api-errors

Overview

Error handling in @cyanheads/mcp-ts-core follows a strict layered pattern: tool and resource handlers throw McpError freely (no try/catch), the handler factory catches and normalizes all errors, and services use ErrorHandler.tryCatch for graceful recovery.

Imports:

import { notFound, validationError, McpError, JsonRpcErrorCode } from '@cyanheads/mcp-ts-core/errors';
import { ErrorHandler } from '@cyanheads/mcp-ts-core/utils';

---

Type-Driven Error Contract (recommended)

The recommended path for new tools and resources. Declare failure modes as a const tuple under errors; the reason union flows into the handler's ctx.fail and TypeScript enforces that you can only fail with a declared reason:

import { tool, z } from '@cyanheads/mcp-ts-core';
import { JsonRpcErrorCode } from '@cyanheads/mcp-ts-core/errors';

export const fetchTool = tool('fetch_articles', {
  description: 'Fetch articles by PMID',
  input: z.object({ pmids: z.array(z.string()).describe('PMIDs') }),
  output: z.object({ articles: z.array(z.unknown()).describe('Articles') }),

  errors: [
    { reason: 'no_match', code: JsonRpcErrorCode.NotFound,
      when: 'No requested PMID returned data',
      recovery: 'Try pubmed_search_articles to discover valid PMIDs first.' },
    { reason: 'queue_full', code: JsonRpcErrorCode.RateLimited,
      when: 'Local request queue is at capacity', retryable: true,
      recovery: 'Wait 30 seconds and retry, or reduce batch size.' },
    { reason: 'ncbi_down', code: JsonRpcErrorCode.ServiceUnavailable,
      when: 'NCBI E-utilities unreachable after retries', retryable: true,
      recovery: 'NCBI is degraded; retry in a few minutes.' },
  ],

  async handler(input, ctx) {
    const articles = await ncbi.fetch(input.pmids);
    if (articles.length === 0) {
      throw ctx.fail('no_match', `None of ${input.pmids.length} PMIDs returned data`);
    }
    // ctx.fail('typo')   ← TypeScript error: 'typo' isn't in the contract
    return { articles };
  },
});

What you get:

Surface	Behavior
Compile time	ctx.fail('typo') is a TS error. Auto-completes declared reasons.
Runtime	ctx.fail(reason, msg?, data?, options?) builds an McpError(contract.code, msg, { ...data, reason }, options) — data.reason is auto-populated from the contract and cannot be overridden by caller-supplied data (spread first, then reason written last), so observers see a stable identifier. options accepts { cause } for ES2022 error chaining.
Lint (startup)	Each code validated against JsonRpcErrorCode. Reasons validated as snake_case + unique within contract. recovery validated as non-empty and ≥ 5 words.
Lint (conformance)	If the handler throw new McpError(JsonRpcErrorCode.X) outside ctx.fail, conformance check warns when X isn't declared.

recovery is opt-in resolution, not auto-population. The contract recovery is required metadata documenting the agent's next move when this failure mode fires (a forcing function for thoughtful guidance — placeholders like "Try again." get flagged by the linter). It does not automatically appear in runtime data.recovery.hint — the framework never injects it without an explicit signal at the throw site. Authors opt in by spreading ctx.recoveryFor('reason') into the data argument, the same way ctx.fail('reason') opts into resolving the contract code. What the author types at the throw site is what flows to the wire, with no hidden transformation; the resolver is just a typed lookup keyed by the same reason the author already typed.

ctx.recoveryFor — opt-in contract resolution

ctx.recoveryFor(reason) returns { recovery: { hint: <contract.recovery> } } for a declared reason, ready to spread into data. Always available on Context (returns {} when no contract is attached or the reason is unknown — spread-safe with no optional chaining). On HandlerContext<R> it tightens to a typed signature constrained to the declared reason union.

export const calculateTool = tool('calculate', {
  // ...
  errors: [
    { reason: 'empty_expression', code: JsonRpcErrorCode.ValidationError,
      when: 'Expression is empty or whitespace-only.',
      recovery: 'Provide a non-empty mathematical expression to evaluate.' },
  ],
  handler(input, ctx) {
    if (!input.expression.trim()) {
      // Static recovery — resolve from the contract.
      throw ctx.fail('empty_expression', undefined, { ...ctx.recoveryFor('empty_expression') });
    }
    // ...
  },
});

Same pattern works inside services that accept ctx:

export class MathService {
  parse(expr: string, ctx: Context) {
    try {
      return mathjs.parse(expr);
    } catch (err) {
      throw validationError(`Parse failed: ${err.message}`, {
        reason: 'parse_failed',
        ...ctx.recoveryFor('parse_failed'),  // {} if calling tool has no matching reason
      });
    }
  }
}

The contract is the single source of truth — write the recovery once, lint validates ≥5 words, the resolver carries it to every throw site that opts in. For runtime-context recovery (interpolating input values, attempted IDs, queue state), override at the throw site:

throw ctx.fail('no_match', `No item ${id}`, {
  recovery: { hint: `No item ${id}; try IDs 1-100 instead.` },
});

ctx.recoveryFor is the first member of a planned family of opt-in resolution helpers. Future contract-bound fields (troubleshootingFor, userMessageFor, …) follow the same shape: single-purpose, spreadable wire-shape, {} fallback when not applicable.

Skip the contract for one-off internal tools or quick prototypes — ctx is plain Context (no fail) and you throw via factories directly. Behavior is identical at the wire; the contract just adds compile-time safety.

Declare contracts inline on each tool, even when similar across tools. The contract is part of the tool's documented public surface — reading one tool definition file should give the full picture (input, output, errors, handler, format). Don't extract a shared errors[] constant or contract module to deduplicate near-identical entries; per-tool repetition is the intended cost of locality, and dynamic recovery hints often need tool-specific runtime context anyway. If a code-cleanup pass suggests consolidating contracts, decline — the duplication is load-bearing for tool-def readability.

Limits of the conformance lint. The conformance and prefer-fail rules scan the handler's source text for throw statements. Errors thrown from called services (e.g. await myService.fetch() raising RateLimited internally) are invisible — the lint only sees what's lexically in the handler. Treat the contract as the advertised failure surface; bubbled-up codes still reach the client correctly via the auto-classifier, just without lint enforcement.

Carrying contract reason from services

Services don't have ctx, so they can't call ctx.fail. To make a service-thrown failure carry the contract's reason on the wire, pass data: { reason: 'X' } to the factory. The framework's auto-classifier preserves data unchanged, so clients see the same error.data.reason they'd see from ctx.fail:

// my-service.ts
throw validationError('Expression cannot be empty.',  { reason: 'empty_expression' });
throw serviceUnavailable('Upstream timeout',          { reason: 'evaluation_timeout' });

// my-tool.tool.ts
errors: [
  { reason: 'empty_expression',   code: JsonRpcErrorCode.ValidationError,
    when: 'Input is empty.',
    recovery: 'Provide a non-empty expression to evaluate.' },
  { reason: 'evaluation_timeout', code: JsonRpcErrorCode.ServiceUnavailable,
    when: 'Upstream exceeded the configured timeout.',
    recovery: 'Simplify the expression or retry the request after a brief delay.' },
]

The handler doesn't catch and re-throw — letting service errors bubble unchanged keeps "logic throws, framework catches" intact. The wire payload still carries code + data.reason, and clients can switch on reason without parsing message text. What's lost is lint-time enforcement that every reason is reachable; compensate with one wire-shape test per reason.

To carry the contract recovery from a service throw, accept ctx and spread the resolver:

throw validationError(message, {
  reason: 'parse_failed',
  ...ctx.recoveryFor('parse_failed'),  // {} when calling tool has no matching reason
});

ctx.recoveryFor is always present on Context (no-op when no contract), so services don't need to know which tool called them — the spread is safe either way.

---

When not to throw

Throw when the server has authoritative classification — auth failure, rate limit, schema violation, upstream 5xx, missing required input. Don't throw when "this looks wrong" depends on intent the server can't see. For mutators, surface raw pre- and post-mutation observable state in the response and let the agent decide whether it matches intent — the server can detect that the file shrunk, but only the agent knows whether it was supposed to. Tell: defensive code justified as a free rider on other work — audit it standalone, and it usually doesn't earn its keep.

---

Error Factories (fallback)

Use when no contract entry fits — ad-hoc throws, tools without a contract, or service-layer code. Shorter than new McpError(...) and self-documenting. All return McpError instances and accept an optional options parameter for error chaining via { cause }.

throw notFound('Item not found', { itemId: '123' });
throw validationError('Missing required field: name', { field: 'name' });
throw unauthorized('Token expired');

// With cause for error chaining
throw serviceUnavailable('API call failed', { url }, { cause: error });

Available factories:

Factory	Code
invalidParams(msg, data?, options?)	InvalidParams (-32602)
invalidRequest(msg, data?, options?)	InvalidRequest (-32600)
notFound(msg, data?, options?)	NotFound (-32001)
forbidden(msg, data?, options?)	Forbidden (-32005)
unauthorized(msg, data?, options?)	Unauthorized (-32006)
validationError(msg, data?, options?)	ValidationError (-32007)
conflict(msg, data?, options?)	Conflict (-32002)
rateLimited(msg, data?, options?)	RateLimited (-32003)
timeout(msg, data?, options?)	Timeout (-32004)
serviceUnavailable(msg, data?, options?)	ServiceUnavailable (-32000)
configurationError(msg, data?, options?)	ConfigurationError (-32008)
internalError(msg, data?, options?)	InternalError (-32603)
serializationError(msg, data?, options?)	SerializationError (-32070) — JSON/XML/parser failures
databaseError(msg, data?, options?)	DatabaseError (-32010)

options is { cause?: unknown } — the standard ES2022 ErrorOptions type.

---

McpError Constructor

For codes not covered by factories (rare — MethodNotFound, ParseError, InitializationFailed, UnknownError):

throw new McpError(code, message?, data?, options?)

• code — a JsonRpcErrorCode enum value
• message — optional human-readable description of the failure
• data — optional structured context (plain object)
• options — optional { cause?: unknown } for error chaining

Example:

import { McpError, JsonRpcErrorCode } from '@cyanheads/mcp-ts-core/errors';

throw new McpError(JsonRpcErrorCode.DatabaseError, 'Connection pool exhausted', {
  pool: 'primary',
});

---

Error Codes

Standard JSON-RPC 2.0 codes:

Code	Value	When to Use
ParseError	-32700	Malformed JSON received
InvalidRequest	-32600	Unsupported operation, missing client capability
MethodNotFound	-32601	Requested method does not exist
InvalidParams	-32602	Bad input, missing required fields, schema validation failure
InternalError	-32603	Unexpected failure, catch-all for programmer errors

Implementation-defined codes (-32000 to -32099):

Code	Value	When to Use
ServiceUnavailable	-32000	External dependency down, upstream failure
NotFound	-32001	Resource, entity, or record doesn't exist
Conflict	-32002	Duplicate key, version mismatch, concurrent modification
RateLimited	-32003	Rate limit exceeded
Timeout	-32004	Operation exceeded time limit
Forbidden	-32005	Authenticated but insufficient scopes/permissions
Unauthorized	-32006	No auth, invalid token, expired credentials
ValidationError	-32007	Business rule violation (not schema — use InvalidParams for that)
ConfigurationError	-32008	Missing env var, invalid config
InitializationFailed	-32009	Server/component startup failure
DatabaseError	-32010	Storage/persistence layer failure
SerializationError	-32070	Data serialization/deserialization failed
UnknownError	-32099	Generic fallback when no other code fits

---

Auto-Classification

When a handler throws a plain Error (or any non-McpError value), the framework classifies it to the most specific JsonRpcErrorCode automatically. This matters when you don't control what a third-party library throws and can't predict its error type.

Use factories or McpError directly when the code must be exact — auto-classification is best-effort pattern matching and not guaranteed for ambiguous messages. For errors from your own code where the code matters, be explicit.

Resolution Order

The framework applies these steps in order — first match wins:

1. McpError instance — error.code is preserved as-is; no classification needed.
2. JS constructor name — matched against a fixed table (e.g. TypeError → ValidationError).
3. Provider-specific patterns — HTTP status codes, AWS exception names, Supabase, OpenRouter. Checked before common patterns because they are more specific (e.g. status code 429 beats the generic rate limit pattern).
4. Common message/name patterns — broad keyword patterns covering auth, not-found, validation, etc. First match wins; order matters.
5. AbortError name — error.name === 'AbortError' → Timeout.
6. Fallback — InternalError.

JS Constructor Name Mappings

Constructor	Mapped Code
SyntaxError	ValidationError
RangeError	ValidationError
URIError	ValidationError
ZodError	ValidationError
ReferenceError	InternalError
EvalError	InternalError
AggregateError	InternalError

TypeError is intentionally excluded from the constructor table — runtime TypeErrors (e.g. "Cannot read property X of undefined") are programmer errors, not validation failures. They fall through to message-pattern matching, then to the InternalError fallback.

Common Message Patterns

Patterns are tested against both the error message and name, case-insensitively. First match wins.

Pattern (regex)	Mapped Code
unauthorized|unauthenticated|not\s+authorized|not.*logged.*in|invalid[\s_-]+token|expired[\s_-]+token	Unauthorized
permission|forbidden|access.*denied|not.*allowed	Forbidden
not found|no such|doesn't exist|couldn't find	NotFound
invalid|validation|malformed|bad request|wrong format|missing\s+(?:required|param|field|input|value|arg)	ValidationError
conflict|already exists|duplicate|unique constraint	Conflict
rate limit|too many requests|throttled	RateLimited
timeout|timed out|deadline exceeded	Timeout
abort(ed)?|cancell?ed	Timeout
service unavailable|bad gateway|gateway timeout|upstream error	ServiceUnavailable
zod|zoderror|schema validation	ValidationError

Provider-Specific Patterns

Checked before common patterns. Cover: AWS exception names, HTTP status codes, DB connection/constraint errors, Supabase JWT/RLS, OpenRouter/LLM quota errors, and low-level network errors.

Pattern	Mapped Code
ThrottlingException|TooManyRequestsException	RateLimited
AccessDenied|UnauthorizedOperation	Forbidden
ResourceNotFoundException	NotFound
status code 401	Unauthorized
status code 403	Forbidden
status code 404	NotFound
status code 409	Conflict
status code 429	RateLimited
status code 5xx	ServiceUnavailable
ECONNREFUSED|connection refused	ServiceUnavailable
ETIMEDOUT|connection timeout	Timeout
unique constraint|duplicate key	Conflict
foreign key constraint	ValidationError
JWT expired	Unauthorized
row level security	Forbidden
insufficient_quota|quota exceeded	RateLimited
model_not_found	NotFound
context_length_exceeded	ValidationError
ENOTFOUND|DNS	ServiceUnavailable
ECONNRESET|connection reset	ServiceUnavailable

---

Where Errors Are Handled

Layer	Pattern
Tool/resource handlers	Throw McpError — no try/catch
Handler factory (tools)	Catches all errors, normalizes to McpError, sets isError: true, mirrors error across both client surfaces (see Error-path parity)
Handler factory (resources)	Catches and re-throws to the SDK, which routes through the JSON-RPC error envelope
Services/setup code	ErrorHandler.tryCatch for graceful recovery

Error-path parity

MCP clients differ in which CallToolResult surface they forward to the agent. Tool errors mirror the success-path format-parity invariant — both surfaces carry the same payload:

Surface	Content	Read by
content[]	Text rendering: Error: <message> (plus Recovery: <hint> when data.recovery.hint is present)	Claude Desktop and other format()-only clients
structuredContent.error	JSON { code, message, data? } carrying the error code, message, and any structured data from the thrown McpError or ZodError	Claude Code and other structuredContent-only clients

Important properties:

• _meta.error is NOT emitted. Error code/data live on structuredContent.error instead. Don't read _meta.error in clients or tests — it doesn't exist.
• data propagation is restricted to explicitly-thrown McpError.data and ZodError.issues. Auto-classified plain errors (TypeError, network errors, etc.) emit code + message only — no data — so internal classification context never leaks to clients.
• Recovery hint mirroring is automatic. When the thrown McpError carries data.recovery.hint, the handler factory appends it to the content[] text so the markdown surface matches the JSON surface. Authors don't need to format the hint manually.

Handler — throw freely, no try/catch:

import { notFound } from '@cyanheads/mcp-ts-core/errors';

export const myTool = tool('my_tool', {
  input: z.object({ id: z.string().describe('Item ID') }),
  output: z.object({ id: z.string(), name: z.string(), status: z.string() }),
  async handler(input, ctx) {
    const item = await db.find(input.id);
    if (!item) {
      throw notFound(`Item not found: ${input.id}`, { id: input.id });
    }
    return item;
  },
});

---

ErrorHandler.tryCatch (Services)

Use ErrorHandler.tryCatch in service code, not in tool handlers. It wraps arbitrary exceptions into McpError and supports structured logging context.

import { ErrorHandler } from '@cyanheads/mcp-ts-core/utils';

// Works with both async and sync functions
const result = await ErrorHandler.tryCatch(
  () => externalApi.fetch(url),
  {
    operation: 'ExternalApi.fetch',
    context: { url },
    errorCode: JsonRpcErrorCode.ServiceUnavailable,
  },
);

const parsed = await ErrorHandler.tryCatch(
  () => JSON.parse(raw),
  {
    operation: 'parseConfig',
    errorCode: JsonRpcErrorCode.ConfigurationError,
  },
);

tryCatch always logs and rethrows — it never swallows errors. The fn argument may be synchronous or return a Promise; both are handled via Promise.resolve(fn()).

Options (Omit<ErrorHandlerOptions, 'rethrow'>):

Option	Type	Required	Purpose
operation	string	Yes	Name logged with the error
context	ErrorContext	No	Extra structured fields merged into the log record; requestId and timestamp receive special treatment
errorCode	JsonRpcErrorCode	No	Code used if the caught error is not already an McpError
input	unknown	No	Input value sanitized and logged alongside the error
critical	boolean	No	Marks the error as critical in logs (default false)
includeStack	boolean	No	Include stack trace in log output (default true)
errorMapper	(error: unknown) => Error	No	Custom transform applied instead of default McpError wrapping

---

HTTP Response → McpError

When you bypass fetchWithTimeout and use raw fetch (typically because you need granular code classification or response body access), use httpErrorFromResponse instead of writing your own status mapping ladder:

import { httpErrorFromResponse } from '@cyanheads/mcp-ts-core/utils';

const response = await fetch(url, { signal: ctx.signal });
if (!response.ok) {
  throw await httpErrorFromResponse(response, {
    service: 'NCBI',                  // included in message
    data: { endpoint, requestId: ctx.requestId },
  });
}

Captures the response body (truncated, configurable limit) and Retry-After header (stored as data.retryAfter) into error.data. The codes it produces line up with withRetry's transient-code set, so retryable responses are retried automatically.

Body reaches the client. error.data is forwarded to the MCP client as structuredContent.error.data (tool errors) or JSON-RPC error.data (resource errors). Upstream 401/403/422 responses sometimes echo token claims, internal user IDs, or schema validation hints — that text becomes client-visible. For sensitive endpoints, pass captureBody: false (or bodyLimit: 0) so the body stays out of data. Defaults remain captureBody: true because most upstreams return useful diagnostic text and silent dropping helps no one debug.

Full status table:

Status	Code
400	InvalidParams
401	Unauthorized
402, 403	Forbidden
404	NotFound
408, 425, 504	Timeout
409, 423, 424	Conflict
422	ValidationError
429	RateLimited
405, 406, 410, 412, 415, 416, 417, 428, 431, 451, 4xx (other)	InvalidRequest
500, 501	InternalError
502, 503, 5xx (other)	ServiceUnavailable

Also exports httpStatusToErrorCode(status) for sync mapping when you don't have a Response object.

---

Handler-Body Lint Rules

The startup linter (bun run lint:mcp and createApp() startup) checks handler bodies for common anti-patterns. All emit warnings (not errors) — they don't block startup but show up in devcheck output.

Rule	Catches
prefer-mcp-error-in-handler	throw new Error(...) inside a handler — use McpError or a factory so the framework returns a specific code
prefer-error-factory	new McpError(JsonRpcErrorCode.NotFound, ...) when notFound(...) exists
preserve-cause-on-rethrow	catch (e) { throw new McpError(...) } without { cause: e }
no-stringify-upstream-error	JSON.stringify(...) inside a thrown message — risks leaking internal traces; use data payload instead

---

Error Contract Lint Rules

The linter validates the structure of errors[] and (when present) cross-checks the handler body against the declared contract.

Structural rules

Rule	Severity	Catches
error-contract-type	error	errors is present but not an array
error-contract-empty	warning	errors: [] — drop the field instead, or declare actual failure modes
error-contract-entry-type	error	An entry isn't an object
error-contract-code-type	error	code missing or not a number
error-contract-code-unknown	error	code isn't a real JsonRpcErrorCode value
error-contract-code-unknown-error	warning	code is JsonRpcErrorCode.UnknownError (the giveup-fallback — pick a more specific code)
error-contract-reason-required	error	reason missing or empty
error-contract-reason-format	warning	reason not snake_case
error-contract-reason-unique	error	Duplicate reason within one contract
error-contract-when-required	error	when missing or empty
error-contract-recovery-required	error	recovery missing or not a string
error-contract-recovery-empty	error	recovery is empty/whitespace-only
error-contract-recovery-min-words	warning	recovery has fewer than 5 words — placeholders like "Try again." or "Check input." get flagged in favor of specific guidance
error-contract-retryable-type	warning	retryable is present but not a boolean

Conformance rules

Rule	Severity	Catches
error-contract-conformance	warning	Handler throws a non-baseline code that isn't in the contract. Suggests adding it to errors[] so the contract is the canonical source of truth for declared failure modes.
error-contract-prefer-fail	warning	Handler throws a code that is in the contract directly (via factory or new McpError) instead of through ctx.fail(reason, …). Encourages routing through the typed helper so observers see consistent data.reason values.

Baseline codes (auto-allowed)

These codes bubble up from anywhere — services, framework utilities, the auto-classifier — and are implicitly always-possible on any tool. They're skipped by the conformance check, so the contract can stay focused on intentional domain failures:

• InternalError — bug, programmer error, truly unexpected
• ServiceUnavailable — upstream/network failures
• Timeout — request deadline exceeded, abort
• ValidationError — schema violations, malformed input
• SerializationError — JSON/XML parse failures

If you want to declare one of these as a domain-specific failure (e.g., a tool that intentionally times out under defined conditions), put it in errors[] anyway — the contract still binds ctx.fail(reason) and the conformance lint will catch undeclared throws. The lint just doesn't require you to enumerate baselines.

When to declare vs. let it bubble

The contract describes the public failure surface — the failures clients/agents can plan around. Modeled after how OpenAPI-driven frameworks treat 5xx: enumerated 4xx for intentional failures, implicit 5xx for infrastructure.

Pattern	Use for
throw ctx.fail('reason', …)	Declared domain failures — typed, contract-checked, data.reason populated
throw notFound(…) / factories	Errors not in the contract; the auto-classifier handles them. Prefer ctx.fail when a matching contract entry exists.
Bubble up from services	Upstream classification already produced an McpError — don't re-wrap