diff --git a/src/components/auth/SignInForm/SignInForm.accessibility.test.tsx b/src/components/auth/SignInForm/SignInForm.accessibility.test.tsx
index cd401f3c..362475b0 100644
--- a/src/components/auth/SignInForm/SignInForm.accessibility.test.tsx
+++ b/src/components/auth/SignInForm/SignInForm.accessibility.test.tsx
@@ -1,5 +1,6 @@
 import { describe, it, expect } from 'vitest';
-import { render } from '@testing-library/react';
+import { render, screen, waitFor } from '@testing-library/react';
+import userEvent from '@testing-library/user-event';
 import { axe } from 'jest-axe';
 import SignInForm from './SignInForm';
 
@@ -11,12 +12,67 @@ describe('SignInForm Accessibility', () => {
     expect(results).toHaveNoViolations();
   });
 
-  it('should have proper ARIA attributes', () => {
-    const { container } = render(<SignInForm />);
+  /**
+   * #857 — a validation error must be tied to the field it is about.
+   *
+   * This test was a generator stub containing only example comments: it rendered
+   * the form and asserted NOTHING, which is the #396 shape the #850 queue is
+   * about, sitting in the file whose job is accessibility.
+   *
+   * What it now pins is the gap it was named for. Every error used to funnel into
+   * one form-level alert with no `id`, and neither input carried `aria-invalid`
+   * or `aria-describedby` — so a screen reader announced "Invalid email address"
+   * with nothing connecting it to the email box.
+   */
+  it('ties a validation error to the field it is about', async () => {
+    render(<SignInForm />);
+
+    const email = screen.getByLabelText(/email/i);
+    // `a@b` is deliberate: `type="email"` ACCEPTS it, so the browser's native
+    // `required`/type validation does not block submission, but `validateEmail`
+    // rejects it (email-validator.ts requires a dot in the domain plus a 2+
+    // alphabetic TLD). Anything the browser itself rejects never reaches React,
+    // and the test would assert against a form that never ran its own validation.
+    await userEvent.type(email, 'a@b');
+    await userEvent.type(screen.getByLabelText(/password/i), 'whatever123');
+    await userEvent.click(screen.getByRole('button', { name: /sign in/i }));
+
+    // Deliberately NOT `findByRole('alert')`. The form carries an empty
+    // `role="alert"` live region, so that query resolves to a container with no
+    // text and the assertion reads `Received: <empty>` — the same trap that made
+    // accessibility.spec.ts:437 unable to see anything (#850). Follow the
+    // association instead, which is the property under test.
+    await waitFor(() => expect(email).toHaveAttribute('aria-invalid', 'true'));
+
+    const describedBy = email.getAttribute('aria-describedby');
+    expect(describedBy, 'email input must point at its error').toBeTruthy();
+
+    const slot = document.getElementById(describedBy!);
+    expect(slot, `no element with id="${describedBy}"`).not.toBeNull();
+    // Non-empty rather than an exact string. The message comes from
+    // `validateEmail` and for `a@b` reads "Invalid or missing top-level domain
+    // (TLD)" — it does not contain the word "email", which is what my first
+    // version of this assertion wrongly assumed. Pinning the copy would break on
+    // any wording change while proving nothing extra; what matters is that the
+    // slot the input points at actually says something.
+    expect(slot!.textContent?.trim()).toBeTruthy();
+    // And it must be announced, not merely present.
+    expect(slot!.querySelector('[role="alert"]')).not.toBeNull();
+  });
+
+  it('leaves the sign-in failure form-level, NOT on the email field', async () => {
+    // Deliberate asymmetry. The post-submit failure is generic to avoid account
+    // enumeration; associating it with the email input would both mislead and
+    // hint at which half of the credentials was wrong. Only client-side
+    // validation is field-scoped.
+    render(<SignInForm />);
+
+    const email = screen.getByLabelText(/email/i);
+    await userEvent.type(email, 'real@example.com');
+    await userEvent.type(screen.getByLabelText(/password/i), 'whatever123');
 
-    // Add specific ARIA attribute tests based on component type
-    // Example: const button = container.querySelector('button');
-    // expect(button).toHaveAttribute('aria-label');
+    expect(email).not.toHaveAttribute('aria-invalid', 'true');
+    expect(email).not.toHaveAttribute('aria-describedby');
   });
 
   it('should be keyboard navigable', () => {
diff --git a/src/components/auth/SignInForm/SignInForm.tsx b/src/components/auth/SignInForm/SignInForm.tsx
index 8214e149..4f3cf495 100644
--- a/src/components/auth/SignInForm/SignInForm.tsx
+++ b/src/components/auth/SignInForm/SignInForm.tsx
@@ -42,6 +42,24 @@ export default function SignInForm({
   const [password, setPassword] = useState('');
   const [rememberMe, setRememberMe] = useState(false);
   const [error, setError] = useState<string | null>(null);
+  /**
+   * The error for ONE named field, kept apart from `error` (#857).
+   *
+   * Everything used to funnel into `error` and render in a single form-level
+   * alert with no `id`, while no input carried `aria-invalid` or
+   * `aria-describedby`. A screen-reader user heard "Invalid email address" with
+   * nothing tying it to the field it was about.
+   *
+   * Only CLIENT-SIDE validation is field-scoped. The post-submit sign-in failure
+   * deliberately stays form-level: that message is generic to avoid account
+   * enumeration, and pinning it to the email input would both mislead and hint
+   * at which half was wrong. Rate limiting and the captcha challenge are not
+   * about one field either.
+   */
+  const [fieldError, setFieldError] = useState<{
+    field: 'email';
+    message: string;
+  } | null>(null);
   const [loading, setLoading] = useState(false);
   const [remainingAttempts, setRemainingAttempts] = useState<number | null>(
     null
@@ -55,11 +73,16 @@ export default function SignInForm({
   const handleSubmit = async (e: React.FormEvent) => {
     e.preventDefault();
     setError(null);
+    setFieldError(null);
 
-    // Enhanced email validation (REQ-SEC-004)
+    // Enhanced email validation (REQ-SEC-004). Field-scoped: this one IS about
+    // the email input, so it is announced against it (#857).
     const emailValidation = validateEmail(email);
     if (!emailValidation.valid) {
-      setError(emailValidation.errors[0] || 'Invalid email address');
+      setFieldError({
+        field: 'email',
+        message: emailValidation.errors[0] || 'Invalid email address',
+      });
       return;
     }
 
@@ -288,12 +311,25 @@ export default function SignInForm({
             type="email"
             value={email}
             onChange={(e) => setEmail(e.target.value)}
-            className="input input-bordered min-h-11 w-full"
+            className={`input input-bordered min-h-11 w-full ${
+              fieldError?.field === 'email' ? 'input-error' : ''
+            }`}
             placeholder="you@example.com"
             autoComplete="email"
             required
             disabled={loading}
+            aria-invalid={fieldError?.field === 'email'}
+            aria-describedby={
+              fieldError?.field === 'email' ? 'email-error' : undefined
+            }
           />
+          {fieldError?.field === 'email' && (
+            <div className="label" id="email-error">
+              <span className="text-error" role="alert" aria-live="polite">
+                {fieldError.message}
+              </span>
+            </div>
+          )}
         </div>
       </div>
 
diff --git a/src/components/auth/SignUpForm/SignUpForm.accessibility.test.tsx b/src/components/auth/SignUpForm/SignUpForm.accessibility.test.tsx
index 8c2218dd..f7974030 100644
--- a/src/components/auth/SignUpForm/SignUpForm.accessibility.test.tsx
+++ b/src/components/auth/SignUpForm/SignUpForm.accessibility.test.tsx
@@ -1,5 +1,6 @@
 import { describe, it, expect } from 'vitest';
-import { render } from '@testing-library/react';
+import { render, screen, waitFor } from '@testing-library/react';
+import userEvent from '@testing-library/user-event';
 import { axe } from 'jest-axe';
 import SignUpForm from './SignUpForm';
 
@@ -11,12 +12,86 @@ describe('SignUpForm Accessibility', () => {
     expect(results).toHaveNoViolations();
   });
 
-  it('should have proper ARIA attributes', () => {
-    const { container } = render(<SignUpForm />);
+  /**
+   * #857 — each validation error must reach the field it is about.
+   *
+   * This was a generator stub containing only example comments: it rendered the
+   * form and asserted nothing, in the file whose job is accessibility.
+   *
+   * SignUpForm is the sharper case of the two. THREE distinct client-side
+   * conditions used to funnel into one form-level alert with no `id` — an invalid
+   * email, a short password, and a mismatched confirmation — so "Passwords do not
+   * match" was announced with nothing saying which of the two password boxes to
+   * fix. Getting the association merely present is not enough here; it has to
+   * land on the RIGHT field, which is what the loop below pins.
+   */
+  it.each([
+    {
+      case: 'invalid email',
+      // `a@b` passes the browser's own `type="email"` check but fails
+      // `validateEmail` (it requires a dot in the domain and a 2+ alpha TLD).
+      // A value the browser rejects never reaches React at all.
+      fill: {
+        email: 'a@b',
+        password: 'LongEnough123',
+        confirm: 'LongEnough123',
+      },
+      field: 'email',
+      others: ['password', 'confirm-password'],
+    },
+    {
+      case: 'short password',
+      fill: { email: 'real@example.com', password: 'short', confirm: 'short' },
+      field: 'password',
+      others: ['email', 'confirm-password'],
+    },
+    {
+      case: 'mismatched confirmation',
+      fill: {
+        email: 'real@example.com',
+        password: 'LongEnough123',
+        confirm: 'Different123',
+      },
+      // Announced against the CONFIRMATION box, not the password: that is the
+      // one the user is being asked to change.
+      field: 'confirm-password',
+      others: ['email', 'password'],
+    },
+  ])('announces the $case against its own field', async (scenario) => {
+    render(<SignUpForm />);
+
+    // Selected by id, not by label text: `getByLabelText(/^password/i)` matches
+    // more than one control here (the strength indicator contributes text), and
+    // the ids are exactly what the aria-describedby wiring under test refers to.
+    const byId = (id: string) =>
+      document.getElementById(id) as HTMLInputElement;
+
+    await userEvent.type(byId('email'), scenario.fill.email);
+    await userEvent.type(byId('password'), scenario.fill.password);
+    await userEvent.type(byId('confirm-password'), scenario.fill.confirm);
+    await userEvent.click(screen.getByRole('button', { name: /sign up/i }));
+
+    const target = byId(scenario.field);
+    await waitFor(() => expect(target).toHaveAttribute('aria-invalid', 'true'));
+
+    const describedBy = target.getAttribute('aria-describedby');
+    expect(describedBy, 'the field must point at its error').toBeTruthy();
+    const slot = document.getElementById(describedBy!);
+    expect(slot, `no element with id="${describedBy}"`).not.toBeNull();
+    expect(slot!.textContent?.trim()).toBeTruthy();
+    expect(slot!.querySelector('[role="alert"]')).not.toBeNull();
 
-    // Add specific ARIA attribute tests based on component type
-    // Example: const button = container.querySelector('button');
-    // expect(button).toHaveAttribute('aria-label');
+    // The half that makes this test worth having: the OTHER fields must stay
+    // clean. An implementation that marked every input invalid would satisfy the
+    // assertions above and still tell the user nothing.
+    for (const other of scenario.others) {
+      const el = byId(other);
+      expect(el, `${other} should not be marked invalid`).not.toHaveAttribute(
+        'aria-invalid',
+        'true'
+      );
+      expect(el).not.toHaveAttribute('aria-describedby');
+    }
   });
 
   it('should be keyboard navigable', () => {
diff --git a/src/components/auth/SignUpForm/SignUpForm.tsx b/src/components/auth/SignUpForm/SignUpForm.tsx
index 2d2892aa..05a54a3f 100644
--- a/src/components/auth/SignUpForm/SignUpForm.tsx
+++ b/src/components/auth/SignUpForm/SignUpForm.tsx
@@ -42,6 +42,23 @@ export default function SignUpForm({
   const [confirmPassword, setConfirmPassword] = useState('');
   const [rememberMe, setRememberMe] = useState(false);
   const [error, setError] = useState<string | null>(null);
+  /**
+   * The error for ONE named field, kept apart from `error` (#857).
+   *
+   * THREE distinct field conditions used to funnel into `error` and render in a
+   * single form-level alert with no `id`: an invalid email, a short password,
+   * and a mismatched confirmation. No input carried `aria-invalid` or
+   * `aria-describedby`, so a screen-reader user heard "Passwords do not match"
+   * with nothing tying it to the field that was wrong.
+   *
+   * Only client-side validation is field-scoped. The captcha challenge, the rate
+   * limit and the sign-up failure from Supabase stay form-level — none is about
+   * one field.
+   */
+  const [fieldError, setFieldError] = useState<{
+    field: 'email' | 'password' | 'confirmPassword';
+    message: string;
+  } | null>(null);
   const [loading, setLoading] = useState(false);
   // (#353) Null until Turnstile solves; cleared on expiry/error so a stale
   // single-use token is never submitted. Always null when CAPTCHA is
@@ -52,11 +69,15 @@ export default function SignUpForm({
   const handleSubmit = async (e: React.FormEvent) => {
     e.preventDefault();
     setError(null);
+    setFieldError(null);
 
-    // Enhanced email validation (REQ-SEC-004)
+    // Enhanced email validation (REQ-SEC-004). Field-scoped (#857).
     const emailValidation = validateEmail(email);
     if (!emailValidation.valid) {
-      setError(emailValidation.errors[0] || 'Invalid email address');
+      setFieldError({
+        field: 'email',
+        message: emailValidation.errors[0] || 'Invalid email address',
+      });
       return;
     }
 
@@ -68,13 +89,21 @@ export default function SignUpForm({
     }
 
     if (password.length < 8) {
-      setError('Password must be at least 8 characters');
+      setFieldError({
+        field: 'password',
+        message: 'Password must be at least 8 characters',
+      });
       return;
     }
 
     // Confirm password match
     if (password !== confirmPassword) {
-      setError('Passwords do not match');
+      // Announced against the CONFIRMATION field, not the password: that is the
+      // one the user is being asked to change.
+      setFieldError({
+        field: 'confirmPassword',
+        message: 'Passwords do not match',
+      });
       return;
     }
 
@@ -211,11 +240,24 @@ export default function SignUpForm({
             type="email"
             value={email}
             onChange={(e) => setEmail(e.target.value)}
-            className="input input-bordered min-h-11 w-full"
+            className={`input input-bordered min-h-11 w-full ${
+              fieldError?.field === 'email' ? 'input-error' : ''
+            }`}
             placeholder="you@example.com"
             required
             disabled={loading}
+            aria-invalid={fieldError?.field === 'email'}
+            aria-describedby={
+              fieldError?.field === 'email' ? 'email-error' : undefined
+            }
           />
+          {fieldError?.field === 'email' && (
+            <div className="label" id="email-error">
+              <span className="text-error" role="alert" aria-live="polite">
+                {fieldError.message}
+              </span>
+            </div>
+          )}
         </div>
       </div>
 
@@ -232,11 +274,24 @@ export default function SignUpForm({
             type="password"
             value={password}
             onChange={(e) => setPassword(e.target.value)}
-            className="input input-bordered min-h-11 w-full"
+            className={`input input-bordered min-h-11 w-full ${
+              fieldError?.field === 'password' ? 'input-error' : ''
+            }`}
             placeholder="••••••••"
             required
             disabled={loading}
+            aria-invalid={fieldError?.field === 'password'}
+            aria-describedby={
+              fieldError?.field === 'password' ? 'password-error' : undefined
+            }
           />
+          {fieldError?.field === 'password' && (
+            <div className="label" id="password-error">
+              <span className="text-error" role="alert" aria-live="polite">
+                {fieldError.message}
+              </span>
+            </div>
+          )}
           {/* Password strength indicator (T042) */}
           <div className="mt-2">
             <PasswordStrengthIndicator password={password} />
@@ -257,11 +312,26 @@ export default function SignUpForm({
             type="password"
             value={confirmPassword}
             onChange={(e) => setConfirmPassword(e.target.value)}
-            className="input input-bordered min-h-11 w-full"
+            className={`input input-bordered min-h-11 w-full ${
+              fieldError?.field === 'confirmPassword' ? 'input-error' : ''
+            }`}
             placeholder="••••••••"
             required
             disabled={loading}
+            aria-invalid={fieldError?.field === 'confirmPassword'}
+            aria-describedby={
+              fieldError?.field === 'confirmPassword'
+                ? 'confirm-password-error'
+                : undefined
+            }
           />
+          {fieldError?.field === 'confirmPassword' && (
+            <div className="label" id="confirm-password-error">
+              <span className="text-error" role="alert" aria-live="polite">
+                {fieldError.message}
+              </span>
+            </div>
+          )}
         </div>
       </div>