diff --git a/.trellis/tasks/09-17-company-loan-principal-repayment/implement.md b/.trellis/tasks/09-17-company-loan-principal-repayment/implement.md
index 59aae2ba..19040c85 100644
--- a/.trellis/tasks/09-17-company-loan-principal-repayment/implement.md
+++ b/.trellis/tasks/09-17-company-loan-principal-repayment/implement.md
@@ -1,3 +1,19 @@
+## 2026-10-11 F：员工与公司组合唯一约束——实施（用户已批准；未提交）
+
+- 迁移 `20261011010000_loan_lender_employee_unique`（迁移总数 83→84）：`CREATE UNIQUE INDEX "loan_lenders_legal_entity_id_employee_id_key" ON loan_lenders(legal_entity_id, employee_id)`；Prisma `@@unique([legal_entity_id, employee_id])`。普通唯一约束，归档行计入、空 `employee_id` 互不冲突，与现行代码规则一致；不改数据、不回填。
+- 冲突提示：新增 `isEmployeeLenderUniqueViolation`（同时识别 `meta.target` 与驱动适配器的约束字段/名称）。从员工创建路径先判它再判请求键重复；旧“确认出借人身份”入口在事务外映射。触到约束时返回原有 409 `loan.employee_lender_conflict`，不报 500，也不误报为 `loan.duplicate_registration`。原员工行锁与代码检查保持不变。
+- 测试：原“直接写入模糊旧数据”的夹具改为断言数据库拒绝（P2002，且被新判定识别）；新增两条无员工出借人可共存、归档关联继续占用组合（应用与数据库两层）。变异 11→12：`legacy_duplicate` 在临时删除索引的隔离库中运行，证明代码检查仍有效，运行后解除重复关联再重建索引；新增 `legacy_duplicate_db_backstop` 在索引存在时运行同一变异，整套夹具通过，证明数据库单独兜底也返回同一冲突提示。
+- 发布要求：部署前在同一发布窗口重跑下节只读计数确认 0 组，部署按 1 个待执行迁移校验。
+- 验证：员工专项 + 变异 12/12（`employee-only-mutations-r1.log`）；完整回归 `full-regression-r1/r2.log` 各退出 0、84 迁移、12 阶段、去重 247 项（新增断言并入原检查项名，项数不变），自有资源清理 PASS；`loanLenderAcknowledgement` 单测 28/28；Prisma validate、前端构建通过。首次后台回归因相对日志路径在并行步骤切换目录后重定向失败、未实际运行，非测试失败。只读复核回执 `review-20261011-codex.json`：无阻断项，20 个内存探针通过；P2 迁移回退说明已补入 PRD F 节。证据在 `artifacts/qa-flow/loan-lender-employee-unique-20261011/`。
+
+## 2026-10-11 F：员工与公司组合唯一约束——存量排查（只读）
+
+用户授权本轮生产只读查询。经生产主机按部署项目标签定位唯一数据库容器，在 `BEGIN TRANSACTION READ ONLY … ROLLBACK` 内只返回计数，不输出 ID、姓名或账号，未写库：出借人共 1 条，关联员工 1 条，归档 0 条；同一公司与员工的重复组合 **0** 组。
+
+代码现状：从员工创建、关联已有出借人、从员工资料刷新三条写入路径均先锁员工行，再用 `assertUniqueEmployeeLender` 拒绝同公司同员工的另一条出借人（`loan.employee_lender_conflict`）；查询有意包含已归档出借人，即现行规则为“同一公司与员工最多一条，归档也计入”。代码中没有归档出借人的路径；员工停用/归档后原关联保留并继续占用。手工创建的无员工出借人 `employee_id` 为空，不要求唯一。
+
+约束建议（待批准后另行实施）：普通唯一约束 `(legal_entity_id, employee_id)`。PostgreSQL 唯一约束中空值互不冲突，无员工出借人不受影响；归档行计入，与现行代码一致，不采用只约束未归档行的部分索引（会放宽规则）。实施需新迁移，部署前须在同一发布窗口重跑本只读计数，确认仍为 0 组。
+
 ## 2026-10-10 E：追回证据刷新提示与员工资料误报（已实施、本地验证完成；未提交）
 
 用户批准两项方案后实施，基线 `91fe31f0`；未提交、推送、部署或连接生产，无新迁移。资料拦截规则先由实施会话自查；2026-10-11 另一会话独立只读复核结论 `REVIEWED_NO_BLOCKER`，无阻断项，补跑 12 组服务层内存探针与电脑/手机 4 组组件浏览器检查，两轮 247 项回归与 11 项变异只核对日志未重跑；回执 `review-20261011.json` 中 7 个文件摘要与提交前工作区一致。复核限制：组件补验的 API、身份为合成替身；上线前打开的旧 v1 预览在上线后保存会被要求重新核对（有意拒绝旧版本）；未跑完整发布门禁。
diff --git a/.trellis/tasks/09-17-company-loan-principal-repayment/prd.md b/.trellis/tasks/09-17-company-loan-principal-repayment/prd.md
index 4f87be80..b12fdb2c 100644
--- a/.trellis/tasks/09-17-company-loan-principal-repayment/prd.md
+++ b/.trellis/tasks/09-17-company-loan-principal-repayment/prd.md
@@ -1,5 +1,21 @@
 # 公司借款台账与本金归还
 
+## 2026-10-11 F：员工与公司组合数据库唯一约束（已实施、本地验证与只读复核完成；未提交）
+
+生产只读排查重复组合为 0 组后，用户批准把“同一公司与员工最多一个出借人（归档计入）”落到数据库。结果见 implement 顶部 F 节。
+
+- [x] AC56：数据库拒绝同一公司与员工的第二个出借人，含归档行；空员工的手工出借人可多条共存。
+- [x] AC57：应用触到约束时返回原有“出借人冲突”提示，不报 500、不误报请求重复；代码检查与员工行锁保持。
+- [x] AC58：借款完整回归通过，并经独立只读复核（涉及迁移）。两轮完整回归各 84 迁移、12 阶段、去重 247 项通过；另一会话只读复核无阻断项、20 个内存探针通过，唯一 P2（缺本迁移回退说明）已在下方补齐。
+- [ ] AC59：发布窗口重跑存量重复计数为 0，并按 1 个待执行迁移部署。
+
+### 迁移回退说明（`20261011010000_loan_lender_employee_unique`）
+
+- **建索引失败**：迁移只有一条 `CREATE UNIQUE INDEX`，单语句原子执行；若发布窗口内出现重复组合导致失败，索引不会部分建立，Prisma 将该迁移记为失败并阻止后续迁移，应用不切换到新版本。处理方式是停止本次发布，回到只读计数重新排查；不在发布中手工删改出借人数据，不改写已应用的迁移历史。失败迁移记录的处理（如标记回滚后重试）须另行批准后按运维手册执行。
+- **应用回退**：回退到上一版应用时**保留**该索引，不删除。上一版代码在员工行锁内已拒绝同一组合，正常路径不会触到约束；只有极端情况越过旧检查时，旧版创建路径会把约束冲突报为“请求重复”(409)，旧身份确认入口会报 500，两者都不会写入重复数据。
+- **撤销约束**：如确需取消该约束，另起新迁移 `DROP INDEX` 并单独批准，不回退或删除本迁移文件。
+
+
 ## 2026-10-10 E：追回证据刷新提示与员工资料误报（已实施、本地验证及独立只读复核完成；未提交）
 
 用户批准两项小问题按方案修复，复用本任务，不含数据库唯一约束（需先查存量重复、单独规划）。结果见 implement 顶部 E 节。
diff --git a/scripts/qa/company-loan-employee-fixture.cjs b/scripts/qa/company-loan-employee-fixture.cjs
index b8940457..cf9bdada 100644
--- a/scripts/qa/company-loan-employee-fixture.cjs
+++ b/scripts/qa/company-loan-employee-fixture.cjs
@@ -136,13 +136,19 @@ module.exports = async function employeeFixture(h) {
   await call(finance, 'POST', `/loan-lenders/${other.id}/identity-confirmation`, { expectedVersion: other.identityVersion,
     employeeId: staff.id, externalPersonConfirmed: false, reason: '合成旧入口重复关联拒绝' }, 409, 'loan.employee_lender_conflict');
   assert.equal((await prisma.loanLender.findUniqueOrThrow({ where: { id: other.id } })).employee_id, null);
-  // Model an old ambiguous dataset directly; this is not an application path.
-  await prisma.loanLender.update({ where: { id: other.id }, data: { employee_id: staff.id } });
-  assert.equal((await source(staff)).linkState, 'conflict');
-  await link(input(staff, await source(staff)), 409, 'loan.employee_lender_conflict');
-  await refresh(lender.id, refreshInput({ ...preview, existingLender: lender }), 409, 'loan.employee_lender_conflict');
-  await prisma.loanLender.update({ where: { id: other.id }, data: { employee_id: null } });
+  // An ambiguous dataset can no longer be written at all: the database rejects
+  // a second lender for the same employee and company, even outside the app.
+  await assert.rejects(prisma.loanLender.update({ where: { id: other.id }, data: { employee_id: staff.id } }),
+    (error) => error.code === 'P2002' && req('./src/services/loanLenderEmployee').isEmployeeLenderUniqueViolation(error));
+  assert.equal((await prisma.loanLender.findUniqueOrThrow({ where: { id: other.id } })).employee_id, null);
+  assert.equal((await source(staff)).linkState, 'reusable');
+  // NULL employee_id never conflicts: several manual lenders coexist in one company.
+  const manualTwin = await manualFor(staff);
+  assert.equal(await prisma.loanLender.count({ where: { legal_entity_id: company.id, employee_id: null, id: { in: [other.id, manualTwin.id] } } }), 2);
+  // An archived link still occupies the pair, in the app and in the database.
   await prisma.loanLender.update({ where: { id: freshLender.id }, data: { archived_at: new Date() } });
+  await assert.rejects(prisma.loanLender.update({ where: { id: manualTwin.id }, data: { employee_id: stale.id } }),
+    (error) => error.code === 'P2002');
   assert.equal((await source(stale)).linkState, 'conflict');
   await link(input(stale, await source(stale)), 409, 'loan.employee_lender_conflict');
   checks.push('legacy_identity_entry_and_ambiguous_or_archived_links_cannot_duplicate');
diff --git a/scripts/qa/company-loan-employee-mutations.cjs b/scripts/qa/company-loan-employee-mutations.cjs
index 82318cab..172d433b 100644
--- a/scripts/qa/company-loan-employee-mutations.cjs
+++ b/scripts/qa/company-loan-employee-mutations.cjs
@@ -16,7 +16,10 @@ module.exports = async function employeeMutations(h, runFixture) {
     // Restores the pre-fix fingerprint (updated_at selected and hashed).
     ['source_version_updated_at', [['status: true, archived_at: true,\n', 'status: true, archived_at: true, updated_at: true,\n'],
       ['"loan-employee-source-v2", legalEntityId, employee.id,', '"loan-employee-source-v2", legalEntityId, employee.id, employee.updated_at,']]],
-    ['legacy_duplicate', 'if (links.some((lender) => lender.id !== lenderId))', 'if (false)'],
+    // The code check is proven with the database constraint removed; with it in
+    // place the same mutant must be fully absorbed, reporting the same conflict.
+    ['legacy_duplicate', 'if (links.some((lender) => lender.id !== lenderId))', 'if (false)', { withoutConstraint: true }],
+    ['legacy_duplicate_db_backstop', 'if (links.some((lender) => lender.id !== lenderId))', 'if (false)', { expectAbsorbed: true }],
     ['employee_lock', 'if (unique.length) await tx.$queryRaw', 'if (false) await tx.$queryRaw'],
     ['employee_lock_fk_compatibility', 'ORDER BY id FOR NO KEY UPDATE', 'ORDER BY id FOR UPDATE'],
     ['in_flight', 'await assertLenderNotInFlight(tx, id);', ''],
@@ -25,7 +28,8 @@ module.exports = async function employeeMutations(h, runFixture) {
     ['required_audit', 'await auditLoan(tx, request, "LOAN_LENDER_EMPLOYEE_CREATED", created.id, {\n          ...requestBinding, employeeSourceRequestKey: input.requestKey,\n        });', ''],
   ];
   const results = [];
-  for (const [name, from, to] of mutations) {
+  const INDEX = 'loan_lenders_legal_entity_id_employee_id_key';
+  for (const [name, from, to, options = {}] of mutations) {
     const edits = Array.isArray(from) ? from : [[from, to]];
     let mutated = source;
     for (const [target, replacement] of edits) {
@@ -37,8 +41,24 @@ module.exports = async function employeeMutations(h, runFixture) {
     mutant._compile(mutated, filename);
     let failure;
     Object.assign(live, mutant.exports);
+    if (options.withoutConstraint) await h.prisma.$executeRawUnsafe(`DROP INDEX "${INDEX}"`);
     try { await runFixture(h); } catch (error) { failure = error; }
-    finally { Object.assign(live, original); }
+    finally {
+      Object.assign(live, original);
+      if (options.withoutConstraint) {
+        // Detach the duplicates this mutant let through, then restore the index.
+        await h.prisma.$executeRawUnsafe(`UPDATE loan_lenders SET employee_id = NULL WHERE id IN (SELECT id FROM (
+          SELECT id, row_number() OVER (PARTITION BY legal_entity_id, employee_id ORDER BY created_at, id) AS n
+          FROM loan_lenders WHERE employee_id IS NOT NULL) ranked WHERE n > 1)`);
+        await h.prisma.$executeRawUnsafe(`CREATE UNIQUE INDEX "${INDEX}" ON loan_lenders (legal_entity_id, employee_id)`);
+      }
+    }
+    if (options.expectAbsorbed) {
+      assert.ok(!failure, `mutation ${name} was not absorbed by the database constraint: ${failure?.message?.split('\n')[0]}`);
+      results.push({ name, absorbedByDatabase: true, witness: 'database_constraint_backstop' });
+      console.log(JSON.stringify({ stage: 'loan_employee_mutation', ...results.at(-1) }));
+      continue;
+    }
     assert.ok(failure, `mutation ${name} survived`);
     // A generic environment, SQL or parsing failure is not a killed guard.
     const wrongAcceptance = failure.code === 'ERR_ASSERTION' && failure.actual === 200 && [403, 409, 500].includes(failure.expected);
diff --git a/server/prisma/migrations/20261011010000_loan_lender_employee_unique/migration.sql b/server/prisma/migrations/20261011010000_loan_lender_employee_unique/migration.sql
new file mode 100644
index 00000000..0fcaafbd
--- /dev/null
+++ b/server/prisma/migrations/20261011010000_loan_lender_employee_unique/migration.sql
@@ -0,0 +1,5 @@
+-- One lender per employee and company, archived rows included. Application
+-- writes already enforce this under the employee row lock; existing data was
+-- checked read-only for duplicates before this migration. NULL employee_id
+-- (manual lenders) never conflicts.
+CREATE UNIQUE INDEX "loan_lenders_legal_entity_id_employee_id_key" ON "loan_lenders"("legal_entity_id", "employee_id");
diff --git a/server/prisma/schema.prisma b/server/prisma/schema.prisma
index 6f3e17c8..f030595b 100644
--- a/server/prisma/schema.prisma
+++ b/server/prisma/schema.prisma
@@ -4020,6 +4020,9 @@ model LoanLender {
 
   @@unique([legal_entity_id, id])
   @@unique([created_by, request_key])
+  // One lender per employee and company, archived rows included; NULL
+  // employee_id (manual lenders) never conflicts.
+  @@unique([legal_entity_id, employee_id])
   @@index([employee_id])
   @@map("loan_lenders")
 }
diff --git a/server/src/routes/companyLoans.js b/server/src/routes/companyLoans.js
index 4b889cfc..1a9b9000 100644
--- a/server/src/routes/companyLoans.js
+++ b/server/src/routes/companyLoans.js
@@ -297,6 +297,9 @@ async function companyLoansRoutes(fastify) {
         identityVersion: result.identity_version, employeeId: body.employeeId, externalPersonConfirmed: body.externalPersonConfirmed,
       });
       return result;
+    }).catch((error) => {
+      if (employeeLenders.isEmployeeLenderUniqueViolation(error)) throw buildBusinessValidationError("loan.employee_lender_conflict", {}, 409);
+      throw error;
     });
     return sendData(reply, request, mapLender(fastify.config, item));
   });
diff --git a/server/src/services/loanLenderEmployee.js b/server/src/services/loanLenderEmployee.js
index 7270616a..874c4210 100644
--- a/server/src/services/loanLenderEmployee.js
+++ b/server/src/services/loanLenderEmployee.js
@@ -32,6 +32,17 @@ async function linkedLenders(db, employeeId, legalEntityId) {
     include: LENDER_INCLUDE, orderBy: { id: "asc" }, take: 2 });
 }
 
+// The database constraint backs the locked check below; a violation reports
+// the same conflict instead of a generic duplicate or a 500.
+const EMPLOYEE_LENDER_UNIQUE = "loan_lenders_legal_entity_id_employee_id_key";
+function isEmployeeLenderUniqueViolation(error) {
+  if (error?.code !== "P2002") return false;
+  const meta = error.meta ?? {};
+  const fields = [meta.target, meta.driverAdapterError?.cause?.constraint?.fields].flat().filter(Boolean).map(String);
+  return fields.includes("employee_id") || fields.includes(EMPLOYEE_LENDER_UNIQUE) ||
+    String(meta.driverAdapterError?.cause?.originalMessage ?? "").includes(EMPLOYEE_LENDER_UNIQUE);
+}
+
 async function assertUniqueEmployeeLender(tx, employeeId, legalEntityId, lenderId) {
   if (!employeeId) return;
   const links = await linkedLenders(tx, employeeId, legalEntityId);
@@ -167,6 +178,7 @@ async function linkEmployeeLender(prisma, config, request, input) {
       });
       if (result) return result;
     } catch (error) {
+      if (isEmployeeLenderUniqueViolation(error)) throw buildBusinessValidationError("loan.employee_lender_conflict", {}, 409);
       if (error?.code === "P2002") throw buildBusinessValidationError("loan.duplicate_registration", {}, 409);
       throw error;
     }
@@ -199,6 +211,6 @@ async function refreshEmployeeLender(tx, config, request, id, input) {
 }
 
 module.exports = {
-  ensureEmployeeBankAccess, lockEmployeeIdentities, assertUniqueEmployeeLender,
+  ensureEmployeeBankAccess, lockEmployeeIdentities, assertUniqueEmployeeLender, isEmployeeLenderUniqueViolation,
   previewEmployeeLender, linkEmployeeLender, refreshEmployeeLender,
 };