Arcjet vs Rein Security
Arcjet vs Rein Security compares the two closest in-process approaches. Teams evaluating a Rein Security alternative usually want HTTP bots, WAF, and rate limiting in the same SDK as tool-call checks.
Of the vendors in agent runtime security, Rein is architecturally closest to Arcjet. Both reject the network proxy: Rein runs as a code-native sidecar next to the application; Arcjet runs inside it. If you're evaluating both, the remaining differences are the sidecar versus a check per code path, and whether HTTP bots, WAF, and rate limiting ship in the same SDK.
Rein sits as a sidecar around the whole agent runtime and maps each action to its business impact. Arcjet is a check on each code path (request handlers, tool calls, jobs) and also covers HTTP bots, WAF, rate limiting, and email validation. Bot detection is an HTTP control in request handlers.
What Rein does
Rein describes an "inside-out" application security approach, deploying as a code-native sidecar that captures every prompt, tool invocation, API call, and resource that an agent touches at runtime. Rein emphasizes Business Impact Analysis, translating each agent action into its operational consequence. It also emphasizes data sovereignty: execution data stays in your organization rather than routing through vendor infrastructure.
Rein was founded in 2024, is based in New York City and Tel Aviv, and raised an $8 million seed round led by Glilot. Rein names Lemonade and HiBob publicly as production customers. The company emerged from stealth in January 2026.
Where they differ
| Area | Rein Security | Arcjet |
|---|---|---|
| Where it runs | Code-native sidecar alongside the agent | Inside the application: request handlers for HTTP, and tool calls, MCP handlers, and jobs for the agent path |
| Integration unit | The agent runtime | The individual code path, one check per action |
| What risk it covers | Agent actions and their business impact | Prompt injection, tool-call abuse, token spend, and in-process PII on the agent path; bots, Shield WAF, rate limiting, and email on HTTP |
| Data handling | Execution data stays in the org | Sensitive-info inspection in-process (built-in local engine, optional on-device ML model). Rate-limit counters, bot reputation, and prompt-injection detection use the Cloud API |
| Who can change policy | Configured with the sidecar deployment | App-specific rules in code. Operational HTTP remote rules (bots, Shield WAF, filters, fixed-window and sliding-window rate limits) in the Arcjet dashboard or through MCP, immediately. Labeled tool-call policies are a separate system. You set token-bucket budgets in application code. |
| Maturity | Emerged from stealth in January 2026 | Shipping SDKs since 2023, JavaScript SDK 1.0 in February 2026, SOC 2 Type 2. JavaScript, TypeScript, and Python documented; Go is pre-release. |
| Non-agent surface | Agent-focused | Also covers HTTP: bots, WAF, rate limiting, email validation |
The open questions worth asking both of us
If you're running a bake-off, these are the questions that differentiate the options, and they're worth asking of Arcjet too:
- Is enforcement pre-action or post-hoc? Does the control block before the side effect executes, or observe and respond after? For Arcjet, the decision returns before the action runs.
- What is the coverage boundary? Which code paths are covered, and which are invisible? For Arcjet, coverage is wherever you put a check: HTTP request handlers, or a tool call or job with no HTTP request.
- Can the control use prior actions in a run when deciding the next one? A correlation ID ties Arcjet decisions to a run so the sequence is reconstructable for investigation.
- What leaves your environment? For Arcjet, sensitive-info inspection is in-process (a built-in local engine, plus an optional on-device ML model for names, addresses, and government or financial identifiers) and the raw body is never sent. Rate-limit counters, bot reputation, and prompt-injection detection do use the Cloud API.
- How is it operated? Can a security team change operational HTTP rules from a dashboard, and still have them enforce inside the application? For Arcjet, yes: dashboard or MCP, immediately. App-specific rules stay in code. Labeled tool-call policies are a separate system.
In the SDK, those two surfaces are protect() (HTTP request handlers) and guard() (tool calls and jobs). When a security check can't finish (a timeout, a transport issue, or an incomplete policy), you choose what happens next. A direct Guard call returns allow with error codes rather than treating the incomplete check as a denial. Vercel AI SDK and LangChain wrappers fail closed unless you opt into continuing on error. HTTP request checks can fail open when Arcjet's cloud can't be reached; that's configurable. If you can't accept a check that didn't finish, look at the error codes. See the Guards reference.
Where Arcjet is the better fit
Choose Arcjet when you want a check on each code path: HTTP routes, tool calls, MCP servers, and queue workers. A security team can change operational HTTP rules from the Arcjet dashboard or MCP. You review app-specific rules, such as per-user token budgets, prompt injection, and PII, with the feature.
If you want a sidecar that captures the full agent runtime and maps each action to business impact, that's Rein's model.
For the wider category, see the AI agent security platform category map.
Learn more: Arcjet Guards · Architecture
Frequently asked questions
How is Arcjet different from Rein Security?
Both run inside the application rather than as a proxy. Rein sits as a sidecar around the agent runtime and emphasizes business impact analysis. Arcjet runs in request handlers and on agent tool calls and jobs, one check per action, and also covers HTTP bots, WAF, rate limiting, and email validation. Bot detection is an HTTP control in request handlers.
Does Arcjet block before the action executes?
Yes. The decision returns before the action runs, so the control is pre-action rather than post-hoc. When a check cannot finish, a direct Guard call returns allow with error codes. Vercel AI SDK and LangChain wrappers fail closed unless you opt into continuing on error. HTTP request checks can fail open when Arcjet's cloud cannot be reached; that is configurable. If you cannot accept a check that did not finish, look at the error codes.
Does Arcjet policy consider prior actions in a run?
A correlation ID tags decisions to a run so the sequence is reconstructable for investigation. Whether a control should use prior actions when deciding the next one is a buyer question to ask every vendor.
What data leaves my environment with Arcjet?
Sensitive-information inspection runs in-process (a built-in local engine, plus an optional on-device ML model for names, addresses, and government or financial identifiers) and the raw body is never sent. Rate-limit counters, bot reputation, and prompt-injection detection do use the Cloud API.
AI runtime security in your code
Protect your AI agent workflows with Arcjet
Arcjet runs inside your application, where it can use runtime context to enforce agent actions and budgets, detect prompt injection, and protect sensitive information before a workflow acts.