Anthropic MCP signing with Atlas e-signatures

Anthropic popularized MCP as the wire format between models and tools. E-sign belongs in that layer when negotiations already happen in Claude.

Atlas is an Anthropic MCP signing server at https://atlaswork.ai/api/mcp. Not a wrapper around another vendor's iframe. Ten tools cover create through extract.

> Share: "Anthropic clients call Atlas MCP tools. Humans Send from review."

Client map

Claude Desktop: add custom connector, sign in, new chat.

Claude Code:

claude mcp add atlas --transport http https://atlaswork.ai/api/mcp \
  --header "Authorization: Bearer YOUR_API_KEY"

Restart Claude Code after adding the server.

Tool design for agents

Anthropic models plan better when tools have narrow jobs:

• Create with send_contract_for_review (returns review_url)
• Repeat with send_contract_from_template
• Observe with check_signing_status and get_envelope
• Act with remind_signer and void_envelope
• Extract with extract_contract_data post-sign

Avoid monolithic "sign_document" tools that hide review gates. Atlas separates prepare from send on purpose.

Upload session for local files

Anthropic clients do not reliably forward arbitrary file bytes to remote MCP servers. For PDF or DOCX on disk:

1. Agent calls upload session start
2. Human opens link and uploads
3. Human confirms in chat
4. Agent completes session

Details in send contract from Claude.

OAuth discovery

Atlas publishes standard OAuth metadata for MCP clients that support dynamic registration:

• Authorization server: https://atlaswork.ai/.well-known/oauth-authorization-server
• Protected resource: https://atlaswork.ai/.well-known/oauth-protected-resource/api/mcp

Claude Desktop handles the OAuth dance. You should never paste secrets into a Claude thread.

Trust ladder

First NDA: review every time. Saved template: send_contract_from_template may auto-send. That policy matches how legal teams actually work.

Comparison to raw REST

Anthropic agents can call REST if you wrap it yourself. MCP removes boilerplate: tool schemas, auth refresh for OAuth clients, and structured results the model reads on the next turn.

For backend workers without a model in the loop, use REST envelope create instead.

Model-agnostic tool schemas

Anthropic models read Atlas tool descriptions on each turn. Keep prompts explicit about sequential parties and file intake method. Haiku-class models follow the same MCP schemas as Opus when the host exposes tools correctly.

Enterprise Anthropic deployments

If your org runs Claude with allowlisted connectors, ask IT to allowlist https://atlaswork.ai/api/mcp and the OAuth metadata URLs. Bearer key mode bypasses OAuth for Claude Code on developer laptops when policy permits API keys.

Audit trail expectations

Every Send and sign event writes to Atlas audit metadata. Anthropic chat logs are separate from signing audit logs. Counsel cares about the Atlas certificate on the signed PDF, not the chat transcript.

Prompt library for legal ops

Keep three prompt templates: NDA upload session, MSA from URL, template repeat send. Anthropic users paste the right template instead of rewriting party lists.

Comparing Sonnet vs Opus for tool use

Both call Atlas MCP reliably when tools are loaded. Opus handles ambiguous party instructions better on complex MSAs.

Claude computer use note

Computer use features do not replace MCP signing tools. Prefer explicit Atlas tool calls over screenshot-driven UI automation for legally binding sends.

Batch operations

For ten NDAs, loop template sends with distinct signer emails. Anthropic models handle sequential tool calls well when you provide a CSV-like list in the prompt.

Funding round closings

Use Anthropic MCP signing during funding closings when counsel emails final PDFs while founders stay in Claude for Q&A. Upload session closes the loop without context switch.

Anthropic users shipping B2B SaaS should pin MSAs as templates after the first successful Claude-assisted send. Repeat MCP calls drop from upload plus review to a single template invocation.

FAQ

Does Atlas accept PDF and DOCX?

Yes. Upload either format when you create an envelope. DOCX files become PDF before anyone signs.

How do I sign in?

Use a Bearer API key from your dashboard settings. MCP connectors in ChatGPT and Claude use OAuth instead.

When do credits get used?

One credit per send, not per upload. You get five free sends when you sign up.

Where should I start?

/mcp and API reference.

Related reading

• MCP setup
• E-signature API guide
• MCP electronic signatures