Generated MCP tool routes cannot emit CallToolResult._meta (Agent.Result metadata is decoded but never projected)

[ScriptedAlchemy]

Summary

Route mode lowers an Agent Document to a CallToolResult through documentToCallToolResult / projectMcpRenderStream (packages/rsc-runtime/src/project-mcp.ts) and registerGeneratedRoutes (packages/agent-bundle/src/mcp-server-runtime.ts). Those produce content, structuredContent (from Agent.Result value), and isError (from a represented Agent.Error), but nothing on the route contract reaches the result's _meta. Agent.Result does accept a metadata?: JsonValue prop, and decodeAgentDocument carries it into the document root (AgentResultNode.metadata), yet the MCP projection ignores it — so there is a documented-looking prop with no wire effect, and no other way for a generated tool route to set per-result _meta.

The defineOperation escape hatch could do this (Mcp.Result _meta={...}), so consumers migrating from createRscMcpServer to conventional routes lose the capability. The MCP Apps convention (ext-apps registerAppTool) stamps _meta.ui.resourceUri on every tool result as well as on the listing; the listing half works in route mode through config._meta, the result half cannot be expressed.

Observed on framework main af1c185.

Repro

// src/mcp/demo/tools/status.tsx
export const config = { _meta: { ui: { resourceUri: 'ui://demo/panel.html' } }, description: 'Status.' } satisfies ToolConfig;
export const inputSchema = z.object({}).strict();
export const resultSchema = z.object({ status: z.literal('ready') }).strict();
export default async function Status() {
  return (
    <Agent.Result metadata={{ ui: { resourceUri: 'ui://demo/panel.html' } }} value={{ status: 'ready' }}>
      <Agent.Text>ready</Agent.Text>
    </Agent.Result>
  );
}

tools/list shows _meta.ui.resourceUri on the tool (good); tools/call returns { content, structuredContent } with no _meta. The runtime primitives show where it is lost:

import { decodeAgentDocument, documentToCallToolResult } from '@agent-bundle/runtime';
import { createElement } from 'react';
const tree = createElement('agent-result', { metadata: { ui: { resourceUri: 'ui://demo/panel.html' } }, value: { status: 'ready' } }, createElement('agent-text', null, 'ready'));
const document = decodeAgentDocument(tree);
console.log(JSON.stringify({ rootMetadata: document.root.metadata, callToolResult: documentToCallToolResult(document) }));
// {"rootMetadata":{"ui":{"resourceUri":"ui://demo/panel.html"}},"callToolResult":{"content":[{"text":"ready","type":"text"}],"structuredContent":{"status":"ready"}}}

document.root.metadata is populated; documentToCallToolResult never reads it.

Expected

Either project AgentResultNode.metadata to CallToolResult._meta (the natural reading of the prop name), or document in docs/framework-mode.md that generated routes have no result _meta and what Agent.Result metadata is for.

Context

Hit while re-porting ScriptedAlchemy/movie-library (five generated servers + an MCP App) onto af1c185; the plugin now relies on listing _meta only and records the missing result _meta as a wire deviation from its previous defineOperation-based build.

[ScriptedAlchemy]

Fixed in #405 (merged as 56b77db).

• Agent.Result metadata now projects to CallToolResult._meta in project-mcp.ts, JSON-snapshotted through the same wire boundary as structuredContent. MCP _meta is an object, so a non-object ('string', an array) fails the projection closed with McpProjectionError('invalid-result-metadata') rather than being dropped or coerced. Listing-level _meta (static config._meta) is unchanged, so the MCP Apps convention stamps _meta.ui.resourceUri on both tools/list and tools/call.
• Related wire fix in the same PR: generated tools advertise outputSchema only when resultSchema describes an object, so text-only routes (resultSchema = z.undefined()) return a bare content result with no structuredContent: {}.
• Evidence: mcp-projector.test.ts and projection/mcp-in-memory.test.ts (projection + in-memory harness _meta, frozen snapshot, fail-closed cases), generated-route-server.test.ts (over real stdio: listing _meta + conditional outputSchema, call _meta + structuredContent). Docs: "What reaches the MCP wire" table in docs/framework-mode.md; the mcp-in-memory harness result exposes _meta.