fix: strip functionCall/functionResponse id and synthetic thoughtSignature for Vertex AI (closes #388) (#414)

open-sse/translator/request/openai-to-vertex.js

@@ -0,0 +1,50 @@
+import { register } from "../index.js";
+import { FORMATS } from "../formats.js";
+import { openaiToGeminiRequest } from "./openai-to-gemini.js";
+
+/**
+ * Post-process a Gemini-format body for Vertex AI compatibility:
+ *
+ * 1. Strip `id` from every `functionCall` and `functionResponse` part.
+ *    Vertex AI rejects requests that include these fields.
+ *
+ * 2. Strip synthetic `thoughtSignature` parts injected by the base translator.
+ *    Vertex rejects fake thought signatures in multi-turn tool-call history;
+ *    only real signatures emitted by Vertex itself should be replayed.
+ */
+function stripVertexIncompatibleFields(body) {
+  if (!body?.contents) return body;
+
+  for (const turn of body.contents) {
+    if (!Array.isArray(turn.parts)) continue;
+
+    // Remove standalone synthetic thoughtSignature parts (text === "" with thoughtSignature)
+    turn.parts = turn.parts.filter(
+      p => !(p.thoughtSignature !== undefined && p.text === "" && !p.thought)
+    );
+
+    for (const part of turn.parts) {
+      // Strip id from functionCall
+      if (part.functionCall && "id" in part.functionCall) {
+        delete part.functionCall.id;
+      }
+      // Strip id from functionResponse
+      if (part.functionResponse && "id" in part.functionResponse) {
+        delete part.functionResponse.id;
+      }
+      // Strip thoughtSignature injected alongside functionCall (synthetic signature)
+      if (part.functionCall && "thoughtSignature" in part) {
+        delete part.thoughtSignature;
+      }
+    }
+  }
+
+  return body;
+}
+
+export function openaiToVertexRequest(model, body, stream, credentials) {
+  const gemini = openaiToGeminiRequest(model, body, stream, credentials);
+  return stripVertexIncompatibleFields(gemini);
+}
+
+register(FORMATS.OPENAI, FORMATS.VERTEX, openaiToVertexRequest, null);