POST /sessions/{session_id}/messages

409 for react/cron, non-interactive, and closed sessions. Returns the created message so the caller can track it by id; a retried send with the same idempotency_key returns the original message.

curl -X POST "https://api.ellipsis.dev/v1/sessions/{session_id}/messages" \
  -H "Authorization: Bearer $ELLIPSIS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"message":"..."}'

{
  "message": {
    "author": "string",
    "body": "string",
    "created_at": "2026-01-01T00:00:00Z",
    "delivered_at": "2026-01-01T00:00:00Z",
    "delivered_turn_id": "string",
    "feed_seq": 0,
    "id": "string",
    "sender_attribution_id": "string",
    "sender_attribution_type": "github_user",
    "session_id": "string",
    "status": "pending"
  }
}

import os

from ellipsis import Ellipsis

client = Ellipsis(api_key=os.environ["ELLIPSIS_API_TOKEN"])

result = client.sessions.send_message("...", "...")
print(result)

{
  "message": {
    "author": "string",
    "body": "string",
    "created_at": "2026-01-01T00:00:00Z",
    "delivered_at": "2026-01-01T00:00:00Z",
    "delivered_turn_id": "string",
    "feed_seq": 0,
    "id": "string",
    "sender_attribution_id": "string",
    "sender_attribution_type": "github_user",
    "session_id": "string",
    "status": "pending"
  }
}

import { Ellipsis } from '@ellipsis-dev/sdk';

const client = new Ellipsis({
  apiKey: process.env.ELLIPSIS_API_TOKEN!,
});

const result = await client.sessions.sendMessage('...', { message: '...' });
console.log(result);

{
  "message": {
    "author": "string",
    "body": "string",
    "created_at": "2026-01-01T00:00:00Z",
    "delivered_at": "2026-01-01T00:00:00Z",
    "delivered_turn_id": "string",
    "feed_seq": 0,
    "id": "string",
    "sender_attribution_id": "string",
    "sender_attribution_type": "github_user",
    "session_id": "string",
    "status": "pending"
  }
}

{
  "properties": {
    "idempotency_key": {
      "anyOf": [
        {
          "maxLength": 128,
          "type": "string"
        },
        {
          "type": "null"
        }
      ]
    },
    "message": {
      "type": "string"
    }
  },
  "required": [
    "message"
  ],
  "type": "object"
}

{
  "properties": {
    "message": {
      "properties": {
        "author": {
          "anyOf": [
            {
              "type": "string"
            },
            {
              "type": "null"
            }
          ]
        },
        "body": {
          "type": "string"
        },
        "created_at": {
          "format": "date-time",
          "type": "string"
        },
        "delivered_at": {
          "anyOf": [
            {
              "format": "date-time",
              "type": "string"
            },
            {
              "type": "null"
            }
          ]
        },
        "delivered_turn_id": {
          "anyOf": [
            {
              "type": "string"
            },
            {
              "type": "null"
            }
          ]
        },
        "feed_seq": {
          "anyOf": [
            {
              "type": "integer"
            },
            {
              "type": "null"
            }
          ]
        },
        "id": {
          "type": "string"
        },
        "sender_attribution_id": {
          "anyOf": [
            {
              "type": "string"
            },
            {
              "type": "null"
            }
          ]
        },
        "sender_attribution_type": {
          "anyOf": [
            {
              "enum": [
                "github_user",
                "linear_user",
                "slack_user",
                "api_key"
              ],
              "type": "string"
            },
            {
              "type": "null"
            }
          ]
        },
        "session_id": {
          "type": "string"
        },
        "status": {
          "enum": [
            "pending",
            "delivered"
          ],
          "type": "string"
        }
      },
      "required": [
        "id",
        "session_id",
        "status",
        "body",
        "created_at"
      ],
      "type": "object"
    }
  },
  "required": [
    "message"
  ],
  "type": "object"
}

POST /sessions/{session_id}/messages

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