diff --git a/.sampo/changesets/haughty-king-tuonetar.md b/.sampo/changesets/haughty-king-tuonetar.md
new file mode 100644
index 0000000..cab9688
--- /dev/null
+++ b/.sampo/changesets/haughty-king-tuonetar.md
@@ -0,0 +1,5 @@
+---
+pypi/gt-i18n: patch
+---
+
+refactor: string registration interface
diff --git a/Makefile b/Makefile
index e61f916..2a38d2b 100644
--- a/Makefile
+++ b/Makefile
@@ -10,6 +10,7 @@ lint-fix:
 	uv run ruff check --fix .
 
 format:
+	uv run ruff check --fix .
 	uv run ruff format .
 
 format-check:
diff --git a/packages/gt-i18n/src/gt_i18n/translation_functions/_extract_variables.py b/packages/gt-i18n/src/gt_i18n/translation_functions/_extract_variables.py
index 949d380..6e8ac91 100644
--- a/packages/gt-i18n/src/gt_i18n/translation_functions/_extract_variables.py
+++ b/packages/gt-i18n/src/gt_i18n/translation_functions/_extract_variables.py
@@ -1,16 +1,27 @@
-"""Filter $-prefixed GT keys from options, returning user variables."""
+"""Filter GT-reserved keys from options, returning user variables."""
 
 from __future__ import annotations
 
+_GT_RESERVED_KEYS = frozenset(
+    {
+        "_context",
+        "_id",
+        "_max_chars",
+        "__hash",
+        "__source",
+        "__fallback",
+    }
+)
+
 
 def extract_variables(options: dict[str, object]) -> dict[str, object]:
-    """Return only user interpolation variables (non-$ keys).
+    """Return only user interpolation variables.
 
     Args:
-        options: The full kwargs dict which may contain ``$context``,
-            ``$id``, ``$max_chars``, etc.
+        options: The full kwargs dict which may contain GT-reserved keys
+            like ``_context``, ``_id``, ``_max_chars``, etc.
 
     Returns:
-        A new dict with only the user-provided variable keys.
+        A new dict with GT-reserved keys removed.
     """
-    return {k: v for k, v in options.items() if not k.startswith("$")}
+    return {k: v for k, v in options.items() if k not in _GT_RESERVED_KEYS}
diff --git a/packages/gt-i18n/src/gt_i18n/translation_functions/_interpolate.py b/packages/gt-i18n/src/gt_i18n/translation_functions/_interpolate.py
index 61eb7c9..9e6dfd5 100644
--- a/packages/gt-i18n/src/gt_i18n/translation_functions/_interpolate.py
+++ b/packages/gt-i18n/src/gt_i18n/translation_functions/_interpolate.py
@@ -25,15 +25,15 @@ def interpolate_message(
 
     Mirrors JS ``interpolateMessage()`` behavior:
 
-    1. Extract user variables (filter out ``$``-prefixed keys).
+    1. Extract user variables (filter out GT-reserved keys).
     2. Extract ``_gt_`` declared variables from the source/fallback.
     3. Condense ``_gt_`` selects to simple refs (only if declared vars exist).
     4. Format with ICU MessageFormat.
-    5. Apply ``$max_chars`` cutoff if specified.
+    5. Apply ``_max_chars`` cutoff if specified.
 
     On error:
-    - If ``$_fallback`` (source) is available, recursively retry with
-      the source message (clearing ``$_fallback`` to prevent infinite loop).
+    - If ``__fallback`` (source) is available, recursively retry with
+      the source message (clearing ``__fallback`` to prevent infinite loop).
     - Otherwise, return the raw message with cutoff applied.
 
     Args:
@@ -44,8 +44,8 @@ def interpolate_message(
     Returns:
         The interpolated string.
     """
-    source = options.get("$_fallback")
-    max_chars = options.get("$max_chars")
+    source = options.get("__fallback")
+    max_chars = options.get("_max_chars")
 
     # Remove GT-related options, keep user variables
     variables = extract_variables(options)
@@ -83,7 +83,7 @@ def interpolate_message(
         if source is not None and isinstance(source, str):
             return interpolate_message(
                 source,
-                {**options, "$_fallback": None},
+                {**options, "__fallback": None},
                 locale,
             )
 
diff --git a/packages/gt-i18n/src/gt_i18n/translation_functions/_is_encoded.py b/packages/gt-i18n/src/gt_i18n/translation_functions/_is_encoded.py
index 8bc3ee2..6d27390 100644
--- a/packages/gt-i18n/src/gt_i18n/translation_functions/_is_encoded.py
+++ b/packages/gt-i18n/src/gt_i18n/translation_functions/_is_encoded.py
@@ -4,5 +4,5 @@
 
 
 def is_encoded_translation_options(decoded_options: dict[str, object]) -> bool:
-    """Return True if the dict contains both ``$_hash`` and ``$_source``."""
-    return bool(decoded_options.get("$_hash") and decoded_options.get("$_source"))
+    """Return True if the dict contains both ``__hash`` and ``__source``."""
+    return bool(decoded_options.get("__hash") and decoded_options.get("__source"))
diff --git a/packages/gt-i18n/src/gt_i18n/translation_functions/_msg.py b/packages/gt-i18n/src/gt_i18n/translation_functions/_msg.py
index 2247f94..6a8e50c 100644
--- a/packages/gt-i18n/src/gt_i18n/translation_functions/_msg.py
+++ b/packages/gt-i18n/src/gt_i18n/translation_functions/_msg.py
@@ -29,15 +29,15 @@ def msg(message: str, **kwargs: object) -> str:
     except Exception:
         return message
 
-    # Build encoded options (preserve all kwargs including $-prefixed)
-    h = kwargs.get("$_hash") or hash_message(
+    # Build encoded options (preserve all kwargs including _-prefixed)
+    h = kwargs.get("__hash") or hash_message(
         message,
-        context=kwargs.get("$context"),  # type: ignore[arg-type]
-        id=kwargs.get("$id"),  # type: ignore[arg-type]
-        max_chars=kwargs.get("$max_chars"),  # type: ignore[arg-type]
+        context=kwargs.get("_context"),  # type: ignore[arg-type]
+        id=kwargs.get("_id"),  # type: ignore[arg-type]
+        max_chars=kwargs.get("_max_chars"),  # type: ignore[arg-type]
     )
 
-    encoded_options = {**kwargs, "$_source": message, "$_hash": h}
+    encoded_options = {**kwargs, "__source": message, "__hash": h}
     options_encoding = base64.b64encode(json.dumps(encoded_options, separators=(",", ":")).encode()).decode()
 
     return f"{interpolated}:{options_encoding}"
diff --git a/packages/gt-i18n/src/gt_i18n/translation_functions/_t.py b/packages/gt-i18n/src/gt_i18n/translation_functions/_t.py
index f5f6bca..405027a 100644
--- a/packages/gt-i18n/src/gt_i18n/translation_functions/_t.py
+++ b/packages/gt-i18n/src/gt_i18n/translation_functions/_t.py
@@ -17,7 +17,7 @@ def t(message: str, **kwargs: object) -> str:
     Args:
         message: The ICU MessageFormat source string.
         **kwargs: Interpolation variables and GT options
-            (``$context``, ``$id``, ``$max_chars``).
+            (``_context``, ``_id``, ``_max_chars``).
 
     Returns:
         The translated and interpolated string.
@@ -31,13 +31,13 @@ def t(message: str, **kwargs: object) -> str:
     translations = manager.get_translations_sync(locale)
     h = hash_message(
         message,
-        context=kwargs.get("$context"),  # type: ignore[arg-type]
-        id=kwargs.get("$id"),  # type: ignore[arg-type]
-        max_chars=kwargs.get("$max_chars"),  # type: ignore[arg-type]
+        context=kwargs.get("_context"),  # type: ignore[arg-type]
+        id=kwargs.get("_id"),  # type: ignore[arg-type]
+        max_chars=kwargs.get("_max_chars"),  # type: ignore[arg-type]
     )
     translated = translations.get(h)
     if translated:
-        return interpolate_message(translated, {**kwargs, "$_fallback": message}, locale)
+        return interpolate_message(translated, {**kwargs, "__fallback": message}, locale)
 
     # No translation found — use source
     return interpolate_message(message, kwargs, locale)
diff --git a/packages/gt-i18n/tests/conftest.py b/packages/gt-i18n/tests/conftest.py
new file mode 100644
index 0000000..ea2e575
--- /dev/null
+++ b/packages/gt-i18n/tests/conftest.py
@@ -0,0 +1,6 @@
+"""Make the tests directory importable so helpers.py can be used."""
+
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).parent))
diff --git a/packages/gt-i18n/tests/helpers.py b/packages/gt-i18n/tests/helpers.py
new file mode 100644
index 0000000..de3af29
--- /dev/null
+++ b/packages/gt-i18n/tests/helpers.py
@@ -0,0 +1,37 @@
+"""Shared test helpers for converting JS-style fixture keys to Python-style."""
+
+from __future__ import annotations
+
+import base64
+import json
+from typing import Any
+
+# Mapping from JS $-prefixed keys to Python _-prefixed keys
+JS_TO_PY_KEY_MAP: dict[str, str] = {
+    "$context": "_context",
+    "$id": "_id",
+    "$max_chars": "_max_chars",
+    "$_source": "__source",
+    "$_hash": "__hash",
+}
+
+
+def convert_keys(d: dict[str, Any]) -> dict[str, Any]:
+    """Convert JS-style $-prefixed keys to Python-style _-prefixed keys."""
+    return {JS_TO_PY_KEY_MAP.get(k, k): v for k, v in d.items()}
+
+
+def convert_encoded_string(encoded: str) -> str:
+    """Re-encode a JS-style encoded message with Python-style keys.
+
+    Takes 'message:base64payload', decodes the payload, converts
+    $-prefixed keys to _-prefixed, and re-encodes.
+    """
+    idx = encoded.rfind(":")
+    if idx == -1:
+        return encoded
+    msg_part = encoded[:idx]
+    payload = json.loads(base64.b64decode(encoded[idx + 1 :]).decode())
+    converted = convert_keys(payload)
+    new_b64 = base64.b64encode(json.dumps(converted, separators=(",", ":")).encode()).decode()
+    return f"{msg_part}:{new_b64}"
diff --git a/packages/gt-i18n/tests/test_extract_variables.py b/packages/gt-i18n/tests/test_extract_variables.py
index a556cff..f874a6c 100644
--- a/packages/gt-i18n/tests/test_extract_variables.py
+++ b/packages/gt-i18n/tests/test_extract_variables.py
@@ -1,15 +1,15 @@
-"""Tests for extract_variables ($-key filtering)."""
+"""Tests for extract_variables (_-key filtering)."""
 
 from gt_i18n.translation_functions._extract_variables import extract_variables
 
 
-def test_filters_dollar_keys() -> None:
-    opts: dict[str, object] = dict({"name": "Alice", "$context": "greeting", "$id": "hello"})
+def test_filters_underscore_keys() -> None:
+    opts: dict[str, object] = dict({"name": "Alice", "_context": "greeting", "_id": "hello"})
     result = extract_variables(opts)
     assert result == {"name": "Alice"}
 
 
-def test_keeps_all_non_dollar() -> None:
+def test_keeps_all_non_underscore() -> None:
     opts = {"name": "Alice", "count": 5, "item": "apples"}
     result = extract_variables(opts)
     assert result == opts
@@ -19,6 +19,12 @@ def test_empty_dict() -> None:
     assert extract_variables({}) == {}
 
 
-def test_all_dollar_keys() -> None:
-    opts = {"$context": "x", "$id": "y", "$max_chars": 10}
+def test_all_underscore_keys() -> None:
+    opts = {"_context": "x", "_id": "y", "_max_chars": 10}
     assert extract_variables(opts) == {}
+
+
+def test_user_underscore_prefixed_keys_preserved() -> None:
+    opts: dict[str, object] = {"_name": "Alice", "_context": "greeting"}
+    result = extract_variables(opts)
+    assert result == {"_name": "Alice"}
diff --git a/packages/gt-i18n/tests/test_fallbacks.py b/packages/gt-i18n/tests/test_fallbacks.py
index 22ae72e..326c758 100644
--- a/packages/gt-i18n/tests/test_fallbacks.py
+++ b/packages/gt-i18n/tests/test_fallbacks.py
@@ -4,6 +4,7 @@
 from pathlib import Path
 
 from gt_i18n import m_fallback, t_fallback
+from helpers import convert_encoded_string, convert_keys
 
 FIXTURES = json.loads((Path(__file__).parent / "fixtures" / "js_msg_parity.json").read_text())
 
@@ -11,7 +12,8 @@
 class TestMFallback:
     def test_encoded_message(self) -> None:
         f = FIXTURES["mFallback"]["encoded"]
-        assert m_fallback(f["input"]) == f["result"]
+        converted_input = convert_encoded_string(f["input"])
+        assert m_fallback(converted_input) == f["result"]
 
     def test_plain_text(self) -> None:
         f = FIXTURES["mFallback"]["plain"]
@@ -33,7 +35,8 @@ def test_plain(self) -> None:
 
     def test_with_vars(self) -> None:
         f = FIXTURES["gtFallback"]["with_vars"]
-        assert t_fallback(f["input"][0], **f["input"][1]) == f["result"]
+        py_args = convert_keys(f["input"][1])
+        assert t_fallback(f["input"][0], **py_args) == f["result"]
 
     def test_declare_var(self) -> None:
         f = FIXTURES["gtFallback"]["declare_var"]
diff --git a/packages/gt-i18n/tests/test_interpolate.py b/packages/gt-i18n/tests/test_interpolate.py
index 3005bee..60a36a1 100644
--- a/packages/gt-i18n/tests/test_interpolate.py
+++ b/packages/gt-i18n/tests/test_interpolate.py
@@ -30,7 +30,7 @@ def test_no_variables() -> None:
 def test_max_chars_cutoff() -> None:
     result = interpolate_message(
         "This is a very long message that should be cut off",
-        {"$max_chars": 10},
+        {"_max_chars": 10},
     )
     assert len(result) <= 10
 
@@ -83,7 +83,7 @@ def test_translated_with_declare_var() -> None:
     """
     source = f"Welcome back, {declare_var('Alice', name='user_name')}!"
     translated = "Bienvenido de nuevo, {_gt_1}!"
-    result = interpolate_message(translated, {"$_fallback": source})
+    result = interpolate_message(translated, {"__fallback": source})
     assert result == "Bienvenido de nuevo, Alice!"
 
 
@@ -95,7 +95,7 @@ def test_translated_multiple_declare_vars() -> None:
         f"{declare_var('oranges', name='item')}"
     )
     translated = "{_gt_1} compró {_gt_2} de {_gt_3}"
-    result = interpolate_message(translated, {"$_fallback": source})
+    result = interpolate_message(translated, {"__fallback": source})
     assert result == "Bob compró 5 de oranges"
 
 
@@ -104,7 +104,7 @@ def test_translated_reordered_vars() -> None:
     source = f"{declare_var('Alice', name='user')} bought {declare_var('apples', name='item')}"
     # Translation reorders: item before user
     translated = "{_gt_2} fueron comprados por {_gt_1}"
-    result = interpolate_message(translated, {"$_fallback": source})
+    result = interpolate_message(translated, {"__fallback": source})
     assert result == "apples fueron comprados por Alice"
 
 
@@ -120,7 +120,7 @@ def test_fallback_retry_on_format_error() -> None:
     """
     bad_translation = "{broken, plural, }"
     source = "Hello, world!"
-    result = interpolate_message(bad_translation, {"$_fallback": source})
+    result = interpolate_message(bad_translation, {"__fallback": source})
     # Should successfully format the source, not return it raw
     assert result == "Hello, world!"
 
@@ -133,7 +133,7 @@ def test_fallback_retry_preserves_user_vars() -> None:
     """
     bad_translation = "{broken, plural, }"
     source = "Hello, {name}!"
-    result = interpolate_message(bad_translation, {"$_fallback": source, "name": "Alice"})
+    result = interpolate_message(bad_translation, {"__fallback": source, "name": "Alice"})
     assert result == "Hello, Alice!"
 
 
@@ -141,7 +141,7 @@ def test_fallback_retry_preserves_declare_vars() -> None:
     """Fallback retry should also handle declare_var in the source."""
     bad_translation = "{broken, plural, }"
     source = f"Hello, {declare_var('Alice', name='user')}!"
-    result = interpolate_message(bad_translation, {"$_fallback": source})
+    result = interpolate_message(bad_translation, {"__fallback": source})
     assert result == "Hello, Alice!"
 
 
@@ -151,7 +151,7 @@ def test_no_fallback_returns_raw_with_cutoff() -> None:
     JS behavior: final catch returns formatCutoff(encodedMsg, {maxChars}).
     """
     bad_msg = "{broken, plural, }"
-    result = interpolate_message(bad_msg, {"$max_chars": 5})
+    result = interpolate_message(bad_msg, {"_max_chars": 5})
     assert len(result) <= 5
 
 
@@ -167,7 +167,7 @@ def test_no_fallback_returns_raw_message() -> None:
 
 def test_cutoff_applied_after_interpolation() -> None:
     """$max_chars truncates after interpolation."""
-    result = interpolate_message("Hello, {name}!", {"name": "Alice", "$max_chars": 8})
+    result = interpolate_message("Hello, {name}!", {"name": "Alice", "_max_chars": 8})
     assert len(result) <= 8
 
 
@@ -175,5 +175,5 @@ def test_cutoff_on_fallback_retry() -> None:
     """$max_chars should still apply when falling back to source."""
     bad_translation = "{broken, plural, }"
     source = "This is a long fallback message"
-    result = interpolate_message(bad_translation, {"$_fallback": source, "$max_chars": 10})
+    result = interpolate_message(bad_translation, {"__fallback": source, "_max_chars": 10})
     assert len(result) <= 10
diff --git a/packages/gt-i18n/tests/test_msg.py b/packages/gt-i18n/tests/test_msg.py
index 961c42e..16b7cc7 100644
--- a/packages/gt-i18n/tests/test_msg.py
+++ b/packages/gt-i18n/tests/test_msg.py
@@ -4,6 +4,7 @@
 from pathlib import Path
 
 from gt_i18n import decode_msg, decode_options, msg
+from helpers import convert_encoded_string, convert_keys
 
 FIXTURES = json.loads((Path(__file__).parent / "fixtures" / "js_msg_parity.json").read_text())
 
@@ -15,50 +16,57 @@ def test_no_options(self) -> None:
 
     def test_with_vars(self) -> None:
         f = FIXTURES["msg"]["with_vars"]
-        result = msg(f["args"][0], **f["args"][1])
+        py_args = convert_keys(f["args"][1])
+        result = msg(f["args"][0], **py_args)
         assert decode_msg(result) == decode_msg(f["result"])
         py_opts = decode_options(result)
         assert py_opts is not None
-        js_opts = FIXTURES["decodeOptions"]["encoded"]["result"]
-        assert py_opts["$_hash"] == js_opts["$_hash"]
-        assert py_opts["$_source"] == js_opts["$_source"]
+        js_opts = convert_keys(FIXTURES["decodeOptions"]["encoded"]["result"])
+        assert py_opts["__hash"] == js_opts["__hash"]
+        assert py_opts["__source"] == js_opts["__source"]
         assert py_opts["name"] == js_opts["name"]
 
     def test_with_context(self) -> None:
         f = FIXTURES["msg"]["with_context"]
-        result = msg(f["args"][0], **f["args"][1])
+        py_args = convert_keys(f["args"][1])
+        result = msg(f["args"][0], **py_args)
         assert ":" in result
         assert decode_msg(result) == "Save"
         opts = decode_options(result)
         assert opts is not None
-        assert opts["$context"] == "button"
+        assert opts["_context"] == "button"
         f_opts = decode_options(f["result"])
         assert f_opts is not None
-        assert opts["$_hash"] == f_opts["$_hash"]
+        f_opts_py = convert_keys(f_opts)
+        assert opts["__hash"] == f_opts_py["__hash"]
 
     def test_with_id(self) -> None:
         f = FIXTURES["msg"]["with_id"]
-        result = msg(f["args"][0], **f["args"][1])
+        py_args = convert_keys(f["args"][1])
+        result = msg(f["args"][0], **py_args)
         assert ":" in result
         assert decode_msg(result) == "Hello"
         opts = decode_options(result)
         assert opts is not None
-        assert opts["$id"] == "greeting"
+        assert opts["_id"] == "greeting"
         f_opts = decode_options(f["result"])
         assert f_opts is not None
-        assert opts["$_hash"] == f_opts["$_hash"]
+        f_opts_py = convert_keys(f_opts)
+        assert opts["__hash"] == f_opts_py["__hash"]
 
     def test_with_id_and_var(self) -> None:
         f = FIXTURES["msg"]["with_id_and_var"]
-        result = msg(f["args"][0], **f["args"][1])
+        py_args = convert_keys(f["args"][1])
+        result = msg(f["args"][0], **py_args)
         assert decode_msg(result) == "Hi, Bob!"
         opts = decode_options(result)
         assert opts is not None
         assert opts["name"] == "Bob"
-        assert opts["$id"] == "greet"
+        assert opts["_id"] == "greet"
         f_opts = decode_options(f["result"])
         assert f_opts is not None
-        assert opts["$_hash"] == f_opts["$_hash"]
+        f_opts_py = convert_keys(f_opts)
+        assert opts["__hash"] == f_opts_py["__hash"]
 
 
 class TestDecodeMsg:
@@ -82,11 +90,13 @@ def test_with_colon(self) -> None:
 class TestDecodeOptions:
     def test_encoded(self) -> None:
         f = FIXTURES["decodeOptions"]["encoded"]
-        result = decode_options(f["input"])
+        converted_input = convert_encoded_string(f["input"])
+        result = decode_options(converted_input)
+        expected = convert_keys(f["result"])
         assert result is not None
-        assert result["$_hash"] == f["result"]["$_hash"]
-        assert result["$_source"] == f["result"]["$_source"]
-        assert result["name"] == f["result"]["name"]
+        assert result["__hash"] == expected["__hash"]
+        assert result["__source"] == expected["__source"]
+        assert result["name"] == expected["name"]
 
     def test_plain_returns_none(self) -> None:
         f = FIXTURES["decodeOptions"]["plain"]