diff --git a/packages/agent/src/compaction/openai.ts b/packages/agent/src/compaction/openai.ts
index df3b8d4f2..ea5e181bf 100644
--- a/packages/agent/src/compaction/openai.ts
+++ b/packages/agent/src/compaction/openai.ts
@@ -38,6 +38,7 @@ import {
 	getOpenAIResponsesHistoryPayload,
 	normalizeResponsesToolCallId,
 } from "@oh-my-pi/pi-ai/utils";
+import { captureOpenAIHttpError } from "@oh-my-pi/pi-ai/utils/openai-http";
 import {
 	CODEX_BASE_URL,
 	getCodexAccountId,
@@ -840,18 +841,19 @@ export async function requestOpenAiRemoteCompaction(
 	});
 
 	if (!response.ok) {
-		const errorText = await response.text().catch(() => "");
+		const cause = await captureOpenAIHttpError(response);
 		logger.warn("OpenAI remote compaction failed", {
 			endpoint,
 			status: response.status,
 			statusText: response.statusText,
-			errorText,
+			errorText: cause.captured.bodyText ?? "",
 		});
 		throw new ProviderHttpError(
 			`Remote compaction failed (${response.status} ${response.statusText})`,
 			response.status,
 			{
 				headers: response.headers,
+				cause,
 			},
 		);
 	}
diff --git a/packages/coding-agent/test/issue-986-compaction-auth-fallback.test.ts b/packages/coding-agent/test/issue-986-compaction-auth-fallback.test.ts
index d463df2e1..ed4c9e5ba 100644
--- a/packages/coding-agent/test/issue-986-compaction-auth-fallback.test.ts
+++ b/packages/coding-agent/test/issue-986-compaction-auth-fallback.test.ts
@@ -81,25 +81,42 @@ describe("issue #986 compaction auth fallback", () => {
 		return { currentModel, fallbackModel };
 	}
 
-	it("falls back across providers when native compaction returns auth_unavailable", async () => {
+	it("falls back across providers when native compaction receives auth_unavailable", async () => {
 		const { currentModel, fallbackModel } = await createSession({ fallbackModelRole: "smol" });
-		const compactSpy = vi.spyOn(compactionModule, "compact").mockImplementation(async (preparation, model) => {
-			if (model.provider === currentModel.provider && model.id === currentModel.id) {
-				throw new Error(
-					"Turn prefix summarization failed: 503 auth_unavailable: no auth available (providers=codex, model=gpt-5.4-mini)",
-				);
-			}
-			if (model.provider !== fallbackModel.provider || model.id !== fallbackModel.id) {
-				throw new Error(`Unexpected compaction model ${model.provider}/${model.id}`);
-			}
-			return {
-				summary: "fallback summary",
-				shortSummary: "fallback short summary",
-				firstKeptEntryId: preparation.firstKeptEntryId,
-				tokensBefore: 42,
-				details: { provider: model.provider },
-			};
-		});
+		const originalCompact = compactionModule.compact;
+		const fetchMock = vi.fn(async () =>
+			Response.json(
+				{ error: { type: "auth_unavailable", message: "no auth available for codex" } },
+				{ status: 503, statusText: "Service Unavailable" },
+			),
+		);
+		const compactSpy = vi
+			.spyOn(compactionModule, "compact")
+			.mockImplementation(async (preparation, model, apiKey, customInstructions, signal, options) => {
+				if (model.provider === currentModel.provider && model.id === currentModel.id) {
+					return originalCompact(
+						{
+							...preparation,
+							settings: { ...preparation.settings, remoteStreamingV2Enabled: false },
+						},
+						model,
+						apiKey,
+						customInstructions,
+						signal,
+						{ ...options, fetch: fetchMock },
+					);
+				}
+				if (model.provider !== fallbackModel.provider || model.id !== fallbackModel.id) {
+					throw new Error(`Unexpected compaction model ${model.provider}/${model.id}`);
+				}
+				return {
+					summary: "fallback summary",
+					shortSummary: "fallback short summary",
+					firstKeptEntryId: preparation.firstKeptEntryId,
+					tokensBefore: 42,
+					details: { provider: model.provider },
+				};
+			});
 		vi.spyOn(modelRegistry, "getApiKey").mockImplementation(async model => {
 			if (model.provider === currentModel.provider && model.id === currentModel.id) return "codex-token";
 			if (model.provider === fallbackModel.provider && model.id === fallbackModel.id) return "anthropic-token";
@@ -109,6 +126,7 @@ describe("issue #986 compaction auth fallback", () => {
 		const result = await session.compact();
 
 		expect(result.summary).toBe("fallback summary");
+		expect(fetchMock).toHaveBeenCalled();
 		expect(compactSpy).toHaveBeenCalledTimes(2);
 		expect(compactSpy.mock.calls.map(([, model]) => `${model.provider}/${model.id}`)).toEqual([
 			`${currentModel.provider}/${currentModel.id}`,