Custos — The bouncer for your AI agents

What it does

Custos is a safety and control layer for AI agents. It sits between an agent and the tools it uses—like APIs, files, databases, shell commands, and MCP servers—and checks every action before it runs. You define what the agent is allowed to do, Custos blocks anything outside those rules, and every action is recorded in a tamper-evident audit trail. It’s open source, works with Python and TypeScript, and is built for teams running AI agents in real-world environments.

Every tool call your AI agent makes is intercepted, policy-checked, and recorded in a cryptographically signed audit ledger. Before execution. Every time.

Every tool call your agent makes — reading files, hitting APIs, running queries — is intercepted, policy-checked, and recorded in a cryptographically signed audit ledger. Before execution. Every time. Custos is infrastructure, not a SaaS dashboard. Drop it in, define a policy, get audited. Every tool call evaluated before execution against a YAML policy. Deny by default. First match wins. No runtime modification needed. Ed25519-signed, SHA-256 hash-chained JSONL. Any modification to any record — including deletion or reordering — breaks the chain instantly. Python signs. Node verifies. Or vice versa. The ledger format is a formal spec — enforced by a cross-language test on every commit.…from sanjaynandanj.github.io

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