diff --git a/apps/ssp/src/app/herregistratie/ui/herregistratie-wizard/herregistratie-wizard.component.ts b/apps/ssp/src/app/herregistratie/ui/herregistratie-wizard/herregistratie-wizard.component.ts
index f2bf768..45c0284 100644
--- a/apps/ssp/src/app/herregistratie/ui/herregistratie-wizard/herregistratie-wizard.component.ts
+++ b/apps/ssp/src/app/herregistratie/ui/herregistratie-wizard/herregistratie-wizard.component.ts
@@ -9,6 +9,7 @@ import {
   WizardPhase,
   naarStapLabel,
 } from '@shared/layout/wizard-shell/wizard-shell.component';
+import { toWizardErrors } from '@shared/layout/wizard-shell/wizard-errors';
 import { ConfirmationComponent } from '@shared/ui/confirmation/confirmation.component';
 import { createStore } from '@shared/application/store';
 import { whenTag } from '@shared/kernel/fp';
@@ -251,12 +252,7 @@ export class HerregistratieWizardComponent {
     }
   });
   /** Current step's field errors, flattened for the shell's error summary. */
-  protected errorList = computed<WizardError[]>(() => {
-    const e = this.editing()?.errors ?? {};
-    return (Object.keys(e) as (keyof typeof e)[])
-      .filter((k) => e[k])
-      .map((k) => ({ id: k, message: e[k]! }));
-  });
+  protected errorList = computed<WizardError[]>(() => toWizardErrors(this.editing()?.errors ?? {}));
 
   constructor() {
     // An explicit seed (stories/tests) wins; otherwise resume the backend draft
diff --git a/apps/ssp/src/app/herregistratie/ui/intake-wizard/intake-wizard.component.ts b/apps/ssp/src/app/herregistratie/ui/intake-wizard/intake-wizard.component.ts
index 2ee3ff1..d6b87e9 100644
--- a/apps/ssp/src/app/herregistratie/ui/intake-wizard/intake-wizard.component.ts
+++ b/apps/ssp/src/app/herregistratie/ui/intake-wizard/intake-wizard.component.ts
@@ -15,6 +15,7 @@ import {
   WizardPhase,
   naarStapLabel,
 } from '@shared/layout/wizard-shell/wizard-shell.component';
+import { toWizardErrors } from '@shared/layout/wizard-shell/wizard-errors';
 import { createStore } from '@shared/application/store';
 import { whenTag } from '@shared/kernel/fp';
 import { BigProfileStore } from '@registratie/application/big-profile.store';
@@ -360,12 +361,9 @@ export class IntakeWizardComponent {
   });
   /** Current step's field errors, flattened for the shell's error summary. The
       field ids match the answer keys, so the summary anchors jump to the field. */
-  protected errorList = computed<WizardError[]>(() => {
-    const e = this.answering()?.errors ?? {};
-    return (Object.keys(e) as (keyof Answers)[])
-      .filter((k) => e[k])
-      .map((k) => ({ id: k, message: e[k]! }));
-  });
+  protected errorList = computed<WizardError[]>(() =>
+    toWizardErrors(this.answering()?.errors ?? {}),
+  );
 
   protected err = (k: keyof Answers) => this.answering()?.errors[k] ?? '';
   protected set = (key: keyof Answers, value: string) =>
diff --git a/apps/ssp/src/app/registratie/domain/registratie-wizard.machine.ts b/apps/ssp/src/app/registratie/domain/registratie-wizard.machine.ts
index d5fd063..67abd47 100644
--- a/apps/ssp/src/app/registratie/domain/registratie-wizard.machine.ts
+++ b/apps/ssp/src/app/registratie/domain/registratie-wizard.machine.ts
@@ -66,7 +66,7 @@ export type DraftField = 'straat' | 'postcode' | 'woonplaats' | 'email';
 
 /** Per-field error map. `antwoorden` holds per-policy-question errors, keyed by
     question id (a step can show several questions). */
-export interface Errors {
+export type Errors = {
   straat?: string;
   postcode?: string;
   woonplaats?: string;
@@ -75,7 +75,7 @@ export interface Errors {
   diploma?: string;
   documenten?: string;
   antwoorden?: Record<string, string>;
-}
+};
 
 export type RegistratieState =
   | { tag: 'Invullen'; draft: Draft; cursor: number; errors: Errors; upload: UploadState }
diff --git a/apps/ssp/src/app/registratie/ui/registratie-wizard/registratie-wizard.component.ts b/apps/ssp/src/app/registratie/ui/registratie-wizard/registratie-wizard.component.ts
index 50a1d4c..21e75b4 100644
--- a/apps/ssp/src/app/registratie/ui/registratie-wizard/registratie-wizard.component.ts
+++ b/apps/ssp/src/app/registratie/ui/registratie-wizard/registratie-wizard.component.ts
@@ -17,6 +17,7 @@ import {
   WizardPhase,
   naarStapLabel,
 } from '@shared/layout/wizard-shell/wizard-shell.component';
+import { toWizardErrors } from '@shared/layout/wizard-shell/wizard-errors';
 import { ASYNC } from '@shared/ui/async/async.component';
 import { AddressFieldsComponent } from '@registratie/ui/address-fields/address-fields.component';
 import { createStore } from '@shared/application/store';
@@ -468,14 +469,7 @@ export class RegistratieWizardComponent {
   /** Current step's errors (incl. per-question), flattened for the error summary. */
   protected errorList = computed<WizardError[]>(() => {
     const e = this.invullen()?.errors ?? {};
-    const out: WizardError[] = [];
-    for (const [k, v] of Object.entries(e)) {
-      if (k !== 'antwoorden' && typeof v === 'string' && v) out.push({ id: k, message: v });
-    }
-    for (const [qid, msg] of Object.entries(e.antwoorden ?? {})) {
-      if (msg) out.push({ id: 'vraag-' + qid, message: msg });
-    }
-    return out;
+    return [...toWizardErrors(e), ...toWizardErrors(e.antwoorden ?? {}, 'vraag-')];
   });
   protected adresSamenvatting = computed(() => {
     const d = this.draft();
diff --git a/docs/project/readable-codebase/RD-20-wizard-errors.md b/docs/project/readable-codebase/RD-20-wizard-errors.md
new file mode 100644
index 0000000..35bc178
--- /dev/null
+++ b/docs/project/readable-codebase/RD-20-wizard-errors.md
@@ -0,0 +1,170 @@
+# RD-20 — `wizard-errors.ts` + spec, adopted by all three wizards
+
+Status: done
+Source: PLAN.md 3c
+
+## Why
+
+Each of the three wizards flattens its per-field error record into the shell's
+`WizardError[]` summary, and each writes the flattening itself. Two of the three are the same
+six lines with a different key type. The third does the same job plus a nested group.
+
+The loop is pure, it has no spec, and it lives in a component — so the one part of the wizard
+that decides what the user sees in the error summary is the one part no test covers.
+
+Extract one pure helper beside `naarStapLabel`, which lives in that folder for exactly this
+reason.
+
+## Read first
+
+- `libs/shared/src/layout/wizard-shell/wizard-shell.component.ts:19-27` — `naarStapLabel` and
+  the `WizardError` interface. The new file sits beside this one.
+- The three call sites, in the order they get easier:
+  - `herregistratie-wizard.component.ts:253-259`
+  - `intake-wizard.component.ts:361-368`
+  - `registratie-wizard.component.ts:468-479` — the one with the nested group.
+- `apps/ssp/src/app/registratie/domain/registratie-wizard.machine.ts:69` — `Errors`, declared
+  as an `interface`. Decision 3 is about this line.
+
+## Decisions (pre-made, don't relitigate)
+
+1. **One helper, two parameters, both used today:**
+
+   ```ts
+   /** Flatten a machine's per-field error record into the shell's summary list.
+       Values that are not a non-empty string are skipped, so a nested group
+       (the registratie wizard's `antwoorden`) is appended by the caller with its
+       own `idPrefix` rather than special-cased here. */
+   export function toWizardErrors(errors: Record<string, unknown>, idPrefix = ''): WizardError[] {
+     const out: WizardError[] = [];
+     for (const [k, v] of Object.entries(errors)) {
+       if (typeof v === 'string' && v) out.push({ id: idPrefix + k, message: v });
+     }
+     return out;
+   }
+   ```
+
+   `Record<string, unknown>` plus the `typeof` guard narrows `v` to `string` with **no cast**,
+   which keeps the file inside the repo's `any`-free rule. Do not reach for a generic key type;
+   the three machines key their errors differently and the helper does not care.
+
+2. **New file `libs/shared/src/layout/wizard-shell/wizard-errors.ts`, plus its spec.** It takes
+   the `WizardError` type from the component file with a **type-only import**:
+
+   ```ts
+   import type { WizardError } from './wizard-shell.component';
+   ```
+
+   `import type` is erased at compile time, so the helper stays pure and its spec needs no
+   TestBed and pulls in no Angular at run time. Moving the `WizardError` interface into the new
+   file would be tidier on paper and would touch four more files for no behaviour; not worth it.
+
+3. **`registratie-wizard.machine.ts:69` changes from `interface Errors` to `type Errors`.**
+   This is required, not cosmetic. TypeScript gives an implicit index signature to a type alias
+   but **not** to an interface, so `Errors` as an interface is not assignable to
+   `Record<string, unknown>` and the call site will not compile. The other two machines already
+   declare their error maps as type aliases (`Partial<Record<…, string>>`), so this also makes
+   the three consistent. Nothing `extends` or `implements` `Errors` — verified.
+
+4. **The three call sites become:**
+
+   | Wizard         | Body                                                                      |
+   | -------------- | ------------------------------------------------------------------------- |
+   | herregistratie | `toWizardErrors(this.editing()?.errors ?? {})`                            |
+   | intake         | `toWizardErrors(this.answering()?.errors ?? {})`                          |
+   | registratie    | `[...toWizardErrors(e), ...toWizardErrors(e.antwoorden ?? {}, 'vraag-')]` |
+
+   The registratie site keeps its `const e = this.invullen()?.errors ?? {};` line. The
+   `vraag-` prefix and the skipping of non-string values together replace its
+   `if (k !== 'antwoorden' && typeof v === 'string' && v)` filter.
+
+5. **Do not touch the three `shellStatus` switches.** The tags genuinely differ per machine and
+   an exhaustive switch is the house style. This ticket is about the error list only.
+
+6. **No story.** The helper is a pure function. Its spec is the test; `wizard-shell.stories.ts`
+   already renders the error summary.
+
+## Files
+
+- `libs/shared/src/layout/wizard-shell/wizard-errors.ts` (new)
+- `libs/shared/src/layout/wizard-shell/wizard-errors.spec.ts` (new)
+- `apps/ssp/src/app/herregistratie/ui/herregistratie-wizard/herregistratie-wizard.component.ts`
+- `apps/ssp/src/app/herregistratie/ui/intake-wizard/intake-wizard.component.ts`
+- `apps/ssp/src/app/registratie/ui/registratie-wizard/registratie-wizard.component.ts`
+- `apps/ssp/src/app/registratie/domain/registratie-wizard.machine.ts` (decision 3, one line)
+- `libs/shared/docs/behaviour-spec.mdx` (regenerated, never hand-edited)
+
+## Steps
+
+1. Write `wizard-errors.ts` per decisions 1 and 2.
+2. Write `wizard-errors.spec.ts`. Cover: a flat record; an empty record; a record whose values
+   are `undefined` or `''` (both skipped); the `idPrefix`; and a record holding a nested object
+   value (skipped, which is what lets decision 4's registratie case work).
+3. Apply decision 3 — one word, `interface` → `type`, and the `{` stays.
+4. Convert the three call sites per decision 4.
+5. Run `npm run gen:behaviour-spec` — the new spec titles otherwise fail the drift check.
+6. `git add -A`, then run the acceptance commands.
+7. Update this ticket's `Status:` to `done` and the README's RD-20 row to `done`.
+8. Commit all of it together.
+
+## Acceptance criteria
+
+Measured against the tree before handover.
+
+```bash
+git grep -c "export function toWizardErrors" -- libs/shared/src/layout/wizard-shell/wizard-errors.ts   # MUST be 1
+git grep -l "toWizardErrors" -- apps libs | wc -l    # is 0 -> MUST be 5 (helper, spec, 3 wizards)
+```
+
+The hand-rolled flattening is gone from all three:
+
+```bash
+git grep -c "filter((k) => e\[k\])" -- apps | awk -F: '{s+=$NF} END {print s+0}'   # is 2 -> MUST be 0
+git grep -c "k !== 'antwoorden'" -- apps | awk -F: '{s+=$NF} END {print s+0}'       # is 1 -> MUST be 0
+```
+
+Decision 3 landed:
+
+```bash
+git grep -c "export interface Errors" -- apps/ssp/src/app/registratie/domain/registratie-wizard.machine.ts   # is 1 -> MUST be 0
+git grep -c "export type Errors" -- apps/ssp/src/app/registratie/domain/registratie-wizard.machine.ts        # MUST be 1
+```
+
+```bash
+npm run ci   # exits 0
+```
+
+## Verification
+
+`npm run ci` is enough. `--full` is **not** required: no story changes, and the new file is
+under `libs/shared/src/layout/`, not `libs/shared/src/ui/`. Regenerating `behaviour-spec.mdx`
+does not trigger `--full` on its own — RD-17 set that precedent.
+
+`npm run lint` inside the gate is what proves the three `/* eslint-disable max-lines */`
+directives are still needed. `reportUnusedDisableDirectives` is `error`, so a directive that
+stops being necessary fails the build.
+
+## Out of scope
+
+- Splitting any wizard into step components. RD-22 (intake) and RD-23 (registratie) own that,
+  and this ticket makes both smaller first.
+- The `shellStatus` switches (decision 5).
+- `change-request-form` and `besluit-form`. Neither builds a `WizardError[]`; they render field
+  errors directly. There is nothing to share.
+
+## Risks
+
+- **The `interface` → `type` change is load-bearing** (decision 3). Skipping it produces
+  "Index signature for type 'string' is missing in type 'Errors'" at the registratie call site,
+  and the tempting wrong fix is to widen the helper's parameter to `object`, which forces an
+  `any` and fails lint.
+- **PLAN says this ticket deletes an `eslint-disable max-lines` from
+  `herregistratie-wizard.component.ts`. That is stale — there is no such directive.** Earlier
+  tickets already brought the file under the budget: it measures ~248 effective lines against a
+  limit of 250. The two disables that do exist (intake, registratie) stay; RD-22 and RD-23
+  remove them. Both files are far above 250 (~362 and ~574 effective), so this ticket's saving
+  cannot make either directive unused.
+- **`behaviour-spec.mdx` drift** from the new spec titles. Run `gen:behaviour-spec` in the same
+  commit, and never edit that file by hand.
+- **Keep `import type`, not a value import** (decision 2). A value import of the component into
+  the helper drags Angular into a pure module and its spec.
diff --git a/docs/project/readable-codebase/README.md b/docs/project/readable-codebase/README.md
index 1d2137f..ef07a59 100644
--- a/docs/project/readable-codebase/README.md
+++ b/docs/project/readable-codebase/README.md
@@ -114,7 +114,7 @@ two. Note that RD-15 exists because 22 abandoned agent worktrees are still on di
 | RD-17 | `successOf`/`successOr` sweep — 10 sites, 8 files                            | 01         |           | done   |
 | RD-18 | Ticket-reference sweep, frontend — 181 refs, 100 files                       | 01         | yes       | done   |
 | RD-19 | Ticket-reference sweep, backend — 370 refs, 86 files                         | 01         |           | done   |
-| RD-20 | `wizard-errors.ts` + spec, adopted by all 3 wizards                          | 02         |           | todo   |
+| RD-20 | `wizard-errors.ts` + spec, adopted by all 3 wizards                          | 02         |           | done   |
 | RD-21 | `rich-text-dom.ts` helpers + spec cases                                      | 02         | yes       | todo   |
 | RD-22 | `intake-wizard` to 3 step components                                         | 08, 20     | yes       | todo   |
 | RD-23 | `registratie-wizard` to 3 steps + the upload-controller move                 | 08, 20     | yes       | todo   |
diff --git a/libs/shared/docs/behaviour-spec.mdx b/libs/shared/docs/behaviour-spec.mdx
index c733132..26cd4ee 100644
--- a/libs/shared/docs/behaviour-spec.mdx
+++ b/libs/shared/docs/behaviour-spec.mdx
@@ -20,7 +20,7 @@ tested where._
 
 Every bullet below is a real test name from the suite — an `it()` title (frontend) or a test
 method name (backend), read as a sentence. Nothing here is hand-written prose: this page
-**is** the suite, reshaped for a business reader. 532 frontend behaviours across
+**is** the suite, reshaped for a business reader. 538 frontend behaviours across
 9 contexts; 261 backend behaviours across 42 test
 classes.
 
@@ -973,6 +973,15 @@ classes.
 - unwraps a Success value
 - is the fallback for every other state
 
+#### toWizardErrors
+
+- flattens a flat record of field errors
+- returns an empty list for an empty record
+- skips a value that is undefined
+- skips a value that is an empty string
+- prefixes every id with idPrefix when given
+- skips a value that is a nested object, so the caller can flatten it separately
+
 #### upload lifecycle messages
 
 - queued → progress → complete
diff --git a/libs/shared/src/layout/wizard-shell/wizard-errors.spec.ts b/libs/shared/src/layout/wizard-shell/wizard-errors.spec.ts
new file mode 100644
index 0000000..26aaad1
--- /dev/null
+++ b/libs/shared/src/layout/wizard-shell/wizard-errors.spec.ts
@@ -0,0 +1,35 @@
+import { describe, it, expect } from 'vitest';
+import { toWizardErrors } from './wizard-errors';
+
+describe('toWizardErrors', () => {
+  it('flattens a flat record of field errors', () => {
+    expect(toWizardErrors({ straat: 'Verplicht', postcode: 'Ongeldig' })).toEqual([
+      { id: 'straat', message: 'Verplicht' },
+      { id: 'postcode', message: 'Ongeldig' },
+    ]);
+  });
+
+  it('returns an empty list for an empty record', () => {
+    expect(toWizardErrors({})).toEqual([]);
+  });
+
+  it('skips a value that is undefined', () => {
+    expect(toWizardErrors({ straat: undefined })).toEqual([]);
+  });
+
+  it('skips a value that is an empty string', () => {
+    expect(toWizardErrors({ straat: '' })).toEqual([]);
+  });
+
+  it('prefixes every id with idPrefix when given', () => {
+    expect(toWizardErrors({ q1: 'Verplicht' }, 'vraag-')).toEqual([
+      { id: 'vraag-q1', message: 'Verplicht' },
+    ]);
+  });
+
+  it('skips a value that is a nested object, so the caller can flatten it separately', () => {
+    expect(toWizardErrors({ straat: 'Verplicht', antwoorden: { q1: 'Verplicht' } })).toEqual([
+      { id: 'straat', message: 'Verplicht' },
+    ]);
+  });
+});
diff --git a/libs/shared/src/layout/wizard-shell/wizard-errors.ts b/libs/shared/src/layout/wizard-shell/wizard-errors.ts
new file mode 100644
index 0000000..09e307e
--- /dev/null
+++ b/libs/shared/src/layout/wizard-shell/wizard-errors.ts
@@ -0,0 +1,13 @@
+import type { WizardError } from './wizard-shell.component';
+
+/** Flatten a machine's per-field error record into the shell's summary list.
+    Values that are not a non-empty string are skipped, so a nested group
+    (the registratie wizard's `antwoorden`) is appended by the caller with its
+    own `idPrefix` rather than special-cased here. */
+export function toWizardErrors(errors: Record<string, unknown>, idPrefix = ''): WizardError[] {
+  const out: WizardError[] = [];
+  for (const [k, v] of Object.entries(errors)) {
+    if (typeof v === 'string' && v) out.push({ id: idPrefix + k, message: v });
+  }
+  return out;
+}