Auth Errors

Overview

Auth errors are thrown when authentication or authorization fails at the request level. These are public errors that inform clients about what action is needed (re-authenticate, authorize an app, etc.). For internal auth infrastructure errors, see Auth Internal Errors.

Error Reference

UnauthorizedError

Thrown when a request is missing valid credentials.

Property	Type	Value
`code`	`string`	`UNAUTHORIZED`
`statusCode`	`number`	`401`
`isPublic`	`boolean`	`true`

new UnauthorizedError(message?: string)

Example:

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

throw new UnauthorizedError('Bearer token expired');

AuthConfigurationError

Thrown when the authentication configuration is invalid (e.g., transparent mode on a parent with multiple child providers).

Property	Type	Value
`code`	`string`	`AUTH_CONFIGURATION_ERROR`
`statusCode`	`number`	`500`
`isPublic`	`boolean`	`true`
`errors`	`string[]`	List of configuration errors
`suggestion`	`string \| undefined`	How to fix the issue

new AuthConfigurationError(message: string, options?: {
  errors?: string[];
  suggestion?: string;
})

Example:

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

throw new AuthConfigurationError('Invalid auth configuration', {
  errors: ['Transparent mode requires exactly one child provider'],
  suggestion: 'Set auth.mode to "local" or "remote" when using multiple providers.',
});

SessionMissingError

Thrown when a request arrives without a valid session. This tells the client it needs to authenticate.

Property	Type	Value
`code`	`string`	`SESSION_MISSING`
`statusCode`	`number`	`401`
`isPublic`	`boolean`	`true`

new SessionMissingError(message?: string)

Example:

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

throw new SessionMissingError();
// "Unauthorized: missing session"

UnsupportedClientVersionError

Thrown when a client connects with an unsupported MCP protocol version.

Property	Type	Value
`code`	`string`	`UNSUPPORTED_CLIENT_VERSION`
`statusCode`	`number`	`400`
`isPublic`	`boolean`	`true`
`clientVersion`	`string`	The version string from the client

new UnsupportedClientVersionError(version: string)

// Factory method
UnsupportedClientVersionError.fromVersion(version: string)

Example:

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

throw UnsupportedClientVersionError.fromVersion('0.1.0');

AuthorizationRequiredError

Thrown when a tool requires app-level authorization the user has not yet granted. Supports progressive/incremental authorization. Behavior depends on session mode:

Stateful: Returns an auth_url link for incremental authorization
Stateless: Returns an unauthorized error (user must re-authenticate)

Property	Type	Value
`code`	`string`	`AUTHORIZATION_REQUIRED`
`statusCode`	`number`	`403`
`isPublic`	`boolean`	`true`
`appId`	`string`	App requiring authorization
`toolId`	`string`	Tool that triggered the requirement
`authUrl`	`string \| undefined`	Auth URL (stateful mode only)
`requiredScopes`	`string[] \| undefined`	Scopes needed
`sessionMode`	`'stateful' \| 'stateless'`	Session mode
`supportsIncremental`	`boolean`	Whether incremental auth is available
`elicitId`	`string \| undefined`	Elicit flow ID
`vaultId`	`string \| undefined`	Vault ID for stateful sessions
`pendingAuthId`	`string \| undefined`	Tracking ID

new AuthorizationRequiredError(params: {
  appId: string;
  toolId: string;
  authUrl?: string;
  requiredScopes?: string[];
  message?: string;
  sessionMode?: 'stateful' | 'stateless';
  elicitId?: string;
  vaultId?: string;
  pendingAuthId?: string;
})

Methods:

Method	Returns	Description
`toMcpError()`	MCP error with `_meta`	Includes authorization metadata for AI agents
`toAuthorizationRequiredData()`	`AuthorizationRequiredData`	Structured data object
`toElicitResponse()`	`ElicitResponse \| null`	Elicit response (stateful only)
`canUseIncrementalAuth()`	`boolean`	Whether link-based auth is available
`getUserFacingMessage()`	`string`	Message with auth link or re-auth instructions

Example (stateful):

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

throw new AuthorizationRequiredError({
  appId: 'slack',
  toolId: 'slack:send_message',
  authUrl: '/oauth/authorize?app=slack',
  sessionMode: 'stateful',
});

Example (stateless):

throw new AuthorizationRequiredError({
  appId: 'slack',
  toolId: 'slack:send_message',
  sessionMode: 'stateless',
});

ToolNotAllowedError

Thrown when a tool is not in the active skill session’s allowlist. Used by the Tool Authorization Guard.

Property	Type	Value
`code`	`string`	`TOOL_NOT_ALLOWED`
`statusCode`	`number`	`400`
`isPublic`	`boolean`	`true`
`toolName`	`string`	The denied tool name
`skillId`	`string \| undefined`	Active skill session ID
`reason`	`string`	`not_in_allowlist`, `denied`, `rate_limited`, or `no_active_skill`
`allowedTools`	`string[]`	List of allowed tool names

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

ToolApprovalRequiredError

Thrown when a tool requires explicit approval before use within a skill session (approval policy mode).

Property	Type	Value
`code`	`string`	`TOOL_APPROVAL_REQUIRED`
`statusCode`	`number`	`400`
`isPublic`	`boolean`	`true`
`toolName`	`string`	Tool requiring approval
`skillId`	`string \| undefined`	Active skill session ID

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

Decorators

API

Errors

Overview

Error Reference

UnauthorizedError

AuthConfigurationError

SessionMissingError

UnsupportedClientVersionError

AuthorizationRequiredError

ToolNotAllowedError

ToolApprovalRequiredError

​Overview

​Error Reference

​UnauthorizedError

​AuthConfigurationError

​SessionMissingError

​UnsupportedClientVersionError

​AuthorizationRequiredError

​ToolNotAllowedError

​ToolApprovalRequiredError

Overview

Error Reference

UnauthorizedError

AuthConfigurationError

SessionMissingError

UnsupportedClientVersionError

AuthorizationRequiredError

ToolNotAllowedError

ToolApprovalRequiredError