diff --git a/changes/2026.bugfix.rst b/changes/2026.bugfix.rst
new file mode 100644
index 000000000..17f161589
--- /dev/null
+++ b/changes/2026.bugfix.rst
@@ -0,0 +1 @@
+Python 3.12.7 introduced an incompatibility with the handling of ``-C``, ``-d`` and other flags that accept values. This incompatibility has been corrected.
diff --git a/src/briefcase/cmdline.py b/src/briefcase/cmdline.py
index 0ed3ac829..e79e30e86 100644
--- a/src/briefcase/cmdline.py
+++ b/src/briefcase/cmdline.py
@@ -22,7 +22,12 @@
 from briefcase.console import MAX_TEXT_WIDTH, Console
 from briefcase.platforms import get_output_formats, get_platforms
 
-from .exceptions import InvalidFormatError, NoCommandError, UnsupportedCommandError
+from .exceptions import (
+    InvalidFormatError,
+    InvalidPlatformError,
+    NoCommandError,
+    UnsupportedCommandError,
+)
 
 COMMANDS = [
     NewCommand,
@@ -103,12 +108,6 @@ def parse_cmdline(args, console: Console | None = None):
         help=argparse.SUPPRESS,
     )
 
-    # To make the UX a little forgiving, we normalize *any* case to the case
-    # actually used to register the platform. This function maps the lower-case
-    # version of the registered name to the actual registered name.
-    def normalize(name):
-        return {n.lower(): n for n in platforms.keys()}.get(name.lower(), name)
-
     # argparse handles `--` specially, so make the passthrough args bypass the parser.
     def parse_known_args(args):
         args, passthrough = split_passthrough(args)
@@ -136,48 +135,42 @@ def parse_known_args(args):
         Command = DevCommand
     elif options.command == "upgrade":
         Command = UpgradeCommand
-
-    # Commands dependent on the platform and format
     else:
-        parser.add_argument(
-            "platform",
-            choices=list(platforms.keys()),
-            default={
+        # Commands dependent on the platform and format. The general form of such a
+        # command is `briefcase <cmd> <platform> <format>`; but the format will be
+        # inferred from the platform if one isn't specified, and the platform will be
+        # inferred from the operating system if it isn't explicitly given.
+        #
+        # <platform> and <format> aren't parsed as regular arguments due to ambiguities
+        # in interpreting those arguments; instead, they're handled directly from the
+        # argument list, with the expectation that they *must* be the first and second
+        # arguments (after the command) if provided. There's no other bare arguments, so
+        # we only need to look for whether the arguments start with "-".
+        if extra and not extra[0].startswith("-"):
+            name = extra.pop(0)
+            # Normalize the platform name to the registered capitalization
+            platform = {n.lower(): n for n in platforms.keys()}.get(name.lower(), name)
+        else:
+            platform = {
                 "darwin": "macOS",
                 "linux": "linux",
                 "win32": "windows",
-            }[sys.platform],
-            metavar="platform",
-            nargs="?",
-            type=normalize,
-            help="The platform to target (one of %(choices)s; default: %(default)s",
-        )
-
-        # <format> is also optional, with the default being platform dependent.
-        # There's no way to encode option-dependent choices, so allow *any*
-        # input, and we'll manually validate.
-        parser.add_argument(
-            "output_format",
-            metavar="format",
-            nargs="?",
-            help="The output format to use (the available output formats are platform dependent)",
-        )
-
-        # Re-parse the arguments, now that we know it is a command that makes use
-        # of platform/output_format.
-        options, extra = parse_known_args(args)
+            }[sys.platform]
 
         # Import the platform module
-        platform_module = platforms[options.platform]
+        try:
+            platform_module = platforms[platform]
+        except KeyError:
+            raise InvalidPlatformError(platform, platforms.keys())
 
         # If the output format wasn't explicitly specified, check to see
         # Otherwise, extract and use the default output_format for the platform.
-        if options.output_format is None:
-            output_format = platform_module.DEFAULT_OUTPUT_FORMAT
+        if extra and not extra[0].startswith("-") and not extra[0] == "--":
+            output_format = extra.pop(0)
         else:
-            output_format = options.output_format
+            output_format = platform_module.DEFAULT_OUTPUT_FORMAT
 
-        output_formats = get_output_formats(options.platform)
+        output_formats = get_output_formats(platform)
 
         # Normalise casing of output_format to be more forgiving.
         output_format = {n.lower(): n for n in output_formats}.get(
@@ -196,7 +189,7 @@ def parse_known_args(args):
             )
         except AttributeError:
             raise UnsupportedCommandError(
-                platform=options.platform,
+                platform=platform,
                 output_format=output_format,
                 command=options.command,
             )
diff --git a/src/briefcase/exceptions.py b/src/briefcase/exceptions.py
index 3197d5092..4f7723ca7 100644
--- a/src/briefcase/exceptions.py
+++ b/src/briefcase/exceptions.py
@@ -26,6 +26,17 @@ def __str__(self):
         return self.msg
 
 
+class InvalidPlatformError(BriefcaseError):
+    def __init__(self, requested, choices):
+        super().__init__(error_code=-20, skip_logfile=True)
+        self.requested = requested
+        self.choices = choices
+
+    def __str__(self):
+        choices = ", ".join(sorted(self.choices, key=str.lower))
+        return f"Invalid platform {self.requested!r}; (choose from: {choices})"
+
+
 class InvalidFormatError(BriefcaseError):
     def __init__(self, requested, choices):
         super().__init__(error_code=-21, skip_logfile=True)
diff --git a/tests/test_cmdline.py b/tests/test_cmdline.py
index 3f6dcc349..616b305cb 100644
--- a/tests/test_cmdline.py
+++ b/tests/test_cmdline.py
@@ -9,6 +9,7 @@
 from briefcase.console import Console, Log, LogLevel
 from briefcase.exceptions import (
     InvalidFormatError,
+    InvalidPlatformError,
     NoCommandError,
     UnsupportedCommandError,
 )
@@ -456,15 +457,10 @@ def test_command_unknown_platform(monkeypatch, logger, console):
     # Pretend we're on macOS, regardless of where the tests run.
     monkeypatch.setattr(sys, "platform", "darwin")
 
-    with pytest.raises(SystemExit) as excinfo:
+    expected_exc_regex = r"Invalid platform 'foobar'; \(choose from: .*\)"
+    with pytest.raises(InvalidPlatformError, match=expected_exc_regex):
         do_cmdline_parse("create foobar".split(), logger, console)
 
-    assert excinfo.value.code == 2
-    assert excinfo.value.__context__.argument_name == "platform"
-    assert excinfo.value.__context__.message.startswith(
-        "invalid choice: 'foobar' (choose from"
-    )
-
 
 def test_command_explicit_platform(monkeypatch, logger, console):
     """``briefcase create linux`` returns linux create app command."""