HTTP API Reference

This page documents HTTP endpoints in the Open Reward Standard.

Base URL

All endpoints are relative to the ORS server base URL:

http://localhost:8080         # Local development
https://your-server.com       # Production

Common Headers

X-Session-ID

Most endpoints require the X-Session-ID header to identify the episode:

X-Session-ID: abc-123-def-456

Required for: /create, /delete, /delete_session, /ping, /{env_name}/call, /{env_name}/prompt, /{env_name}/task_tools Not required for: /create_session, discovery endpoints, /health

Discovery Endpoints

These endpoints provide information about available environments, tools, and tasks. They generally do not require session IDs, with one exception: listing tools may require an active session when the environment defines task-specific tools (see below).

GET /health

Health check endpoint. Request:

GET /health

Response (200 OK):

{
  "status": "ok"
}

Use case: Verify server is running and responsive.

GET /list_environments

List all available environments on this server. Request:

GET /list_environments

Response (200 OK):

[
  "math",
  "code_env",
  "web_nav"
]

Returns an array of environment names (strings). Each name can be used in /{env_name}/* endpoints. Use case: Discover what environments are available before creating a session.

GET /{env_name}/tools

List all tools available in the specified environment. Request:

GET /math/tools

Response (200 OK):

{
  "tools": [
    {
      "name": "submit",
      "description": "Submit an answer to the math problem",
      "input_schema": {
        "type": "object",
        "properties": {
          "answer": {
            "type": "string",
            "description": "Your answer to the problem"
          }
        },
        "required": ["answer"]
      }
    }
  ]
}

Response Schema:

tools: Array of ToolSpec objects
- name: Tool identifier (string)
- description: Human-readable description (string)
- input_schema: JSON Schema defining tool parameters (object, optional)

Error Responses:

404 Not Found: Environment name not recognized

Use case: Discover what actions the agent can take before starting an episode.

Some environments define task-specific tools (using @tool(shared=False)) that are only available within an active session. Use the GET /{env_name}/task_tools endpoint (which requires an active session) to get the combined set of shared and task-specific tools. This sessionless /{env_name}/tools endpoint returns only shared tools.

GET /{env_name}/splits

List all task splits available in the environment. Request:

GET /math/splits

Response (200 OK):

[
  {
    "name": "train",
    "type": "train"
  },
  {
    "name": "test",
    "type": "test"
  }
]

Response Schema:

Array of Split objects
- name: Split identifier (string)
- type: Split category - “train”, “validation”, or “test” (string)

Error Responses:

404 Not Found: Environment name not recognized

Use case: Determine which task sets are available for training vs evaluation.

POST /{env_name}/tasks

List tasks for a specific split. Request:

POST /math/tasks
Content-Type: application/json

{
  "split": "train"
}

Request Schema:

split: Split name to query (string, required)

Response (200 OK):

{
  "tasks": [
    {
      "question": "What is 2+2?",
      "answer": "4"
    },
    {
      "question": "If x + 5 = 12, what is x?",
      "answer": "7"
    }
  ],
  "env_name": "math"
}

Response Schema:

tasks: Array of task objects (structure is environment-specific)
env_name: Environment name (string)

Error Responses:

400 Bad Request: Invalid split name
404 Not Found: Environment name not recognized

Use case: Get the list of tasks to iterate through during training or evaluation. Note: Task structure is environment-specific. Math environments have questions and answers. Coding environments have problem descriptions and test cases.

POST /{env_name}/num_tasks

Get the number of tasks for a specific split. Request:

POST /math/num_tasks
Content-Type: application/json

{
  "split": "train"
}

Request Schema:

split: Split name to query (string, required)

Response (200 OK):

{
  "num_tasks": 500
}

Error Responses:

400 Bad Request: Invalid split name
404 Not Found: Environment name not recognized

Use case: Check the size of a task set without downloading all tasks. Useful for pagination with /{env_name}/task_range.

POST /{env_name}/task

Get a single task by split and index. Request:

POST /math/task
Content-Type: application/json

{
  "split": "train",
  "index": 0
}

Request Schema:

split: Split name (string, required)
index: Zero-based task index (integer, required)

Response (200 OK):

{
  "task": {
    "question": "What is 2+2?",
    "answer": "4"
  }
}

Error Responses:

400 Bad Request: Invalid split name or invalid index (out of range)
404 Not Found: Environment name not recognized

Use case: Fetch individual tasks for lazy loading or selective evaluation, without downloading the full task list.

POST /{env_name}/task_range

Get tasks for a range of indices. Request:

POST /math/task_range
Content-Type: application/json

{
  "split": "train",
  "start": 0,
  "stop": 10
}

Request Schema:

split: Split name (string, required)
start: Start index, inclusive (integer, optional, default: 0). Supports negative indices.
stop: Stop index, exclusive (integer, optional, default: num_tasks). Supports negative indices.

Response (200 OK):

{
  "tasks": [
    {"question": "What is 2+2?", "answer": "4"},
    {"question": "If x + 5 = 12, what is x?", "answer": "7"}
  ]
}

Error Responses:

400 Bad Request: Invalid split name or invalid index range
404 Not Found: Environment name not recognized

Use case: Paginate through large task sets. Follows Python range()/slice conventions: start is inclusive, stop is exclusive. Negative indices are resolved relative to the total number of tasks.

Session Management

These endpoints manage episode lifecycle.

POST /create_session

Generate a new session ID. Request:

POST /create_session

Response (200 OK):

{
  "sid": "abc-123-def-456"
}

Response Schema:

sid: Session identifier (string, UUID format)

Use case: First step in episode lifecycle. Get a session ID to use in subsequent requests. Note: This endpoint only generates an ID. The episode instance is created by /create.

POST /create

Create an episode instance for a specific task. Request:

POST /create
X-Session-ID: abc-123-def-456
Content-Type: application/json

{
  "env_name": "math",
  "task_spec": {
    "question": "What is 2+2?",
    "answer": "4"
  },
  "secrets": {
    "api_key": "sk-..."
  }
}

Request Headers:

X-Session-ID: Session identifier from /create_session (required)

Request Schema:

env_name: Environment to instantiate (string, optional). Defaults to the first registered environment if omitted. If the server hosts a single environment, requests to unrecognized paths are redirected to it automatically.
task_spec: Task-specific data passed to the environment constructor (object). Provide either task_spec or both split and index, not both.
split: Split name to resolve the task from (string). Must be provided together with index.
index: Task index within the split (integer). Must be provided together with split.
secrets: Secrets to pass to environment (object, optional, default: )

You must provide exactly one of: task_spec (pass the full task object directly) or split + index (let the server resolve the task via its get_task() method). Providing both or neither will return a 400 error.

Response (200 OK):

{
  "sid": "abc-123-def-456"
}

Error Responses:

400 Bad Request: Session already exists, or missing X-Session-ID header
404 Not Found: Environment name not recognized

Use case: Start an episode by initializing the environment with a specific task. This calls the environment’s setup() method. Important: The environment initialization happens asynchronously. Subsequent requests wait for setup to complete.

POST /ping

Keep the session alive to prevent timeout. Request:

POST /ping
X-Session-ID: abc-123-def-456

Request Headers:

X-Session-ID: Session identifier (required)

Response (200 OK):

{
  "status": "ok"
}

Error Responses:

400 Bad Request: Missing X-Session-ID header
404 Not Found: Session not found

Use case: Sessions timeout after 15 minutes of inactivity. Call /ping periodically to keep the session alive. The reference SDK pings every 10 seconds; at minimum, ping well before the 15-minute timeout.

POST /delete

Delete an episode and clean up resources. Request:

POST /delete
X-Session-ID: abc-123-def-456

Request Headers:

X-Session-ID: Session identifier (required)

Response (200 OK):

{
  "sid": "abc-123-def-456"
}

Error Responses:

400 Bad Request: Missing X-Session-ID header
404 Not Found: Session not found

Use case: Clean up after episode completes. This calls the environment’s teardown() method. Important: Always call /delete when done with an episode to free server resources.

POST /delete_session

Optional cleanup endpoint for session ID. Request:

POST /delete_session
X-Session-ID: abc-123-def-456

Request Headers:

X-Session-ID: Session identifier (required)

Response (200 OK):

{
  "sid": "abc-123-def-456"
}

Use case: Optional cleanup after /delete. In practice, /delete is sufficient.

Episode Interaction

These endpoints interact with an active episode.

GET /{env_name}/task_tools

List all tools (shared + task-specific) available in the current session. Request:

GET /math/task_tools
X-Session-ID: abc-123-def-456

Request Headers:

X-Session-ID: Session identifier (required)

Response (200 OK):

{
  "tools": [
    {
      "name": "submit",
      "description": "Submit an answer",
      "input_schema": { "type": "object", "properties": { "answer": { "type": "string" } }, "required": ["answer"] }
    },
    {
      "name": "get_hint",
      "description": "Get a hint for this specific task",
      "input_schema": null
    }
  ]
}

Response Schema:

tools: Array of ToolSpec objects (shared tools + task-specific tools combined)

Error Responses:

400 Bad Request: Missing X-Session-ID header
404 Not Found: Session not found
410 Gone: Session was deleted

Use case: Get the full set of tools available for the current task, including task-specific tools that are not returned by the sessionless /{env_name}/tools endpoint. Tools marked with @tool(shared=False) or returned by list_task_tools() only appear here.

GET /{env_name}/prompt

Get the initial prompt for the current task. Request:

GET /math/prompt
X-Session-ID: abc-123-def-456

Request Headers:

X-Session-ID: Session identifier (required)

Response (200 OK):

[
  {
    "text": "What is 2+2?",
    "detail": null,
    "type": "text"
  }
]

Response Schema:

Array of Block objects
- TextBlock: {"text": "...", "detail": null, "type": "text"}
- ImageBlock: {"data": "base64...", "mimeType": "image/png", "detail": null, "type": "image"}

Error Responses:

400 Bad Request: Missing X-Session-ID header
404 Not Found: Session not found (no active environment for this session ID)
410 Gone: Session was deleted

Use case: Get the initial state/instructions for the episode. Call this after /create and before calling tools. Note: Prompts can be multi-modal (text + images). The environment is resolved by the session, not by the {env_name} path segment. The path parameter is not validated against the session’s actual environment.

POST /{env_name}/call

Call a tool in the environment. Request:

POST /math/call
X-Session-ID: abc-123-def-456
Accept: text/event-stream
Content-Type: application/json

{
  "name": "submit",
  "input": {
    "answer": "4"
  },
  "task_id": "optional-trace-id"
}

Request Headers:

X-Session-ID: Session identifier (required)
Accept: Should be text/event-stream (recommended). The server returns SSE regardless, but setting this header is good practice for client clarity.

Request Schema:

name: Tool name to call (string, required)
input: Tool-specific parameters (object, required)
task_id: Identifier for reconnection and tracing (string, optional). If provided on a reconnect, the server returns the result of the original task instead of starting a new one. Completed results linger for 60 seconds, enabling recovery from dropped connections.

Response (200 OK - Server-Sent Events): Small response (fits in one event):

event: task_id
data: 877bb56c594e4a0f921ad55c439a3762

event: end
data: {"ok": true, "output": {"blocks": [{"text": "Correct!", "detail": null, "type": "text"}], "metadata": null, "reward": 1.0, "finished": true}}

Large response (>4 KB, chunked):

event: task_id
data: 877bb56c594e4a0f921ad55c439a3762

event: chunk
data: {"ok": true, "output": {"blocks": [{"text": "Very long out

event: end
data: put...", "detail": null, "type": "text"}], "metadata": null, "reward": 0.5, "finished": false}}

SSE Event Types:

Event	Data	Description
`task_id`	Task identifier string	Always sent first. Use this ID to reconnect if the connection drops.
`chunk`	Partial JSON string	Intermediate chunk for responses exceeding 4 KB. Concatenate all `chunk` data with the final `end` data to reconstruct the full JSON.
`end`	JSON string (or final chunk)	Final event. Contains the complete response JSON, or the last chunk if the response was split.
`error`	Error message string	Sent if the `task_id` is unknown (on reconnect) or the task raised an exception. No `end` event follows.

Response Schema (reconstructed from SSE data): On success:

{
  "ok": true,
  "output": {
    "blocks": [
      {"text": "Tool output", "detail": null, "type": "text"}
    ],
    "metadata": null,
    "reward": 1.0,
    "finished": true
  }
}

On error:

{
  "ok": false,
  "error": "Error message"
}

Error Responses:

400 Bad Request: Missing X-Session-ID, invalid tool name, or invalid input
404 Not Found: Session not found, environment mismatch, or tool not found
410 Gone: Session was deleted

Use case: Execute an action in the environment. This is the core interaction loop. Important:

Responses are delivered via Server-Sent Events (see Tool Execution with SSE section above)
The finished field indicates if the episode is complete
The reward field provides RL feedback signal

Complete Episode Flow

Here’s a complete example showing all endpoints in sequence:

# 1. Health check
curl http://localhost:8080/health
# → {"status": "ok"}

# 2. List environments
curl http://localhost:8080/list_environments
# → ["math"]

# 3. List tools
curl http://localhost:8080/math/tools
# → {"tools": [{"name": "submit", ...}]}

# 4. List splits
curl http://localhost:8080/math/splits
# → [{"name": "train", "type": "train"}, ...]

# 5. List tasks
curl -X POST http://localhost:8080/math/tasks \
  -H "Content-Type: application/json" \
  -d '{"split": "train"}'
# → {"tasks": [...], "env_name": "math"}

# 6. Create session ID
curl -X POST http://localhost:8080/create_session
# → {"sid": "abc-123"}

# 7. Create episode (option A: pass task_spec directly)
curl -X POST http://localhost:8080/create \
  -H "X-Session-ID: abc-123" \
  -H "Content-Type: application/json" \
  -d '{"env_name": "math", "task_spec": {"question": "What is 2+2?"}, "secrets": {}}'
# → {"sid": "abc-123"}

# 7b. Or create episode (option B: resolve by split + index)
# curl -X POST http://localhost:8080/create \
#   -H "X-Session-ID: abc-123" \
#   -H "Content-Type: application/json" \
#   -d '{"env_name": "math", "split": "train", "index": 0, "secrets": {}}'
# → {"sid": "abc-123"}

# 8. Get prompt
curl http://localhost:8080/math/prompt \
  -H "X-Session-ID: abc-123"
# → [{"text": "What is 2+2?", "detail": null, "type": "text"}]

# 9. Call tool (SSE response)
curl -X POST http://localhost:8080/math/call \
  -H "X-Session-ID: abc-123" \
  -H "Accept: text/event-stream" \
  -H "Content-Type: application/json" \
  -d '{"name": "submit", "input": {"answer": "4"}}'
# → SSE stream with result

# 10. Delete episode
curl -X POST http://localhost:8080/delete \
  -H "X-Session-ID: abc-123"
# → {"sid": "abc-123"}

Error Handling

Standard HTTP Status Codes

200 OK: Request succeeded
400 Bad Request: Invalid input (missing header, invalid JSON, etc.)
404 Not Found: Resource not found (environment, session, tool)
410 Gone: Session was deleted
500 Internal Server Error: Server error during processing

Error Response Format

For HTTP errors (non-SSE):

{
  "detail": "Error message explaining what went wrong"
}

For tool execution errors (SSE):

{
  "ok": false,
  "error": "Tool execution failed: reason"
}

Rate Limiting and Timeouts

Session Timeout

Sessions automatically expire after 15 minutes of inactivity:

“Activity” means any request with that session’s X-Session-ID
Use /ping to keep sessions alive
Timeout is reset after each request

Request Timeout

Individual requests may have server-specific timeouts. All tool calls use SSE, which keeps connections alive with periodic pings during long-running execution.

Next Steps

Data Types

Detailed schemas for all request/response objects

Implementing a Client

Build an ORS client using these endpoints

Implementing a Server

Build an ORS server that implements these endpoints

Reference Implementation: The ORS Python SDK implements this complete API.

Getting Started

Specification

Core Concepts

Implementation Guides

Comparison

Base URL

Common Headers

X-Session-ID

Discovery Endpoints

GET /health

GET /list_environments

GET /{env_name}/tools

GET /{env_name}/splits

POST /{env_name}/tasks

POST /{env_name}/num_tasks

POST /{env_name}/task

POST /{env_name}/task_range

Session Management

POST /create_session

POST /create

POST /ping

POST /delete

POST /delete_session

Episode Interaction

GET /{env_name}/task_tools

GET /{env_name}/prompt

POST /{env_name}/call

Complete Episode Flow

Error Handling

Standard HTTP Status Codes

Error Response Format

Rate Limiting and Timeouts

Session Timeout

Request Timeout

Next Steps

Data Types

Implementing a Client

Implementing a Server

Getting Started

Specification

Core Concepts

Implementation Guides

Comparison

​Base URL

​Common Headers

​X-Session-ID

​Discovery Endpoints

​GET /health

​GET /list_environments

​GET /{env_name}/tools

​GET /{env_name}/splits

​POST /{env_name}/tasks

​POST /{env_name}/num_tasks

​POST /{env_name}/task

​POST /{env_name}/task_range

​Session Management

​POST /create_session

​POST /create

​POST /ping

​POST /delete

​POST /delete_session

​Episode Interaction

​GET /{env_name}/task_tools

​GET /{env_name}/prompt

​POST /{env_name}/call

​Complete Episode Flow

​Error Handling

​Standard HTTP Status Codes

​Error Response Format

​Rate Limiting and Timeouts

​Session Timeout

​Request Timeout

​Next Steps

Data Types

Implementing a Client

Implementing a Server

Base URL

Common Headers

X-Session-ID

Discovery Endpoints

GET /health

GET /list_environments

GET /{env_name}/tools

GET /{env_name}/splits

POST /{env_name}/tasks

POST /{env_name}/num_tasks

POST /{env_name}/task

POST /{env_name}/task_range

Session Management

POST /create_session

POST /create

POST /ping

POST /delete

POST /delete_session

Episode Interaction

GET /{env_name}/task_tools

GET /{env_name}/prompt

POST /{env_name}/call

Complete Episode Flow

Error Handling

Standard HTTP Status Codes

Error Response Format

Rate Limiting and Timeouts

Session Timeout

Request Timeout

Next Steps