URL: https://gofastmcp.com/apps/fastmcp-app
Title: FastMCPApp - FastMCP

Documentation Index
Fetch the complete documentation index at:
/llms.txt
Use this file to discover all available pages before exploring further.
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FAQ
On this page
A minimal interactive app
Why not just @mcp.tool(app=True)?
@app.ui() — entry points
@app.tool() — backend tools
CallTool — UI → backend
Handling results
result_key shorthand
Actions
Loading states
Forms
Manual forms
Forms from Pydantic models
Composition and namespacing
Mounting
Running standalone
A full example: contact manager
Next steps
Apps
FastMCPApp
Copy page
Wire an interactive UI to backend tools with managed visibility and composition safety.
Copy page
New in version
3.2.0
Prefab
is under active development with frequent breaking changes. FastMCP sets a minimum
prefab-ui
version but does not pin an upper bound —
pin
prefab-ui
to a specific version in your own dependencies
before deploying.
Search a list, fill out a form, click save, the list updates. That pattern — UI that reads and writes data on the server — needs two things: backend tools that actually do the work, and a way to call them from the UI.
FastMCPApp
handles the wiring.
You’ll build up to the contacts app above by the end of this page. Let’s start with something smaller.
​
A minimal interactive app
The smallest interactive app: a form that saves a note, and a list that updates when the user submits.
from
prefab_ui
.
actions
import
SetState
,
ShowToast
from
prefab_ui
.
actions
.
mcp
import
CallTool
from
prefab_ui
.
app
import
PrefabApp
from
prefab_ui
.
components
import
(
Badge
,
Button
,
Column
,
ForEach
,
Form
,
Heading
,
Input
,
Row
,
Separator
,
Text
,
)
from
prefab_ui
.
rx
import
RESULT
from
fastmcp
import
FastMCP
,
FastMCPApp
app
=
FastMCPApp
(
"
Notes
"
)
notes_db
:
list
[
dict
]
=
[]
@
app
.
tool
()
def
add_note
(
title
:
str
,
body
:
str
)
->
list
[
dict
]:
"""
Save a note and return all notes.
"""
notes_db
.
append
({
"
title
"
:
title
,
"
body
"
:
body
})
return
list
(
notes_db
)
@
app
.
ui
()
def
notes_app
()
->
PrefabApp
:
"""
Open the notes app.
"""
with
Column
(
gap
=
6
,
css_class
=
"
p-6
"
)
as
view
:
Heading
(
"
Notes
"
)
with
ForEach
(
"
notes
"
)
as
note
:
with
Row
(
gap
=
2
,
align
=
"
center
"
):
Text
(
note
.
title
,
css_class
=
"
font-semibold
"
)
Badge
(
note
.
body
)
Separator
()
with
Form
(
on_submit
=
CallTool
(
"
add_note
"
,
on_success
=[
SetState
(
"
notes
"
,
RESULT
),
ShowToast
(
"
Note saved!
"
,
variant
=
"
success
"
),
],
on_error
=
ShowToast
(
"
Failed to save
"
,
variant
=
"
error
"
),
)
):
Input
(
name
=
"
title
"
,
label
=
"
Title
"
,
required
=
True
)
Input
(
name
=
"
body
"
,
label
=
"
Body
"
,
required
=
True
)
Button
(
"
Add Note
"
)
return
PrefabApp
(
view
=
view
,
state
={
"
notes
"
:
list
(
notes_db
)})
mcp
=
FastMCP
(
"
Notes Server
"
,
providers
=[
app
])
The model sees one tool:
notes_app
. Calling it opens the UI. When the user submits the form,
CallTool("add_note")
fires, the server saves the note, returns the updated list, and
SetState("notes", RESULT)
writes that list back into state.
ForEach("notes")
re-renders. The model never sees
add_note
— it’s UI-only.
​
Why not just
@mcp.tool(app=True)
?
A fair question. Any
Interactive Tool
can call a server tool — there’s nothing stopping you from putting
CallTool("add_note")
inside a regular
@mcp.tool(app=True)
. It works for one or two tools. Things get harder once the app grows:
Which tools should the model see, and which are UI-only?
What happens to
CallTool("add_note")
when you mount this server under a namespace and the tool becomes
notes_add_note
?
How do you keep it all wired correctly as you compose servers?
FastMCPApp
owns these concerns. Entry points register as model-visible. Backend tools register as UI-only by default. Backend tools get globally stable identifiers that survive namespacing, and
CallTool
accepts function references, so references stay valid when you compose servers.
The rest of this page covers each piece in turn.
​
@app.ui()
— entry points
Entry points are what the model sees. They return a
PrefabApp
and default to
visibility=["model"]
, showing up in the LLM tool list but not callable from within the UI.
@
app
.
ui
()
def
dashboard
()
->
PrefabApp
:
"""
The model calls this to open the dashboard.
"""
with
Column
(
gap
=
4
,
css_class
=
"
p-6
"
)
as
view
:
Heading
(
"
Dashboard
"
)
...
return
PrefabApp
(
view
=
view
)
@app.ui()
supports the same options as
@mcp.tool
:
name
,
description
,
title
,
tags
,
icons
,
auth
, and
timeout
.
​
@app.tool()
— backend tools
Backend tools do the work. By default they’re visible only to the UI (
visibility=["app"]
), not the model.
@
app
.
tool
()
def
save_contact
(
name
:
str
,
email
:
str
)
->
list
[
dict
]:
"""
Save a contact and return the updated list.
"""
db
.
append
({
"
name
"
:
name
,
"
email
"
:
email
})
return
list
(
db
)
If you want a tool callable by both the model and the UI, pass
model=True
:
@
app
.
tool
(
model
=
True
)
def
list_contacts
()
->
list
[
dict
]:
"""
Both the model and the UI can call this.
"""
return
list
(
db
)
Backend tools support
name
,
description
,
auth
, and
timeout
.
​
CallTool
— UI → backend
CallTool
is how the UI invokes a backend tool. Pass the tool’s name (or a direct function reference):
from
prefab_ui
.
actions
.
mcp
import
CallTool
CallTool
(
"
save_contact
"
,
arguments
={
"
name
"
:
"
Alice
"
,
"
email
"
:
"
alice@example.com
"
})
# Or a function reference — resolves to a stable global key
CallTool
(
save_contact
,
arguments
={
...
})
Arguments can reference state with
Rx
:
from
prefab_ui
.
rx
import
STATE
CallTool
(
"
search
"
,
arguments
={
"
query
"
:
STATE
.
search_term
})
​
Handling results
Server calls are async. Use
on_success
and
on_error
callbacks:
from
prefab_ui
.
actions
import
SetState
,
ShowToast
from
prefab_ui
.
rx
import
RESULT
CallTool
(
"
save_contact
"
,
on_success
=[
SetState
(
"
contacts
"
,
RESULT
),
ShowToast
(
"
Saved!
"
,
variant
=
"
success
"
),
],
on_error
=
ShowToast
(
"
Something went wrong
"
,
variant
=
"
error
"
),
)
RESULT
is a reactive reference to the tool’s return value, available inside
on_success
.
ERROR
(from
prefab_ui.rx
) is the counterpart inside
on_error
. Callbacks can be a single action or a list; they execute in order and short-circuit on error.
​
result_key
shorthand
When a tool’s return value should replace a state key, use
result_key
:
CallTool
(
"
list_contacts
"
,
result_key
=
"
contacts
"
)
# same as:
CallTool
(
"
list_contacts
"
,
on_success
=
SetState
(
"
contacts
"
,
RESULT
))
​
Actions
CallTool
is one of several actions. Actions attach to handlers like
on_click
,
on_submit
, and
on_change
.
Client-side actions run instantly in the browser, no server round-trip:
from
prefab_ui
.
actions
import
SetState
,
ToggleState
,
AppendState
,
PopState
,
ShowToast
SetState
(
"
count
"
,
42
)
ToggleState
(
"
expanded
"
)
AppendState
(
"
items
"
,
{
"
name
"
:
"
New Item
"
})
PopState
(
"
items
"
,
0
)
ShowToast
(
"
Done!
"
,
variant
=
"
success
"
)
Pass a list to chain actions:
Button
(
"
Reset
"
,
on_click
=[
SetState
(
"
query
"
,
""
),
SetState
(
"
results
"
,
[]),
ShowToast
(
"
Cleared
"
),
],
)
​
Loading states
A common pattern: disable a button and show a spinner while a call is in flight.
from
prefab_ui
.
rx
import
Rx
saving
=
Rx
(
"
saving
"
)
Button
(
saving
.
then
(
"
Saving...
"
,
"
Save
"
),
disabled
=
saving
,
on_click
=[
SetState
(
"
saving
"
,
True
),
CallTool
(
"
save_data
"
,
on_success
=[
SetState
(
"
saving
"
,
False
),
SetState
(
"
result
"
,
RESULT
),
ShowToast
(
"
Saved!
"
,
variant
=
"
success
"
),
],
on_error
=[
SetState
(
"
saving
"
,
False
),
ShowToast
(
"
Failed
"
,
variant
=
"
error
"
),
],
),
],
)
# PrefabApp(view=view, state={"saving": False, ...})
​
Forms
Forms collect input and submit it to a tool. When submitted, named input values become the tool’s arguments.
​
Manual forms
from
prefab_ui
.
components
import
Form
,
Input
,
Select
,
SelectOption
,
Textarea
,
Button
with
Form
(
on_submit
=
CallTool
(
"
create_ticket
"
,
on_success
=
ShowToast
(
"
Ticket created!
"
,
variant
=
"
success
"
),
)
):
Input
(
name
=
"
title
"
,
label
=
"
Title
"
,
required
=
True
)
with
Select
(
name
=
"
priority
"
,
label
=
"
Priority
"
):
SelectOption
(
"
Low
"
,
value
=
"
low
"
)
SelectOption
(
"
Medium
"
,
value
=
"
medium
"
)
SelectOption
(
"
High
"
,
value
=
"
high
"
)
Textarea
(
name
=
"
description
"
,
label
=
"
Description
"
)
Button
(
"
Create Ticket
"
)
On submit,
CallTool
receives
{"title": ..., "priority": ..., "description": ...}
.
​
Forms from Pydantic models
For structured input,
Form.from_model()
generates the whole form — inputs, labels, validation:
from
typing
import
Literal
from
pydantic
import
BaseModel
,
Field
class
BugReport
(
BaseModel
):
title
:
str
=
Field
(
title
=
"
Bug Title
"
)
severity
:
Literal
[
"
low
"
,
"
medium
"
,
"
high
"
,
"
critical
"
]
=
Field
(
title
=
"
Severity
"
,
default
=
"
medium
"
)
description
:
str
=
Field
(
title
=
"
Description
"
)
@
app
.
ui
()
def
report_bug
()
->
PrefabApp
:
with
Column
(
gap
=
4
,
css_class
=
"
p-6
"
)
as
view
:
Heading
(
"
Report a Bug
"
)
Form
.
from_model
(
BugReport
,
on_submit
=
CallTool
(
"
create_bug
"
,
on_success
=
ShowToast
(
"
Bug filed!
"
,
variant
=
"
success
"
),
),
)
return
PrefabApp
(
view
=
view
)
@
app
.
tool
()
def
create_bug
(
data
:
BugReport
)
->
str
:
return
f
"Created:
{
data
.
title
}
"
str
becomes a text input,
Literal
becomes a select,
bool
becomes a checkbox. Field titles and defaults are respected.
​
Composition and namespacing
The reason
FastMCPApp
exists — and why you’d pick it over plain
@mcp.tool(app=True)
with string-based
CallTool
— is composition safety.
When you mount a server under a namespace, tool names get prefixed:
platform
=
FastMCP
(
"
Platform
"
)
platform
.
mount
(
"
contacts
"
,
contacts_server
)
# "save_contact" becomes "contacts_save_contact"
CallTool("save_contact")
would now be broken. But
CallTool(save_contact)
with a function reference resolves to a globally stable identifier that bypasses the namespace. Your app works the same whether standalone or mounted.
​
Mounting
FastMCPApp
is a Provider. Add it to a server with
providers=
or
add_provider
:
mcp
=
FastMCP
(
"
Platform
"
,
providers
=[
app
])
# or
mcp
=
FastMCP
(
"
Platform
"
)
mcp
.
add_provider
(
app
)
Multiple apps can coexist; each gets its own global keys, so there’s no collision even if two apps have a tool named
save
.
mcp
=
FastMCP
(
"
Platform
"
,
providers
=[
contacts_app
,
inventory_app
,
billing_app
])
​
Running standalone
For development,
FastMCPApp
has a
run()
shortcut that wraps itself in a temporary
FastMCP
server:
app
=
FastMCPApp
(
"
Contacts
"
)
# ... register tools ...
if
__name__
==
"
__main__
"
:
app
.
run
()
​
A full example: contact manager
This brings everything together — entry point, backend tools, Pydantic form, manual form, state, actions, and multi-visibility.
from
__future__
import
annotations
from
typing
import
Literal
from
prefab_ui
.
actions
import
SetState
,
ShowToast
from
prefab_ui
.
actions
.
mcp
import
CallTool
from
prefab_ui
.
app
import
PrefabApp
from
prefab_ui
.
components
import
(
Badge
,
Button
,
Column
,
ForEach
,
Form
,
Heading
,
Input
,
Muted
,
Row
,
Separator
,
Text
,
)
from
prefab_ui
.
rx
import
RESULT
,
Rx
from
pydantic
import
BaseModel
,
Field
from
fastmcp
import
FastMCP
,
FastMCPApp
contacts_db
:
list
[
dict
]
=
[
{
"
name
"
:
"
Arthur Dent
"
,
"
email
"
:
"
arthur@earth.com
"
,
"
category
"
:
"
Customer
"
},
{
"
name
"
:
"
Ford Prefect
"
,
"
email
"
:
"
ford@betelgeuse.org
"
,
"
category
"
:
"
Partner
"
},
]
class
ContactModel
(
BaseModel
):
name
:
str
=
Field
(
title
=
"
Full Name
"
,
min_length
=
1
)
email
:
str
=
Field
(
title
=
"
Email
"
)
category
:
Literal
[
"
Customer
"
,
"
Vendor
"
,
"
Partner
"
,
"
Other
"
]
=
"
Other
"
app
=
FastMCPApp
(
"
Contacts
"
)
@
app
.
tool
()
def
save_contact
(
data
:
ContactModel
)
->
list
[
dict
]:
"""
Save a new contact and return the updated list.
"""
contacts_db
.
append
(
data
.
model_dump
())
return
list
(
contacts_db
)
@
app
.
tool
()
def
search_contacts
(
query
:
str
)
->
list
[
dict
]:
"""
Filter contacts by name or email.
"""
q
=
query
.
lower
()
return
[
c
for
c
in
contacts_db
if
q
in
c
[
"
name
"
].
lower
()
or
q
in
c
[
"
email
"
].
lower
()
]
@
app
.
tool
(
model
=
True
)
def
list_contacts
()
->
list
[
dict
]:
"""
Return all contacts. Visible to both the model and the UI.
"""
return
list
(
contacts_db
)
@
app
.
ui
()
def
contact_manager
()
->
PrefabApp
:
"""
Open the contact manager.
"""
with
Column
(
gap
=
6
,
css_class
=
"
p-6
"
)
as
view
:
Heading
(
"
Contacts
"
)
with
ForEach
(
"
contacts
"
)
as
contact
:
with
Row
(
gap
=
2
,
align
=
"
center
"
):
Text
(
contact
.
name
,
css_class
=
"
font-medium
"
)
Muted
(
contact
.
email
)
Badge
(
contact
.
category
)
Separator
()
Heading
(
"
Add Contact
"
,
level
=
3
)
Form
.
from_model
(
ContactModel
,
on_submit
=
CallTool
(
"
save_contact
"
,
on_success
=[
SetState
(
"
contacts
"
,
RESULT
),
ShowToast
(
"
Contact saved!
"
,
variant
=
"
success
"
),
],
on_error
=
ShowToast
(
"
Failed to save
"
,
variant
=
"
error
"
),
),
)
Separator
()
Heading
(
"
Search
"
,
level
=
3
)
with
Form
(
on_submit
=
CallTool
(
"
search_contacts
"
,
arguments
={
"
query
"
:
Rx
(
"
query
"
)},
on_success
=
SetState
(
"
contacts
"
,
RESULT
),
)
):
Input
(
name
=
"
query
"
,
placeholder
=
"
Search by name or email...
"
)
Button
(
"
Search
"
)
return
PrefabApp
(
view
=
view
,
state
={
"
contacts
"
:
list
(
contacts_db
)})
mcp
=
FastMCP
(
"
Contacts Server
"
,
providers
=[
app
])
if
__name__
==
"
__main__
"
:
mcp
.
run
()
See all 100 lines
Also available as a runnable server at
examples/apps/contacts/contacts_server.py
.
​
Next steps
Interactive Tools
— the building blocks: charts, tables, dashboards, reactive state
Examples
— complete working servers
Development
— preview and test app tools locally
Prefab UI docs
— full component reference
Quickstart
Previous
Interactive Tools
Next
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