URL: https://gofastmcp.com/more/settings
Title: Settings - FastMCP

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Logging
Transport & HTTP
Error Handling
Client
CLI & Display
Tasks (Docket)
Advanced
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Settings
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Configure FastMCP behavior through environment variables or a .env file.
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FastMCP uses
pydantic-settings
for configuration. Every setting is available as an environment variable with a
FASTMCP_
prefix. Settings are loaded from environment variables and from a
.env
file (see the
Tasks (Docket)
section for a caveat about nested settings in
.env
files).
# Set via environment
export
FASTMCP_LOG_LEVEL
=
DEBUG
export
FASTMCP_PORT
=
3000
# Or use a .env file (loaded automatically)
echo
"
FASTMCP_LOG_LEVEL=DEBUG
"
>>
.env
You can change which
.env
file is loaded by setting the
FASTMCP_ENV_FILE
environment variable (defaults to
.env
). Because this controls which file is loaded, it must be set as an environment variable — it cannot be set inside a
.env
file itself.
​
Logging
Environment Variable
Type
Default
Description
FASTMCP_LOG_LEVEL
Literal["DEBUG", "INFO", "WARNING", "ERROR", "CRITICAL"]
INFO
Log level for FastMCP’s own logging output. Case-insensitive.
FASTMCP_LOG_ENABLED
bool
true
Enable or disable FastMCP logging entirely.
FASTMCP_CLIENT_LOG_LEVEL
Literal["debug", "info", "notice", "warning", "error", "critical", "alert", "emergency"]
None
Default minimum log level for messages sent to MCP clients via
context.log()
. When set, messages below this level are suppressed. Individual clients can override this per-session using the MCP
logging/setLevel
request.
FASTMCP_ENABLE_RICH_LOGGING
bool
true
Use rich formatting for log output. Set to
false
for plain Python logging.
FASTMCP_ENABLE_RICH_TRACEBACKS
bool
true
Use rich tracebacks for errors.
FASTMCP_DEPRECATION_WARNINGS
bool
true
Show deprecation warnings.
​
Transport & HTTP
These control how the server listens when running with an HTTP transport.
Environment Variable
Type
Default
Description
FASTMCP_TRANSPORT
Literal["stdio", "http", "sse", "streamable-http"]
stdio
Default transport.
FASTMCP_HOST
str
127.0.0.1
Host to bind to.
FASTMCP_PORT
int
8000
Port to bind to.
FASTMCP_SSE_PATH
str
/sse
Path for SSE endpoint.
FASTMCP_MESSAGE_PATH
str
/messages/
Path for SSE message endpoint.
FASTMCP_STREAMABLE_HTTP_PATH
str
/mcp
Path for Streamable HTTP endpoint.
FASTMCP_STATELESS_HTTP
bool
false
Enable stateless HTTP mode (new transport per request). Useful for multi-worker deployments.
FASTMCP_JSON_RESPONSE
bool
false
Use JSON responses instead of SSE for Streamable HTTP.
FASTMCP_DEBUG
bool
false
Enable debug mode.
​
Error Handling
Environment Variable
Type
Default
Description
FASTMCP_MASK_ERROR_DETAILS
bool
false
Mask error details before sending to clients. When enabled, only messages from explicitly raised
ToolError
,
ResourceError
, or
PromptError
are included in responses.
FASTMCP_STRICT_INPUT_VALIDATION
bool
false
Strictly validate tool inputs against the JSON schema. When disabled, compatible inputs are coerced (e.g., the string
"10"
becomes the integer
10
).
FASTMCP_MOUNTED_COMPONENTS_RAISE_ON_LOAD_ERROR
bool
false
Raise errors when loading mounted components instead of logging warnings.
​
Client
Environment Variable
Type
Default
Description
FASTMCP_CLIENT_INIT_TIMEOUT
float | None
None
Timeout in seconds for the client initialization handshake. Set to
0
or leave unset to disable.
FASTMCP_CLIENT_DISCONNECT_TIMEOUT
float
5
Maximum time in seconds to wait for a clean disconnect before giving up.
FASTMCP_CLIENT_RAISE_FIRST_EXCEPTIONGROUP_ERROR
bool
true
When an
ExceptionGroup
is raised, re-raise the first error directly instead of the group. Simplifies debugging but may mask secondary errors.
​
CLI & Display
Environment Variable
Type
Default
Description
FASTMCP_SHOW_SERVER_BANNER
bool
true
Show the server banner on startup. Also controllable via
--no-banner
or
server.run(show_banner=False)
.
FASTMCP_CHECK_FOR_UPDATES
Literal["stable", "prerelease", "off"]
stable
Update checking on CLI startup.
stable
checks stable releases only,
prerelease
includes pre-releases,
off
disables checking.
​
Tasks (Docket)
These configure the
Docket
task queue used by
server tasks
. All use the
FASTMCP_DOCKET_
prefix.
When setting Docket values in a
.env
file, use a
double
underscore:
FASTMCP_DOCKET__URL
(not
FASTMCP_DOCKET_URL
). This is because
.env
values are resolved through the parent
Settings
class, which uses
__
as its nested delimiter. As regular environment variables (e.g.,
export
), the single-underscore form
FASTMCP_DOCKET_URL
works fine.
Environment Variable
Type
Default
Description
FASTMCP_DOCKET_NAME
str
fastmcp
Queue name. Servers and workers sharing the same name and backend URL share a task queue.
FASTMCP_DOCKET_URL
str
memory://
Backend URL. Use
memory://
for single-process or
redis://host:port/db
for distributed workers.
FASTMCP_DOCKET_WORKER_NAME
str | None
None
Worker name. Auto-generated if unset.
FASTMCP_DOCKET_CONCURRENCY
int
10
Maximum concurrent tasks per worker.
FASTMCP_DOCKET_REDELIVERY_TIMEOUT
timedelta
300s
If a worker doesn’t complete a task within this time, it’s redelivered to another worker.
FASTMCP_DOCKET_RECONNECTION_DELAY
timedelta
5s
Delay between reconnection attempts when the worker loses its backend connection.
FASTMCP_DOCKET_MINIMUM_CHECK_INTERVAL
timedelta
50ms
How frequently the worker polls for new tasks. Lower values reduce latency at the cost of more CPU usage.
​
Advanced
Environment Variable
Type
Default
Description
FASTMCP_HOME
Path
Platform default
Data directory for FastMCP. Defaults to the platform-specific user data directory.
FASTMCP_ENV_FILE
str
.env
Path to the
.env
file to load settings from. Must be set as an environment variable (see above).
FASTMCP_SERVER_DEPENDENCIES
list[str]
[]
Additional dependencies to install in the server environment.
FASTMCP_DECORATOR_MODE
Literal["function", "object"]
function
Controls what
@tool
,
@resource
, and
@prompt
decorators return.
function
returns the original function (default);
object
returns component objects (deprecated, will be removed).
FASTMCP_TEST_MODE
bool
false
Enable test mode.
MCP JSON Configuration 🤝 FastMCP
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