fix: stream thinking output immediately

src/output.ts

@@ -554,7 +554,7 @@ class TextOutputFormatter implements OutputFormatter {
       }
       case "agent_thought_chunk": {
         if (update.content.type === "text") {
-          this.thoughtBuffer += update.content.text;
+          this.writeThoughtChunk(update.content.text);
         }
         return;
       }
@@ -655,6 +655,14 @@ class TextOutputFormatter implements OutputFormatter {
     this.write(text);
   }
 
+  private writeThoughtChunk(text: string): void {
+    if (!text) {
+      return;
+    }
+    this.beginSection("thought");
+    this.writeLine(this.dim(`[thinking] ${truncate(collapseWhitespace(text), MAX_THOUGHT_CHARS)}`));
+  }
+
   private flushThoughtBuffer(): void {
     const thought = truncate(collapseWhitespace(this.thoughtBuffer), MAX_THOUGHT_CHARS);
     this.thoughtBuffer = "";

test/output.test.ts

@@ -53,18 +53,22 @@ function doneResult(stopReason: string): unknown {
   };
 }
 
-test("text formatter batches thought chunks from ACP notifications", () => {
+test("text formatter streams thought chunks from ACP notifications immediately", () => {
   const writer = new CaptureWriter();
   const formatter = createOutputFormatter("text", { stdout: writer });
 
   formatter.onAcpMessage(thoughtChunk("Investigating ") as never);
+  assert.match(writer.toString(), /\[thinking\] Investigating/);
+
   formatter.onAcpMessage(thoughtChunk("the issue") as never);
+  assert.equal((writer.toString().match(/\[thinking\]/g) ?? []).length, 2);
+
   formatter.onAcpMessage(messageChunk("Done.") as never);
   formatter.onAcpMessage(doneResult("end_turn") as never);
 
   const output = writer.toString();
-  assert.equal((output.match(/\[thinking\]/g) ?? []).length, 1);
-  assert.match(output, /\[thinking\] Investigating the issue/);
+  assert.match(output, /\[thinking\] Investigating/);
+  assert.match(output, /\[thinking\] the issue/);
   assert.match(output, /\[done\] end_turn/);
 });