diff --git a/core/http/react-ui/src/components/addons/AppsView.jsx b/core/http/react-ui/src/components/addons/AppsView.jsx
index 77d656f6a..f8bd5c793 100644
--- a/core/http/react-ui/src/components/addons/AppsView.jsx
+++ b/core/http/react-ui/src/components/addons/AppsView.jsx
@@ -267,7 +267,7 @@ function AppCard({ app, t, language, actions, onStartStop }) {
 }
 
 function ActionResult({ outcome, t }) {
-  const text = describeOutcome(outcome.outcome, t)
+  const text = describeOutcome(outcome.outcome, t, outcome.error)
   if (!text) return null
   return <p className="ad-card__note" data-tone="error" role="alert">{text}</p>
 }
diff --git a/core/http/react-ui/src/components/addons/BrowseView.jsx b/core/http/react-ui/src/components/addons/BrowseView.jsx
index f52373ea5..8f70f38fb 100644
--- a/core/http/react-ui/src/components/addons/BrowseView.jsx
+++ b/core/http/react-ui/src/components/addons/BrowseView.jsx
@@ -86,7 +86,7 @@ export default function BrowseView({ api, adapter }) {
           </div>
           {pending && <UnknownNotice addonId={entry.id} actions={actions} />}
           {!pending && st && !working && st.outcome !== 'completed' && (
-            <p className="ad-entry__note" data-tone="error" role="alert">{describeOutcome(st.outcome, t)}</p>
+            <p className="ad-entry__note" data-tone="error" role="alert">{describeOutcome(st.outcome, t, st.error)}</p>
           )}
           {st && st.outcome === 'completed' && <p className="ad-entry__note" data-tone="ok" role="status">{t('browse.installedNotEnabled')}</p>}
         </div>
diff --git a/core/http/react-ui/src/components/addons/InstalledView.jsx b/core/http/react-ui/src/components/addons/InstalledView.jsx
index 0e214342f..894755c9e 100644
--- a/core/http/react-ui/src/components/addons/InstalledView.jsx
+++ b/core/http/react-ui/src/components/addons/InstalledView.jsx
@@ -194,7 +194,7 @@ function latest(actions, addon, verbs) {
 function outcomeText(result, t) {
   const type = result && result.error && result.error.type
   if (type && UPDATE_TYPES.has(type)) return t(`updates.errors.${type}`)
-  return describeOutcome(result.outcome, t)
+  return describeOutcome(result.outcome, t, result.error)
 }
 
 function UpdateFact({ view, t }) {
diff --git a/core/http/react-ui/src/components/addons/errors.js b/core/http/react-ui/src/components/addons/errors.js
index a3c23799a..6999e4a57 100644
--- a/core/http/react-ui/src/components/addons/errors.js
+++ b/core/http/react-ui/src/components/addons/errors.js
@@ -1,11 +1,20 @@
+// The reason the server gave for a refusal, as sent. AddonsError falls back to
+// the kind when the body had no message, and that is not a reason.
+function serverReason(error) {
+  const m = error && typeof error.message === 'string' ? error.message.trim() : ''
+  return m && m !== error.kind ? m : ''
+}
+
 // One message per kind of answer, the same on every page. The server's words
-// are never shown as success and never hidden behind a generic "failed".
+// are never shown as success and never hidden behind a generic "failed". A
+// refusal (403 or 400) shows the server's own reason when the body has one,
+// untranslated, and the generic text only when it has none.
 export function describeError(error, t) {
   const kind = error && error.kind
   switch (kind) {
     case 'auth': return t('errors.auth')
-    case 'denied': return t('errors.denied')
-    case 'invalid': return t('errors.invalid')
+    case 'denied': return serverReason(error) || t('errors.denied')
+    case 'invalid': return serverReason(error) || t('errors.invalid')
     case 'unknown_outcome': return t('errors.unknownOutcome')
     case 'network': return t('errors.network')
     default: return t('errors.unavailable')
@@ -14,13 +23,13 @@ export function describeError(error, t) {
 
 // The words for the outcome of an action the user started. `completed` has
 // none: the list on the page shows the new state.
-export function describeOutcome(outcome, t) {
+export function describeOutcome(outcome, t, error) {
   switch (outcome) {
     case 'completed': return ''
     case 'failed': return t('actions.failed')
     case 'unknown': return t('actions.unknown')
     case 'not_applied': return t('actions.notApplied')
     case 'blocked': return t('actions.blocked')
-    default: return describeError({ kind: outcome }, t)
+    default: return describeError({ kind: outcome, message: error && error.message }, t)
   }
 }
diff --git a/core/http/react-ui/tests/unit/addons.test.mjs b/core/http/react-ui/tests/unit/addons.test.mjs
index 34f6b9bc4..455da2e96 100644
--- a/core/http/react-ui/tests/unit/addons.test.mjs
+++ b/core/http/react-ui/tests/unit/addons.test.mjs
@@ -12,6 +12,7 @@ import {
 import { createAddonsApi, createAddonsCatalogAdapter, AddonsError } from '../../src/utils/addonsApi.js'
 import { runLifecycle, settleOperation } from '../../src/components/addons/lifecycleFlow.js'
 import { createActionBook, effectOutcome } from '../../src/components/addons/actionBook.js'
+import { describeError, describeOutcome } from '../../src/components/addons/errors.js'
 
 const json = (status, body) => ({ ok: status >= 200 && status < 300, status, json: async () => body })
 
@@ -262,3 +263,18 @@ test('a completed status that left the add-on unchanged is not reported as done'
   assert.equal(effectOutcome('failed', false), 'failed')
   assert.equal(effectOutcome('unknown', false), 'unknown')
 })
+
+test('a refusal shows the server reason and falls back to the generic text', () => {
+  const t = key => `T:${key}`
+  const withBody = new AddonsError(classifyResponse(403, { error: { type: 'addon_error', message: 'Same-origin browser request required' } }))
+  assert.equal(describeError(withBody, t), 'Same-origin browser request required')
+  assert.equal(describeOutcome('denied', t, withBody), 'Same-origin browser request required')
+  const noBody = new AddonsError(classifyResponse(403, null))
+  assert.equal(describeError(noBody, t), 'T:errors.denied')
+  assert.equal(describeOutcome('denied', t, noBody), 'T:errors.denied')
+  assert.equal(describeOutcome('denied', t), 'T:errors.denied')
+  const invalid = new AddonsError(classifyResponse(400, { error: { message: 'Bad id' } }))
+  assert.equal(describeError(invalid, t), 'Bad id')
+  // Other kinds keep their own words.
+  assert.equal(describeError(new AddonsError(classifyResponse(401, { error: { message: 'x' } })), t), 'T:errors.auth')
+})