fix(context): preserve leading underscores in _snake_to_camel key conversion (#2414)

core/wren/src/wren/context.py

@@ -76,9 +76,20 @@ See https://docs.getwren.ai/oss/engine/get_started/installation for full setup.
 
 
 def _snake_to_camel(name: str) -> str:
-    """Convert snake_case to camelCase."""
-    parts = name.split("_")
-    return parts[0] + "".join(w.capitalize() for w in parts[1:])
+    """Convert snake_case to camelCase, preserving any leading underscores.
+
+    Keys like ``_instructions`` are sentinel carriers consumed downstream
+    under that exact name. The naive ``"_instructions".split("_")`` yields
+    ``["", "instructions"]`` → ``"Instructions"`` — the leading underscore is
+    lost and the first real word is capitalized, mangling the key. Strip and
+    re-attach the leading underscore run so it round-trips.
+    """
+    stripped = name.lstrip("_")
+    prefix = name[: len(name) - len(stripped)]
+    parts = stripped.split("_")
+    if not stripped:
+        return name
+    return prefix + parts[0] + "".join(w.capitalize() for w in parts[1:])
 
 
 def _convert_keys(obj: Any) -> Any:

core/wren/tests/unit/test_context.py

@@ -43,6 +43,24 @@ def test_snake_to_camel():
     assert _snake_to_camel("name") == "name"
 
 
+def test_snake_to_camel_preserves_leading_underscore():
+    # Sentinel keys like `_instructions` must keep their leading underscore
+    # and not capitalize the first real word (the naive split mangled this
+    # into "Instructions").
+    assert _snake_to_camel("_instructions") == "_instructions"
+    assert _snake_to_camel("_dbt_tests") == "_dbtTests"
+    assert _snake_to_camel("__double") == "__double"
+
+
+def test_convert_keys_does_not_mangle_nested_instructions():
+    # A nested `_instructions` carrier must survive the camelCase pass intact;
+    # the top-level pop workarounds elsewhere don't protect nested ones.
+    obj = {"models": [{"name": "m", "_instructions": "do x"}]}
+    result = _convert_keys(obj)
+    assert result["models"][0]["_instructions"] == "do x"
+    assert "Instructions" not in result["models"][0]
+
+
 def test_convert_keys_nested():
     obj = {
         "table_reference": {"catalog": "c", "schema_name": "s"},