fix(claude): forced tool_choice 400 on cc/ OAuth route

convertOpenAIToolChoice mapped {type:"function"} verbatim and cloakClaudeTools
left tool_choice.name unsuffixed, both rejected by Claude on the cc/ path.
Map forced-function to {type:"tool",name}, allowlist Claude-valid types, and
suffix tool_choice.name when it targets a renamed client tool.

Fixes #1592

Co-authored-by: Cursor <cursoragent@cursor.com>

open-sse/translator/request/openai-to-claude.js

@@ -291,15 +291,35 @@ function getContentBlocksFromMessage(msg, toolNameMap = new Map()) {
   return blocks;
 }
 
-// Convert OpenAI tool choice to Claude format
+// Convert OpenAI tool choice to Claude format.
+// Claude only accepts tool_choice.type of "auto" | "any" | "tool" | "none";
+// anything else (e.g. OpenAI's "function") triggers a 400, so we never pass an
+// unrecognized type through.
+const CLAUDE_TOOL_CHOICE_TYPES = new Set(["auto", "any", "tool", "none"]);
+
 function convertOpenAIToolChoice(choice) {
   if (!choice) return { type: "auto" };
-  if (typeof choice === "object" && choice.type) return choice;
-  if (choice === "auto" || choice === "none") return { type: "auto" };
-  if (choice === "required") return { type: "any" };
-  if (typeof choice === "object" && choice.function) {
-    return { type: "tool", name: choice.function.name };
+
+  // OpenAI string forms: "auto" | "none" | "required"
+  if (typeof choice === "string") {
+    if (choice === "required") return { type: "any" };
+    return { type: "auto" }; // "auto", "none", or anything unexpected
   }
+
+  if (typeof choice === "object") {
+    // OpenAI forced tool: { type: "function", function: { name } }.
+    // Checked before the native pass-through below, because the OpenAI shape
+    // also carries a `.type` ("function") that Claude rejects.
+    if (choice.function?.name) {
+      return { type: "tool", name: choice.function.name };
+    }
+    // Already Claude-native — only pass through types Claude actually accepts,
+    // so a malformed or unknown type can never leak into the upstream request.
+    if (CLAUDE_TOOL_CHOICE_TYPES.has(choice.type)) {
+      return choice;
+    }
+  }
+
   return { type: "auto" };
 }
 

open-sse/utils/claudeCloaking.js

@@ -35,13 +35,16 @@ export function cloakClaudeTools(body) {
   const tools = body.tools;
   if (!tools || tools.length === 0) return { body, toolNameMap: null };
 
+  const suffix = (name) => `${name}${CLAUDE_TOOL_SUFFIX}`;
   const toolNameMap = new Map();
+  const clientToolNames = new Set();
   const clientDeclarations = [];
 
   // All client tools get renamed with suffix
   for (const tool of tools) {
-    const suffixed = `${tool.name}${CLAUDE_TOOL_SUFFIX}`;
+    const suffixed = suffix(tool.name);
     toolNameMap.set(suffixed, tool.name);
+    clientToolNames.add(tool.name);
     clientDeclarations.push({ ...tool, name: suffixed });
   }
 
@@ -51,17 +54,27 @@ export function cloakClaudeTools(body) {
   // Rename tool_use in message history (all client tools get suffix)
   const renamedMessages = body.messages?.map(msg => {
     if (!Array.isArray(msg.content)) return msg;
-    const renamedContent = msg.content.map(block => {
-      if (block.type === "tool_use") {
-        return { ...block, name: `${block.name}${CLAUDE_TOOL_SUFFIX}` };
-      }
-      return block;
-    });
+    const renamedContent = msg.content.map(block =>
+      block.type === "tool_use" ? { ...block, name: suffix(block.name) } : block
+    );
     return { ...msg, content: renamedContent };
   });
 
+  const cloakedBody = { ...body, tools: allTools, messages: renamedMessages || body.messages };
+
+  // A forced tool_choice ({ type: "tool", name }) must point at the suffixed
+  // tool name, otherwise Claude rejects it: "Tool '<name>' not found in provided tools".
+  // Only rewrite when the choice targets one of the client tools we actually
+  // renamed — never a decoy/built-in name (those are sent unsuffixed).
+  if (
+    body.tool_choice?.type === "tool" &&
+    clientToolNames.has(body.tool_choice.name)
+  ) {
+    cloakedBody.tool_choice = { ...body.tool_choice, name: suffix(body.tool_choice.name) };
+  }
+
   return {
-    body: { ...body, tools: allTools, messages: renamedMessages || body.messages },
+    body: cloakedBody,
     toolNameMap: toolNameMap.size > 0 ? toolNameMap : null
   };
 }