diff --git a/fastapi/exception_handlers.py b/fastapi/exception_handlers.py
index 475dd7bdd9..7e039f2b4d 100644
--- a/fastapi/exception_handlers.py
+++ b/fastapi/exception_handlers.py
@@ -9,6 +9,16 @@ from starlette.status import WS_1008_POLICY_VIOLATION
 
 
 async def http_exception_handler(request: Request, exc: HTTPException) -> Response:
+    """
+    Default exception handler for `HTTPException`.
+
+    Returns a JSON response with the exception `detail` and the appropriate
+    status code and headers. For status codes that do not allow a response body
+    (1xx, 204, 205, 304), a plain `Response` without a body is returned.
+
+    Read more about it in the
+    [FastAPI docs for Handling Errors](https://fastapi.tiangolo.com/tutorial/handling-errors/).
+    """
     headers = getattr(exc, "headers", None)
     if not is_body_allowed_for_status_code(exc.status_code):
         return Response(status_code=exc.status_code, headers=headers)
@@ -20,6 +30,16 @@ async def http_exception_handler(request: Request, exc: HTTPException) -> Respon
 async def request_validation_exception_handler(
     request: Request, exc: RequestValidationError
 ) -> JSONResponse:
+    """
+    Default exception handler for `RequestValidationError`.
+
+    Returns a 422 JSON response containing the validation error details produced
+    when the request data (path parameters, query parameters, headers, or body)
+    does not match the endpoint's declared schema.
+
+    Read more about it in the
+    [FastAPI docs for Handling Errors](https://fastapi.tiangolo.com/tutorial/handling-errors/#override-request-validation-exceptions).
+    """
     return JSONResponse(
         status_code=422,
         content={"detail": jsonable_encoder(exc.errors())},
@@ -29,6 +49,12 @@ async def request_validation_exception_handler(
 async def websocket_request_validation_exception_handler(
     websocket: WebSocket, exc: WebSocketRequestValidationError
 ) -> None:
+    """
+    Default exception handler for `WebSocketRequestValidationError`.
+
+    Closes the WebSocket connection with a 1008 (Policy Violation) close code
+    and includes the validation error details as the close reason.
+    """
     await websocket.close(
         code=WS_1008_POLICY_VIOLATION, reason=jsonable_encoder(exc.errors())
     )
diff --git a/fastapi/security/utils.py b/fastapi/security/utils.py
index 8ee66fd381..7e7f8e5fb9 100644
--- a/fastapi/security/utils.py
+++ b/fastapi/security/utils.py
@@ -1,6 +1,12 @@
 def get_authorization_scheme_param(
     authorization_header_value: str | None,
 ) -> tuple[str, str]:
+    """
+    Parse an ``Authorization`` header value into its scheme and parameter.
+
+    Returns a tuple of ``(scheme, credentials)``. If the header is ``None``
+    or empty, returns ``("", "")``.
+    """
     if not authorization_header_value:
         return "", ""
     scheme, _, param = authorization_header_value.partition(" ")
diff --git a/fastapi/utils.py b/fastapi/utils.py
index 12eaa2bf08..60186e3b6a 100644
--- a/fastapi/utils.py
+++ b/fastapi/utils.py
@@ -24,6 +24,13 @@ if TYPE_CHECKING:  # pragma: nocover
 
 
 def is_body_allowed_for_status_code(status_code: int | str | None) -> bool:
+    """
+    Return ``True`` if the given HTTP status code allows a response body.
+
+    Status codes 1xx, 204, 205, and 304 do not allow a body per the HTTP
+    specification. Wildcard codes (``"1XX"``-``"5XX"``) and ``"default"``
+    always return ``True``.
+    """
     if status_code is None:
         return True
     # Ref: https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.1.0.md#patterned-fields-1
@@ -41,6 +48,7 @@ def is_body_allowed_for_status_code(status_code: int | str | None) -> bool:
 
 
 def get_path_param_names(path: str) -> set[str]:
+    """Extract the set of path parameter names from a URL path template."""
     return set(re.findall("{(.*?)}", path))
 
 
@@ -63,6 +71,13 @@ def create_model_field(
     alias: str | None = None,
     mode: Literal["validation", "serialization"] = "validation",
 ) -> ModelField:
+    """
+    Create a Pydantic ``ModelField`` for use in request/response validation.
+
+    Raises ``FastAPIError`` if the given type cannot produce a valid Pydantic
+    schema, and ``PydanticV1NotSupportedError`` if a ``pydantic.v1`` model is
+    passed.
+    """
     if annotation_is_pydantic_v1(type_):
         raise PydanticV1NotSupportedError(
             "pydantic.v1 models are no longer supported by FastAPI."
@@ -93,6 +108,7 @@ def generate_operation_id_for_path(
 
 
 def generate_unique_id(route: "APIRoute") -> str:
+    """Generate a unique OpenAPI ``operationId`` for the given route."""
     operation_id = f"{route.name}{route.path_format}"
     operation_id = re.sub(r"\W", "_", operation_id)
     assert route.methods
@@ -101,6 +117,12 @@ def generate_unique_id(route: "APIRoute") -> str:
 
 
 def deep_dict_update(main_dict: dict[Any, Any], update_dict: dict[Any, Any]) -> None:
+    """
+    Recursively merge ``update_dict`` into ``main_dict`` in place.
+
+    Nested dicts are merged recursively and nested lists are concatenated;
+    all other values are overwritten.
+    """
     for key, value in update_dict.items():
         if (
             key in main_dict