diff --git a/headroom/transforms/content_router.py b/headroom/transforms/content_router.py
index 4a4bbc8d0..a6c0ab411 100644
--- a/headroom/transforms/content_router.py
+++ b/headroom/transforms/content_router.py
@@ -1449,6 +1449,7 @@ class ContentRouter(Transform):
         Returns:
             RouterCompressionResult with compressed content and routing metadata.
         """
+        context = context or ""
         debug_enabled = logger.isEnabledFor(logging.DEBUG)
         request_debug = (
             {
diff --git a/headroom/transforms/diff_compressor.py b/headroom/transforms/diff_compressor.py
index 95885f651..4d1e2984e 100644
--- a/headroom/transforms/diff_compressor.py
+++ b/headroom/transforms/diff_compressor.py
@@ -103,7 +103,7 @@ class DiffCompressor:
         )
 
     def compress(self, content: str, context: str = "") -> DiffCompressionResult:
-        r = self._rust.compress(content, context)
+        r = self._rust.compress(content, context or "")
         cache_key: str | None = r.cache_key
         if cache_key is not None:
             # Mirror log_compressor.py + search_compressor.py: when the
diff --git a/tests/test_transforms/test_content_router.py b/tests/test_transforms/test_content_router.py
index 7d53e584a..16c3e7f39 100644
--- a/tests/test_transforms/test_content_router.py
+++ b/tests/test_transforms/test_content_router.py
@@ -9,6 +9,7 @@ Comprehensive tests covering:
 """
 
 import json
+import logging
 from types import SimpleNamespace
 
 import pytest
@@ -472,6 +473,23 @@ class TestContentRouter:
         assert result.original == content
         assert result.strategy_used is not None
 
+    def test_compress_diff_accepts_none_context_with_debug(self, router, caplog):
+        """None context is normalized before debug logging and compressor dispatch."""
+
+        class FakeDiffCompressor:
+            def compress(self, content, context):
+                assert context == ""
+                return SimpleNamespace(compressed="diff summary")
+
+        diff = "diff --git a/file.py b/file.py\n@@ -1 +1 @@\n-old\n+new\n"
+        router._diff_compressor = FakeDiffCompressor()
+
+        caplog.set_level(logging.DEBUG, logger="headroom.transforms.content_router")
+        result = router.compress(diff, context=None)
+
+        assert result.compressed == "diff summary"
+        assert result.strategy_used == CompressionStrategy.DIFF
+
     def test_name_property(self, router):
         """Router has correct name."""
         assert router.name == "content_router"
diff --git a/tests/test_transforms/test_diff_compressor.py b/tests/test_transforms/test_diff_compressor.py
index 522434767..8490d1536 100644
--- a/tests/test_transforms/test_diff_compressor.py
+++ b/tests/test_transforms/test_diff_compressor.py
@@ -24,6 +24,30 @@ from headroom.transforms.diff_compressor import (
 )
 
 
+def _fake_diff_result(compressed: str = "compressed") -> DiffCompressionResult:
+    return DiffCompressionResult(
+        compressed=compressed,
+        original_line_count=1,
+        compressed_line_count=1,
+        files_affected=1,
+        additions=0,
+        deletions=0,
+        hunks_kept=1,
+        hunks_removed=0,
+    )
+
+
+class _FakeRustDiffCompressor:
+    def __init__(self) -> None:
+        self.contexts: list[str] = []
+
+    def compress(self, content: str, context: str):
+        if context is None:
+            raise AssertionError("Rust diff compressor received None context")
+        self.contexts.append(context)
+        return _fake_diff_result(content)
+
+
 class TestContextReduction:
     """Tests for context line reduction."""
 
@@ -374,6 +398,32 @@ class TestEdgeCases:
         assert result.compressed is not None
 
 
+class TestContextNormalization:
+    """Tests for the Python-to-Rust diff compressor boundary."""
+
+    def test_none_and_omitted_context_become_empty_string(self) -> None:
+        compressor = object.__new__(DiffCompressor)
+        fake_rust = _FakeRustDiffCompressor()
+        compressor._rust = fake_rust
+
+        diff = "diff --git a/file.py b/file.py\n--- a/file.py\n+++ b/file.py\n"
+
+        compressor.compress(diff, context=None)
+        compressor.compress(diff)
+
+        assert fake_rust.contexts == ["", ""]
+
+    def test_non_empty_context_passes_through_unchanged(self) -> None:
+        compressor = object.__new__(DiffCompressor)
+        fake_rust = _FakeRustDiffCompressor()
+        compressor._rust = fake_rust
+
+        diff = "diff --git a/file.py b/file.py\n--- a/file.py\n+++ b/file.py\n"
+        compressor.compress(diff, context="question context")
+
+        assert fake_rust.contexts == ["question context"]
+
+
 class TestConfigOptions:
     """Tests for configuration options."""