The Resource Changed While the Agent Was Thinking

An application reads a resource, then spends time preparing an answer from it. During that time, the resource changes. The answer may now depend on an older copy. A resource subscription gives the application a way to learn which URI needs another read.

A subscription names the resource

In the Model Context Protocol (MCP), each resource has a URI. Clients retrieve its contents with resources/read. A server can advertise support for resource-specific update notifications through its subscribe capability. The resource specification describes these as separate operations: reading gets content; subscribing requests notice of changes.

To watch a resource, the client sends subscriptions/listen with its URI in resourceSubscriptions. That request opens a stream for notifications. The server first acknowledges the subscription and states which requested notifications it agreed to send. The client should check that acknowledgment before the initial read; reading first would leave a gap in which a change could go unnoticed. The subscription specification shows the request and acknowledgment.

The update identifies what changed

When a watched resource changes, the server sends notifications/resources/updated. The notification includes the resource URI and a subscription ID. Its example contains no replacement content. The client can use the URI to request the resource again with resources/read. The resource specification shows both messages.

There is a related signal for a different event. notifications/resources/list_changed concerns the list of available resources. The server may support list changes and resource-specific updates independently. A client watching a document should therefore handle an update to that document separately from a change to the resource list. The resource specification defines both capabilities.

What to do

1. Check that the server advertises resource subscription support. Open subscriptions/listen for the URI and check that the acknowledgment includes the requested subscription.
2. Read the URI whose contents the application needs with resources/read. Keep the subscription active while using that content. The subscription specification describes this check.
3. When an update names the URI, mark the earlier copy stale and call resources/read again before using that content in a pending answer.
4. If the stream closes, restore the subscription. A client using stdio must send a new listen request after reconnecting. The subscription specification describes how subscriptions end.