fix: decode Composer cursor thinking output (#1310)

open-sse/executors/cursor.js

@@ -41,6 +41,19 @@ const debugLog = (...args) => {
   if (CURSOR_STREAM_DEBUG) console.log(...args);
 };
 
+function isComposerModel(model) {
+  const modelId = String(model || "").split("/").pop();
+  return /^composer(?:-|$)/i.test(modelId);
+}
+
+function visibleComposerContentFromThinking(thinking) {
+  if (!thinking) return "";
+  const endTag = "</think>";
+  const endIdx = thinking.lastIndexOf(endTag);
+  if (endIdx < 0) return "";
+  return thinking.slice(endIdx + endTag.length).trimStart();
+}
+
 function decompressPayload(payload, flags) {
   // Check if payload is JSON error (starts with {"error")
   if (payload.length > 10 && payload[0] === 0x7b && payload[1] === 0x22) {
@@ -264,6 +277,7 @@ export class CursorExecutor extends BaseExecutor {
 
     let offset = 0;
     let totalContent = "";
+    let totalThinking = "";
     const toolCalls = [];
     const toolCallsMap = new Map(); // Track streaming tool calls by ID
     const finalizedIds = new Set();
@@ -373,8 +387,14 @@ export class CursorExecutor extends BaseExecutor {
       }
 
       if (result.text) totalContent += result.text;
+      if (result.thinking) totalThinking += result.thinking;
     }
 
+    const visibleComposerContent = isComposerModel(model)
+      ? visibleComposerContentFromThinking(totalThinking)
+      : "";
+    const finalContent = totalContent || visibleComposerContent;
+
     debugLog(
       `[CURSOR BUFFER] Parsed ${frameCount} frames, toolCallsMap size: ${toolCallsMap.size}, finalized toolCalls: ${toolCalls.length}`
     );
@@ -400,14 +420,14 @@ export class CursorExecutor extends BaseExecutor {
 
     const message = {
       role: "assistant",
-      content: totalContent || null
+      content: finalContent || null
     };
 
     if (toolCalls.length > 0) {
       message.tool_calls = toolCalls;
     }
 
-    const usage = estimateUsage(body, totalContent.length, FORMATS.OPENAI);
+    const usage = estimateUsage(body, finalContent.length, FORMATS.OPENAI);
 
     const completion = {
       id: responseId,
@@ -435,6 +455,8 @@ export class CursorExecutor extends BaseExecutor {
     const chunks = [];
     let offset = 0;
     let totalContent = "";
+    let totalThinking = "";
+    let emittedComposerThinkingContentLength = 0;
     const toolCalls = [];
     const toolCallsMap = new Map(); // Track streaming tool calls by ID
     const finalizedIds = new Set();
@@ -635,6 +657,34 @@ export class CursorExecutor extends BaseExecutor {
           })}\n\n`
         );
       }
+
+      if (isComposerModel(model) && result.thinking) {
+        totalThinking += result.thinking;
+        const visibleContent = visibleComposerContentFromThinking(totalThinking);
+        if (visibleContent.length > emittedComposerThinkingContentLength) {
+          const deltaContent = visibleContent.slice(emittedComposerThinkingContentLength);
+          emittedComposerThinkingContentLength = visibleContent.length;
+          totalContent += deltaContent;
+          chunks.push(
+            `data: ${JSON.stringify({
+              id: responseId,
+              object: "chat.completion.chunk",
+              created,
+              model,
+              choices: [
+                {
+                  index: 0,
+                  delta:
+                    chunks.length === 0 && toolCalls.length === 0
+                      ? { role: "assistant", content: deltaContent }
+                      : { content: deltaContent },
+                  finish_reason: null
+                }
+              ]
+            })}\n\n`
+          );
+        }
+      }
     }
 
     debugLog(

tests/unit/cursor-composer-thinking.test.js

@@ -0,0 +1,86 @@
+import { describe, it, expect } from "vitest";
+
+import { CursorExecutor } from "../../open-sse/executors/cursor.js";
+import { encodeField, wrapConnectRPCFrame } from "../../open-sse/utils/cursorProtobuf.js";
+
+const LEN = 2;
+
+function cursorResponseFrame({ text = "", thinking = "" }) {
+  const responseFields = [];
+
+  if (text) {
+    responseFields.push(encodeField(1, LEN, text));
+  }
+
+  if (thinking) {
+    const thinkingMessage = encodeField(1, LEN, thinking);
+    responseFields.push(encodeField(25, LEN, thinkingMessage));
+  }
+
+  const response = Buffer.concat(responseFields.map((field) => Buffer.from(field)));
+  const envelope = encodeField(2, LEN, response);
+  return Buffer.from(wrapConnectRPCFrame(envelope));
+}
+
+function parseSSE(text) {
+  return text
+    .split("\n\n")
+    .filter((chunk) => chunk.startsWith("data: "))
+    .map((chunk) => chunk.slice("data: ".length))
+    .filter((data) => data !== "[DONE]")
+    .map((data) => JSON.parse(data));
+}
+
+describe("CursorExecutor Composer thinking-field responses", () => {
+  it("uses visible content after </think> for non-streaming Composer responses", async () => {
+    const executor = new CursorExecutor();
+    const buffer = cursorResponseFrame({
+      thinking: "private reasoning that must not leak</think>OK",
+    });
+
+    const response = executor.transformProtobufToJSON(buffer, "cu/composer-2.5", {
+      messages: [{ role: "user", content: "reply OK" }],
+    });
+    const payload = await response.json();
+
+    expect(payload.choices[0].message.content).toBe("OK");
+    expect(JSON.stringify(payload)).not.toContain("private reasoning");
+    expect(payload.usage.completion_tokens).toBeGreaterThan(0);
+  });
+
+  it("streams only visible content after </think> for Composer responses", async () => {
+    const executor = new CursorExecutor();
+    const buffer = Buffer.concat([
+      cursorResponseFrame({ thinking: "private reasoning" }),
+      cursorResponseFrame({ thinking: " that must not leak</think>O" }),
+      cursorResponseFrame({ thinking: "K" }),
+    ]);
+
+    const response = executor.transformProtobufToSSE(buffer, "composer-2.5-fast", {
+      messages: [{ role: "user", content: "reply OK" }],
+    });
+    const events = parseSSE(await response.text());
+    const content = events
+      .map((event) => event.choices?.[0]?.delta?.content || "")
+      .join("");
+
+    expect(content).toBe("OK");
+    expect(JSON.stringify(events)).not.toContain("private reasoning");
+    expect(events.at(-1).usage.completion_tokens).toBeGreaterThan(0);
+  });
+
+  it("does not treat thinking as visible output for non-Composer models", async () => {
+    const executor = new CursorExecutor();
+    const buffer = cursorResponseFrame({
+      thinking: "private reasoning</think>SHOULD_NOT_APPEAR",
+    });
+
+    const response = executor.transformProtobufToJSON(buffer, "gpt-5.3-codex", {
+      messages: [{ role: "user", content: "hi" }],
+    });
+    const payload = await response.json();
+
+    expect(payload.choices[0].message.content).toBeNull();
+    expect(JSON.stringify(payload)).not.toContain("SHOULD_NOT_APPEAR");
+  });
+});