> ## Documentation Index
> Fetch the complete documentation index at: https://agentclientprotocol.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Session Setup
> Creating and loading sessions
Sessions represent a specific conversation or thread between the [Client](/protocol/v1/overview#client) and [Agent](/protocol/v1/overview#agent). Each session maintains its own context, conversation history, and state, allowing multiple independent interactions with the same Agent.
Before creating a session, Clients **MUST** first complete the [initialization](/protocol/v1/initialization) phase to establish protocol compatibility and capabilities.
If the Agent requires authentication, `session/new` may fail with an `auth_required` error until the Client completes the [authentication flow](/protocol/v1/draft/authentication).
```mermaid theme={null}
sequenceDiagram
participant Client
participant Agent
Note over Agent,Client: Initialized
alt
Client->>Agent: session/new
Note over Agent: Create session context
Note over Agent: Connect to MCP servers
Agent-->>Client: session/new response (sessionId)
else
Client->>Agent: session/load (sessionId)
Note over Agent: Restore session context
Note over Agent: Connect to MCP servers
Note over Agent,Client: Replay conversation history...
Agent->>Client: session/update
Agent->>Client: session/update
Note over Agent,Client: All content streamed
Agent-->>Client: session/load response
else
Client->>Agent: session/resume (sessionId)
Note over Agent: Restore session context
Note over Agent: Connect to MCP servers
Agent-->>Client: session/resume response
end
Note over Client,Agent: Ready for prompts
```
## Creating a Session
Clients create a new session by calling the `session/new` method with:
* The [working directory](#working-directory) for the session
* A list of [MCP servers](#mcp-servers) the Agent should connect to
```json theme={null}
{
"jsonrpc": "2.0",
"id": 1,
"method": "session/new",
"params": {
"cwd": "/home/user/project",
"mcpServers": [
{
"name": "filesystem",
"command": "/path/to/mcp-server",
"args": ["--stdio"],
"env": []
}
]
}
}
```
The Agent **MUST** respond with a unique [Session ID](#session-id) that identifies this conversation:
```json theme={null}
{
"jsonrpc": "2.0",
"id": 1,
"result": {
"sessionId": "sess_abc123def456"
}
}
```
## Loading Sessions
Agents that support the `loadSession` capability allow Clients to resume previous conversations with history replay. This feature enables persistence across restarts and sharing sessions between different Client instances.
### Checking Support
Before attempting to load a session, Clients **MUST** verify that the Agent supports this capability by checking the `loadSession` field in the `initialize` response:
```json highlight={7} theme={null}
{
"jsonrpc": "2.0",
"id": 0,
"result": {
"protocolVersion": 1,
"agentCapabilities": {
"loadSession": true
}
}
}
```
If `loadSession` is `false` or not present, the Agent does not support loading sessions and Clients **MUST NOT** attempt to call `session/load`.
### Loading a Session
To load an existing session, Clients **MUST** call the `session/load` method with:
* The [Session ID](#session-id) to resume
* [MCP servers](#mcp-servers) to connect to
* The [working directory](#working-directory)
```json theme={null}
{
"jsonrpc": "2.0",
"id": 1,
"method": "session/load",
"params": {
"sessionId": "sess_789xyz",
"cwd": "/home/user/project",
"mcpServers": [
{
"name": "filesystem",
"command": "/path/to/mcp-server",
"args": ["--mode", "filesystem"],
"env": []
}
]
}
}
```
The Agent **MUST** replay the entire conversation to the Client in the form of `session/update` notifications (like `session/prompt`).
For example, a user message from the conversation history:
```json theme={null}
{
"jsonrpc": "2.0",
"method": "session/update",
"params": {
"sessionId": "sess_789xyz",
"update": {
"sessionUpdate": "user_message_chunk",
"messageId": "msg_user_8f7a1",
"content": {
"type": "text",
"text": "What's the capital of France?"
}
}
}
}
```
Followed by the agent's response:
```json theme={null}
{
"jsonrpc": "2.0",
"method": "session/update",
"params": {
"sessionId": "sess_789xyz",
"update": {
"sessionUpdate": "agent_message_chunk",
"messageId": "msg_agent_c42b9",
"content": {
"type": "text",
"text": "The capital of France is Paris."
}
}
}
}
```
If the Agent provides message IDs during replay, each `messageId` is an opaque, unique identifier for the replayed message.
When **all** the conversation entries have been streamed to the Client, the
Agent **MUST** respond to the original `session/load` request.
```json theme={null}
{
"jsonrpc": "2.0",
"id": 1,
"result": {}
}
```
The response **MAY** also include initial mode, model, or session configuration
state when those features are supported by the Agent.
The Client can then continue sending prompts as if the session was never
interrupted.
## Resuming Sessions
Agents that advertise `sessionCapabilities.resume` allow Clients to reconnect to
an existing session without replaying the conversation history.
### Checking Support
Before attempting to resume a session, Clients **MUST** verify that the Agent
supports this capability by checking for the `sessionCapabilities.resume` field
in the `initialize` response:
```json highlight={7-9} theme={null}
{
"jsonrpc": "2.0",
"id": 0,
"result": {
"protocolVersion": 1,
"agentCapabilities": {
"sessionCapabilities": {
"resume": {}
}
}
}
}
```
If `sessionCapabilities.resume` is not present, the Agent does not support
resuming sessions and Clients **MUST NOT** attempt to call `session/resume`.
### Resuming a Session
To resume an existing session without replaying prior messages, Clients
**MUST** call the `session/resume` method with:
* The [Session ID](#session-id) to resume
* [MCP servers](#mcp-servers) to connect to
* The [working directory](#working-directory)
```json theme={null}
{
"jsonrpc": "2.0",
"id": 2,
"method": "session/resume",
"params": {
"sessionId": "sess_789xyz",
"cwd": "/home/user/project",
"mcpServers": [
{
"name": "filesystem",
"command": "/path/to/mcp-server",
"args": ["--mode", "filesystem"],
"env": []
}
]
}
}
```
Unlike `session/load`, the Agent **MUST NOT** replay the conversation history
via `session/update` notifications before responding. Instead, it restores the
session context, reconnects to the requested MCP servers, and returns once the
session is ready to continue.
```json theme={null}
{
"jsonrpc": "2.0",
"id": 2,
"result": {}
}
```
The response **MAY** also include initial mode, model, or session configuration
state when those features are supported by the Agent.
## Closing Active Sessions
Agents that advertise `sessionCapabilities.close` allow Clients to tell the
Agent to cancel any ongoing work for a session and free any resources
associated with that active session.
### Checking Support
Before attempting to close a session, Clients **MUST** verify that the Agent
supports this capability by checking the `sessionCapabilities.close` field in
the `initialize` response:
```json highlight={7-9} theme={null}
{
"jsonrpc": "2.0",
"id": 0,
"result": {
"protocolVersion": 1,
"agentCapabilities": {
"sessionCapabilities": {
"close": {}
}
}
}
}
```
If `sessionCapabilities.close` is not present, the Agent does not support
closing sessions and Clients **MUST NOT** attempt to call `session/close`.
### Closing a Session
To close an active session, Clients **MUST** call the `session/close` method
with the session ID:
```json theme={null}
{
"jsonrpc": "2.0",
"id": 2,
"method": "session/close",
"params": {
"sessionId": "sess_789xyz"
}
}
```
The ID of the active session to close.
The Agent **MUST** cancel any ongoing work for that session as if
[`session/cancel`](/protocol/v1/prompt-turn#cancellation) had been called, then free the
resources associated with the session.
On success, the Agent responds with an empty result object:
```json theme={null}
{
"jsonrpc": "2.0",
"id": 2,
"result": {}
}
```
Agents MAY return an error if the session does not exist or is not currently
active.
## Additional Workspace Roots
Agents that advertise `sessionCapabilities.additionalDirectories` allow Clients
to include `additionalDirectories` on supported session lifecycle requests to
expand the session's effective filesystem root set. Supported stable lifecycle
requests include `session/new`, `session/load`, and `session/resume`.
```json theme={null}
{
"jsonrpc": "2.0",
"id": 2,
"method": "session/load",
"params": {
"sessionId": "sess_789xyz",
"cwd": "/home/user/project",
"additionalDirectories": [
"/home/user/shared-lib",
"/home/user/product-docs"
],
"mcpServers": []
}
}
```
When present, `additionalDirectories` has the following behavior:
* `cwd` remains the primary working directory and the base for relative paths
* each `additionalDirectories` entry **MUST** be an absolute path
* omitting the field or providing an empty array activates no additional roots for the resulting session
* on `session/load` and `session/resume`, Clients must send the full intended additional-root list again; that list may differ from any previous or reported list as long as the request `cwd` matches the session's `cwd`, and omitting the field or providing an empty array does not restore stored roots implicitly
Clients **MUST** only send `additionalDirectories` when the Agent advertises `sessionCapabilities.additionalDirectories`.
## Session ID
The session ID returned by `session/new` is a unique identifier for the conversation context.
Clients use this ID to:
* Send prompt requests via `session/prompt`
* Cancel ongoing operations via `session/cancel`
* Load previous sessions via `session/load` (if the Agent supports the `loadSession` capability)
* Resume previous sessions via `session/resume` (if the Agent supports the `sessionCapabilities.resume` capability)
* Close active sessions via `session/close` (if the Agent supports the `sessionCapabilities.close` capability)
## Working Directory
The `cwd` (current working directory) parameter establishes the primary file system context for the session. This directory:
* **MUST** be an absolute path
* **MUST** be used for the session regardless of where the Agent subprocess was spawned
* **MUST** remain the base for relative-path resolution
* **MUST** be part of the session's effective root set
When `sessionCapabilities.additionalDirectories` is in use, the session's effective root set is `[cwd, ...additionalDirectories]`. This root set **SHOULD** serve as a boundary for tool operations on the file system.
## MCP Servers
The [Model Context Protocol (MCP)](https://modelcontextprotocol.io) allows Agents to access external tools and data sources. When creating a session, Clients **MAY** include connection details for MCP servers that the Agent should connect to.
MCP servers can be connected to using different transports. All Agents **MUST** support the stdio transport, while HTTP and SSE transports are optional capabilities that can be checked during initialization.
While they are not required to by the spec, new Agents **SHOULD** support the HTTP transport to ensure compatibility with modern MCP servers.
### Transport Types
#### Stdio Transport
All Agents **MUST** support connecting to MCP servers via stdio (standard input/output). This is the default transport mechanism.
A human-readable identifier for the server
The absolute path to the MCP server executable
Command-line arguments to pass to the server
Environment variables to set when launching the server
The name of the environment variable.
The value of the environment variable.
Example stdio transport configuration:
```json theme={null}
{
"name": "filesystem",
"command": "/path/to/mcp-server",
"args": ["--stdio"],
"env": [
{
"name": "API_KEY",
"value": "secret123"
}
]
}
```
#### HTTP Transport
When the Agent supports `mcpCapabilities.http`, Clients can specify MCP servers configurations using the HTTP transport.
Must be `"http"` to indicate HTTP transport
A human-readable identifier for the server
The URL of the MCP server
HTTP headers to include in requests to the server
The name of the HTTP header.
The value to set for the HTTP header.
Example HTTP transport configuration:
```json theme={null}
{
"type": "http",
"name": "api-server",
"url": "https://api.example.com/mcp",
"headers": [
{
"name": "Authorization",
"value": "Bearer token123"
},
{
"name": "Content-Type",
"value": "application/json"
}
]
}
```
#### SSE Transport
When the Agent supports `mcpCapabilities.sse`, Clients can specify MCP servers configurations using the SSE transport.
This transport was deprecated by the MCP spec.
Must be `"sse"` to indicate SSE transport
A human-readable identifier for the server
The URL of the SSE endpoint
HTTP headers to include when establishing the SSE connection
The name of the HTTP header.
The value to set for the HTTP header.
Example SSE transport configuration:
```json theme={null}
{
"type": "sse",
"name": "event-stream",
"url": "https://events.example.com/mcp",
"headers": [
{
"name": "X-API-Key",
"value": "apikey456"
}
]
}
```
### Checking Transport Support
Before using HTTP or SSE transports, Clients **MUST** verify the Agent's capabilities during initialization:
```json highlight={7-10} theme={null}
{
"jsonrpc": "2.0",
"id": 0,
"result": {
"protocolVersion": 1,
"agentCapabilities": {
"mcpCapabilities": {
"http": true,
"sse": true
}
}
}
}
```
If `mcpCapabilities.http` is `false` or not present, the Agent does not support HTTP transport.
If `mcpCapabilities.sse` is `false` or not present, the Agent does not support SSE transport.
Agents **SHOULD** connect to all MCP servers specified by the Client.
Clients **MAY** use this ability to provide tools directly to the underlying language model by including their own MCP server.