> ## Documentation Index
> Fetch the complete documentation index at: https://developers.myhero.so/llms.txt
> Use this file to discover all available pages before exploring further.

# Post a message to a legacy SSE session

> Delivers a JSON-RPC message to an active legacy SSE session identified by the `sessionId` query parameter.

The server pushes the response back over the SSE stream.

Deprecated — prefer the Streamable HTTP transport.



## OpenAPI

````yaml post /mcp/messages
openapi: 3.1.0
info:
  title: HERO API
  version: current
  description: |-
    The HERO REST API.

    See https://developers.myhero.so for the rendered docs.
servers:
  - url: https://app.myhero.so
security: []
paths:
  /mcp/messages:
    post:
      tags:
        - MCP
      summary: Post a message to a legacy SSE session
      description: >-
        Delivers a JSON-RPC message to an active legacy SSE session identified
        by the `sessionId` query parameter.


        The server pushes the response back over the SSE stream.


        Deprecated — prefer the Streamable HTTP transport.
      operationId: mcpSseMessage
      parameters:
        - schema:
            type: string
            minLength: 1
          required: true
          name: sessionId
          in: query
      requestBody:
        content:
          application/json:
            schema:
              type: object
              properties:
                jsonrpc:
                  type: string
                  enum:
                    - '2.0'
                method:
                  type: string
                params: {}
                result: {}
                error:
                  type: object
                  properties:
                    code:
                      type: integer
                    message:
                      type: string
                    data: {}
                  required:
                    - code
                    - message
                id:
                  anyOf:
                    - type: string
                    - type: number
                    - type: 'null'
              required:
                - jsonrpc
      responses:
        '200':
          description: Message accepted.
          content:
            application/json:
              schema:
                type: object
                properties: {}
        '400':
          description: sessionId query param missing.
          content:
            application/json:
              schema:
                type: object
                properties:
                  error:
                    type: string
                    description: |-
                      Error class name (e.g.

                      ValidationError, NotFound)
                  message:
                    type: string
                    description: Human-readable error message
                  details:
                    type: array
                    items: {}
                required:
                  - error
                  - message
        '403':
          description: Session belongs to a different user.
          content:
            application/json:
              schema:
                type: object
                properties:
                  error:
                    type: string
                    description: |-
                      Error class name (e.g.

                      ValidationError, NotFound)
                  message:
                    type: string
                    description: Human-readable error message
                  details:
                    type: array
                    items: {}
                required:
                  - error
                  - message
        '404':
          description: No active session for the given session ID.
          content:
            application/json:
              schema:
                type: object
                properties:
                  error:
                    type: string
                    description: |-
                      Error class name (e.g.

                      ValidationError, NotFound)
                  message:
                    type: string
                    description: Human-readable error message
                  details:
                    type: array
                    items: {}
                required:
                  - error
                  - message
      security:
        - oauth2: []
components:
  securitySchemes:
    oauth2:
      type: oauth2
      flows:
        authorizationCode:
          authorizationUrl: https://app.myhero.so/oauth/authorize
          tokenUrl: https://app.myhero.so/oauth/token
          scopes: {}

````