email-thread-mcp

MCP server (stdio) with three deterministic tools for an accounts-receivable (AR) email agent: no LLM inside, no network access, no credentials.

Use

Any MCP client that starts stdio servers can run it with uvx:

{
  "mcpServers": {
    "email-thread-mcp": {
      "command": "uvx",
      "args": ["email-thread-mcp@0.4.1"]
    }
  }
}

Pin a version: a client that keeps the server running may otherwise serve a stale cached build.

Each tool returns one JSON object. Structured arguments (ar_mailboxes, categories, output, envelope, schema, allowed_categories, thread) may be passed as values or as text: clients that render arguments through a template engine first can send JSON or a Python-style repr, and both are accepted.

A typical agent calls parse_email_thread first, as preprocessing, and uses latest_message, reply_to_name and thread_context in later steps; it calls validate_output last, on its final envelope.

Use assemble_output instead when a workflow engine can build the envelope itself. A model that copies identifiers, invoice numbers or a draft into a final JSON answer changes some of them; a template that renders each value with a JSON filter does not. Its argument names (envelope, schema, allowed_categories, thread) are chosen not to clash with the usual step outputs, since some engines fill an argument from a context variable of the same name. A failed check does not raise: it is reported in validation, so the caller still gets the output to review. An argument that is still an unrendered template (a variable it uses was not set) raises an error that says so.

For the reply, two fields save the drafting model from reading the whole thread:

• reply_to_greeting: how to address reply_to_name. If our earlier message opened "Dear Mr. Berg,", it is Mr. Berg for Jan Berg; otherwise the name as given. No honorific is added that no message states.
• our_signature: the signature block of our quoted message, to sign the reply with. Our message is the one from ar_mailboxes or, without them, the one signed by the person the customer greets ("Dear Ms. Weiss" → Clara Weiss's message). Of that person's quoted messages, the fullest block is used, since a short follow-up is often signed with the name alone. Phone, email, URL and address lines are dropped; null when no quoted message is ours. If parsing fails, falling back to the plain-text body before the first quote marker keeps the agent working.

Language support

Inbound mail may be in any language, so the parser treats its three kinds of miss differently:

vocab.py covers EN, DE, FR, NL, IT, ES, PT, PL, CS, SV, DA, NO, FI. Only EN is verified against real threads; the others are built from Outlook's localized reply headers. A language outside the list still splits, and parse_info.warnings names what was not understood, e.g. quoted message 1: header labels not recognised: Kimden, Gönderildi, Kime, Konu. Search traces for those warnings to see which language to add next.

To add or verify a language: add its entry to vocab.py and a case to tests/test_languages.py. Once a real thread in it (anonymised as below) parses correctly, add it as a fixture and set verified.

Untrusted input

The body is written by the customer, and anyone can send one. The parser:

• drops text a reader cannot see (CSS display:none, visibility:hidden, font-size:0, opacity:0, mso-hide:all, the hidden attribute) and zero-width, bidirectional and control characters, and reports both in parse_info.warnings;
• bounds its own cost: the body is cut at 2M raw / 300k text characters, and dates beyond 50 parser calls per email stay null.

It does not make the content trustworthy. The quoted history in particular is plain text the customer can edit, including a fake message "from" the AR team (role recipient). Prompts must treat latest_message and thread_context as data, never as instructions.

Release

1. Bump version in pyproject.toml, then uv run pytest.
2. Commit, tag v<version>, push both. Never move or reuse a tag or a version.
3. uv build, check that the sdist holds only src/, README.md and pyproject.toml, then uv publish.
4. Check: uvx email-thread-mcp@<version> starts and waits on stdin.

Develop

uv sync
uv run pytest            # 14 real Outlook threads, synthetic variants, 13 languages, hostile input
uv run email-thread-mcp # stdio server

Real customer threads stay out of this repository. Locally, tests/fixtures/ and tests/test_client_threads.py (both gitignored) hold anonymised real threads and their expected results; the suite runs them when present. Replace every real address and name before using a thread there.