URL: https://gofastmcp.com/servers/providers/custom
Title: Custom Providers - FastMCP

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When to Build Custom
Providers vs Middleware
The Provider Interface
What Providers Return
Registering Providers
A Simple Provider
Lifecycle Management
Full Example: API-Backed Resources
MCP Providers
Custom Providers
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Build providers that source components from any data source
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New in version
3.0.0
Custom providers let you source components from anywhere - databases, APIs, configuration systems, or dynamic runtime logic. If you can write Python code to fetch or generate a component, you can wrap it in a provider.
​
When to Build Custom
The built-in providers handle common cases: decorators (
LocalProvider
), composition (
FastMCPProvider
), and proxying (
ProxyProvider
). Build a custom provider when your components come from somewhere else:
Database-backed tools
: Admin users define tools in a database, and your server exposes them dynamically
API-backed resources
: Resources that fetch content from external services on demand
Configuration-driven components
: Components loaded from YAML/JSON config files at startup
Multi-tenant systems
: Different users see different tools based on their permissions
Plugin systems
: Third-party code registers components at runtime
​
Providers vs Middleware
Both providers and
middleware
can influence what components a client sees, but they work at different levels.
Providers
are objects that source components. They make it easy to reason about where tools, resources, and prompts come from - a database, another server, an API.
Middleware
intercepts individual requests. It’s well-suited for request-specific decisions like logging, rate limiting, or authentication.
You
could
use middleware to dynamically add tools based on request context. But it’s often cleaner to have a provider source all possible tools, then use middleware or
visibility controls
to filter what each request can see. This separation makes it easier to reason about how components are sourced and how they interact with other server machinery.
​
The Provider Interface
A provider implements protected
_list_*
methods that return available components. The public
list_*
methods handle transforms automatically - you override the underscore-prefixed versions:
from
collections
.
abc
import
Sequence
from
fastmcp
.
server
.
providers
import
Provider
from
fastmcp
.
tools
import
Tool
from
fastmcp
.
resources
import
Resource
from
fastmcp
.
prompts
import
Prompt
class
MyProvider
(
Provider
):
async
def
_list_tools
(
self
)
->
Sequence
[
Tool
]:
"""
Return all tools this provider offers.
"""
return
[]
async
def
_list_resources
(
self
)
->
Sequence
[
Resource
]:
"""
Return all resources this provider offers.
"""
return
[]
async
def
_list_prompts
(
self
)
->
Sequence
[
Prompt
]:
"""
Return all prompts this provider offers.
"""
return
[]
You only need to implement the methods for component types you provide. The base class returns empty sequences by default.
The
_get_*
methods (
_get_tool
,
_get_resource
,
_get_prompt
) have default implementations that search through the list results. Override them only if you can fetch individual components more efficiently than iterating the full list.
​
What Providers Return
Providers return component objects that are ready to use. When a client calls a tool, FastMCP invokes the tool’s function - your provider isn’t involved in execution. This means the
Tool
,
Resource
, or
Prompt
you return must actually work.
The easiest way to create components is from functions:
from
fastmcp
.
tools
import
Tool
def
add
(
a
:
int
,
b
:
int
)
->
int
:
"""
Add two numbers.
"""
return
a
+
b
tool
=
Tool
.
from_function
(
add
)
The function’s type hints become the input schema, and the docstring becomes the description. You can override these:
tool
=
Tool
.
from_function
(
add
,
name
=
"
calculator_add
"
,
description
=
"
Add two integers together
"
)
Similar
from_function
methods exist for
Resource
and
Prompt
.
​
Registering Providers
Add providers when creating the server:
mcp
=
FastMCP
(
"
MyServer
"
,
providers
=[
DatabaseProvider
(
db_url
),
ConfigProvider
(
config_path
),
]
)
Or add them after creation:
mcp
=
FastMCP
(
"
MyServer
"
)
mcp
.
add_provider
(
DatabaseProvider
(
db_url
))
​
A Simple Provider
Here’s a minimal provider that serves tools from a dictionary:
from
collections
.
abc
import
Callable
,
Sequence
from
fastmcp
import
FastMCP
from
fastmcp
.
server
.
providers
import
Provider
from
fastmcp
.
tools
import
Tool
class
DictProvider
(
Provider
):
def
__init__
(
self
,
tools
:
dict
[
str
,
Callable
]):
super
().
__init__
()
self
.
_tools
=
[
Tool
.
from_function
(
fn
,
name
=
name
)
for
name
,
fn
in
tools
.
items
()
]
async
def
_list_tools
(
self
)
->
Sequence
[
Tool
]:
return
self
.
_tools
Use it like this:
def
add
(
a
:
int
,
b
:
int
)
->
int
:
"""
Add two numbers.
"""
return
a
+
b
def
multiply
(
a
:
int
,
b
:
int
)
->
int
:
"""
Multiply two numbers.
"""
return
a
*
b
mcp
=
FastMCP
(
"
Calculator
"
,
providers
=[
DictProvider
({
"
add
"
:
add
,
"
multiply
"
:
multiply
})
])
​
Lifecycle Management
Providers often need to set up connections when the server starts and clean them up when it stops. Override the
lifespan
method:
from
contextlib
import
asynccontextmanager
from
collections
.
abc
import
AsyncIterator
,
Sequence
class
DatabaseProvider
(
Provider
):
def
__init__
(
self
,
db_url
:
str
):
super
().
__init__
()
self
.
db_url
=
db_url
self
.
db
=
None
@
asynccontextmanager
async
def
lifespan
(
self
)
->
AsyncIterator
[
None
]:
self
.
db
=
await
connect_database
(
self
.
db_url
)
try
:
yield
finally
:
await
self
.
db
.
close
()
async
def
_list_tools
(
self
)
->
Sequence
[
Tool
]:
rows
=
await
self
.
db
.
fetch
(
"
SELECT * FROM tools
"
)
return
[
self
.
_make_tool
(
row
)
for
row
in
rows
]
FastMCP calls your provider’s
lifespan
during server startup and shutdown. The connection is available to your methods while the server runs.
​
Full Example: API-Backed Resources
Here’s a complete provider that fetches resources from an external REST API:
from
contextlib
import
asynccontextmanager
from
collections
.
abc
import
AsyncIterator
,
Sequence
from
fastmcp
.
server
.
providers
import
Provider
from
fastmcp
.
resources
import
Resource
import
httpx
class
ApiResourceProvider
(
Provider
):
"""
Provides resources backed by an external API.
"""
def
__init__
(
self
,
base_url
:
str
,
api_key
:
str
):
super
().
__init__
()
self
.
base_url
=
base_url
self
.
api_key
=
api_key
self
.
client
=
None
@
asynccontextmanager
async
def
lifespan
(
self
)
->
AsyncIterator
[
None
]:
self
.
client
=
httpx
.
AsyncClient
(
base_url
=
self
.
base_url
,
headers
={
"
Authorization
"
:
f
"Bearer
{self
.
api_key
}
"
}
)
try
:
yield
finally
:
await
self
.
client
.
aclose
()
async
def
_list_resources
(
self
)
->
Sequence
[
Resource
]:
response
=
await
self
.
client
.
get
(
"
/resources
"
)
response
.
raise_for_status
()
return
[
self
.
_make_resource
(
item
)
for
item
in
response
.
json
()[
"
items
"
]
]
def
_make_resource
(
self
,
data
:
dict
)
->
Resource
:
resource_id
=
data
[
"
id
"
]
async
def
read_content
()
->
str
:
response
=
await
self
.
client
.
get
(
f
"/resources/
{
resource_id
}
/content"
)
return
response
.
text
return
Resource
.
from_function
(
read_content
,
uri
=
f
"api://resources/
{
resource_id
}
"
,
name
=
data
[
"
name
"
],
description
=
data
.
get
(
"
description
"
,
""
),
mime_type
=
data
.
get
(
"
mime_type
"
,
"
text/plain
"
)
)
Register it like any other provider:
from
fastmcp
import
FastMCP
mcp
=
FastMCP
(
"
API Resources
"
,
providers
=[
ApiResourceProvider
(
"
https://api.example.com
"
,
"
my-api-key
"
)
])
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