Hexum MCP for Codex

Last updated 29 August 2026

Hexum is a hosted MCP server at https://hexum.dev/mcp. It does not replace Codex. Keep the editor. Mint a hexum_live_ key at /app after usage billing is on.

Install Hexum MCP in Codex in 3 steps

  1. Open /app, turn usage billing on, create a key. The secret is shown once.
  2. codex mcp add hexum --url https://hexum.dev/mcp --header "Authorization: Bearer hexum_live_…" then codex mcp list.
  3. Reload MCP. The agent discovers hexum_skill, hexum_shrink, hexum_expand, hexum_splice, hexum_check from tools/list.
mcp.json
{
  "mcpServers": {
    "hexum": {
      "serverUrl": "https://hexum.dev/mcp",
      "headers": { "Authorization": "Bearer hexum_live_…" }
    }
  }
}

If the host shows AUTH, the key is missing, invalid, or revoked. If it shows BILLING, turn billing on at /app. Full session: A session. Other agents: Hexum MCP install for every agent.

FAQ

What is Hexum?

Hexum is an MCP server for Cursor, Claude Code, and other coding agents. It sits between the agent and the model: hexum_shrink sends this turn instead of the repository, hexum_check fails the turn when architecture.hexum is broken, and you keep your editor. On MBPP, context dropped about 85% with the same tests passing.

Does Hexum replace Cursor or Claude Code?

No. Hexum is a hosted MCP server. You keep Cursor, Claude Code, Antigravity, Codex, or Grok. Hexum is not an IDE and not a model.

How do I add Hexum to Codex?

codex mcp add hexum --url https://hexum.dev/mcp --header "Authorization: Bearer hexum_live_…" then codex mcp list. Billing must be on at /app or MCP returns BILLING 403.

How does Hexum reduce tokens?

The agent calls hexum_shrink before it reads widely. The model is sent this turn, not the repository. On MBPP, measured context dropped about 85% with the same tests passing. Example turn: 18.2k → 3.1k tokens, 41 files → 6. See /benchmarks and /methodology.

Why did MCP return BILLING or AUTH?

AUTH means the key is missing, invalid, or revoked. BILLING (HTTP 403) means the key is valid but usage billing is off — or, on a team, Teams billing is off. Turn billing on at /app, then retry.

Does Hexum store my repo?

No. Zero Data Retention is on Usage and Teams. MCP payloads are not stored. Hexum does not train on your code. Policy: /zdr.