Improve compaction

Signed-off-by: David Gageot <david.gageot@docker.com>

Author: dgageot

pkg/runtime/runtime.go

@@ -988,8 +988,25 @@ func (r *LocalRuntime) RunStream(ctx context.Context, sess *session.Session) <-c
 
 			if m != nil && r.sessionCompaction {
 				if sess.InputTokens+sess.OutputTokens > int64(float64(contextLimit)*0.9) {
+					messageCountBefore := len(sess.Messages)
 					r.Summarize(ctx, sess, "", events)
 					events <- TokenUsage(sess.ID, r.currentAgent, sess.InputTokens, sess.OutputTokens, sess.InputTokens+sess.OutputTokens, contextLimit, sess.Cost)
+
+					// Check if compaction actually produced a summary by comparing
+					// message count (Compact appends a Summary item on success).
+					if len(sess.Messages) > messageCountBefore {
+						// After compaction, the summary is the last item. GetMessages
+						// starts collecting conversation messages after the last summary,
+						// so there would be zero conversation messages. Providers
+						// (e.g. Anthropic) reject requests with no non-system messages,
+						// so we add a continuation message to bridge the gap.
+						sess.AddMessage(session.ImplicitUserMessage("The conversation was automatically compacted. Please continue where you left off."))
+
+						// Reset token counters so the compaction check doesn't trigger
+						// again on the next iteration (before the model reports new usage).
+						sess.InputTokens = 0
+						sess.OutputTokens = 0
+					}
 				}
 			}
 

pkg/runtime/runtime_test.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io"
 	"reflect"
+	"strings"
 	"sync"
 	"testing"
 	"time"
@@ -75,6 +76,18 @@ func (m *mockStream) Recv() (chat.MessageStreamResponse, error) {
 
 func (m *mockStream) Close() { m.closed = true }
 
+// errorStream always returns an error on Recv, simulating a failed model call.
+type errorStream struct {
+	err    error
+	closed bool
+}
+
+func (e *errorStream) Recv() (chat.MessageStreamResponse, error) {
+	return chat.MessageStreamResponse{}, e.err
+}
+
+func (e *errorStream) Close() { e.closed = true }
+
 type streamBuilder struct{ responses []chat.MessageStreamResponse }
 
 func newStreamBuilder() *streamBuilder {
@@ -725,6 +738,205 @@ func TestCompaction(t *testing.T) {
 	require.NotEqual(t, -1, compactionStartIdx, "expected a SessionCompaction start event")
 }
 
+// capturingQueueProvider extends queueProvider to also capture the messages
+// passed to each CreateChatCompletionStream call.
+type capturingQueueProvider struct {
+	queueProvider
+	calls [][]chat.Message // messages sent on each call
+}
+
+func (p *capturingQueueProvider) CreateChatCompletionStream(_ context.Context, msgs []chat.Message, _ []tools.Tool) (chat.MessageStream, error) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	p.calls = append(p.calls, msgs)
+	if len(p.streams) == 0 {
+		return &mockStream{}, nil
+	}
+	s := p.streams[0]
+	p.streams = p.streams[1:]
+	return s, nil
+}
+
+func TestCompaction_ContinuationMessageSent(t *testing.T) {
+	// After auto-compaction, the runtime must inject a continuation user
+	// message so that the model receives at least one non-system message.
+	// This prevents providers (e.g. Anthropic) from rejecting the request
+	// with "messages: Field required".
+
+	// Stream 1: initial response that pushes usage above 90% of context.
+	mainStream := newStreamBuilder().
+		AddContent("Hello there").
+		AddStopWithUsage(101, 0). // will exceed 90% of 100
+		Build()
+
+	// Stream 2: summary generation (used by the compactor runtime).
+	summaryStream := newStreamBuilder().
+		AddContent("summary of conversation").
+		AddStopWithUsage(1, 1).
+		Build()
+
+	// Stream 3: the model call that happens after compaction.
+	postCompactionStream := newStreamBuilder().
+		AddContent("I'll continue.").
+		AddStopWithUsage(5, 3).
+		Build()
+
+	prov := &capturingQueueProvider{
+		queueProvider: queueProvider{
+			id:      "test/mock-model",
+			streams: []chat.MessageStream{mainStream, summaryStream, postCompactionStream},
+		},
+	}
+
+	root := agent.New("root", "You are a test agent", agent.WithModel(prov))
+	tm := team.New(team.WithAgents(root))
+
+	rt, err := NewLocalRuntime(tm, WithSessionCompaction(true), WithModelStore(mockModelStoreWithLimit{limit: 100}))
+	require.NoError(t, err)
+
+	// First RunStream: establishes baseline usage.
+	sess := session.New(session.WithUserMessage("Start"))
+	for range rt.RunStream(t.Context(), sess) {
+	}
+
+	// Second RunStream: will trigger compaction because usage > 90%.
+	sess.AddMessage(session.UserMessage("Again"))
+	for range rt.RunStream(t.Context(), sess) {
+	}
+
+	// The third model call (stream 3 = index 2 from the provider's perspective,
+	// but calls[1] because first RunStream used calls[0]) should contain
+	// a user message with the continuation prompt.
+	require.GreaterOrEqual(t, len(prov.calls), 2, "expected at least 2 model calls in second RunStream")
+
+	// The last captured call is the post-compaction model invocation.
+	lastCallMsgs := prov.calls[len(prov.calls)-1]
+
+	// Find the continuation user message.
+	var foundContinuation bool
+	for _, msg := range lastCallMsgs {
+		if msg.Role == chat.MessageRoleUser && msg.Content == "The conversation was automatically compacted. Please continue where you left off." {
+			foundContinuation = true
+			break
+		}
+	}
+	require.True(t, foundContinuation, "expected continuation user message after compaction; messages: %v", lastCallMsgs)
+
+	// Also check the summary is in the system messages.
+	var foundSummary bool
+	for _, msg := range lastCallMsgs {
+		if msg.Role == chat.MessageRoleSystem && strings.Contains(msg.Content, "summary of conversation") {
+			foundSummary = true
+			break
+		}
+	}
+	require.True(t, foundSummary, "expected session summary in system messages after compaction")
+}
+
+func TestCompaction_TokenCountersResetAfterSuccess(t *testing.T) {
+	// After successful compaction the token counters must be reset to zero
+	// to prevent the compaction check from triggering again immediately.
+
+	mainStream := newStreamBuilder().
+		AddContent("Hello").
+		AddStopWithUsage(101, 0).
+		Build()
+
+	summaryStream := newStreamBuilder().
+		AddContent("summary").
+		AddStopWithUsage(1, 1).
+		Build()
+
+	// Post-compaction stream with LOW usage.
+	postCompactionStream := newStreamBuilder().
+		AddContent("Continuing").
+		AddStopWithUsage(10, 5).
+		Build()
+
+	prov := &queueProvider{
+		id:      "test/mock-model",
+		streams: []chat.MessageStream{mainStream, summaryStream, postCompactionStream},
+	}
+
+	root := agent.New("root", "You are a test agent", agent.WithModel(prov))
+	tm := team.New(team.WithAgents(root))
+
+	rt, err := NewLocalRuntime(tm, WithSessionCompaction(true), WithModelStore(mockModelStoreWithLimit{limit: 100}))
+	require.NoError(t, err)
+
+	sess := session.New(session.WithUserMessage("Start"))
+	for range rt.RunStream(t.Context(), sess) {
+	}
+
+	sess.AddMessage(session.UserMessage("Again"))
+
+	var events []Event
+	for ev := range rt.RunStream(t.Context(), sess) {
+		events = append(events, ev)
+	}
+
+	// Count how many times compaction started — should be exactly once.
+	var compactionCount int
+	for _, ev := range events {
+		if e, ok := ev.(*SessionCompactionEvent); ok && e.Status == "started" {
+			compactionCount++
+		}
+	}
+	require.Equal(t, 1, compactionCount, "compaction should trigger exactly once, not loop")
+
+	// Token counters should reflect the post-compaction model call, not the old values.
+	assert.Equal(t, int64(10), sess.InputTokens, "InputTokens should be from post-compaction call")
+	assert.Equal(t, int64(5), sess.OutputTokens, "OutputTokens should be from post-compaction call")
+}
+
+func TestCompaction_FailedCompactionNoStrayMessage(t *testing.T) {
+	// When compaction fails (summary runtime errors), no continuation
+	// message should be added and the original messages should be preserved.
+
+	// Stream 1: initial response with high usage.
+	mainStream := newStreamBuilder().
+		AddContent("Hello").
+		AddStopWithUsage(101, 0).
+		Build()
+
+	// Stream 2: summary generation FAILS — the stream returns an error.
+	failingStream := &errorStream{err: fmt.Errorf("simulated API error during summary")}
+
+	prov := &queueProvider{
+		id:      "test/mock-model",
+		streams: []chat.MessageStream{mainStream, failingStream},
+	}
+
+	root := agent.New("root", "You are a test agent", agent.WithModel(prov))
+	tm := team.New(team.WithAgents(root))
+
+	rt, err := NewLocalRuntime(tm, WithSessionCompaction(true), WithModelStore(mockModelStoreWithLimit{limit: 100}))
+	require.NoError(t, err)
+
+	sess := session.New(session.WithUserMessage("Start"))
+	for range rt.RunStream(t.Context(), sess) {
+	}
+
+	sess.AddMessage(session.UserMessage("Again"))
+
+	for range rt.RunStream(t.Context(), sess) {
+	}
+
+	// No summary was appended, so no continuation message should exist.
+	for _, item := range sess.Messages {
+		if item.IsMessage() && item.Message.Message.Content == "The conversation was automatically compacted. Please continue where you left off." {
+			t.Fatal("found stray continuation message after failed compaction")
+		}
+	}
+
+	// No summary items should exist.
+	for _, item := range sess.Messages {
+		if item.Summary != "" {
+			t.Fatal("found summary item after failed compaction")
+		}
+	}
+}
+
 func TestSessionWithoutUserMessage(t *testing.T) {
 	stream := newStreamBuilder().AddContent("OK").AddStopWithUsage(1, 1).Build()