feat: enhance JSON parse errors with line/column info and snippets

column positions and error snippets to JSON decode errors for
better debugging experience. Updates error location format and provides
context around the problematic JSON.

fastapi/routing.py

@@ -85,7 +85,8 @@ def _prepare_response_content(
     exclude_none: bool = False,
 ) -> Any:
     if isinstance(res, BaseModel):
-        read_with_orm_mode = getattr(_get_model_config(res), "read_with_orm_mode", None)
+        read_with_orm_mode = getattr(
+            _get_model_config(res), "read_with_orm_mode", None)
         if read_with_orm_mode:
             # Let from_orm extract the data from this model instead of converting
             # it now to a dict.
@@ -164,7 +165,8 @@ async def serialize_response(
                 exclude_none=exclude_none,
             )
         if is_coroutine:
-            value, errors_ = field.validate(response_content, {}, loc=("response",))
+            value, errors_ = field.validate(
+                response_content, {}, loc=("response",))
         else:
             value, errors_ = await run_in_threadpool(
                 field.validate, response_content, {}, loc=("response",)
@@ -219,7 +221,8 @@ def get_request_handler(
     dependant: Dependant,
     body_field: Optional[ModelField] = None,
     status_code: Optional[int] = None,
-    response_class: Union[Type[Response], DefaultPlaceholder] = Default(JSONResponse),
+    response_class: Union[Type[Response],
+                          DefaultPlaceholder] = Default(JSONResponse),
     response_field: Optional[ModelField] = None,
     response_model_include: Optional[IncEx] = None,
     response_model_exclude: Optional[IncEx] = None,
@@ -232,7 +235,8 @@ def get_request_handler(
 ) -> Callable[[Request], Coroutine[Any, Any, Response]]:
     assert dependant.call is not None, "dependant.call must be a function"
     is_coroutine = asyncio.iscoroutinefunction(dependant.call)
-    is_body_form = body_field and isinstance(body_field.field_info, params.Form)
+    is_body_form = body_field and isinstance(
+        body_field.field_info, params.Form)
     if isinstance(response_class, DefaultPlaceholder):
         actual_response_class: Type[Response] = response_class.value
     else:
@@ -251,7 +255,8 @@ def get_request_handler(
                         body_bytes = await request.body()
                         if body_bytes:
                             json_body: Any = Undefined
-                            content_type_value = request.headers.get("content-type")
+                            content_type_value = request.headers.get(
+                                "content-type")
                             if not content_type_value:
                                 json_body = await request.json()
                             else:
@@ -266,14 +271,32 @@ def get_request_handler(
                             else:
                                 body = body_bytes
             except json.JSONDecodeError as e:
+                lines_before = e.doc[: e.pos].split("\n")
+                line_number = len(lines_before)
+                column_number = len(
+                    lines_before[-1]) + 1 if lines_before else 1
+
+                start_pos = max(0, e.pos - 40)
+                end_pos = min(len(e.doc), e.pos + 40)
+                error_snippet = e.doc[start_pos:end_pos]
+                if start_pos > 0:
+                    error_snippet = "..." + error_snippet
+                if end_pos < len(e.doc):
+                    error_snippet = error_snippet + "..."
+
                 validation_error = RequestValidationError(
                     [
                         {
                             "type": "json_invalid",
-                            "loc": ("body", e.pos),
-                            "msg": "JSON decode error",
-                            "input": {},
-                            "ctx": {"error": e.msg},
+                            "loc": ("body", line_number, column_number),
+                            "msg": f"JSON decode error - {e.msg} at line {line_number}, column {column_number}",
+                            "input": error_snippet,
+                            "ctx": {
+                                "error": e.msg,
+                                "position": e.pos,
+                                "line": line_number,
+                                "column": column_number,
+                            },
                         }
                     ],
                     body=e.doc,
@@ -336,10 +359,12 @@ def get_request_handler(
                             exclude_none=response_model_exclude_none,
                             is_coroutine=is_coroutine,
                         )
-                        response = actual_response_class(content, **response_args)
+                        response = actual_response_class(
+                            content, **response_args)
                         if not is_body_allowed_for_status_code(response.status_code):
                             response.body = b""
-                        response.headers.raw.extend(solved_result.response.headers.raw)
+                        response.headers.raw.extend(
+                            solved_result.response.headers.raw)
             if errors:
                 validation_error = RequestValidationError(
                     _normalize_errors(errors), body=body
@@ -400,12 +425,15 @@ class APIWebSocketRoute(routing.WebSocketRoute):
         self.endpoint = endpoint
         self.name = get_name(endpoint) if name is None else name
         self.dependencies = list(dependencies or [])
-        self.path_regex, self.path_format, self.param_convertors = compile_path(path)
-        self.dependant = get_dependant(path=self.path_format, call=self.endpoint)
+        self.path_regex, self.path_format, self.param_convertors = compile_path(
+            path)
+        self.dependant = get_dependant(
+            path=self.path_format, call=self.endpoint)
         for depends in self.dependencies[::-1]:
             self.dependant.dependencies.insert(
                 0,
-                get_parameterless_sub_dependant(depends=depends, path=self.path_format),
+                get_parameterless_sub_dependant(
+                    depends=depends, path=self.path_format),
             )
         self._flat_dependant = get_flat_dependant(self.dependant)
         self._embed_body_fields = _should_embed_body_fields(
@@ -489,7 +517,8 @@ class APIRoute(routing.Route):
         self.tags = tags or []
         self.responses = responses or {}
         self.name = get_name(endpoint) if name is None else name
-        self.path_regex, self.path_format, self.param_convertors = compile_path(path)
+        self.path_regex, self.path_format, self.param_convertors = compile_path(
+            path)
         if methods is None:
             methods = ["GET"]
         self.methods: Set[str] = {method.upper() for method in methods}
@@ -529,13 +558,15 @@ class APIRoute(routing.Route):
             self.response_field = None  # type: ignore
             self.secure_cloned_response_field = None
         self.dependencies = list(dependencies or [])
-        self.description = description or inspect.cleandoc(self.endpoint.__doc__ or "")
+        self.description = description or inspect.cleandoc(
+            self.endpoint.__doc__ or "")
         # if a "form feed" character (page break) is found in the description text,
         # truncate description text to the content preceding the first "form feed"
         self.description = self.description.split("\f")[0].strip()
         response_fields = {}
         for additional_status_code, response in self.responses.items():
-            assert isinstance(response, dict), "An additional response must be a dict"
+            assert isinstance(
+                response, dict), "An additional response must be a dict"
             model = response.get("model")
             if model:
                 assert is_body_allowed_for_status_code(additional_status_code), (
@@ -547,16 +578,19 @@ class APIRoute(routing.Route):
                 )
                 response_fields[additional_status_code] = response_field
         if response_fields:
-            self.response_fields: Dict[Union[int, str], ModelField] = response_fields
+            self.response_fields: Dict[Union[int,
+                                             str], ModelField] = response_fields
         else:
             self.response_fields = {}
 
         assert callable(endpoint), "An endpoint must be a callable"
-        self.dependant = get_dependant(path=self.path_format, call=self.endpoint)
+        self.dependant = get_dependant(
+            path=self.path_format, call=self.endpoint)
         for depends in self.dependencies[::-1]:
             self.dependant.dependencies.insert(
                 0,
-                get_parameterless_sub_dependant(depends=depends, path=self.path_format),
+                get_parameterless_sub_dependant(
+                    depends=depends, path=self.path_format),
             )
         self._flat_dependant = get_flat_dependant(self.dependant)
         self._embed_body_fields = _should_embed_body_fields(
@@ -623,7 +657,8 @@ class APIRouter(routing.Router):
     def __init__(
         self,
         *,
-        prefix: Annotated[str, Doc("An optional path prefix for the router.")] = "",
+        prefix: Annotated[str, Doc(
+            "An optional path prefix for the router.")] = "",
         tags: Annotated[
             Optional[List[Union[str, Enum]]],
             Doc(
@@ -1124,7 +1159,8 @@ class APIRouter(routing.Router):
         self,
         router: Annotated["APIRouter", Doc("The `APIRouter` to include.")],
         *,
-        prefix: Annotated[str, Doc("An optional path prefix for the router.")] = "",
+        prefix: Annotated[str, Doc(
+            "An optional path prefix for the router.")] = "",
         tags: Annotated[
             Optional[List[Union[str, Enum]]],
             Doc(

tests/test_json_error_improvements.py

@@ -0,0 +1,103 @@
+from fastapi import FastAPI
+from fastapi.testclient import TestClient
+from pydantic import BaseModel
+
+app = FastAPI()
+
+
+class Item(BaseModel):
+    name: str
+    price: float
+    description: str = None
+
+
+@app.post("/items/")
+async def create_item(item: Item):
+    return item
+
+
+client = TestClient(app)
+
+
+def test_json_decode_error_single_line():
+    response = client.post(
+        "/items/",
+        content='{"name": "Test", "price": None}',
+        headers={"Content-Type": "application/json"},
+    )
+
+    assert response.status_code == 422
+    error = response.json()["detail"][0]
+
+    assert error["loc"] == ["body", 1, 27]
+    assert "line 1" in error["msg"]
+    assert "column 27" in error["msg"]
+    assert error["ctx"]["line"] == 1
+    assert error["ctx"]["column"] == 27
+    assert "None" in error["input"]
+
+
+def test_json_decode_error_multiline():
+    invalid_json = """
+{
+  "name": "Test",
+  "price": 'invalid'
+}"""
+
+    response = client.post(
+        "/items/", content=invalid_json, headers={"Content-Type": "application/json"}
+    )
+
+    assert response.status_code == 422
+    error = response.json()["detail"][0]
+
+    assert error["loc"] == ["body", 4, 12]
+    assert "line 4" in error["msg"]
+    assert "column 12" in error["msg"]
+    assert error["ctx"]["line"] == 4
+    assert error["ctx"]["column"] == 12
+    assert "invalid" in error["input"]
+
+
+def test_json_decode_error_shows_snippet():
+    long_json = '{"very_long_field_name_here": "some value", "another_field": invalid}'
+
+    response = client.post(
+        "/items/", content=long_json, headers={"Content-Type": "application/json"}
+    )
+
+    assert response.status_code == 422
+    error = response.json()["detail"][0]
+
+    assert "..." in error["input"]
+    assert "invalid" in error["input"]
+    assert len(error["input"]) <= 83
+
+
+def test_json_decode_error_empty_body():
+    response = client.post(
+        "/items/", content="", headers={"Content-Type": "application/json"}
+    )
+
+    assert response.status_code == 422
+    error = response.json()["detail"][0]
+
+    # Empty body is handled differently, not as a JSON decode error
+    assert error["loc"] == ["body"]
+    assert error["type"] == "missing"
+
+
+def test_json_decode_error_unclosed_brace():
+    response = client.post(
+        "/items/",
+        content='{"name": "Test"',
+        headers={"Content-Type": "application/json"},
+    )
+
+    assert response.status_code == 422
+    error = response.json()["detail"][0]
+
+    assert "line" in error["msg"].lower()
+    assert "column" in error["msg"].lower()
+    assert error["type"] == "json_invalid"
+    assert "position" in error["ctx"]

tests/test_tutorial/test_body/test_tutorial001.py

@@ -60,7 +60,8 @@ def test_post_with_str_float_description(client: TestClient):
 def test_post_with_str_float_description_tax(client: TestClient):
     response = client.post(
         "/items/",
-        json={"name": "Foo", "price": "50.5", "description": "Some Foo", "tax": 0.3},
+        json={"name": "Foo", "price": "50.5",
+              "description": "Some Foo", "tax": 0.3},
     )
     assert response.status_code == 200
     assert response.json() == {
@@ -206,11 +207,14 @@ def test_post_broken_body(client: TestClient):
             "detail": [
                 {
                     "type": "json_invalid",
-                    "loc": ["body", 1],
-                    "msg": "JSON decode error",
-                    "input": {},
+                    "loc": ["body", 1, 2],
+                    "msg": "JSON decode error - Expecting property name enclosed in double quotes at line 1, column 2",
+                    "input": "{some broken json}",
                     "ctx": {
-                        "error": "Expecting property name enclosed in double quotes"
+                        "error": "Expecting property name enclosed in double quotes",
+                        "position": 1,
+                        "line": 1,
+                        "column": 2,
                     },
                 }
             ]