diff --git a/lib/request/response-handler.ts b/lib/request/response-handler.ts
index a36f89ae..8899f270 100644
--- a/lib/request/response-handler.ts
+++ b/lib/request/response-handler.ts
@@ -349,6 +349,34 @@ function notifyResponseId(
 	}
 }
 
+function truncateDiagnosticText(value: unknown, maxLength = 400): string | undefined {
+	if (typeof value !== "string") return undefined;
+	const trimmed = value.trim();
+	if (trimmed.length === 0) return undefined;
+	return trimmed.length > maxLength ? `${trimmed.slice(0, maxLength)}...` : trimmed;
+}
+
+function logStreamDiagnostics(finalResponse: unknown): void {
+	if (!LOGGING_ENABLED || !isRecord(finalResponse)) {
+		return;
+	}
+
+	const responseId = extractResponseId(finalResponse);
+	const phase = getStringField(finalResponse, "phase");
+	const commentaryText = truncateDiagnosticText(finalResponse.commentary_text);
+	const finalAnswerText = truncateDiagnosticText(finalResponse.final_answer_text);
+	const reasoningSummaryText = truncateDiagnosticText(finalResponse.reasoning_summary_text);
+	if (phase || commentaryText || finalAnswerText || reasoningSummaryText) {
+		logRequest("stream-diagnostics", {
+			...(responseId ? { responseId } : {}),
+			...(phase ? { phase } : {}),
+			...(commentaryText ? { commentaryText } : {}),
+			...(finalAnswerText ? { finalAnswerText } : {}),
+			...(reasoningSummaryText ? { reasoningSummaryText } : {}),
+		});
+	}
+}
+
 function maybeCaptureResponseEvent(
 	state: ParsedResponseState,
 	data: SSEEventData,
@@ -539,6 +567,8 @@ export async function convertSseToJson(
 			});
 		}
 
+		logStreamDiagnostics(finalResponse);
+
 		// Return as plain JSON (not SSE)
 		const jsonHeaders = new Headers(headers);
 		jsonHeaders.set('content-type', 'application/json; charset=utf-8');
diff --git a/test/request-transformer.test.ts b/test/request-transformer.test.ts
index bb360838..91dcdb18 100644
--- a/test/request-transformer.test.ts
+++ b/test/request-transformer.test.ts
@@ -2067,7 +2067,6 @@ describe('Request Transformer Module', () => {
 					'Removed 1 plan-mode-only tool definition(s) because collaboration mode is default',
 				);
 			});
-
 			it('removes tool_search tools when the selected model lacks search capability', async () => {
 				const body: RequestBody = {
 					model: 'gpt-5-nano',
@@ -2151,7 +2150,6 @@ describe('Request Transformer Module', () => {
 					},
 				]);
 			});
-
 			it('filters unsupported tools from nested namespaces without dropping supported descendants', async () => {
 				const body: RequestBody = {
 					model: 'gpt-5-nano',
diff --git a/test/response-handler-logging.test.ts b/test/response-handler-logging.test.ts
index a295b754..70cfab80 100644
--- a/test/response-handler-logging.test.ts
+++ b/test/response-handler-logging.test.ts
@@ -1,4 +1,4 @@
-import { describe, expect, it, vi } from "vitest";
+import { beforeEach, describe, expect, it, vi } from "vitest";
 
 const logRequestMock = vi.fn();
 
@@ -14,6 +14,10 @@ vi.mock("../lib/logger.js", () => ({
 }));
 
 describe("response handler logging branch", () => {
+	beforeEach(() => {
+		logRequestMock.mockClear();
+	});
+
 	it("logs full stream content when logging is enabled", async () => {
 		const { convertSseToJson } = await import("../lib/request/response-handler.js");
 		const response = new Response(
@@ -29,4 +33,33 @@ describe("response handler logging branch", () => {
 			expect.objectContaining({ fullContent: expect.stringContaining("response.done") }),
 		);
 	});
+
+	it("logs parsed phase and reasoning summary diagnostics when semantic SSE fields are present", async () => {
+		const { convertSseToJson } = await import("../lib/request/response-handler.js");
+		const response = new Response(
+			[
+				'data: {"type":"response.created","response":{"id":"resp_diag_123","object":"response"}}',
+				'data: {"type":"response.output_item.added","output_index":0,"item":{"id":"msg_123","type":"message","role":"assistant","phase":"commentary"}}',
+				'data: {"type":"response.output_text.done","output_index":0,"content_index":0,"text":"Thinking...","phase":"commentary"}',
+				'data: {"type":"response.output_item.done","output_index":0,"item":{"id":"msg_123","type":"message","role":"assistant","phase":"final_answer"}}',
+				'data: {"type":"response.output_text.done","output_index":0,"content_index":1,"text":"Done.","phase":"final_answer"}',
+				'data: {"type":"response.reasoning_summary_text.done","output_index":1,"summary_index":0,"text":"Need more context."}',
+				'data: {"type":"response.done","response":{"id":"resp_diag_123","object":"response"}}',
+				"",
+			].join("\n"),
+		);
+
+		const result = await convertSseToJson(response, new Headers());
+		expect(result.status).toBe(200);
+		expect(logRequestMock).toHaveBeenCalledWith(
+			"stream-diagnostics",
+			expect.objectContaining({
+				responseId: "resp_diag_123",
+				phase: "final_answer",
+				commentaryText: "Thinking...",
+				finalAnswerText: "Done.",
+				reasoningSummaryText: "Need more context.",
+			}),
+		);
+	});
 });