diff --git a/README.md b/README.md
index 0a72900..fa26c35 100644
--- a/README.md
+++ b/README.md
@@ -12,6 +12,7 @@ cd first-commit-ai
 pip install -e ".[dev]"
 pytest
 python -m first_commit_ai --mock "hi"
+python -m first_commit_ai --mock --json "hi"
 ```
 
 You should see a `[mock]` reply. That means the wiring works before you spend a token.
diff --git a/src/first_commit_ai/cli.py b/src/first_commit_ai/cli.py
index 2c6eb15..1921371 100644
--- a/src/first_commit_ai/cli.py
+++ b/src/first_commit_ai/cli.py
@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import argparse
+import json
 import sys
 
 from first_commit_ai.client import DEFAULT_SYSTEM, ChatClient
@@ -35,6 +36,11 @@ def build_parser() -> argparse.ArgumentParser:
         metavar="TEXT",
         help=f"Override the system prompt (default: {DEFAULT_SYSTEM!r})",
     )
+    p.add_argument(
+        "--json",
+        action="store_true",
+        help="Print the reply as a JSON object (reply, mock, system)",
+    )
     return p
 
 
@@ -44,7 +50,11 @@ def main(argv: list[str] | None = None) -> int:
     if args.system is not None:
         client.system = args.system
     try:
-        print(client.chat(args.prompt))
+        reply = client.chat(args.prompt)
+        if args.json:
+            print(json.dumps({"reply": reply, "mock": bool(args.mock), "system": client.system}))
+        else:
+            print(reply)
     except Exception as exc:  # noqa: BLE001 — teach failures clearly
         print(f"error: {exc}", file=sys.stderr)
         return 1
diff --git a/tests/test_cli.py b/tests/test_cli.py
index e8be9b8..7f633e9 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -1,7 +1,20 @@
+import json
+
 from first_commit_ai.cli import build_parser, main
 from first_commit_ai.client import DEFAULT_SYSTEM, ChatClient
 
 
+def test_json_output_structure(capsys):
+    code = main(["--mock", "--json", "hi"])
+    captured = capsys.readouterr()
+    assert code == 0
+    data = json.loads(captured.out)
+    assert data["reply"].startswith("[mock]")
+    assert "hi" in data["reply"]
+    assert data["mock"] is True
+    assert data["system"] == DEFAULT_SYSTEM
+
+
 def test_mock_chat_is_deterministic():
     client = ChatClient(mock=True)
     out = client.chat("hi")