation call was refused, for example by argument validation or by the operation.
`read_unavailable`	A query, status or recovery call failed, including on invalid arguments. This says nothing about a write’s outcome.
`result_too_large`	The result is larger than the tool result limit.
`result_encoding_failed`	The result contains a value that can’t be encoded, such as a non-finite number.
`OperationRequestConflict`	The request key was already used with different arguments or by a different caller.
`OperationDefinitionChanged`	The operation’s contract differs from the one the tool was published with.
`OperationRetired`	The operation no longer accepts new requests.
`OperationResultExpired`	The operation’s retained result has expired.

If the data behind a read changes while its result is being released, the endpoint retries that read up to three times in total, rechecking the grant each time. Writes are never retried by the endpoint.

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MCP Endpoint Limits

Limits

Supported methods

Errors

HTTP errors

Tool errors

​Limits

​Supported methods

​Errors

​HTTP errors

​Tool errors

Limits

Supported methods

Errors

HTTP errors

Tool errors