Streamable HTTP Powers Real-Time AI Tool Interaction via MCP

The Model Context Protocol (MCP) stands as an open standard designed to seamlessly link artificial intelligence models—acting as clients—with external tools and diverse data sources, which function as servers. While local integrations can leverage a straightforward standard input/output mechanism, remote interactions across networks rely on a sophisticated HTTP-based communication layer. This is where Streamable HTTP comes into play, a modern transport introduced in early 2025 as an evolution of earlier approaches, specifically engineered to manage streaming interactions between AI clients and their remote tools. Essentially, Streamable HTTP empowers MCP to transmit and receive data over the internet in a continuous, real-time flow, moving beyond the traditional single request-response paradigm. This innovation is fundamental to how remote MCP servers operate, enabling AI agents to engage with web services in a fluid and highly interactive manner.

At its core, Streamable HTTP is an HTTP-based communication framework that facilitates streaming responses and bidirectional communication over a single HTTP connection. From the client’s perspective, this typically involves sending requests via HTTP POST. The server, in turn, can respond either with a conventional single JSON message or by initiating a live event stream using Server-Sent Events (SSE) to deliver multiple messages over time. A key design feature is the server’s use of one unified HTTP endpoint—for instance, https://example.com/mcp—which accommodates both POST requests for sending commands and GET requests for establishing a persistent listening stream. This single-endpoint architecture significantly simplifies implementation compared to older, more complex multi-endpoint schemes. Crucially, Streamable HTTP maintains compatibility with existing web infrastructure, such as proxies and load balancers, by built upon standard HTTP protocols (POST, GET) and SSE, while simultaneously enabling long-lived data streams. In essence, it provides a controlled mechanism for MCP to stream data over HTTP, using SSE to deliver continuous server messages whenever necessary.

To fully grasp the mechanics of Streamable HTTP, it’s helpful to trace the sequential steps of a typical client-server interaction, from initial setup to termination. The process begins with the client initiating a session by sending an HTTP POST request to the server’s designated MCP endpoint. This foundational request carries information about the client’s capabilities and the desired protocol version. Upon successful processing, the server responds with an HTTP 200 OK status, critically including a unique Mcp-Session-Id in its headers. This identifier is the cornerstone for maintaining the session’s state, and the client must include it in all subsequent requests to preserve context. The response body also confirms the successful setup and details the server’s capabilities, such as available tools.

Following session establishment, the client typically opens a secondary communication channel for server-initiated messages, known as the Announcement Channel. This is achieved by sending an HTTP GET request to the same endpoint, again including the Mcp-Session-Id and signaling its intent to receive event streams via the Accept: text/event-stream header. The server responds with HTTP 200 OK and a Content-Type: text/event-stream header, keeping the TCP connection open indefinitely. This persistent connection allows the server to push JSON-RPC requests or notifications to the client at any time, independently of the client’s own command requests.

The true power of Streamable HTTP becomes evident when handling long-running tasks that require real-time updates. When a client needs to execute such a task, for example, running a data analysis script, it sends another HTTP POST request containing the relevant command, complete with the Mcp-Session-Id. Recognizing this as a potentially long operation, the server immediately responds with an HTTP 200 OK status and a Content-Type: text/event-stream header. This action keeps the connection for this specific POST request open, transforming its response body into a dedicated SSE stream. As the task progresses, the server sends real-time updates as SSE events on this stream. Once the task is complete, the final result is sent as the last SSE event, and this specific stream is then closed, neatly scoping all progress updates and the final result to the initiating request.

Crucially, the two channels operate independently. While a long-running POST transaction is underway, the server can still communicate with the client about unrelated matters. For instance, if a new tool is added to the server, it can construct a notification and send it as an SSE event not on the active POST response stream, but over the persistent GET connection established earlier. This demonstrates the Announcement Channel in action, delivering session-scoped, general notifications decoupled from any specific client-initiated command.

The protocol is also designed for robustness against network failures. If the client’s long-lived GET connection (the Announcement Channel) drops, the client’s networking library detects the disconnection and immediately issues a new HTTP GET request. This request includes both the Mcp-Session-Id and a Last-Event-ID header, indicating the last successfully processed event. The server uses this information to replay any messages sent after that event that the client might have missed, seamlessly restoring the Announcement Channel without data loss. This layered approach to state — where the Mcp-Session-Id creates a durable application-layer session and open HTTP connections provide ephemeral transport-level stream state — enables such resilience, allowing the protocol to rebuild the ephemeral transport state using the durable application state whenever disruptions occur.

Finally, when the client completes all its tasks, it explicitly terminates the session by sending an HTTP DELETE request to the /mcp endpoint, including the Mcp-Session-Id. The server validates the ID, cleans up any associated session state, closes the persistent GET connection, and invalidates the session ID, responding with an HTTP 200 OK to formally conclude the communication lifecycle.

Streamable HTTP is a cornerstone of MCP’s ability to connect AI agents with tools over the internet in a rich, interactive manner. By extending HTTP to be stream-friendly and leveraging SSE for continuous updates, it provides the flexibility essential for AI systems that need to stream outputs, manage long-running tasks, and maintain real-time synchronization. Unlike the traditional HTTP request-response model, Streamable HTTP keeps the conversation alive: an AI agent can initiate a task with a tool and immediately begin receiving progress updates or results, while the tool can also send alerts or ask for clarification on the same persistent communication line. This approach significantly enhances reliability through session resumption capabilities and improves efficiency by avoiding unnecessary always-on connections, all while remaining fully compatible with established web standards. In practice, Streamable HTTP enables scenarios like a coding assistant streaming live logs from a remote build tool or a data analysis agent querying a database and receiving results incrementally. It effectively marries the simplicity of HTTP with the power of streaming, fulfilling MCP’s promise of acting as a “USB-C for AI tools” by providing a unified, robust interface for streaming context and data between AI and the world.