feat: make the FastMCP access log opt-in (#243)

* feat: make the FastMCP access log opt-in

Closes #240

* review: warn at config construction, per branch, and on the contradiction

* feat: remove logging_turn_off_middleware instead of shimming it

Author: lesnik512

docs/integrations/fastmcp.md

@@ -47,8 +47,26 @@ def greet_person(person_name: str) -> str:
     return f"Hello, {person_name}!"
 ```
 
-Set `logging_turn_off_middleware=True` on the config to disable the per-MCP-message
-access log middleware. Set `health_checks_enabled=False` to omit the health route.
+## Logging
+
+The per-MCP-message access log is **off by default**, matching FastAPI and Litestar. Turn it on
+explicitly:
+
+```python
+FastMcpConfig(
+    service_name="microservice",
+    fastmcp_logging_middleware_enabled=True,
+)
+```
+
+Enabled, each message is logged with its `method`, `source` and `type`, plus `duration` in
+nanoseconds. A message that raises is logged at exception level and the exception is re-raised.
+
+This replaces `logging_turn_off_middleware`, which has been removed. Setting it now raises
+`TypeError`: the default flipped from on to off, so a service that configured the old field has to
+decide again rather than upgrade past the change unnoticed.
+
+Set `health_checks_enabled=False` to omit the health route.
 
 Teardown is wired through FastMCP's provider lifecycle — `bootstrapper.teardown()`
 runs automatically when the FastMCP server's ASGI lifespan shuts down (i.e. when

docs/introduction/configuration.md

@@ -255,6 +255,15 @@ FastAPI ships no access log of its own, so lite-bootstrap provides one. It is **
 See [the FastAPI integration guide](../integrations/fastapi.md#logging) for what gets logged and why
 it defaults to off.
 
+### Structlog FastMCP
+
+The per-MCP-message access log is **off by default**:
+
+- `fastmcp_logging_middleware_enabled` - turn on the access log (default: `False`). Replaces
+  `logging_turn_off_middleware`, which has been removed.
+
+See [the FastMCP integration guide](../integrations/fastmcp.md#logging) for what gets logged.
+
 ### Structlog FastStream
 
 When using FastStream, the structlog logger is automatically injected into the broker so that all broker

lite_bootstrap/bootstrappers/fastmcp_bootstrapper.py

@@ -81,7 +81,7 @@ async def on_message(
 @dataclasses.dataclass(kw_only=True, slots=True, frozen=True)
 class FastMcpConfig(HealthChecksConfig, LoggingConfig, PrometheusConfig, PyroscopeConfig, SentryConfig):
     application: "FastMCP[typing.Any]" = dataclasses.field(default_factory=_make_fastmcp)
-    logging_turn_off_middleware: bool = False
+    fastmcp_logging_middleware_enabled: bool = False
 
 
 @dataclasses.dataclass(kw_only=True)
@@ -132,7 +132,7 @@ class FastMcpLoggingInstrument(LoggingInstrument):
 
     def bootstrap(self) -> None:
         super().bootstrap()
-        if self.bootstrap_config.logging_turn_off_middleware:
+        if not self.bootstrap_config.fastmcp_logging_middleware_enabled:
             return
         self.bootstrap_config.application.add_middleware(FastMcpLoggingMiddleware())
 

tests/test_fastmcp_bootstrap.py

@@ -13,7 +13,10 @@
 from lite_bootstrap import BootstrapperNotReadyError, FastMcpBootstrapper, FastMcpConfig
 from lite_bootstrap.bootstrappers.fastmcp_bootstrapper import FastMcpLoggingMiddleware
 from lite_bootstrap.exceptions import ConfigurationError
-from tests.conftest import emulate_package_missing, emulate_package_missing_with_module_reload
+from tests.conftest import (
+    emulate_package_missing,
+    emulate_package_missing_with_module_reload,
+)
 
 
 def test_fastmcp_config_default_application() -> None:
@@ -221,24 +224,41 @@ def _find_mcp_logging_middleware(application: "FastMCP") -> list[FastMcpLoggingM
     return [m for m in application.middleware if isinstance(m, FastMcpLoggingMiddleware)]
 
 
-def test_fastmcp_logging_middleware_is_mounted_by_default() -> None:
-    config = _make_test_config()
+def _mounted_logging_middleware(config: FastMcpConfig) -> list[FastMcpLoggingMiddleware]:
     bootstrapper = FastMcpBootstrapper(bootstrap_config=config)
     application = bootstrapper.bootstrap()
     try:
-        assert len(_find_mcp_logging_middleware(application)) == 1
+        return _find_mcp_logging_middleware(application)
     finally:
         bootstrapper.teardown()
 
 
-def test_fastmcp_logging_middleware_disabled_via_flag() -> None:
-    config = _make_test_config(logging_turn_off_middleware=True)
-    bootstrapper = FastMcpBootstrapper(bootstrap_config=config)
-    application = bootstrapper.bootstrap()
-    try:
-        assert _find_mcp_logging_middleware(application) == []
-    finally:
-        bootstrapper.teardown()
+def test_fastmcp_logging_middleware_is_not_mounted_by_default() -> None:
+    assert _mounted_logging_middleware(_make_test_config()) == []
+
+
+def test_fastmcp_logging_middleware_is_mounted_when_enabled() -> None:
+    config = _make_test_config(fastmcp_logging_middleware_enabled=True)
+    assert len(_mounted_logging_middleware(config)) == 1
+
+
+def test_fastmcp_config_rejects_the_removed_turn_off_field() -> None:
+    """The superseded name is gone: setting it fails at construction rather than silently.
+
+    The default flipped, so a shim could not have been silent anyway, and a service that set the old
+    field has to re-decide rather than upgrade past the change without noticing.
+    """
+    with pytest.raises(TypeError, match="logging_turn_off_middleware"):
+        _make_test_config(logging_turn_off_middleware=True)
+
+
+@pytest.mark.parametrize("overrides", [{}, {"fastmcp_logging_middleware_enabled": True}], ids=["default", "enabled"])
+def test_fastmcp_logging_config_warns_nothing(overrides: dict[str, bool]) -> None:
+    with warnings.catch_warnings(record=True) as caught:
+        warnings.simplefilter("always")
+        _make_test_config(**overrides)
+
+    assert [str(one.message) for one in caught] == []
 
 
 @pytest.mark.parametrize(