Answer step defaults to low reasoning effort (issue #134)

Product chose A after the A/B: low cut answer-step reasoning tokens ~45%
and answer time 34s -> 22s with no quality loss. Unset now means "low";
CONSULT_ANSWER_REASONING_EFFORT=off restores the pre-change request body,
and the old "unset keeps the body" guarantee moves to "off".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/mastra/consultation-tools.ts

@@ -177,22 +177,26 @@ export const MAX_EVIDENCE_LOOKUPS_PER_TURN = 3;
 export const CONSULTATION_WINDOW_CALC_TOOL_ID = "run-jyotish-window-consultation";
 
 /**
- * Answer-step reasoning effort. Unset or blank keeps today's request body.
- * An illegal value warns once and is treated as unset.
+ * Answer-step reasoning effort. Unset or blank means "low" (issue #134: the
+ * A/B halved reasoning tokens and cut answer time ~37% with no quality loss).
+ * "off" sends no reasoning_effort, i.e. the pre-#134 request body.
+ * An illegal value warns once and is treated as "off".
  */
 const CONSULT_ANSWER_REASONING_EFFORTS = ["low", "medium", "high", "max"] as const;
+export const CONSULT_ANSWER_REASONING_EFFORT_DEFAULT: ProviderReasoningEffort = "low";
 let consultAnswerReasoningEffortWarned = false;
 
 export function consultAnswerReasoningEffort(): ProviderReasoningEffort | undefined {
   const raw = process.env.CONSULT_ANSWER_REASONING_EFFORT;
-  if (raw === undefined || raw.trim() === "") return undefined;
+  if (raw === undefined || raw.trim() === "") return CONSULT_ANSWER_REASONING_EFFORT_DEFAULT;
   const value = raw.trim();
+  if (value === "off") return undefined;
   if ((CONSULT_ANSWER_REASONING_EFFORTS as readonly string[]).includes(value)) {
     return value as ProviderReasoningEffort;
   }
   if (!consultAnswerReasoningEffortWarned) {
     consultAnswerReasoningEffortWarned = true;
-    console.warn(`CONSULT_ANSWER_REASONING_EFFORT=${value} is not low, medium, high, or max; leaving reasoning effort unset.`);
+    console.warn(`CONSULT_ANSWER_REASONING_EFFORT=${value} is not off, low, medium, high, or max; leaving reasoning effort unset.`);
   }
   return undefined;
 }

frontend/tests/answer-effort-switch-20261013.test.ts

@@ -51,9 +51,12 @@ test("an illegal effort warns once and stays unset", () => {
   }
 });
 
-test("unset effort keeps the pre-change request bodies", async () => {
+// 原值: 不设环境变量时请求体与改动前逐字节相同
+// 新值: 设成 "off" 时请求体与改动前逐字节相同；不设时默认 "low"（见下一条）
+// 原因: issue #134 产品选 A，答题步默认低思考量；"off" 是一行回退
+test("off keeps the pre-change request bodies", async () => {
   const previous = process.env.CONSULT_ANSWER_REASONING_EFFORT;
-  delete process.env.CONSULT_ANSWER_REASONING_EFFORT;
+  process.env.CONSULT_ANSWER_REASONING_EFFORT = "off";
   try {
     const natal = await captureStreamBodies({
       settings: consultationGenerationSettings(MODEL),
@@ -76,6 +79,43 @@ test("unset effort keeps the pre-change request bodies", async () => {
   }
 });
 
+test("unset or blank effort defaults to low", () => {
+  const previous = process.env.CONSULT_ANSWER_REASONING_EFFORT;
+  try {
+    delete process.env.CONSULT_ANSWER_REASONING_EFFORT;
+    assert.equal(consultAnswerReasoningEffort(), "low");
+    process.env.CONSULT_ANSWER_REASONING_EFFORT = "  ";
+    assert.equal(consultAnswerReasoningEffort(), "low");
+    process.env.CONSULT_ANSWER_REASONING_EFFORT = "off";
+    assert.equal(consultAnswerReasoningEffort(), undefined);
+  } finally {
+    if (previous === undefined) delete process.env.CONSULT_ANSWER_REASONING_EFFORT;
+    else process.env.CONSULT_ANSWER_REASONING_EFFORT = previous;
+  }
+});
+
+test("unset effort puts low on the answer step only", async () => {
+  const previous = process.env.CONSULT_ANSWER_REASONING_EFFORT;
+  delete process.env.CONSULT_ANSWER_REASONING_EFFORT;
+  try {
+    const natal = await captureStreamBodies({
+      settings: consultationGenerationSettings(MODEL),
+      prepareStep: (input) => consultationNatalPrepareStep({
+        stepNumber: input.stepNumber,
+        answerReady: input.stepNumber >= 1,
+        model: MODEL,
+      }),
+      steps: 2,
+    }) as Record<string, unknown>[];
+    assert.deepEqual(natal[0], fixture.natal[0]);
+    assert.equal(natal[1]?.reasoning_effort, "low");
+    assert.deepEqual(withoutEffort(natal[1]), fixture.natal[1]);
+  } finally {
+    if (previous === undefined) delete process.env.CONSULT_ANSWER_REASONING_EFFORT;
+    else process.env.CONSULT_ANSWER_REASONING_EFFORT = previous;
+  }
+});
+
 test("low effort is only on the answer step", async () => {
   const previous = process.env.CONSULT_ANSWER_REASONING_EFFORT;
   process.env.CONSULT_ANSWER_REASONING_EFFORT = "low";