fix(claude): preserve intentional prefill from non-messages[] source formats

translateRequest detected the client's terminal role only from messages[],
so a Gemini contents[] or Responses input[] body ending on an explicit
model/assistant turn (real prefill) read as undefined and got the
"Continue." restoration appended, clobbering the prefill. Map the trailing
role per source shape; only model/assistant counts as prefill — every other
tail stays undefined so the emptied-turn fix still applies.

Co-Authored-By: Claude Code <noreply@anthropic.com>

open-sse/translator/index.js

@@ -9,6 +9,7 @@ import { applyThinking, captureThinking } from "./concerns/thinkingUnified.js";
 import { captureSessionId } from "../utils/sessionManager.js";
 import { AntigravityExecutor } from "../executors/antigravity.js";
 import { PROVIDERS } from "../providers/index.js";
+import { ROLE, GEMINI_ROLE } from "./schema/roles.js";
 
 // Registry for translators. Lazy-init guards against circular-import order:
 // translator modules call register() (side-effect) before this module's body runs.
@@ -49,12 +50,26 @@ function stripContentTypes(body, stripList = []) {
   }
 }
 
+// Role the client's conversation actually ended on, in the source format's own
+// shape — not every source uses messages[] (Gemini/Antigravity: contents[],
+// Responses/Codex: input[]). Only an explicit trailing model/assistant turn is
+// real prefill and must reach ensureTrailingUserTurn as ROLE.ASSISTANT; every
+// other tail (including no role, e.g. a function output) stays undefined so
+// the emptied-turn fix still applies.
+function detectClientLastRole(body) {
+  if (Array.isArray(body?.messages)) return body.messages[body.messages.length - 1]?.role;
+  const items = Array.isArray(body?.contents) ? body.contents : Array.isArray(body?.input) ? body.input : null;
+  if (!items) return undefined;
+  const role = items[items.length - 1]?.role;
+  return role === ROLE.ASSISTANT || role === GEMINI_ROLE.MODEL ? ROLE.ASSISTANT : undefined;
+}
+
 // Translate request: source -> openai -> target
 export function translateRequest(sourceFormat, targetFormat, model, body, stream = true, credentials = null, provider = null, reqLogger = null, stripList = [], connectionId = null, clientTool = null) {
   ensureInitialized();
   let result = body;
   // Role the client actually ended on, before any translator drops an emptied turn.
-  const clientLastRole = Array.isArray(body?.messages) ? body.messages[body.messages.length - 1]?.role : undefined;
+  const clientLastRole = detectClientLastRole(body);
 
   // Strip explicit content types (opt-in via strip[] in PROVIDER_MODELS entry)
   stripContentTypes(result, stripList);

tests/unit/claude-trailing-user-turn-source-formats.test.js

@@ -0,0 +1,42 @@
+// Non-messages[] sources (Gemini contents[], Responses input[]) carry the client's
+// terminal role in their own shape. An explicit trailing model/assistant turn is real
+// prefill and must survive translation to Claude; an emptied trailing user turn must
+// still get the "Continue." restoration.
+import { describe, it, expect } from "vitest";
+import { translateRequest } from "../../open-sse/translator/index.js";
+
+const roles = (body) => body.messages.map((m) => m.role);
+
+describe("trailing user turn: non-messages[] source formats", () => {
+  it("keeps a Gemini trailing model turn (real prefill)", () => {
+    const out = translateRequest("gemini", "claude", "claude-sonnet-4-5", {
+      contents: [
+        { role: "user", parts: [{ text: "hi" }] },
+        { role: "model", parts: [{ text: "The answer is" }] },
+      ],
+    }, false);
+    expect(roles(out)).toEqual(["user", "assistant"]);
+  });
+
+  it("keeps a Responses trailing assistant message (real prefill)", () => {
+    const out = translateRequest("openai-responses", "claude", "claude-sonnet-4-5", {
+      model: "claude-sonnet-4-5",
+      input: [
+        { role: "user", content: "hi" },
+        { role: "assistant", content: "The answer is" },
+      ],
+    }, false);
+    expect(roles(out)).toEqual(["user", "assistant"]);
+  });
+
+  it("still restores a user turn when a Gemini trailing user turn is emptied", () => {
+    const out = translateRequest("gemini", "claude", "claude-sonnet-4-5", {
+      contents: [
+        { role: "user", parts: [{ text: "hi" }] },
+        { role: "model", parts: [{ text: "hello" }] },
+        { role: "user", parts: [] },
+      ],
+    }, false);
+    expect(roles(out)).toEqual(["user", "assistant", "user"]);
+  });
+});