Fix pre-parse JSON flag detection (#212)

CHANGELOG.md

@@ -30,6 +30,11 @@ and versions are tracked in the repo-root `VERSION` file.
   values, and header-style `key: value` arguments before they reach logs or
   persisted history.
 
+### Fixed
+
+- Honor combined positive/negative JSON option declarations and explicit
+  `--no-json` values when deciding whether pre-parse errors use JSON output.
+
 ### Added
 
 - Add a framework choice guide, five-minute evaluation path, and clearer

lib/python/base_cli/app.py

@@ -2901,18 +2901,26 @@ def _json_requested(args: list[str], lifecycle_options: LifecycleOptions) -> boo
     option = lifecycle_options.json
     if option is None:
         return False
-    if option.default is True:
-        return True
+
+    positive_declarations, negative_declarations = _lifecycle_flag_declarations(option)
+    explicit_value: bool | None = None
+    for argument in args:
+        if any(
+            argument == declaration or argument.startswith(f"{declaration}=") for declaration in positive_declarations
+        ):
+            explicit_value = True
+        elif any(
+            argument == declaration or argument.startswith(f"{declaration}=") for declaration in negative_declarations
+        ):
+            explicit_value = False
+    if explicit_value is not None:
+        return explicit_value
+
     if option.envvar is not None:
         envvars = (option.envvar,) if isinstance(option.envvar, str) else option.envvar
         if any(os.environ.get(name, "").lower() in {"1", "true", "yes", "on"} for name in envvars):
             return True
-    declarations = tuple(declaration for declaration in option.param_decls if declaration.startswith(("-", "/")))
-    return any(
-        argument == declaration or argument.startswith(f"{declaration}=")
-        for argument in args
-        for declaration in declarations
-    )
+    return option.default is True
 
 
 def _captured_stdout(output_capture: io.StringIO | None) -> str:

tests/test_json_contracts.py

@@ -215,6 +215,48 @@ def main(ctx: base_cli.Context, x: bool) -> None:
             "hello from combined flags\n",
         )
 
+    def test_json_preparse_errors_honor_combined_positive_declaration(self) -> None:
+        app = base_cli.App(
+            name="json-combined-declaration",
+            log_to_file=False,
+            lifecycle_options=base_cli.LifecycleOptions(
+                json=base_cli.LifecycleOption("--json/--no-json"),
+            ),
+        )
+
+        @app.command()
+        def main(ctx: base_cli.Context) -> None:
+            del ctx
+
+        with tempfile.TemporaryDirectory() as home:
+            result = base_cli.testing.invoke(app, ["--json", "--unknown"], home=Path(home))
+
+        self.assertEqual(result.exit_code, 2)
+        envelope = json.loads(result.stdout)
+        self.assertEqual(envelope["schema"], "base-cli.error")
+        self.assertEqual(result.stderr, "")
+        self.assertIn("No such option", envelope["message"])
+
+    def test_json_preparse_errors_honor_explicit_negative_declaration_over_true_default(self) -> None:
+        app = base_cli.App(
+            name="json-negative-declaration",
+            log_to_file=False,
+            lifecycle_options=base_cli.LifecycleOptions(
+                json=base_cli.LifecycleOption("--json/--no-json", default=True),
+            ),
+        )
+
+        @app.command()
+        def main(ctx: base_cli.Context) -> None:
+            del ctx
+
+        with tempfile.TemporaryDirectory() as home:
+            result = base_cli.testing.invoke(app, ["--no-json", "--unknown"], home=Path(home))
+
+        self.assertEqual(result.exit_code, 2)
+        self.assertEqual(result.stdout, "")
+        self.assertIn("No such option", result.stderr or result.output)
+
     def test_json_mode_captures_default_map_values(self) -> None:
         app = base_cli.App(
             name="json-default-map",