diff --git a/.changeset/replace-existing-field-notice.md b/.changeset/replace-existing-field-notice.md
new file mode 100644
index 0000000..f21e2de
--- /dev/null
+++ b/.changeset/replace-existing-field-notice.md
@@ -0,0 +1,5 @@
+---
+"@onkernel/managed-auth-react": minor
+---
+
+Show a notice on fields whose saved value the site rejected, driven by the `replace_existing` flag on managed-auth fields. Adds the `fieldReplaceExistingNotice` label and the `inputReplaceNotice` appearance slot.
diff --git a/packages/managed-auth-react/src/appearance/types.ts b/packages/managed-auth-react/src/appearance/types.ts
index 16eb621..1d004a5 100644
--- a/packages/managed-auth-react/src/appearance/types.ts
+++ b/packages/managed-auth-react/src/appearance/types.ts
@@ -71,6 +71,8 @@ export interface AppearanceElements {
   input?: ElementValue;
   /** Small hint text under an input. */
   inputHint?: ElementValue;
+  /** Notice under an input whose saved value was rejected. */
+  inputReplaceNotice?: ElementValue;
   /** Password show/hide toggle button. */
   passwordToggle?: ElementValue;
   /** All buttons — base. */
diff --git a/packages/managed-auth-react/src/components/UnifiedAuthForm.tsx b/packages/managed-auth-react/src/components/UnifiedAuthForm.tsx
index 49395f0..cfc4749 100644
--- a/packages/managed-auth-react/src/components/UnifiedAuthForm.tsx
+++ b/packages/managed-auth-react/src/components/UnifiedAuthForm.tsx
@@ -300,6 +300,11 @@ export function UnifiedAuthForm({
               }
             />
           )}
+          {field.replace_existing && (
+            <p {...slot("inputReplaceNotice", "kma-input-replace-notice")}>
+              {l.fieldReplaceExistingNotice}
+            </p>
+          )}
           {field.hint && (
             <p {...slot("inputHint", "kma-input-hint")}>{field.hint}</p>
           )}
diff --git a/packages/managed-auth-react/src/lib/types.ts b/packages/managed-auth-react/src/lib/types.ts
index be0cdb9..fe08275 100644
--- a/packages/managed-auth-react/src/lib/types.ts
+++ b/packages/managed-auth-react/src/lib/types.ts
@@ -35,6 +35,7 @@ export interface DiscoveredField {
   type: "text" | "email" | "password" | "tel" | "code" | "totp";
   placeholder?: string;
   required?: boolean;
+  replace_existing?: boolean;
   hint?: string;
   linked_mfa_type?: MFAType;
 }
@@ -73,6 +74,7 @@ export interface ManagedAuthField {
     | "text";
   label?: string;
   required?: boolean;
+  replace_existing?: boolean;
   hint?: string;
   observed_selector?: string | null;
 }
diff --git a/packages/managed-auth-react/src/localization/defaults.ts b/packages/managed-auth-react/src/localization/defaults.ts
index 01ccebc..7cb4ed7 100644
--- a/packages/managed-auth-react/src/localization/defaults.ts
+++ b/packages/managed-auth-react/src/localization/defaults.ts
@@ -42,6 +42,8 @@ export const DEFAULT_LOCALIZATION: Localizer = {
   orDivider: "or",
   passwordShow: "Show password",
   passwordHide: "Hide password",
+  fieldReplaceExistingNotice:
+    "The saved value was rejected. Enter a new one to continue.",
   credentialSafetyNotice:
     "Your credentials are encrypted and sent directly from your browser. They are never shared with anyone or any LLM.",
   mfaTypeLabels: {
diff --git a/packages/managed-auth-react/src/localization/types.ts b/packages/managed-auth-react/src/localization/types.ts
index 1c01c71..d4402e7 100644
--- a/packages/managed-auth-react/src/localization/types.ts
+++ b/packages/managed-auth-react/src/localization/types.ts
@@ -39,6 +39,7 @@ export interface Localization {
   orDivider?: string;
   passwordShow?: string;
   passwordHide?: string;
+  fieldReplaceExistingNotice?: string;
   credentialSafetyNotice?: string;
   /** MFA type labels. */
   mfaTypeLabels?: Partial<Record<MFAType, string>>;
diff --git a/packages/managed-auth-react/src/session/state.test.ts b/packages/managed-auth-react/src/session/state.test.ts
index 81cbf01..a6cf31d 100644
--- a/packages/managed-auth-react/src/session/state.test.ts
+++ b/packages/managed-auth-react/src/session/state.test.ts
@@ -239,3 +239,94 @@ describe("normalizeManagedAuthState", () => {
     });
   });
 });
+
+describe("replace_existing", () => {
+  test("carries the replacement requirement from canonical fields", () => {
+    const state = managedAuthState({
+      fields: [
+        {
+          id: "field_password",
+          ref: "password",
+          type: "password",
+          label: "Password",
+          replace_existing: true,
+        },
+      ],
+    });
+
+    const derived = normalizeManagedAuthState(state).discovered_fields;
+    expect(derived?.[0].replace_existing).toBe(true);
+  });
+
+  test("leaves the replacement requirement unset when canonical omits it", () => {
+    const state = managedAuthState({
+      fields: [
+        {
+          id: "field_password",
+          ref: "password",
+          type: "password",
+          label: "Password",
+        },
+      ],
+    });
+
+    const derived = normalizeManagedAuthState(state).discovered_fields;
+    expect(derived?.[0].replace_existing).toBeUndefined();
+  });
+
+  test("passes the replacement requirement through legacy fields", () => {
+    const state = managedAuthState({
+      discovered_fields: [
+        {
+          name: "password",
+          label: "Password",
+          type: "password",
+          replace_existing: true,
+        },
+      ],
+    });
+
+    const derived = normalizeManagedAuthState(state).discovered_fields;
+    expect(derived?.[0].replace_existing).toBe(true);
+  });
+
+  // Strict on the whole projected field, not a subset: a canonical property that
+  // the form renders but this projection forgets is invisible to every consumer.
+  test("projects the exact field shape the form renders", () => {
+    const state = managedAuthState({
+      fields: [
+        {
+          id: "field_password",
+          ref: "password",
+          type: "password",
+          label: "Password",
+          required: false,
+          replace_existing: true,
+        },
+      ],
+      discovered_fields: [
+        {
+          name: "password",
+          type: "password",
+          label: "Password",
+          placeholder: "Enter password",
+          hint: "Use the password for this account",
+        },
+      ],
+    });
+
+    expect(normalizeManagedAuthState(state).discovered_fields).toStrictEqual([
+      {
+        id: "field_password",
+        ref: "password",
+        name: "field_password",
+        type: "password",
+        label: "Password",
+        placeholder: "Enter password",
+        required: false,
+        replace_existing: true,
+        hint: "Use the password for this account",
+      },
+    ]);
+  });
+});
diff --git a/packages/managed-auth-react/src/session/state.ts b/packages/managed-auth-react/src/session/state.ts
index 93783fe..04f2e7d 100644
--- a/packages/managed-auth-react/src/session/state.ts
+++ b/packages/managed-auth-react/src/session/state.ts
@@ -56,17 +56,25 @@ function fieldTypeToDiscoveredType(
 
 function fieldsFromCanonical(
   fields: ManagedAuthField[],
+  legacyFields: DiscoveredField[] | null | undefined,
 ): DiscoveredField[] | null {
   if (!fields.length) return null;
-  return fields.map((field) => ({
-    id: field.id,
-    ref: field.ref,
-    name: field.id,
-    type: fieldTypeToDiscoveredType(field),
-    label: field.label || field.ref,
-    required: field.required ?? true,
-    hint: field.hint,
-  }));
+  return fields.map((field) => {
+    const legacyField = legacyFields?.find(
+      (candidate) => candidate.name === field.ref,
+    );
+    return {
+      id: field.id,
+      ref: field.ref,
+      name: field.id,
+      type: fieldTypeToDiscoveredType(field),
+      label: field.label || field.ref,
+      placeholder: legacyField?.placeholder,
+      required: field.required ?? true,
+      replace_existing: field.replace_existing,
+      hint: field.hint ?? legacyField?.hint,
+    };
+  });
 }
 
 function ssoButtonsFromCanonical(
@@ -176,7 +184,7 @@ export function normalizeManagedAuthState(
   return {
     ...state,
     discovered_fields: hasCanonicalFields
-      ? fieldsFromCanonical(state.fields ?? [])
+      ? fieldsFromCanonical(state.fields ?? [], state.discovered_fields)
       : state.discovered_fields,
     pending_sso_buttons: hasCanonicalChoices
       ? ssoButtonsFromCanonical(state.choices ?? [])
diff --git a/packages/managed-auth-react/src/styles/styles.css b/packages/managed-auth-react/src/styles/styles.css
index be0412e..1cbc6e0 100644
--- a/packages/managed-auth-react/src/styles/styles.css
+++ b/packages/managed-auth-react/src/styles/styles.css
@@ -604,6 +604,12 @@
   color: var(--kma-color-muted-foreground);
 }
 
+.kma-input-replace-notice {
+  margin: 0;
+  font-size: var(--kma-font-size-sm);
+  color: var(--kma-color-danger);
+}
+
 .kma-password-toggle {
   position: absolute;
   right: 10px;