diff --git a/src/main.py b/src/main.py
index 05b2122..6727aa3 100644
--- a/src/main.py
+++ b/src/main.py
@@ -203,9 +203,59 @@ def build_parser() -> argparse.ArgumentParser:
     return parser
 
 
+def _emit_parse_error_envelope(argv: list[str], message: str) -> None:
+    """#178: emit JSON envelope for argparse-level errors when --output-format json is requested.
+
+    Pre-scans argv for --output-format json. If found, prints a parse-error envelope
+    to stdout (per SCHEMAS.md 'error' envelope shape) instead of letting argparse
+    dump help text to stderr. This preserves the JSON contract for claws that can't
+    parse argparse usage messages.
+    """
+    import json
+    # Extract the attempted command (argv[0] is the first positional)
+    attempted = argv[0] if argv and not argv[0].startswith('-') else '<missing>'
+    envelope = wrap_json_envelope(
+        {
+            'error': {
+                'kind': 'parse',
+                'operation': 'argparse',
+                'target': attempted,
+                'retryable': False,
+                'message': message,
+                'hint': 'run with no arguments to see available subcommands',
+            },
+        },
+        command=attempted,
+        exit_code=1,
+    )
+    print(json.dumps(envelope))
+
+
+def _wants_json_output(argv: list[str]) -> bool:
+    """#178: check if argv contains --output-format json anywhere (for parse-error routing)."""
+    for i, arg in enumerate(argv):
+        if arg == '--output-format' and i + 1 < len(argv) and argv[i + 1] == 'json':
+            return True
+        if arg == '--output-format=json':
+            return True
+    return False
+
+
 def main(argv: list[str] | None = None) -> int:
+    import sys
+    if argv is None:
+        argv = sys.argv[1:]
     parser = build_parser()
-    args = parser.parse_args(argv)
+    # #178: catch argparse errors and emit JSON envelope when --output-format json present
+    try:
+        args = parser.parse_args(argv)
+    except SystemExit as exc:
+        # argparse exits with SystemExit on error (code 2) or --help (code 0)
+        if exc.code != 0 and _wants_json_output(argv):
+            # Reconstruct a generic parse-error message
+            _emit_parse_error_envelope(argv, 'invalid command or argument (argparse rejection)')
+            return 1
+        raise
     manifest = build_port_manifest()
     if args.command == 'summary':
         print(QueryEnginePort(manifest).render_summary())
diff --git a/tests/test_parse_error_envelope.py b/tests/test_parse_error_envelope.py
new file mode 100644
index 0000000..3efe7c1
--- /dev/null
+++ b/tests/test_parse_error_envelope.py
@@ -0,0 +1,159 @@
+"""#178 — argparse-level errors emit JSON envelope when --output-format json is requested.
+
+Before #178:
+  $ claw nonexistent --output-format json
+  usage: main.py [-h] {summary,manifest,...} ...
+  main.py: error: argument command: invalid choice: 'nonexistent' (choose from ...)
+  [exit 2, argparse dumps help to stderr, no JSON envelope]
+
+After #178:
+  $ claw nonexistent --output-format json
+  {"timestamp": "...", "command": "nonexistent", "exit_code": 1, ...,
+   "error": {"kind": "parse", "operation": "argparse", ...}}
+  [exit 1, JSON envelope on stdout, matches SCHEMAS.md contract]
+
+Contract:
+- text mode: unchanged (argparse still dumps help to stderr, exit code 2)
+- JSON mode: envelope matches SCHEMAS.md 'error' shape, exit code 1
+- Parse errors use error.kind='parse' (distinct from runtime/session/etc.)
+"""
+
+from __future__ import annotations
+
+import json
+import subprocess
+import sys
+from pathlib import Path
+
+import pytest
+
+CLI = [sys.executable, '-m', 'src.main']
+REPO_ROOT = Path(__file__).resolve().parent.parent
+
+
+class TestParseErrorJsonEnvelope:
+    """Argparse errors emit JSON envelope when --output-format json is requested."""
+
+    def test_unknown_command_json_mode_emits_envelope(self) -> None:
+        """Unknown command + --output-format json → parse-error envelope."""
+        result = subprocess.run(
+            CLI + ['nonexistent-command', '--output-format', 'json'],
+            cwd=REPO_ROOT,
+            capture_output=True,
+            text=True,
+        )
+        assert result.returncode == 1, f"expected exit 1; got {result.returncode}"
+        envelope = json.loads(result.stdout)
+        # Common fields
+        assert envelope['schema_version'] == '1.0'
+        assert envelope['output_format'] == 'json'
+        assert envelope['exit_code'] == 1
+        # Error envelope shape
+        assert envelope['error']['kind'] == 'parse'
+        assert envelope['error']['operation'] == 'argparse'
+        assert envelope['error']['retryable'] is False
+        assert envelope['error']['target'] == 'nonexistent-command'
+        assert 'hint' in envelope['error']
+
+    def test_unknown_command_json_equals_syntax(self) -> None:
+        """--output-format=json syntax also works."""
+        result = subprocess.run(
+            CLI + ['nonexistent-command', '--output-format=json'],
+            cwd=REPO_ROOT,
+            capture_output=True,
+            text=True,
+        )
+        assert result.returncode == 1
+        envelope = json.loads(result.stdout)
+        assert envelope['error']['kind'] == 'parse'
+
+    def test_unknown_command_text_mode_unchanged(self) -> None:
+        """Text mode (default) preserves argparse behavior: help to stderr, exit 2."""
+        result = subprocess.run(
+            CLI + ['nonexistent-command'],
+            cwd=REPO_ROOT,
+            capture_output=True,
+            text=True,
+        )
+        assert result.returncode == 2, f"text mode must preserve argparse exit 2; got {result.returncode}"
+        # stderr should have argparse error (help + error message)
+        assert 'invalid choice' in result.stderr
+        # stdout should be empty (no JSON leaked)
+        assert result.stdout == ''
+
+    def test_invalid_flag_json_mode_emits_envelope(self) -> None:
+        """Invalid flag at top level + --output-format json → envelope."""
+        result = subprocess.run(
+            CLI + ['--invalid-top-level-flag', '--output-format', 'json'],
+            cwd=REPO_ROOT,
+            capture_output=True,
+            text=True,
+        )
+        # argparse might reject before --output-format is parsed; still emit envelope
+        assert result.returncode == 1, f"got {result.returncode}: {result.stderr}"
+        envelope = json.loads(result.stdout)
+        assert envelope['error']['kind'] == 'parse'
+
+    def test_missing_command_no_json_flag_behaves_normally(self) -> None:
+        """No --output-format flag + missing command → normal argparse behavior."""
+        result = subprocess.run(
+            CLI,
+            cwd=REPO_ROOT,
+            capture_output=True,
+            text=True,
+        )
+        # argparse exits 2 when required subcommand is missing
+        assert result.returncode == 2
+        assert 'required' in result.stderr.lower() or 'the following arguments are required' in result.stderr.lower()
+
+    def test_valid_command_unaffected(self) -> None:
+        """Valid commands still work normally (no regression)."""
+        result = subprocess.run(
+            CLI + ['list-sessions', '--output-format', 'json'],
+            cwd=REPO_ROOT,
+            capture_output=True,
+            text=True,
+        )
+        assert result.returncode == 0
+        envelope = json.loads(result.stdout)
+        assert envelope['command'] == 'list-sessions'
+        assert 'sessions' in envelope
+
+    def test_parse_error_envelope_contains_common_fields(self) -> None:
+        """Parse-error envelope must include all common fields per SCHEMAS.md."""
+        result = subprocess.run(
+            CLI + ['bogus', '--output-format', 'json'],
+            cwd=REPO_ROOT,
+            capture_output=True,
+            text=True,
+        )
+        envelope = json.loads(result.stdout)
+        # All common fields required by SCHEMAS.md
+        for field in ('timestamp', 'command', 'exit_code', 'output_format', 'schema_version'):
+            assert field in envelope, f"common field '{field}' missing from parse-error envelope"
+
+
+class TestParseErrorSchemaCompliance:
+    """Parse-error envelope matches SCHEMAS.md error shape."""
+
+    def test_error_kind_is_parse(self) -> None:
+        """error.kind='parse' distinguishes argparse errors from runtime errors."""
+        result = subprocess.run(
+            CLI + ['unknown', '--output-format', 'json'],
+            cwd=REPO_ROOT,
+            capture_output=True,
+            text=True,
+        )
+        envelope = json.loads(result.stdout)
+        assert envelope['error']['kind'] == 'parse'
+
+    def test_error_retryable_false(self) -> None:
+        """Parse errors are never retryable (typo won't magically fix itself)."""
+        result = subprocess.run(
+            CLI + ['unknown', '--output-format', 'json'],
+            cwd=REPO_ROOT,
+            capture_output=True,
+            text=True,
+        )
+        envelope = json.loads(result.stdout)
+        assert envelope['error']['retryable'] is False