Anthropic API proxy with OAuth credential rotation and Claude Code fingerprinting

Sniffs a real Claude Code request on startup to capture exact HTTP headers, then replays them for all proxied requests. Injects the billing header with per-request SHA256 fingerprint into the system prompt. Uses utls with Chrome TLS fingerprint to pass Cloudflare's bot detection on api.anthropic.com. Supports both streaming (SSE) and non-streaming modes, round-robin credential selection with automatic failover, and loading OAuth tokens from both cli-proxy-api auth files and native ~/.claude/.credentials.json.
2026-04-09 21:05:32 +02:00
commit c4c1d4daa4
17 changed files with 1417 additions and 0 deletions
@@ -0,0 +1,90 @@
+package proxy
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"encoding/json"
+	"fmt"
+	"unicode/utf16"
+
+	"github.com/tidwall/gjson"
+	"github.com/tidwall/sjson"
+)
+
+const fingerprintSalt = "59cf53e54c78"
+
+func computeFingerprint(firstUserMessage string, version string) string {
+	indices := []int{4, 7, 20}
+	runes := utf16.Encode([]rune(firstUserMessage))
+	var chars string
+	for _, i := range indices {
+		if i < len(runes) {
+			chars += string(rune(runes[i]))
+		} else {
+			chars += "0"
+		}
+	}
+	input := fingerprintSalt + chars + version
+	hash := sha256.Sum256([]byte(input))
+	return hex.EncodeToString(hash[:])[:3]
+}
+
+func extractFirstUserMessage(body []byte) string {
+	messages := gjson.GetBytes(body, "messages")
+	if !messages.Exists() || !messages.IsArray() {
+		return ""
+	}
+	for _, msg := range messages.Array() {
+		if msg.Get("role").String() != "user" {
+			continue
+		}
+		content := msg.Get("content")
+		if content.Type == gjson.String {
+			return content.String()
+		}
+		if content.IsArray() {
+			for _, block := range content.Array() {
+				if block.Get("type").String() == "text" {
+					return block.Get("text").String()
+				}
+			}
+		}
+		break
+	}
+	return ""
+}
+
+func buildBillingHeader(body []byte, version string) string {
+	userMsg := extractFirstUserMessage(body)
+	fp := computeFingerprint(userMsg, version)
+	return fmt.Sprintf("x-anthropic-billing-header: cc_version=%s.%s; cc_entrypoint=cli; cch=00000;", version, fp)
+}
+
+func injectBillingHeader(body []byte, version string) []byte {
+	header := buildBillingHeader(body, version)
+	billingBlock := map[string]interface{}{"type": "text", "text": header}
+	billingJSON, _ := json.Marshal(billingBlock)
+
+	existing := gjson.GetBytes(body, "system")
+	if !existing.Exists() {
+		body, _ = sjson.SetRawBytes(body, "system", []byte("["+string(billingJSON)+"]"))
+		return body
+	}
+
+	if existing.IsArray() {
+		items := make([]json.RawMessage, 0, len(existing.Array())+1)
+		items = append(items, billingJSON)
+		for _, item := range existing.Array() {
+			items = append(items, json.RawMessage(item.Raw))
+		}
+		systemJSON, _ := json.Marshal(items)
+		body, _ = sjson.SetRawBytes(body, "system", systemJSON)
+		return body
+	}
+
+	origText := existing.String()
+	origBlock := map[string]string{"type": "text", "text": origText}
+	origJSON, _ := json.Marshal(origBlock)
+	body, _ = sjson.SetRawBytes(body, "system", []byte("["+string(billingJSON)+","+string(origJSON)+"]"))
+	return body
+}