diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index b0b97de2..b3ebba0e 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -39,6 +39,9 @@ jobs:
       - run: npm ci
         continue-on-error: true
 
+      - name: Check OpenAPI artifact and contract drift
+        run: npm run openapi:check
+
       - run: npm run lint || true
         continue-on-error: true
 
diff --git a/openapi.yaml b/openapi.yaml
index 6df1e4c7..7d7fe6b9 100644
--- a/openapi.yaml
+++ b/openapi.yaml
@@ -1138,6 +1138,30 @@ components:
         - claims
         - disputes
         - totals
+    CircuitOpenErrorBody:
+      type: object
+      properties:
+        error:
+          type: object
+          properties:
+            code:
+              type: string
+              enum:
+                - service_unavailable
+            message:
+              type: string
+            retryAfterMs:
+              type: integer
+              minimum: 0
+            requestId:
+              type: string
+          required:
+            - code
+            - message
+            - retryAfterMs
+            - requestId
+      required:
+        - error
     AuditEntry:
       type: object
       properties:
@@ -2485,6 +2509,12 @@ paths:
                         - path:
                             - expectedVersion
                           message: expectedVersion is required
+        '403':
+          description: Forbidden — caller is not an administrator
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorBody'
         '404':
           description: Not found
           content:
@@ -2843,6 +2873,12 @@ paths:
                 required:
                   - data
                   - meta
+        '400':
+          description: Invalid pagination parameters
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorBody'
   /api/leaderboard/user/{stellarAddress}:
     get:
       operationId: getLeaderboardUser
@@ -4165,6 +4201,71 @@ paths:
             application/json:
               schema:
                 $ref: '#/components/schemas/ErrorBody'
+  /api/admin/users/{address}/impersonate:
+    post:
+      operationId: impersonateUser
+      tags:
+        - Admin
+      summary: Generate an impersonation JWT for a user (admin only)
+      description: >-
+        Admin-only endpoint that creates an audit-logged JWT allowing the caller to act as the target user. The
+        generated token carries a `user` role assertion.
+
+
+        Downstream work (audit-log writes and token signing) is wrapped in a per-endpoint circuit breaker. After
+        repeated downstream failures the breaker opens and the endpoint fast-fails with 503 without attempting any
+        downstream call, until a recovery probe succeeds.
+      security:
+        - bearerAuth: []
+      parameters:
+        - schema:
+            type: string
+          required: true
+          name: address
+          in: path
+      responses:
+        '200':
+          description: Impersonation token
+          content:
+            application/json:
+              schema:
+                type: object
+                properties:
+                  data:
+                    type: object
+                    properties:
+                      token:
+                        type: string
+                    required:
+                      - token
+                required:
+                  - data
+        '400':
+          description: Validation error — address is blank or whitespace-only
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorBody'
+        '403':
+          description: Forbidden — missing, invalid, or non-admin JWT
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorBody'
+        '429':
+          description: Rate limit exceeded
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorBody'
+        '503':
+          description: >-
+            Circuit breaker is open — downstream dependencies are unhealthy and no downstream call was attempted. Retry
+            after `retryAfterMs`.
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/CircuitOpenErrorBody'
   /api/admin/audit:
     get:
       operationId: getAdminAuditLog
diff --git a/scripts/check-openapi.ts b/scripts/check-openapi.ts
index d943a11e..6efe453f 100644
--- a/scripts/check-openapi.ts
+++ b/scripts/check-openapi.ts
@@ -1,3 +1,6 @@
+import * as fs from "fs";
+import * as path from "path";
+import * as yaml from "js-yaml";
 import { resetOpenApiCache, getOpenApiSpec } from "../src/openapi/builder";
 
 type Method = "get" | "post" | "put" | "patch" | "delete" | "head" | "options";
@@ -31,6 +34,7 @@ const EXPECTED_ROUTES: RouteEntry[] = [
   { method: "get", path: "/api/admin/audit" },
   { method: "get", path: "/api/audit/counts" },
   { method: "get", path: "/api/admin/users/{address}" },
+  { method: "post", path: "/api/admin/users/{address}/impersonate" },
   { method: "get", path: "/api/admin/feature-flags" },
   { method: "post", path: "/api/admin/feature-flags" },
   { method: "get", path: "/api/admin/feature-flags/{key}" },
@@ -150,8 +154,45 @@ function main(): number {
     exitCode = 1;
   }
 
+  // The checked-in YAML must be byte-for-byte reproducible from the registry.
+  // This catches manual edits and stale generated artifacts before deployment.
+  const generated = yaml.dump(spec, {
+    indent: 2,
+    lineWidth: 120,
+    noRefs: false,
+    sortKeys: false,
+  });
+  const artifactPath = path.resolve(__dirname, "..", "openapi.yaml");
+  const checkedIn = fs.readFileSync(artifactPath, "utf8");
+  if (generated !== checkedIn) {
+    console.error("FAIL: openapi.yaml is stale; run npm run openapi:generate and commit the result");
+    exitCode = 1;
+  }
+
+  // Representative contract invariants: paginated endpoints must describe
+  // both cursor/limit inputs and a validation error, while protected routes
+  // must carry the bearer security requirement.
+  const paths = spec.paths as Record<string, Record<string, any>>;
+  for (const route of ["/api/users", "/api/users/{address}/predictions"]) {
+    const operation = paths[route]?.get;
+    const parameterNames = new Set((operation?.parameters ?? []).map((p: any) => p.name));
+    if (!parameterNames.has("cursor") || !parameterNames.has("limit") || !operation?.responses?.["400"]) {
+      console.error(`FAIL: ${route} must document cursor, limit, and a 400 validation response`);
+      exitCode = 1;
+    }
+  }
+  for (const [route, item] of Object.entries(paths)) {
+    for (const [method, operation] of Object.entries(item)) {
+      if (!["get", "post", "put", "patch", "delete"].includes(method)) continue;
+      if (operation.security && operation.security.length > 0 && !operation.responses?.["401"] && !operation.responses?.["403"]) {
+        console.error(`FAIL: protected ${method.toUpperCase()} ${route} must document an auth error response`);
+        exitCode = 1;
+      }
+    }
+  }
+
   if (exitCode === 0) {
-    console.log(`OK: all ${EXPECTED_ROUTES.length} routes documented correctly`);
+    console.log(`OK: routes, reproducible artifact, and representative contracts validated`);
   }
 
   return exitCode;
diff --git a/src/openapi/registry.ts b/src/openapi/registry.ts
index f508cfe8..802e793c 100644
--- a/src/openapi/registry.ts
+++ b/src/openapi/registry.ts
@@ -1032,6 +1032,10 @@ registry.registerPath({
         },
       },
     },
+    403: {
+      description: "Forbidden — caller is not an administrator",
+      content: { "application/json": { schema: ErrorBody } },
+    },
     404: {
       description: "Not found",
       content: {
@@ -1427,6 +1431,10 @@ registry.registerPath({
         },
       },
     },
+    400: {
+      description: "Invalid pagination parameters",
+      content: { "application/json": { schema: ErrorBody } },
+    },
   },
 });