fix(kiro): strip leaked <thinking> tags from content stream (#2158)

CodeWhisperer leaks literal <thinking> blocks into assistantResponseEvent,
duplicating reasoning already routed via reasoningContentEvent. Track
inThinking state to strip these tags during SSE transform, handling split
chunks across tag boundaries.

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

tests/unit/kiro-thinking-strip.test.js

@@ -0,0 +1,124 @@
+import { describe, it, expect } from "vitest";
+import { KiroExecutor } from "../../open-sse/executors/kiro.js";
+
+function createMockFrame(eventType, payloadObj) {
+  const payloadStr = JSON.stringify(payloadObj);
+  const payloadBytes = new TextEncoder().encode(payloadStr);
+
+  const headerName = ":event-type";
+  const headerNameBytes = new TextEncoder().encode(headerName);
+  const headerValueBytes = new TextEncoder().encode(eventType);
+
+  // nameLen(1) + name + type(1) + valueLen(2) + value
+  const headerLength = 1 + headerNameBytes.length + 1 + 2 + headerValueBytes.length;
+  const totalLength = 12 + headerLength + payloadBytes.length + 4;
+
+  const buffer = new Uint8Array(totalLength);
+  const view = new DataView(buffer.buffer);
+
+  view.setUint32(0, totalLength, false);
+  view.setUint32(4, headerLength, false);
+
+  let offset = 12;
+  buffer[offset++] = headerNameBytes.length;
+  buffer.set(headerNameBytes, offset);
+  offset += headerNameBytes.length;
+
+  buffer[offset++] = 7; // String type
+  view.setUint16(offset, headerValueBytes.length, false);
+  offset += 2;
+  buffer.set(headerValueBytes, offset);
+  offset += headerValueBytes.length;
+
+  buffer.set(payloadBytes, offset);
+  
+  return buffer;
+}
+
+async function readAllSSE(stream) {
+  const reader = stream.getReader();
+  const decoder = new TextDecoder();
+  let result = "";
+  while (true) {
+    const { done, value } = await reader.read();
+    if (done) break;
+    result += decoder.decode(value, { stream: true });
+  }
+  return result;
+}
+
+describe("KiroExecutor thinking tag stripping", () => {
+  it("strips <thinking> tags from assistantResponseEvent", async () => {
+    const executor = new KiroExecutor();
+    
+    // Create frames
+    const f1 = createMockFrame("assistantResponseEvent", { content: "Here is my answer. <thinking>Let me think..." });
+    const f2 = createMockFrame("assistantResponseEvent", { content: "still thinking...</thinking> Yes, 42." });
+    
+    const readableStream = new ReadableStream({
+      start(controller) {
+        controller.enqueue(f1);
+        controller.enqueue(f2);
+        controller.close();
+      }
+    });
+
+    const mockResponse = { body: readableStream };
+    const transformedResponse = executor.transformEventStreamToSSE(mockResponse, "claude-test");
+    
+    const output = await readAllSSE(transformedResponse.body);
+    
+    // Check that we got chat.completion.chunk outputs
+    expect(output).toContain("chat.completion.chunk");
+    // Ensure the thinking parts are gone
+    expect(output).not.toContain("<thinking>");
+    expect(output).not.toContain("Let me think...");
+    expect(output).not.toContain("still thinking...");
+    expect(output).not.toContain("</thinking>");
+    
+    // Check that the normal content is preserved
+    // Parse the data chunks
+    const dataLines = output.split("\n").filter(line => line.startsWith("data: "));
+    const contents = dataLines.map(line => {
+      if (line.includes("[DONE]")) return "";
+      try {
+        return JSON.parse(line.slice(6)).choices[0].delta.content || "";
+      } catch {
+        return "";
+      }
+    });
+    
+    const fullText = contents.join("");
+    expect(fullText).toBe("Here is my answer.  Yes, 42.");
+  });
+
+  it("handles empty content after stripping when hasReasoningContent is true", async () => {
+    const executor = new KiroExecutor();
+    
+    const f0 = createMockFrame("reasoningContentEvent", { text: "I am reasoning" });
+    const f1 = createMockFrame("assistantResponseEvent", { content: "<thinking>purely thinking...</thinking>" });
+    
+    const readableStream = new ReadableStream({
+      start(controller) {
+        controller.enqueue(f0);
+        controller.enqueue(f1);
+        controller.close();
+      }
+    });
+
+    const mockResponse = { body: readableStream };
+    const transformedResponse = executor.transformEventStreamToSSE(mockResponse, "claude-test");
+    
+    const output = await readAllSSE(transformedResponse.body);
+    
+    const dataLines = output.split("\n").filter(line => line.startsWith("data: ") && !line.includes("[DONE]"));
+    const objects = dataLines.map(line => JSON.parse(line.slice(6)));
+    
+    // First chunk should have reasoning_content
+    expect(objects[0].choices[0].delta.reasoning_content).toBe("I am reasoning");
+    
+    // We shouldn't get an empty content chunk from f1 since it was entirely stripped and reasoning was present
+    const contentChunks = objects.filter(obj => obj.choices[0].delta.content !== undefined);
+    expect(contentChunks.length).toBe(0);
+  });
+});