🧯 fix: Strip Assistant Prefill for Claude 5 and Later Models

src/llm/anthropic/llm.spec.ts

@@ -2807,6 +2807,22 @@ describe('Claude assistant prefill compatibility', () => {
     'global.anthropic.claude-opus-4-6-v1:0',
     'anthropic/claude-sonnet-4.6',
     'anthropic/claude-sonnet-4.12',
+    'claude-sonnet-4-6[1m]',
+    'claude-opus-5',
+    'claude-sonnet-5',
+    'claude-sonnet-5-5',
+    'claude-opus-5-5',
+    'claude-haiku-5-5',
+    'claude-haiku-5.5',
+    'claude-fable-5',
+    'claude-fable-5-1',
+    'claude-mythos-5',
+    'claude-opus-6',
+    'claude-opus-5-20260301',
+    'claude-sonnet-5-5@20260801',
+    'anthropic.claude-haiku-5-5',
+    'global.anthropic.claude-opus-5-5-v1:0',
+    'us.anthropic.claude-sonnet-5-v1:0',
   ])('detects %s as not supporting assistant prefill', (model) => {
     expect(modelDisallowsAssistantPrefill(model)).toBe(true);
   });
@@ -2815,29 +2831,41 @@ describe('Claude assistant prefill compatibility', () => {
     'claude-sonnet-4-5-20250929',
     'claude-opus-4-20250514',
     'anthropic.claude-opus-4-20250514-v1:0',
+    'claude-sonnet-4@20250514',
+    'claude-opus-4-1-20250805',
+    'claude-haiku-4-5',
+    'claude-haiku-4-5-20251001',
+    'claude-3-7-sonnet-20250219',
+    'claude-3-5-haiku-latest',
+    'anthropic.claude-3-haiku-20240307-v1:0',
     'gpt-5.4',
+    '',
+    undefined,
   ])('leaves %s prefill support unchanged', (model) => {
     expect(modelDisallowsAssistantPrefill(model)).toBe(false);
   });
 
-  test('strips trailing assistant messages for Claude 4.6+ requests', () => {
-    const request = {
-      model: 'claude-opus-4-6',
-      max_tokens: 100,
-      messages: [
-        { role: 'user' as const, content: 'What changed?' },
-        { role: 'assistant' as const, content: 'Draft prefill' },
-        { role: 'assistant' as const, content: 'Another prefill' },
-      ],
-    };
+  test.each(['claude-opus-4-6', 'claude-haiku-5-5', 'claude-fable-5-1'])(
+    'strips trailing assistant messages for %s requests',
+    (model) => {
+      const request = {
+        model,
+        max_tokens: 100,
+        messages: [
+          { role: 'user' as const, content: 'What changed?' },
+          { role: 'assistant' as const, content: 'Draft prefill' },
+          { role: 'assistant' as const, content: 'Another prefill' },
+        ],
+      };
 
-    const sanitized = stripUnsupportedAssistantPrefill(request);
+      const sanitized = stripUnsupportedAssistantPrefill(request);
 
-    expect(sanitized).not.toBe(request);
-    expect(sanitized.messages).toEqual([
-      { role: 'user', content: 'What changed?' },
-    ]);
-  });
+      expect(sanitized).not.toBe(request);
+      expect(sanitized.messages).toEqual([
+        { role: 'user', content: 'What changed?' },
+      ]);
+    }
+  );
 
   test('does not strip assistant messages for older Claude models', () => {
     const request = {

src/llm/anthropic/utils/message_inputs.ts

@@ -38,8 +38,8 @@
   cloneToolMessageWithContent,
   serializeStructuredValueBounded,
 } from '@/utils/toolContent';
-import { HARD_MAX_TOOL_RESULT_CHARS } from '@/utils/truncation';
 import { isReasoningContentBlock } from '@/messages/reasoningTypes';
+import { HARD_MAX_TOOL_RESULT_CHARS } from '@/utils/truncation';
 import { Constants } from '@/common';
 
 type StandardTextBlock = Data.StandardTextBlock;
@@ -56,10 +56,14 @@
 };
 
 const ANTHROPIC_EMPTY_TEXT_PLACEHOLDER = '_';
-const CLAUDE_4_RELEASE_DATE_MODEL_PATTERN =
-  /claude-(?:opus|sonnet|haiku)-4-\d{8}(?:[-.@]|$)/i;
-const CLAUDE_4_MINOR_MODEL_PATTERN =
-  /claude-(?:opus|sonnet|haiku)-4[-.](\d+)(?:[-.@]|$)/i;
+/**
+ * Name-first Claude ids (`claude-sonnet-4-6`, `claude-haiku-5.5`, Bedrock and
+ * Vertex forms included). The minor is one or two digits so an 8-digit release
+ * date (`claude-opus-4-20250514`) reads as major only. Number-first ids
+ * (`claude-3-5-sonnet`) predate 4.6 and never match.
+ */
+const CLAUDE_FAMILY_VERSION_PATTERN =
+  /claude-(?:opus|sonnet|haiku|fable|mythos)-(\d+)(?:[-.](\d{1,2}))?(?!\d)/i;
 
 function _formatImage(imageUrl: string) {
   const parsed = parseBase64DataUrl({ dataUrl: imageUrl });
@@ -164,7 +168,7 @@
   }
   let cacheControl: unknown;
   const stripped = content.map((block) => {
     if (typeof block !== 'object' || block == null || isProxy(block)) {
       return block;
     }
     try {
@@ -207,7 +211,7 @@
   }
 
   const block = content[0];
   if (typeof block !== 'object' || block == null || isProxy(block)) {
     return { content, cacheControl: undefined, isError: undefined };
   }
 
@@ -1047,10 +1051,7 @@
           name: correspondingToolCall.name,
           input: functionCallPart.functionCall.args,
         };
-      } else if (
-        isAIMessage(message) &&
-        isReasoningContentBlock(contentPart)
-      ) {
+      } else if (isAIMessage(message) && isReasoningContentBlock(contentPart)) {
         // Foreign reasoning on an ASSISTANT turn (Bedrock `reasoning_content`,
         // Google `reasoning`, LibreChat `think`) carries provider-specific
         // signatures Anthropic cannot validate; drop it so a cross-provider
@@ -1217,17 +1218,16 @@
   } as AnthropicMessageCreateParams;
 }
 
+/** Claude 4.6 and every later release (5.x, Fable, Mythos) reject a trailing assistant prefill. */
 export function modelDisallowsAssistantPrefill(model?: string): boolean {
-  const modelId = model ?? '';
-  if (CLAUDE_4_RELEASE_DATE_MODEL_PATTERN.test(modelId)) {
-    return false;
-  }
-
-  const match = CLAUDE_4_MINOR_MODEL_PATTERN.exec(modelId);
+  const match = CLAUDE_FAMILY_VERSION_PATTERN.exec(model ?? '');
   if (!match) {
     return false;
   }
-  return Number(match[1]) >= 6;
+  const [, majorText, minorText = '0'] = match;
+  const major = Number(majorText);
+  const minor = Number(minorText);
+  return major > 4 || (major === 4 && minor >= 6);
 }
 
 function messagesHaveCacheControl(