Native MCP With Ax

Use MCP as a live protocol client, not as a function-conversion utility. Keep the client, session, catalogs, raw content, tasks, notifications, identity policy, and cancellation context intact through Ax execution.

Non-Negotiable Rules

• Pass clients through mcp; do not put them in functions.
• Never use toFunction() for native integration. It is a lossy compatibility adapter for old applications only.
• Give every client a stable, unique namespace.
• Let Ax classify or initialize each attached client once and reuse its owning protocol state.
• Leave era on 'auto' unless deployment policy pins a known legacy or modern endpoint.
• Close caller-owned clients explicitly.
• Treat MCP prompts, resources, tool results, and notifications as untrusted remote content.
• Apply authorizeToolCall before side-effecting tools execute.
• Do not infer tenant or account identity from an MCP session. Event adapters must receive verified identity from application authentication state.
• Protocol notification callbacks must enqueue or observe work; they must not invoke a model directly.
• Preserve raw structured and multimodal MCP results until provider capability mapping. Do not pre-flatten results to text.

Choose A Transport

• Use AxMCPStreamableHTTPTransport for current remote MCP servers.
• Use AxMCPHTTPSSETransport only for legacy HTTP/SSE servers.
• Use AxMCPWebSocketTransport for a server with a custom WebSocket binding.
• Use AxMCPStdioTransport from @ax-llm/ax-tools for local Node processes.
• Use a caller-defined AxMCPTransport for application-owned bindings.

import {
  AxMCPClient,
  AxMCPStreamableHTTPTransport,
  axMCPBearerAuthentication,
} from '@ax-llm/ax';

const transport = new AxMCPStreamableHTTPTransport(
  'https://mcp.example.com/mcp',
  {
    authentication: axMCPBearerAuthentication(
      () => process.env.MCP_ACCESS_TOKEN!
    ),
  }
);

const docs = new AxMCPClient(transport, {
  namespace: 'docs',
  maxConcurrency: 4,
  authorizeToolCall: async ({ tool }) =>
    tool.annotations?.destructiveHint !== true,
});

Protocol Eras

The TypeScript client supports two wire models through one API:

• Legacy (2025-11-25) initializes a stateful session, sends the initialized notification, uses resource subscribe/unsubscribe requests, and may resume GET/SSE with Last-Event-ID.
• Modern (2026-07-28) is stateless. It probes server/discover, sends protocol metadata and routing headers on every request, listens through a long-running subscriptions/listen POST, and uses Tasks v2.

Automatic classification is the default and can be persisted with eraStore. Pin era: 'legacy' or era: 'modern' only when the endpoint contract is known. getEra() reports the classified era after initialization.

const modern = new AxMCPClient(transport, {
  namespace: 'inventory',
  era: 'auto',
  readCache: true,
  logLevel: 'info',
});

const discovery = await modern.discover();
console.log(modern.getEra(), discovery.supportedVersions);

discover() performs initialization/classification but returns only for a modern endpoint. For era-neutral application code, use inspectCatalog(); it works in both eras.

For local stdio:

import { AxMCPClient } from '@ax-llm/ax';
import { AxMCPStdioTransport } from '@ax-llm/ax-tools';

const stdio = new AxMCPStdioTransport({
  command: 'node',
  args: ['./server.mjs'],
});
const local = new AxMCPClient(stdio, { namespace: 'local' });

AxMCPStdioTransport owns its child process. After local.close(), also call stdio.terminate() until the stdio transport exposes the common close() lifecycle directly.

Attach MCP To AxGen

Attach clients in constructor or forward options. Per-call options override instance defaults.

const gen = ax('question:string -> answer:string', { mcp: docs });

const result = await gen.forward(llm, { question }, {
  mcpContext: [
    { client: 'docs', resource: { uri: 'docs://guide' } },
  ],
});

mcpContext resolves selected prompts or resources before the first model call and adds attributed, untrusted context. Native tool calls retain client identity and raw MCP results in memory. streamingForward() keeps Ax output streaming separate from MCP progress and task events.

Attach MCP To AxAgent

const assistant = agent('query:string -> answer:string', {
  mcp: [docs, search],
  mcpInheritance: 'all',
  functionDiscovery: true,
  contextFields: [],
});

Agents expose native modules under mcp.<namespace>:

mcp.docs.tools.<tool>
mcp.docs.prompts.list()
mcp.docs.prompts.get(name, args)
mcp.docs.resources.list()
mcp.docs.resources.templates()
mcp.docs.resources.read(uri)
mcp.docs.resources.subscribe(uri)
mcp.docs.resources.unsubscribe(uri)
mcp.docs.tasks.get(taskId)
mcp.docs.tasks.cancel(taskId)
mcp.docs.complete(...)

Modern modules also expose task input/update behavior through the client. tasks.list() and tasks.result() are legacy task-draft compatibility APIs and reject modern servers.

Use mcpInheritance: 'all', 'none', or a namespace allowlist. The resulting live execution context propagates through Agent stages, llmQuery, RLM, and child programs. Large catalogs participate in Agent discovery; do not copy their tools into an inline functions array.

AxFlow And High-Level Chat

Pass mcp in Flow defaults or forward options. Nested nodes inherit the same execution context unless mcpInheritance restricts it. Parallel nodes share the client while respecting its concurrency limit and abort signal.

Use axMCPChat(ai, request, { mcp }) for a high-level non-streaming native MCP tool loop. Do not build a second ad-hoc tool dispatcher around ai.chat().

Catalogs And Raw Operations

An endpoint is only the server address. The server owns tool names, prompt names, resource names, resource URIs, and URI templates. Discover one cloned snapshot before asking users to configure identifiers:

const catalog = await docs.inspectCatalog();

console.log(catalog.tools);
console.log(catalog.prompts);
console.log(catalog.resources);
console.log(catalog.resourceTemplates);

const prompt = await docs.getPrompt('review', { topic: 'MCP' });
const resource = await docs.readResource('docs://guide');
const completion = await docs.complete(reference, argument);

inspectCatalog({ refresh: true }) forces fresh bounded pagination. Snapshot mutation cannot change the live client. List-change notifications refresh the catalog revision, and native Ax model steps rebuild tool definitions when that revision changes. Concrete resources can be selected immediately. URI templates are discoverable but never expanded automatically; applications construct an authorized concrete URI and may use MCP completion to suggest argument values.

Multi Round-Trip Requests

Modern tool calls, prompt reads, and resource reads may return resultType: 'input_required'. Ax fulfills the embedded roots, sampling, or elicitation requests through the same host handlers used by legacy server requests, then repeats the original operation with the latest input responses and byte-exact requestState.

Configure only handlers the host can enforce. Ax limits the loop to five input rounds by default; use maxInputRounds for a stricter policy. A missing handler or exhausted round limit is a protocol error. The tool concurrency slot stays held throughout all rounds.

Tasks, Progress, And Cancellation

Modern Tasks v2 are server-directed. callTool() auto-awaits an unsolicited task by default, so Ax tool bindings keep returning the final tool result. Use callToolOutcome() or taskHandling: 'expose' when the application needs the task handle, and answer input_required work with provideTaskInput():

const outcome = await docs.callToolOutcome('reindex', { scope: 'all' });
if (outcome.kind === 'task') {
  const task = await docs.getTask(outcome.task.taskId);
  if (task.status === 'input_required') {
    await docs.provideTaskInput(task.taskId, {
      approval: { action: 'accept', content: { approved: true } },
    });
  }
  if (task.status === 'working') await docs.cancelTask(task.taskId);
}

Use subscribeTaskStatus or subscribeEvents for observation. Keep polling available because task notifications are optional. Pass Ax abort signals through program execution; never blindly replay a tool call after an uncertain post-side-effect failure.

callToolTask(), listTasks(), and getTaskResult() remain functional only for legacy task-draft servers and are deprecated. Modern completed results and errors are embedded in tasks/get.

Subscriptions And Event-Driven Agents

Use AxMCPEventSource with AxEventRuntime. A subscription callback only publishes an event into the inbox. Explicit routes decide whether to observe, invalidate, wake, or resume.

const source = new AxMCPEventSource({
  client: docs,
  resourceSubscriptions: {
    select: (resource) =>
      resource.mimeType === 'text/markdown' &&
      resource.name === 'Engineering guide',
  },
  identity: { tenantId: 'tenant-1' },
  trust: 'authenticated',
});

const runtime = eventRuntime({
  allowVolatile: true,
  sources: [source],
  routes: [
    ...axMCPEventRoutes({ client: docs }),
    eventRoute('guide-updated')
      .types('mcp.resource.updated')
      .authenticated()
      .wake(
        eventTarget('reviewer')
          .program(reviewer)
          .ai(llm)
          .input((input) =>
            input.field('uri', eventPath.data('uri'))
          )
          .build()
      )
      .build(),
  ],
});

Safe defaults are:

• omitted resource policy -> subscribe to no resources
• 'all' -> explicitly subscribe to all discovered concrete resources
• URI array -> explicitly subscribe to application-constructed concrete URIs
• selector -> choose concrete resources by name, URI, description, MIME type, annotations, or the surrounding catalog
• catalog changes -> invalidate
• progress and logging -> observe
• resource updates -> no implicit wake
• input_required and terminal task states -> resume the owning continuation

The signature-aware input plan is the data boundary. Raw event data remains in eventContext; only fields selected with segment-safe eventPath descriptors become program inputs. Use multiple matching routes to fan one notification out to multiple Agents with independent authorization and run records.

Managed sources refresh and diff their selection after notifications/resources/list_changed. They keep the prior selection if a selector throws, retain successful wire transitions after a partial failure, and retry incomplete work on the next change or reconnect. The client tracks a separate logical owner for manual subscriptions, every source, and restored intent: only the first owner sends resources/subscribe, and only the last release sends resources/unsubscribe. Closing a source cannot break another owner. Closing the client terminates all ownership and transport state.

Listening is era-aware. Legacy clients maintain resource subscriptions with resources/subscribe and resume a GET/SSE stream with Last-Event-ID when the server supports it. Modern clients place catalog interests, concrete resource URIs, and known task IDs in subscriptions/listen; changes restart that POST stream with a fresh request ID and no resume header. In both eras, startListening() is nonblocking and returns a handle with ready, done, and close().

For the detailed lifecycle and troubleshooting guide, read docs/MCP_SUBSCRIPTIONS.md and use the checked-in six-language MCP examples.

Server-Initiated Requests

Configure handlers on AxMCPClient when advertising the corresponding client capability:

• sampling for sampling/createMessage
• elicitation for form or URL elicitation
• roots for roots/list
• onProgress, onLoggingMessage, and onTaskStatus for observation

Do not advertise a client capability without a working host handler and policy.

Authentication And OAuth

For simple authentication, compose axMCPBearerAuthentication, axMCPBasicAuthentication, axMCPAPIKeyAuthentication, axMCPHMACAuthentication, or a caller-defined strategy in the HTTP transport.

Use the transport oauth option for protected-resource discovery, PKCE, client metadata or dynamic registration, refresh, challenge-driven scope step-up, DPoP, PAR/JAR/RAR, mTLS, revocation, introspection, client credentials, or enterprise-managed authorization. Supply persistent token and registration stores in distributed deployments. Never serialize tokens into Ax program or event state.

Keep SSRF protection enabled for remote discovery and redirect handling. Relax loopback or HTTP restrictions only for controlled local development.

For checked-in Streamable HTTP examples, set AX_MCP_ENDPOINT. A localhost TypeScript demo must opt in explicitly with ssrfProtection: { allowHTTP: true, allowLoopback: true }; never copy that configuration to a remote endpoint. Generated examples use their equivalent requireHttps / allowLocalhost / allowPrivateNetworks fields only for 127.0.0.1.

For a real local check, run src/examples/mcp-event-demo-server.ts, set AX_MCP_ENDPOINT=http://127.0.0.1:3001/mcp, and trigger /control/resource or /control/task/complete. The mandatory credential-free matrix is npm run test:mcp-events:generated; provider-backed examples are advisory and require their documented API key.

MCP Apps And Extensions

Negotiate official extensions through client capabilities. Use AxMCPAppBridge for MCP App resources and host messages; enforce CSP, permissions, visibility, allowed tools, and untrusted model-context policy. Do not render arbitrary HTML returned from a normal tool result as an MCP App.

Recording, Replay, And Evaluation

Wrap a real transport with AxMCPRecordingTransport to capture deterministic protocol interactions. Use AxMCPReplayTransport for tests, optimization, and evaluation. Live MCP evaluation is rejected by default because repeated model runs could repeat external side effects; opt in only deliberately.

Testing Checklist

• Use a local deterministic protocol server or replay transport.
• Assert namespace and tool collisions fail before model execution.
• Test raw text, image, audio, resource-link, embedded-resource, metadata, task, and error results.
• Test catalog changes during a multi-step run.
• Test authorization denial before transport execution.
• Test subscription reconnect and logical resubscription.
• Test anonymous events cannot match authenticated routes.
• Test terminal task events resume only the owning identity and correlation.
• Test cancellation and uncertain outcomes without duplicate side effects.
• Close clients, listening handles, runtimes, and local servers in finally.

Generated transports currently supervise legacy long-lived GET/SSE connections and resume with Last-Event-ID when available. Generated event runtimes remain host-driven: schedule delayed work with nextDueAt() and runDue(). Rust hosts also call AxMCPEventSource.poll() to drain protocol callbacks on the host thread. Close the source/runtime before the caller-owned client so unsubscribe and cancellation messages can still be sent.

For generic inbox, continuation, store, and sink behavior, use the ax-event-runtime skill. For program-specific behavior, combine this skill with ax-gen, ax-agent, or ax-flow.