From 8b6b5f1348b26da8c03af3c3d0d445690f017bd2 Mon Sep 17 00:00:00 2001
From: Stefan Age <age@adobe.com>
Date: Wed, 6 May 2026 18:46:33 -0500
Subject: [PATCH] feat(types,spec): add tools field to McpUiHostCapabilities

McpUiAppCapabilities already allows widgets to declare tools: {}
to signal they expose oncalltool/onlisttools, but McpUiHostCapabilities
had no corresponding field. This left the handshake one-sided: widgets
could declare intent but had no way to check whether the host would
honor it, and hosts had no spec-defined way to advertise support.

Add tools?: { listChanged?: boolean } to McpUiHostCapabilities in both
the TypeScript types and the draft spec, with MUST-level requirements on
what a host advertising this capability is expected to implement:
- Call tools/list on the app after connect and surface results to the LLM
- Route matching LLM tool calls back to the app via tools/call
- Re-query tools/list on notifications/tools/list_changed (if listChanged: true)

Fixes: #655
---
 specification/draft/apps.mdx | 12 ++++++++++++
 src/spec.types.ts            |  5 +++++
 2 files changed, 17 insertions(+)

diff --git a/specification/draft/apps.mdx b/specification/draft/apps.mdx
index 6c810d63..502d4224 100644
--- a/specification/draft/apps.mdx
+++ b/specification/draft/apps.mdx
@@ -660,6 +660,18 @@ interface HostCapabilities {
     /** Host supports resources/list_changed notifications. */
     listChanged?: boolean;
   };
+  /**
+   * Host supports calling tools declared by the app (oncalltool/onlisttools).
+   * When present, the host MUST:
+   * 1. Call tools/list on the app after connect and surface results to the LLM.
+   * 2. Route LLM tool calls matching app-declared tool names back to the app via tools/call.
+   * 3. If listChanged is true: re-query tools/list when it receives a
+   *    notifications/tools/list_changed notification from the app.
+   */
+  tools?: {
+    /** Host will re-query tools/list when it receives notifications/tools/list_changed from the app. */
+    listChanged?: boolean;
+  };
   /** Host accepts log messages. */
   logging?: {};
   /** Sandbox configuration applied by the host. */
diff --git a/src/spec.types.ts b/src/spec.types.ts
index 9f83fa97..1c275d9b 100644
--- a/src/spec.types.ts
+++ b/src/spec.types.ts
@@ -508,6 +508,11 @@ export interface McpUiHostCapabilities {
     /** @description Host supports resources/list_changed notifications. */
     listChanged?: boolean;
   };
+  /** @description Host supports calling tools declared by the app (oncalltool/onlisttools). When present, the host will call tools/list on the app after connect, surface the results to the LLM, and route matching tool calls back to the app via tools/call. */
+  tools?: {
+    /** @description Host supports tools/list_changed notifications from the app and will re-query tools/list when received. */
+    listChanged?: boolean;
+  };
   /** @description Host accepts log messages. */
   logging?: {};
   /** @description Sandbox configuration applied by the host. */