fix: drop assistant prefill for cc bridge

open-sse/services/claudeCodeCompatible.ts

@@ -270,6 +270,31 @@ function buildClaudeCodeCompatibleMessages(messages: MessageLike[]) {
     merged.push({ role: message.role, content: [...message.content] });
   }
 
+  // CC-compatible sites we tested reject assistant-prefill shaped requests even
+  // when Anthropic would normally allow them. Keep assistant/model history, but
+  // drop trailing assistant turns so the upstream request ends on a user turn.
+  while (merged.length > 0 && merged[merged.length - 1].role === "assistant") {
+    merged.pop();
+  }
+
+  if (merged.length === 0) {
+    const fallbackText = converted
+      .flatMap((message) => message.content)
+      .map((block) => toNonEmptyString(block.text))
+      .filter(Boolean)
+      .join("\n")
+      .trim();
+
+    if (fallbackText) {
+      return [
+        {
+          role: "user" as const,
+          content: [{ type: "text", text: fallbackText, cache_control: { type: "ephemeral" } }],
+        },
+      ];
+    }
+  }
+
   for (let i = merged.length - 1; i >= 0; i--) {
     if (merged[i].role !== "user") continue;
     const lastBlock = merged[i].content[merged[i].content.length - 1];

tests/unit/cc-compatible-provider.test.mjs

@@ -51,7 +51,7 @@ test.after(() => {
   fs.rmSync(TEST_DATA_DIR, { recursive: true, force: true });
 });
 
-test("buildClaudeCodeCompatibleRequest keeps order/text while mapping unsupported roles", () => {
+test("buildClaudeCodeCompatibleRequest keeps prior role history while dropping trailing assistant prefill", () => {
   const payload = buildClaudeCodeCompatibleRequest({
     sourceBody: {
       reasoning_effort: "xhigh",
@@ -79,6 +79,7 @@ test("buildClaudeCodeCompatibleRequest keeps order/text while mapping unsupporte
         { role: "user", content: [{ type: "text", text: "u1" }, { type: "image_url" }] },
         { role: "model", content: "a1" },
         { role: "user", content: [{ type: "text", text: "u2" }, { type: "tool_result" }] },
+        { role: "model", content: "prefill" },
       ],
     },
     model: "claude-sonnet-4-6",
@@ -120,6 +121,26 @@ test("buildClaudeCodeCompatibleRequest keeps order/text while mapping unsupporte
   assert.equal(JSON.parse(payload.metadata.user_id).session_id, "session-1");
 });
 
+test("buildClaudeCodeCompatibleRequest falls back to a user turn when the source only has assistant/model text", () => {
+  const payload = buildClaudeCodeCompatibleRequest({
+    sourceBody: {
+      messages: [{ role: "model", content: "draft" }],
+    },
+    normalizedBody: {
+      messages: [{ role: "model", content: "draft" }],
+    },
+    model: "claude-sonnet-4-6",
+    sessionId: "session-only-assistant",
+  });
+
+  assert.deepEqual(payload.messages, [
+    {
+      role: "user",
+      content: [{ type: "text", text: "draft", cache_control: { type: "ephemeral" } }],
+    },
+  ]);
+});
+
 test("buildClaudeCodeCompatibleRequest honors token priority fields", () => {
   const payload = buildClaudeCodeCompatibleRequest({
     sourceBody: { max_completion_tokens: 321 },