Enum McpErrorCode 

#[non_exhaustive]
#[repr(i32)]
pub enum McpErrorCode {
    ConnectionClosed = -32_000,
    HeaderMismatch = -32_020,
    ResourceNotFound = -32_002,
    MissingRequiredClientCapability = -32_021,
    UnsupportedProtocolVersion = -32_022,
    SessionNotFound = -32_005,
    SessionRequired = -32_006,
    Forbidden = -32_007,
    AlreadySubscribed = -32_008,
    NotSubscribed = -32_009,
    RequestTimeout = -32_010,
    UrlElicitationRequired = -32_042,
}

MCP-specific error codes (in the -32000 to -32099 range).

Codes marked spec are assigned by an MCP SEP. Others are implementation-defined within the JSON-RPC server-error range; using them is permitted by JSON-RPC 2.0 but they are not part of the MCP wire spec.

Recently-changed assignments:

• HeaderMismatch (SEP-2243), MissingRequiredClientCapability, and UnsupportedProtocolVersion (both SEP-2575) were originally assigned -32001/-32003/-32004 by their SEP documents. The upstream error-code allocation policy (spec PR modelcontextprotocol#2907, 2026-06) renumbered them to -32020/-32021/-32022 in the draft schema, which is canonical. This is a breaking change on the wire within the experimental 2026-07-28 surface.
• AlreadySubscribed and NotSubscribed previously occupied -32003 and -32004; they moved to -32008 and -32009 when those slots were believed to be spec-assigned. They stay at -32008/-32009.
• RequestTimeout previously occupied -32001 and moved to -32010 for the same reason. It stays at -32010.

Variants (Non-exhaustive)

Non-exhaustive enums could have additional variants added in future. Therefore, when matching against variants of non-exhaustive enums, an extra wildcard arm must be added to account for any future variants.

ConnectionClosed = -32_000

Transport connection was closed.

HeaderMismatch = -32_020

SEP-2243: HTTP headers (e.g. Mcp-Method, Mcp-Name, Mcp-Param-*) do not match the corresponding values in the request body, or a required header is missing or malformed.

Originally -32001 in the SEP text; renumbered to -32020 by the upstream error-code allocation (spec PR modelcontextprotocol#2907).

Use JsonRpcError::header_mismatch to construct.

ResourceNotFound = -32_002

👎Deprecated since 0.12.0:

SEP-2164 reassigned resource-not-found to InvalidParams (-32602). Use JsonRpcError::resource_not_found or ErrorCode::InvalidParams.

Resource not found.

Deprecated: SEP-2164 (FINAL) moves this to the standard JSON-RPC InvalidParams code (-32602). The JsonRpcError::resource_not_found constructor now emits -32602. The enum variant is retained so existing pattern matches on McpErrorCode::ResourceNotFound keep compiling, but the variant’s numeric value is no longer what the constructor produces.

MissingRequiredClientCapability = -32_021

SEP-2575: client capabilities advertised on the request do not include a capability required by the called method.

Originally -32003 in the SEP text; renumbered to -32021 by the upstream error-code allocation (spec PR modelcontextprotocol#2907).

UnsupportedProtocolVersion = -32_022

SEP-2575: server does not support the protocol version the client requested (via MCP-Protocol-Version header or per-request _meta).

Originally -32004 in the SEP text; renumbered to -32022 by the upstream error-code allocation (spec PR modelcontextprotocol#2907).

Use JsonRpcError::unsupported_protocol_version to construct.

SessionNotFound = -32_005

Session not found or expired – client should re-initialize.

SEP-2567 deprecates sessions entirely. This code stays for the 2025-11-25 protocol path; sessionless deployments will never emit it.

SessionRequired = -32_006

Session ID is required but was not provided.

SEP-2567 deprecates sessions entirely. Same caveat as SessionNotFound.

Forbidden = -32_007

Access forbidden (insufficient scope or authorization).

AlreadySubscribed = -32_008

Resource already subscribed. Moved from -32003 when that slot was believed to be spec-assigned; stays at -32008.

NotSubscribed = -32_009

Resource not subscribed (for unsubscribe). Moved from -32004 when that slot was believed to be spec-assigned; stays at -32009.

RequestTimeout = -32_010

Request exceeded timeout. Moved from -32001 when that slot was believed to be spec-assigned; stays at -32010.

UrlElicitationRequired = -32_042

URL elicitation is required before processing the request.

Implementations

impl McpErrorCode

pub fn code(self) -> i32

Return this MCP-specific error code as its JSON-RPC integer value.

Trait Implementations

impl Clone for McpErrorCode

fn clone(&self) -> McpErrorCode

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for McpErrorCode

impl Debug for McpErrorCode

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl Eq for McpErrorCode

impl PartialEq for McpErrorCode

fn eq(&self, other: &McpErrorCode) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl StructuralPartialEq for McpErrorCode