diff --git a/frontend/src/components/EntityForm.ts b/frontend/src/components/EntityForm.ts
index 716150ab..09a59f35 100644
--- a/frontend/src/components/EntityForm.ts
+++ b/frontend/src/components/EntityForm.ts
@@ -22,6 +22,63 @@ import { customElement, property, state } from 'lit/decorators.js';
 
 const ENTITY_ID_RE = /^[a-z][a-z0-9_]*\.[a-z][a-z0-9_]*$/;
 
+/**
+ * Known Home Assistant domain prefixes. We don't reject unknown domains
+ * (the API accepts any matching the regex), but unknown ones get a
+ * warning so the operator sees what's standard. Add new domains here
+ * as integrations land.
+ */
+const KNOWN_DOMAINS = new Set([
+    'sensor', 'binary_sensor', 'switch', 'light', 'climate', 'cover',
+    'fan', 'media_player', 'lock', 'camera', 'vacuum', 'humidifier',
+    'water_heater', 'scene', 'script', 'automation', 'input_boolean',
+    'input_number', 'input_text', 'input_select', 'input_datetime',
+    'person', 'device_tracker', 'zone', 'sun', 'weather', 'calendar',
+    'remote', 'siren', 'select', 'number', 'text', 'button',
+    'homeassistant', 'homecore', 'group', 'notify', 'tts', 'alarm_control_panel',
+]);
+
+type FieldValidity = { ok: true } | { ok: false; level: 'err' | 'warn'; msg: string };
+
+function validateEntityId(id: string): FieldValidity {
+    const trimmed = id.trim();
+    if (!trimmed) return { ok: false, level: 'err', msg: 'required' };
+    if (!ENTITY_ID_RE.test(trimmed)) {
+        return {
+            ok: false,
+            level: 'err',
+            msg: 'must match domain.snake_case (lowercase, digits, underscores)',
+        };
+    }
+    const domain = trimmed.split('.')[0]!;
+    if (!KNOWN_DOMAINS.has(domain)) {
+        return {
+            ok: false,
+            level: 'warn',
+            msg: `unknown domain "${domain}" — HA-standard domains include sensor / light / switch / binary_sensor / climate`,
+        };
+    }
+    return { ok: true };
+}
+
+function validateState(s: string): FieldValidity {
+    if (!s.trim()) return { ok: false, level: 'err', msg: 'required' };
+    return { ok: true };
+}
+
+function validateAttrs(raw: string): FieldValidity {
+    if (!raw.trim()) return { ok: true }; // empty = {}
+    try {
+        const parsed = JSON.parse(raw);
+        if (typeof parsed !== 'object' || Array.isArray(parsed) || parsed === null) {
+            return { ok: false, level: 'err', msg: 'must be a JSON object (not array, not scalar)' };
+        }
+        return { ok: true };
+    } catch (e) {
+        return { ok: false, level: 'err', msg: `JSON parse: ${e instanceof Error ? e.message : String(e)}` };
+    }
+}
+
 @customElement('hc-entity-form')
 export class EntityForm extends LitElement {
     @property({ type: String }) entityId = '';
@@ -31,6 +88,10 @@ export class EntityForm extends LitElement {
 
     @state() private _attrs = '';
     @state() private _err: string | null = null;
+    /** Per-field live validity. `null` = haven't typed yet (no decoration). */
+    @state() private _idValid: FieldValidity | null = null;
+    @state() private _stateValid: FieldValidity | null = null;
+    @state() private _attrsValid: FieldValidity | null = null;
 
     static styles = css`
         :host { display: block; font-family: var(--hc-font-sans, 'Outfit', system-ui, sans-serif); color: var(--hc-text, #e6eaee); }
@@ -45,6 +106,14 @@ export class EntityForm extends LitElement {
         }
         input:focus, textarea:focus { outline: 2px solid hsl(185 80% 50% / 0.5); border-color: var(--hc-primary, #19d4e5); }
         input[disabled] { opacity: 0.5; cursor: not-allowed; }
+        input.invalid, textarea.invalid { border-color: hsl(0 60% 50%); }
+        input.warn, textarea.warn { border-color: hsl(38 80% 55%); }
+        .field-status { font-size: 11px; margin-top: 4px; display: flex; align-items: center; gap: 6px; }
+        .field-status.ok { color: hsl(150 60% 55%); }
+        .field-status.err { color: hsl(0 70% 70%); }
+        .field-status.warn { color: hsl(38 80% 65%); }
+        .field-status .sigil { display: inline-block; width: 12px; text-align: center; font-weight: 700; }
+        button.primary[disabled] { background: hsl(220 15% 20%); color: var(--hc-text-muted, #7b899d); border-color: var(--hc-border, #2a323e); cursor: not-allowed; }
         textarea { min-height: 90px; resize: vertical; }
         .hint { font-size: 11px; color: var(--hc-text-muted, #7b899d); margin-top: 4px; }
         .err { margin-top: 10px; padding: 10px; border: 1px solid #b35a5a; border-radius: 6px; background: hsl(0 35% 12%); color: #f0c0c0; font-size: 12px; }
@@ -70,6 +139,47 @@ export class EntityForm extends LitElement {
         }
     }
 
+    /** Allow the host (Dashboard) to surface a server-side error inline. */
+    public setSubmitError(msg: string | null): void {
+        this._err = msg;
+    }
+
+    /** True iff every field is valid (warnings are OK, errors block). Public so the host can bind a disabled state on the submit button. */
+    public isValid(): boolean {
+        const checks = [
+            validateEntityId(this.entityId),
+            validateState(this.state),
+            validateAttrs(this._attrs),
+        ];
+        return !checks.some((c) => !c.ok && c.level === 'err');
+    }
+
+    private _onIdInput(v: string) {
+        this.entityId = v;
+        this._idValid = validateEntityId(v);
+    }
+    private _onStateInput(v: string) {
+        this.state = v;
+        this._stateValid = validateState(v);
+    }
+    private _onAttrsInput(v: string) {
+        this._attrs = v;
+        this._attrsValid = validateAttrs(v);
+    }
+
+    private _statusLine(label: string, v: FieldValidity | null) {
+        if (v === null) return html``;
+        if (v.ok) return html`<div class="field-status ok"><span class="sigil">✓</span>${label} OK</div>`;
+        return html`<div class="field-status ${v.level}">
+            <span class="sigil">${v.level === 'warn' ? '!' : '✗'}</span>${v.msg}
+        </div>`;
+    }
+
+    private _fieldClass(v: FieldValidity | null): string {
+        if (v === null || v.ok) return '';
+        return v.level;
+    }
+
     /** Public — call from host to trigger validation + emit submit event. */
     public requestSubmit(): void { this._submit(); }
 
@@ -118,21 +228,27 @@ export class EntityForm extends LitElement {
             <form @submit=${(e: Event) => { e.preventDefault(); this._submit(); }}>
                 <label for="eid">entity_id</label>
                 <input id="eid" .value=${this.entityId}
+                       class=${this._fieldClass(this._idValid)}
                        ?disabled=${this.editing}
-                       @input=${(e: Event) => (this.entityId = (e.target as HTMLInputElement).value)}
+                       @input=${(e: Event) => this._onIdInput((e.target as HTMLInputElement).value)}
                        placeholder="light.kitchen_ceiling" />
                 <div class="hint">format: <code>domain.snake_case</code> — domain like sensor / light / switch / binary_sensor</div>
+                ${this._statusLine('entity_id', this._idValid)}
 
                 <label for="state">state</label>
                 <input id="state" .value=${this.state}
-                       @input=${(e: Event) => (this.state = (e.target as HTMLInputElement).value)}
+                       class=${this._fieldClass(this._stateValid)}
+                       @input=${(e: Event) => this._onStateInput((e.target as HTMLInputElement).value)}
                        placeholder="on / off / 42 / 14.5 / detected" />
+                ${this._statusLine('state', this._stateValid)}
 
                 <label for="attrs">attributes (JSON object)</label>
                 <textarea id="attrs" .value=${this._attrs}
-                          @input=${(e: Event) => (this._attrs = (e.target as HTMLTextAreaElement).value)}
+                          class=${this._fieldClass(this._attrsValid)}
+                          @input=${(e: Event) => this._onAttrsInput((e.target as HTMLTextAreaElement).value)}
                           placeholder='{ "friendly_name": "Kitchen Ceiling", "brightness": 230 }'></textarea>
                 <div class="hint">optional; leave blank for <code>{}</code></div>
+                ${this._statusLine('attributes', this._attrsValid)}
 
                 ${this._err ? html`<div class="err">${this._err}</div>` : ''}
             </form>
diff --git a/frontend/src/pages/Dashboard.ts b/frontend/src/pages/Dashboard.ts
index 3d7a7fae..321e53b1 100644
--- a/frontend/src/pages/Dashboard.ts
+++ b/frontend/src/pages/Dashboard.ts
@@ -173,6 +173,8 @@ export class Dashboard extends LitElement {
     private async _onSubmit(e: CustomEvent<{ entity_id: string; state: string; attributes: Record<string, unknown> }>) {
         const { entity_id, state, attributes } = e.detail;
         const wasEditing = this.editingState !== null;
+        // Clear any previous server-side error before the next attempt.
+        this._form?.setSubmitError(null);
         try {
             const resp = await fetch(`/api/states/${encodeURIComponent(entity_id)}`, {
                 method: 'POST',
@@ -182,14 +184,21 @@ export class Dashboard extends LitElement {
                 },
                 body: JSON.stringify({ state, attributes }),
             });
-            if (!resp.ok) throw new Error(`HTTP ${resp.status}: ${await resp.text()}`);
+            if (!resp.ok) {
+                // Surface the server message inline in the form, not at
+                // the top of the page — the form is what the user is
+                // looking at.
+                const body = await resp.text();
+                this._form?.setSubmitError(`server rejected (${resp.status}): ${body || resp.statusText}`);
+                return;
+            }
             this.modalOpen = false;
             this.editingState = null;
             this.submitToast = `${wasEditing ? 'Updated' : 'Created'} ${entity_id} = ${state}`;
             window.setTimeout(() => (this.submitToast = null), 3000);
             await this.refresh();
         } catch (err) {
-            this.error = err instanceof Error ? err.message : String(err);
+            this._form?.setSubmitError(err instanceof Error ? err.message : String(err));
         }
     }