diff --git a/package-lock.json b/package-lock.json
index 60692b7..4cb5170 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "@hellocoop/mockin",
-  "version": "3.1.0",
+  "version": "3.1.1",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "@hellocoop/mockin",
-      "version": "3.1.0",
+      "version": "3.1.1",
       "license": "MIT",
       "dependencies": {
         "@fastify/cors": "^11.3.0",
diff --git a/package.json b/package.json
index 2590780..6b82fb1 100644
--- a/package.json
+++ b/package.json
@@ -1,7 +1,7 @@
 {
   "name": "@hellocoop/mockin",
   "private": false,
-  "version": "3.1.0",
+  "version": "3.1.1",
   "description": "Hellō Mock Login OpenID Connect Server",
   "engines": {
     "node": ">=22"
diff --git a/src/aauth/issue-auth-token.js b/src/aauth/issue-auth-token.js
index 1172f31..8baa770 100644
--- a/src/aauth/issue-auth-token.js
+++ b/src/aauth/issue-auth-token.js
@@ -115,7 +115,15 @@ export async function issueAuthToken({
     const cfg = getConfig()
     const iat = Math.floor(Date.now() / 1000)
     let lifetime = Math.min(cfg.token_lifetime || MAX_AUTH_TOKEN_TTL, MAX_AUTH_TOKEN_TTL)
-    if (Number.isFinite(presented_exp)) {
+    if (Number.isFinite(presented_exp) && lifetime > 0) {
+        // Never outlive the presented token (-11 §Auth Token Structure), and
+        // never let a presented token that is nearly expired turn a positive
+        // lifetime into a negative one — one second is still a live token.
+        //
+        // The floor applies to the clamp only. A NEGATIVE `token_lifetime` is
+        // the mock switch for minting an already-expired token, which the
+        // challenge-on-401 suites depend on; flooring that to 1 would issue a
+        // valid token and quietly break them.
         lifetime = Math.max(1, Math.min(lifetime, presented_exp - iat))
     }
     const { identity, resource } = classifyScopes(scope)
diff --git a/test/aauth/token.identity.spec.js b/test/aauth/token.identity.spec.js
index 4e05835..5f81ead 100644
--- a/test/aauth/token.identity.spec.js
+++ b/test/aauth/token.identity.spec.js
@@ -139,6 +139,29 @@ describe('AAuth auth_token_endpoint — identity flow (no R3)', function () {
         expect(claims.exp - claims.iat).to.equal(3600)
     })
 
+    it('a negative token_lifetime still mints an expired token', async function () {
+        // The switch exists so a resource-side suite can drive its
+        // challenge-on-401 path. Clamping to the presented token's exp must
+        // not floor a deliberately negative lifetime up to 1s — that issues a
+        // live token and the challenge never fires.
+        // Mint the person token FIRST — token_lifetime shapes it too, and an
+        // expired presented token is a different (correct) 400.
+        const { agentToken, body } = await personAndResourceToken(fastify, {
+            resource: { scope: 'openid' },
+        })
+        await fastify.inject({
+            method: 'PUT',
+            url: '/mock/aauth',
+            headers: { 'content-type': 'application/json' },
+            payload: JSON.stringify({ token_lifetime: -60 }),
+        })
+        const response = await postAuthToken(fastify, { body, agentToken })
+        expect(response.statusCode).to.equal(200)
+        const claims = decodeJwt(response.json().auth_token)
+        expect(claims.exp - claims.iat).to.equal(-60)
+        expect(claims.exp).to.be.below(Math.floor(Date.now() / 1000))
+    })
+
     it('never outlives the presented token (agent token 600s → auth token ≤ 600s)', async function () {
         const { response, personClaims } = await postToken({
             person: { agentToken: await mintAgentToken({ ttl: 600 }) },