Transport Errors

Overview

Transport errors are thrown by the MCP transport layer — the component responsible for managing HTTP connections, sessions, and message framing. Most are internal errors except UnsupportedContentTypeError, which is a client-facing 400 error.

Error Reference

MethodNotImplementedError

Thrown when calling an abstract or placeholder method that has not been implemented.

Property	Type	Value
`code`	`string`	`METHOD_NOT_IMPLEMENTED`
`statusCode`	`number`	`500`
`isPublic`	`boolean`	`false`

new MethodNotImplementedError(className?: string, methodName?: string)

Example:

import { MethodNotImplementedError } from '@frontmcp/sdk';

throw new MethodNotImplementedError('CustomTransport', 'send');
// "CustomTransport.send() is not implemented"

UnsupportedTransportTypeError

Thrown when an unsupported transport type is specified in configuration.

Property	Type	Value
`code`	`string`	`UNSUPPORTED_TRANSPORT_TYPE`
`statusCode`	`number`	`500`
`isPublic`	`boolean`	`false`

new UnsupportedTransportTypeError(transportType: string)

Example:

import { UnsupportedTransportTypeError } from '@frontmcp/sdk';

throw new UnsupportedTransportTypeError('websocket');
// "Unsupported transport type: "websocket""

TransportBusRequiredError

Thrown when a transport bus is required but not provided.

Property	Type	Value
`code`	`string`	`TRANSPORT_BUS_REQUIRED`
`statusCode`	`number`	`500`
`isPublic`	`boolean`	`false`

new TransportBusRequiredError()

InvalidTransportSessionError

Thrown when a transport session is invalid or missing.

Property	Type	Value
`code`	`string`	`INVALID_TRANSPORT_SESSION`
`statusCode`	`number`	`500`
`isPublic`	`boolean`	`false`

new InvalidTransportSessionError(message: string)

Example:

import { InvalidTransportSessionError } from '@frontmcp/sdk';

throw new InvalidTransportSessionError('Session ID missing from request headers');

TransportNotConnectedError

Thrown when an operation is attempted on a transport that is not connected.

Property	Type	Value
`code`	`string`	`TRANSPORT_NOT_CONNECTED`
`statusCode`	`number`	`500`
`isPublic`	`boolean`	`false`

new TransportNotConnectedError(context?: string)

Example:

import { TransportNotConnectedError } from '@frontmcp/sdk';

throw new TransportNotConnectedError('SSE channel closed');

TransportAlreadyStartedError

Thrown when attempting to start a transport that is already running.

Property	Type	Value
`code`	`string`	`TRANSPORT_ALREADY_STARTED`
`statusCode`	`number`	`500`
`isPublic`	`boolean`	`false`

new TransportAlreadyStartedError(message?: string)

UnsupportedContentTypeError

Thrown when a request uses an unsupported Content-Type header. This is a public error since it represents a client-side issue.

Property	Type	Value
`code`	`string`	`UNSUPPORTED_CONTENT_TYPE`
`statusCode`	`number`	`400`
`isPublic`	`boolean`	`true`
`contentType`	`string`	The unsupported content type

new UnsupportedContentTypeError(contentType: string)

Example:

import { UnsupportedContentTypeError } from '@frontmcp/sdk';

throw new UnsupportedContentTypeError('text/xml');
// "Unsupported content-type: text/xml"

TransportServiceNotAvailableError

Thrown when the transport service is requested but not registered (typically a misconfigured server bootstrap).

Property	Type	Value
`code`	`string`	`TRANSPORT_SERVICE_NOT_AVAILABLE`
`isPublic`	`boolean`	`false`

SessionClaimConflictError

Thrown in HA deployments when another pod claims an active session ID. The local pod must drop its in-memory state for that session.

Property	Type	Value
`code`	`string`	`SESSION_CLAIM_CONFLICT`
`isPublic`	`boolean`	`false`

import { SessionClaimConflictError } from '@frontmcp/sdk';

throw new SessionClaimConflictError(sessionId);

Decorators

API

Errors

Overview

Error Reference

MethodNotImplementedError

UnsupportedTransportTypeError

TransportBusRequiredError

InvalidTransportSessionError

TransportNotConnectedError

TransportAlreadyStartedError

UnsupportedContentTypeError

TransportServiceNotAvailableError

SessionClaimConflictError

​Overview

​Error Reference

​MethodNotImplementedError

​UnsupportedTransportTypeError

​TransportBusRequiredError

​InvalidTransportSessionError

​TransportNotConnectedError

​TransportAlreadyStartedError

​UnsupportedContentTypeError

​TransportServiceNotAvailableError

​SessionClaimConflictError

Overview

Error Reference

MethodNotImplementedError

UnsupportedTransportTypeError

TransportBusRequiredError

InvalidTransportSessionError

TransportNotConnectedError

TransportAlreadyStartedError

UnsupportedContentTypeError

TransportServiceNotAvailableError

SessionClaimConflictError