How Streamable HTTP Transport Manages Session State via the Mcp-Session-Id Header

The Streamable HTTP transport maintains session continuity by automatically capturing the mcp-session-id response header from the first request and injecting it as the Mcp-Session-Id request header on all subsequent calls.

The Streamable HTTP transport implementation in the junjiem/dify-plugin-tools-mcp_sse repository provides stateful session management over stateless HTTP connections. Unlike traditional cookie-based sessions, this transport uses an explicit header mechanism defined by the MCP protocol specification to correlate requests with server-side session contexts.

Session State Initialization

When instantiating McpStreamableHttpClient in utils/mcp_client.py, the client begins with no session context. The __init__ method initializes self.session_id to None, indicating that the client has not yet established a session with the remote MCP server.

self.session_id = None          # ← stored per-client instance

Source: [utils/mcp_client.py](https://github.com/junjiem/dify-plugin-tools-mcp_sse/blob/main/utils/mcp_client.py#L42-L44)

This initialization occurs per client instance, meaning each McpStreamableHttpClient object maintains its own isolated session state throughout its lifetime.

Sending the Mcp-Session-Id Header

The send_message method implements conditional header injection before dispatching HTTP POST requests. The client checks the current session state and only includes the Mcp-Session-Id header when a session has been established.

if self.session_id:
    headers["Mcp-Session-Id"] = self.session_id

Source: [utils/mcp_client.py](https://github.com/junjiem/dify-plugin-tools-mcp_sse/blob/main/utils/mcp_client.py#L52-L54)

First request behavior: The initial request (typically an initialize call) transmits without the Mcp-Session-Id header, signaling to the server that a new session must be created.

Subsequent request behavior: All following requests automatically include the cached session identifier, allowing the server to associate the request with the existing session context for authentication, tool registration state, or conversation history.

Receiving and Persisting Session Identifiers

After receiving an HTTP response, the client examines the response headers for the mcp-session-id field. When present, the value overrides the current self.session_id, ensuring the client always uses the most recent session token provided by the server.

if "mcp-session-id" in response.headers:
    self.session_id = response.headers.get("mcp-session-id")

Source: [utils/mcp_client.py](https://github.com/junjiem/dify-plugin-tools-mcp_sse/blob/main/utils/mcp_client.py#L67-L68)

This extraction occurs on every response, allowing servers to rotate session IDs if necessary while maintaining transparent continuity from the client's perspective.

Session Lifecycle and Persistence

The session ID persists for the entire lifetime of the McpStreamableHttpClient instance. Calling close() shuts down the underlying httpx.AsyncClient connection pool but does not clear the stored session identifier. This design ensures that if the underlying transport reconnects, the logical MCP session remains intact.

The transport achieves stateless-on-the-wire, stateful-in-practice semantics: individual HTTP requests remain independently resolvable, but the header mechanism provides the server with the context needed to maintain continuity across the conversation.

Practical Implementation Example

The following example demonstrates configuring and using the Streamable HTTP transport with automatic session management:

from utils.mcp_client import McpClients

# Configuration for Streamable HTTP transport

servers_cfg = {
    "my_mcp": {
        "url": "https://example.com/mcp",
        "transport": "streamable_http",   # ← selects McpStreamableHttpClient

        "headers": {"Authorization": "Bearer <token>"},
        "timeout": 30,
    }
}

# Initialize client wrapper (creates McpStreamableHttpClient internally)

clients = McpClients(servers_config=servers_cfg)

# Access the underlying Streamable HTTP client

stream_client = clients._clients["my_mcp"]   # type: McpStreamableHttpClient

# First call - no session header sent; server creates session

response1 = stream_client.send_message({
    "jsonrpc": "2.0",
    "method": "initialize",
    "params": {}
})

# Second call - automatically includes Mcp-Session-Id header

response2 = stream_client.send_message({
    "jsonrpc": "2.0",
    "method": "tools/list",
    "params": {}
})

In this workflow, response1 triggers the server to generate a session ID, which send_message extracts and stores. The second call transparently includes this ID in the Mcp-Session-Id header, ensuring the server recognizes it as part of the same logical session.

Summary

• Header-based state: The Streamable HTTP transport uses the Mcp-Session-Id request header to maintain session continuity without cookies or URL parameters.
• Automatic lifecycle: McpStreamableHttpClient in utils/mcp_client.py handles session creation (first request), storage (instance variable), and propagation (subsequent requests) automatically.
• Persistent identifiers: Session IDs survive connection closures and persist until the client instance is garbage collected.
• Protocol compliance: The implementation follows the MCP Streamable HTTP specification by reading mcp-session-id from responses and sending Mcp-Session-Id in requests.

Frequently Asked Questions

What happens if the server returns a new mcp-session-id in a later response?

The client automatically updates its internal self.session_id variable with the new value on every response processing cycle. This allows servers to implement session rotation or renewal strategies while maintaining transparent compatibility with existing client instances.

Does closing the HTTP client clear the session state?

No. According to the implementation in utils/mcp_client.py, invoking close() only terminates the underlying httpx.AsyncClient connection pool. The self.session_id attribute retains its value, meaning a reopened or reused client instance would continue sending the same session identifier.

How does Streamable HTTP session management differ from SSE transport?

While Streamable HTTP uses explicit Mcp-Session-Id headers to correlate discrete HTTP requests, SSE (Server-Sent Events) transport typically maintains session state through a persistent long-lived connection. The Streamable HTTP approach更适合 stateless proxy environments and load-balanced deployments where connections may route to different backend instances.

Is the Mcp-Session-Id header case-sensitive in the implementation?

The client implementation normalizes header handling through httpx, which treats headers case-insensitively for retrieval. However, when sending, the code explicitly uses the Pascal-Kebab case Mcp-Session-Id to comply with the MCP protocol specification, while checking for the lowercase mcp-session-id key in response dictionaries to accommodate server variations.

{
  "mcpServers": {
    "instagit": {
      "command": "npx",
      "args": ["-y", "instagit@latest"]
    }
  }
}