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FastMCP: Support dynamic annotation updates #1475
Description
Problem
FastMCP will support resource annotations (audience, priority, lastModified) via the @mcp.resource() decorator (see #1468). However, annotations are set once at decoration time and cannot be updated as the server runs.
This is problematic for lastModified especially. If you annotate a file resource, there's no easy way to keep the timestamp current as the file changes:
@mcp.resource("file://docs/readme.md", annotations=Annotations(
audience=["user"],
lastModified="2025-01-12T15:00:58Z"
))
def read_readme() -> str:
return open("readme.md").read()Once deployed, this annotation is stale. If the file updates tomorrow, clients see outdated metadata.
Solution
Allow annotations to be updated dynamically. Possible approaches:
- Provide a method to update resource annotations at runtime
- Allow annotation callables/lambdas that compute values on each
list_resources()call - Return fresh annotations from resource handler functions
This would enable:
- Accurate
lastModifiedtimestamps from file mtimes - Dynamic audience/priority based on server state
- Better integration with external data sources
Issue written by Claude, reviewed by @maxisbey
Activity
fennb commentedon Oct 14, 2025
Hey @maxisbey huh, I didn't really think about this when implementing #1468, good catch!
Given that #1468 is approved, will it get merged first or does this issue become a blocking factor? (the only reason I ask is that I'm working on something that's depending on the functionality 🙂).
maxisbey commentedon Oct 15, 2025
Sorry totally missed this comment!
Yea definitely wasn't a blocker. It's kind of trickier to solve and I felt it was a bit out of scope for the PR you had which added value as-is. Not super sure on this the best way to solve yet which is why I made the issue :)