diff --git a/apps/api/src/routes/admin/index.ts b/apps/api/src/routes/admin/index.ts
index ae793bd..895c867 100644
--- a/apps/api/src/routes/admin/index.ts
+++ b/apps/api/src/routes/admin/index.ts
@@ -7,6 +7,7 @@ import { adminAiRoutes } from "./ai"
 import { adminConfRoutes } from "./conf"
 import { adminContestRoutes } from "./contest"
 import { adminProblemSetRoutes } from "./problemset"
+import { adminTagRoutes } from "./tag"
 import { adminAnnouncementRoutes } from "./announcement"
 import { adminTutorialRoutes } from "./tutorial"
 
@@ -26,5 +27,6 @@ adminRoutes.route("/", adminAiRoutes)
 adminRoutes.route("/", adminConfRoutes)
 adminRoutes.route("/", adminContestRoutes)
 adminRoutes.route("/", adminProblemSetRoutes)
+adminRoutes.route("/", adminTagRoutes)
 adminRoutes.route("/", adminAnnouncementRoutes)
 adminRoutes.route("/", adminTutorialRoutes)
diff --git a/apps/api/src/routes/admin/tag.ts b/apps/api/src/routes/admin/tag.ts
new file mode 100644
index 0000000..cda77fb
--- /dev/null
+++ b/apps/api/src/routes/admin/tag.ts
@@ -0,0 +1,293 @@
+import {
+  acTrendSchema,
+  adminTagSchema,
+  batchProblemTagRequestSchema,
+  batchProblemTagResponseSchema,
+  generateFlowchartRequestSchema,
+  generateFlowchartResponseSchema,
+  renameTagRequestSchema,
+  renameTagResponseSchema,
+  stuckProblemSchema,
+} from "@oj2/contract"
+import { and, asc, countDistinct, count, desc, eq, gte, ilike, inArray, isNull, lte, ne, sql } from "drizzle-orm"
+import { Hono } from "hono"
+
+import { requireProblemPermission, requireTeacher, type AppEnv } from "../../auth/middleware"
+import type { AuthUser } from "../../auth/session"
+import { db, schema } from "../../db"
+import { failure, success } from "../../http"
+import { JudgeStatus } from "../../judge/status"
+import { completeChat } from "../../services/ai"
+import { queryInteger, rounded } from "../helpers"
+
+export const adminTagRoutes = new Hono<AppEnv>()
+
+const ACCEPTED = [JudgeStatus.ACCEPTED, JudgeStatus.AST_CHECK_FAILED]
+const FAILED = [JudgeStatus.WRONG_ANSWER, JudgeStatus.COMPILE_ERROR, JudgeStatus.RUNTIME_ERROR]
+
+/** 能管所有题目：超管，或 problemPermission 为 All */
+function canManageAllProblems(user: AuthUser) {
+  return user.adminType === "Super Admin" || user.problemPermission === "All"
+}
+
+// ---------------------------------------------------------------- 标签
+
+adminTagRoutes.get("/problem-tags", requireProblemPermission, async (c) => {
+  const keyword = c.req.query("keyword")?.trim()
+  const rows = await db.select({
+    id: schema.problemTag.id,
+    name: schema.problemTag.name,
+    problemCount: countDistinct(schema.problemTags.problemId),
+  }).from(schema.problemTag)
+    .leftJoin(schema.problemTags, eq(schema.problemTags.problemtagId, schema.problemTag.id))
+    .where(keyword ? ilike(schema.problemTag.name, `%${keyword}%`) : undefined)
+    .groupBy(schema.problemTag.id, schema.problemTag.name)
+    // 后台标签管理要看到 problemCount=0 的标签（正是要清理的那些），
+    // 所以这里用 leftJoin 且不加 having —— oj 侧的 /problem-tags 才过滤 >0
+    .orderBy(desc(countDistinct(schema.problemTags.problemId)), asc(schema.problemTag.name))
+  return success(c, rows.map((row) => adminTagSchema.parse(row)))
+})
+
+adminTagRoutes.put("/problem-tags/:id", requireProblemPermission, async (c) => {
+  const id = queryInteger(c.req.param("id"), 0, { min: 1 })
+  const parsed = renameTagRequestSchema.safeParse(await c.req.json().catch(() => null))
+  if (!parsed.success) return failure(c, 400, "invalid-request", "标签名不能为空")
+  const name = parsed.data.name
+
+  const [tag] = await db.select().from(schema.problemTag).where(eq(schema.problemTag.id, id)).limit(1)
+  if (!tag) return failure(c, 404, "tag-not-found", "标签不存在，请刷新后重试")
+
+  const [target] = await db.select().from(schema.problemTag)
+    .where(and(sql`lower(${schema.problemTag.name}) = lower(${name})`, ne(schema.problemTag.id, id))).limit(1)
+
+  if (!target) {
+    await db.update(schema.problemTag).set({ name }).where(eq(schema.problemTag.id, id))
+    return success(c, renameTagResponseSchema.parse({ merged: false, id, name, affectedCount: 0 }))
+  }
+
+  // 改名撞上已有标签，视为合并：题目关系转移过去，原标签删除
+  const affected = await db.transaction(async (tx) => {
+    const links = await tx.select({ problemId: schema.problemTags.problemId })
+      .from(schema.problemTags).where(eq(schema.problemTags.problemtagId, id))
+    const already = new Set((await tx.select({ problemId: schema.problemTags.problemId })
+      .from(schema.problemTags).where(eq(schema.problemTags.problemtagId, target.id)))
+      .map((row) => row.problemId))
+    // 只给还没挂目标标签的题目补关系，否则会撞 (problem_id, problemtag_id) 唯一约束
+    const missing = links.filter((link) => !already.has(link.problemId))
+    if (missing.length) {
+      await tx.insert(schema.problemTags).values(missing.map((link) => ({
+        problemId: link.problemId,
+        problemtagId: target.id,
+      })))
+    }
+    await tx.delete(schema.problemTags).where(eq(schema.problemTags.problemtagId, id))
+    await tx.delete(schema.problemTag).where(eq(schema.problemTag.id, id))
+    return links.length
+  })
+  return success(c, renameTagResponseSchema.parse({
+    merged: true, id: target.id, name: target.name, affectedCount: affected,
+  }))
+})
+
+adminTagRoutes.delete("/problem-tags/:id", requireProblemPermission, async (c) => {
+  const id = queryInteger(c.req.param("id"), 0, { min: 1 })
+  // 中间表是 NO ACTION 外键，得先清关系再删标签
+  const deleted = await db.transaction(async (tx) => {
+    await tx.delete(schema.problemTags).where(eq(schema.problemTags.problemtagId, id))
+    return tx.delete(schema.problemTag).where(eq(schema.problemTag.id, id))
+      .returning({ id: schema.problemTag.id })
+  })
+  if (deleted.length === 0) return failure(c, 404, "tag-not-found", "标签不存在，请刷新后重试")
+  return success(c, null)
+})
+
+adminTagRoutes.post("/problems/batch-tag", requireProblemPermission, async (c) => {
+  const parsed = batchProblemTagRequestSchema.safeParse(await c.req.json().catch(() => null))
+  if (!parsed.success) {
+    return failure(c, 400, "invalid-request", parsed.error.issues[0]?.message ?? "参数错误")
+  }
+  const user = c.get("user")!
+  const filters = [inArray(schema.problem.id, parsed.data.problemIds), isNull(schema.problem.contestId)]
+  if (!canManageAllProblems(user)) filters.push(eq(schema.problem.createdById, user.id))
+  const problems = await db.select({ id: schema.problem.id }).from(schema.problem).where(and(...filters))
+  if (problems.length === 0) return failure(c, 404, "no-problems", "没有可操作的题目")
+
+  // 去重且大小写不敏感，与旧 resolve_tags / find_tags 一致
+  const wanted: string[] = []
+  const seen = new Set<string>()
+  for (const raw of parsed.data.tagNames) {
+    const name = raw.trim()
+    if (!name || seen.has(name.toLowerCase())) continue
+    seen.add(name.toLowerCase())
+    wanted.push(name)
+  }
+
+  const tagIds = await db.transaction(async (tx) => {
+    const ids: number[] = []
+    for (const name of wanted) {
+      const [existing] = await tx.select({ id: schema.problemTag.id }).from(schema.problemTag)
+        .where(sql`lower(${schema.problemTag.name}) = lower(${name})`).limit(1)
+      if (existing) { ids.push(existing.id); continue }
+      // 添加时按需新建标签，移除时只认已有标签 —— 否则「移除」会顺手造出一堆空标签
+      if (parsed.data.action === "add") {
+        const [created] = await tx.insert(schema.problemTag).values({ name })
+          .returning({ id: schema.problemTag.id })
+        ids.push(created!.id)
+      }
+    }
+    return ids
+  })
+  if (tagIds.length === 0) return failure(c, 404, "no-tags", "没有匹配的标签")
+
+  const problemIds = problems.map((problem) => problem.id)
+  await db.transaction(async (tx) => {
+    if (parsed.data.action === "remove") {
+      await tx.delete(schema.problemTags).where(and(
+        inArray(schema.problemTags.problemId, problemIds),
+        inArray(schema.problemTags.problemtagId, tagIds),
+      ))
+      return
+    }
+    const existing = await tx.select().from(schema.problemTags).where(and(
+      inArray(schema.problemTags.problemId, problemIds),
+      inArray(schema.problemTags.problemtagId, tagIds),
+    ))
+    const have = new Set(existing.map((row) => `${row.problemId}:${row.problemtagId}`))
+    const rows = []
+    for (const problemId of problemIds) {
+      for (const tagId of tagIds) {
+        if (!have.has(`${problemId}:${tagId}`)) rows.push({ problemId, problemtagId: tagId })
+      }
+    }
+    if (rows.length) await tx.insert(schema.problemTags).values(rows)
+  })
+
+  return success(c, batchProblemTagResponseSchema.parse({
+    problemCount: problems.length,
+    tagCount: tagIds.length,
+  }))
+})
+
+// ---------------------------------------------------------------- 题目可见性
+
+adminTagRoutes.put("/problems/:id/visibility", requireProblemPermission, async (c) => {
+  const id = queryInteger(c.req.param("id"), 0, { min: 1 })
+  const [problem] = await db.select({ id: schema.problem.id, visible: schema.problem.visible, createdById: schema.problem.createdById })
+    .from(schema.problem).where(eq(schema.problem.id, id)).limit(1)
+  // 旧后端这里的 `self.error(...)` 少写了 return，题目不存在时会继续往下跑并抛
+  // AttributeError（500）。这里正常返回 404。
+  if (!problem) return failure(c, 404, "problem-not-found", "题目不存在")
+  const user = c.get("user")!
+  if (!canManageAllProblems(user) && problem.createdById !== user.id) {
+    return failure(c, 404, "problem-not-found", "题目不存在")
+  }
+  await db.update(schema.problem).set({ visible: !problem.visible }).where(eq(schema.problem.id, id))
+  return success(c, { visible: !problem.visible })
+})
+
+// ---------------------------------------------------------------- 卡点题目 / AC 趋势
+
+adminTagRoutes.get("/problems/stuck", requireTeacher, async (c) => {
+  const failedFilter = sql`filter (where ${inArray(schema.submission.result, FAILED)})`
+  const rows = await db.select({
+    displayId: schema.problem.displayId,
+    title: schema.problem.title,
+    total: count(),
+    accepted: sql<number>`count(*) filter (where ${inArray(schema.submission.result, ACCEPTED)})`.mapWith(Number),
+    failed: sql<number>`count(*) ${failedFilter}`.mapWith(Number),
+    failedUsers: sql<number>`count(distinct ${schema.submission.userId}) ${failedFilter}`.mapWith(Number),
+  }).from(schema.submission)
+    .innerJoin(schema.problem, eq(schema.submission.problemId, schema.problem.id))
+    .groupBy(schema.problem.id, schema.problem.displayId, schema.problem.title)
+    .having(sql`count(distinct ${schema.submission.userId}) ${failedFilter} > 0`)
+    .orderBy(desc(sql`count(distinct ${schema.submission.userId}) ${failedFilter}`))
+    .limit(40)
+  return success(c, rows.map((row) => stuckProblemSchema.parse({
+    problemId: row.displayId,
+    problemTitle: row.title,
+    total: row.total,
+    failed: row.failed,
+    failedUsers: row.failedUsers,
+    acRate: row.total ? rounded((row.accepted / row.total) * 100, 1) : 0,
+  })))
+})
+
+adminTagRoutes.get("/problems/ac-trend", requireTeacher, async (c) => {
+  const currentYear = new Date().getFullYear()
+  // 参数按旧后端的口径夹逼：越界一律回落到默认值，不报错
+  let sinceYear = queryInteger(c.req.query("sinceYear"), 2023)
+  if (sinceYear < 2022 || sinceYear > currentYear) sinceYear = 2023
+  let untilYear = queryInteger(c.req.query("untilYear"), currentYear)
+  if (untilYear < sinceYear || untilYear > currentYear) untilYear = currentYear - 1
+  let minPerYear = queryInteger(c.req.query("minPerYear"), 100)
+  if (![50, 100, 200].includes(minPerYear)) minPerYear = 100
+
+  const year = sql<number>`extract(year from ${schema.submission.createTime})`.mapWith(Number)
+  const rows = await db.select({
+    problemId: schema.problem.id,
+    displayId: schema.problem.displayId,
+    title: schema.problem.title,
+    year,
+    total: count(),
+    accepted: sql<number>`count(*) filter (where ${inArray(schema.submission.result, ACCEPTED)})`.mapWith(Number),
+  }).from(schema.submission)
+    .innerJoin(schema.problem, eq(schema.submission.problemId, schema.problem.id))
+    .where(and(
+      isNull(schema.submission.contestId),
+      gte(sql`extract(year from ${schema.submission.createTime})`, sinceYear),
+      lte(sql`extract(year from ${schema.submission.createTime})`, untilYear),
+    ))
+    .groupBy(schema.problem.id, schema.problem.displayId, schema.problem.title, year)
+    .orderBy(asc(schema.problem.id), asc(year))
+
+  const required = new Set<number>()
+  for (let y = sinceYear; y <= untilYear; y += 1) required.add(y)
+
+  const grouped = new Map<number, { displayId: string; title: string; yearly: typeof rows }>()
+  for (const row of rows) {
+    const bucket = grouped.get(row.problemId)
+    if (bucket) bucket.yearly.push(row)
+    else grouped.set(row.problemId, { displayId: row.displayId, title: row.title, yearly: [row] })
+  }
+
+  const result = []
+  for (const entry of grouped.values()) {
+    const years = new Set(entry.yearly.map((row) => row.year))
+    // 每一年都得有数据，且每年提交量都超过门槛 —— 否则趋势没有可比性
+    if (![...required].every((y) => years.has(y))) continue
+    if (!entry.yearly.every((row) => row.total > minPerYear)) continue
+    result.push(acTrendSchema.parse({
+      problemId: entry.displayId,
+      problemTitle: entry.title,
+      yearly: entry.yearly
+        .map((row) => ({
+          year: row.year,
+          total: row.total,
+          accepted: row.accepted,
+          acRate: row.total ? rounded((row.accepted / row.total) * 100, 1) : 0,
+        }))
+        .sort((left, right) => left.year - right.year),
+    }))
+  }
+  return success(c, result)
+})
+
+// ---------------------------------------------------------------- Python → 流程图
+
+adminTagRoutes.post("/problems/flowchart", requireProblemPermission, async (c) => {
+  const parsed = generateFlowchartRequestSchema.safeParse(await c.req.json().catch(() => null))
+  if (!parsed.success) return failure(c, 400, "invalid-request", "python 代码不能为空")
+  try {
+    const flowchart = await completeChat(
+      `你是一个可以将Python代码转换为mermaid的助手。
+请将用户提供的Python代码转换为 Mermaid 纯文本。
+注意括号内的内容用引号包裹，如果本身就有引号，请注意双引号和单引号的问题。
+请只返回 mermaid 代码，连 \`\`\` 都不需要。`,
+      parsed.data.python,
+    )
+    return success(c, generateFlowchartResponseSchema.parse({ flowchart }))
+  } catch (error) {
+    console.error("Flowchart generation failed", error)
+    return failure(c, 502, "ai-unavailable", "生成失败，请稍后再试")
+  }
+})
diff --git a/apps/web/src/admin/api.ts b/apps/web/src/admin/api.ts
index 1745df4..9efb238 100644
--- a/apps/web/src/admin/api.ts
+++ b/apps/web/src/admin/api.ts
@@ -72,11 +72,11 @@ export function editProblem(problem: AdminProblem | BlankProblem) {
 }
 
 export function toggleProblemVisible(problemID: number) {
-  return http.put("admin/problem/visible", { id: problemID })
+  return legacyResponse(api2.put(`admin/problems/${problemID}/visibility`))
 }
 
 export function generateFlowchartFromPythonCode(python: string) {
-  return http.post("admin/problem/flowchart", { python })
+  return legacyResponse(api2.post("admin/problems/flowchart", { python }))
 }
 
 export function editContestProblem(problem: AdminProblem | BlankProblem) {
@@ -93,20 +93,22 @@ export function getContestProblem(id: number) {
 
 // 标签管理
 export function getTagAdminList(keyword = "") {
-  return http.get<AdminTag[]>("admin/problem/tag", { params: { keyword } })
+  return legacyResponse<AdminTag[]>(
+    api2.get("admin/problem-tags", { params: { keyword } }),
+  )
 }
 
 export function renameTag(id: number, name: string) {
-  return http.put<{
+  return legacyResponse<{
     merged: boolean
     id: number
     name: string
     affected_count: number
-  }>("admin/problem/tag", { id, name })
+  }>(api2.put(`admin/problem-tags/${id}`, { name }))
 }
 
 export function deleteTag(id: number) {
-  return http.delete("admin/problem/tag", { params: { id } })
+  return api2.delete(`admin/problem-tags/${id}`)
 }
 
 export function batchTagProblems(
@@ -114,9 +116,12 @@ export function batchTagProblems(
   tagNames: string[],
   action: "add" | "remove",
 ) {
-  return http.post<{ problem_count: number; tag_count: number }>(
-    "admin/problem/batch_tag",
-    { problem_ids: problemIds, tag_names: tagNames, action },
+  return legacyResponse<{ problem_count: number; tag_count: number }>(
+    api2.post("admin/problems/batch-tag", {
+      problemIds,
+      tagNames,
+      action,
+    }),
   )
 }
 
@@ -670,7 +675,7 @@ export function removeUserFromProblemSet(problemSetId: number, userId: number) {
 
 // 学生卡点分析
 export function getStuckProblems() {
-  return http.get("admin/problem/stuck")
+  return legacyResponse(api2.get("admin/problems/stuck"))
 }
 
 export function getTopACTrend(params: {
@@ -678,7 +683,15 @@ export function getTopACTrend(params: {
   until_year: number
   min_per_year: number
 }) {
-  return http.get("admin/problem/top_ac_trend", { params })
+  return legacyResponse(
+    api2.get("admin/problems/ac-trend", {
+      params: {
+        sinceYear: params.since_year,
+        untilYear: params.until_year,
+        minPerYear: params.min_per_year,
+      },
+    }),
+  )
 }
 
 // AI 学习分析报告
diff --git a/docs/specs/phase3-coverage.md b/docs/specs/phase3-coverage.md
index 620d0e7..acfe15e 100644
--- a/docs/specs/phase3-coverage.md
+++ b/docs/specs/phase3-coverage.md
@@ -147,3 +147,34 @@
 
 1. 本报告的对照关系带人工判断成分，若某条对应有异议，以实际业务行为为准。
 2. `utils/download.ts` 仍指向旧后端的 `/api/admin`（blob 下载），只被 admin 侧两个页面用，随阶段 4 一起切。
+
+---
+
+## 阶段 5 切换必做项（阶段 4 施工时发现，记在这里以免忘）
+
+**导入数据后必须重置全部序列。** 本地库是按显式 id 从生产导入的，
+`problem_tag_id_seq` 停在 6 而表里 max(id)=87，于是第一次新建标签就撞
+`duplicate key value violates unique constraint "problem_tag_pkey"`（500）。
+
+生产切换若沿用同一个库则不受影响（序列本来就是对的）；但只要有任何一步是
+「导出 → 导入到新库」，就必须补这一句：
+
+```sql
+do $$
+declare r record; mx bigint;
+begin
+  for r in
+    select split_part(pg_get_serial_sequence(quote_ident(t.table_name), c.column_name), '.', 2) as seqname,
+           t.table_name, c.column_name
+    from information_schema.tables t
+    join information_schema.columns c on c.table_name = t.table_name
+    join pg_sequences s on s.sequencename = split_part(pg_get_serial_sequence(quote_ident(t.table_name), c.column_name), '.', 2)
+    where t.table_schema = 'public'
+  loop
+    execute format('select coalesce(max(%I),0) from %I', r.column_name, r.table_name) into mx;
+    execute format('select setval(%L, greatest(%s, 1))', r.seqname, mx);
+  end loop;
+end $$;
+```
+
+症状很隐蔽：读全部正常，只有**写**才炸，而且是导入后第一次写才炸。
diff --git a/packages/contract/src/admin.ts b/packages/contract/src/admin.ts
index dc6a446..0a92862 100644
--- a/packages/contract/src/admin.ts
+++ b/packages/contract/src/admin.ts
@@ -433,3 +433,57 @@ export const adminProblemSetProgressSchema = z.object({
   totalProblemsCount: z.number().int(),
   totalScore: z.number().int(),
 })
+
+// ---------------------------------------------------------------- 标签与题目分析
+
+export const adminTagSchema = z.object({
+  id: z.number().int(),
+  name: z.string(),
+  problemCount: z.number().int(),
+})
+
+export const renameTagRequestSchema = z.object({ name: z.string().trim().min(1).max(64) })
+
+export const renameTagResponseSchema = z.object({
+  /** 改名撞上已有标签时视为合并：题目关系转移过去、原标签删除 */
+  merged: z.boolean(),
+  id: z.number().int(),
+  name: z.string(),
+  affectedCount: z.number().int(),
+})
+
+export const batchProblemTagRequestSchema = z.object({
+  problemIds: z.array(z.number().int().positive()).min(1),
+  tagNames: z.array(z.string()).min(1),
+  action: z.enum(["add", "remove"]),
+})
+
+export const batchProblemTagResponseSchema = z.object({
+  problemCount: z.number().int(),
+  tagCount: z.number().int(),
+})
+
+export const stuckProblemSchema = z.object({
+  problemId: z.string(),
+  problemTitle: z.string(),
+  total: z.number().int(),
+  failed: z.number().int(),
+  failedUsers: z.number().int(),
+  acRate: z.number(),
+})
+
+export const acTrendYearSchema = z.object({
+  year: z.number().int(),
+  total: z.number().int(),
+  accepted: z.number().int(),
+  acRate: z.number(),
+})
+
+export const acTrendSchema = z.object({
+  problemId: z.string(),
+  problemTitle: z.string(),
+  yearly: z.array(acTrendYearSchema),
+})
+
+export const generateFlowchartRequestSchema = z.object({ python: z.string().min(1).max(64 * 1024) })
+export const generateFlowchartResponseSchema = z.object({ flowchart: z.string() })