ALTWEB — signed context capsules

Write markdown. Ship it as a self-contained, verifiable, optionally encrypted artifact. Let AI agents load it only after the signature checks out.

AI agents run on plain-text context: instructions, personas, skills, memory files. None of it has provenance — anything that can write those files can poison them. ALTWEB gives context a chain of custody, and makes the loader refuse anything that lacks one:

• Capsule — markdown compiled into a single .altweb.html file (or URL): content compressed (deflate), optionally encrypted (AES-256-GCM), optionally signed (ECDSA P-256). Self-contained — opens in any browser, verifies offline, needs no server: you hand someone a file, not a database.

• Verified context loading — the altweb-context MCP server loads a capsule into your agent only when the signature is valid and the signer's public key is in your trust file. Unsigned, tampered, or untrusted capsules are refused at load time, with an explicit reason. Refusal is the default: an empty trust file rejects everything, signed or not.

you write MD ──► altweb compile --sign ──► capsule (.altweb.html / URL)
                                              │
agent asks for context ──► altweb-context ──► verify signature + trust
                                              │
                              trusted ──► markdown injected
                          everything else ──► REFUSED (reason)

Packages

Related MCP server: context-diamond

Quickstart

npm install
npm run build

# create your signing identity (deterministic from a passphrase; only the
# public key + fingerprint are stored, in ~/.altweb/identity.json)
node packages/cli/dist/altweb.mjs keygen --save

# write, compile, sign
echo "# My agent's operating notes" > notes.md
node packages/cli/dist/altweb.mjs compile notes.md -o notes.altweb.html --sign

# verify anywhere, offline
node packages/cli/dist/altweb.mjs verify notes.altweb.html

Wire the loader into an MCP client (Claude Code example):

claude mcp add altweb-context -- node "$(pwd)/packages/mcp/dist/altweb-context.mjs"

Trust a signer by adding its full public key to ~/.altweb/trusted-keys.json (the UNTRUSTED_KEY refusal message hands you the ready-made entry; the short fingerprint is a human label, not the trust anchor):

{ "keys": [ { "name": "Me", "publicKey": "<base64url SPKI>", "fingerprint": "ab:12:..." } ] }

What a signature proves — and what it does not

A valid signature proves who authored the capsule and that the bytes are intact. It does not make the content safe or true. The trust file is your policy; keep it short.

Pick a long passphrase. Identities derive deterministically from your passphrase with a fixed global salt (that is what makes them portable with nothing stored) — so the passphrase's entropy is the entire security of the identity. Use a 16+ character diceware-style phrase; the tooling enforces a minimum strength.

Security

Content is sanitized with DOMPurify on decode; artifacts carry a CSP; the codec validates structure with zod. See site/ docs → Security model for the full write-up, including the encrypted-capsule caveat (the signature covers the decrypted payload, so verification completes after decryption).

Credits

Built on excellent open source: Novel (Apache-2.0) and Tiptap (MIT) for the editor; DOMPurify, marked, pako, zod, @noble/curves in the engine. See NOTICE.

License

ALTWEB is dual-licensed:

• Open source: AGPL-3.0-or-later. Free to use, study, modify, and share — with one core obligation: if you modify ALTWEB and distribute it or run it as a network service (e.g. hosting altweb-context for others), you must release your modified source under the AGPL.

• Commercial: by agreement. To use ALTWEB in a closed-source product, or as a hosted service without publishing your changes, you need a separate commercial license. See COMMERCIAL.md.

Copyright © 2026 Daniel C. ȘOIMU. Bundled third-party components keep their own (permissive) licenses — see NOTICE.