Cover input error messages

tests/data/malformed/bad.graphql

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+type Query {
+  user:
+}

tests/data/malformed/bad_openapi.yaml

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+openapi: 3.0.0
+info:
+  title: Bad
+paths: [

tests/data/malformed/dangling_local_ref.json

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+{"$ref": "#/definitions/Missing"}

tests/data/malformed/enum_as_dict.json

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+{
+  "type": "string",
+  "enum": {"one": 1}
+}

tests/data/malformed/missing_external_ref.json

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+{"$ref": "missing.json"}

tests/data/malformed/non_dict_root.yaml

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+- not
+- a
+- mapping

tests/data/malformed/pointer_through_scalar.json

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+{
+  "definitions": {
+    "Name": "scalar"
+  },
+  "$ref": "#/definitions/Name/properties/value"
+}

tests/data/malformed/pointer_through_scalar_openapi.yaml

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+openapi: 3.0.0
+info:
+  title: Bad
+  version: '1.0'
+paths: {}
+components:
+  schemas:
+    Name: scalar
+    UseName:
+      $ref: '#/components/schemas/Name/properties/value'

tests/data/malformed/required_as_string.json

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+{
+  "type": "object",
+  "required": "name",
+  "properties": {
+    "name": {"type": "string"}
+  }
+}

tests/data/malformed/truncated_jsonschema.json

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+{"type": "object", "properties": {

tests/data/malformed/wrong_type_properties.json

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+{
+  "type": "object",
+  "properties": []
+}

tests/main/test_error_messages.py

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+"""Regression tests for malformed CLI input diagnostics."""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+import pytest
+
+from datamodel_code_generator.__main__ import Exit
+from tests.main.conftest import DATA_PATH, InputFileTypeLiteral, run_main_and_assert
+
+if TYPE_CHECKING:
+    from pathlib import Path
+
+
+MALFORMED_DATA_PATH = DATA_PATH / "malformed"
+TRACEBACK_HEADER = "Traceback (most recent call last)"
+
+ERROR_CASES: tuple[tuple[str, InputFileTypeLiteral, str], ...] = (
+    ("truncated_jsonschema.json", "jsonschema", TRACEBACK_HEADER),
+    ("bad_openapi.yaml", "openapi", TRACEBACK_HEADER),
+    ("bad.graphql", "graphql", TRACEBACK_HEADER),
+    ("non_dict_root.yaml", "openapi", TRACEBACK_HEADER),
+    ("pointer_through_scalar_openapi.yaml", "openapi", TRACEBACK_HEADER),
+    (
+        "pointer_through_scalar.json",
+        "jsonschema",
+        "Error at schema path 'pointer_through_scalar.json/#/definitions/Name': ValidationError",
+    ),
+    ("wrong_type_properties.json", "jsonschema", "Error at schema path 'wrong_type_properties.json'"),
+    ("required_as_string.json", "jsonschema", "Error at schema path 'required_as_string.json'"),
+    ("enum_as_dict.json", "jsonschema", "Error at schema path 'enum_as_dict.json'"),
+    ("missing_external_ref.json", "jsonschema", "$ref file not found:"),
+    ("empty_jsonschema.json", "jsonschema", "Models not found in the input data"),
+)
+
+MISSING_INPUT_CASES: tuple[tuple[InputFileTypeLiteral | None, str], ...] = (
+    (None, "File not found"),
+    # Explicit input types currently bypass auto-detection's missing-file check.
+    # Keep this pinned until the diagnostic behavior is intentionally changed.
+    ("jsonschema", TRACEBACK_HEADER),
+)
+
+TOLERATED_CASES: tuple[tuple[str, InputFileTypeLiteral], ...] = (("dangling_local_ref.json", "jsonschema"),)
+
+
+@pytest.mark.parametrize(
+    ("fixture_name", "input_file_type", "expected_stderr_contains"),
+    ERROR_CASES,
+    ids=[fixture_name for fixture_name, _, _ in ERROR_CASES],
+)
+def test_malformed_input_error_messages(
+    fixture_name: str,
+    input_file_type: InputFileTypeLiteral,
+    expected_stderr_contains: str,
+    output_file: Path,
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    """Malformed inputs keep their current stderr signal and non-zero exit code."""
+    run_main_and_assert(
+        input_path=MALFORMED_DATA_PATH / fixture_name,
+        output_path=output_file,
+        input_file_type=input_file_type,
+        expected_exit=Exit.ERROR,
+        capsys=capsys,
+        expected_stderr_contains=expected_stderr_contains,
+        output_should_not_exist=True,
+    )
+
+
+@pytest.mark.parametrize(
+    ("input_file_type", "expected_stderr_contains"),
+    MISSING_INPUT_CASES,
+    ids=[input_file_type or "auto" for input_file_type, _ in MISSING_INPUT_CASES],
+)
+def test_missing_input_error_messages(
+    input_file_type: InputFileTypeLiteral | None,
+    expected_stderr_contains: str,
+    output_file: Path,
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    """Missing local input paths keep their current CLI diagnostics."""
+    run_main_and_assert(
+        input_path=MALFORMED_DATA_PATH / "missing.json",
+        output_path=output_file,
+        input_file_type=input_file_type,
+        expected_exit=Exit.ERROR,
+        capsys=capsys,
+        expected_stderr_contains=expected_stderr_contains,
+        output_should_not_exist=True,
+    )
+
+
+@pytest.mark.parametrize(
+    ("fixture_name", "input_file_type"),
+    TOLERATED_CASES,
+    ids=[fixture_name for fixture_name, _ in TOLERATED_CASES],
+)
+def test_malformed_input_tolerated_behavior(
+    fixture_name: str,
+    input_file_type: InputFileTypeLiteral,
+    output_file: Path,
+) -> None:
+    """Document malformed inputs that are intentionally tolerated until stricter diagnostics exist."""
+    run_main_and_assert(
+        input_path=MALFORMED_DATA_PATH / fixture_name,
+        output_path=output_file,
+        input_file_type=input_file_type,
+        expected_exit=Exit.OK,
+    )