diff --git a/client/src/modules/version.js b/client/src/modules/version.js
index a0fef7e..138f1b4 100644
--- a/client/src/modules/version.js
+++ b/client/src/modules/version.js
@@ -11,6 +11,9 @@
 // that the two files can drift, so a test asserts they agree
 // (client/test/moduleRegistry.test.js) rather than trusting a bump to remember
 // both.
+// 1.19.0 — an event action may declare `demoRefuses(params)` (DEMO_MODE.md §3.4,
+// DM18): the demo refuses a step at save. Server-side only; the refusal reaches
+// the browser as an ordinary DEMO_LOCKED answer. This file bumps for the reason at the top.
 // 1.18.0 — the public demo (DEMO_MODE.md §3.2): `demoLocks` in module.json,
 // `ctx.demoMode`, `api.onDemoSeed`. Server-side; the client half is core's own
 // banner and refusal toast, which an installed module's requests reach through
@@ -102,4 +105,4 @@
 // but the two halves state ONE version: a module declares a single coreApi range
 // and is served one chunk, so a client that claimed 1.0.0 while the server
 // answered 1.1.0 would be two answers to one question.
-export const MODULE_API_VERSION = '1.18.0'
+export const MODULE_API_VERSION = '1.19.0'
diff --git a/server/engagement-triggers.json b/server/engagement-triggers.json
index a748e2e..eba8631 100644
--- a/server/engagement-triggers.json
+++ b/server/engagement-triggers.json
@@ -1,6 +1,6 @@
 {
   "_comment": "Generated event-trigger inventory - the authoritative freeze of CORE's engagement contract (docs/website/ENGAGEMENT.md 4.3). Regenerate with `npm run engagement:manifest` in website/server. A renamed variable, a changed type or a widened ceiling breaks stored templates and rules, so the diff here is the review signal. A module ships its own copy in its bundle; this file never contains one.",
-  "moduleApiVersion": "1.18.0",
+  "moduleApiVersion": "1.19.0",
   "triggers": [
     {
       "id": "event.phase.changed",
diff --git a/server/src/model/events/eventDefinitions.model.js b/server/src/model/events/eventDefinitions.model.js
index d968312..2613b00 100644
--- a/server/src/model/events/eventDefinitions.model.js
+++ b/server/src/model/events/eventDefinitions.model.js
@@ -29,6 +29,8 @@ const spec = require('../../events/spec')
 const authorize = require('../../events/authorize')
 const verifier = require('../../events/verify')
 const registries = require('../../modules/registries')
+const { isDemo } = require('../../demo/mode')
+const { CODE: DEMO_LOCKED } = require('../../demo/locks')
 const { slugify, uniqueSlug } = require('../teams/teamSlug')
 const { cleanBody } = require('../../utils/sanitizeHtml')
 const log = require('../../utils/logger')('events')
@@ -181,6 +183,8 @@ async function create(input, userId, { role = null } = {}) {
   if (!result.ok) return result
   const floor = checkRoleFloor(result.definition.spec, role)
   if (floor) return floor
+  const refused = await checkDemoRefusals(result.definition.spec)
+  if (refused) return refused
   const id = await db.insert({ ...result.definition, created_by: userId })
   return { ok: true, id, definition: await db.getById(id) }
 }
@@ -203,6 +207,8 @@ async function save(id, input, userId, { role = null } = {}) {
   if (!result.ok) return result
   const floor = checkRoleFloor(result.definition.spec, role)
   if (floor) return floor
+  const refused = await checkDemoRefusals(result.definition.spec)
+  if (refused) return refused
   await db.update(id, { ...result.definition, updated_by: userId })
   return { ok: true, id, definition: await db.getById(id) }
 }
@@ -228,6 +234,11 @@ async function publish(id, userId) {
   })
   if (!checked.ok) return { ok: false, status: 400, errors: checked.errors }
 
+  // Asked again here, as the registries are: a draft saved before the module's
+  // list grew must not become a version that runs (DM18).
+  const refused = await checkDemoRefusals(checked.spec)
+  if (refused) return refused
+
   const publishable = spec.publishable(checked.spec)
   if (!publishable.ok) {
     return {
@@ -365,6 +376,37 @@ function checkRoleFloor(specValue, role) {
   }
 }
 
+/**
+ * The public demo's save-time refusal (MODULE_API 1.19.0, DEMO_MODE.md §3.4,
+ * DM18): each step whose action declares `demoRefuses(params)` is asked, and
+ * the first sentence back refuses the whole save `403 DEMO_LOCKED`. Null off
+ * the demo, where nothing is asked.
+ *
+ * A hook that throws refuses too, with core's sentence: this is a lock, and a
+ * lock that cannot decide fails closed, as the route lock does (§3.3).
+ */
+async function checkDemoRefusals(specValue) {
+  if (!isDemo()) return null
+  for (const phase of specValue?.phases || []) {
+    for (const step of phase.steps || []) {
+      const action = registries.eventAction(step.actionId)
+      if (!action || !action.demoRefuses) continue
+      let said
+      try {
+        said = await action.demoRefuses(step.params || {})
+      } catch (err) {
+        log.error('demoRefuses failed', { action: action.id, message: err.message })
+        said = `"${action.label}" could not be checked, so it is turned off on the demo site.`
+      }
+      if (typeof said === 'string' && said.trim()) {
+        const message = said.trim()
+        return { ok: false, status: 403, code: DEMO_LOCKED, message, errors: [message] }
+      }
+    }
+  }
+  return null
+}
+
 /**
  * Dry-run a definition, and record the pass when there is a version to record it
  * on (Phase 6).
@@ -545,6 +587,7 @@ module.exports = {
   archive,
   verify,
   checkRoleFloor,
+  checkDemoRefusals,
   isTimezone,
   MIN_GRACE_SECONDS,
   MAX_GRACE_SECONDS,
diff --git a/server/src/modules/registries.js b/server/src/modules/registries.js
index 8bdf12c..e6ab5c6 100644
--- a/server/src/modules/registries.js
+++ b/server/src/modules/registries.js
@@ -445,14 +445,14 @@ async function resolveAudience(id, params = {}) {
 /**
  * Every declaration WITHOUT its callables — what the admin catalog serves.
  *
- * `perform`, `revert`, `reconcile`, `cost` and `progress` are stripped for the same reason `resolve` is
+ * `perform`, `revert`, `reconcile`, `cost`, `progress` and `demoRefuses` are stripped for the same reason `resolve` is
  * stripped from an audience and `handler` from a slash command: this is the
  * object that leaves the process, and the browser's whole relationship with an
  * action is naming one by id. §F's "a module registers actions server-side and
  * adds no routes for them" is only true if the functions never ride out.
  */
 const allEventActions = () =>
-  [...eventActions.values()].map(({ perform, revert, reconcile, cost, progress, ...rest }) => rest)
+  [...eventActions.values()].map(({ perform, revert, reconcile, cost, progress, demoRefuses, ...rest }) => rest)
 
 /** One declaration, callables included. The runner's lookup (Phase 2). */
 const eventAction = (id) => eventActions.get(id) || null
@@ -1128,7 +1128,7 @@ function checkActionParam(actionId, entry, seen, call = 'registerEventActions')
 }
 
 /**
- * `registerEventActions([{ id, label, risk, reversible, version, budgetMs, cost, params, perform, revert, reconcile, progress }])`.
+ * `registerEventActions([{ id, label, risk, reversible, version, budgetMs, cost, params, perform, revert, reconcile, progress, demoRefuses }])`.
  *
  * A typed verb core may ask a registrant to carry out. Everything decidable from
  * the argument alone is decided here, at the call; the collision — is this id
@@ -1211,6 +1211,13 @@ function checkEventActionShape(entry) {
   if (a.progress !== undefined && typeof a.progress !== 'function') {
     throw new Error(`registerEventActions: ${a.id} progress must be a function`)
   }
+  // 1.19.0, optional: the public demo's save-time refusal (DEMO_MODE.md §3.4,
+  // DM18). Asked only with DEMO_MODE on, by the definition save path; a
+  // sentence back refuses the save `403 DEMO_LOCKED`. The module still checks
+  // at run, because a definition can predate the list it is checked against.
+  if (a.demoRefuses !== undefined && typeof a.demoRefuses !== 'function') {
+    throw new Error(`registerEventActions: ${a.id} demoRefuses must be a function of its params`)
+  }
 
   const version = a.version === undefined ? 1 : a.version
   if (!Number.isInteger(version) || version < 1) {
@@ -1244,6 +1251,7 @@ function checkEventActionShape(entry) {
     revert: a.revert || null,
     reconcile: a.reconcile || null,
     progress: a.progress || null,
+    demoRefuses: a.demoRefuses || null,
   }
 }
 
diff --git a/server/src/modules/version.js b/server/src/modules/version.js
index 00db8cf..6c69e52 100644
--- a/server/src/modules/version.js
+++ b/server/src/modules/version.js
@@ -9,6 +9,15 @@
 // Deliberately separate from PROTOCOL_VERSION (which versions the shard wire and
 // has nothing to say about a website module) and from any module's own version.
 
+// 1.19.0 — the public demo refuses a step at SAVE (docs/website/DEMO_MODE.md
+// §3.4, DM18). An event action may declare `demoRefuses(params)`, returning a
+// sentence or null. With DEMO_MODE on, core asks it for every step when a
+// definition is created, saved or published, and a sentence refuses the whole
+// write `403 DEMO_LOCKED` with that sentence as `message`; a hook that throws
+// refuses too. Off the demo it is validated and never called. One optional
+// action member, so minor. Module-uo declares `^1.10.0` and declares none.
+// Module-Rust's console-command deny list is the first user.
+//
 // 1.18.0 — the public demo (docs/website/DEMO_MODE.md §3.2, §3.4, §7.3; website#222).
 // Three additions, each optional:
 //   • `demoLocks` in module.json — `[{ method, path }]`, writes only, each path
@@ -245,6 +254,6 @@
 // an admin action a module performs belongs in core's one audit log, the
 // extension slot needs the user its prefix names, and §2.7 forbids a module
 // reading core's `APP_BASE_URL` for itself. Additions only, so minor.
-const MODULE_API_VERSION = '1.18.0'
+const MODULE_API_VERSION = '1.19.0'
 
 module.exports = { MODULE_API_VERSION }
diff --git a/server/src/router/v1/admin/events.controller.js b/server/src/router/v1/admin/events.controller.js
index 1e16af5..165304b 100644
--- a/server/src/router/v1/admin/events.controller.js
+++ b/server/src/router/v1/admin/events.controller.js
@@ -42,6 +42,16 @@ const asId = (raw) => {
   return Number.isInteger(n) && n > 0 ? n : null
 }
 
+/**
+ * A refused save's body: the errors, plus `code` and `message` when the model
+ * set them. The demo's step refusal does (DM18), so the client's one
+ * `DEMO_LOCKED` toast fires as it does for a route lock.
+ */
+const refusal = (result) => ({
+  errors: result.errors,
+  ...(result.code ? { code: result.code, message: result.message } : {}),
+})
+
 /**
  * The shape a definition takes on the wire.
  *
@@ -413,7 +423,7 @@ exports.listVersions = async (req, res) => {
 /** POST /api/v1/admin/events */
 exports.create = async (req, res) => {
   const result = await definitions.create(req.body, req.user.id, { role: req.user.role })
-  if (!result.ok) return res.status(result.status || 400).json({ errors: result.errors })
+  if (!result.ok) return res.status(result.status || 400).json(refusal(result))
   await activity.log({
     req,
     action: 'event.definition.created',
@@ -427,7 +437,7 @@ exports.update = async (req, res) => {
   const id = asId(req.params.id)
   if (!id) return res.status(400).json({ error: 'bad event id' })
   const result = await definitions.save(id, req.body, req.user.id, { role: req.user.role })
-  if (!result.ok) return res.status(result.status || 400).json({ errors: result.errors })
+  if (!result.ok) return res.status(result.status || 400).json(refusal(result))
   await activity.log({
     req,
     action: 'event.definition.updated',
@@ -441,7 +451,7 @@ exports.publish = async (req, res) => {
   const id = asId(req.params.id)
   if (!id) return res.status(400).json({ error: 'bad event id' })
   const result = await definitions.publish(id, req.user.id)
-  if (!result.ok) return res.status(result.status || 400).json({ errors: result.errors })
+  if (!result.ok) return res.status(result.status || 400).json(refusal(result))
   await activity.log({
     req,
     action: 'event.definition.published',
diff --git a/server/swagger/swagger-output.json b/server/swagger/swagger-output.json
index 1fca305..88c14eb 100644
--- a/server/swagger/swagger-output.json
+++ b/server/swagger/swagger-output.json
@@ -18553,7 +18553,7 @@
           },
           "description": {
             "type": "string",
-            "example": "The public demo refused a write (DEMO_MODE.md §3). Only on a deployment with `DEMO_MODE=true`. `keys` is present on `PUT /admin/settings` and names the settings the demo keeps."
+            "example": "The public demo refused a write (DEMO_MODE.md §3). Only on a deployment with `DEMO_MODE=true`. `keys` is present on `PUT /admin/settings` and names the settings the demo keeps. `errors` is present when an event save or publish is refused because a step's action refused its params (MODULE_API 1.19.0 `demoRefuses`, DM18); `message` is then the module's sentence."
           },
           "properties": {
             "type": "object",
@@ -18615,6 +18615,33 @@
                     }
                   }
                 }
+              },
+              "errors": {
+                "type": "object",
+                "properties": {
+                  "type": {
+                    "type": "string",
+                    "example": "array"
+                  },
+                  "items": {
+                    "type": "object",
+                    "properties": {
+                      "type": {
+                        "type": "string",
+                        "example": "string"
+                      }
+                    }
+                  },
+                  "example": {
+                    "type": "array",
+                    "example": [
+                      "\"quit\" is turned off on the demo site."
+                    ],
+                    "items": {
+                      "type": "string"
+                    }
+                  }
+                }
               }
             }
           },
diff --git a/server/swagger/swagger.js b/server/swagger/swagger.js
index a48d084..c96d9bc 100644
--- a/server/swagger/swagger.js
+++ b/server/swagger/swagger.js
@@ -109,11 +109,12 @@ const doc = {
       DemoLocked: {
         type: 'object',
         description:
-          'The public demo refused a write (DEMO_MODE.md §3). Only on a deployment with `DEMO_MODE=true`. `keys` is present on `PUT /admin/settings` and names the settings the demo keeps.',
+          'The public demo refused a write (DEMO_MODE.md §3). Only on a deployment with `DEMO_MODE=true`. `keys` is present on `PUT /admin/settings` and names the settings the demo keeps. `errors` is present when an event save or publish is refused because a step\'s action refused its params (MODULE_API 1.19.0 `demoRefuses`, DM18); `message` is then the module\'s sentence.',
         properties: {
           message: { type: 'string', example: 'This is turned off on the demo site.' },
           code: { type: 'string', enum: ['DEMO_LOCKED'] },
           keys: { type: 'array', items: { type: 'string' }, example: ['site_mode'] },
+          errors: { type: 'array', items: { type: 'string' }, example: ['"quit" is turned off on the demo site.'] },
         },
         required: ['message', 'code'],
       },
diff --git a/server/test/demoStepRefusal.test.js b/server/test/demoStepRefusal.test.js
new file mode 100644
index 0000000..317677d
--- /dev/null
+++ b/server/test/demoStepRefusal.test.js
@@ -0,0 +1,154 @@
+// ── The demo refuses a step at save (MODULE_API 1.19.0, DEMO_MODE.md §3.4) ──
+//
+// DM18: an event action may declare `demoRefuses(params)`. With DEMO_MODE on,
+// the definition save path asks it for every step at create, save and publish,
+// and a sentence back refuses the write `403 DEMO_LOCKED`. Off the demo it is
+// never called. The definitions table is stubbed on the module object the model
+// holds; the pool points at a closed port.
+process.env.DB_HOST = '127.0.0.1'
+process.env.DB_PORT = '59999'
+
+const { test, beforeEach, afterEach, after } = require('node:test')
+const assert = require('node:assert/strict')
+
+const db = require('../src/utils/db')
+const registries = require('../src/modules/registries')
+const definitions = require('../src/model/events/eventDefinitions.model')
+const definitionsDb = require('../src/model/events/eventDefinitions.db')
+const controller = require('../src/router/v1/admin/events.controller')
+
+after(() => db.close())
+
+let asked
+
+const register = (extra = {}) => {
+  const api = registries.stage('demo')
+  api.registerEventActions([{
+    id: 'demo.console.run',
+    label: 'Run console commands',
+    risk: 'irreversible',
+    reversible: 'none',
+    params: [{ name: 'command', type: 'string', required: true, example: 'say hi' }],
+    perform: async () => ({ ok: true }),
+    demoRefuses: (params) => {
+      asked.push(params)
+      return params.command === 'quit' ? '"quit" is turned off on the demo site.' : null
+    },
+    ...extra,
+  }])
+  registries.apply(api.staged)
+}
+
+const input = (command) => ({
+  title: 'Console night',
+  spec: {
+    schedule: { kind: 'manual' },
+    phases: [{ key: 'main', label: 'Main', steps: [{ actionId: 'demo.console.run', params: { command } }] }],
+  },
+})
+
+const stubTable = (t, existing = null) => {
+  const wrote = []
+  t.mock.method(definitionsDb, 'list', async () => [])
+  t.mock.method(definitionsDb, 'insert', async (row) => { wrote.push(row); return 7 })
+  t.mock.method(definitionsDb, 'update', async (_id, row) => { wrote.push(row) })
+  t.mock.method(definitionsDb, 'getById', async () => existing || { id: 7, state: 'draft', title: 'Console night', spec: input('say hi').spec })
+  return wrote
+}
+
+beforeEach(() => {
+  asked = []
+  registries._reset()
+  registries.registerCore()
+})
+afterEach(() => {
+  registries._reset()
+  delete process.env.DEMO_MODE
+})
+
+test('demoRefuses must be a function, and never rides out to the catalog', () => {
+  assert.throws(() => register({ demoRefuses: 'quit' }), /demoRefuses must be a function/)
+  registries._reset()
+  register()
+  assert.equal(typeof registries.eventAction('demo.console.run').demoRefuses, 'function')
+  const served = registries.allEventActions().find((a) => a.id === 'demo.console.run')
+  assert.equal('demoRefuses' in served, false)
+})
+
+test('off the demo the hook is never asked, and a denied command saves', async (t) => {
+  register()
+  const wrote = stubTable(t)
+  const made = await definitions.create(input('quit'), 1, { role: 'admin' })
+  assert.equal(made.ok, true)
+  assert.equal(wrote.length, 1)
+  assert.deepEqual(asked, [])
+})
+
+test('on the demo a create naming a denied command is refused 403 DEMO_LOCKED and writes nothing', async (t) => {
+  process.env.DEMO_MODE = 'true'
+  register()
+  const wrote = stubTable(t)
+  const made = await definitions.create(input('quit'), 1, { role: 'admin' })
+  assert.deepEqual(made, {
+    ok: false,
+    status: 403,
+    code: 'DEMO_LOCKED',
+    message: '"quit" is turned off on the demo site.',
+    errors: ['"quit" is turned off on the demo site.'],
+  })
+  assert.equal(wrote.length, 0)
+  // The hook is handed the step's checked params.
+  assert.deepEqual(asked, [{ command: 'quit' }])
+})
+
+test('on the demo a command the hook allows saves', async (t) => {
+  process.env.DEMO_MODE = 'true'
+  register()
+  const wrote = stubTable(t)
+  assert.equal((await definitions.create(input('say hi'), 1, { role: 'admin' })).ok, true)
+  assert.equal((await definitions.save(7, input('say hi'), 1, { role: 'admin' })).ok, true)
+  assert.equal(wrote.length, 2)
+})
+
+test('on the demo a save and a publish are refused too', async (t) => {
+  process.env.DEMO_MODE = 'true'
+  register()
+  const wrote = stubTable(t, { id: 7, state: 'draft', title: 'Console night', spec: input('quit').spec })
+  const saved = await definitions.save(7, input('quit'), 1, { role: 'admin' })
+  assert.equal(saved.status, 403)
+  assert.equal(saved.code, 'DEMO_LOCKED')
+  // A draft written before the list grew: publish asks again.
+  const published = await definitions.publish(7, 1)
+  assert.equal(published.status, 403)
+  assert.equal(published.code, 'DEMO_LOCKED')
+  assert.equal(wrote.length, 0)
+})
+
+test('a hook that throws refuses with core\'s sentence (fails closed)', async (t) => {
+  process.env.DEMO_MODE = 'true'
+  register({ demoRefuses: () => { throw new Error('boom') } })
+  stubTable(t)
+  const made = await definitions.create(input('say hi'), 1, { role: 'admin' })
+  assert.equal(made.status, 403)
+  assert.equal(made.code, 'DEMO_LOCKED')
+  assert.match(made.message, /"Run console commands" could not be checked/)
+})
+
+test('the controller answers the refusal with code and message, so the toast fires', async (t) => {
+  process.env.DEMO_MODE = 'true'
+  register()
+  stubTable(t)
+  let status
+  let body
+  const res = {
+    status(s) { status = s; return this },
+    json(b) { body = b; return this },
+  }
+  await controller.create({ body: input('quit'), user: { id: 1, role: 'admin' } }, res)
+  assert.equal(status, 403)
+  assert.deepEqual(body, {
+    errors: ['"quit" is turned off on the demo site.'],
+    code: 'DEMO_LOCKED',
+    message: '"quit" is turned off on the demo site.',
+  })
+})
diff --git a/server/test/eventModuleContract.test.js b/server/test/eventModuleContract.test.js
index aebbfcd..fe37060 100644
--- a/server/test/eventModuleContract.test.js
+++ b/server/test/eventModuleContract.test.js
@@ -112,7 +112,7 @@ test('the version a module declares against satisfies ^1.10.0', () => {
   // 1.10.0 loading — Module-uo declares exactly `^1.10.0` — and this is the
   // assertion that says it does. The number itself is pinned in version.js.
   assert.ok(semver.satisfies(MODULE_API_VERSION, '^1.10.0'), MODULE_API_VERSION)
-  assert.equal(MODULE_API_VERSION, '1.18.0')
+  assert.equal(MODULE_API_VERSION, '1.19.0')
 })
 
 test('a module registers actions, budgets, leases and option sources', () => {