fix(adk): bound summarization model input

adk/middlewares/summarization/summarization.go

@@ -75,6 +75,11 @@ type TypedConfig[M adk.MessageType] struct {
 
 	// Trigger specifies the conditions that activate summarization.
 	// Optional. Defaults to triggering when total tokens exceed 160k.
+	//
+	// ContextTokens also bounds the default summarization model input. When
+	// history exceeds that limit, the default input builder keeps the newest
+	// contiguous messages that fit and evicts older messages. A custom
+	// GenModelInput retains full control and is not windowed.
 	Trigger *TriggerCondition
 
 	// EmitInternalEvents indicates whether internal events should be emitted during summarization,
@@ -372,7 +377,7 @@ func (m *TypedMiddleware[M]) shouldSummarize(ctx context.Context, input *TypedTo
 
 func (m *TypedMiddleware[M]) getTriggerContextTokens() int {
 	const defaultTriggerContextTokens = 160000
-	if m.cfg.Trigger != nil {
+	if m.cfg.Trigger != nil && m.cfg.Trigger.ContextTokens > 0 {
 		return m.cfg.Trigger.ContextTokens
 	}
 	return defaultTriggerContextTokens
@@ -433,12 +438,7 @@ func defaultTypedTokenCounter[M adk.MessageType](_ context.Context, input *Typed
 
 	var incrementTokens int
 	for _, msg := range input.Messages[incrementStart:] {
-		switch m := any(msg).(type) {
-		case *schema.Message:
-			incrementTokens += estimateMessageTokens(m)
-		case *schema.AgenticMessage:
-			incrementTokens += estimateAgenticMessageTokens(m)
-		}
+		incrementTokens += estimateTypedMessageTokens(msg)
 	}
 
 	for _, tl := range input.Tools {
@@ -481,6 +481,17 @@ func estimateTokenBytes(tokens int) int {
 	return tokens * 4
 }
 
+func estimateTypedMessageTokens[M adk.MessageType](msg M) int {
+	switch m := any(msg).(type) {
+	case *schema.Message:
+		return estimateMessageTokens(m)
+	case *schema.AgenticMessage:
+		return estimateAgenticMessageTokens(m)
+	default:
+		return 0
+	}
+}
+
 func (m *TypedMiddleware[M]) summarize(ctx context.Context, originalMsgs []M) (M, []M, error) {
 	var zero M
 	_, contextMsgs := splitSystemAndContextMsgs(originalMsgs)
@@ -614,6 +625,9 @@ func (m *TypedMiddleware[M]) buildSummarizationModelInput(ctx context.Context, o
 		return input, nil
 	}
 
+	contextMsgs = windowMessagesByTokenLimit(contextMsgs, m.getTriggerContextTokens()-
+		estimateTypedMessageTokens(sysInstruction)-estimateTypedMessageTokens(userInstruction))
+
 	input := make([]M, 0, len(contextMsgs)+2)
 	input = append(input, sysInstruction)
 	input = append(input, contextMsgs...)
@@ -622,6 +636,29 @@ func (m *TypedMiddleware[M]) buildSummarizationModelInput(ctx context.Context, o
 	return input, nil
 }
 
+// windowMessagesByTokenLimit returns the newest contiguous message window that
+// fits within tokenLimit. Keeping a suffix preserves the latest user intent and
+// tool-call/result ordering while preventing an already-over-limit history from
+// being sent unchanged to the summarization model.
+func windowMessagesByTokenLimit[M adk.MessageType](messages []M, tokenLimit int) []M {
+	if tokenLimit <= 0 || len(messages) == 0 {
+		return nil
+	}
+
+	total := 0
+	start := len(messages)
+	for i := len(messages) - 1; i >= 0; i-- {
+		tokens := estimateTypedMessageTokens(messages[i])
+		if total+tokens > tokenLimit {
+			break
+		}
+		total += tokens
+		start = i
+	}
+
+	return messages[start:]
+}
+
 func (m *TypedMiddleware[M]) getModelInstructions() (M, M) {
 	userInstruction := m.cfg.UserInstruction
 	if userInstruction == "" {

adk/middlewares/summarization/summarization_test.go

@@ -575,6 +575,23 @@ func TestMiddlewareShouldSummarize(t *testing.T) {
 		assert.False(t, triggered)
 	})
 
+	t.Run("message-only trigger does not enable zero token threshold", func(t *testing.T) {
+		mw := &TypedMiddleware[*schema.Message]{
+			cfg: &Config{
+				Trigger: &TriggerCondition{ContextMessages: 3},
+			},
+		}
+
+		triggered, err := mw.shouldSummarize(ctx, &TokenCounterInput{
+			Messages: []adk.Message{
+				schema.UserMessage("msg1"),
+				schema.UserMessage("msg2"),
+			},
+		})
+		assert.NoError(t, err)
+		assert.False(t, triggered)
+	})
+
 	t.Run("returns true when over threshold", func(t *testing.T) {
 		mw := &TypedMiddleware[*schema.Message]{
 			cfg: &Config{
@@ -1022,6 +1039,27 @@ func TestMiddlewareBuildSummarizationModelInput(t *testing.T) {
 		assert.True(t, found, "should contain context message")
 	})
 
+	t.Run("windows default input to context token threshold", func(t *testing.T) {
+		mw := &TypedMiddleware[*schema.Message]{
+			cfg: &Config{
+				Trigger: &TriggerCondition{},
+			},
+		}
+		systemInstruction, userInstruction := mw.getModelInstructions()
+		mw.cfg.Trigger.ContextTokens = estimateTypedMessageTokens(systemInstruction) +
+			estimateTypedMessageTokens(userInstruction) + 3
+
+		oldMessage := schema.UserMessage(strings.Repeat("old", 20))
+		recentMessage := schema.UserMessage("recent")
+		contextMsgs := []adk.Message{oldMessage, recentMessage}
+
+		input, err := mw.buildSummarizationModelInput(ctx, contextMsgs, contextMsgs)
+		require.NoError(t, err)
+		require.Len(t, input, 3)
+		assert.Same(t, recentMessage, input[1])
+		assert.NotContains(t, input, oldMessage)
+	})
+
 	t.Run("uses GenModelInput", func(t *testing.T) {
 		expectedInput := []adk.Message{
 			schema.UserMessage("custom input"),
@@ -1074,6 +1112,18 @@ func TestMiddlewareBuildSummarizationModelInput(t *testing.T) {
 	})
 }
 
+func TestWindowMessagesByTokenLimit(t *testing.T) {
+	oldMessage := schema.UserMessage(strings.Repeat("o", 80))
+	recentMessage := schema.UserMessage("recent")
+	messages := []adk.Message{oldMessage, recentMessage}
+
+	window := windowMessagesByTokenLimit(messages, estimateTypedMessageTokens(recentMessage))
+	require.Len(t, window, 1)
+	assert.Same(t, recentMessage, window[0])
+
+	assert.Nil(t, windowMessagesByTokenLimit(messages, 0))
+}
+
 func TestMiddlewareSummarize(t *testing.T) {
 	ctx := context.Background()