api-context— agent skill

Overview

Every tool and resource handler receives a single Context (ctx) argument. It provides request identity, structured logging, tenant-scoped storage, optional protocol capabilities (elicitation, sampling), cancellation, and task progress — 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 {
  // Identity & tracing
  readonly requestId: string;       // Unique per request, auto-generated
  readonly timestamp: string;       // ISO 8601 request start time
  readonly tenantId?: string;       // From JWT 'tid' claim; 'default' in stdio mode
  readonly traceId?: string;        // OTEL trace ID (present when OTEL enabled)
  readonly spanId?: string;         // OTEL span ID (present when OTEL enabled)
  readonly auth?: AuthContext;      // Parsed auth claims (clientId, scopes, sub)

  // Structured logging — auto-includes requestId, traceId, tenantId
  readonly log: ContextLogger;

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

  // Optional protocol capabilities (undefined when client doesn't support them)
  readonly elicit?: (message: string, schema: z.ZodObject<z.ZodRawShape>) => Promise<ElicitResult>;
  readonly sample?: (messages: SamplingMessage[], opts?: SamplingOpts) => Promise<CreateMessageResult>;

  // Cancellation
  readonly signal: AbortSignal;

  // Task progress — present only when tool is defined with task: true
  readonly progress?: ContextProgress;

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

Identity fields

ctx.log

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

Methods

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

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 mode — defaults to 'default').
• Keys are tenant-prefixed internally; handlers never need to namespace manually.
• 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.elicit / ctx.sample

Both are optional — undefined when the connected client doesn't support the capability. Check for presence before calling. A simple truthiness check is enough; no type guards needed.

ctx.elicit — ask the user for structured input

Presents a form to the user via the MCP elicitation protocol. The user fills in a Zod-validated schema and returns an action (accept, decline, or cancel).

if (ctx.elicit) {
  const result = await ctx.elicit(
    'Which output format do you want?',
    z.object({
      format: z.enum(['json', 'csv', 'markdown']).describe('Output format'),
      includeHeaders: z.boolean().default(true).describe('Include column headers'),
    }),
  );

  if (result.action === 'accept') {
    // result.content is Record<string, string | number | boolean | string[]> | undefined
    await produceOutput(result.content?.format as string, result.content?.includeHeaders as boolean);
  } else {
    // 'decline' or 'cancel' — user opted out
    throw new McpError(JsonRpcErrorCode.InvalidRequest, 'User declined input');
  }
}

ElicitResult (from @modelcontextprotocol/sdk/types.js):

// Actual SDK type — a flat object, not a discriminated union
interface ElicitResult {
  action: 'accept' | 'decline' | 'cancel';
  // Present when action === 'accept'; values are primitives or string arrays
  content?: Record<string, string | number | boolean | string[]>;
}

Note: content is not typed against the Zod schema you pass — it is a Record of primitives. Validate content against your schema manually (e.g. MySchema.parse(result.content)) when action === 'accept'.

Convention: Only call ctx.elicit from tool handlers, not from services.

ctx.sample — request an LLM completion from the client

Requests a completion from the client's LLM via the MCP sampling protocol. Useful for AI-assisted tool behavior without managing a separate LLM provider.

if (ctx.sample) {
  const result = await ctx.sample(
    [
      { role: 'user', content: { type: 'text', text: `Summarize: ${data}` } },
    ],
    { maxTokens: 500 },
  );
  return { summary: result.content.text };
}

SamplingOpts:

interface SamplingOpts {
  includeContext?: 'none' | 'thisServer' | 'allServers';
  maxTokens?: number;
  modelPreferences?: Record<string, unknown>;
  stopSequences?: string[];
  temperature?: number;
}

Convention: Only call ctx.sample from tool handlers, not from services.

ctx.signal

Standard AbortSignal. Present on every context. Set when the client cancels the request or when a task tool is cancelled.

// 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);
}

In task tools (task: true), the framework signals ctx.signal when the client sends a cancellation request.

ctx.progress

Present only when the tool definition includes task: true. Undefined for standard (non-task) tools and all resource handlers.

Methods

Usage

const asyncCountdown = tool('async_countdown', {
  description: 'Count down from a number with progress updates.',
  task: true,
  input: z.object({
    count: z.number().int().positive().describe('Number to count down from'),
    delayMs: z.number().default(1000).describe('Delay between counts in ms'),
  }),
  output: z.object({
    finalCount: z.number().describe('Final count value'),
    message: z.string().describe('Completion message'),
  }),

  async handler(input, ctx) {
    await ctx.progress!.setTotal(input.count);

    for (let i = input.count; i > 0; i--) {
      if (ctx.signal.aborted) break;

      await ctx.progress!.update(`Counting: ${i}`);
      await new Promise(resolve => setTimeout(resolve, input.delayMs));
      await ctx.progress!.increment();
    }

    return { finalCount: 0, message: 'Countdown complete' };
  },
});

Note: Use the non-null assertion (ctx.progress!) when accessing inside a task: true handler — the type is ContextProgress | undefined even though it's guaranteed present at runtime. TypeScript cannot narrow based on the task flag.

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.

Quick reference