diff --git a/includes/class-tmdo-schema-registry.php b/includes/class-tmdo-schema-registry.php
index 45ef898..93b390c 100644
--- a/includes/class-tmdo-schema-registry.php
+++ b/includes/class-tmdo-schema-registry.php
@@ -67,6 +67,13 @@ class TMDO_Schema_Registry {
 	 */
 	private array $warm_fields = array();
 
+	/**
+	 * Providers already told the Hot zone is frozen (once per request).
+	 *
+	 * @var array<string, true>
+	 */
+	private array $hot_deprecation_flagged = array();
+
 	/**
 	 * Private constructor — use instance() to get the singleton.
 	 */
@@ -138,6 +145,7 @@ class TMDO_Schema_Registry {
 		switch ( $config['zone'] ) {
 			case 'hot':
 				$this->hot_columns[ $config['post_type'] ][ $config['column'] ] = $config['data_type'];
+				$this->flag_hot_deprecation( $provider, $config['meta_key'] );
 				break;
 
 			case 'warm':
@@ -159,6 +167,29 @@ class TMDO_Schema_Registry {
 		}
 	}
 
+	/**
+	 * The Hot zone is frozen (ADR-003): fields still register so existing hot
+	 * data stays readable, but each provider gets one notice per request.
+	 *
+	 * @param string $provider Provider name.
+	 * @param string $meta_key First hot meta_key seen from it.
+	 */
+	private function flag_hot_deprecation( string $provider, string $meta_key ): void {
+		if ( isset( $this->hot_deprecation_flagged[ $provider ] ) ) {
+			return;
+		}
+		$this->hot_deprecation_flagged[ $provider ] = true;
+		_doing_it_wrong(
+			__CLASS__ . '::register',
+			sprintf(
+				'Provider "%1$s" registered Hot zone field "%2$s". The Hot zone is frozen; register an entity group (TMDO_Entity_Registry::register_group) instead.',
+				esc_html( $provider ),
+				esc_html( $meta_key )
+			),
+			esc_html( TMDO_VERSION )
+		);
+	}
+
 	/**
 	 * Bulk register multiple fields.
 	 *
diff --git a/tests/bootstrap.php b/tests/bootstrap.php
index 2c897b0..ccef6bc 100644
--- a/tests/bootstrap.php
+++ b/tests/bootstrap.php
@@ -231,7 +231,7 @@ if ( ! function_exists( 'esc_sql' ) ) {
 	}
 }
 if ( ! function_exists( '_doing_it_wrong' ) ) {
-	function _doing_it_wrong( string $fn, string $msg, string $ver ): void {}
+	function _doing_it_wrong( string $fn, string $msg, string $ver ): void { $GLOBALS['_doing_it_wrong_calls'][] = array( $fn, $msg ); }
 }
 if ( ! function_exists( 'get_option' ) ) {
 	function get_option( string $key, $default = false ) { return $GLOBALS['_wp_options'][ $key ] ?? $default; }
diff --git a/tests/unit/SchemaRegistryTest.php b/tests/unit/SchemaRegistryTest.php
index 73cfe93..91c0af7 100644
--- a/tests/unit/SchemaRegistryTest.php
+++ b/tests/unit/SchemaRegistryTest.php
@@ -18,6 +18,30 @@ class SchemaRegistryTest extends TestCase {
 		$instance->setValue( null, null );
 
 		$this->registry = WPDO_Schema_Registry::instance();
+		$GLOBALS['_doing_it_wrong_calls'] = array();
+	}
+
+	// ── Hot zone deprecation (ADR-003) ──────────────────────────────────────
+
+	public function test_hot_registration_is_flagged_once_per_provider(): void {
+		foreach ( array( 'p1', 'p1', 'p2' ) as $i => $provider ) {
+			$this->registry->register( $provider, array( 'post_type' => 'x', 'meta_key' => "k{$i}", 'zone' => 'hot' ) );
+		}
+		$this->registry->register( 'p1', array( 'post_type' => 'x', 'meta_key' => 'k9' ) ); // zone defaults to hot.
+
+		$calls = $GLOBALS['_doing_it_wrong_calls'];
+		$this->assertCount( 2, $calls );
+		$this->assertStringContainsString( 'p1', $calls[0][1] );
+		$this->assertStringContainsString( 'p2', $calls[1][1] );
+		$this->assertSame( 'hot', $this->registry->get_field_zone( 'x', 'k9' ), 'still registered: frozen, not rejected' );
+	}
+
+	public function test_other_zones_are_not_flagged(): void {
+		$this->registry->register( 'p', array( 'post_type' => 'x', 'meta_key' => 'c', 'zone' => 'cold' ) );
+		$this->registry->register( 'p', array( 'post_type' => 'x', 'meta_key' => 'w', 'zone' => 'warm' ) );
+		$this->registry->register( 'p', array( 'post_type' => 'x', 'meta_key' => 'a', 'zone' => 'archive' ) );
+
+		$this->assertSame( array(), $GLOBALS['_doing_it_wrong_calls'] );
 	}
 
 	// ── register() ──────────────────────────────────────────────────────────