plugin-mcp drops MCP tool `_meta` when decoding tools/list

[mapleeit]

Summary

@executor-js/plugin-mcp silently drops each tool's MCP _meta object while decoding tools/list. Hosts that embed the plugin as their MCP client never see that field, even though the MCP spec reserves it as opaque metadata on Tool.

Observed on @executor-js/plugin-mcp@1.5.40; main still has the same ListedTool schema.

Spec

MCP Tool includes _meta as a reserved, implementation-defined map (Tools). Servers use it for host-only routing / policy hints that must not be interpreted by the model and must not be stuffed into the closed annotations set (title, readOnlyHint, destructiveHint, …).

Examples we see on the wire:

{
  "name": "time_get_current_time",
  "description": "Get the current time",
  "inputSchema": { "type": "object" },
  "_meta": {
    "serverName": "time",
    "shortDescription": "Current time",
    "defer_loading": false
  }
}

Where it is stripped

packages/plugins/mcp/src/sdk/manifest.ts decodes each list entry with a closed Effect Schema.Struct. Excess keys are ignored:

const ListedTool = Schema.Struct({
  name: Schema.String,
  description: Schema.optional(Schema.NullOr(Schema.String)),
  inputSchema: Schema.optional(Schema.Unknown),
  parameters: Schema.optional(Schema.Unknown),
  outputSchema: Schema.optional(Schema.Unknown),
  annotations: Schema.optional(McpToolAnnotations),
  // `_meta` is not declared, so decode drops it
});

extractManifestFromListToolsResult then copies only the declared fields onto McpToolManifestEntry, so _meta is gone before toToolDef.

Pagination is already opaque (ListToolsPage.tools is Schema.Array(Schema.Unknown)); the loss happens on the per-entry decode after pages are merged.

Why hosts cannot recover it

mcpPlugin() has no hook that exposes the raw tools/list result. connections.refresh() returns already-constructed Tool values. Wrapping the exported extractManifestFromListToolsResult does not help, because the plugin's internal callers import the local function.

Executor Tool / ToolDef also has no _meta field. The plugin already persists the real MCP tool name in the existing annotations.mcp stamp (toToolDef in packages/plugins/mcp/src/sdk/plugin.ts), which is the natural bag to carry opaque _meta as well.

Note: McpStampSchema / readStamp currently decode annotations.mcp as { toolName, upstream? } only. If stamp round-trips through that schema, _meta would be stripped a second time unless the stamp schema is extended too.

Suggested fix (small)

1. Add _meta: Schema.optional(Schema.Unknown) to ListedTool and McpToolManifestEntry.
2. Copy _meta in extractManifestFromListToolsResult.
3. Stamp it onto annotations.mcp._meta in toToolDef (Executor Tool has no _meta field).
4. Add _meta to McpStampSchema if stamps are re-decoded.

Happy to send a PR if this direction looks right.

Repro

import { extractManifestFromListToolsResult } from "@executor-js/plugin-mcp/core";

const meta = { serverName: "time", shortDescription: "Current time" };

const manifest = extractManifestFromListToolsResult({
  tools: [{
    name: "time_get_current_time",
    description: "Get the current time",
    inputSchema: { type: "object" },
    _meta: meta,
  }],
});

// expected: manifest.tools[0]._meta === meta
// actual:   `_meta` is missing