diff --git a/headroom/proxy/semantic_cache_key.py b/headroom/proxy/semantic_cache_key.py
index cbef6dcd6..267eddb36 100644
--- a/headroom/proxy/semantic_cache_key.py
+++ b/headroom/proxy/semantic_cache_key.py
@@ -21,11 +21,20 @@ def compute_semantic_cache_key(
     model: str,
     **key_fields: Any,
 ) -> str:
-    """Compute the proxy semantic-cache key from generation-shaping inputs."""
+    """Compute the proxy semantic-cache key from generation-shaping inputs.
+
+    ``cache_control`` is stripped from ``messages`` as well as the shaping
+    fields: it is a prompt-caching directive for the upstream provider that
+    never changes the generated completion, so a moved breakpoint must not
+    fragment the key. Messages are the primary key component and, on the
+    Anthropic path, the most common place a client (e.g. Claude Code) moves a
+    breakpoint between turns, so leaving them un-stripped defeated the strip for
+    the field that matters most.
+    """
     normalized = json.dumps(
         {
             "model": model,
-            "messages": messages,
+            "messages": strip_cache_control(messages),
             **{k: strip_cache_control(v) for k, v in key_fields.items()},
         },
         sort_keys=True,
diff --git a/headroom/proxy/semantic_cache_key_policy.py b/headroom/proxy/semantic_cache_key_policy.py
index 1fef49a7c..3b7d3625d 100644
--- a/headroom/proxy/semantic_cache_key_policy.py
+++ b/headroom/proxy/semantic_cache_key_policy.py
@@ -21,11 +21,20 @@ def compute_semantic_cache_key(
     model: str,
     **key_fields: Any,
 ) -> str:
-    """Compute a stable cache key from request content and shaping fields."""
+    """Compute a stable cache key from request content and shaping fields.
+
+    ``cache_control`` is stripped from ``messages`` as well as the shaping
+    fields: it is a prompt-caching directive for the upstream provider that
+    never changes the generated completion, so a moved breakpoint must not
+    fragment the key. Messages are the primary key component and, on the
+    Anthropic path, the most common place a client (e.g. Claude Code) moves a
+    breakpoint between turns, so leaving them un-stripped defeated the strip for
+    the field that matters most.
+    """
     normalized = json.dumps(
         {
             "model": model,
-            "messages": messages,
+            "messages": strip_cache_control(messages),
             **{k: strip_cache_control(v) for k, v in key_fields.items()},
         },
         sort_keys=True,
diff --git a/tests/test_proxy_semantic_cache_key.py b/tests/test_proxy_semantic_cache_key.py
index e6601cd0d..94db0600b 100644
--- a/tests/test_proxy_semantic_cache_key.py
+++ b/tests/test_proxy_semantic_cache_key.py
@@ -106,6 +106,36 @@ def test_tools_cache_control_ignored():
     assert _key(cache, tools=tools_cc) == _key(cache, tools=tools_plain)
 
 
+def test_message_cache_control_breakpoint_move_same_key():
+    """A moved cache_control breakpoint on a *message* must not fragment the key.
+
+    Messages are the primary key component and, on the Anthropic path, the most
+    common place a client (e.g. Claude Code) moves a breakpoint between turns.
+    cache_control is a prompt-caching directive that never changes the
+    completion, so the two requests must share a cache entry — previously only
+    system/tools breakpoints were stripped, so a message breakpoint missed.
+    """
+    cache = SemanticCache()
+    messages_with_cc = [
+        {
+            "role": "user",
+            "content": [{"type": "text", "text": "hello", "cache_control": {"type": "ephemeral"}}],
+        }
+    ]
+    messages_without_cc = [{"role": "user", "content": [{"type": "text", "text": "hello"}]}]
+    assert cache._compute_key(messages_with_cc, MODEL) == cache._compute_key(
+        messages_without_cc, MODEL
+    )
+
+
+def test_message_content_change_still_distinct_key():
+    """Stripping cache_control must not collapse genuinely different messages."""
+    cache = SemanticCache()
+    a = [{"role": "user", "content": [{"type": "text", "text": "hello"}]}]
+    b = [{"role": "user", "content": [{"type": "text", "text": "goodbye"}]}]
+    assert cache._compute_key(a, MODEL) != cache._compute_key(b, MODEL)
+
+
 # --- behavioral get/set: collision prevented end to end -----------------------
 
 
diff --git a/tests/test_proxy_semantic_cache_key_policy.py b/tests/test_proxy_semantic_cache_key_policy.py
index e106779bc..04239d0ac 100644
--- a/tests/test_proxy_semantic_cache_key_policy.py
+++ b/tests/test_proxy_semantic_cache_key_policy.py
@@ -29,6 +29,20 @@ def test_semantic_cache_key_ignores_moved_cache_control() -> None:
     )
 
 
+def test_semantic_cache_key_ignores_moved_message_cache_control() -> None:
+    """cache_control on a message must not fragment the key either (#327 intent)."""
+    messages_with_cc = [
+        {
+            "role": "user",
+            "content": [{"type": "text", "text": "hello", "cache_control": {"type": "ephemeral"}}],
+        }
+    ]
+    messages_without_cc = [{"role": "user", "content": [{"type": "text", "text": "hello"}]}]
+    assert compute_semantic_cache_key(messages_with_cc, MODEL) == (
+        compute_semantic_cache_key(messages_without_cc, MODEL)
+    )
+
+
 def test_strip_cache_control_recurses_through_dicts_and_lists() -> None:
     assert strip_cache_control(
         {