diff --git a/core/src/commonMain/kotlin/com/google/adk/kt/summarizer/EventsCompactionConfig.kt b/core/src/commonMain/kotlin/com/google/adk/kt/summarizer/EventsCompactionConfig.kt
index e4aa3420..2e583518 100644
--- a/core/src/commonMain/kotlin/com/google/adk/kt/summarizer/EventsCompactionConfig.kt
+++ b/core/src/commonMain/kotlin/com/google/adk/kt/summarizer/EventsCompactionConfig.kt
@@ -27,11 +27,19 @@ package com.google.adk.kt.summarizer
  * @property summarizer The [EventSummarizer] used to produce the compaction summary. When `null`,
  *   the runner defaults it to an [LlmEventSummarizer] backed by the root agent's model, requiring
  *   the root to be an `LlmAgent` (otherwise it throws at construction).
+ * @property tokenThreshold The most recent prompt token count that triggers
+ *   intra-invocation token-threshold (tail-retention) compaction before an LLM call. Must be
+ *   strictly positive when set, and must be set together with [eventRetentionSize].
+ * @property eventRetentionSize The number of most recent events kept raw (un-compacted) when
+ *   token-threshold compaction fires; the older events are summarized into a single event. Must be
+ *   non-negative when set, and must be set together with [tokenThreshold].
  */
 data class EventsCompactionConfig(
   val compactionInterval: Int? = null,
   val overlapSize: Int? = null,
   val summarizer: EventSummarizer? = null,
+  val tokenThreshold: Int? = null,
+  val eventRetentionSize: Int? = null,
 ) {
   init {
     if (compactionInterval != null) {
@@ -46,8 +54,23 @@ data class EventsCompactionConfig(
       "compactionInterval and overlapSize must be set together or both null " +
         "(got compactionInterval=$compactionInterval, overlapSize=$overlapSize)."
     }
+    if (tokenThreshold != null) {
+      require(tokenThreshold > 0) { "tokenThreshold must be > 0, but was $tokenThreshold." }
+    }
+    if (eventRetentionSize != null) {
+      require(eventRetentionSize >= 0) {
+        "eventRetentionSize must be >= 0, but was $eventRetentionSize."
+      }
+    }
+    require((tokenThreshold == null) == (eventRetentionSize == null)) {
+      "tokenThreshold and eventRetentionSize must be set together or both null " +
+        "(got tokenThreshold=$tokenThreshold, eventRetentionSize=$eventRetentionSize)."
+    }
   }
 
   /** Returns true when sliding-window compaction is fully configured. */
   fun hasSlidingWindowConfig(): Boolean = compactionInterval != null && overlapSize != null
+
+  /** Returns true when token-threshold compaction is fully configured. */
+  fun hasTokenThresholdConfig(): Boolean = tokenThreshold != null && eventRetentionSize != null
 }
diff --git a/core/src/commonTest/kotlin/com/google/adk/kt/summarizer/EventsCompactionConfigTest.kt b/core/src/commonTest/kotlin/com/google/adk/kt/summarizer/EventsCompactionConfigTest.kt
index 6202e909..4eff4c5e 100644
--- a/core/src/commonTest/kotlin/com/google/adk/kt/summarizer/EventsCompactionConfigTest.kt
+++ b/core/src/commonTest/kotlin/com/google/adk/kt/summarizer/EventsCompactionConfigTest.kt
@@ -29,6 +29,7 @@ class EventsCompactionConfigTest {
     val config = EventsCompactionConfig()
 
     assertFalse(config.hasSlidingWindowConfig())
+    assertFalse(config.hasTokenThresholdConfig())
   }
 
   @Test
@@ -83,6 +84,77 @@ class EventsCompactionConfigTest {
     }
   }
 
+  @Test
+  fun construct_validTokenThreshold_succeeds() {
+    val config =
+      EventsCompactionConfig(
+        tokenThreshold = 50000,
+        eventRetentionSize = 5,
+        summarizer = NoopSummarizer,
+      )
+
+    assertTrue(config.hasTokenThresholdConfig())
+    assertFalse(config.hasSlidingWindowConfig())
+    assertEquals(50000, config.tokenThreshold)
+    assertEquals(5, config.eventRetentionSize)
+  }
+
+  @Test
+  fun construct_eventRetentionSizeZero_succeeds() {
+    val config = EventsCompactionConfig(tokenThreshold = 50000, eventRetentionSize = 0)
+
+    assertTrue(config.hasTokenThresholdConfig())
+  }
+
+  @Test
+  fun construct_bothStrategiesConfigured_succeeds() {
+    val config =
+      EventsCompactionConfig(
+        compactionInterval = 3,
+        overlapSize = 1,
+        tokenThreshold = 50000,
+        eventRetentionSize = 5,
+      )
+
+    assertTrue(config.hasSlidingWindowConfig())
+    assertTrue(config.hasTokenThresholdConfig())
+  }
+
+  @Test
+  fun construct_tokenThresholdZero_throwsIllegalArgumentException() {
+    assertFailsWith<IllegalArgumentException> {
+      EventsCompactionConfig(tokenThreshold = 0, eventRetentionSize = 5)
+    }
+  }
+
+  @Test
+  fun construct_tokenThresholdNegative_throwsIllegalArgumentException() {
+    assertFailsWith<IllegalArgumentException> {
+      EventsCompactionConfig(tokenThreshold = -1, eventRetentionSize = 5)
+    }
+  }
+
+  @Test
+  fun construct_eventRetentionSizeNegative_throwsIllegalArgumentException() {
+    assertFailsWith<IllegalArgumentException> {
+      EventsCompactionConfig(tokenThreshold = 50000, eventRetentionSize = -1)
+    }
+  }
+
+  @Test
+  fun construct_tokenThresholdWithoutEventRetentionSize_throwsIllegalArgumentException() {
+    assertFailsWith<IllegalArgumentException> {
+      EventsCompactionConfig(tokenThreshold = 50000, eventRetentionSize = null)
+    }
+  }
+
+  @Test
+  fun construct_eventRetentionSizeWithoutTokenThreshold_throwsIllegalArgumentException() {
+    assertFailsWith<IllegalArgumentException> {
+      EventsCompactionConfig(tokenThreshold = null, eventRetentionSize = 5)
+    }
+  }
+
   private object NoopSummarizer : EventSummarizer {
     override suspend fun summarizeEvents(events: List<Event>): Event? = null
   }