ceos/fastmcp
1
0
Fork 0
mirror of https://github.com/PrefectHQ/fastmcp.git synced 2026-08-23 14:04:18 +02:00
Code Issues Projects Releases Packages Wiki Activity

Document mounted server state store isolation in upgrade guide (#3236)

Browse source 
* Document mounted server state store isolation in upgrade guide

* Add missing FastMCP import to upgrade guide example
This commit is contained in:
Jeremiah Lowin 2026-02-19 12:05:50 -05:00 • committed by GitHub
commit 390a11d7d2
No known key found for this signature in database
GPG key ID: B5690EEEBB952194
1 changed files with 16 additions and 0 deletions
Download patch file Download diff file Expand all files Collapse all files

16
docs/getting-started/upgrading/from-fastmcp-2.mdx
View file

@ -56,6 +56,8 @@ BREAKING CHANGES (will crash at import or runtime):
3. ASYNC STATE: ctx.set_state() and ctx.get_state() are now async (must be awaited).
State values must be JSON-serializable unless serializable=False is passed.
Each FastMCP instance has its own state store, so serializable state set by parent middleware isn't visible to mounted tools by default.
Fix: pass the same session_state_store to both servers, or use serializable=False (request-scoped state is always shared).
4. PROMPTS: mcp.types.PromptMessage replaced by fastmcp.prompts.Message.
Before: PromptMessage(role="user", content=TextContent(type="text", text="Hello"))
@ -222,6 +224,20 @@ State values must also be JSON-serializable by default (dicts, lists, strings, n
await ctx.set_state("client", my_http_client, serializable=False)
```
**Mounted servers have isolated state stores**
Each `FastMCP` instance has its own state store. In v2 this wasn't noticeable because mounted tools ran in the parent's context, but in v3's provider architecture each server is isolated. Non-serializable state (`serializable=False`) is request-scoped and automatically shared across mount boundaries. For serializable state, pass the same `session_state_store` to both servers:
```python
from fastmcp import FastMCP
from key_value.aio.stores.memory import MemoryStore
store = MemoryStore()
parent = FastMCP("Parent", session_state_store=store)
child = FastMCP("Child", session_state_store=store)
parent.mount(child, namespace="child")
```
**Auth provider environment variables removed**
In v2, auth providers like `GitHubProvider` could auto-load configuration from environment variables with a `FASTMCP_SERVER_AUTH_*` prefix. This magic has been removed — pass values explicitly: