From 9ff021e42b4739b2721ec4f1bd28a0a526686255 Mon Sep 17 00:00:00 2001
From: Tam Nhu Tran <kaitran.ntt@gmail.com>
Date: Mon, 16 Mar 2026 14:03:51 -0400
Subject: [PATCH] fix(cursor): harden anthropic translation edge cases

---
 src/cursor/cursor-anthropic-translator.ts     | 22 +++++-
 .../cursor-anthropic-translator.test.ts       | 78 +++++++++++++++++++
 2 files changed, 97 insertions(+), 3 deletions(-)

diff --git a/src/cursor/cursor-anthropic-translator.ts b/src/cursor/cursor-anthropic-translator.ts
index 30c8db5b..ce245bb7 100644
--- a/src/cursor/cursor-anthropic-translator.ts
+++ b/src/cursor/cursor-anthropic-translator.ts
@@ -13,6 +13,9 @@ export interface TranslatedAnthropicRequest {
   messages: CursorOpenAIMessage[];
 }
 
+const TOOL_RESULT_SERIALIZATION_FALLBACK = '[unserializable content]';
+const TOOL_USE_ARGUMENTS_FALLBACK = '{}';
+
 function assertObject(value: unknown, label: string): Record<string, unknown> {
   if (typeof value !== 'object' || value === null) {
     throw new Error(`${label} must be an object`);
@@ -20,6 +23,19 @@ function assertObject(value: unknown, label: string): Record<string, unknown> {
   return value as Record<string, unknown>;
 }
 
+function safeJsonStringify(value: unknown, fallback: string): string {
+  try {
+    const serialized = JSON.stringify(value);
+    return typeof serialized === 'string' ? serialized : fallback;
+  } catch {
+    return fallback;
+  }
+}
+
+function createFallbackToolId(messageIndex: number, blockIndex: number): string {
+  return `toolu_ccs_fallback_${messageIndex}_${blockIndex}`;
+}
+
 function flattenTextContent(content: unknown, label: string): string {
   if (typeof content === 'string') {
     return content;
@@ -49,7 +65,7 @@ function toToolResultContent(content: unknown, label: string): string {
   if (Array.isArray(content)) {
     return flattenTextContent(content, label);
   }
-  return JSON.stringify(content);
+  return safeJsonStringify(content, TOOL_RESULT_SERIALIZATION_FALLBACK);
 }
 
 function mapThinkingToReasoningEffort(
@@ -125,11 +141,11 @@ export function translateAnthropicRequest(raw: unknown): TranslatedAnthropicRequ
           id:
             typeof parsed.id === 'string' && parsed.id.length > 0
               ? parsed.id
-              : `toolu_${messageIndex}_${blockIndex}`,
+              : createFallbackToolId(messageIndex, blockIndex),
           type: 'function',
           function: {
             name: typeof parsed.name === 'string' ? parsed.name : 'tool',
-            arguments: JSON.stringify(parsed.input ?? {}),
+            arguments: safeJsonStringify(parsed.input ?? {}, TOOL_USE_ARGUMENTS_FALLBACK),
           },
         });
         return;
diff --git a/tests/unit/cursor/cursor-anthropic-translator.test.ts b/tests/unit/cursor/cursor-anthropic-translator.test.ts
index 8ae0e619..63d91201 100644
--- a/tests/unit/cursor/cursor-anthropic-translator.test.ts
+++ b/tests/unit/cursor/cursor-anthropic-translator.test.ts
@@ -89,6 +89,67 @@ describe('translateAnthropicRequest', () => {
       { role: 'tool', tool_call_id: 'toolu_1', content: 'done' },
     ]);
   });
+
+  it('uses a distinct fallback prefix for missing tool_use ids', () => {
+    const translated = translateAnthropicRequest({
+      messages: [
+        {
+          role: 'assistant',
+          content: [{ type: 'tool_use', name: 'search', input: { q: 'x' } }],
+        },
+      ],
+    });
+
+    expect(translated.messages[0]?.tool_calls?.[0]?.id).toBe('toolu_ccs_fallback_0_0');
+  });
+
+  it('falls back when tool_result content cannot be serialized', () => {
+    const circular: Record<string, unknown> = {};
+    circular.self = circular;
+
+    const translated = translateAnthropicRequest({
+      messages: [
+        {
+          role: 'user',
+          content: [{ type: 'tool_result', tool_use_id: 'toolu_1', content: circular }],
+        },
+      ],
+    });
+
+    expect(translated.messages).toEqual([
+      {
+        role: 'tool',
+        tool_call_id: 'toolu_1',
+        content: '[unserializable content]',
+      },
+    ]);
+  });
+
+  it('falls back when tool_use input cannot be serialized', () => {
+    const circular: Record<string, unknown> = {};
+    circular.self = circular;
+
+    const translated = translateAnthropicRequest({
+      messages: [
+        {
+          role: 'assistant',
+          content: [{ type: 'tool_use', name: 'search', input: circular }],
+        },
+      ],
+    });
+
+    expect(translated.messages[0]).toEqual({
+      role: 'assistant',
+      content: '',
+      tool_calls: [
+        {
+          id: 'toolu_ccs_fallback_0_0',
+          type: 'function',
+          function: { name: 'search', arguments: '{}' },
+        },
+      ],
+    });
+  });
 });
 
 describe('createAnthropicProxyResponse', () => {
@@ -166,4 +227,21 @@ describe('createAnthropicProxyResponse', () => {
     expect(body).toContain('"type":"text_delta"');
     expect(body).toContain('event: message_stop');
   });
+
+  it('emits Anthropic-style error events when SSE translation fails', async () => {
+    const oversizedChunk = `data: ${'x'.repeat(1024 * 1024 + 32)}`;
+
+    const transformed = await createAnthropicProxyResponse(
+      new Response(oversizedChunk, {
+        status: 200,
+        headers: { 'Content-Type': 'text/event-stream' },
+      })
+    );
+
+    const body = await transformed.text();
+    expect(body).toContain('event: error');
+    expect(body).toContain('"type":"error"');
+    expect(body).toContain('"error":{"type":"api_error"');
+    expect(body).toContain('Failed to translate Cursor SSE response');
+  });
 });