feat: warn when output extension does not match --format

src/vectrify/main.py

@@ -1,6 +1,7 @@
 import logging
 import os
 import sys
+from pathlib import Path
 
 from vectrify.cli import parse_args
 from vectrify.dashboard import Dashboard
@@ -52,6 +53,19 @@ def determine_provider_and_model(args) -> tuple[str, str]:
     return provider, model
 
 
+def format_extension_warning(
+    output_path: str, fmt: str, expected_ext: str
+) -> str | None:
+    """Return a warning if the output extension doesn't match --format, else None."""
+    actual = Path(output_path).suffix.lower()
+    if actual == expected_ext:
+        return None
+    return (
+        f"Output path '{output_path}' has extension '{actual or '(none)'}' but "
+        f"--format {fmt} produces '{expected_ext}' files; writing it anyway."
+    )
+
+
 def main():
     args = parse_args()
     provider, model = determine_provider_and_model(args)
@@ -71,6 +85,10 @@ def main():
     else:
         plugin = SvgPlugin()
 
+    mismatch = format_extension_warning(args.output, args.format, plugin.file_extension)
+    if mismatch:
+        logger.warning(mismatch)
+
     stats = SearchStats(
         strategy_name=args.strategy,
         model_name=model,

src/vectrify/tests/test_main.py

@@ -2,7 +2,7 @@
 
 import pytest
 
-from vectrify.main import determine_provider_and_model
+from vectrify.main import determine_provider_and_model, format_extension_warning
 
 _KEYS = ("OPENAI_API_KEY", "ANTHROPIC_API_KEY", "GEMINI_API_KEY")
 
@@ -54,3 +54,30 @@ def test_explicit_model_is_preserved(monkeypatch):
     provider, model = determine_provider_and_model(_args(model="custom-model"))
     assert provider == "openai"
     assert model == "custom-model"
+
+
+@pytest.mark.parametrize(
+    ("output", "fmt", "ext"),
+    [
+        ("out.svg", "svg", ".svg"),
+        ("out.dot", "graphviz", ".dot"),
+        ("OUT.SVG", "svg", ".svg"),  # extension check is case-insensitive
+    ],
+)
+def test_extension_match_no_warning(output, fmt, ext):
+    assert format_extension_warning(output, fmt, ext) is None
+
+
+@pytest.mark.parametrize(
+    ("output", "fmt", "ext"),
+    [
+        ("out.svg", "graphviz", ".dot"),
+        ("out.dot", "svg", ".svg"),
+        ("out", "typst", ".typ"),  # no extension at all
+    ],
+)
+def test_extension_mismatch_warns(output, fmt, ext):
+    msg = format_extension_warning(output, fmt, ext)
+    assert msg is not None
+    assert fmt in msg
+    assert ext in msg