fix(claude): avoid racy fingerprint downgrades

internal/runtime/executor/claude_device_profile.go

@@ -31,6 +31,8 @@ var (
 	claudeDeviceProfileCache            = make(map[string]claudeDeviceProfileCacheEntry)
 	claudeDeviceProfileCacheMu          sync.RWMutex
 	claudeDeviceProfileCacheCleanupOnce sync.Once
+
+	claudeDeviceProfileBeforeCandidateStore func(claudeDeviceProfile)
 )
 
 type claudeCLIVersion struct {
@@ -257,13 +259,25 @@ func resolveClaudeDeviceProfile(auth *cliproxyauth.Auth, apiKey string, headers
 	cachedValid := hasCached && entry.expire.After(now) && entry.profile.UserAgent != ""
 	claudeDeviceProfileCacheMu.RUnlock()
 
-	if hasCandidate && (!cachedValid || shouldUpgradeClaudeDeviceProfile(candidate, entry.profile)) {
-		newEntry := claudeDeviceProfileCacheEntry{
+	if hasCandidate {
+		if claudeDeviceProfileBeforeCandidateStore != nil {
+			claudeDeviceProfileBeforeCandidateStore(candidate)
+		}
+
+		claudeDeviceProfileCacheMu.Lock()
+		entry, hasCached = claudeDeviceProfileCache[cacheKey]
+		cachedValid = hasCached && entry.expire.After(now) && entry.profile.UserAgent != ""
+		if cachedValid && !shouldUpgradeClaudeDeviceProfile(candidate, entry.profile) {
+			entry.expire = now.Add(claudeDeviceProfileTTL)
+			claudeDeviceProfileCache[cacheKey] = entry
+			claudeDeviceProfileCacheMu.Unlock()
+			return entry.profile
+		}
+
+		claudeDeviceProfileCache[cacheKey] = claudeDeviceProfileCacheEntry{
 			profile: candidate,
 			expire:  now.Add(claudeDeviceProfileTTL),
 		}
-		claudeDeviceProfileCacheMu.Lock()
-		claudeDeviceProfileCache[cacheKey] = newEntry
 		claudeDeviceProfileCacheMu.Unlock()
 		return candidate
 	}

internal/runtime/executor/claude_executor_test.go

@@ -8,7 +8,9 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"strings"
+	"sync"
 	"testing"
+	"time"
 
 	"github.com/gin-gonic/gin"
 	"github.com/klauspost/compress/zstd"
@@ -206,6 +208,97 @@ func TestApplyClaudeHeaders_DoesNotDowngradeConfiguredBaselineOnFirstClaudeClien
 	assertClaudeFingerprint(t, newerClaudeReq.Header, "claude-cli/2.1.71 (external, cli)", "0.81.0", "v24.6.0", "Linux", "x64")
 }
 
+func TestResolveClaudeDeviceProfile_RechecksCacheBeforeStoringCandidate(t *testing.T) {
+	resetClaudeDeviceProfileCache()
+	stabilize := true
+
+	cfg := &config.Config{
+		ClaudeHeaderDefaults: config.ClaudeHeaderDefaults{
+			UserAgent:              "claude-cli/2.1.60 (external, cli)",
+			PackageVersion:         "0.70.0",
+			RuntimeVersion:         "v22.0.0",
+			OS:                     "MacOS",
+			Arch:                   "arm64",
+			StabilizeDeviceProfile: &stabilize,
+		},
+	}
+	auth := &cliproxyauth.Auth{
+		ID: "auth-racy-upgrade",
+		Attributes: map[string]string{
+			"api_key": "key-racy-upgrade",
+		},
+	}
+
+	lowPaused := make(chan struct{})
+	releaseLow := make(chan struct{})
+	var pauseOnce sync.Once
+	var releaseOnce sync.Once
+
+	claudeDeviceProfileBeforeCandidateStore = func(candidate claudeDeviceProfile) {
+		if candidate.UserAgent != "claude-cli/2.1.62 (external, cli)" {
+			return
+		}
+		pauseOnce.Do(func() { close(lowPaused) })
+		<-releaseLow
+	}
+	t.Cleanup(func() {
+		claudeDeviceProfileBeforeCandidateStore = nil
+		releaseOnce.Do(func() { close(releaseLow) })
+	})
+
+	lowResultCh := make(chan claudeDeviceProfile, 1)
+	go func() {
+		lowResultCh <- resolveClaudeDeviceProfile(auth, "key-racy-upgrade", http.Header{
+			"User-Agent":                  []string{"claude-cli/2.1.62 (external, cli)"},
+			"X-Stainless-Package-Version": []string{"0.74.0"},
+			"X-Stainless-Runtime-Version": []string{"v24.3.0"},
+			"X-Stainless-Os":              []string{"Linux"},
+			"X-Stainless-Arch":            []string{"x64"},
+		}, cfg)
+	}()
+
+	select {
+	case <-lowPaused:
+	case <-time.After(2 * time.Second):
+		t.Fatal("timed out waiting for lower candidate to pause before storing")
+	}
+
+	highResult := resolveClaudeDeviceProfile(auth, "key-racy-upgrade", http.Header{
+		"User-Agent":                  []string{"claude-cli/2.1.63 (external, cli)"},
+		"X-Stainless-Package-Version": []string{"0.75.0"},
+		"X-Stainless-Runtime-Version": []string{"v24.4.0"},
+		"X-Stainless-Os":              []string{"MacOS"},
+		"X-Stainless-Arch":            []string{"arm64"},
+	}, cfg)
+	releaseOnce.Do(func() { close(releaseLow) })
+
+	select {
+	case lowResult := <-lowResultCh:
+		if lowResult.UserAgent != "claude-cli/2.1.63 (external, cli)" {
+			t.Fatalf("lowResult.UserAgent = %q, want %q", lowResult.UserAgent, "claude-cli/2.1.63 (external, cli)")
+		}
+		if lowResult.PackageVersion != "0.75.0" {
+			t.Fatalf("lowResult.PackageVersion = %q, want %q", lowResult.PackageVersion, "0.75.0")
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatal("timed out waiting for lower candidate result")
+	}
+
+	if highResult.UserAgent != "claude-cli/2.1.63 (external, cli)" {
+		t.Fatalf("highResult.UserAgent = %q, want %q", highResult.UserAgent, "claude-cli/2.1.63 (external, cli)")
+	}
+
+	cached := resolveClaudeDeviceProfile(auth, "key-racy-upgrade", http.Header{
+		"User-Agent": []string{"curl/8.7.1"},
+	}, cfg)
+	if cached.UserAgent != "claude-cli/2.1.63 (external, cli)" {
+		t.Fatalf("cached.UserAgent = %q, want %q", cached.UserAgent, "claude-cli/2.1.63 (external, cli)")
+	}
+	if cached.PackageVersion != "0.75.0" {
+		t.Fatalf("cached.PackageVersion = %q, want %q", cached.PackageVersion, "0.75.0")
+	}
+}
+
 func TestApplyClaudeHeaders_DisableDeviceProfileStabilization(t *testing.T) {
 	resetClaudeDeviceProfileCache()