diff --git a/cmd/olliesrv/internal/agent/history.go b/cmd/olliesrv/internal/agent/history.go
index df0d3ed..1c2d24a 100644
--- a/cmd/olliesrv/internal/agent/history.go
+++ b/cmd/olliesrv/internal/agent/history.go
@@ -38,7 +38,8 @@ Be concise. Capture what another LLM needs to seamlessly continue.`
 // Used for multi-agent session restore where each agent has separate messages.
 func RestoreHistoryFromMessages(messages []backend.Message) *History {
 	s := &History{
-		messages: messages,
+		messages:     messages,
+		summaryCache: make(map[string]string),
 	}
 	for i := range s.messages {
 		if s.messages[i].Role == "assistant" && s.messages[i].ID == "" {
@@ -65,6 +66,10 @@ func RestoreHistoryFromMessages(messages []backend.Message) *History {
 type History struct {
 	goal     string
 	messages []backend.Message
+	// summaryCache reuses validated summaries for identical tool-result content
+	// during the lifetime of this agent. SummaryApplied persists per-message
+	// state across session restore; this cache avoids new LLM calls for repeats.
+	summaryCache map[string]string
 	// Cumulative usage tracking.
 	TotalInputTokens         int
 	TotalCachedInputTokens   int
@@ -85,7 +90,7 @@ type History struct {
 // newHistory creates a new empty History. The caller is responsible for
 // appending the initial user message via appendUserMessage.
 func newHistory(goal string) *History {
-	return &History{goal: goal}
+	return &History{goal: goal, summaryCache: make(map[string]string)}
 }
 
 func toolMessageID(m backend.Message) string {
@@ -96,6 +101,14 @@ func toolMessageID(m backend.Message) string {
 	return "tool:content:" + hex.EncodeToString(h[:])
 }
 
+func toolSummaryHash(m backend.Message) string {
+	h := sha256.New()
+	h.Write([]byte(m.ToolCallID))
+	h.Write([]byte{0})
+	h.Write([]byte(m.Content))
+	return hex.EncodeToString(h.Sum(nil))
+}
+
 func (s *History) history() []backend.Message {
 	return s.messages
 }
@@ -404,6 +417,9 @@ func (s *History) estimateTokens() int {
 // using a single batched LLM call. Messages in the last hotTailSize slots
 // are left verbatim.
 func (s *History) stripCold(ctx context.Context, b backend.Backend) (backend.Usage, bool) {
+	if s.summaryCache == nil {
+		s.summaryCache = make(map[string]string)
+	}
 	hot := len(s.messages) - hotTailSize
 	if hot < 0 {
 		return backend.Usage{}, false
@@ -411,10 +427,10 @@ func (s *History) stripCold(ctx context.Context, b backend.Backend) (backend.Usa
 
 	// Collect cold-zone tool results exceeding 200 chars.
 	type item struct {
-		id        int
 		idx       int // index into s.messages
 		orig      string
 		messageID string
+		hash      string
 	}
 	var items []item
 	for i := range s.messages {
@@ -433,7 +449,12 @@ func (s *History) stripCold(ctx context.Context, b backend.Backend) (backend.Usa
 				m.ID = id
 			}
 			if !m.SummaryApplied {
-				items = append(items, item{id: len(items), idx: i, orig: m.Content})
+				if summary, ok := s.summaryCache[toolSummaryHash(*m)]; ok {
+					m.Content = summary
+					m.SummaryApplied = true
+					continue
+				}
+				items = append(items, item{idx: i, orig: m.Content, messageID: id, hash: toolSummaryHash(*m)})
 			}
 		}
 	}
@@ -443,9 +464,9 @@ func (s *History) stripCold(ctx context.Context, b backend.Backend) (backend.Usa
 
 	// Build a single prompt listing each item.
 	var sb strings.Builder
-	sb.WriteString("Compress each of the following tool results to 1-3 sentences preserving key facts, file paths, values, and findings.\n\nReturn a JSON object where each key is the id and the value is the summary, for example:\n{\"0\": \"summarized result 0\", \"1\": \"summarized result 1\"}\n\nOnly include keys that were in the input.\n\n")
+	sb.WriteString("Compress each of the following tool results to 1-3 sentences preserving key facts, file paths, values, and findings.\n\nReturn ONLY a valid JSON object. Do not use Markdown fences, prose, or any text outside the JSON object. Each key must be exactly one of the supplied message IDs and each value must be a string summary, for example:\n{\"tool:call-123\": \"summarized result\", \"tool:call-456\": \"another summary\"}\n\nOnly include keys that were in the input.\n\n")
 	for _, it := range items {
-		fmt.Fprintf(&sb, "---\nid=%d:\n%s\n", it.id, it.orig)
+		fmt.Fprintf(&sb, "---\nid=%s:\n%s\n", it.messageID, it.orig)
 	}
 
 	ch, err := b.ChatStream(ctx, []backend.Message{{Role: "user", Content: sb.String()}}, nil, backend.GenerationParams{})
@@ -478,8 +499,8 @@ func (s *History) stripCold(ctx context.Context, b backend.Backend) (backend.Usa
 
 	// Apply summaries back into history.
 	for _, it := range items {
-		key := fmt.Sprintf("%d", it.id)
-		if summary, ok := summaries[key]; ok && summary != "" {
+		if summary, ok := summaries[it.messageID]; ok && summary != "" {
+			s.summaryCache[it.hash] = summary
 			s.messages[it.idx].Content = summary
 			s.messages[it.idx].SummaryApplied = true
 		}
diff --git a/cmd/olliesrv/internal/agent/history_summary_test.go b/cmd/olliesrv/internal/agent/history_summary_test.go
index fbcb036..a829071 100644
--- a/cmd/olliesrv/internal/agent/history_summary_test.go
+++ b/cmd/olliesrv/internal/agent/history_summary_test.go
@@ -54,12 +54,56 @@ func TestStripColdSkipsAlreadySummarized(t *testing.T) {
 	}
 }
 
+func TestStripColdReusesSummaryCache(t *testing.T) {
+	b := &summaryBackend{}
+	h := newHistory("test")
+	for i := 0; i < 9; i++ {
+		m := backend.Message{Role: "user", Content: "hot"}
+		if i < 1 {
+			m = backend.Message{Role: "tool", ToolCallID: "tc-0", Content: strings.Repeat("same result ", 30)}
+		}
+		h.messages = append(h.messages, m)
+	}
+	if _, _ = h.stripCold(context.Background(), b); b.calls != 1 {
+		t.Fatalf("first stripCold calls = %d, want 1", b.calls)
+	}
+	original := strings.Repeat("same result ", 30)
+	h.messages[0].Content = original
+	h.messages[0].SummaryApplied = false
+	if _, _ = h.stripCold(context.Background(), b); b.calls != 1 {
+		t.Fatalf("cached stripCold calls = %d, want 1", b.calls)
+	}
+	if !h.messages[0].SummaryApplied || h.messages[0].Content != "summary" {
+		t.Fatalf("cached summary was not applied: %#v", h.messages[0])
+	}
+}
+func TestStripColdSkipsAlreadySummarizedAfterRestore(t *testing.T) {
+	b := &summaryBackend{}
+	h := RestoreHistoryFromMessages([]backend.Message{
+		{Role: "tool", ToolCallID: "tc-0", Content: strings.Repeat("summary ", 30), SummaryApplied: true},
+		{Role: "user", Content: "hot"},
+		{Role: "user", Content: "hot"},
+		{Role: "user", Content: "hot"},
+		{Role: "user", Content: "hot"},
+		{Role: "user", Content: "hot"},
+		{Role: "user", Content: "hot"},
+		{Role: "user", Content: "hot"},
+		{Role: "user", Content: "hot"},
+	})
+	if _, _ = h.stripCold(context.Background(), b); b.calls != 0 {
+		t.Fatalf("restored summarized history calls = %d, want 0", b.calls)
+	}
+}
+
 type summaryBackend struct{ calls int }
 
 func (b *summaryBackend) ChatStream(_ context.Context, messages []backend.Message, _ []backend.Tool, _ backend.GenerationParams) (<-chan backend.StreamEvent, error) {
 	b.calls++
+	if len(messages) != 1 || !strings.Contains(messages[0].Content, "id=tool:tc-0:") {
+		return nil, fmt.Errorf("summary request did not use stable message ID")
+	}
 	out := make(chan backend.StreamEvent, 1)
-	out <- backend.StreamEvent{Content: `{"0":"summary"}`, Done: true}
+	out <- backend.StreamEvent{Content: `{"tool:tc-0":"summary"}`, Done: true}
 	close(out)
 	return out, nil
 }