fix: align manual native compaction fallback

(cherry picked from commit 2d5397a52f6feaeee4136fe4e7d8271309043b75)

packages/coding-agent/src/session/session-maintenance.ts

@@ -1460,10 +1460,18 @@ export class SessionMaintenance {
 		const candidates =
 			precomputedCandidates ?? this.#getCompactionModelCandidates(this.#host.modelRegistry.getAvailable());
 		const telemetry = resolveTelemetry(this.#host.agent.telemetry, this.#host.sessionId());
+		let nativeCompactionFailure: { error: NativeCompactionError; provider: string } | undefined;
 
 		for (const candidate of candidates) {
 			const apiKey = await this.#host.modelRegistry.getApiKey(candidate, this.#host.sessionId());
 			if (!apiKey) continue;
+			if (
+				nativeCompactionFailure &&
+				(candidate.provider !== nativeCompactionFailure.provider ||
+					!shouldUseProviderNativeCompaction(candidate, preparation.settings))
+			) {
+				throw nativeCompactionFailure.error;
+			}
 
 			try {
 				return await compact(
@@ -1501,12 +1509,17 @@ export class SessionMaintenance {
 					},
 				);
 			} catch (error) {
-				if (!AIError.is(AIError.classify(error, candidate.api), AIError.Flag.AuthFailed)) {
-					throw error;
+				const id = AIError.classify(error instanceof NativeCompactionError ? error.cause : error, candidate.api);
+				if (AIError.is(id, AIError.Flag.AuthFailed)) continue;
+				if (error instanceof NativeCompactionError) {
+					nativeCompactionFailure ??= { error, provider: candidate.provider };
+					continue;
 				}
+				throw error;
 			}
 		}
 
+		if (nativeCompactionFailure) throw nativeCompactionFailure.error;
 		throw this.#buildCompactionAuthError();
 	}
 

packages/coding-agent/test/issue-986-compaction-auth-fallback.test.ts

@@ -373,6 +373,35 @@ describe("issue #986 compaction auth fallback", () => {
 		]);
 	});
 
+	it("tries same-provider native candidates during manual compaction before crossing providers", async () => {
+		const { crossProviderModel, currentModel, sameProviderModel } = await createAutoNativeFallbackSession();
+		const attemptedModels: string[] = [];
+		vi.spyOn(compactionModule, "compact").mockImplementation(async (preparation, model) => {
+			attemptedModels.push(`${model.provider}/${model.id}`);
+			if (model.provider === currentModel.provider && model.id === currentModel.id) {
+				throw new compactionModule.NativeCompactionError(new Error("native manual compaction failed"));
+			}
+			if (model.provider === sameProviderModel.provider && model.id === sameProviderModel.id) {
+				return {
+					summary: "same-provider manual summary",
+					shortSummary: "same-provider manual",
+					firstKeptEntryId: preparation.firstKeptEntryId,
+					tokensBefore: 42,
+				};
+			}
+			throw new Error(`Unexpected compaction model ${model.provider}/${model.id}`);
+		});
+
+		const result = await session.compact();
+
+		expect(result.summary).toBe("same-provider manual summary");
+		expect(attemptedModels).toEqual([
+			`${currentModel.provider}/${currentModel.id}`,
+			`${sameProviderModel.provider}/${sameProviderModel.id}`,
+		]);
+		expect(attemptedModels).not.toContain(`${crossProviderModel.provider}/${crossProviderModel.id}`);
+	});
+
 	it("falls back across providers when native compaction receives auth_unavailable", async () => {
 		const { currentModel, fallbackModel } = await createSession({ fallbackModelRole: "smol" });
 		const originalCompact = compactionModule.compact;