feat(translator): lossless passthrough via CLI tool + provider pairing

Add clientDetector utility to identify CLI tools (Claude Code, Gemini CLI,
Antigravity, Codex) from request headers. When the CLI tool and provider
are a native pair, skip all translation — only swap model and Bearer token.

Made-with: Cursor

open-sse/handlers/chatCore.js

@@ -15,6 +15,7 @@ import { buildRequestDetail, extractRequestConfig } from "./chatCore/requestDeta
 import { handleForcedSSEToJson } from "./chatCore/sseToJsonHandler.js";
 import { handleNonStreamingResponse } from "./chatCore/nonStreamingHandler.js";
 import { handleStreamingResponse, buildOnStreamComplete } from "./chatCore/streamingHandler.js";
+import { detectClientTool, isNativePassthrough } from "../utils/clientDetector.js";
 
 /**
  * Core chat handler - shared between SSE and Worker
@@ -56,14 +57,26 @@ export async function handleChatCore({ body, modelInfo, credentials, log, onCred
   reqLogger.logRawRequest(body);
   log?.debug?.("FORMAT", `${sourceFormat} → ${targetFormat} | stream=${stream}`);
 
-  let translatedBody = translateRequest(sourceFormat, targetFormat, model, body, stream, credentials, provider, reqLogger, modelCaps);
-  if (!translatedBody) {
-    trackPendingRequest(model, provider, connectionId, false, true);
-    return createErrorResult(HTTP_STATUS.BAD_REQUEST, `Failed to translate request for ${sourceFormat} → ${targetFormat}`);
+  // Native passthrough: CLI tool and provider are the same ecosystem
+  // Skip all translation/normalization — only model and Bearer are swapped
+  const clientTool = detectClientTool(clientRawRequest?.headers || {}, body);
+  const passthrough = isNativePassthrough(clientTool, provider);
+
+  let translatedBody;
+  let toolNameMap;
+  if (passthrough) {
+    log?.debug?.("PASSTHROUGH", `${clientTool} → ${provider} | native lossless`);
+    translatedBody = { ...body, model };
+  } else {
+    translatedBody = translateRequest(sourceFormat, targetFormat, model, body, stream, credentials, provider, reqLogger, modelCaps);
+    if (!translatedBody) {
+      trackPendingRequest(model, provider, connectionId, false, true);
+      return createErrorResult(HTTP_STATUS.BAD_REQUEST, `Failed to translate request for ${sourceFormat} → ${targetFormat}`);
+    }
+    toolNameMap = translatedBody._toolNameMap;
+    delete translatedBody._toolNameMap;
+    translatedBody.model = model;
   }
-  const toolNameMap = translatedBody._toolNameMap;
-  delete translatedBody._toolNameMap;
-  translatedBody.model = model;
 
   const executor = getExecutor(provider);
   trackPendingRequest(model, provider, connectionId, true);

open-sse/utils/clientDetector.js

@@ -0,0 +1,53 @@
+/**
+ * Detect CLI tool identity from request headers/body.
+ * Used to determine if a request can be passed through losslessly.
+ */
+
+// Map of CLI tool identifiers to provider IDs they are "native" to
+const NATIVE_PAIRS = {
+  "claude":      ["claude", "anthropic"],
+  "gemini-cli":  ["gemini-cli"],
+  "antigravity": ["antigravity"],
+  "codex":       ["codex"],
+};
+
+/**
+ * Detect which CLI tool is making the request.
+ * Returns one of: "claude" | "gemini-cli" | "antigravity" | "codex" | null
+ * @param {object} headers - Lowercase header key/value object
+ * @param {object} body    - Parsed request body
+ */
+export function detectClientTool(headers = {}, body = {}) {
+  const ua = (headers["user-agent"] || "").toLowerCase();
+  const xApp = (headers["x-app"] || "").toLowerCase();
+
+  // Antigravity: detected via body field (not header)
+  if (body.userAgent === "antigravity") return "antigravity";
+
+  // Claude Code / Claude CLI
+  if (ua.includes("claude-cli") || ua.includes("claude-code") || xApp === "cli") return "claude";
+
+  // Gemini CLI
+  if (ua.includes("gemini-cli")) return "gemini-cli";
+
+  // Codex CLI
+  if (ua.includes("codex-cli")) return "codex";
+
+  return null;
+}
+
+/**
+ * Check if this CLI tool + provider pair should be passed through losslessly.
+ * @param {string|null} clientTool - Result of detectClientTool()
+ * @param {string} provider        - Provider ID (e.g. "claude", "gemini-cli")
+ */
+export function isNativePassthrough(clientTool, provider) {
+  if (!clientTool) return false;
+  const nativeProviders = NATIVE_PAIRS[clientTool];
+  if (!nativeProviders) return false;
+  // Support anthropic-compatible-* variants
+  const normalizedProvider = provider.startsWith("anthropic-compatible")
+    ? "anthropic"
+    : provider;
+  return nativeProviders.includes(normalizedProvider);
+}