fix(teacher): 区分词元预算与传输字节避免短问题被拦截

electron/coding-teacher/cloud-runner.ts

@@ -169,9 +169,10 @@ export function prepareCloudTeacher(
     ? undefined
     : createTeacherReadTools(access);
   return {
-    // Leave room for routine JSON escaping. The exact wire size is checked below
-    // so unusually escape-heavy material cannot silently exceed the input budget.
-    inputLimit: Math.max(0, topic.definition.limits.max_input_tokens - 256),
+    // The cloud query contract is byte-bounded. Compile against the exact JSON
+    // envelope instead of reserving a fixed amount for unpredictable escaping.
+    inputLimit: topic.definition.limits.max_input_tokens,
+    measureInput: (messages: TeacherModelMessage[]) => Buffer.byteLength(JSON.stringify({ messages }), 'utf8'),
     async run(
       messages: TeacherModelMessage[],
       signal: AbortSignal,
@@ -186,7 +187,7 @@ export function prepareCloudTeacher(
         throw new TeacherError(
           422,
           'teacher_context_too_long',
-          '问题、引用或老师指令超过上下文预算，请缩短引用或新建话题。'
+          '本次问题或引用超过上下文预算，请缩短问题或引用后重试。'
         );
       }
       const currentRequest = topic.requests.find((item) => item.id === requestId);