URL: https://gofastmcp.com/servers/lifespan
Title: Lifespans - FastMCP

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Basic Usage
Accessing Lifespan Context
Composing Lifespans
Backwards Compatibility
With FastAPI
Extensibility
Lifespans
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Server-level setup and teardown with composable lifespans
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New in version
3.0.0
Lifespans let you run code once when the server starts and clean up when it stops. Unlike per-session handlers, lifespans run exactly once regardless of how many clients connect.
​
Basic Usage
Use the
@lifespan
decorator to define a lifespan:
from
fastmcp
import
FastMCP
from
fastmcp
.
server
.
lifespan
import
lifespan
@
lifespan
async
def
app_lifespan
(
server
):
# Setup: runs once when server starts
print
(
"
Starting up...
"
)
try
:
yield
{
"
started_at
"
:
"
2024-01-01
"
}
finally
:
# Teardown: runs when server stops
print
(
"
Shutting down...
"
)
mcp
=
FastMCP
(
"
MyServer
"
,
lifespan
=
app_lifespan
)
The dict you yield becomes the
lifespan context
, accessible from tools.
Always use
try/finally
for cleanup code to ensure it runs even if the server is cancelled.
​
Accessing Lifespan Context
Access the lifespan context in tools via
ctx.lifespan_context
:
from
fastmcp
import
FastMCP
,
Context
from
fastmcp
.
server
.
lifespan
import
lifespan
@
lifespan
async
def
app_lifespan
(
server
):
# Initialize shared state
data
=
{
"
users
"
:
[
"
alice
"
,
"
bob
"
]}
yield
{
"
data
"
:
data
}
mcp
=
FastMCP
(
"
MyServer
"
,
lifespan
=
app_lifespan
)
@
mcp
.
tool
def
list_users
(
ctx
:
Context
)
->
list
[
str
]:
data
=
ctx
.
lifespan_context
[
"
data
"
]
return
data
[
"
users
"
]
​
Composing Lifespans
Compose multiple lifespans with the
|
operator:
from
fastmcp
import
FastMCP
from
fastmcp
.
server
.
lifespan
import
lifespan
@
lifespan
async
def
config_lifespan
(
server
):
config
=
{
"
debug
"
:
True
,
"
version
"
:
"
1.0
"
}
yield
{
"
config
"
:
config
}
@
lifespan
async
def
data_lifespan
(
server
):
data
=
{
"
items
"
:
[]}
yield
{
"
data
"
:
data
}
# Compose with |
mcp
=
FastMCP
(
"
MyServer
"
,
lifespan
=
config_lifespan
|
data_lifespan
)
Composed lifespans:
Enter in order (left to right)
Exit in reverse order (right to left)
Merge their context dicts (later values overwrite earlier on conflict)
​
Backwards Compatibility
Existing
@asynccontextmanager
lifespans still work when passed directly to FastMCP:
from
contextlib
import
asynccontextmanager
from
fastmcp
import
FastMCP
@
asynccontextmanager
async
def
legacy_lifespan
(
server
):
yield
{
"
key
"
:
"
value
"
}
mcp
=
FastMCP
(
"
MyServer
"
,
lifespan
=
legacy_lifespan
)
To compose an
@asynccontextmanager
function with
@lifespan
functions, wrap it with
ContextManagerLifespan
:
from
contextlib
import
asynccontextmanager
from
fastmcp
.
server
.
lifespan
import
lifespan
,
ContextManagerLifespan
@
asynccontextmanager
async
def
legacy_lifespan
(
server
):
yield
{
"
legacy
"
:
True
}
@
lifespan
async
def
new_lifespan
(
server
):
yield
{
"
new
"
:
True
}
# Wrap the legacy lifespan explicitly for composition
combined
=
ContextManagerLifespan
(
legacy_lifespan
)
|
new_lifespan
​
With FastAPI
When mounting FastMCP into FastAPI, use
combine_lifespans
to run both your app’s lifespan and the MCP server’s lifespan:
from
contextlib
import
asynccontextmanager
from
fastapi
import
FastAPI
from
fastmcp
import
FastMCP
from
fastmcp
.
utilities
.
lifespan
import
combine_lifespans
@
asynccontextmanager
async
def
app_lifespan
(
app
):
print
(
"
FastAPI starting...
"
)
yield
print
(
"
FastAPI shutting down...
"
)
mcp
=
FastMCP
(
"
Tools
"
)
mcp_app
=
mcp
.
http_app
()
app
=
FastAPI
(
lifespan
=
combine_lifespans
(
app_lifespan
,
mcp_app
.
lifespan
))
app
.
mount
(
"
/mcp
"
,
mcp_app
)
See the
FastAPI integration guide
for full details.
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