diff --git a/src/routes/admin/+page.svelte b/src/routes/admin/+page.svelte
index 03fbed3..0ea8801 100644
--- a/src/routes/admin/+page.svelte
+++ b/src/routes/admin/+page.svelte
@@ -18,10 +18,14 @@
   // Unit form state
   let unitForm = $state<Partial<Unit>>({});
   let unitFormNew = $state(false);
+  let unitFormSnapshot = $state<Partial<Unit>>({});
+  let unitFieldError = $state<string | null>(null);  // which field has error
 
   // Group form state
   let groupForm = $state<Partial<Group>>({});
   let groupFormNew = $state(false);
+  let groupFormSnapshot = $state<Partial<Group>>({});
+  let groupFieldError = $state<string | null>(null); // which field has error
 
   // ── Load data ────────────────────────────────────────────────────────────────
   async function loadData() {
@@ -58,8 +62,11 @@
   function selectUnit(unit: Unit) {
     selectedItem = { type: 'unit', id: unit.id };
     unitForm = { ...unit };
+    unitFormSnapshot = { ...unit };
     unitFormNew = false;
     groupFormNew = false;
+    unitFieldError = null;
+    groupFieldError = null;
     formError = null;
     formSuccess = null;
   }
@@ -67,26 +74,147 @@
   function selectGroup(group: Group) {
     selectedItem = { type: 'group', id: group.id };
     groupForm = { ...group };
+    groupFormSnapshot = { ...group };
     groupFormNew = false;
     unitFormNew = false;
+    unitFieldError = null;
+    groupFieldError = null;
     formError = null;
     formSuccess = null;
   }
 
-  function startNewUnit(groupId?: string) {
+  // ── Dirty checks ─────────────────────────────────────────────────────────────
+  function isUnitDirty(): boolean {
+    if (!unitFormNew && selectedItem === null) return false;
+    const s = unitFormSnapshot;
+    return (
+      unitForm.label !== s.label ||
+      unitForm.labelPlural !== s.labelPlural ||
+      unitForm.symbol !== s.symbol ||
+      unitForm.toBase !== s.toBase ||
+      unitForm.group !== s.group ||
+      (unitForm.description ?? '') !== (s.description ?? '')
+    );
+  }
+
+  function isGroupDirty(): boolean {
+    if (!groupFormNew && selectedItem === null) return false;
+    const s = groupFormSnapshot;
+    return (
+      groupForm.label !== s.label ||
+      groupForm.id !== s.id
+    );
+  }
+
+  // ── Validate unit form ───────────────────────────────────────────────────────
+  function validateUnitForm(): { valid: boolean; firstError: string | null } {
+    const f = unitForm;
+    if (!f.id || !/^[a-z][a-z0-9-]*$/.test(f.id)) return { valid: false, firstError: 'unit-id' };
+    if (!f.label) return { valid: false, firstError: 'unit-label' };
+    if (!f.labelPlural) return { valid: false, firstError: 'unit-labelPlural' };
+    if (!f.symbol) return { valid: false, firstError: 'unit-symbol' };
+    if (typeof f.toBase !== 'number' || f.toBase <= 0) return { valid: false, firstError: 'unit-toBase' };
+    return { valid: true, firstError: null };
+  }
+
+  function validateGroupForm(): { valid: boolean; firstError: string | null } {
+    const f = groupForm;
+    if (!f.id || !/^[a-z][a-z0-9-]*$/.test(f.id)) return { valid: false, firstError: 'group-id' };
+    if (!f.label) return { valid: false, firstError: 'group-label' };
+    return { valid: true, firstError: null };
+  }
+
+  // ── Auto-save current form if dirty ──────────────────────────────────────────
+  async function autoSaveCurrentForm(): Promise<boolean> {
+    if (unitFormNew || (selectedItem?.type === 'unit')) {
+      if (!isUnitDirty()) return true;
+      const v = validateUnitForm();
+      if (!v.valid) {
+        formError = 'Please complete the current form before opening a new one.';
+        unitFieldError = v.firstError;
+        return false;
+      }
+      try {
+        const res = unitFormNew
+          ? await fetch('/admin/api/units', {
+              method: 'POST',
+              headers: { 'Content-Type': 'application/json' },
+              body: JSON.stringify(unitForm)
+            })
+          : await fetch(`/admin/api/units/${selectedItem?.id}`, {
+              method: 'PUT',
+              headers: { 'Content-Type': 'application/json' },
+              body: JSON.stringify(unitForm)
+            });
+        if (!res.ok) {
+          formError = await extractError(res, 'Auto-save failed');
+          return false;
+        }
+        await loadData();
+        return true;
+      } catch {
+        formError = 'Network error during auto-save';
+        return false;
+      }
+    }
+    if (groupFormNew || (selectedItem?.type === 'group')) {
+      if (!isGroupDirty()) return true;
+      const v = validateGroupForm();
+      if (!v.valid) {
+        formError = 'Please complete the current form before opening a new one.';
+        groupFieldError = v.firstError;
+        return false;
+      }
+      try {
+        const res = groupFormNew
+          ? await fetch('/admin/api/groups', {
+              method: 'POST',
+              headers: { 'Content-Type': 'application/json' },
+              body: JSON.stringify(groupForm)
+            })
+          : await fetch(`/admin/api/groups/${selectedItem?.id}`, {
+              method: 'PUT',
+              headers: { 'Content-Type': 'application/json' },
+              body: JSON.stringify(groupForm)
+            });
+        if (!res.ok) {
+          formError = await extractError(res, 'Auto-save failed');
+          return false;
+        }
+        await loadData();
+        return true;
+      } catch {
+        formError = 'Network error during auto-save';
+        return false;
+      }
+    }
+    return true;
+  }
+
+  async function startNewUnit(groupId?: string) {
+    const ok = await autoSaveCurrentForm();
+    if (!ok) return;
     selectedItem = null;
     unitForm = { group: groupId ?? null, toBase: 1, symbol: '', label: '', labelPlural: '', id: '' };
+    unitFormSnapshot = { group: groupId ?? null, toBase: 1, symbol: '', label: '', labelPlural: '', id: '' };
     unitFormNew = true;
     groupFormNew = false;
+    unitFieldError = null;
+    groupFieldError = null;
     formError = null;
     formSuccess = null;
   }
 
-  function startNewGroup() {
+  async function startNewGroup() {
+    const ok = await autoSaveCurrentForm();
+    if (!ok) return;
     selectedItem = null;
     groupForm = { id: '', label: '', baseUnitId: '', toUniversal: 1 };
+    groupFormSnapshot = { id: '', label: '', baseUnitId: '', toUniversal: 1 };
     groupFormNew = true;
     unitFormNew = false;
+    unitFieldError = null;
+    groupFieldError = null;
     formError = null;
     formSuccess = null;
   }
@@ -149,6 +277,13 @@
   async function saveUnit() {
     formError = null;
     formSuccess = null;
+    unitFieldError = null;
+    const v = validateUnitForm();
+    if (!v.valid) {
+      formError = 'Please fix the highlighted field.';
+      unitFieldError = v.firstError;
+      return;
+    }
     try {
       const res = unitFormNew
         ? await fetch('/admin/api/units', {
@@ -167,6 +302,10 @@
         const created = (await res.json()) as Unit;
         unitFormNew = false;
         selectedItem = { type: 'unit', id: created.id ?? '' };
+        unitForm = { ...created };
+        unitFormSnapshot = { ...created };
+      } else {
+        unitFormSnapshot = { ...unitForm };
       }
       await loadData();
     } catch { formError = 'Network error'; }
@@ -188,6 +327,13 @@
   async function saveGroup() {
     formError = null;
     formSuccess = null;
+    groupFieldError = null;
+    const v = validateGroupForm();
+    if (!v.valid) {
+      formError = 'Please fix the highlighted field.';
+      groupFieldError = v.firstError;
+      return;
+    }
     try {
       const res = groupFormNew
         ? await fetch('/admin/api/groups', {
@@ -206,6 +352,10 @@
         const created = (await res.json()) as Group;
         groupFormNew = false;
         selectedItem = { type: 'group', id: created.id ?? '' };
+        groupForm = { ...created };
+        groupFormSnapshot = { ...created };
+      } else {
+        groupFormSnapshot = { ...groupForm };
       }
       await loadData();
     } catch { formError = 'Network error'; }
@@ -434,19 +584,23 @@
       <h3>{unitFormNew ? 'New Unit' : 'Edit Unit'}</h3>
       <label>
         ID {#if !unitFormNew}<small class="muted">(readonly)</small>{/if}
-        <input type="text" bind:value={unitForm.id} readonly={!unitFormNew} placeholder="e.g. my-unit" />
+        <input type="text" bind:value={unitForm.id} readonly={!unitFormNew} placeholder="e.g. my-unit"
+          class:field-error={unitFieldError === 'unit-id'} />
       </label>
       <label>
         Label
-        <input type="text" bind:value={unitForm.label} placeholder="e.g. My Unit" />
+        <input type="text" bind:value={unitForm.label} placeholder="e.g. My Unit"
+          class:field-error={unitFieldError === 'unit-label'} />
       </label>
       <label>
         Label (plural)
-        <input type="text" bind:value={unitForm.labelPlural} placeholder="e.g. My Units" />
+        <input type="text" bind:value={unitForm.labelPlural} placeholder="e.g. My Units"
+          class:field-error={unitFieldError === 'unit-labelPlural'} />
       </label>
       <label>
         Symbol
-        <input type="text" bind:value={unitForm.symbol} placeholder="e.g. mu" />
+        <input type="text" bind:value={unitForm.symbol} placeholder="e.g. mu"
+          class:field-error={unitFieldError === 'unit-symbol'} />
       </label>
       <label>
         Group
@@ -459,7 +613,8 @@
       </label>
       <label>
         To Base
-        <input type="number" bind:value={unitForm.toBase} min="0.000001" step="any" />
+        <input type="number" bind:value={unitForm.toBase} min="0.000001" step="any"
+          class:field-error={unitFieldError === 'unit-toBase'} />
         {#if toBaseHint}
           <small class="tobase-hint">{toBaseHint}</small>
         {/if}
@@ -479,11 +634,13 @@
       <h3>{groupFormNew ? 'New Group' : 'Edit Group'}</h3>
       <label>
         ID {#if !groupFormNew}<small class="muted">(readonly)</small>{/if}
-        <input type="text" bind:value={groupForm.id} readonly={!groupFormNew} placeholder="e.g. my-group" />
+        <input type="text" bind:value={groupForm.id} readonly={!groupFormNew} placeholder="e.g. my-group"
+          class:field-error={groupFieldError === 'group-id'} />
       </label>
       <label>
         Label
-        <input type="text" bind:value={groupForm.label} placeholder="e.g. My Group" />
+        <input type="text" bind:value={groupForm.label} placeholder="e.g. My Group"
+          class:field-error={groupFieldError === 'group-label'} />
       </label>
       <label>
         Base Unit
@@ -792,6 +949,9 @@
   }
   .form-error { color: var(--pico-del-color, #e74c3c); background: rgba(231,76,60,0.1); }
   .form-success { color: var(--pico-ins-color, #2ecc71); background: rgba(46,204,113,0.1); }
+  .field-error {
+    border-color: var(--pico-form-element-invalid-border-color) !important;
+  }
   .tobase-hint {
     display: block;
     color: var(--pico-muted-color);