URL: https://gofastmcp.com/servers/authorization
Title: Authorization - FastMCP

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Auth Checks
require_scopes
restrict_tag
Combining Checks
Custom Auth Checks
Async Auth Checks
Error Handling
Component-Level Authorization
Server-Level Authorization
Component Auth + Middleware
Tag-Based Global Authorization
Accessing Tokens in Tools
Reference
AccessToken
AuthContext
Imports
Auth
Authorization
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Control access to components using callable-based authorization checks that filter visibility and enforce permissions.
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New in version
3.0.0
Authorization controls what authenticated users can do with your FastMCP server. While
authentication
verifies identity (who you are), authorization determines access (what you can do). FastMCP provides a callable-based authorization system that works at both the component level and globally via middleware.
The authorization model centers on a simple concept: callable functions that receive context about the current request and return
True
to allow access or
False
to deny it. Multiple checks combine with AND logic, meaning all checks must pass for access to be granted.
Authorization relies on OAuth tokens which are only available with HTTP transports (SSE, Streamable HTTP). In STDIO mode, there’s no OAuth mechanism, so
get_access_token()
returns
None
and all auth checks are skipped.
When an
AuthProvider
is configured, all requests to the MCP endpoint must carry a valid token—unauthenticated requests are rejected at the transport level before any auth checks run. Authorization checks therefore differentiate between authenticated users based on their scopes and claims, not between authenticated and unauthenticated users.
​
Auth Checks
An auth check is any callable that accepts an
AuthContext
and returns a boolean. Auth checks can be synchronous or asynchronous, so checks that need to perform async operations (like reading server state or calling external services) work naturally.
from
fastmcp
.
server
.
auth
import
AuthContext
def
my_custom_check
(
ctx
:
AuthContext
)
->
bool
:
# ctx.token is AccessToken | None
# ctx.component is the Tool, Resource, or Prompt being accessed
return
ctx
.
token
is
not
None
and
"
special
"
in
ctx
.
token
.
scopes
FastMCP provides two built-in auth checks that cover common authorization patterns.
​
require_scopes
Scope-based authorization checks that the token contains all specified OAuth scopes. When multiple scopes are provided, all must be present (AND logic).
from
fastmcp
import
FastMCP
from
fastmcp
.
server
.
auth
import
require_scopes
mcp
=
FastMCP
(
"
Scoped Server
"
)
@
mcp
.
tool
(
auth
=
require_scopes
(
"
admin
"
))
def
admin_operation
()
->
str
:
"""
Requires the 'admin' scope.
"""
return
"
Admin action completed
"
@
mcp
.
tool
(
auth
=
require_scopes
(
"
read
"
,
"
write
"
))
def
read_write_operation
()
->
str
:
"""
Requires both 'read' AND 'write' scopes.
"""
return
"
Read/write action completed
"
​
restrict_tag
Tag-based restrictions apply scope requirements conditionally. If a component has the specified tag, the token must have the required scopes. Components without the tag are unaffected.
from
fastmcp
import
FastMCP
from
fastmcp
.
server
.
auth
import
restrict_tag
from
fastmcp
.
server
.
middleware
import
AuthMiddleware
mcp
=
FastMCP
(
"
Tagged Server
"
,
middleware
=[
AuthMiddleware
(
auth
=
restrict_tag
(
"
admin
"
,
scopes
=[
"
admin
"
]))
]
)
@
mcp
.
tool
(
tags
={
"
admin
"
})
def
admin_tool
()
->
str
:
"""
Tagged 'admin', so requires 'admin' scope.
"""
return
"
Admin only
"
@
mcp
.
tool
(
tags
={
"
public
"
})
def
public_tool
()
->
str
:
"""
Not tagged 'admin', so no scope required by the restriction.
"""
return
"
Anyone can access
"
​
Combining Checks
Multiple auth checks can be combined by passing a list. All checks must pass for authorization to succeed (AND logic).
from
fastmcp
import
FastMCP
from
fastmcp
.
server
.
auth
import
require_scopes
mcp
=
FastMCP
(
"
Combined Auth Server
"
)
@
mcp
.
tool
(
auth
=[
require_scopes
(
"
admin
"
),
require_scopes
(
"
write
"
)])
def
secure_admin_action
()
->
str
:
"""
Requires both 'admin' AND 'write' scopes.
"""
return
"
Secure admin action
"
​
Custom Auth Checks
Any callable that accepts
AuthContext
and returns
bool
can serve as an auth check. This enables authorization logic based on token claims, component metadata, or external systems.
from
fastmcp
import
FastMCP
from
fastmcp
.
server
.
auth
import
AuthContext
mcp
=
FastMCP
(
"
Custom Auth Server
"
)
def
require_premium_user
(
ctx
:
AuthContext
)
->
bool
:
"""
Check for premium user status in token claims.
"""
if
ctx
.
token
is
None
:
return
False
return
ctx
.
token
.
claims
.
get
(
"
premium
"
,
False
)
is
True
def
require_access_level
(
minimum_level
:
int
):
"""
Factory function for level-based authorization.
"""
def
check
(
ctx
:
AuthContext
)
->
bool
:
if
ctx
.
token
is
None
:
return
False
user_level
=
ctx
.
token
.
claims
.
get
(
"
level
"
,
0
)
return
user_level
>=
minimum_level
return
check
@
mcp
.
tool
(
auth
=
require_premium_user
)
def
premium_feature
()
->
str
:
"""
Only for premium users.
"""
return
"
Premium content
"
@
mcp
.
tool
(
auth
=
require_access_level
(
5
))
def
advanced_feature
()
->
str
:
"""
Requires access level 5 or higher.
"""
return
"
Advanced feature
"
​
Async Auth Checks
Auth checks can be
async
functions, which is useful when the authorization decision depends on asynchronous operations like reading server state or querying external services.
from
fastmcp
import
FastMCP
from
fastmcp
.
server
.
auth
import
AuthContext
mcp
=
FastMCP
(
"
Async Auth Server
"
)
async
def
check_user_permissions
(
ctx
:
AuthContext
)
->
bool
:
"""
Async auth check that reads server state.
"""
if
ctx
.
token
is
None
:
return
False
user_id
=
ctx
.
token
.
claims
.
get
(
"
sub
"
)
# Async operations work naturally in auth checks
permissions
=
await
fetch_user_permissions
(
user_id
)
return
"
admin
"
in
permissions
@
mcp
.
tool
(
auth
=
check_user_permissions
)
def
admin_tool
()
->
str
:
return
"
Admin action completed
"
Sync and async checks can be freely combined in a list — each check is handled according to its type.
​
Error Handling
Auth checks can raise exceptions for explicit denial with custom messages:
AuthorizationError
: Propagates with its custom message, useful for explaining why access was denied
Other exceptions
: Masked for security (logged internally, treated as denial)
from
fastmcp
.
server
.
auth
import
AuthContext
from
fastmcp
.
exceptions
import
AuthorizationError
def
require_verified_email
(
ctx
:
AuthContext
)
->
bool
:
"""
Require verified email with explicit denial message.
"""
if
ctx
.
token
is
None
:
raise
AuthorizationError
(
"
Authentication required
"
)
if
not
ctx
.
token
.
claims
.
get
(
"
email_verified
"
):
raise
AuthorizationError
(
"
Email verification required
"
)
return
True
​
Component-Level Authorization
The
auth
parameter on decorators controls visibility and access for individual components. When auth checks fail for the current request, the component is hidden from list responses and direct access returns not-found.
from
fastmcp
import
FastMCP
from
fastmcp
.
server
.
auth
import
require_scopes
mcp
=
FastMCP
(
"
Component Auth Server
"
)
@
mcp
.
tool
(
auth
=
require_scopes
(
"
write
"
))
def
write_tool
()
->
str
:
"""
Only visible to users with 'write' scope.
"""
return
"
Written
"
@
mcp
.
resource
(
"
secret://data
"
,
auth
=
require_scopes
(
"
read
"
))
def
secret_resource
()
->
str
:
"""
Only visible to users with 'read' scope.
"""
return
"
Secret data
"
@
mcp
.
prompt
(
auth
=
require_scopes
(
"
admin
"
))
def
admin_prompt
()
->
str
:
"""
Only visible to users with 'admin' scope.
"""
return
"
Admin prompt content
"
Component-level
auth
controls both visibility (list filtering) and access (direct lookups return not-found for unauthorized requests). Additionally use
AuthMiddleware
to apply server-wide authorization rules and get explicit
AuthorizationError
responses on unauthorized execution attempts.
​
Server-Level Authorization
For server-wide authorization enforcement, use
AuthMiddleware
. This middleware applies auth checks globally to all components—filtering list responses and blocking unauthorized execution with explicit
AuthorizationError
responses.
from
fastmcp
import
FastMCP
from
fastmcp
.
server
.
auth
import
require_scopes
from
fastmcp
.
server
.
middleware
import
AuthMiddleware
mcp
=
FastMCP
(
"
Enforced Auth Server
"
,
middleware
=[
AuthMiddleware
(
auth
=
require_scopes
(
"
api
"
))]
)
@
mcp
.
tool
def
any_tool
()
->
str
:
"""
Requires 'api' scope to see AND call.
"""
return
"
Protected
"
​
Component Auth + Middleware
Component-level
auth
and
AuthMiddleware
work together as complementary layers. The middleware applies server-wide rules to all components, while component-level auth adds per-component requirements. Both layers are checked—all checks must pass.
from
fastmcp
import
FastMCP
from
fastmcp
.
server
.
auth
import
require_scopes
,
restrict_tag
from
fastmcp
.
server
.
middleware
import
AuthMiddleware
mcp
=
FastMCP
(
"
Layered Auth Server
"
,
middleware
=[
AuthMiddleware
(
auth
=
restrict_tag
(
"
admin
"
,
scopes
=[
"
admin
"
]))
]
)
# Requires "write" scope (component-level)
# Also requires "admin" scope if tagged "admin" (middleware-level)
@
mcp
.
tool
(
auth
=
require_scopes
(
"
write
"
),
tags
={
"
admin
"
})
def
admin_write
()
->
str
:
"""
Requires both 'write' AND 'admin' scopes.
"""
return
"
Admin write
"
# Requires "write" scope (component-level only)
@
mcp
.
tool
(
auth
=
require_scopes
(
"
write
"
))
def
user_write
()
->
str
:
"""
Requires 'write' scope.
"""
return
"
User write
"
​
Tag-Based Global Authorization
A common pattern uses
restrict_tag
with
AuthMiddleware
to apply scope requirements based on component tags.
from
fastmcp
import
FastMCP
from
fastmcp
.
server
.
auth
import
restrict_tag
from
fastmcp
.
server
.
middleware
import
AuthMiddleware
mcp
=
FastMCP
(
"
Tag-Based Auth Server
"
,
middleware
=[
AuthMiddleware
(
auth
=
restrict_tag
(
"
admin
"
,
scopes
=[
"
admin
"
])),
AuthMiddleware
(
auth
=
restrict_tag
(
"
write
"
,
scopes
=[
"
write
"
])),
]
)
@
mcp
.
tool
(
tags
={
"
admin
"
})
def
delete_all_data
()
->
str
:
"""
Requires 'admin' scope.
"""
return
"
Deleted
"
@
mcp
.
tool
(
tags
={
"
write
"
})
def
update_record
(
id
:
str
,
data
:
str
)
->
str
:
"""
Requires 'write' scope.
"""
return
f
"Updated
{
id
}
"
@
mcp
.
tool
def
read_record
(
id
:
str
)
->
str
:
"""
No tag restrictions, accessible to all.
"""
return
f
"Record
{
id
}
"
​
Accessing Tokens in Tools
Tools can access the current authentication token using
get_access_token()
from
fastmcp.server.dependencies
. This enables tools to make decisions based on user identity or permissions beyond simple authorization checks.
from
fastmcp
import
FastMCP
from
fastmcp
.
server
.
dependencies
import
get_access_token
mcp
=
FastMCP
(
"
Token Access Server
"
)
@
mcp
.
tool
def
personalized_greeting
()
->
str
:
"""
Greet the user based on their token claims.
"""
token
=
get_access_token
()
if
token
is
None
:
return
"
Hello, guest!
"
name
=
token
.
claims
.
get
(
"
name
"
,
"
user
"
)
return
f
"Hello,
{
name
}
!"
@
mcp
.
tool
def
user_dashboard
()
->
dict
:
"""
Return user-specific data based on token.
"""
token
=
get_access_token
()
if
token
is
None
:
return
{
"
error
"
:
"
Not authenticated
"
}
return
{
"
client_id
"
:
token
.
client_id
,
"
scopes
"
:
token
.
scopes
,
"
claims
"
:
token
.
claims
,
}
​
Reference
​
AccessToken
The
AccessToken
object contains information extracted from the OAuth token.
Property
Type
Description
token
str
The raw token string
client_id
str | None
OAuth client identifier
scopes
list[str]
Granted OAuth scopes
expires_at
datetime | None
Token expiration time
claims
dict[str, Any]
All JWT claims or custom token data
​
AuthContext
The
AuthContext
dataclass is passed to all auth check functions.
Property
Type
Description
token
AccessToken | None
Current access token, or
None
if unauthenticated
component
Tool | Resource | Prompt
The component being accessed
Access to the component object enables authorization decisions based on metadata like tags, name, or custom properties.
from
fastmcp
.
server
.
auth
import
AuthContext
def
require_matching_tag
(
ctx
:
AuthContext
)
->
bool
:
"""
Require a scope matching each of the component's tags.
"""
if
ctx
.
token
is
None
:
return
False
user_scopes
=
set
(
ctx
.
token
.
scopes
)
return
ctx
.
component
.
tags
.
issubset
(
user_scopes
)
​
Imports
from
fastmcp
.
server
.
auth
import
(
AccessToken
,
# Token with .token, .client_id, .scopes, .expires_at, .claims
AuthContext
,
# Context with .token, .component
AuthCheck
,
# Type alias: sync or async Callable[[AuthContext], bool]
require_scopes
,
# Built-in: requires specific scopes
restrict_tag
,
# Built-in: tag-based scope requirements
run_auth_checks
,
# Utility: run checks with AND logic
)
from
fastmcp
.
server
.
middleware
import
AuthMiddleware
Multiple Auth Sources
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