fix(ai): improved reasoning fallback logic for equidistant tiers

- Update fallback selection to prefer higher effort tiers when distance is equal.
- Add test case verifying fallback from unsupported "medium" effort to "high".

packages/ai/src/providers/openai-reasoning-fallback.ts

@@ -230,14 +230,16 @@ function nearestEnabledReasoningFallback(currentEffort: string, allowed: Set<str
 	if (currentRank === undefined) return undefined;
 	let best: string | undefined;
 	let bestDistance = Number.POSITIVE_INFINITY;
+	let bestRank = Number.NEGATIVE_INFINITY;
 	for (const candidate of allowedEnabled) {
 		if (candidate === current) continue;
 		const candidateRank = REASONING_VALUE_RANK[candidate];
 		if (candidateRank === undefined) continue;
 		const distance = Math.abs(candidateRank - currentRank);
-		if (distance < bestDistance) {
+		if (distance < bestDistance || (distance === bestDistance && candidateRank > bestRank)) {
 			best = candidate;
 			bestDistance = distance;
+			bestRank = candidateRank;
 		}
 	}
 	return best;

packages/ai/test/openai-reasoning-effort-fallback.test.ts

@@ -71,6 +71,19 @@ function invalidReasoningResponse(param: "reasoning_effort" | "reasoning.effort"
 		{ status: 400, headers: { "content-type": "application/json" } },
 	);
 }
+function invalidMediumReasoningResponse(): Response {
+	return new Response(
+		JSON.stringify({
+			error: {
+				message: 'reasoning.effort: Invalid option: expected one of "high"|"low"|"minimal"|"none"',
+				type: "invalid_request_error",
+				param: "reasoning.effort",
+			},
+		}),
+		{ status: 400, headers: { "content-type": "application/json" } },
+	);
+}
+
 function pipeDelimitedReasoningEffortResponse(): Response {
 	return new Response(
 		JSON.stringify({
@@ -271,6 +284,30 @@ describe("OpenAI reasoning effort fallback retry", () => {
 		expect(bodies.map(body => (body.reasoning as { effort?: string } | undefined)?.effort)).toEqual(["max", "xhigh"]);
 	});
 
+	it("retries medium as high when medium is missing and high is the closest upper tier", async () => {
+		const bodies: Record<string, unknown>[] = [];
+		const fetchMock: FetchImpl = Object.assign(
+			async (_input: string | URL | Request, init?: RequestInit): Promise<Response> => {
+				const body = parseJsonBody(init);
+				bodies.push(body);
+				return bodies.length === 1 ? invalidMediumReasoningResponse() : createResponsesSseResponse();
+			},
+			{ preconnect: fetch.preconnect },
+		);
+
+		const result = await streamOpenAIResponses(createResponsesModel(), testContext, {
+			apiKey: "test-key",
+			fetch: fetchMock,
+			reasoning: "medium",
+		}).result();
+
+		expect(result.stopReason).toBe("stop");
+		expect(bodies.map(body => (body.reasoning as { effort?: string } | undefined)?.effort)).toEqual([
+			"medium",
+			"high",
+		]);
+	});
+
 	it("retries Azure Responses xhigh as provider max", async () => {
 		const bodies: Record<string, unknown>[] = [];
 		const fetchMock: FetchImpl = Object.assign(