[Platform][Anthropic] Add prompt caching for tool definitions

Add cache_control marker to the last tool definition, creating an
additional cache breakpoint between system prompt and messages.
Tool definitions are identical across requests, making them an
effective caching target that reduces input token costs.

Respects the existing cacheRetention setting (none/short/long).

ModelClient.php

@@ -63,6 +63,7 @@ final class ModelClient implements ModelClientInterface
 
         if (isset($options['tools'])) {
             $options['tool_choice'] = ['type' => 'auto'];
+            $options['tools'] = $this->injectToolsCacheControl($options['tools']);
         }
 
         if (isset($options['thinking'])) {
@@ -90,6 +91,33 @@ final class ModelClient implements ModelClientInterface
         ]));
     }
 
+    /**
+     * Injects a prompt-caching marker on the last tool definition.
+     *
+     * This creates an additional cache breakpoint after all tool definitions,
+     * so the prefix "system → tools" can be cached independently of the
+     * messages that follow.  Tool definitions are typically identical across
+     * requests, making this a very effective caching target.
+     *
+     * @param list<array<string, mixed>> $tools Normalised tool definitions
+     *
+     * @return list<array<string, mixed>>
+     */
+    private function injectToolsCacheControl(array $tools): array
+    {
+        if ('none' === $this->cacheRetention || [] === $tools) {
+            return $tools;
+        }
+
+        $cacheControl = 'long' === $this->cacheRetention
+            ? ['type' => 'ephemeral', 'ttl' => '1h']
+            : ['type' => 'ephemeral'];
+
+        $tools[\count($tools) - 1]['cache_control'] = $cacheControl;
+
+        return $tools;
+    }
+
     /**
      * Injects prompt-caching markers into the normalised message payload.
      *