Create message

Sends a steering message to a running session — the endpoint a human or another agent interrupts through.

POST /v1/agent/sessions/{id}/message

Agent → · All API references → · Get started →

Address	`https://api.hanzo.ai/v1/agent/sessions/{id}/message`
Method	`POST`
Operation	`post_agent_sessions_by_id_message`
Auth	`Authorization: Bearer $HANZO_API_KEY`

Sends a steering message to a running session — the endpoint a human or another agent interrupts through. It requires a message or a payload; the other three commands do not.

A chat turn's run READS it: the message is handed to the model as the person's next words at its next step, and a message that arrives while the model is writing its answer gets a step of its own before the answer stands. A turn that has already answered takes no more and answers 409 — what is said after an answer is a new turn.

Request

4 fields, body application/json (required).

Field	In	Type	Required	Description
`id`	path	string	yes	ID is the session to steer, from the path.
`id`	body	string	—	ID is the session to steer, from the path.
`message`	body	string	—	Message is free text for the running agent, up to 16 KiB.
`payload`	body	any	—	Payload is a structured argument for the command: any valid JSON up to 64 KiB, scanned for credentials before it is stored (a hit refuses the command with 422 and the findings).

Response

Status	Body	Meaning
`200`	agent.controlResult	ok
`default`	problem-details	refused

200 body — 11 fields.

Field	In	Type	Always	Description
`command`	body	string	—	Command is the verb that was recorded: pause, resume, stop or message.
`event`	body	agent.eventView	—
`event.actor`	body	string	—	Actor is who produced the turn.
`event.createdAt`	body	string	—	CreatedAt is when the turn was recorded, RFC 3339 in UTC to the second.
`event.id`	body	string	—	ID is the event's own handle, minted as "evt_" + 32 hex characters.
`event.kind`	body	string	—	Kind is what the turn IS, from a closed six: message (a model turn), tool-call, spawn (a subagent started), log, status, control (a steering command the running surface consumes).
`event.payload`	body	any	—	Payload is the turn's body, embedded as JSON rather than as a string — whatever the writer sent, up to 64 KiB, checked for well-formedness and scanned for credentials before it was stored.
`event.seq`	body	integer (int64)	—	Seq is the turn's position in this session's log: monotonic from 1, assigned by the store inside the insert, and unique PER SESSION rather than globally.
`event.sessionId`	body	string	—	SessionID is the session this turn belongs to.
`forwarded`	body	boolean	—	Forwarded is whether the command also reached the durable-execution engine.
`run`	body	string	—	Run is the run a resume started, under the same session.

Failure carries the platform error shape — see Errors.

Examples

CLI

SDK

HTTP

MCP

hanzo agent sessions message <id>

.ts

.py

.go

.rs

.java

import { Configuration, AgentApi } from 'hanzoai';

const api = new AgentApi(new Configuration({ accessToken: process.env.HANZO_API_KEY }));
const { data } = await api.postAgentSessionsByIdMessage({ id: 'id', id: "<id>", message: "<message>" });

from hanzoai.cloud import ApiClient, Configuration
from hanzoai.cloud.api import AgentApi

client = ApiClient(Configuration(access_token=os.environ["HANZO_API_KEY"]))
result = AgentApi(client).post_agent_sessions_by_id_message(id='id', id="<id>", message="<message>")

cfg := hanzoai.NewConfiguration()
cfg.AddDefaultHeader("Authorization", "Bearer "+os.Getenv("HANZO_API_KEY"))
client := hanzoai.NewAPIClient(cfg)

resp, _, err := client.AgentAPI.PostAgentSessionsByIdMessage(context.Background()).Execute()
if err != nil {
	return err
}

use hanzo_client::apis::{configuration::Configuration, agent_api};

let mut cfg = Configuration::new();
cfg.bearer_access_token = std::env::var("HANZO_API_KEY").ok();

let result = agent_api::post_agent_sessions_by_id_message(&cfg, Default::default()).await?;

import ai.hanzo.cloud.ApiClient;
import ai.hanzo.cloud.api.AgentApi;

ApiClient client = new ApiClient();
client.setBearerToken(System.getenv("HANZO_API_KEY"));

var result = new AgentApi(client).postAgentSessionsByIdMessage();

curl -X POST https://api.hanzo.ai/v1/agent/sessions/<id>/message \
  -H "Authorization: Bearer $HANZO_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
       "id": "<id>",
       "message": "<message>"
     }'

MCP reaches agent through the agent tool, which names its 38 operations with its own verbs — this one among them, under a name only MCP declares. describe explains any of them:

curl -X POST https://api.hanzo.ai/v1/mcp \
  -H "Content-Type: application/json" \
  -d '{
       "jsonrpc": "2.0",
       "id": 1,
       "method": "tools/call",
       "params": {
         "name": "describe",
         "arguments": {
           "op": "list_agents"
         }
       }
     }'

Agent API · All Hanzo APIs · Interactive reference

Was this page useful?

PreviousCreate events NextCreate pause