diff --git a/open-sse/executors/cursor.js b/open-sse/executors/cursor.js
index 2873782..0c92b6e 100644
--- a/open-sse/executors/cursor.js
+++ b/open-sse/executors/cursor.js
@@ -132,7 +132,10 @@ export class CursorExecutor extends BaseExecutor {
     const messages = body.messages || [];
     const tools = body.tools || [];
     const reasoningEffort = body.reasoning_effort || null;
-    return generateCursorBody(messages, model, tools, reasoningEffort);
+    // Detect Claude Code UA to force Agent mode (issue #643)
+    const ua = credentials?.rawHeaders?.["user-agent"] || "";
+    const forceAgentMode = ua.includes("claude-cli") || ua.includes("claude-code") || ua.includes("Claude Code");
+    return generateCursorBody(messages, model, tools, reasoningEffort, forceAgentMode);
   }
 
   async makeFetchRequest(url, headers, body, signal, proxyOptions = null) {
diff --git a/open-sse/translator/request/openai-to-cursor.js b/open-sse/translator/request/openai-to-cursor.js
index a36f25c..1e3aa9c 100644
--- a/open-sse/translator/request/openai-to-cursor.js
+++ b/open-sse/translator/request/openai-to-cursor.js
@@ -169,8 +169,10 @@ function convertMessages(messages) {
 
 export function buildCursorRequest(model, body, stream, credentials) {
   const messages = convertMessages(body.messages || []);
+
   // Strip fields irrelevant to Cursor (OpenAI/Anthropic-specific)
   const { user, metadata, tool_choice, stream_options, system, ...rest } = body;
+
   return {
     ...rest,
     messages,
diff --git a/open-sse/utils/cursorProtobuf.js b/open-sse/utils/cursorProtobuf.js
index e6f7f8b..c870921 100644
--- a/open-sse/utils/cursorProtobuf.js
+++ b/open-sse/utils/cursorProtobuf.js
@@ -447,9 +447,9 @@ export function encodeMcpTool(tool) {
 
 // ==================== REQUEST BUILDING ====================
 
-export function encodeRequest(messages, modelName, tools = [], reasoningEffort = null) {
+export function encodeRequest(messages, modelName, tools = [], reasoningEffort = null, forceAgentMode = false) {
   const hasTools = tools?.length > 0;
-  const isAgentic = hasTools;
+  const isAgentic = hasTools || forceAgentMode;
   const formattedMessages = [];
   const messageIds = [];
   const normalizedMessages = [];
@@ -583,8 +583,8 @@ export function encodeRequest(messages, modelName, tools = [], reasoningEffort =
   );
 }
 
-export function buildChatRequest(messages, modelName, tools = [], reasoningEffort = null) {
-  return encodeField(FIELD.REQUEST, WIRE_TYPE.LEN, encodeRequest(messages, modelName, tools, reasoningEffort));
+export function buildChatRequest(messages, modelName, tools = [], reasoningEffort = null, forceAgentMode = false) {
+  return encodeField(FIELD.REQUEST, WIRE_TYPE.LEN, encodeRequest(messages, modelName, tools, reasoningEffort, forceAgentMode));
 }
 
 /**
@@ -646,10 +646,10 @@ export function wrapConnectRPCFrame(payload, compress = false) {
   return frame;
 }
 
-export function generateCursorBody(messages, modelName, tools = [], reasoningEffort = null) {
-  log("BODY", `Generating: ${messages.length} msgs, model=${modelName}, tools=${tools.length}, reasoning=${reasoningEffort || "none"}`);
+export function generateCursorBody(messages, modelName, tools = [], reasoningEffort = null, forceAgentMode = false) {
+  log("BODY", `Generating: ${messages.length} msgs, model=${modelName}, tools=${tools.length}, reasoning=${reasoningEffort || "none"}, forceAgentMode=${forceAgentMode}`);
   
-  const protobuf = buildChatRequest(messages, modelName, tools, reasoningEffort);
+  const protobuf = buildChatRequest(messages, modelName, tools, reasoningEffort, forceAgentMode);
   const framed = wrapConnectRPCFrame(protobuf, false); // Cursor doesn't support compressed requests
   
   log("BODY", `Protobuf=${protobuf.length}B, Framed=${framed.length}B`);