title: Client. Message Client

A2A Node SDK - v0.1.0 / Modules / Client / MessageClient

Class: MessageClient

Client.MessageClient

Client for sending and receiving messages between A2A agents

The MessageClient provides methods for sending messages to agents and streaming real-time communication. It also provides access to related TaskClient and AgentClient instances for convenience.

Example

const messageClient = new MessageClient({ baseUrl: 'https://a2a-server.example.com' });
 
// Send a simple text message to an agent
const messageId = await messageClient.sendMessage([
  { type: 'text', content: 'What is the weather in New York?' }
], 'weather-agent');

Hierarchy

• EventEmitter

  ↳ MessageClient

Table of contents

Constructors

• constructor

Properties

• agents
• captureRejectionSymbol
• captureRejections
• defaultMaxListeners
• errorMonitor
• tasks

Methods

• [captureRejectionSymbol]
• addAbortListener
• addListener
• emit
• eventNames
• getEventListeners
• getMaxListeners
• getMaxListeners
• listenerCount
• listenerCount
• listeners
• off
• on
• on
• once
• once
• prependListener
• prependOnceListener
• rawListeners
• removeAllListeners
• removeListener
• sendMessage
• setMaxListeners
• setMaxListeners
• streamMessage

Constructors

constructor

• new MessageClient(options): MessageClient

Creates a new MessageClient instance

Parameters

Returns

MessageClient

Overrides

EventEmitter.constructor

Properties

agents

• Readonly agents: AgentClient

Agent client for discovering and interacting with agents

captureRejectionSymbol

▪ Static Readonly captureRejectionSymbol: typeof captureRejectionSymbol

Value: Symbol.for('nodejs.rejection')

See how to write a custom rejection handler.

Since

v13.4.0, v12.16.0

Inherited from

EventEmitter.captureRejectionSymbol

captureRejections

▪ Static captureRejections: boolean

Value: boolean

Change the default captureRejections option on all new EventEmitter objects.

Since

v13.4.0, v12.16.0

Inherited from

EventEmitter.captureRejections

defaultMaxListeners

▪ Static defaultMaxListeners: number

By default, a maximum of 10 listeners can be registered for any single event. This limit can be changed for individual EventEmitter instances using the emitter.setMaxListeners(n) method. To change the default for allEventEmitter instances, the events.defaultMaxListeners property can be used. If this value is not a positive number, a RangeError is thrown.

Take caution when setting the events.defaultMaxListeners because the change affects all EventEmitter instances, including those created before the change is made. However, calling emitter.setMaxListeners(n) still has precedence over events.defaultMaxListeners.

This is not a hard limit. The EventEmitter instance will allow more listeners to be added but will output a trace warning to stderr indicating that a “possible EventEmitter memory leak” has been detected. For any single EventEmitter, the emitter.getMaxListeners() and emitter.setMaxListeners() methods can be used to temporarily avoid this warning:

import { EventEmitter } from 'node:events';
const emitter = new EventEmitter();
emitter.setMaxListeners(emitter.getMaxListeners() + 1);
emitter.once('event', () => {
  // do stuff
  emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0));
});

The --trace-warnings command-line flag can be used to display the stack trace for such warnings.

The emitted warning can be inspected with process.on('warning') and will have the additional emitter, type, and count properties, referring to the event emitter instance, the event’s name and the number of attached listeners, respectively. Its name property is set to 'MaxListenersExceededWarning'.

Since

v0.11.2

Inherited from

EventEmitter.defaultMaxListeners

errorMonitor

▪ Static Readonly errorMonitor: typeof errorMonitor

This symbol shall be used to install a listener for only monitoring 'error' events. Listeners installed using this symbol are called before the regular 'error' listeners are called.

Installing a listener using this symbol does not change the behavior once an 'error' event is emitted. Therefore, the process will still crash if no regular 'error' listener is installed.

Since

v13.6.0, v12.17.0

Inherited from

EventEmitter.errorMonitor

tasks

• Readonly tasks: TaskClient

Task client for managing tasks

Methods

[captureRejectionSymbol]

▸ [captureRejectionSymbol]<K>(error, event, ...args): void

Type parameters

Parameters

Returns

void

Inherited from

EventEmitter.[captureRejectionSymbol]

addAbortListener

▸ addAbortListener(signal, resource): Disposable

Listens once to the abort event on the provided signal.

Listening to the abort event on abort signals is unsafe and may lead to resource leaks since another third party with the signal can call e.stopImmediatePropagation(). Unfortunately Node.js cannot change this since it would violate the web standard. Additionally, the original API makes it easy to forget to remove listeners.

This API allows safely using AbortSignals in Node.js APIs by solving these two issues by listening to the event such that stopImmediatePropagation does not prevent the listener from running.

Returns a disposable so that it may be unsubscribed from more easily.

import { addAbortListener } from 'node:events';
 
function example(signal) {
  let disposable;
  try {
    signal.addEventListener('abort', (e) => e.stopImmediatePropagation());
    disposable = addAbortListener(signal, (e) => {
      // Do something when signal is aborted.
    });
  } finally {
    disposable?.[Symbol.dispose]();
  }
}

Parameters

Returns

Disposable

Disposable that removes the abort listener.

Since

v20.5.0

Inherited from

EventEmitter.addAbortListener

addListener

▸ addListener<K>(eventName, listener): this

Alias for emitter.on(eventName, listener).

Type parameters

Parameters

Returns

this

Since

v0.1.26

Inherited from

EventEmitter.addListener

emit

▸ emit<K>(eventName, ...args): boolean

Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each.

Returns true if the event had listeners, false otherwise.

import { EventEmitter } from 'node:events';
const myEmitter = new EventEmitter();
 
// First listener
myEmitter.on('event', function firstListener() {
  console.log('Helloooo! first listener');
});
// Second listener
myEmitter.on('event', function secondListener(arg1, arg2) {
  console.log(`event with parameters ${arg1}, ${arg2} in second listener`);
});
// Third listener
myEmitter.on('event', function thirdListener(...args) {
  const parameters = args.join(', ');
  console.log(`event with parameters ${parameters} in third listener`);
});
 
console.log(myEmitter.listeners('event'));
 
myEmitter.emit('event', 1, 2, 3, 4, 5);
 
// Prints:
// [
//   [Function: firstListener],
//   [Function: secondListener],
//   [Function: thirdListener]
// ]
// Helloooo! first listener
// event with parameters 1, 2 in second listener
// event with parameters 1, 2, 3, 4, 5 in third listener

Type parameters

Parameters

Returns

boolean

Since

v0.1.26

Inherited from

EventEmitter.emit

eventNames

▸ eventNames(): (string | symbol)[]

Returns an array listing the events for which the emitter has registered listeners. The values in the array are strings or Symbols.

import { EventEmitter } from 'node:events';
 
const myEE = new EventEmitter();
myEE.on('foo', () => {});
myEE.on('bar', () => {});
 
const sym = Symbol('symbol');
myEE.on(sym, () => {});
 
console.log(myEE.eventNames());
// Prints: [ 'foo', 'bar', Symbol(symbol) ]

Returns

(string | symbol)[]

Since

v6.0.0

Inherited from

EventEmitter.eventNames

getEventListeners

▸ getEventListeners(emitter, name): Function[]

Returns a copy of the array of listeners for the event named eventName.

For EventEmitters this behaves exactly the same as calling .listeners on the emitter.

For EventTargets this is the only way to get the event listeners for the event target. This is useful for debugging and diagnostic purposes.

import { getEventListeners, EventEmitter } from 'node:events';
 
{
  const ee = new EventEmitter();
  const listener = () => console.log('Events are fun');
  ee.on('foo', listener);
  console.log(getEventListeners(ee, 'foo')); // [ [Function: listener] ]
}
{
  const et = new EventTarget();
  const listener = () => console.log('Events are fun');
  et.addEventListener('foo', listener);
  console.log(getEventListeners(et, 'foo')); // [ [Function: listener] ]
}

Parameters

Returns

Function[]

Since

v15.2.0, v14.17.0

Inherited from

EventEmitter.getEventListeners

getMaxListeners

▸ getMaxListeners(emitter): number

Returns the currently set max amount of listeners.

For EventEmitters this behaves exactly the same as calling .getMaxListeners on the emitter.

For EventTargets this is the only way to get the max event listeners for the event target. If the number of event handlers on a single EventTarget exceeds the max set, the EventTarget will print a warning.

import { getMaxListeners, setMaxListeners, EventEmitter } from 'node:events';
 
{
  const ee = new EventEmitter();
  console.log(getMaxListeners(ee)); // 10
  setMaxListeners(11, ee);
  console.log(getMaxListeners(ee)); // 11
}
{
  const et = new EventTarget();
  console.log(getMaxListeners(et)); // 10
  setMaxListeners(11, et);
  console.log(getMaxListeners(et)); // 11
}

Parameters

Returns

number

Since

v19.9.0

Inherited from

EventEmitter.getMaxListeners

getMaxListeners

▸ getMaxListeners(): number

Returns the current max listener value for the EventEmitter which is either set by emitter.setMaxListeners(n) or defaults to EventEmitter.defaultMaxListeners.

Returns

number

Since

v1.0.0

Inherited from

EventEmitter.getMaxListeners

listenerCount

▸ listenerCount(emitter, eventName): number

A class method that returns the number of listeners for the given eventName registered on the given emitter.

import { EventEmitter, listenerCount } from 'node:events';
 
const myEmitter = new EventEmitter();
myEmitter.on('event', () => {});
myEmitter.on('event', () => {});
console.log(listenerCount(myEmitter, 'event'));
// Prints: 2

Parameters

Returns

number

Since

v0.9.12

Deprecated

Since v3.2.0 - Use listenerCount instead.

Inherited from

EventEmitter.listenerCount

listenerCount

▸ listenerCount<K>(eventName, listener?): number

Returns the number of listeners listening for the event named eventName. If listener is provided, it will return how many times the listener is found in the list of the listeners of the event.

Type parameters

Parameters

Returns

number

Since

v3.2.0

Inherited from

EventEmitter.listenerCount

listeners

▸ listeners<K>(eventName): Function[]

Returns a copy of the array of listeners for the event named eventName.

server.on('connection', (stream) => {
  console.log('someone connected!');
});
console.log(util.inspect(server.listeners('connection')));
// Prints: [ [Function] ]

Type parameters

Parameters

Returns

Function[]

Since

v0.1.26

Inherited from

EventEmitter.listeners

off

▸ off<K>(eventName, listener): this

Alias for emitter.removeListener().

Type parameters

Parameters

Returns

this

Since

v10.0.0

Inherited from

EventEmitter.off

on

▸ on(emitter, eventName, options?): AsyncIterator<any[], any, any>

import { on, EventEmitter } from 'node:events';
import process from 'node:process';
 
const ee = new EventEmitter();
 
// Emit later on
process.nextTick(() => {
  ee.emit('foo', 'bar');
  ee.emit('foo', 42);
});
 
for await (const event of on(ee, 'foo')) {
  // The execution of this inner block is synchronous and it
  // processes one event at a time (even with await). Do not use
  // if concurrent execution is required.
  console.log(event); // prints ['bar'] [42]
}
// Unreachable here

Returns an AsyncIterator that iterates eventName events. It will throw if the EventEmitter emits 'error'. It removes all listeners when exiting the loop. The value returned by each iteration is an array composed of the emitted event arguments.

An AbortSignal can be used to cancel waiting on events:

import { on, EventEmitter } from 'node:events';
import process from 'node:process';
 
const ac = new AbortController();
 
(async () => {
  const ee = new EventEmitter();
 
  // Emit later on
  process.nextTick(() => {
    ee.emit('foo', 'bar');
    ee.emit('foo', 42);
  });
 
  for await (const event of on(ee, 'foo', { signal: ac.signal })) {
    // The execution of this inner block is synchronous and it
    // processes one event at a time (even with await). Do not use
    // if concurrent execution is required.
    console.log(event); // prints ['bar'] [42]
  }
  // Unreachable here
})();
 
process.nextTick(() => ac.abort());

Use the close option to specify an array of event names that will end the iteration:

import { on, EventEmitter } from 'node:events';
import process from 'node:process';
 
const ee = new EventEmitter();
 
// Emit later on
process.nextTick(() => {
  ee.emit('foo', 'bar');
  ee.emit('foo', 42);
  ee.emit('close');
});
 
for await (const event of on(ee, 'foo', { close: ['close'] })) {
  console.log(event); // prints ['bar'] [42]
}
// the loop will exit after 'close' is emitted
console.log('done'); // prints 'done'

Parameters

Returns

AsyncIterator<any[], any, any>

An AsyncIterator that iterates eventName events emitted by the emitter

Since

v13.6.0, v12.16.0

Inherited from

EventEmitter.on

▸ on(emitter, eventName, options?): AsyncIterator<any[], any, any>

Parameters

Returns

AsyncIterator<any[], any, any>

Inherited from

EventEmitter.on

on

▸ on<K>(eventName, listener): this

Adds the listener function to the end of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.

server.on('connection', (stream) => {
  console.log('someone connected!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

By default, event listeners are invoked in the order they are added. The emitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

import { EventEmitter } from 'node:events';
const myEE = new EventEmitter();
myEE.on('foo', () => console.log('a'));
myEE.prependListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   a

Type parameters

Parameters

Returns

this

Since

v0.1.101

Inherited from

EventEmitter.on

once

▸ once(emitter, eventName, options?): Promise<any[]>

Creates a Promise that is fulfilled when the EventEmitter emits the given event or that is rejected if the EventEmitter emits 'error' while waiting. The Promise will resolve with an array of all the arguments emitted to the given event.

This method is intentionally generic and works with the web platform EventTarget interface, which has no special'error' event semantics and does not listen to the 'error' event.

import { once, EventEmitter } from 'node:events';
import process from 'node:process';
 
const ee = new EventEmitter();
 
process.nextTick(() => {
  ee.emit('myevent', 42);
});
 
const [value] = await once(ee, 'myevent');
console.log(value);
 
const err = new Error('kaboom');
process.nextTick(() => {
  ee.emit('error', err);
});
 
try {
  await once(ee, 'myevent');
} catch (err) {
  console.error('error happened', err);
}

The special handling of the 'error' event is only used when events.once() is used to wait for another event. If events.once() is used to wait for the ‘error' event itself, then it is treated as any other kind of event without special handling:

import { EventEmitter, once } from 'node:events';
 
const ee = new EventEmitter();
 
once(ee, 'error')
  .then(([err]) => console.log('ok', err.message))
  .catch((err) => console.error('error', err.message));
 
ee.emit('error', new Error('boom'));
 
// Prints: ok boom

An AbortSignal can be used to cancel waiting for the event:

import { EventEmitter, once } from 'node:events';
 
const ee = new EventEmitter();
const ac = new AbortController();
 
async function foo(emitter, event, signal) {
  try {
    await once(emitter, event, { signal });
    console.log('event emitted!');
  } catch (error) {
    if (error.name === 'AbortError') {
      console.error('Waiting for the event was canceled!');
    } else {
      console.error('There was an error', error.message);
    }
  }
}
 
foo(ee, 'foo', ac.signal);
ac.abort(); // Abort waiting for the event
ee.emit('foo'); // Prints: Waiting for the event was canceled!

Parameters

Returns

Promise<any[]>

Since

v11.13.0, v10.16.0

Inherited from

EventEmitter.once

▸ once(emitter, eventName, options?): Promise<any[]>

Parameters

Returns

Promise<any[]>

Inherited from

EventEmitter.once

once

▸ once<K>(eventName, listener): this

Adds a one-time listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.

server.once('connection', (stream) => {
  console.log('Ah, we have our first user!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

By default, event listeners are invoked in the order they are added. The emitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

import { EventEmitter } from 'node:events';
const myEE = new EventEmitter();
myEE.once('foo', () => console.log('a'));
myEE.prependOnceListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   a

Type parameters

Parameters

Returns

this

Since

v0.3.0

Inherited from

EventEmitter.once

prependListener

▸ prependListener<K>(eventName, listener): this

Adds the listener function to the beginning of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.

server.prependListener('connection', (stream) => {
  console.log('someone connected!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

Type parameters

Parameters

Returns

this

Since

v6.0.0

Inherited from

EventEmitter.prependListener

prependOnceListener

▸ prependOnceListener<K>(eventName, listener): this

Adds a one-timelistener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.

server.prependOnceListener('connection', (stream) => {
  console.log('Ah, we have our first user!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

Type parameters

Parameters

Returns

this

Since

v6.0.0

Inherited from

EventEmitter.prependOnceListener

rawListeners

▸ rawListeners<K>(eventName): Function[]

Returns a copy of the array of listeners for the event named eventName, including any wrappers (such as those created by .once()).

import { EventEmitter } from 'node:events';
const emitter = new EventEmitter();
emitter.once('log', () => console.log('log once'));
 
// Returns a new Array with a function `onceWrapper` which has a property
// `listener` which contains the original listener bound above
const listeners = emitter.rawListeners('log');
const logFnWrapper = listeners[0];
 
// Logs "log once" to the console and does not unbind the `once` event
logFnWrapper.listener();
 
// Logs "log once" to the console and removes the listener
logFnWrapper();
 
emitter.on('log', () => console.log('log persistently'));
// Will return a new Array with a single function bound by `.on()` above
const newListeners = emitter.rawListeners('log');
 
// Logs "log persistently" twice
newListeners[0]();
emitter.emit('log');

Type parameters

Parameters

Returns

Function[]

Since

v9.4.0

Inherited from

EventEmitter.rawListeners

removeAllListeners

▸ removeAllListeners(eventName?): this

Removes all listeners, or those of the specified eventName.

It is bad practice to remove listeners added elsewhere in the code, particularly when the EventEmitter instance was created by some other component or module (e.g. sockets or file streams).

Returns a reference to the EventEmitter, so that calls can be chained.

Parameters

Returns

this

Since

v0.1.26

Inherited from

EventEmitter.removeAllListeners

removeListener

▸ removeListener<K>(eventName, listener): this

Removes the specified listener from the listener array for the event named eventName.

const callback = (stream) => {
  console.log('someone connected!');
};
server.on('connection', callback);
// ...
server.removeListener('connection', callback);

removeListener() will remove, at most, one instance of a listener from the listener array. If any single listener has been added multiple times to the listener array for the specified eventName, then removeListener() must be called multiple times to remove each instance.

Once an event is emitted, all listeners attached to it at the time of emitting are called in order. This implies that any removeListener() or removeAllListeners() calls after emitting and before the last listener finishes execution will not remove them fromemit() in progress. Subsequent events behave as expected.

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
 
const callbackA = () => {
  console.log('A');
  myEmitter.removeListener('event', callbackB);
};
 
const callbackB = () => {
  console.log('B');
};
 
myEmitter.on('event', callbackA);
 
myEmitter.on('event', callbackB);
 
// callbackA removes listener callbackB but it will still be called.
// Internal listener array at time of emit [callbackA, callbackB]
myEmitter.emit('event');
// Prints:
//   A
//   B
 
// callbackB is now removed.
// Internal listener array [callbackA]
myEmitter.emit('event');
// Prints:
//   A

Because listeners are managed using an internal array, calling this will change the position indices of any listener registered after the listener being removed. This will not impact the order in which listeners are called, but it means that any copies of the listener array as returned by the emitter.listeners() method will need to be recreated.

When a single function has been added as a handler multiple times for a single event (as in the example below), removeListener() will remove the most recently added instance. In the example the once('ping') listener is removed:

import { EventEmitter } from 'node:events';
const ee = new EventEmitter();
 
function pong() {
  console.log('pong');
}
 
ee.on('ping', pong);
ee.once('ping', pong);
ee.removeListener('ping', pong);
 
ee.emit('ping');
ee.emit('ping');

Returns a reference to the EventEmitter, so that calls can be chained.

Type parameters

Parameters

Returns

this

Since

v0.1.26

Inherited from

EventEmitter.removeListener

sendMessage

▸ sendMessage(parts, agentId): Promise<string>

Sends a message synchronously to an agent

This method sends a message to a specified agent and waits for the server to acknowledge receipt. It validates the message parts before sending and handles network errors appropriately.

Parameters

Returns

Promise<string>

Promise resolving to the message ID assigned by the server

Throws

If there’s a network issue contacting the server

Throws

If the message parts are invalid

Example

// Send a text message
const textMessageId = await messageClient.sendMessage([
  { type: 'text', content: 'Hello, agent!' }
], 'assistant-agent');
 
// Send a message with multiple parts
const multipartMessageId = await messageClient.sendMessage([
  { type: 'text', content: 'Here is the data you requested' },
  { 
    type: 'data', 
    content: { temperature: 72, humidity: 65 },
    schema: 'weather-data'
  }
], 'weather-agent');

setMaxListeners

▸ setMaxListeners(n?, ...eventTargets): void

import { setMaxListeners, EventEmitter } from 'node:events';
 
const target = new EventTarget();
const emitter = new EventEmitter();
 
setMaxListeners(5, target, emitter);

Parameters

Returns

void

Since

v15.4.0

Inherited from

EventEmitter.setMaxListeners

setMaxListeners

▸ setMaxListeners(n): this

By default EventEmitters will print a warning if more than 10 listeners are added for a particular event. This is a useful default that helps finding memory leaks. The emitter.setMaxListeners() method allows the limit to be modified for this specific EventEmitter instance. The value can be set to Infinity (or 0) to indicate an unlimited number of listeners.

Returns a reference to the EventEmitter, so that calls can be chained.

Parameters

Returns

this

Since

v0.3.5

Inherited from

EventEmitter.setMaxListeners

streamMessage

▸ streamMessage(parts, agentId, options): Promise<void>

Streams messages to and from an agent

This method establishes a real-time streaming connection with an agent, allowing for continuous message exchange with automatic error handling.

Parameters

Returns

Promise<void>

Promise that resolves when the stream completes

Throws

If there’s a network issue establishing the stream

Throws

If the message parts are invalid

Example

await messageClient.streamMessage(
  [{ type: 'text', content: 'Tell me a story about dragons' }],
  'storyteller-agent',
  {
    onMessage: (data) => console.log('Received:', data),
    onError: (error) => console.error('Stream error:', error),
    onComplete: () => console.log('Stream completed')
  }
);