guard

LLM-evaluated command gate for AI agents. Every command gets evaluated by a fast LLM call before execution. Approved commands run normally. Denied commands return an explanation.

$ guard run ls -la /etc/nginx/
drwxr-xr-x 8 root root 4096 Mar 10 14:22 .
-rw-r--r-- 1 root root 1482 Mar 10 14:22 nginx.conf

$ guard run rm -rf /etc/nginx/
DENIED: Recursive deletion of system config directory.

Why

AI agents (Claude Code, Codex, Aider, OpenHands, CrewAI, etc.) increasingly need command execution access for debugging, log analysis, and ops tasks. A single hallucinated rm -rf or kubectl delete namespace can take down production.

Guard sits between the agent and the shell. Every command gets evaluated by an LLM (openai/gpt-5.4-nano via OpenRouter by default) before it reaches the system. The LLM analyzes intent and risk, not just pattern matching, so it catches obfuscated attacks and novel command chains that static policies miss.

Cost is negligible: each evaluation uses roughly 3600 prompt + 45 completion tokens, costing about $0.0005 per decision with openai/gpt-5.4-nano. A full 45-case CTF adversarial benchmark runs for about $0.02 total.

Install

cargo install --path .

Or download a release artifact and install the binary manually.

See INSTALL.md for install options and DEPLOYMENT.md for service deployment.

Quick start

# Set your API key (OpenRouter, or any OpenAI-compatible endpoint)
export SSH_GUARD_LLM_API_KEY="your-key-here"

# Start the server
guard server start &

# Execute commands through the guard
guard run uptime
guard run cat /var/log/syslog
guard run ps aux

# These will be denied:
guard run rm -rf /tmp/*
guard run sudo su

To test policy decisions without executing approved commands, start a separate dry-run server:

guard server start --dry-run --socket .cache/guard-dry-run.sock
guard server connect --socket .cache/guard-dry-run.sock bash -- -c 'sudo id'

Modes

Set via SSH_GUARD_MODE:

Mode	Description
`readonly`	Read-only evaluation. Blocks file writes, system or service state changes, package changes, privilege escalation, reverse shells, and obfuscated payloads. Allows routine inspection commands like `ls`, `ps`, `cat`, `grep`, `df`.
`safe`	Permissive administrative evaluation. Allows visible, bounded troubleshooting and admin work, but blocks credential-material reads, destructive operations, privilege escalation, unauthorized network pivots, and hidden payloads.
`paranoid`	Restrictive. Blocks writes, sensitive file reads, network connections, shells, interpreters, side-channel execution, and chained commands. Only allows basic non-sensitive inspection (`id`, `hostname`, `pwd`, `ls`, `ps`, `df`, limited `git status`).

Use readonly when agents should investigate production state without changing it. Use safe when an operator is supervising real sysadmin work and wants bounded writes, targeted restarts, and ordinary maintenance to be possible while still blocking destructive or credential-seeking commands. Use paranoid for untrusted agents, adversarial testing, or first contact with an unfamiliar workspace where even broad reads and network access should be treated as risky.

SSH_GUARD_MODE=safe guard run sudo systemctl status ssh --no-pager  # allowed
SSH_GUARD_MODE=paranoid guard run sudo systemctl status ssh --no-pager  # denied

All modes evaluate sudo by the underlying command:

guard run sudo ls /etc/nginx/        # readonly: allowed (read operation)
guard run sudo rm -rf /etc/nginx/    # readonly: denied  (destructive)
guard run sudo systemctl restart app # safe: allowed     (targeted restart)

Consequence-gated execution

By default an approved command runs immediately. That is one gate for every action, regardless of whether the action is a read, a reversible change, or an irreversible destruction. Consequence gating (opt-in, --gate consequence) makes the gate depend on the consequence of the command, turning the binary allow/deny into a graduated trust ladder:

class	when	gate
`reversible`	read-only / idempotent / trivially undone, low risk	runs immediately
`recoverable`	a mutation with a known inverse	runs behind an auto-revert envelope: applied, then reverted unless an operator confirms in time
`irreversible`	destruction or no clean inverse (or high risk, or uncertain)	held for operator approval; not executed until approved

With gating on, the LLM keeps deciding APPROVE/DENY exactly as before (the deny rules are unchanged) and additionally classifies the reversibility of commands it approves. The daemon routes on that class. Classification is fail-safe: reversibility can only raise the gate, never lower it, and a missing or uncertain class is held, never run. Operator-authored allows (static policy, trusted verbs) and the LLM-uncertain path are separated: the open-ended LLM path is gated; the operator-vetted surface is not.

Gating is meaningful only where the daemon UID differs from the agent's (so the agent cannot approve its own held command). It requires a Unix-socket listener and is refused with --tcp-port; on Windows it is unavailable (run guard in WSL/Linux). Approval, denial, confirmation, and manual revert are daemon-UID only — the operator decides the irreversible steps, never the agent.

guard server start --gate consequence --exec-as-caller \
  --socket /run/guard/guard.sock --verbs /etc/guard/verbs.yaml

# Reversible: runs now.
guard run ls -la /etc/nginx/

# Recoverable: applied behind an auto-revert envelope.
guard run --revert "systemctl stop app" --confirm-within 900 \
  systemctl restart app
# PROVISIONAL (containment envelope): ... handle: 3f9c...
#   confirm: guard confirm 3f9c...   (else auto-reverts)
guard confirm 3f9c...     # operator: keep it

# Irreversible: held for operator approval, not executed.
guard run rm -rf /var/data
# HELD for operator approval: ... handle: a1b2...
#   approve: guard approve a1b2...
guard approvals                 # operator: review the queue
guard approve a1b2...           # operator: execute the exact held command
guard deny a1b2...              # operator: reject it

A free-form --revert is itself policy-evaluated at arm time, so an agent cannot smuggle an arbitrary command into the rollback slot. A recoverable command with no usable revert is held, not run unconfined. Held commands fail closed: an unattended queue denies on a TTL rather than stalling. Held and provisional state survives a daemon restart, and a revert never runs unattended at boot — a past-deadline provisional becomes needs_operator_decision for explicit handling. Inspect state with guard provisionals and guard approvals.

Verbs: the typed interface

guard run <anything> is a single, all-powerful entry point. For high-value operations, expose verbs instead: named, typed operations the operator defines in a catalog (--verbs <yaml>), each with a fixed binary, an argument template with pattern-validated parameters, a declared consequence class, and a rollback. The agent calls the verb; it never composes raw shell.

guard verb list
# restart-service [recoverable] trusted revertable — Restart a systemd unit
#     --param unit=<^[a-zA-Z0-9@._-]+$>

guard verb run restart-service --param unit=nginx

Each {param} renders as exactly one argument (no shell, no word-splitting), and a value may not begin with - unless the parameter opts in, so parameter and flag injection are structurally impossible. The catalog is operator-only and hot-reloaded on change; agents cannot add or alter verbs. A trusted verb skips the LLM (a deterministic allow path); its declared class still drives the gate. See examples/verbs.yaml.

Configuration

Defaults vs. opt-ins

Guard ships with an LLM-only evaluation pipeline: a single call to openai/gpt-5.4-nano via OpenRouter, function-calling based, with two retries before failing closed. No static allow or deny lists are loaded. No fallback model chain is active. This default is production-ready for the common case.

Two opt-in features exist for deployments with specific constraints:

Static allow/deny lists via --policy <yaml>. Short-circuits the LLM for deterministic safe or unsafe patterns. See examples/ for allow-policy.yaml, deny-policy.yaml, and hybrid-policy.yaml.
Fallback model chain via SSH_GUARD_LLM_MODELS. Fails over to alternate providers after the primary exhausts its retries. See examples/fallback-models.env.
Learned static rules via --learn-rules. Repeated low-risk LLM approvals can promote conservative exact static allow rules so repeated calls return immediately. Misses and risky commands still fall through to the LLM.

Enable either only when a concrete latency or uptime constraint forces it.

Environment variables

All configuration via environment variables, CLI flags, or .env files.

Guard walks up from your current directory to / looking for .env files (closest wins), so you can scope config per project.

Variable	Default	Description
`SSH_GUARD_LLM_API_KEY` / `OPENROUTER_API_KEY`	(none)	LLM API key (required). `OPENROUTER_API_KEY` is the conventional name and is accepted for compatibility.
`SSH_GUARD_LLM_API_URL` / `SSH_GUARD_API_URL`	`https://openrouter.ai/api/v1/chat/completions`	Any OpenAI-compatible endpoint
`SSH_GUARD_LLM_MODELS`	(unset)	Optional comma-separated fallback chain (e.g. `openai/gpt-5.4-nano,meta-llama/llama-4-maverick`). When set, overrides `--llm-model` and is tried in order, each with its own retry budget. Primary model when unset: `openai/gpt-5.4-nano`.
`SSH_GUARD_LLM_RETRIES`	`2`	Retries per model on transient failures (429, timeouts, parse errors). 1-2.
`SSH_GUARD_LLM_TIMEOUT` / `SSH_GUARD_TIMEOUT`	`30`	LLM call timeout in seconds.
`SSH_GUARD_AUTH_TOKEN`	(none)	Shared token for TCP clients. Use this for loopback TCP daemons instead of passing `--auth-token` on the command line.
`SSH_GUARD_ADMIN_TOKEN`	(none)	Separate token for TCP admin RPCs such as `guard grant`, `guard session show`, and the full `guard status`. The Windows launcher generates and stores one automatically.
`SSH_GUARD_MODE`	`readonly`	`readonly`, `safe`, or `paranoid`
`SSH_GUARD_DRY_RUN`	`false`	Evaluate policy but do not execute approved commands. Useful for prompt and policy testing.
`SSH_GUARD_LEARN_RULES`	`false`	Learn static allows from repeated low-risk LLM approvals.
`SSH_GUARD_LEARN_MIN_APPROVALS`	`2`	Approvals required before promotion.
`SSH_GUARD_LEARN_MAX_RISK`	`2`	Highest LLM risk score eligible for promotion.
`SSH_GUARD_LEARN_SHIMS`	`suggest`	`off`, `suggest`, or `create` service shims for learned SSH/API wrappers.
`SSH_GUARD_PROMPT_APPEND`	(none)	Path to additive prompt file (appended to base prompt)
`SSH_GUARD_GPG_RECIPIENT`	(none)	GPG recipient for the `local` secret backend
`SSH_GUARD_BACKEND`	(auto)	Secret backend (`pass`, `env`, `local`). Auto prefers `pass`; otherwise it falls back to non-persistent `env` and logs a warning.
`SSH_GUARD_GATE`	`off`	Consequence gating: `off` or `consequence`. Requires a Unix-socket listener.
`SSH_GUARD_VERBS`	(none)	Path to the verb catalog YAML. Hot-reloaded on change.

The primary model is openai/gpt-5.4-nano via OpenRouter by default. Set it per-invocation with --llm-model <slug>. To configure a true fallback chain across providers, use SSH_GUARD_LLM_MODELS (comma-separated) or --llm-models. --llm-timeout <seconds> controls the per-call HTTP timeout.

See .env.example for a copyable template.

Examples

Basic server with readonly mode

Start the guard server and execute commands through it:

export SSH_GUARD_LLM_API_KEY="sk-or-v1-..."

guard server start --socket .cache/guard.sock &
guard config set-server .cache/guard.sock

# Allowed: routine inspection
guard run hostname
# guard-host

guard run ps aux
# USER  PID %CPU %MEM    VSZ   RSS TTY STAT START   TIME COMMAND
# root    1  0.0  0.0   4624  3456 ?   Ss   10:00   0:00 /sbin/init

# Denied: destructive
guard run rm -rf /
# DENIED: Recursive deletion of root filesystem

# Denied: obfuscated attack
guard run bash -c 'eval $(echo cm0gLXJmIC8= | base64 -d)'
# DENIED: Base64-decoded payload piped through eval

When using guard server connect directly (rather than guard run), target arguments are forwarded after the target binary: guard server connect --socket .cache/guard.sock df -h.

Safe mode

Safe mode allows visible, bounded administration while still blocking direct credential-material reads and obvious escalation paths:

SSH_GUARD_MODE=safe guard server start --socket .cache/guard.sock &

# Allowed: ordinary inspection and work files
guard run cat /etc/hosts
# 127.0.0.1 localhost

guard run cp README.md .cache/readme-copy
# copied

# Denied: credential material
guard run cat /app/.env
# DENIED: Credential material read

# Still denied: privilege escalation
guard run sudo su
# DENIED: Escalation to root shell

Paranoid mode for untrusted agents

Paranoid mode locks down to basic read-only inspection:

SSH_GUARD_MODE=paranoid guard server start --socket .cache/guard.sock &

# Allowed: basic system state
guard run id
# uid=1000(agent) gid=1000(agent) groups=1000(agent)

guard run df -h
# Filesystem      Size  Used Avail Use% Mounted on
# /dev/sda1       100G   45G   55G  45% /

# Denied: file reading
guard run cat /etc/passwd
# DENIED: File reading blocked in paranoid mode

# Denied: environment inspection
guard run env
# DENIED: Environment variable dump blocked in paranoid mode

Static deny policy

For fast-reject of known-bad patterns without an LLM call, add a static deny policy:

guard server start --policy examples/deny-policy.yaml --socket .cache/guard.sock &

Static patterns are checked first. If a command matches a deny pattern, it is rejected immediately without an LLM call. Commands that pass static policy are then evaluated by the LLM.

See examples/deny-policy.yaml for a reference policy with documented limitations of static glob matching.

Learned static rules

For services with repetitive low-risk calls, enable learned rules:

guard server start \
  --learn-rules \
  --learn-min-approvals 2 \
  --learn-max-risk 2 \
  --learn-shims suggest \
  --socket .cache/guard.sock &

When the LLM repeatedly approves the same low-risk command shape, guard writes a learned allow rule to the state directory and future identical calls bypass the LLM. Learned rules never deny or wildcard over service verbs; misses, session-prompted calls, and commands with destructive shell-control tokens fall back to normal evaluation. For SSH API wrappers, guard may also mention a shorter service shim such as opnsense-api; with --learn-shims create, promoted rules create that wrapper in the configured shim directory, and the same approved operation is covered through the shim name.

Custom system prompt

Replace the built-in prompt entirely for a specific deployment:

guard server start --system-prompt /etc/guard/my-prompt.txt --socket .cache/guard.sock &

Or place a prompt file at ~/.config/guard/system-prompt.txt to override automatically.

Additive prompt

Append environment-specific instructions to the built-in prompt without replacing it:

# Via CLI flag
guard server start --system-prompt-append /etc/guard/extra-rules.txt &

# Or via environment variable
SSH_GUARD_PROMPT_APPEND=/etc/guard/extra-rules.txt guard server start &

Example additive prompt (extra-rules.txt):

Additional rules for this environment:

- This server runs a PostgreSQL database. Allow SELECT queries via psql but deny DROP, DELETE, or TRUNCATE.
- The /opt/app directory contains the application. Allow reads but deny writes.
- Allow docker ps and docker logs but deny docker exec, docker run, and docker rm.

The additive prompt is appended to whichever base prompt is active (readonly, safe, paranoid, or custom), letting operators customize behavior without maintaining a full prompt fork.

Session grants

Session grants hand a specific agent narrow extra permissions for a specific run, without relaxing the global mode. The agent identifies its session by the GUARD_SESSION env var; every guard run (and guard server connect) reads that env var and forwards it as the session token in the request. Operators attach allow/deny patterns, prose intent, and prompt context to that token.

The simplest flow is guard session new, which mints a token and (optionally) grants it in one round trip, printing an eval-friendly export line:

# Operator: mint a session, grant it, capture the token in the current shell
eval "$(guard session new \
  --allow 'mkdir /tmp/job-42*' \
  --allow 'rm /tmp/job-42/scratch*' \
  --prompt 'This session is preparing /tmp/job-42 as scratch space.' \
  --ttl 3600)"

# Now any agent launched from this shell inherits GUARD_SESSION
claude
# or
GUARD_SESSION="$GUARD_SESSION" my-agent

Inside the agent's process tree, every guard run call automatically picks up GUARD_SESSION from the inherited environment, so the model itself does not need to know or pass the token explicitly — it is bound to the shell that launched the agent.

To grant rules to an existing token (e.g. one the agent already has):

guard session grant <token> --allow '<glob>' --deny '<glob>' [--ttl N] [--prompt TEXT] [--auto-amend]

There is also a top-level shorthand. With a quoted prose description it mints and grants a fresh session, again printing an eval-friendly export line:

eval "$(guard grant --ttl 3600 --static-only 'readonly access to nextcloud resources in the morgaesis-dev kube cluster, not secrets, with write access for scaling replicas and editing ingresses')"

For an existing token, pass the token first:

guard grant <token> "readonly access to nextcloud resources in the morgaesis-dev kube cluster, not secrets, with write access for scaling replicas and editing ingresses"

Prose grants are compiled at grant time into conservative static rules when guard recognizes the domain. The first compiler handles Kubernetes: it infers namespaces such as nextcloud, optional contexts such as morgaesis-dev, adds hard denies for shell-control, secret access, token creation, raw kubeconfig reads, exec, cp, port-forward, and deletes, then adds namespace-scoped read, scale, and ingress/reverse-proxy rules implied by the prose. Safe command examples in backticks are added as exact static allows. Unrecognized prose is still stored as session LLM context, but does not create broad static globs.

Session allow/deny patterns use guard's shell-style glob matcher, not regex. *, ?, and bracket classes are supported, but the match is against the flat reconstructed command line; it does not understand shell quoting, Kubernetes resource schemas, or argument semantics. Generated rules therefore use broad globs sparingly: for example, Kubernetes prose grants may add namespace-bounded get * -n nextcloud and describe * -n nextcloud read globs, backed by explicit secret and mutating-resource denies. Automatic amendments do not add globs at all; they add exact binary + argv rules, so literal * or [ characters in an appealed command do not become wildcards.

Matching deny patterns win over allow patterns, and by default everything that does not match a session rule falls through to the normal evaluator with the session prose/prompt appended. Prose grants enable auto_amend by default so fresh low-risk LLM fallback approvals can add exact session allows, and fresh high-risk LLM denials can add exact session denies. Use --no-auto-amend to keep fallback non-mutating, or --auto-amend to opt a manual --allow/--deny grant into the same behavior. Cache hits, static policy hits, and learned-rule hits never amend a session; session fallback also does not promote global learned rules. Add --static-only (alias --no-llm-fallback) to guard grant, guard session grant, or guard session new to deny any session-rule miss instead of falling through to the LLM; static-only grants disable auto-amend.

To ask for a one-off amendment without executing the command, appeal it:

guard appeal --session <token> kubectl get httproute -n nextcloud
# or, when GUARD_SESSION is already set:
guard appeal kubectl get httproute -n nextcloud
# equivalent explicit session subcommand:
guard session appeal <token> kubectl get httproute -n nextcloud

An appeal runs the evaluator with the session context and then either amends an exact allow, amends an exact deny for a high-risk denial, or refuses to amend. It exits nonzero when the appealed command remains denied. Appeals are admin RPCs, like grant/revoke/show, because they can change durable authorization state.

Session grants are persisted in the daemon state database and survive daemon restarts by default. The default path is the XDG state dir ($XDG_STATE_HOME/guard/state.db or ~/.local/state/guard/state.db); override it with --state-db or SSH_GUARD_STATE_DB. guard session revoke <token> is daemon-UID-only; guard session list is visible over the Unix socket to exec-allowed local users, but it redacts the bearer token, rule bodies, and prompt text unless the caller is the daemon UID.

For operator forensics, guard session show <token> is daemon-UID-only and prints the full prompt, aggregate allow/deny and exec outcome counts, source breakdown (llm, cache, static_policy, session_allow, session_deny, session_static_only, validation), a risk histogram for LLM-evaluated calls, and a bounded recent interaction log. Those summaries are loaded from the state database, so they remain available after a service restart within the configured retention window.

Per-run secret injection

guard run can request stored secrets for one approved command without requiring a shim or persistent tool config. The daemon resolves the secret values immediately before exec, injects them into the child environment, and includes those values in exact-match output redaction.

guard secrets add/list/remove are Unix-socket-only, per-user operations. Any exec-allowed local user can manage their own secret namespace; TCP clients cannot use secret storage or --secret, because the namespace is keyed from Unix peer credentials. When the daemon UID runs guard secrets list, it gets an aggregate names-only view across every UID namespace; duplicate key names can appear more than once and are intentionally not annotated with ownership in the default list output.

For daemon-side migration and cleanup, use guard secrets list --detailed as the daemon UID. That view includes uid= for namespaced entries and origin=legacy for pre-namespace flat secrets that still need operator migration.

Upgrade note: pre-namespace flat secrets are no longer served through normal per-user guard secrets list / guard run --secret paths. Migrate them before rollout:

pass: move guard/<key> to guard/u<uid>/<key>
env: rename GUARD_SECRET_<KEY> to GUARD_SECRET_U<uid>_<KEY>
local: rewrite the flat YAML { KEY: value } into { <uid>: { KEY: value } }

After migration, verify with guard secrets list as the target user and guard run --secret KEY ....

For a stored secret with a shell-safe name, --secret NAME injects $NAME:

guard run \
  --secret OPNSENSE_API_KEY \
  --secret OPNSENSE_API_SECRET \
  --secret OPNSENSE_USERNAME \
  ssh opnsense-host 'configctl system status'

For a stored secret with dashes, slashes, or lowercase names, bare --secret derives an uppercase env var by replacing separators with underscores:

guard run --secret opnsense-apikey-secret \
  sh -c 'opnsense-tool --key "$OPNSENSE_APIKEY_SECRET"'

Map a different environment variable name to a stored secret key with ENV_VAR=secret-name:

guard run --secret OPNSENSE_API_KEY=atlas/opnsense-apikey ssh opnsense-host uptime

Plain per-run environment values are also supported for non-secret settings:

guard run --env OPNSENSE_HOST=opnsense-host --secret OPNSENSE_API_KEY \
  sh -c 'ssh "$OPNSENSE_HOST" uptime'

Admin authorization

Session admin RPCs (session new / grant / revoke, plus the privileged subset of status) are restricted to the daemon's own UID on Unix sockets. session list is the Unix-socket exception: exec-allowed local callers may see that grants exist, when they were granted, and when they expire, but the daemon redacts the session token, allow/deny patterns, and prompt text unless the caller is the daemon UID. On TCP transports, non-Ping admin RPCs require the separate SSH_GUARD_ADMIN_TOKEN; the ordinary TCP exec SSH_GUARD_AUTH_TOKEN is not enough to mint grants.

The non-privileged guard status (run as your normal user or any other exec-allowed UID, or over TCP without the admin token) returns only client + server version, uptime, evaluation mode, and dry-run state. It is a liveness probe — enough to confirm the connection works and what mode the evaluator is in, but nothing that would help fingerprint the deployment or escalate privilege.

The --prompt / --prompt-file flags attach a free-form context fragment that is appended to the LLM system prompt under a Session context: heading for evaluator calls made under that token. Prose grants use the same context path after static rule synthesis. Use prompt/prose for guidance the static glob patterns cannot express. The decision cache is bypassed when a session prompt is in play, because cached verdicts were made under the base prompt and may not hold under the extended context.

Because grants are now durable, broad sessions deserve the same care as any other persistent authorization state. Prefer explicit TTLs for elevated sessions, and treat allow=["*"] as an operator override that must be revoked intentionally rather than something a daemon restart will clear for you. Generated prose rules intentionally stay narrow; if guard cannot infer a safe static rule, it relies on LLM fallback or denies under --static-only.

Execution identity

By default the daemon executes approved commands as its own service identity. If you want guard to act as a local secret broker and redactor for per-user files such as ~/.aws/config or ~/.cmk/config, start a root-owned daemon with --exec-as-caller and only a Unix socket listener. In that mode guard authenticates the caller by Unix peer credentials and drops the child process to that UID before exec, so the command runs with the caller's filesystem access instead of the daemon's or root's. TCP listeners are intentionally incompatible with this mode because a token is not a trustworthy local UID.

Agent integration

Point your agent's command execution at guard run instead of direct execution.

MCP server

Guard can run as a stdio MCP server so agents call a tool instead of shelling out:

guard config set-server ~/.guard/guard.sock
guard mcp serve

The server exposes a guard_run tool:

{
  "binary": "ps",
  "args": ["aux"]
}

Optional per-call environment and secret references mirror guard run:

{
  "binary": "sh",
  "args": ["-lc", "[ -n \"$OPNSENSE_APIKEY_SECRET\" ] && echo set"],
  "secrets": ["opnsense-apikey-secret"],
  "secretEnv": {
    "OPNSENSE_API_KEY": "atlas/opnsense-apikey"
  },
  "env": {
    "TARGET_HOST": "opnsense-prod"
  }
}

Response:

{
  "allowed": true,
  "reason": "Read-only process listing",
  "exit_code": 0,
  "stdout": "USER  PID ...",
  "stderr": null
}

Denied commands return a normal MCP tool result with allowed: false and the denial reason. Transport or daemon failures still use isError: true.

Claude Code (CLAUDE.md)

# Command Execution

Use the guard MCP server for all command execution.
Never use interactive sessions.

OpenHands / SWE-Agent

export SSH_GUARD_LLM_API_KEY="..."
export SSH_GUARD_MODE=readonly
alias ssh='guard run ssh'

LangChain / CrewAI tool definition

import subprocess

def guarded_command(command: str, args: list[str]) -> str:
    """Execute a command through the guard."""
    result = subprocess.run(
        ["guard", "run", command] + args,
        capture_output=True, text=True, timeout=60,
    )
    if result.returncode != 0:
        return f"DENIED: {result.stderr.strip()}"
    return result.stdout

Security model

Guard provides defense in depth through three layers:

Environment isolation (env_clear): Child processes inherit only safe environment variables (PATH, HOME, USER, LANG, TERM, etc.) plus any per-run or tool-configured variables the caller explicitly requested.
Output redaction: Known secret values (API keys, auth tokens, tool secrets, per-run injected secrets) are exact-match redacted from stdout/stderr before returning to the agent. Regex patterns catch *_TOKEN, *_KEY, *_SECRET, *_PASSWORD, PEM blocks, and JWTs.
LLM evaluation: Each command is analyzed for destructive intent, privilege escalation, reverse shells, obfuscated payloads, tool side-channel abuse, and prompt injection. The LLM evaluates the full command including all chained parts.

Audit logging

Guard logs all decisions via tracing. Configure log level with RUST_LOG:

RUST_LOG=info guard server start    # decisions + token usage
RUST_LOG=debug guard server start   # verbose request/response logging

LLM token usage is logged per evaluation:

[LLM_USAGE] model=openai/gpt-5.4-nano attempt=1 prompt_tokens=3594 completion_tokens=47 total_tokens=3641 status=ok

For this local deployment model, the audit source of truth is the daemon's structured tracing output, typically collected by journald. The SQLite state database is for session state and queryable session history, not for replacing your log pipeline. guard session show is an operator view over that persisted session history; it should complement journald, not replace it.

Limitations

Not a sandbox. Guard is a policy gate, not an isolation boundary. Defense-in-depth (seccomp, read-only FS, restricted users, network segmentation) is still needed for adversarial environments.
No interactive sessions. Agents get command execution only.
LLM latency. Each command adds ~0.5-2s for the LLM call.
Fail-closed. If the LLM call fails or returns unparseable output, the command is denied.

License

MIT

Name		Name	Last commit message	Last commit date
Latest commit History 127 Commits
.github		.github
config		config
ctf		ctf
deployment		deployment
examples		examples
scripts		scripts
src		src
tests		tests
.env.example		.env.example
.gitignore		.gitignore
.markdownlint.yaml		.markdownlint.yaml
.pre-commit-config.yaml		.pre-commit-config.yaml
ARCHITECTURE.md		ARCHITECTURE.md
BASHCONTROL.md		BASHCONTROL.md
Cargo.lock		Cargo.lock
Cargo.toml		Cargo.toml
DEPLOYMENT.md		DEPLOYMENT.md
DEVELOPMENT.md		DEVELOPMENT.md
INSTALL.md		INSTALL.md
LICENSE		LICENSE
README.md		README.md
ROADMAP.md		ROADMAP.md
SECURITY.md		SECURITY.md
build.rs		build.rs
demo.tape		demo.tape
deny.toml		deny.toml

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guard

Why

Install

Quick start

Modes

Consequence-gated execution

Verbs: the typed interface

Configuration

Defaults vs. opt-ins

Environment variables

Examples

Basic server with readonly mode

Safe mode

Paranoid mode for untrusted agents

Static deny policy

Learned static rules

Custom system prompt

Additive prompt

Session grants

Per-run secret injection

Admin authorization

Execution identity

Agent integration

MCP server

Security model

Audit logging

Limitations

License

About

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