diff --git a/python/selfie-lib/selfie_lib/CommentTracker.py b/python/selfie-lib/selfie_lib/CommentTracker.py
index e8ee1bf0..8e23e0c5 100644
--- a/python/selfie-lib/selfie_lib/CommentTracker.py
+++ b/python/selfie-lib/selfie_lib/CommentTracker.py
@@ -1,19 +1,11 @@
-from typing import Dict, Iterable, Tuple
+from typing import Dict, Iterable, Tuple, Sequence, TypeVar, Callable
+from abc import ABC, abstractmethod
 from enum import Enum, auto
 import threading
-from selfie_lib.TypedPath import TypedPath
-from selfie_lib.Slice import Slice
-
-
-# Placeholder implementations for CallStack, SnapshotFileLayout, and FS
-class CallStack:
-    pass
 
-
-class SnapshotFileLayout:
-    def sourcePathForCall(self, location) -> "TypedPath":
-        # Placeholder return or raise NotImplementedError
-        raise NotImplementedError("sourcePathForCall is not implemented")
+from selfie_lib.Slice import Slice
+from selfie_lib.TypedPath import TypedPath
+from selfie_lib.WriteTracker import CallStack, SnapshotFileLayout
 
 
 class WritableComment(Enum):
diff --git a/python/selfie-lib/selfie_lib/WriteTracker.py b/python/selfie-lib/selfie_lib/WriteTracker.py
index 37e2cd9b..579a2afd 100644
--- a/python/selfie-lib/selfie_lib/WriteTracker.py
+++ b/python/selfie-lib/selfie_lib/WriteTracker.py
@@ -1,13 +1,49 @@
-from typing import List, Optional, Generic, TypeVar, Dict
-from selfie_lib.CommentTracker import SnapshotFileLayout
+from typing import List, Optional, Generic, TypeVar, Dict, cast, Callable, Sequence
 from abc import ABC, abstractmethod
 import inspect, threading
 from functools import total_ordering
 
+from selfie_lib.SourceFile import SourceFile
+from selfie_lib.Literals import LiteralValue
+from selfie_lib.TypedPath import TypedPath
+
+
 T = TypeVar("T")
 U = TypeVar("U")
 
 
+class FS(ABC):
+    @abstractmethod
+    def file_walk(self, typed_path, walk: Callable[[Sequence["TypedPath"]], T]) -> T:
+        pass
+
+    def file_read(self, typed_path) -> str:
+        return self.file_read_binary(typed_path).decode()
+
+    def file_write(self, typed_path, content: str):
+        self.file_write_binary(typed_path, content.encode())
+
+    @abstractmethod
+    def file_read_binary(self, typed_path) -> bytes:
+        pass
+
+    @abstractmethod
+    def file_write_binary(self, typed_path, content: bytes):
+        pass
+
+    @abstractmethod
+    def assert_failed(self, message: str, expected=None, actual=None) -> Exception:
+        pass
+
+
+class SnapshotFileLayout:
+    def __init__(self, fs: FS):
+        self.fs = fs
+
+    def sourcePathForCall(self, location) -> "TypedPath":
+        raise NotImplementedError("sourcePathForCall is not implemented")
+
+
 @total_ordering
 class CallLocation:
     def __init__(self, file_name: Optional[str], line: int):
@@ -132,3 +168,34 @@ def recordInternal(
 class DiskWriteTracker(WriteTracker[T, U]):
     def record(self, key: T, snapshot: U, call: CallStack, layout: SnapshotFileLayout):
         super().recordInternal(key, snapshot, call, layout)
+
+
+class InlineWriteTracker(WriteTracker[CallLocation, LiteralValue]):
+    def record(
+        self,
+        key: CallLocation,
+        snapshot: LiteralValue,
+        call: CallStack,
+        layout: SnapshotFileLayout,
+    ):
+        super().recordInternal(key, snapshot, call, layout)
+
+        file = layout.sourcePathForCall(key)
+        if snapshot.expected is not None:
+            content = SourceFile(file.name, layout.fs.file_read(file))
+            try:
+                snapshot = cast(LiteralValue, snapshot)
+                parsed_value = content.parse_to_be_like(key.line).parse_literal(
+                    snapshot.format
+                )
+            except Exception as e:
+                raise AssertionError(
+                    f"Error while parsing the literal at {key.ide_link(layout)}. Please report this error at https://github.com/diffplug/selfie",
+                    e,
+                )
+            if parsed_value != snapshot.expected:
+                raise layout.fs.assert_failed(
+                    f"Selfie cannot modify the literal at {key.ide_link(layout)} because Selfie has a parsing bug. Please report this error at https://github.com/diffplug/selfie",
+                    snapshot.expected,
+                    parsed_value,
+                )