diff --git a/headroom/proxy/handlers/anthropic.py b/headroom/proxy/handlers/anthropic.py
index ede5e72d7..7e01915e5 100644
--- a/headroom/proxy/handlers/anthropic.py
+++ b/headroom/proxy/handlers/anthropic.py
@@ -1873,27 +1873,31 @@ class AnthropicHandlerMixin:
                 # dropping the gate cannot start injecting into non-CCR
                 # conversations.
                 if configured_inject_tool:
-                    from headroom.proxy.helpers import (
-                        apply_session_sticky_ccr_tool,
-                        has_new_ccr_markers,
-                    )
+                    from headroom.proxy.helpers import apply_session_sticky_ccr_tool
 
-                    # #1850: markers replayed from the previously-forwarded
-                    # prefix (overlay_cached_prefix) are historical; only
-                    # markers NEW this turn may drive a first-time injection,
-                    # else a replayed marker injects the tool into a session
-                    # that never actually compressed.
-                    has_new_compressed_content = has_new_ccr_markers(
-                        current_detected_hashes=injector.detected_hashes,
-                        previous_forwarded_messages=prefix_tracker.get_last_forwarded_messages(),
-                        provider="anthropic",
-                    )
+                    # Inject whenever the request carries ANY CCR marker, new or
+                    # replayed from the frozen prefix. #1850 narrowed the
+                    # first-time gate to markers created THIS turn to avoid
+                    # arming a session that never compressed, but a replayed
+                    # marker is exactly as unredeemable as a fresh one: the agent
+                    # redeems hashes it was handed turns ago (project instructions
+                    # can even tell it to), and if `headroom_retrieve` is absent
+                    # Anthropic rejects the whole request with 400 "Tool reference
+                    # 'headroom_retrieve' not found in available tools" (#2766). A
+                    # present marker means the session HAS compressed, so this
+                    # cannot start injecting into non-CCR conversations. It is also
+                    # the cache-stable choice: toggling the tool in and out of the
+                    # tools array between turns is what busts the tools cache
+                    # segment, whereas injecting consistently whenever markers
+                    # exist keeps it stable. The `SessionCcrTracker` is
+                    # per-process, so a proxy restart mid-conversation makes live
+                    # sessions look fresh again, which is what re-armed the 400.
                     tools, ccr_tool_injected = apply_session_sticky_ccr_tool(
                         provider="anthropic",
                         session_id=session_id,
                         request_id=request_id,
                         existing_tools=tools,
-                        has_compressed_content_this_turn=has_new_compressed_content,
+                        has_compressed_content_this_turn=injector.has_compressed_content,
                     )
                     if ccr_tool_injected:
                         logger.debug(
diff --git a/tests/test_proxy/test_anthropic_ccr_deferred_injection.py b/tests/test_proxy/test_anthropic_ccr_deferred_injection.py
index edccdbee2..46a334113 100644
--- a/tests/test_proxy/test_anthropic_ccr_deferred_injection.py
+++ b/tests/test_proxy/test_anthropic_ccr_deferred_injection.py
@@ -592,16 +592,17 @@ def test_cache_mode_compresses_delta_but_replays_cached_prefix_when_markers_are_
         assert response.status_code == 200
         assert len(captured.get("compression_calls", [])) == 1
         forwarded = captured["body"]
-        # Tool injection is deferred (no CCR tool this turn), but the frozen
-        # prefix was cached COMPRESSED last turn. Replay it byte-identical so the
-        # prompt cache still hits instead of busting on original bytes (#1850);
-        # the historical marker does not force tool injection back on. Tool absent
-        # AND cache intact.
+        # The frozen prefix was cached COMPRESSED last turn, so it is replayed
+        # byte-identical to keep the prompt cache warm. The replayed marker is
+        # still redeemable this turn, so `headroom_retrieve` MUST be present or
+        # Anthropic 400s "Tool reference 'headroom_retrieve' not found" (#2766);
+        # injecting it whenever a marker exists is itself cache-stable (toggling
+        # is what busts the tools segment). Message prefix replayed AND tool present.
         assert forwarded["messages"] == previous_forwarded_messages
-        assert "tools" not in forwarded
+        assert [tool["name"] for tool in forwarded["tools"]] == ["headroom_retrieve"]
 
 
-def test_cache_mode_exact_prefix_replay_forwards_cached_compressed_prefix_when_tool_injection_is_deferred(
+def test_cache_mode_exact_prefix_replay_forwards_cached_compressed_prefix_and_injects_retrieve_tool(
     monkeypatch,
 ) -> None:
     captured: dict[str, object] = {}
@@ -684,11 +685,13 @@ def test_cache_mode_exact_prefix_replay_forwards_cached_compressed_prefix_when_t
         assert response.status_code == 200
         assert captured.get("compression_calls", []) == []
         forwarded = captured["body"]
-        # Deferred injection (no CCR tool), single frozen message cached
-        # COMPRESSED last turn: replay it so the cache holds instead of busting
-        # on original bytes (#1850). Tool absent AND cache intact.
+        # Single frozen message cached COMPRESSED last turn: replay it
+        # byte-identical so the cache holds instead of busting on original bytes.
+        # The replayed marker is still redeemable, so `headroom_retrieve` must be
+        # present this turn or Anthropic 400s "Tool reference 'headroom_retrieve'
+        # not found" (#2766). Message prefix replayed AND tool present.
         assert forwarded["messages"] == previous_forwarded_messages
-        assert "tools" not in forwarded
+        assert [tool["name"] for tool in forwarded["tools"]] == ["headroom_retrieve"]
 
 
 def test_token_mode_cached_messages_skip_cache_update_when_pipeline_result_is_unchanged(