MCP Inspector SSE Reconnect Bug: Missing Last-Event-ID

Hey guys! Today, we're diving into a tricky bug that's been causing some headaches in the Model Context Protocol (MCP) world, specifically within the mcp-inspector. If you're working with Streamable HTTP (SSE) and MCP, you'll definitely want to hear about this. It's all about a missing header during reconnects, and it's messing with the ability to smoothly resume streams. Let's break it down!

The Bug: Last-Event-ID Header MIA

The core issue revolves around the Last-Event-ID header. When using mcp-inspector with an MCP server over SSE, the client, for some reason, isn't sending this crucial header during reconnects. Now, why is this a big deal? Well, the Last-Event-ID header is the key to resuming an SSE stream from where it left off. It tells the server the ID of the last event the client received, so the server knows exactly where to pick up. Without it, the server is left in the dark, and the stream can't resume correctly, breaking the MCP protocol's resume functionality. This can lead to lost events and inconsistencies, which is definitely not what we want.

The importance of SSE reconnect functionality cannot be overstated, especially in scenarios where network interruptions are common. Imagine a real-time data feed providing critical updates; a dropped connection shouldn't mean losing valuable information. The Last-Event-ID header is precisely the mechanism designed to prevent such data loss. It ensures that when a client reconnects after a disruption, it can seamlessly resume the stream from the point of interruption, guaranteeing data integrity and continuity. This is a fundamental aspect of SSE and is crucial for applications that rely on a continuous, uninterrupted flow of data. The absence of this header during reconnects effectively disables this crucial feature, rendering the system vulnerable to data loss and inconsistencies. Therefore, addressing this bug is not just about fixing a technical glitch; it's about safeguarding the reliability and integrity of the entire data streaming process.

To further understand the implications of this missing header, it's essential to delve into the MCP specification for Streamable HTTP/SSE. The specification explicitly describes support for the Last-Event-ID header, highlighting its role in ensuring seamless stream resumption. This underscores the fact that the current behavior of mcp-inspector deviates from the intended functionality as defined by the protocol. The specification serves as a contract between the client and the server, outlining the expected behavior during SSE communication. By omitting the Last-Event-ID header, mcp-inspector is essentially violating this contract, leading to the observed issues with stream resumption. This discrepancy between the intended behavior and the actual implementation highlights the severity of the bug and the need for a prompt resolution. The MCP specification clearly lays out the importance of this header, and its absence directly contradicts the protocol's design, emphasizing the urgency of addressing this issue to ensure compliance and proper functionality.

The impact of this bug extends beyond mere technical inconvenience; it has significant implications for the overall reliability and robustness of applications that rely on MCP and SSE. In many real-world scenarios, network connectivity can be unpredictable, and temporary disruptions are inevitable. Without the Last-Event-ID header, these disruptions can lead to data loss and inconsistencies, potentially compromising the integrity of the application. For instance, consider a system that uses MCP and SSE to deliver real-time updates to users. If a user experiences a brief network outage, they might miss critical updates if the client fails to send the Last-Event-ID header upon reconnection. This can result in a degraded user experience and, in some cases, even lead to incorrect or outdated information being displayed. Therefore, fixing this bug is not just about adhering to the MCP specification; it's about ensuring that applications built on this technology can withstand the challenges of real-world network conditions and provide a consistently reliable experience for users. The robustness of the system hinges on the correct implementation of SSE resumption, making this bug a critical issue to address.

How to Reproduce the Issue

Want to see this bug in action? Here’s how you can reproduce it:

1. First, you'll need to start up an MCP server that supports SSE and makes sure it emits events with an id: field. This id field is what the Last-Event-ID header will be referencing.
2. Next, connect to the server using mcp-inspector. This is where the action happens.
3. Now, for the fun part: interrupt the connection. You can do this by restarting the server or even just disabling and re-enabling your network connection. Simulate a brief network hiccup.
4. You'll notice that mcp-inspector automatically tries to reconnect, which is good! However, here's the catch: the HTTP request doesn't include the Last-Event-ID header. This is where the bug manifests itself.

This step-by-step process allows you to witness firsthand the absence of the Last-Event-ID header during reconnection attempts. By simulating a network interruption, you can observe how the mcp-inspector fails to properly resume the stream, highlighting the importance of this header in maintaining data continuity. The ability to reproduce the bug is crucial for understanding its impact and for verifying the effectiveness of any proposed solutions. This practical demonstration underscores the need for a fix and provides a clear validation method for ensuring that the issue is resolved.

Expected Behavior

So, what should happen? Well, on an SSE reconnect, mcp-inspector should be a good citizen and include the Last-Event-ID header. This header needs to contain the id of the last event that was successfully received. This way, the server knows exactly where to pick up the stream and continue sending events from the correct point. It's like telling the server,