Transport Errors

Errors for transport layer failures including connection, session, and content type issues.

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"