diff --git a/communication/src/main/java/datadog/communication/serialization/GenerationalUtf8Cache.java b/communication/src/main/java/datadog/communication/serialization/GenerationalUtf8Cache.java
index 099ff56e0e0..9dd1c3b2818 100644
--- a/communication/src/main/java/datadog/communication/serialization/GenerationalUtf8Cache.java
+++ b/communication/src/main/java/datadog/communication/serialization/GenerationalUtf8Cache.java
@@ -1,5 +1,6 @@
 package datadog.communication.serialization;
 
+import datadog.trace.api.function.BackgroundOnly;
 import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
 import java.nio.charset.StandardCharsets;
 import javax.annotation.concurrent.ThreadSafe;
@@ -15,6 +16,11 @@
  * String#getBytes(java.nio.charset.Charset)}.
  *
  * <p>The cache is thread safe.
+ *
+ * <p>{@link BackgroundOnly}: the eden/tenured promotion and recalibration bookkeeping costs more
+ * than the {@code getBytes} call it replaces, so the saving only shows up as reduced allocation/GC
+ * pressure on the thread that pays it -- confine it to the background serializer thread, not an
+ * application thread.
  */
 /*
  * Cache works by using a 2-level promotion based scheme.
diff --git a/communication/src/main/java/datadog/communication/serialization/SimpleUtf8Cache.java b/communication/src/main/java/datadog/communication/serialization/SimpleUtf8Cache.java
index 8eb12d48465..4af30539053 100644
--- a/communication/src/main/java/datadog/communication/serialization/SimpleUtf8Cache.java
+++ b/communication/src/main/java/datadog/communication/serialization/SimpleUtf8Cache.java
@@ -1,5 +1,6 @@
 package datadog.communication.serialization;
 
+import datadog.trace.api.function.BackgroundOnly;
 import java.nio.charset.StandardCharsets;
 import javax.annotation.concurrent.ThreadSafe;
 
@@ -13,6 +14,11 @@
  * String#getBytes(java.nio.charset.Charset)}.
  *
  * <p>The cache is thread safe.
+ *
+ * <p>{@link BackgroundOnly}: the bookkeeping (hit counting, LFU eviction scan) costs more than the
+ * {@code getBytes} call it replaces, so the saving only shows up as reduced allocation/GC pressure
+ * on the thread that pays it -- confine it to the background serializer thread, not an application
+ * thread.
  */
 /*
  * Thread safety is achieved through using CacheEntry objects where the key data
diff --git a/internal-api/src/main/java/datadog/trace/api/cache/DDCache.java b/internal-api/src/main/java/datadog/trace/api/cache/DDCache.java
index d1cd8024847..17888187a83 100644
--- a/internal-api/src/main/java/datadog/trace/api/cache/DDCache.java
+++ b/internal-api/src/main/java/datadog/trace/api/cache/DDCache.java
@@ -1,8 +1,15 @@
 package datadog.trace.api.cache;
 
+import datadog.trace.api.function.ForegroundSafe;
 import java.util.function.BiConsumer;
 import java.util.function.Function;
 
+/**
+ * {@link ForegroundSafe}: every implementation looks up a key via a small, bounded number of probes
+ * (no growth, no eviction bookkeeping beyond overwriting a slot) -- cheap enough to call from an
+ * application thread.
+ */
+@ForegroundSafe
 public interface DDCache<K, V> {
   /**
    * Look up or create and store a value in the cache.