feat: use message LCP cache affinity keys

apps/api/internal/runner/cache_affinity_test.go

@@ -1,6 +1,9 @@
 package runner
 
-import "testing"
+import (
+	"strings"
+	"testing"
+)
 
 func TestBuildCacheAffinityKeyUsesExplicitKeyFirst(t *testing.T) {
 	body := map[string]any{
@@ -10,7 +13,7 @@ func TestBuildCacheAffinityKeyUsesExplicitKeyFirst(t *testing.T) {
 	}
 
 	got := buildCacheAffinityKey("chat.completions", "text_generate", body)
-	if got == "" || got[:9] != "explicit:" {
+	if !strings.HasPrefix(got, "explicit:") {
 		t.Fatalf("expected explicit cache affinity key, got %q", got)
 	}
 }
@@ -20,7 +23,7 @@ func TestBuildCacheAffinityKeyUsesSessionContinuity(t *testing.T) {
 		"session_id": "conversation-1",
 	})
 
-	if got == "" || got[:11] != "session_id:" {
+	if !strings.HasPrefix(got, "session_id:") {
 		t.Fatalf("expected session cache affinity key, got %q", got)
 	}
 }
@@ -36,7 +39,35 @@ func TestBuildCacheAffinityKeyFallsBackToPromptHash(t *testing.T) {
 
 	first := buildCacheAffinityKey("chat.completions", "text_generate", body)
 	second := buildCacheAffinityKey("chat.completions", "text_generate", body)
-	if first == "" || first != second || first[:7] != "prompt:" {
-		t.Fatalf("expected stable prompt hash cache affinity key, first=%q second=%q", first, second)
+	if first == "" || first != second || !strings.HasPrefix(first, "prompt_lcp:") {
+		t.Fatalf("expected stable prompt LCP cache affinity key, first=%q second=%q", first, second)
+	}
+}
+
+func TestBuildCacheAffinityKeysUsesMessagePrefixLCPKeys(t *testing.T) {
+	first := buildCacheAffinityKeys("chat.completions", "text_generate", map[string]any{
+		"messageHashes": []any{"system-hash", "user-hash"},
+		"tools":         []any{map[string]any{"type": "function", "function": map[string]any{"name": "lookup"}}},
+	})
+	next := buildCacheAffinityKeys("chat.completions", "text_generate", map[string]any{
+		"messageHashes": []any{"system-hash", "user-hash", "assistant-hash", "tool-hash"},
+		"tools":         []any{map[string]any{"type": "function", "function": map[string]any{"name": "lookup"}}},
+	})
+
+	if !strings.HasPrefix(first.Primary, "prompt_lcp:") {
+		t.Fatalf("expected LCP prompt key, got %+v", first)
+	}
+	if len(first.Record) != 2 || len(next.Lookup) != 4 {
+		t.Fatalf("expected per-message prefix keys, first=%+v next=%+v", first, next)
+	}
+	found := false
+	for _, key := range next.Lookup {
+		if key == first.Primary {
+			found = true
+			break
+		}
+	}
+	if !found {
+		t.Fatalf("next lookup keys should include first request prefix key, first=%+v next=%+v", first, next)
 	}
 }