Superhuman MCP with DeepSeek Harness

Superhuman Mail runs a remote MCP server at https://mcp.mail.superhuman.com/mcp. This guide covers what that server requires, why DSH cannot complete its interactive sign-in on its own, the stdio bridge Superhuman documents for desktop clients, and how to check the connection without letting an agent touch your inbox.

Last checked 2026-10-13 · English-only guide · checked against the sources listed at the end

Short answer. The Mail MCP server is remote and authenticates with an interactive browser sign-in that depends on the Superhuman Mail Chrome extension. DSH's MCP client drives stdio commands and streamable-HTTP endpoints with static headers, so reach this server through a local stdio bridge and keep send actions behind approval. Superhuman Docs MCP is a separate product with its own page and its own plan question.

Scope: DeepSeek Harness only. The row below is a Cordis patch for DSH's MCP client. Claude Desktop, Cursor, Windsurf and other clients use their own config format, so follow that service's own setup page for those — these steps do not apply there.

At a glance

Server: Superhuman Mail MCP Server (Superhuman help centre, “Superhuman AI” section)
Endpoint: https://mcp.mail.superhuman.com/mcp
Transport: Remote MCP over HTTP; the desktop instructions reach it through the mcp-remote stdio bridge
Authentication: Interactive sign-in; the Superhuman Mail Chrome extension is listed as a prerequisite “for authentication”
Plan: Superhuman Mail with Ask AI enabled, Business plan or higher; the server is not available on Starter
Managed accounts: Enterprise or managed accounts may need an administrator to enable MCP access first
Agent capabilities: Search mail and calendar, draft in your voice, create or update events, send mail, read open statuses, manage reminders and labels
DSH fit: Local stdio bridge — the remote URL alone has no way to finish interactive sign-in inside DSH
Last checked: 2026-10-13

Step 1 — Confirm the account, plan and client prerequisites

Superhuman lists four prerequisites before the Mail MCP server will connect. Check them first, because the first three cause most failed setups:

A Superhuman Mail account with Ask AI enabled.
The Superhuman Mail Chrome extension installed, because it is what authenticates the connection.
An MCP-capable AI client — in our case DSH.
A Business plan or higher. The help centre states the MCP Server is not available on the Starter plan.

If your account is Enterprise or managed, an administrator may have to enable MCP access before you can connect; the help centre routes that request through a customer success manager or hello@superhuman.com. Not every organisation needs this step.

Step 2 — Read the tool surface before you grant access

The server exposes read and write tools across mail and calendar. The ones worth knowing before you connect:

Read: list_accounts, list_threads, get_thread, get_message, query_email_and_calendar, get_read_statuses, get_availability, list_drafts, list_reminders, list_labels, list_splits.

Write: create_or_update_draft, create_or_update_event, create_or_update_reminder, update_thread, trash_thread, discard_draft, discard_reminder, unsubscribe, update_personalization.

Sends mail: send_draft — supports Smart Send, scheduled send and undo send. The help centre advises enabling per-call confirmation in your AI client before you use it, because it can send without delay.

Two behaviour details matter for prompts. Drafts created through MCP appear in Superhuman Mail, but drafts created in Gmail or Outlook do not appear there yet, and the reverse holds too. And add_account returns a sign-in link: never share that link, and never open one somebody sends you — the help centre makes the same point.

Step 3 — Reach the remote server from DSH through a stdio bridge

DSH's MCP client configuration supports two transports: a spawned child process over stdio, and streamable HTTP with a static header map. There is no interactive OAuth step in that client, and Superhuman's Mail endpoint authenticates through a browser sign-in that depends on the Chrome extension, so the remote URL cannot be wired up directly. Run a local bridge instead:

# cordis.patch.yml — passed with: dsh web --patch ./cordis.patch.yml
- insert:
    - id: mcp-superhuman-mail
      name: '@deepseek-ai/dsh-mcp-client'
      config:
        serverName: superhuman-mail
        transport: stdio
        command: npx
        args: ['-y', 'mcp-remote', 'https://mcp.mail.superhuman.com/mcp']
        env: {}
        cwd: !!js process.cwd()

The command shape is Superhuman's own: its help centre shows npx -y mcp-remote https://mcp.mail.superhuman.com/mcp for desktop clients. Adapted to DSH, the bridge holds the sign-in state and DSH launches and stops it with the plugin lifecycle. DSH deliberately removes ambient credential-looking variables before starting a child process, so the token store belongs to the bridge rather than to your shell environment.

To keep the row across runs, merge it into a user patch layer — $DSH_HOME/profiles/<name>/cordis.patch.yml for one profile, or $DSH_HOME/cordis.patch.yml for every profile on the machine. Merge into the existing file rather than overwriting it.

Verification status: the prerequisites, endpoint, tool list and bridge command come from Superhuman's help centre, read 2026-10-13. We did not run this against a paid Mail account, so treat the DSH row as a documented adaptation rather than a tested recipe.

Verify with read-only calls first

dsh --profile web --dump-config and confirm your mcp-superhuman-mail row is in the booted tree.
Prompt: “Use the superhuman-mail tools to list my accounts. Do not send, draft, or delete anything.” Expect list_accounts to return the seats linked to your session.
Negative test: temporarily point args at a package name that does not exist, confirm no mcp__superhuman-mail__* tools are listed, then revert. This proves you are looking at the bridge's output rather than a cached tool list.
Only then enable a write path, and leave per-call confirmation on.

Tool discovery is asynchronous, so wait for the provider's tools to appear before sending the first prompt.

Troubleshooting

Sign-in never completes, or you are bounced back to sign in: Superhuman's own troubleshooting blames a stale cached extension: open chrome://extensions, turn on Developer mode, click Update, and retry — reinstall the Mail extension from the Chrome Web Store if that does not help. When linking an extra account, the screen should read “Sign in to link this account to your assistant”.
No mcp__superhuman-mail__* tools appear: Check that the bridge package can be fetched (npx needs network access on first run), confirm the row id and serverName are unique among live clients, then reload. Initial discovery is asynchronous.
Node errors about node:path or “File is not defined”: The help centre ties these to Node 18 or earlier and recommends Node 20.18.1 or later (its Claude Desktop walkthrough uses nvm which 20 to pin the binary). DSH itself requires Node ^22.19.0 or newer, which also satisfies that floor.
The agent asks for confirmation on every action: Expected, and worth keeping. This server can send mail, so approval prompts are the safety net, not noise.

Limits and risk notes. The server can send email and change calendar events — enable per-call confirmation. Treat an add_account sign-in link as a secret. Superhuman's help centre states there is currently no usage limit for the Mail MCP Server and that this may change. Plan requirements are the Mail server's; Docs MCP is documented separately at superhuman.com/docs/features/docs-mcp with its own plan question, so do not assume Mail's terms carry over.

FAQ

Can DeepSeek Harness connect straight to the Superhuman Mail MCP URL?

Not on its own. The remote server authenticates through an interactive sign-in that depends on the Superhuman Mail Chrome extension, while DSH's MCP client supports stdio commands and streamable-HTTP endpoints with static headers. Reach it through a local stdio bridge instead, and treat the bridge as the component that holds the sign-in state.

Which Superhuman plan do I need for Mail MCP?

The help centre lists a Business plan or higher and states that the MCP Server is not available on the Starter plan, together with a Mail account that has Ask AI enabled. On Enterprise or managed accounts an administrator may still need to enable MCP access.

Can the agent send email by itself?

Yes, if the send tool is enabled: the tool surface includes send_draft, which supports Smart Send, scheduled send and undo send. Enable per-call confirmation in your client before using it, and start with read-only prompts such as list_accounts or a thread search.

Is Superhuman Mail MCP the same as Superhuman Docs MCP?

No. Mail MCP works with your inbox and calendar. Docs MCP is a separate product for documents and tables, documented on its own page with its own plan question, so check that page rather than assuming Mail's requirements apply.

Sources

Every claim above was checked against these pages on 2026-10-13. Third-party material is labelled where it is used.

Superhuman help centre — “Superhuman Mail MCP Server” — Prerequisites, endpoint, bridge command, tool list, multi-account notes, troubleshooting read 2026-10-13
Superhuman Mail — “Email MCP Server for AI Workflows” — Product positioning, supported assistants read 2026-10-13
Superhuman — “Docs MCP” — Separate Docs product, permissions model read 2026-10-13
DeepSeek Harness docs — “Connect a third-party memory MCP server” — Transport configuration, tool naming, credential scrubbing read 2026-10-13
DeepSeek Harness docs — “Plugin Config Catalog” — stdio and streamable-HTTP config fields, reconnect defaults read 2026-10-13

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Verification status

These guides document configuration; they are not reports of a live run. Endpoints, prerequisites, tool names and billing rules were read from the sources listed above on 2026-10-13. We did not sign in to any of the four services, did not run a paid generation, and did not send mail, so treat every config row as a documented recipe to test in your own environment. If a service changes an endpoint or its plan terms, that operator's own page wins.