gateway: report cache writes in Chat usage (#429)

Claude Code reports cache writes, and the gateway retains them in Usage. Usage.chat() leaves them out of the response, so OpenAI-compatible clients count those tokens as ordinary input. Emit prompt_tokens_details.cache_write_tokens and keep prompt_tokens as the total prompt.

TestClaudeChatCacheWrites drives Claude stream usage through both Chat response modes. It fails without the serializer change. go vet, the gateway/redaction race tests, the provider account race tests, the no-GUI build, and go test ./... pass, except TestPluginModelListThroughAccountProxy, TestPluginProxy and TestPluginSOCKSProxy: they fail on main too on the test machine, whose DNS resolves .invalid names. A local live client run records cache writes on a cold turn and cache reads on a later turn.

internal/gateway/chat.go

@@ -506,7 +506,7 @@ func (u cUsage) usage() Usage {
 func (u Usage) chat() map[string]any {
 	in := u.prompt()
 	return map[string]any{"prompt_tokens": in, "completion_tokens": u.Output, "total_tokens": in + u.Output,
-		"prompt_tokens_details":     map[string]any{"cached_tokens": u.CacheRead},
+		"prompt_tokens_details":     map[string]any{"cached_tokens": u.CacheRead, "cache_write_tokens": u.CacheWrite},
 		"completion_tokens_details": map[string]any{"reasoning_tokens": u.Reasoning}}
 }
 

internal/gateway/claude_chat_usage_test.go

@@ -0,0 +1,70 @@
+package gateway
+
+import (
+	"encoding/json"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/yetone/magpie/internal/provider"
+)
+
+func TestClaudeChatCacheWrites(t *testing.T) {
+	// Claude reports the TTL split at message_start, but omits it in the
+	// final delta. Both Chat response modes must retain the write count.
+	lines := []string{
+		`{"type":"stream_event","event":{"type":"message_start","message":{"id":"m1","model":"claude-haiku-4-5","usage":{"input_tokens":10,"cache_creation_input_tokens":6416,"cache_read_input_tokens":0,"cache_creation":{"ephemeral_5m_input_tokens":0,"ephemeral_1h_input_tokens":6416},"output_tokens":4}}}}`,
+		`{"type":"stream_event","event":{"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":"OK"}}}`,
+		`{"type":"stream_event","event":{"type":"message_delta","delta":{"stop_reason":"end_turn"},"usage":{"input_tokens":10,"cache_creation_input_tokens":6416,"cache_read_input_tokens":0,"output_tokens":40,"output_tokens_details":{"thinking_tokens":33}}}}`,
+		`{"type":"stream_event","event":{"type":"message_stop"}}`,
+	}
+	for _, c := range []struct {
+		name   string
+		stream bool
+	}{{"stream", true}, {"non-stream", false}} {
+		t.Run(c.name, func(t *testing.T) {
+			run := &subscriptionRun{}
+			segment := run.attach()
+			go run.readOutput(strings.NewReader(strings.Join(lines, "\n") + "\n"))
+			rec := httptest.NewRecorder()
+			req := &Request{Model: "claude-haiku-4-5", Stream: c.stream}
+			var usage Usage
+			code, msg := relay(rec, httptest.NewRequest("POST", "/v1/chat/completions", nil), provider.Chat, "Claude Code", req, segment, &usage,
+				func() {}, func(string, string, bool) {})
+			if code != 200 || msg != "" {
+				t.Fatalf("reply: %d %s", code, msg)
+			}
+			var reply struct {
+				Usage *struct {
+					Prompt     int `json:"prompt_tokens"`
+					Completion int `json:"completion_tokens"`
+					Total      int `json:"total_tokens"`
+					Details    struct {
+						Read  int  `json:"cached_tokens"`
+						Write *int `json:"cache_write_tokens"`
+					} `json:"prompt_tokens_details"`
+				} `json:"usage"`
+			}
+			if c.stream {
+				for _, line := range strings.Split(rec.Body.String(), "\n") {
+					data, ok := strings.CutPrefix(line, "data: ")
+					if !ok || data == "[DONE]" {
+						continue
+					}
+					if err := json.Unmarshal([]byte(data), &reply); err != nil {
+						t.Fatal(err)
+					}
+				}
+			} else if err := json.Unmarshal(rec.Body.Bytes(), &reply); err != nil {
+				t.Fatal(err)
+			}
+			u := reply.Usage
+			if u == nil || u.Details.Write == nil || *u.Details.Write != 6416 {
+				t.Fatalf("cache writes missing or changed: %s", rec.Body.String())
+			}
+			if u.Prompt != 6426 || u.Completion != 40 || u.Total != 6466 || u.Details.Read != 0 {
+				t.Fatalf("token counts: %+v", u)
+			}
+		})
+	}
+}