feat: MCP app sampling support via stock SDK types (alt to #477) (#530)

* feat: MCP app sampling support via stock SDK types

Alternative to #477. Instead of forking a stripped-down sampling schema,
this reuses the stock `CreateMessageRequest` / `CreateMessageResult` /
`CreateMessageResultWithTools` types from `@modelcontextprotocol/sdk`
directly — same pattern already used for `tools/call`.

- Spec: `sampling/createMessage` listed under `### Standard MCP Messages`,
  `sampling?: { tools?: {} }` added to `HostCapabilities` (mirrors MCP
  `ClientCapabilities.sampling` shape for easy pass-through).
- SDK: `App.createSamplingMessage()` with overloads that narrow the return
  type based on whether `params.tools` is set; `AppBridge.oncreatesamplingmessage`
  setter; `CreateMessageRequest`/`CreateMessageResult*` added to the
  `AppRequest`/`AppResult` unions.
- Picks up SEP-1577 tool-calling support (`tools`, `toolChoice`, `tool_use`
  content blocks, `stopReason: "toolUse"`, array content) for free — unblocks
  the "nested agents" use case motivated by the original PR.

https://claude.ai/code/session_01ENGWTtsfcyP4S6fTWtUMAh

* feat(app): guard createSamplingMessage before handshake completes

---------

Co-authored-by: Claude <noreply@anthropic.com>

src/generated/schema.json

@@ -501,6 +501,19 @@
             }
           },
           "additionalProperties": false
+        },
+        "sampling": {
+          "description": "Host supports LLM sampling (sampling/createMessage) from the view.\nMirrors the MCP `ClientCapabilities.sampling` shape so hosts can pass it through.",
+          "type": "object",
+          "properties": {
+            "tools": {
+              "description": "Host supports tool use via `tools` and `toolChoice` parameters.",
+              "type": "object",
+              "properties": {},
+              "additionalProperties": false
+            }
+          },
+          "additionalProperties": false
         }
       },
       "additionalProperties": false
@@ -2841,6 +2854,19 @@
                 }
               },
               "additionalProperties": false
+            },
+            "sampling": {
+              "description": "Host supports LLM sampling (sampling/createMessage) from the view.\nMirrors the MCP `ClientCapabilities.sampling` shape so hosts can pass it through.",
+              "type": "object",
+              "properties": {
+                "tools": {
+                  "description": "Host supports tool use via `tools` and `toolChoice` parameters.",
+                  "type": "object",
+                  "properties": {},
+                  "additionalProperties": false
+                }
+              },
+              "additionalProperties": false
             }
           },
           "additionalProperties": false,

src/generated/schema.ts

@@ -558,6 +558,24 @@ export const McpUiHostCapabilitiesSchema = z.object({
   message: McpUiSupportedContentBlockModalitiesSchema.optional().describe(
     "Host supports receiving content messages (ui/message) from the view.",
   ),
+  /**
+   * @description Host supports LLM sampling (sampling/createMessage) from the view.
+   * Mirrors the MCP `ClientCapabilities.sampling` shape so hosts can pass it through.
+   */
+  sampling: z
+    .object({
+      /** @description Host supports tool use via `tools` and `toolChoice` parameters. */
+      tools: z
+        .object({})
+        .optional()
+        .describe(
+          "Host supports tool use via `tools` and `toolChoice` parameters.",
+        ),
+    })
+    .optional()
+    .describe(
+      "Host supports LLM sampling (sampling/createMessage) from the view.\nMirrors the MCP `ClientCapabilities.sampling` shape so hosts can pass it through.",
+    ),
 });
 
 /**