fix: strip trailing assistant prefill

voidsteed · voidsteed · commit 05282717fd61 · 2026-05-29T10:54:18.000-07:00
diff --git a/src/routes/messages/non-stream-translation.ts b/src/routes/messages/non-stream-translation.ts
@@ -1,3 +1,5 @@
+import consola from "consola"
+
 import { state } from "~/lib/state"
 import {
   type ChatCompletionResponse,
@@ -30,12 +32,11 @@ import { mapOpenAIStopReasonToAnthropic } from "./utils"
 export function translateToOpenAI(
   payload: AnthropicMessagesPayload,
 ): ChatCompletionsPayload {
+  const messages = stripTrailingAssistantPrefill(payload.messages)
+
   return {
     model: translateModelName(payload.model),
-    messages: translateAnthropicMessagesToOpenAI(
-      payload.messages,
-      payload.system,
-    ),
+    messages: translateAnthropicMessagesToOpenAI(messages, payload.system),
     max_tokens: payload.max_tokens,
     stop: payload.stop_sequences,
     stream: payload.stream,
@@ -48,6 +49,24 @@ export function translateToOpenAI(
   }
 }
 
+function stripTrailingAssistantPrefill(
+  messages: Array<AnthropicMessage>,
+): Array<AnthropicMessage> {
+  let end = messages.length
+  while (end > 0 && messages[end - 1].role === "assistant") {
+    end--
+  }
+
+  if (end === messages.length) {
+    return messages
+  }
+
+  consola.warn(
+    `Stripped ${messages.length - end} trailing assistant prefill message(s) unsupported by Copilot chat completions`,
+  )
+  return messages.slice(0, end)
+}
+
 /**
  * Pre-process the Anthropic payload to handle async / proxy-side concerns
  * (currently: PDF document block extraction). Returns a new payload with
diff --git a/tests/anthropic-request.test.ts b/tests/anthropic-request.test.ts
@@ -84,6 +84,7 @@ describe("Anthropic to OpenAI translation logic", () => {
           role: "assistant",
           content: "The weather in Boston is sunny and 75&deg;F.",
         },
+        { role: "user", content: "Thanks." },
       ],
       temperature: 0.7,
       max_tokens: 150,
@@ -144,6 +145,7 @@ describe("Anthropic to OpenAI translation logic", () => {
             { type: "text", text: "2+2 equals 4." },
           ],
         },
+        { role: "user", content: "Thanks." },
       ],
       max_tokens: 100,
     }
@@ -183,6 +185,16 @@ describe("Anthropic to OpenAI translation logic", () => {
             },
           ],
         },
+        {
+          role: "user",
+          content: [
+            {
+              type: "tool_result",
+              tool_use_id: "call_123",
+              content: "72&deg;F and sunny",
+            },
+          ],
+        },
       ],
       max_tokens: 100,
     }
diff --git a/tests/translation.test.ts b/tests/translation.test.ts
@@ -332,6 +332,10 @@ describe("translateToOpenAI (request side)", () => {
             { type: "text", text: "4" },
           ],
         },
+        {
+          role: "user",
+          content: "thanks",
+        },
       ],
     }
     const result = translateToOpenAI(payload)
@@ -366,6 +370,116 @@ describe("translateToOpenAI (request side)", () => {
       },
     ])
   })
+
+  test("strips a single trailing assistant prefill", () => {
+    const payload: AnthropicMessagesPayload = {
+      model: "claude-opus-4-5",
+      max_tokens: 100,
+      messages: [
+        { role: "user", content: "Write a greeting" },
+        { role: "assistant", content: "Hello" },
+      ],
+    }
+
+    const result = translateToOpenAI(payload)
+
+    expect(result.messages).toEqual([
+      { role: "user", content: "Write a greeting" },
+    ])
+  })
+
+  test("strips multiple trailing assistant prefill turns", () => {
+    const payload: AnthropicMessagesPayload = {
+      model: "claude-opus-4-5",
+      max_tokens: 100,
+      messages: [
+        { role: "user", content: "Summarize" },
+        { role: "assistant", content: "Draft one" },
+        { role: "assistant", content: "Draft two" },
+      ],
+    }
+
+    const result = translateToOpenAI(payload)
+
+    expect(result.messages).toEqual([{ role: "user", content: "Summarize" }])
+  })
+
+  test("strips trailing assistant tool-call prefill", () => {
+    const payload: AnthropicMessagesPayload = {
+      model: "claude-opus-4-5",
+      max_tokens: 100,
+      messages: [
+        { role: "user", content: "Read a file" },
+        {
+          role: "assistant",
+          content: [
+            {
+              type: "tool_use",
+              id: "toolu_123",
+              name: "Read",
+              input: { file_path: "README.md" },
+            },
+          ],
+        },
+      ],
+    }
+
+    const result = translateToOpenAI(payload)
+
+    expect(result.messages).toEqual([{ role: "user", content: "Read a file" }])
+  })
+
+  test("keeps assistant tool call when followed by tool result", () => {
+    const payload: AnthropicMessagesPayload = {
+      model: "claude-opus-4-5",
+      max_tokens: 100,
+      messages: [
+        { role: "user", content: "Read a file" },
+        {
+          role: "assistant",
+          content: [
+            {
+              type: "tool_use",
+              id: "toolu_123",
+              name: "Read",
+              input: { file_path: "README.md" },
+            },
+          ],
+        },
+        {
+          role: "user",
+          content: [
+            {
+              type: "tool_result",
+              tool_use_id: "toolu_123",
+              content: "# Project",
+            },
+          ],
+        },
+      ],
+    }
+
+    const result = translateToOpenAI(payload)
+
+    expect(result.messages).toEqual([
+      { role: "user", content: "Read a file" },
+      {
+        role: "assistant",
+        content: null,
+        tool_calls: [
+          {
+            id: "toolu_123",
+            type: "function",
+            function: {
+              name: "Read",
+              arguments: JSON.stringify({ file_path: "README.md" }),
+            },
+          },
+        ],
+      },
+      { role: "tool", tool_call_id: "toolu_123", content: "# Project" },
+    ])
+  })
 })
 
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