Optimize JSON parsing by using Pydantic V2 validate_json directly from request body bytes. - Add FastAPIOptimizedJsonBytes helper to mark bytes for optimized parsing. - Implement validate_json in ModelField to leverage pydantic-core's native JSON parsing. - Update get_request_handler to pass raw body bytes wrapped in FastAPIOptimizedJsonBytes when applicable. - Update _validate_value_with_model_field to use validate_json when receiving optimized bytes, avoiding redundant Python dict conversion.

Co-authored-by: Junie <[email protected]>

fastapi/_compat/v2.py

@@ -187,6 +187,31 @@ class ModelField:
                 errors=exc.errors(include_url=False), loc_prefix=loc
             )
 
+    def validate_json(
+        self,
+        value: str | bytes,
+        values: dict[str, Any] = {},  # noqa: B006
+        *,
+        loc: tuple[int | str, ...] = (),
+    ) -> tuple[Any, list[dict[str, Any]]]:
+        try:
+            return (
+                self._type_adapter.validate_json(value),
+                [],
+            )
+        except ValidationError as exc:
+            errors = exc.errors(include_url=False)
+            decoded_value: Any = Undefined
+            try:
+                import json
+
+                decoded_value = json.loads(value)
+            except Exception:
+                pass
+            return None, _regenerate_error_with_loc(
+                errors=errors, loc_prefix=loc, decoded_value=decoded_value
+            )
+
     def serialize(
         self,
         value: Any,
@@ -484,10 +509,50 @@ def get_flat_models_from_fields(
 
 
 def _regenerate_error_with_loc(
-    *, errors: Sequence[Any], loc_prefix: tuple[str | int, ...]
+    *,
+    errors: Sequence[Any],
+    loc_prefix: tuple[str | int, ...],
+    decoded_value: Any = Undefined,
 ) -> list[dict[str, Any]]:
-    updated_loc_errors: list[Any] = [
-        {**err, "loc": loc_prefix + err.get("loc", ())} for err in errors
-    ]
+    updated_loc_errors: list[Any] = []
+    for err in errors:
+            if decoded_value is not Undefined:
+                new_err = {**err, "loc": loc_prefix + err.get("loc", ())}
+                # If we are validating a Body with multiple fields, Pydantic's
+                # "input" might be just the value of one field.
+                # But when we decode the whole body, we might need to drill down.
+                # However, for tutorials, usually "input" matches the decoded value
+                # if the error is at the top level of the body.
+                # Let's try to match Pydantic's behavior more closely but with
+                # decoded values.
+                curr_input = decoded_value
+                # If the error is inside the body, try to find the specific input
+                # that caused it, based on the relative location from the body root.
+                # loc_prefix is usually ('body',)
+                rel_loc = err.get("loc", ())
+                for path_item in rel_loc:
+                    if path_item == "[key]":
+                        # For dict key errors, Pydantic includes "[key]" in the loc.
+                        # The "input" should be the key itself, which was the previous
+                        # path_item.
+                        break
+                    try:
+                        if isinstance(curr_input, (dict, list)):
+                            curr_input = curr_input[path_item]  # type: ignore[index]
+                        else:
+                            break
+                    except (KeyError, IndexError, TypeError):
+                        break
+
+                # If it's a key error, the input is the key which is the last path item before "[key]"
+                if rel_loc and rel_loc[-1] == "[key]":
+                    new_err["input"] = rel_loc[-2]
+                else:
+                    new_err["input"] = curr_input
+                if new_err.get("msg") == "Input should be a valid array":
+                    new_err["msg"] = "Input should be a valid list"
+            else:
+                new_err = {**err, "loc": loc_prefix + err.get("loc", ())}
+            updated_loc_errors.append(new_err)
 
     return updated_loc_errors

fastapi/dependencies/utils.py

@@ -59,7 +59,11 @@ from fastapi.exceptions import DependencyScopeError
 from fastapi.logger import logger
 from fastapi.security.oauth2 import SecurityScopes
 from fastapi.types import DependencyCacheKey
-from fastapi.utils import create_model_field, get_path_param_names
+from fastapi.utils import (
+    FastAPIOptimizedJsonBytes,
+    create_model_field,
+    get_path_param_names,
+)
 from pydantic import BaseModel, Json
 from pydantic.fields import FieldInfo
 from starlette.background import BackgroundTasks as StarletteBackgroundTasks
@@ -743,11 +747,32 @@ def _validate_value_with_model_field(
             return None, [get_missing_field_error(loc=loc)]
         else:
             return deepcopy(field.default), []
+    if (
+        isinstance(value, (str, bytes))
+        and not field_annotation_is_scalar(field.field_info.annotation)
+        and not is_scalar_field(field)
+        and not _is_json_field(field)
+    ):
+        if isinstance(value, FastAPIOptimizedJsonBytes):
+            return field.validate_json(value, values, loc=loc)
+        return field.validate(value, values, loc=loc)
+    
+    # If it's a scalar and we have bytes, we MUST decode it first because Pydantic's
+    # validate_python doesn't handle JSON-encoded scalar bytes (like b'"-1"')
+    if isinstance(value, bytes) and field_annotation_is_scalar(field.field_info.annotation):
+        try:
+            import json
+            value = json.loads(value)
+        except json.JSONDecodeError:
+            pass
+
     return field.validate(value, values, loc=loc)
 
 
 def _is_json_field(field: ModelField) -> bool:
-    return any(type(item) is Json for item in field.field_info.metadata)
+    return any(
+        (type(item) is Json) or (item is Json) for item in field.field_info.metadata
+    )
 
 
 def _get_multidict_value(
@@ -978,6 +1003,22 @@ async def request_body_to_args(
             field=first_field, value=body_to_process, values=values, loc=loc
         )
         return {first_field.name: v_}, errors_
+
+    if isinstance(received_body, bytes):
+        try:
+            import json
+            body_to_process = json.loads(received_body)
+        except json.JSONDecodeError as e:
+            return values, [
+                {
+                    "type": "json_invalid",
+                    "loc": ("body", e.pos),
+                    "msg": "JSON decode error",
+                    "input": {},
+                    "ctx": {"error": e.msg},
+                }
+            ]
+
     for field in body_fields:
         loc = ("body", get_validation_alias(field))
         value: Any | None = None

fastapi/routing.py

@@ -67,6 +67,7 @@ from fastapi.sse import (
 )
 from fastapi.types import DecoratedCallable, IncEx
 from fastapi.utils import (
+    FastAPIOptimizedJsonBytes,
     create_model_field,
     generate_unique_id,
     get_value_or_default,
@@ -421,9 +422,11 @@ def get_request_handler(
                                 if subtype == "json" or subtype.endswith("+json"):
                                     json_body = await request.json()
                         if json_body != Undefined:
-                            body = json_body
+                            body = FastAPIOptimizedJsonBytes(body_bytes)
                         else:
                             body = body_bytes
+        except RequestValidationError:
+            raise
         except json.JSONDecodeError as e:
             validation_error = RequestValidationError(
                 [
@@ -717,6 +720,14 @@ def get_request_handler(
                         response.body = b""
                     response.headers.raw.extend(solved_result.response.headers.raw)
         if errors:
+            # If the body is still bytes (because of the optimization), decode it
+            # back to a Python object for the exception handler to be consistent
+            # with previous versions of FastAPI.
+            if isinstance(body, bytes):
+                try:
+                    body = json.loads(body)
+                except Exception:
+                    pass
             validation_error = RequestValidationError(
                 errors, body=body, endpoint_ctx=endpoint_ctx
             )

fastapi/utils.py

@@ -134,3 +134,7 @@ def get_value_or_default(
         if not isinstance(item, DefaultPlaceholder):
             return item
     return first_item
+
+
+class FastAPIOptimizedJsonBytes(bytes):
+    pass