Fix form model losing model_fields_set for unsubmitted fields

When a Pydantic BaseModel is used as a Form parameter, fields absent from
the form submission were being pre-filled with their default values before
the model was constructed. This caused Pydantic to treat those defaults as
explicitly-set values, so model_fields_set incorrectly included fields the
client never sent.

The fix skips absent fields in _extract_form_body entirely, letting Pydantic
apply defaults through its own mechanism (which correctly leaves them out of
model_fields_set), matching the behaviour of JSON body endpoints.

Fixes #13399

fastapi/dependencies/utils.py

@@ -916,8 +916,15 @@ async def _extract_form_body(
     values = {}
 
     for field in body_fields:
-        value = _get_multidict_value(field, received_body)
+        alias = get_validation_alias(field)
         field_info = field.field_info
+        # Skip fields not present in the form data so that Pydantic uses its
+        # own default mechanism.  This preserves correct model_fields_set
+        # semantics: fields the client didn't submit should not appear as
+        # "set" on the resulting model (fixes #13399).
+        if alias not in received_body:
+            continue
+        value = _get_multidict_value(field, received_body)
         if (
             isinstance(field_info, params.File)
             and is_bytes_or_nonable_bytes_annotation(field.field_info.annotation)
@@ -936,7 +943,7 @@ async def _extract_form_body(
                 results.append(await sub_value.read())
             value = serialize_sequence_value(field=field, value=results)
         if value is not None:
-            values[get_validation_alias(field)] = value
+            values[alias] = value
     field_aliases = {get_validation_alias(field) for field in body_fields}
     for key in received_body.keys():
         if key not in field_aliases:

tests/test_form_model_fields_set.py

@@ -0,0 +1,81 @@
+"""
+Tests for issue #13399: Form models should preserve correct model_fields_set.
+
+Fields not submitted in the form data must NOT appear in model_fields_set,
+matching the behaviour of JSON body endpoints.
+"""
+
+from typing import Annotated
+
+from fastapi import FastAPI, Form
+from fastapi.testclient import TestClient
+from pydantic import BaseModel
+
+app = FastAPI()
+
+
+class ExampleModel(BaseModel):
+    field_1: bool = True
+    field_2: str = "default"
+    field_3: int | None = None
+
+
+@app.post("/body")
+async def body_endpoint(model: ExampleModel):
+    return {"fields_set": sorted(model.model_fields_set)}
+
+
+@app.post("/form")
+async def form_endpoint(model: Annotated[ExampleModel, Form()]):
+    return {"fields_set": sorted(model.model_fields_set)}
+
+
+client = TestClient(app)
+
+
+def test_json_empty_body_no_fields_set():
+    """Baseline: JSON body with no fields → fields_set should be empty."""
+    resp = client.post("/body", json={})
+    assert resp.status_code == 200, resp.text
+    assert resp.json()["fields_set"] == []
+
+
+def test_form_empty_submission_no_fields_set():
+    """Form with no fields submitted → fields_set should be empty (like JSON)."""
+    resp = client.post("/form", data={})
+    assert resp.status_code == 200, resp.text
+    assert resp.json()["fields_set"] == []
+
+
+def test_form_partial_submission_only_submitted_fields_set():
+    """Only fields actually submitted appear in fields_set."""
+    resp = client.post("/form", data={"field_1": "false"})
+    assert resp.status_code == 200, resp.text
+    assert resp.json()["fields_set"] == ["field_1"]
+
+
+def test_form_all_fields_submitted():
+    """All submitted fields appear in fields_set."""
+    resp = client.post(
+        "/form", data={"field_1": "false", "field_2": "hello", "field_3": "42"}
+    )
+    assert resp.status_code == 200, resp.text
+    assert resp.json()["fields_set"] == ["field_1", "field_2", "field_3"]
+
+
+def test_form_defaults_still_applied():
+    """Default values are still applied for fields not submitted."""
+    resp = client.post("/form", data={})
+    assert resp.status_code == 200, resp.text
+    # The endpoint just returns fields_set; check defaults via a richer endpoint
+    # by confirming 200 and correct fields_set — defaults are tested in
+    # test_forms_single_model.py.
+
+
+def test_form_fields_set_matches_json_behavior():
+    """Form and JSON endpoints must agree on fields_set for equivalent inputs."""
+    json_resp = client.post("/body", json={"field_2": "hi"})
+    form_resp = client.post("/form", data={"field_2": "hi"})
+    assert json_resp.status_code == 200
+    assert form_resp.status_code == 200
+    assert json_resp.json()["fields_set"] == form_resp.json()["fields_set"]

tests/test_forms_single_model.py

@@ -99,13 +99,13 @@ def test_no_data():
                 "type": "missing",
                 "loc": ["body", "username"],
                 "msg": "Field required",
-                "input": {"tags": ["foo", "bar"], "with": "nothing"},
+                "input": {},
             },
             {
                 "type": "missing",
                 "loc": ["body", "lastname"],
                 "msg": "Field required",
-                "input": {"tags": ["foo", "bar"], "with": "nothing"},
+                "input": {},
             },
         ]
     }