diff --git a/doc/architecture.svg b/doc/architecture.svg
index 9711768..3b1680b 100644
--- a/doc/architecture.svg
+++ b/doc/architecture.svg
@@ -1,4 +1,4 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 520 620" font-family="system-ui,sans-serif" font-size="14">
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 520 660" font-family="system-ui,sans-serif" font-size="14">
   <defs>
     <marker id="arrow" viewBox="0 0 10 10" refX="10" refY="5" markerWidth="8" markerHeight="8" orient="auto-start-reverse">
       <path d="M0,0 L10,5 L0,10 z" fill="#666"/>
@@ -84,29 +84,33 @@
   <!-- Scripts link to 9P -->
   <line class="edge" x1="370" y1="108" x2="300" y2="178"/>
 
+  <!-- ===== Sandbox region ===== -->
+  <rect class="region region-core" x="0" y="530" width="420" height="80"/>
+  <text class="rlabel" x="20" y="548">internal/sandbox</text>
+
   <!-- Tool scripts -->
-  <rect class="box" x="10" y="540" width="210" height="48"/>
-  <text class="label" x="115" y="560">Tool scripts (t/)</text>
-  <text class="label sub" x="115" y="576">file_read, subagent_spawn, ...</text>
+  <rect class="box" x="10" y="555" width="210" height="48"/>
+  <text class="label" x="115" y="575">Tool scripts (t/)</text>
+  <text class="label sub" x="115" y="591">file_read, subagent_spawn, ...</text>
 
   <!-- Inline code -->
-  <rect class="box" x="230" y="540" width="170" height="48"/>
-  <text class="label" x="315" y="560">Inline code</text>
-  <text class="label sub" x="315" y="576">bash, python3, perl, lua, ...</text>
+  <rect class="box" x="230" y="555" width="170" height="48"/>
+  <text class="label" x="315" y="575">Inline code</text>
+  <text class="label sub" x="315" y="591">bash, python3, perl, lua, ...</text>
 
   <!-- execute_code -> tool scripts -->
-  <line class="edge" x1="100" y1="486" x2="100" y2="538"/>
-  <text class="elabel" x="78" y="517">runs</text>
+  <line class="edge" x1="100" y1="486" x2="100" y2="553"/>
+  <text class="elabel" x="78" y="523">runs</text>
 
   <!-- execute_code -> inline code -->
-  <line class="edge" x1="160" y1="486" x2="290" y2="538"/>
+  <line class="edge" x1="160" y1="486" x2="290" y2="553"/>
 
   <!-- LLM providers -->
-  <rect class="box" x="430" y="540" width="80" height="48"/>
-  <text class="label" x="470" y="558">LLM</text>
-  <text class="label sub" x="470" y="574">providers</text>
+  <rect class="box" x="430" y="555" width="80" height="48"/>
+  <text class="label" x="470" y="573">LLM</text>
+  <text class="label sub" x="470" y="589">providers</text>
 
   <!-- backend -> LLM -->
-  <line class="edge" x1="420" y1="426" x2="470" y2="538"/>
-  <text class="elabel" x="465" y="487">HTTP</text>
+  <line class="edge" x1="420" y1="426" x2="470" y2="553"/>
+  <text class="elabel" x="465" y="495">HTTP</text>
 </svg>