api-context

name	api-context
description	Canonical reference for the unified `Context` object passed to every tool and resource handler in `@cyanheads/mcp-ts-core`. Covers the full interface, its `RequestContext` base, all sub-APIs (`ctx.log`, `ctx.state`, `ctx.requestInput`, `ctx.inputs`, `ctx.enrich`, `ctx.content`), and when to use each.
metadata	{"author":"cyanheads","version":"2.1","audience":"external","type":"reference"}

Overview

Every tool and resource handler receives a single Context (ctx) argument. It provides request identity, structured logging, tenant-scoped storage, multi-round-trip input collection, and cancellation — all auto-correlated to the current request.

The framework auto-instruments every handler call (OTel span, duration, payload metrics). Use ctx.log for domain-specific logging and ctx.state for storage inside handlers. Use the global logger and StorageService directly only in lifecycle/background code (setup(), services).

---

Context interface

import type { Context } from '@cyanheads/mcp-ts-core';

interface Context extends RequestContext {
  // Identity & tracing (inherited from RequestContext — see § RequestContext)
  readonly requestId: string;       // Unique per request, auto-generated
  readonly timestamp: string;       // ISO 8601 request start time
  readonly tenantId?: string;       // JWT 'tid' claim; 'default' for stdio and HTTP+MCP_AUTH_MODE=none
  readonly sessionId?: string;      // Mcp-Session-Id (HTTP stateful/auto); undefined elsewhere unless opted in
  readonly traceId?: string;        // Trace containing this handler execution
  readonly spanId?: string;         // The handler's own execution span
  readonly auth?: AuthContext;      // Parsed auth claims (clientId, scopes, sub)
  readonly operation?: string;      // Label for the operation this context belongs to
  readonly extra?: Readonly<Record<string, unknown>>;  // Correlation bag — the one open field

  // Structured logging — auto-includes requestId, traceId, tenantId.
  // Dual-sink: Pino on the server, plus notifications/message to the client.
  readonly log: ContextLogger;

  // Tenant-scoped key-value storage
  readonly state: ContextState;

  // Multi-round-trip input — always present, both eras (see § ctx.requestInput)
  readonly requestInput: RequestInputFn;   // (spec) => never — suspends and asks the caller
  readonly inputs: ContextInputs;          // reader over a retried request's responses

  // List-changed / resource-updated notifications — wired in every handler ctx;
  // delivery is request-scoped (see § list-changed notifications)
  readonly notifyResourceListChanged?: () => void;
  readonly notifyResourceUpdated?: (uri: string) => void;
  readonly notifyPromptListChanged?: () => void;
  readonly notifyToolListChanged?: () => void;

  // Cancellation
  readonly signal: AbortSignal;

  // Raw URI — present only for resource handlers
  readonly uri?: URL;

  // Agent-facing success-path enrichment — accumulates notices, query echo, totals
  // onto the request; reaches structuredContent + content[]. Always present (no-op
  // when no `enrichment` block), strictly typed on HandlerContext<R, E> against the
  // declared fields. Kind-tagged helpers: enrich.notice / .total / .echo.
  readonly enrich: Enrich;

  // Non-text content blocks (image/audio bytes) for the calling model — prepended
  // to content[] after format() runs, never placed in structuredContent. Always
  // present (no-op when never called). Helpers: content.image / .audio; content(block)
  // pushes a raw ContentBlock.
  readonly content: ContentCollect;

  // Opt-in contract resolver — always present (returns {} when no contract is attached
  // or the reason is unknown), strictly typed on HandlerContext<R> against declared reasons.
  recoveryFor(reason: string): { recovery: { hint: string } } | {};
}

ctx.fail is on HandlerContext<R>, not Context. When a definition declares errors: [...], the handler receives HandlerContext<R> = Context & { fail: TypedFail<R>; recoveryFor: TypedRecoveryFor<R> } — both the typed fail and the strictly-typed recoveryFor live on the intersection. The bare Context.recoveryFor is the loose, always-present resolver. See ctx.fail and ctx.recoveryFor below.

Identity fields

Field	Always present	Source
requestId	Yes	Auto-generated UUID per request
timestamp	Yes	ISO 8601, request start
tenantId	Stdio and HTTP+MCP_AUTH_MODE=none (as 'default'); JWT tid claim in HTTP+jwt/oauth	JWT / single-tenant default
sessionId	HTTP stateful / auto mode; undefined for stdio and stateless HTTP unless opted in	Mcp-Session-Id header (or server-minted) — see § ctx.sessionId
traceId	When OTEL enabled	Trace containing this handler execution
spanId	When OTEL enabled	The active tool_execution:* / resource_read:* span
auth	When auth enabled	Parsed JWT claims

---

RequestContext — the one canonical request shape

Context extends RequestContext. There is a single request-shape type; the handler-facing Context adds handler-only surfaces (log, state, signal, requestInput, inputs, enrich, content, uri) on top of it and redeclares none of the identity fields. A handler's ctx is therefore assignable anywhere a RequestContext is — services, storage, the framework logger — with no slice helper and no cast.

import { requestContextService, withExtra } from '@cyanheads/mcp-ts-core/utils';
import type { RequestContext } from '@cyanheads/mcp-ts-core/utils';

// A service typed against RequestContext accepts a handler ctx directly.
async function fetchUser(id: string, ctx: RequestContext) { /* … */ }
await fetchUser('123', ctx);   // ctx is a Context — no conversion

Closed by design

RequestContext has no index signature. Its fields are exactly: auth, extra, operation, requestId, sessionId, spanId, tenantId, timestamp, traceId. A misspelled canonical field (tenatId) is a compile error instead of a silently-ignored key.

Operation-specific correlation data goes in extra — the one deliberate open bag (Readonly<Record<string, unknown>>). The logger flattens extra into the emitted line, so log output looks the same as a top-level spread.

Adding correlation data

Three supported ways, most common first:

// 1. Per-log-call metadata — the common case. Nothing lands on the context.
ctx.log.info('Retrying upstream call', { attempt, url });

// 2. A copy of this context carrying extra fields. `withExtra` MERGES into any
//    bag the parent already had; a hand-written `{ ...ctx, extra: {…} }` replaces it.
logger.warning('Retrying upstream call', withExtra(ctx, { attempt, url }));

// 3. A derived context for a sub-operation. `additionalContext` lands on `extra`,
//    merged over whatever the parent already carried.
const childCtx = requestContextService.createRequestContext({
  parentContext: ctx,
  operation: 'processItem',
  additionalContext: { itemId: item.id },   // → childCtx.extra.itemId
});

// Reading an ad-hoc key back off a context:
const itemId = childCtx.extra?.itemId;

createRequestContext(params) takes a closed parameter object — additionalContext, operation, parentContext, tenantId — and nothing else; a key it doesn't declare is a compile error rather than an arbitrary passthrough.

Never re-open the shape to get past a type error: no index signature, no widening a parameter back to Record<string, unknown>, no as cast. A { ...ctx, someKey } object literal that fails to compile is the signal to move someKey into extra, not to loosen the type.

ErrorContext (the ErrorHandler call's context) is Partial<RequestContext> and is closed the same way — put ad-hoc keys under extra via withExtra, or pass them in the ErrorHandler call's own context field.

RequestContextLike is a deprecated alias for RequestContext, kept for one minor. Replace every use with RequestContext, and collapse any RequestContextLike | RequestContext parameter union to plain RequestContext.

---

ctx.log

Request-scoped structured logger. Every log line is automatically annotated with requestId, traceId, and tenantId — no manual spreading needed.

Dual-sink. Each call writes to Pino and mirrors onto the MCP wire as a notifications/message (the framework advertises the logging capability, and the SDK filters by the level the client set via logging/setLevel). The wire payload is { message, ...data }; ctx.log.error adds error: <message>. Delivery is fire-and-forget — a client that never upgraded to SSE, set a higher level, or already disconnected drops the notification, and a failed send never fails the handler. Treat ctx.log as client-visible: it is no longer a server-only sink, so don't log anything there you wouldn't put in a tool result.

Methods

Method	Level
ctx.log.debug(msg, data?)	Verbose debugging
ctx.log.info(msg, data?)	Normal operational events
ctx.log.notice(msg, data?)	Significant but non-error events
ctx.log.warning(msg, data?)	Recoverable issues, unexpected states
ctx.log.error(msg, error?, data?)	Errors (second arg is the Error object)

Usage

// Basic
ctx.log.info('Processing query', { query: input.query });

// With error object (second arg)
ctx.log.error('Failed to fetch upstream', error, { url, statusCode });

// Debug detail
ctx.log.debug('Cache miss', { key, ttl });

ctx.log vs global logger

Use	Where
ctx.log	Inside tool/resource handlers — auto-correlated to the request
core.logger / logger	In setup(), service constructors, background tasks — no request context available

The global logger is imported from @cyanheads/mcp-ts-core/utils. In handlers, prefer ctx.log.

---

ctx.state

Tenant-scoped key-value storage. Delegates to StorageService with automatic tenantId scoping — data written under tenant A is invisible to tenant B.

Interface

interface ContextState {
  get<T = unknown>(key: string): Promise<T | null>;
  get<T>(key: string, schema: ZodType<T>): Promise<T | null>;  // runtime-validated
  set(key: string, value: unknown, opts?: { ttl?: number }): Promise<void>;
  delete(key: string): Promise<void>;
  deleteMany(keys: string[]): Promise<number>;
  getMany<T = unknown>(keys: string[]): Promise<Map<string, T>>;
  setMany(entries: Map<string, unknown>, opts?: { ttl?: number }): Promise<void>;
  list(prefix?: string, opts?: { cursor?: string; limit?: number }): Promise<{
    items: Array<{ key: string; value: unknown }>;
    cursor?: string;  // opaque base64url; omitted on last page
  }>;
}

Usage

// Store — accepts any serializable value, no manual JSON.stringify needed
await ctx.state.set('item/123', { name: 'Widget', count: 42 });
await ctx.state.set('session/xyz', token, { ttl: 3600 }); // TTL in seconds

// Retrieve — generic type assertion or Zod-validated
const item = await ctx.state.get<Item>('item/123');       // T | null (type assertion)
const safe = await ctx.state.get('item/123', ItemSchema);  // T | null (runtime validated)

// Delete
await ctx.state.delete('item/123');

// Batch operations
const values = await ctx.state.getMany<Item>(['item/1', 'item/2']); // Map<string, T>
await ctx.state.setMany(new Map([['a', 1], ['b', 2]]));
const deleted = await ctx.state.deleteMany(['item/1', 'item/2']);    // number

// List with prefix + pagination
const page = await ctx.state.list('item/', { cursor, limit: 20 });
for (const { key, value } of page.items) { /* ... */ }
if (page.cursor) { /* more pages available */ }

Behavior notes

• Throws McpError(InvalidRequest) if tenantId is missing. Won't happen in stdio (any auth mode) or HTTP+MCP_AUTH_MODE=none — both default to 'default'. Can happen in HTTP+MCP_AUTH_MODE=jwt/oauth when the token lacks a tid claim (intentional fail-closed: distinct authenticated callers must not silently share state).
• Keys are tenant-prefixed internally; handlers never need to namespace manually.
• Key charset: ^[a-zA-Z0-9_.\-/]+$, 1024 chars max, no ... Slashes are the namespace separator — a colon (item:123) throws McpError(ValidationError) on every call. The rule covers list prefixes and every key in a batch operation. createMockContext().state enforces it identically, so an illegal key fails in the test rather than in a deployment.
• Workers persistence: The in-memory provider loses data on cold starts. Use cloudflare-kv, cloudflare-r2, or cloudflare-d1 for durable storage in Workers.

---

ctx.sessionId

Optional HTTP session identifier. Surfaced when the request carries a durable session — handlers use it as a discovery / scoping key on top of tenant-keyed ctx.state, not as an authorization principal.

When it's defined

Transport / mode	ctx.sessionId
stdio (any auth)	undefined
HTTP, MCP_SESSION_MODE=stateless	undefined (default) — see opt-in
HTTP, stateful / auto, MCP_AUTH_MODE=none	session token; possession = access (no identity binding)
HTTP, stateful / auto, MCP_AUTH_MODE=jwt / oauth	session token, identity-bound — hijack mismatches are rejected by SessionStore.isValidForIdentity before the handler runs

In stateful / auto mode, the value mirrors the Mcp-Session-Id HTTP header (or a server-minted token for new sessions). Each subsequent request from the same client reuses it; reconnects after disconnect bind to the same session as long as it hasn't expired.

The in-flight SSE stream is resumable too: stateful sessions carry a bounded event store, so a client reconnecting with Last-Event-ID gets the frames it missed replayed before the live stream resumes. On by default — selecting the session mode is the opt-in — with MCP_HTTP_RESUMABILITY=false as the kill switch and retention capped by both event count and TTL (api-config has the knobs). The buffer is released when its session is evicted.

Stateless-mode opt-in

In stateless HTTP mode the SDK still hands the framework a freshly generated token for every request, but it has request-lifetime semantics (no SessionStore, no continuity). The framework hides this from handlers by default — ctx.sessionId is undefined so any handler treating it as durable fails closed.

To surface the per-request token anyway, opt in via createApp:

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

await createApp({
  tools: [...],
  context: {
    exposeStatelessSessionId: true,
  },
});

Use this only when downstream code is structured around ctx.sessionId and accepts that the value changes per-request. For generic per-request correlation, use ctx.requestId (always present, no opt-in).

Capability-token model

Surfacing sessionId does not change the framework's capability-as-token rule (possession of an opaque ID grants access — see CLAUDE.md/AGENTS.md # Core Rules). It is an opt-in discovery-scoping axis, not an access boundary.

• Tokens shared across sessions (e.g. df_<uuid> handed from Agent A to Agent B) still resolve on the receiving side. The lookup key is the token, not the session.
• Session-scoped enumeration (e.g. dataframe_describe returning only items registered by the current session) is a per-server pattern: maintain a session-keyed lookup of known names, gate list-all on it, but route direct lookups against the shared backing store.

This matches deployments like brapi-mcp-server under MCP_AUTH_MODE=none: each session gets its own _connect alias surface and its own dataframe_describe enumeration scope, while any agent holding a df_<uuid> token can query it directly across session boundaries.

Recipes

Strict — fail closed when no session is present:

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

if (!ctx.sessionId) {
  throw invalidRequest('Session required for this operation.');
}
await ctx.state.set(`session/${ctx.sessionId}/${baseKey}`, value);

Lax — fall back to tenant-shared key:

const sessionKey = ctx.sessionId
  ? `session/${ctx.sessionId}/${baseKey}`
  : baseKey;
await ctx.state.set(sessionKey, value);

Reading the matching log correlation field. The framework's auto-instrumented logs always carry the raw SDK session token (even in stateless mode, for tracing) under the sessionId field. Don't read ctx.sessionId and pass it to ctx.log — the logger already has it.

Behavior notes

• Not a tenant boundary. ctx.state is still tenant-scoped. Building session-scoped state is the consumer's responsibility — prefix with session/${ctx.sessionId}/ as shown above.
• Protocol revision. Sessions belong to the 2025-era arm, which negotiates its revision through initialize and carries Mcp-Session-Id. The 2026-07-28 revision has no session at all — it is per-request, selected by the request's own _meta envelope — so ctx.sessionId is undefined for every request served on that leg.
• Worker bundle. Workers use the same HTTP transport plumbing; session behavior matches Node HTTP.

---

ctx.requestInput / ctx.inputs

Always present, on every transport and both protocol eras. A handler that needs something the caller didn't supply returns ctx.requestInput(...) and is re-entered with the answers on ctx.inputs — there is no mid-handler await for user input.

ctx.requestInput(spec) never returns: it throws an InputRequiredSignal that the tool, resource, and prompt handler factories catch and convert into the protocol's input_required result. It bypasses the error classifier entirely — no span, no log, no isError.

One code path serves both eras. A 2026-07-28 client fulfils the embedded requests and retries the call; for a 2025-era session the SDK's legacy shim fulfils the same returns by issuing real elicitation/create / sampling/createMessage / roots/list round trips and re-entering the handler itself.

The shape of a multi-round-trip handler

Read ctx.inputs first, request only what is still missing, and write the call in return position so TypeScript narrows the line below it.

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

const Confirm = z.object({
  confirm: z.boolean().describe('Whether to proceed with the deletion.'),
});

export const deletePath = tool('delete_path', {
  description: 'Delete a path after confirming with the user.',
  input: z.object({ path: z.string().describe('Path to delete') }),
  output: z.object({ deleted: z.string().describe('The path that was deleted') }),
  annotations: { destructiveHint: true },

  handler(input, ctx) {
    // A declined or cancelled prompt is a dead end, not a round to retry —
    // re-asking loops until the round budget runs out.
    const view = ctx.inputs.view('confirm');
    if (view.kind === 'elicit' && view.action !== 'accept') {
      throw validationError(`User ${view.action} the confirmation.`);
    }

    const answer = ctx.inputs.accepted('confirm', Confirm);
    if (!answer) {
      return ctx.requestInput({
        inputRequests: {
          confirm: inputRequired.elicit({
            message: `Delete ${input.path}?`,
            requestedSchema: Confirm,
          }),
        },
      });
    }
    // `answer` is narrowed here.
    if (!answer.confirm) throw validationError('Deletion not confirmed.');

    return { deleted: remove(input.path) };
  },
});

ctx.requestInput returns never, so return ctx.requestInput(...) type-checks against any output type. Calling it as a bare statement works at runtime — and is the only option from a service-layer helper — but TypeScript will not narrow across it.

Building the embedded requests

inputRequired is re-exported from the main entry. Its per-kind constructors build the entries of inputRequests:

Constructor	Wire request	Notes
inputRequired.elicit({ message, requestedSchema })	elicitation/create (form)	requestedSchema accepts a Zod schema; shapes the restricted elicitation JSON Schema can't express throw a TypeError before anything is sent
inputRequired.elicitUrl({ message, url })	elicitation/create (URL)	Authorization flows, hosted forms. On 2026-07-28 URL mode rides the same multi-round-trip flow; the 2025-era elicitationId is not part of that shape — correlate with your own identifier inside requestState
inputRequired.createMessage(params)	sampling/createMessage	Ask the client's model
inputRequired.listRoots()	roots/list	Ask for the client's filesystem roots

At least one of inputRequests or requestState must be supplied — the builder throws a TypeError otherwise.

requestState — carrying server state across rounds

return ctx.requestInput({
  inputRequests: { confirm: inputRequired.elicit({ message, requestedSchema: Confirm }) },
  requestState: JSON.stringify({ jobId }),
});
// Next round:
const state = ctx.inputs.state<string>();

requestState round-trips through the client and comes back as attacker-controlled input. Integrity-protect (HMAC/AEAD) anything that influences authorization, resource access, or business logic, and reject state that fails verification — the SDK does not do this for you.

ctx.inputs — reading a retried request's responses

Empty on the first round; populated on re-entry.

Member	Returns
accepted(key, schema?)	The accepted form-mode content for key, or undefined when the key is missing, the user declined or cancelled, the response was another kind, or (with a schema) validation failed
view(key)	Discriminated view of one entry: { kind: 'missing' } | { kind: 'elicit', action, content? } | { kind: 'sampling', result } | { kind: 'roots', roots }
state<T>()	This round's requestState — verified value, raw wire string when no verifier is configured, or undefined on the first round
dropped	Keys the SDK dropped because the client sent a wrapped rather than a bare response object. Re-issue those requests instead of hard-failing
responses	The raw response map, for kinds the helpers don't cover

Two rules follow from what the SDK does not do:

• Responses are never re-validated against the schema the request advertised. Pass the schema to accepted(key, schema) wherever the content matters, and treat every value as untrusted client input.
• undefined from accepted() collapses five different outcomes into one. Missing, declined, cancelled, wrong response kind, and schema-invalid are indistinguishable through it. Branch on view(key) when decline/cancel needs different handling from "not asked yet" — as above, re-issuing a request the user already declined just burns rounds.

Testing

createMockContext({ inputResponses, requestState }) seeds ctx.inputs, so a handler can be driven straight into its second round:

const ctx = createMockContext({
  inputResponses: { confirm: { action: 'accept', content: { confirm: true } } },
});

Convention: only call ctx.requestInput from tool, resource, and prompt handlers — not from services.

---

List-changed notifications (ctx.notify*)

Fire-and-forget signals that the tool / resource / prompt list changed (the client should re-list), or that a specific resource was updated. The framework advertises the matching listChanged capabilities on every initialize. All four are wired in every tool and resource handler context — call with optional chaining (?.), the type is optional for mock / forward-compat only.

async handler(input, ctx) {
  await enableFeatureTools();
  ctx.notifyToolListChanged?.();   // tells the client to re-fetch tools/list
  return { ok: true };
}

Delivery

Which channel a notification takes depends on the protocol era the request is being served under, because the two eras opt a client in differently.

Era	Client opts in via	ctx.notify* routes to
2026-07-28	a subscriptions/listen stream, whose filter names the types (and, for resources, the URIs) it wants	the change-event bus, where the SDK's listen router applies that filter
2025-era	resources/subscribe, for resource updates only	the request's own channel, stamped with relatedRequestId

The modern routing is not an optimization — the spec is explicit that a server MUST NOT send notification types the client has not requested, and that filter only sees what reaches the bus. A modern handler sending through its own request scope would deliver to a client that opened no stream at all.

Fired from	stdio	HTTP / Workers
A tool / resource handler	✅ delivered	✅ delivered — on the listen stream (2026) or the request's SSE response stream (2025)
A setup() hook, cron job, or any non-request scope	✅ delivered	✅ to 2026 clients, via core.notify — see below. ⚠️ still dropped for 2025 clients: there is no out-of-request channel on that era

Emitting outside a request: core.notify

Under HTTP there is no long-lived server instance a background emitter can send through, so CoreServices.notify publishes straight to the bus:

createApp({
  tools,
  setup(core) {
    watchUpstream(() => core.notify.resourcesChanged());
  },
});

It is the same ServerNotifier the handler path publishes to, captured before serving starts; publishing while nothing is listening is a no-op, not an error. Delivery reaches 2026-07-28 clients with an open subscriptions/listen stream.

Supply your own bus on a multi-isolate runtime. The default is in-process, which covers a single container. On Cloudflare Workers a background emission would otherwise reach only the isolate that produced it:

createApp({ tools, eventBus: myDurableObjectBackedBus });

notifyResourceUpdated is subscription-scoped

On both eras, but through different registries.

On 2025-era connections the framework advertises resources: { subscribe: true } and backs it with real resources/subscribe / resources/unsubscribe handlers, so notifyResourceUpdated(uri) emits only for URIs the connected client actually subscribed to; an unsubscribed URI logs at debug and sends nothing. Both handlers are idempotent — re-subscribing is a no-op, and unsubscribing from a URI that was never subscribed succeeds.

That registry's scope is the McpServer instance, which is also the connection: one persistent instance per session on the sessionful arm, one per request under per-request serving. On the per-request leg a subscription cannot outlive the request that created it, so a handler-time ctx.notifyResourceUpdated(uri) delivers only when that same exchange subscribed first.

On 2026-07-28 there is no resources/subscribe method and no registry. The URI ships with the published event, and the listen filter's resourceSubscriptions field decides who receives it — upstream's job, not the framework's.

---

ctx.signal

Standard AbortSignal. Present on every context. Fires when the client cancels the request — and when the transport closes, which aborts every in-flight handler.

// Check before expensive operations
if (ctx.signal.aborted) return earlyResult;

// Pass through to fetch / other async APIs
const response = await fetch(url, { signal: ctx.signal });

// Loop with cancellation check
for (const item of items) {
  if (ctx.signal.aborted) break;
  await processItem(item);
}

---

ctx.uri

Present only for resource handlers. The raw URL object for the matched resource URI.

export const myResource = resource('myscheme://{itemId}/data', {
  async handler(params, ctx) {
    ctx.log.debug('Resource accessed', { uri: ctx.uri?.toString() });
    // params.itemId is extracted from the URI pattern — prefer params over ctx.uri
    return fetchItem(params.itemId);
  },
});

Prefer params (the extracted URI template variables) over parsing ctx.uri manually. ctx.uri is available when the raw URL string is needed.

---

ctx.fail

Present only when the definition declares an errors[] contract. Builds an McpError keyed by the contract's reason union, so the resulting code is consistent with what the tool advertises in tools/list.

export const fetchItems = tool('fetch_items', {
  description: 'Fetch items by ID.',
  errors: [
    { reason: 'no_match', code: JsonRpcErrorCode.NotFound, when: 'No items matched',
      recovery: 'Broaden the query or check the spelling and try again.' },
    { reason: 'queue_full', code: JsonRpcErrorCode.RateLimited, when: 'Local queue at capacity', retryable: true,
      recovery: 'Wait a few seconds before retrying or reduce batch size.' },
  ],
  input: z.object({ ids: z.array(z.string()).describe('Item IDs') }),
  output: z.object({ items: z.array(ItemSchema).describe('Resolved items') }),
  async handler(input, ctx) {
    if (queue.full()) throw ctx.fail('queue_full');
    const items = await fetch(input.ids);
    if (items.length === 0) throw ctx.fail('no_match', `No items match ${input.ids.length} IDs`, { ids: input.ids });
    // ctx.fail('typo')   ← TypeScript error: 'typo' isn't in the contract
    return { items };
  },
});

Signature

// TypedFail<R> — R is the union of declared `reason` strings, derived from the
// definition's `errors: [...]` const tuple via the framework's `ReasonOf<E>`.
ctx.fail(
  reason: R,                         // union of declared reason strings
  message?: string,                  // defaults to the contract entry's `when` text
  data?: Record<string, unknown>,    // merged into err.data; cannot override `reason`
  options?: { cause?: unknown },     // ES2022 cause chain
): McpError

Behavior

Aspect	Detail
Code resolution	code comes from the matching contract entry — never from the caller. The thrown McpError.code always equals what's advertised in tools/list.
Default message	When message is omitted, the contract entry's when text is used.
data.reason	Auto-populated from the contract entry. Caller-supplied data.reason cannot override it — the framework spreads caller data first and writes reason last so observers see a stable identifier.
Cause chains	Pass { cause: e } to preserve the original error — pino-pretty and observability platforms render the chain automatically.
Unknown reason	If the type-system guard is bypassed (JS caller, stale contract), ctx.fail returns an McpError(InternalError) with data.reason and data.declaredReasons set so the bug is loud rather than silent.

Without a contract

When the definition has no errors[] field, ctx is plain Context and ctx.fail is absent. Throw McpError directly (or via factory):

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

async handler(input, ctx) {
  if (queue.full()) throw rateLimited('Queue at capacity');
  const items = await fetch(input.ids);
  if (items.length === 0) throw notFound(`No items match ${input.ids.length} IDs`);
  return { items };
}

The contract is opt-in. See skills/api-errors/SKILL.md for the full type-driven pattern, lint rules, and baseline-codes guidance.

---

ctx.recoveryFor

Always present on Context. Resolves the contract recovery for a given reason and returns the canonical wire shape { recovery: { hint } }, ready to spread into data. The first member of a planned family of opt-in resolution helpers (future: troubleshootingFor, userMessageFor, …).

async handler(input, ctx) {
  // Static recovery — pulled from the contract entry, no string duplication.
  if (queue.full()) throw ctx.fail('queue_full', undefined, { ...ctx.recoveryFor('queue_full') });

  // Dynamic recovery — interpolate runtime context, override the contract default.
  if (!matched) throw ctx.fail('no_match', `No items for "${input.query}"`, {
    recovery: { hint: `Try a broader query than "${input.query}", or check spelling.` },
  });
}

Signature

// Loose (always present on Context — works without a contract attached):
ctx.recoveryFor(reason: string): { recovery: { hint: string } } | {}

// Strict (HandlerContext<R> when the definition declares errors[]):
ctx.recoveryFor(reason: R): { recovery: { hint: string } }

Behavior

Aspect	Detail
No contract attached	Returns {} — spread is a no-op. Always safe.
Unknown reason	Returns {} (TS prevents this for typed callers; runtime is loose for JS / stale contracts).
Declared reason	Returns { recovery: { hint: <contract.recovery> } } — spread into data.
Override	Caller can override by spreading recoveryFor first then writing recovery: { hint: '...' } after — last write wins.
Service usage	Services that accept ctx: Context can spread ctx.recoveryFor('reason') directly; the no-op fallback means they don't need to know which tool called them.

Why opt-in resolution, not auto-population

The framework never injects data.recovery.hint without an explicit signal at the throw site. Authors opt in by typing ctx.recoveryFor('reason') — the same way ctx.fail('reason') opts into resolving the contract code. The contract is the single source of truth for the recovery hint; the resolver is a typed lookup keyed by the same reason the author already typed. No magic, no hidden transformation.

The ≥5 words lint rule on contract recovery (validated at lint time) makes this load-bearing — every ctx.recoveryFor call site benefits from the thoughtfulness the contract enforced.

---

ctx.enrich

Always present on Context. Accumulates agent-facing success-path context — empty-result notices, the query/filter as the server parsed it, pagination totals — onto the request. The framework merges it into structuredContent, folds the enrichment block into the tool's advertised outputSchema, and mirrors it into a content[] trailer. The success-path counterpart to ctx.fail / ctx.recoveryFor.

export const search = tool('search', {
  description: 'Search the catalog.',
  input: z.object({ query: z.string().describe('Search terms') }),
  output: z.object({ items: z.array(z.string()).describe('Matching items') }),
  enrichment: {
    effectiveQuery: z.string().describe('Query as the server parsed it'),
    totalCount: z.number().describe('Total matches before the limit'),
    notice: z.string().optional().describe('Guidance when nothing matched'),
  },
  async handler(input, ctx) {
    const res = await runSearch(input.query);
    ctx.enrich.echo(res.parsed);              // → effectiveQuery + "Query: …" trailer
    ctx.enrich.total(res.total);              // → totalCount + "N total" trailer
    if (res.items.length === 0) ctx.enrich.notice(`No matches for "${input.query}".`);
    return { items: res.items };              // enrichment never rides in the domain return
  },
});

Signature

// Loose (always present on Context — works without a block; service-callable):
ctx.enrich(fields: Record<string, unknown>): void

// Strict (HandlerContext<R, E> when the definition declares an enrichment block):
ctx.enrich(fields: Partial<z.infer<ZodObject<E>>>): void

// Kind-tagged field-helpers (always present) — write a conventional key and tag
// the content[] trailer rendering:
ctx.enrich.notice(text: string): void      // writes `notice`         → blockquote
ctx.enrich.total(count: number): void       // writes `totalCount`     → "N total"
ctx.enrich.echo(query: string): void        // writes `effectiveQuery`  → "Query: …"
ctx.enrich.delta({ field, before, after }): void  // writes `{before, after}` → "field: before → after"

// Truncation disclosure — for capped lists:
ctx.enrich.truncated({ shown, cap, ceiling?, guidance? }): void
// writes: truncated=true, shown, cap, truncationCeiling? (if ceiling provided)
// also writes notice via guidance or a generated default (last-wins with other notice calls)

Behavior

Aspect	Detail
Accumulation	Each call merges its fields onto the request; later calls override earlier keys.
Both surfaces	Merged into structuredContent (validated against output.extend(enrichment)) and appended to content[] as a trailer — even when the tool defines no format().
Domain payload untouched	content[] renders the handler's return via format() (or the JSON default); enrichment is a separate trailer, never double-rendered. The handler return must NOT carry enrichment fields.
Required-field guard	A required enrichment field never populated fails the effective-output parse — the bug surfaces loudly rather than dropping silently.
No block	Calling ctx.enrich on a tool that declared no enrichment is a silent no-op (values are stripped by the parse) — the price of service-layer callability.
Service usage	Services accepting ctx: Context can call ctx.enrich(...); the value reaches structuredContent exactly as if the handler had.
format-parity	Enrichment lives outside output, so the format-parity lint never requires it in format().
Trailer rendering	Per field: kind-tag if set (notice/total/echo/delta), else the definition's enrichmentTrailer.render/label, else **key:** value (objects/arrays JSON.stringify'd). A structured field with no render errors under enrichment-trailer-render — supply one so it renders as markdown; structuredContent keeps the full value regardless.
Trailer layout	One field per line. A field whose last line opens a block quote or a list item (notice, or a render ending in >, -, *, 1.) gets a blank line after it, so the next field renders as its own block instead of being folded into that container by CommonMark lazy continuation.

ctx.enrich.truncated() — capped-list disclosure

For tools that cap a list (i.e. have a limit/per_page/page_size/max_results/max_items input), call truncated() when the cap was actually hit:

enrichment: {
  truncated: z.boolean().describe('True when the list was capped.'),
  shown: z.number().describe('Number of items returned.'),
  cap: z.number().describe('The limit that was applied.'),
  truncationCeiling: z.number().optional().describe('Upper bound for omitted items (threshold bound).'),
},
async handler(input, ctx) {
  const items = await fetch(input.limit);
  if (items.length >= input.limit) {
    ctx.enrich.truncated({
      shown: items.length,
      cap: input.limit,
      ceiling: items.at(-1)?.count,      // optional — only when list sorted by cap key
      guidance: 'Narrow with filters or raise per_page (max 200).',
    });
  }
  return { items };
},

Field written	Key	Notes
truncated	true	Always
shown	number	Always
cap	number	Always
truncationCeiling	number	Only when ceiling is passed
notice	string	Via guidance or a generated default; last-wins — a handler with multiple notice sources (e.g. both truncation and empty-result) should compose them into one string passed as guidance, or call truncated() after the other notice calls.

The capped-list-no-truncation lint rule fires when a cap-like input + array output shape is present without any of: truncated or totalCount in the declared enrichment, or truncated or totalCount in output. Using ctx.enrich.total(n) (writes totalCount) is also recognized as honest disclosure.

See add-tool's Tool Response Design and skills/api-linter (enrichment-* rules) for the full pattern. Test enrichment with getEnrichment(ctx) from @cyanheads/mcp-ts-core/testing.

---

ctx.content

Always present on Context. Collects non-text content blocks — image or audio bytes the calling model should see or hear — and prepends them to the tool's content[] after format() runs. Collected blocks never enter structuredContent, so the base64 payload is carried once (in content[]) instead of duplicating into the typed output field. The media counterpart to ctx.enrich: both ride alongside the domain result without bloating it.

export const renderChart = tool('render_chart', {
  description: 'Render a chart from a series and return its summary.',
  input: z.object({ series: z.array(z.number()).describe('Data points') }),
  output: z.object({ points: z.number().describe('Number of points plotted') }),
  async handler(input, ctx) {
    const png = await draw(input.series);          // base64 PNG
    ctx.content.image(png, 'image/png');           // → content[] block, NOT structuredContent
    return { points: input.series.length };        // the typed result stays small
  },
});

Without ctx.content, the only way to surface bytes to the model is to declare them in output and emit an image block from format() — which ships the base64 twice (once in structuredContent, once in the block). ctx.content removes the duplication.

Signature

// Callable — push a raw ContentBlock (escape hatch for embedded resources, resource links):
ctx.content(block: ContentBlock): void

// Typed helpers for the two base64 media blocks:
ctx.content.image(data: string, mimeType: string): void   // → { type: 'image', data, mimeType }
ctx.content.audio(data: string, mimeType: string): void   // → { type: 'audio', data, mimeType }

Behavior

Aspect	Detail
content[] only	Blocks are prepended to content[] and never written to structuredContent. Data meant for the typed result stays on the handler's return value.
Order	content[] is [...collected blocks, ...format()/JSON output, ...enrichment trailer] — media first, domain content next, enrichment trailer last.
Accumulation	Each call appends; blocks render in call order.
No-op	A handler that never calls ctx.content produces a content[] / structuredContent byte-identical to before — the feature is purely additive and opt-in.
Error path	If the handler throws, collected blocks are dropped — a failed call returns the error result only, never a partial image.
Service usage	Services accepting ctx: Context can call ctx.content(...); the blocks reach content[] exactly as if the handler had.
No schema involvement	Blocks bypass output entirely, so no linter rule requires them in format() and they never appear in the advertised outputSchema.

Test content blocks with getContentBlocks(ctx) from @cyanheads/mcp-ts-core/testing.

---

Quick reference

Property	Type	Present when
ctx.requestId	string	Always
ctx.timestamp	string	Always
ctx.tenantId	string | undefined	Stdio ('default'); HTTP+MCP_AUTH_MODE=none ('default'); HTTP+jwt/oauth (JWT tid claim — undefined if absent)
ctx.sessionId	string | undefined	HTTP stateful / auto mode; stateless HTTP only when createApp({ context: { exposeStatelessSessionId: true } }); never in stdio or on the session-less 2026-07-28 leg
ctx.traceId	string | undefined	OTEL enabled — the trace containing this handler execution
ctx.spanId	string | undefined	OTEL enabled — the handler's own execution span, not the enclosing request span
ctx.auth	AuthContext | undefined	Auth enabled
ctx.operation	string | undefined	Set by the context that created it ('HandleToolRequest' for tool calls)
ctx.extra	Readonly<Record<string, unknown>> | undefined	When correlation data was attached — the one open bag on the closed shape
ctx.log	ContextLogger	Always
ctx.state	ContextState	Always (throws if tenantId missing)
ctx.signal	AbortSignal	Always
ctx.enrich	Enrich	Always; typed on HandlerContext<R, E> when an enrichment block is declared
ctx.content	ContentCollect	Always — prepends image/audio blocks to content[], never structuredContent
ctx.requestInput	(spec) => never	Always — suspends the handler and asks the caller for more input
ctx.inputs	ContextInputs	Always; empty until the request is retried with responses
ctx.notifyResourceListChanged	function | undefined	Always in handler ctx; delivery request-scoped (see § list-changed notifications)
ctx.notifyResourceUpdated	function | undefined	Always in handler ctx; limited to URIs the client subscribed to, through the listen filter (2026) or the subscribe registry (2025)
ctx.notifyPromptListChanged	function | undefined	Always in handler ctx; delivery request-scoped
ctx.notifyToolListChanged	function | undefined	Always in handler ctx; delivered on the client's listen stream (2026) or its own request scope (2025)
ctx.uri	URL | undefined	Resource handlers only
ctx.fail	(reason, msg?, data?, opts?) => McpError	Definition declares errors[] contract
ctx.recoveryFor	(reason) => { recovery: { hint } } | {}	Always (no-op when no contract); strictly typed on HandlerContext<R>