@@ -19,6 +19,7 @@ like that" is not one.
 
 | Job | Use | Never |
 | --- | --- | --- |
+| Explain a field, dialog or row | Its label, placeholder and options (`Access keys (long-lived)`); `error` for validation, `disabledReason` for a locked control, the dialog's Setup guide for steps | A `hint`, form `Modal` `description`, written `SettingsRow`/`DetailHeader` subtitle or data `hint` (`check:platform-design` `helper-copy`; a line that must stay is named in `HELPER_COPY_OWNERS`) |
 | First read of a view | Render nothing for data-backed content while pending (static chrome renders at once), `FailureNotice` when the read fails, `EmptyState` when it settles empty. Read the query's `status`, not an empty list. An action's own pending label (`pendingLabel`, a Load action's `Loading…`) is feedback, not loading copy | A skeleton, shimmer, placeholder rows or loading copy (`check:platform-design` `loading-copy`); a pending hint, placeholder or detail line; treating empty as loading |
 | Failed read | `FailureNotice`, labelled Retry. `intent='warning'` where the read partly answered | A hand-built retry button or `InlineNotice` + `Button` |
 | Failed background refresh | `FailureNotice` above the content it keeps | Replacing content already on screen |
 
@@ -13,7 +13,7 @@ paths:
 
 **Do not add subtitles, helper text, or descriptive copy beneath headings, labels, cards, or settings by default.** Prefer one concise, self-explanatory heading or label. Only add supporting copy when the user explicitly asks for it, or when it is necessary to prevent misunderstanding or error — and never use it to restate the heading.
 
-This applies to app UI outside the Platform: settings rows, modals, panels, cards, list rows, empty states, form fields, and section headers. Platform views (`app/(product)`, `platform`) follow `apps/sim/platform/AGENTS.md`; marketing surfaces (`app/(landing)`, docs) follow `constitution.md`.
+This applies to app UI outside the Platform: settings rows, modals, panels, cards, list rows, empty states, form fields, and section headers. Platform views (`app/(product)`, `platform`) follow the stricter one-line-per-control rule in `apps/sim/platform/AGENTS.md`, which `check:platform-design` enforces (`helper-copy`); marketing surfaces (`app/(landing)`, docs) follow `constitution.md`.
 
 **Carve-out — settings section metadata.** `SettingsNavigationItem.description` in `components/settings/navigation.ts` stays required, and `SettingsPanel` keeps rendering it as the page subtitle. Settings sections are reached through a nav list where the description is the only thing distinguishing adjacent sections, so it earns its place by the "prevents misunderstanding" test. Keep those descriptions verb-first and one line, per `sim-settings-pages.md`. Everything else on a settings page — inline `<p>` blurbs under section headings, field hints, modal bodies, row subtitles — follows the default rule above.
 
@@ -32,14 +32,14 @@ This applies to app UI outside the Platform: settings rows, modals, panels, card
 <ChipModalField title='Workspace name' />
 ```
 
-If a heading needs a subtitle to be understood, the heading is wrong. Fix the heading — don't append a second line.
+If a heading needs a subtitle to be understood, the heading is wrong. Fix the heading — don't append a second line. For a field, bake the meaning into the line itself first: a format into the placeholder (`us-gov-west-1`), a bound or scope into the label (`Idle timeout (hours, min 1)`), a trade-off into the option (`Access keys (long-lived)`), or point to the dialog's setup guide.
 
 ## When supporting copy earns its place
 
 Keep (or add) a description only when it carries information the label cannot, and its absence would cause a mistake:
 
 - **Irreversible or destructive consequences** — "Deleting this workspace removes every workflow and log. This cannot be undone."
-- **A non-obvious format, unit, or bound** — "Comma-separated. Max 50 domains.", "Cost per 1M input tokens."
+- **A non-obvious format, unit, or bound the label, placeholder and options cannot hold** — "Comma-separated. Max 50 domains.", "Cost per 1M input tokens."
 - **A security or access implication** — "This key is shown once and grants full workspace access."
 - **A state the user cannot otherwise see** — "Inherited from your organization's policy."
 - **Instructional copy that advances a flow** — "We sent a 6-digit code to you@example.com."
 
@@ -17,6 +17,7 @@ like that" is not one.
 
 | Job | Use | Never |
 | --- | --- | --- |
+| Explain a field, dialog or row | Its label, placeholder and options (`Access keys (long-lived)`); `error` for validation, `disabledReason` for a locked control, the dialog's Setup guide for steps | A `hint`, form `Modal` `description`, written `SettingsRow`/`DetailHeader` subtitle or data `hint` (`check:platform-design` `helper-copy`; a line that must stay is named in `HELPER_COPY_OWNERS`) |
 | First read of a view | Render nothing for data-backed content while pending (static chrome renders at once), `FailureNotice` when the read fails, `EmptyState` when it settles empty. Read the query's `status`, not an empty list. An action's own pending label (`pendingLabel`, a Load action's `Loading…`) is feedback, not loading copy | A skeleton, shimmer, placeholder rows or loading copy (`check:platform-design` `loading-copy`); a pending hint, placeholder or detail line; treating empty as loading |
 | Failed read | `FailureNotice`, labelled Retry. `intent='warning'` where the read partly answered | A hand-built retry button or `InlineNotice` + `Button` |
 | Failed background refresh | `FailureNotice` above the content it keeps | Replacing content already on screen |
 
@@ -9,7 +9,7 @@ globs: ["apps/sim/components/**/*.tsx","apps/sim/ee/**/*.tsx","apps/sim/app/_she
 
 **Do not add subtitles, helper text, or descriptive copy beneath headings, labels, cards, or settings by default.** Prefer one concise, self-explanatory heading or label. Only add supporting copy when the user explicitly asks for it, or when it is necessary to prevent misunderstanding or error — and never use it to restate the heading.
 
-This applies to app UI outside the Platform: settings rows, modals, panels, cards, list rows, empty states, form fields, and section headers. Platform views (`app/(product)`, `platform`) follow `apps/sim/platform/AGENTS.md`; marketing surfaces (`app/(landing)`, docs) follow `constitution.md`.
+This applies to app UI outside the Platform: settings rows, modals, panels, cards, list rows, empty states, form fields, and section headers. Platform views (`app/(product)`, `platform`) follow the stricter one-line-per-control rule in `apps/sim/platform/AGENTS.md`, which `check:platform-design` enforces (`helper-copy`); marketing surfaces (`app/(landing)`, docs) follow `constitution.md`.
 
 **Carve-out — settings section metadata.** `SettingsNavigationItem.description` in `components/settings/navigation.ts` stays required, and `SettingsPanel` keeps rendering it as the page subtitle. Settings sections are reached through a nav list where the description is the only thing distinguishing adjacent sections, so it earns its place by the "prevents misunderstanding" test. Keep those descriptions verb-first and one line, per `sim-settings-pages.md`. Everything else on a settings page — inline `<p>` blurbs under section headings, field hints, modal bodies, row subtitles — follows the default rule above.
 
@@ -28,14 +28,14 @@ This applies to app UI outside the Platform: settings rows, modals, panels, card
 <ChipModalField title='Workspace name' />
 ```
 
-If a heading needs a subtitle to be understood, the heading is wrong. Fix the heading — don't append a second line.
+If a heading needs a subtitle to be understood, the heading is wrong. Fix the heading — don't append a second line. For a field, bake the meaning into the line itself first: a format into the placeholder (`us-gov-west-1`), a bound or scope into the label (`Idle timeout (hours, min 1)`), a trade-off into the option (`Access keys (long-lived)`), or point to the dialog's setup guide.
 
 ## When supporting copy earns its place
 
 Keep (or add) a description only when it carries information the label cannot, and its absence would cause a mistake:
 
 - **Irreversible or destructive consequences** — "Deleting this workspace removes every workflow and log. This cannot be undone."
-- **A non-obvious format, unit, or bound** — "Comma-separated. Max 50 domains.", "Cost per 1M input tokens."
+- **A non-obvious format, unit, or bound the label, placeholder and options cannot hold** — "Comma-separated. Max 50 domains.", "Cost per 1M input tokens."
 - **A security or access implication** — "This key is shown once and grants full workspace access."
 - **A state the user cannot otherwise see** — "Inherited from your organization's policy."
 - **Instructional copy that advances a flow** — "We sent a 6-digit code to you@example.com."
 
@@ -39,6 +39,8 @@ Use `ActivityRow` for a first-line glyph, avatar or artwork stack, title/detail
 
 Inputs and textareas share the neutral field frame, resting border and 8px text inset. Hover changes only the frame border; the fill remains unchanged. Focus alone preserves resting paint. Required placeholders describe the expected input without replacing the label. `viewOnly` supplies native read-only behavior and an always-visible shared copy action in the sidebar’s 24px trailing slot. Textareas place the same action at the top-right. Copy feedback remains visible for four seconds before returning to the copy action. Read-only fields show a not-allowed cursor without dimming, while their copy action remains interactive. TextArea defaultSize selects regular (76px), expanded (112px) or large (240px); skill instruction editors use large. Invite recipient fields share the regular textarea height and insets. Quiet `SearchField` is the explicit focus exception: its subtle underline starts at the typing edge, excluding the leading icon and trailing controls, and appears on hover or focus while the background remains unchanged. Counts inherit adjacent label metrics, use ordinary inline spacing and align on the text baseline. Disabled dropdown triggers and items retain their resting opacity, ink, border and fill with a not-allowed cursor; disabled behavior prevents activation, and hover/press never repaints them. Optional disabledReason descriptions use the shared tooltip and remain available to keyboard users. Supply them for non-obvious restrictions such as inherited access, protected administrators or unavailable prerequisites. Button and Checkbox expose the same optional reason; DataTable forwards selectionDisabledReason from its row metadata. Reuse the actual policy reason rather than a generic disabled message, and keep the control disabled while its explanation is focused. Secret reveal and copy use the same 24px icon action and field inset; text labels belong in their accessible names and tooltips.
 
+One line per control. A field, form dialog, settings row or page header carries its meaning in its label, placeholder and options, never in a second line beneath it: no field `hint`, no form `Modal` `description`, no written `SettingsRow` or `DetailHeader` subtitle, and no `hint` in the field data a dialog renders. Put a format in the placeholder (`us-gov-west-1`), a bound or scope in the label (`Idle timeout (hours, min 1)`) or an option (`Access keys (long-lived)`), point to the dialog's Setup guide, or delete the line. A validation message is an `error`; a disabled control's reason is its `disabledReason`; a preference's explanation belongs in its existing About tooltip. Only a line nothing else can hold stays (a one-time secret, an irreversible consequence, a security or privacy exposure, a state the viewer cannot otherwise see), named with that reason in `HELPER_COPY_OWNERS`; `check:platform-design` (`helper-copy`) fails any other.
+
 Toolbar choices use `ChoiceMenu`, the same compact quiet button and menu as Home's activity period. A toolbar action uses the existing quiet compact Button or IconButton. Page-level actions live in the control bar beneath the title and description; an action for a particular record belongs with that record. `Select` and `SelectField` provide bordered field chrome only for labeled forms, including modal forms. Immediate preferences, row roles and toolbar filters use compact text triggers without borders or chevrons. Searchable, multiple-value and date-range pickers retain their specialized behavior with their text presentation composed from the same compact quiet Button; do not place a full-width field picker in a control bar. Reuse the existing owner for the interaction and compare its actual trigger with Home before accepting it.
 
 Button-based field pickers use the owning Base UI Select.Label or Combobox.Label, through FieldFrame’s label slot, so clicking a caption focuses its control without closing and reopening the popup. A calendar popover uses a noninteractive caption explicitly associated with its trigger and displayed value. Text inputs retain native labels. Preserve selected values or counts in accessible names or descriptions alongside field hints and errors.
 
@@ -1131,3 +1131,65 @@ it('fails the route and copy rules: live-only-route, fixture-branch-in-view, fix
     rmSync(root, { recursive: true, force: true })
   }
 }, 30_000)
+
+it('fails [helper-copy] for a hint, a form subtitle or a static row subtitle, and passes reasons, errors and confirm bodies', () => {
+  const { root, put, run } = productFixture()
+  const view = 'app/(product)/o/[organizationId]/home/_components/key-form/key-form.tsx'
+  try {
+    put(
+      view,
+      `import { SelectField, TextField } from '@sim/emcn/field'
+      import { Modal } from '@sim/emcn/modal'
+      import { SettingsRow } from '@sim/emcn/settings-section'
+      import { Picker } from '@/app/(product)/o/[organizationId]/home/_components/picker/picker'
+      export function KeyForm({ note, removing }: { note?: string; removing: boolean }) {
+        return <>
+          <Modal title='Add key' description='Keys are encrypted and stored securely.'>
+            <TextField label='Name' placeholder='my-key' hint='Lowercase letters only.' />
+            <TextField label='Region' placeholder='us-east-1' error='Enter a region.' />
+            <SelectField label='Plan' options={[]} disabledReason='Upgrade first.' aria-description='Billing plan' />
+            <Picker label='Owner' hint={note} />
+            <Picker label='Team' hint='Pick the owning team.' />
+            <SettingsRow label='Theme' description='Select your preferred color theme' />
+            <SettingsRow label='Status' description={removing ? 'Removing' : 'Active'} />
+          </Modal>
+          <Modal variant='confirm' title='Remove key' description='This cannot be undone.' />
+        </>
+      }
+      export const FIELDS = [
+        { id: 'stsRegion', label: 'STS region', hint: 'Only needed for GovCloud keys.' },
+        { id: 'roleArn', label: 'Role ARN', hint: '' },
+      ]`
+    )
+    put(
+      'app/(product)/o/[organizationId]/home/_components/picker/picker.tsx',
+      `export function Picker({ label, hint }: { label: string; hint?: string }) { return <p title={hint}>{label}</p> }`
+    )
+    const result = run()
+    expect(result.status).toBe(1)
+    const helperCopy = result.output.split('\n').filter((line) => line.includes('[helper-copy]'))
+    for (const line of [7, 8, 12, 13, 20])
+      expect(
+        helperCopy.some((finding) => finding.startsWith(`${view}:${line} [helper-copy]`))
+      ).toBe(true)
+    expect(helperCopy).toHaveLength(5)
+    expect(helperCopy.join('\n')).toContain('bake it into the placeholder, label or option')
+    expect(result.output).toContain(`${view}#Name`)
+
+    writeAllowlists(root, {
+      HELPER_COPY_OWNERS: {
+        [`${view}#Add key`]: 'The fixture keeps its dialog subtitle to prove a grant',
+        [`${view}#Name`]: 'The fixture keeps its field hint to prove a grant',
+        [`${view}#Team`]: 'The fixture keeps its wrapper hint to prove a grant',
+        [`${view}#Theme`]: 'The fixture keeps its row subtitle to prove a grant',
+        [`${view}#stsRegion`]: 'The fixture keeps its descriptor hint to prove a grant',
+        [`${view}#Unused`]: 'Grants nothing, so the check names it stale',
+      },
+    })
+    const granted = run()
+    expect(granted.output).not.toContain('[helper-copy]')
+    expect(granted.output).toContain(`HELPER_COPY_OWNERS names ${view}#Unused`)
+  } finally {
+    rmSync(root, { recursive: true, force: true })
+  }
+}, 30_000)
@@ -20,7 +20,9 @@ import { ORGANIZATION_ROUTES, PRODUCT_ROUTES } from './platform-product-routes'
 import { ruleDocumentation } from './platform-rule-docs'
 import {
   classifyPage,
+  type HelperCopyKind,
   hasAsciiEllipsis,
+  helperCopy,
   inFixtureBranch,
   isFixtureModeBranch,
   isLoadingCopy,
@@ -1449,6 +1451,59 @@ for (const file of sourceFiles) {
     })
   }
 }
+/**
+ * Field data the Platform renders that lives outside its tree: the credential descriptors behind its
+ * connect dialogs, the OAuth client settings they ask for, and the generator of its integration
+ * catalog. A `hint` written there reaches a Platform field as surely as one written in a view.
+ */
+const HINT_DATA_MODULES: ReadonlySet<string> = new Set([
+  'lib/credentials/client-credential-accounts/descriptors.ts',
+  'lib/credentials/config-service-accounts/descriptors.ts',
+  'lib/credentials/token-service-accounts/descriptors.ts',
+  'lib/oauth/oauth.ts',
+  '../../scripts/platform-integration-connections.ts',
+])
+const HELPER_COPY_MESSAGES: Record<HelperCopyKind, string> = {
+  'field-hint': 'A field has no hint line',
+  'dialog-subtitle': 'A form dialog has no subtitle',
+  'row-subtitle': 'A settings row or page header has no written subtitle',
+  'data-hint': 'Field data carries no hint',
+}
+/**
+ * `helper-copy`: a field, form dialog, settings row or page header says what it needs in its own
+ * label, placeholder or options, never in a second line beneath. A line that must stay (a one-time
+ * secret warning, a consequence nothing else can show) is named in `HELPER_COPY_OWNERS` as
+ * `<file>#<label, title or field id>` with its reason.
+ */
+function checkHelperCopy(file: string) {
+  const content = read(file)
+  const source = ts.createSourceFile(
+    file,
+    content,
+    ts.ScriptTarget.Latest,
+    true,
+    file.endsWith('.tsx') ? ts.ScriptKind.TSX : ts.ScriptKind.TS
+  )
+  for (const { kind, node, subject } of helperCopy(source)) {
+    const entry = `${file}#${subject}`
+    if (grants('HELPER_COPY_OWNERS', entry)) continue
+    const line = source.getLineAndCharacterOfPosition(node.getStart(source)).line + 1
+    report(
+      'helper-copy',
+      `${file}:${line}`,
+      `${HELPER_COPY_MESSAGES[kind]}: bake it into the placeholder, label or option, or delete it. A line that must stay goes in HELPER_COPY_OWNERS as ${entry} with its reason`
+    )
+  }
+}
+for (const file of listedFiles) {
+  if (
+    /\.tsx?$/.test(file) &&
+    !/\.test\.tsx?$/.test(file) &&
+    !file.startsWith('platform/checks/') &&
+    (inRouteTree(file) || file.startsWith(platform) || HINT_DATA_MODULES.has(file))
+  )
+    checkHelperCopy(file)
+}
 for (const [file, sheet] of sheets) checkCss(file, sheet)
 for (const row of evidence) {
   if (