> ## Documentation Index
> Fetch the complete documentation index at: https://ngrok.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Guard Agent Tool Calls with Firewall

> Create a Firewall policy, test it on one request, apply it to access keys, and switch it from Dry run to Enforce.

In this guide, you create a [Firewall](https://ngrok.com/docs/ai-gateway/concepts/firewall.md) policy that stops your agents from force-pushing, test it on one request, watch what it catches in Dry run, and then enforce it on your access keys.

## What you'll need

* An [ngrok.ai](https://app.ngrok.ai) account and an [access key](https://ngrok.com/docs/ai-gateway/concepts/access-keys.md).
* An agent or app that calls `/v1/chat/completions`, `/v1/responses`, `/v1/messages`, or Gemini `generateContent` through the gateway with tools.
* To apply a policy to some keys and not others, an [access key configuration](https://ngrok.com/docs/ai-gateway/guides/access-key-configurations.md) for those keys.

## 1. Create a policy

1. In [app.ngrok.ai](https://app.ngrok.ai), go to **Firewall** and click **New firewall policy**.
2. Enter a **Name**, such as `Coding agent guard`.
3. Under **Mode**, select **Dry run**.
   New policies start in **Enforce**, so switch now and enforce after you've tested.
4. Under **On error**, keep **Block** to block what Firewall can't check once you enforce, or select **Continue** to let it through.

## 2. Add rules

The new policy comes with one example rule under **Response rules**.
Edit it or remove it, then click **Add rule** for each rule you need.

For each rule, pick:

* **Rule name**: a short description.
  The agent sees it when the rule blocks something, so make it clear, such as `Force push`.
* **Type**: **Tool names**, **Regex**, or **Decision**. See [Rule types](https://ngrok.com/docs/ai-gateway/concepts/firewall.md#rule-types).
* **Tier**: **Hard block**, **Soft block**, or **Allow**. See [Tiers](https://ngrok.com/docs/ai-gateway/concepts/firewall.md#tiers).
* **Reads**: for a request rule, which parts of the request the rule reads.

Put a rule under **Request rules** to check what the agent sends before it reaches the model.
Put it under **Response rules** to check each tool call the model returns.

For a decision rule, describe what to catch in plain language, and leave **Min. confidence %** empty to use the default: 60 for a block rule and 70 for an allow rule.
Raise it if the rule blocks too often, and lower it if it misses things.

Under **Environment details**, add facts that help decision rules judge a call, such as `Deploy target: staging only` or `Trusted repo: github.com/acme/app`.
Environment details aren't rules, so they never block anything by themselves.

### Example rules

| Goal | Rule | Match |
| - | - | - |
| Block force pushes | Response rule, Regex, Hard block | `git push .*(--force\|-f)\b` |
| Block deleting the whole filesystem | Response rule, Regex, Hard block | `rm -rf /["\s]` |
| Stop pushes to the wrong repo | Response rule, Decision, Hard block, with the environment detail `Trusted repo: github.com/acme/app` | "Pushes code to a repository other than the trusted repo." |
| Stop agents from sending AWS keys | Response rule, Regex, Hard block | `AKIA[0-9A-Z]{16}` |
| Block a whole MCP server | Response rule, Tool names, Hard block | `mcp__payments__*` |
| Block requests that offer a tool | Request rule that reads **Tool definitions**, Tool names, Hard block | `delete_*` |
| Stop destructive commands nobody asked for | Response rule, Decision, Soft block | "Deletes files, drops databases, or rewrites history, and the user didn't ask for it." |
| Allow cleanup of the build folder | Response rule, Regex, Allow | `rm -rf \./build/` |
| Catch prompt injection in fetched pages | Request rule that reads **Tool results**, Decision, Hard block | "Text that tells the assistant to ignore its instructions or to send data somewhere." |
| Catch AWS keys in tool output | Request rule that reads **Tool results**, Regex, Hard block | `AKIA[0-9A-Z]{16}` |

Regex rules are case-sensitive.
Start a pattern with `(?i)` to ignore case.

Click **Create policy** to save it.

## 3. Try it in one request

Before you apply the policy to any keys, test it on a single request.
Under **Try it in one request**, the policy editor shows a `plugins` field that carries the same rules.
Add it to the body of a chat completions or messages request, and the gateway checks that request with those rules.

This request offers the model a `bash` tool and asks it to push, so the model returns a tool call for the rule to check:

```bash theme={null}
curl https://gateway.ngrok.ai/v1/chat/completions \
  -H "Authorization: Bearer $NGROK_AI_ACCESS_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "gpt-4o",
    "messages": [{ "role": "user", "content": "Force push my changes to main." }],
    "tools": [{
      "type": "function",
      "function": {
        "name": "bash",
        "description": "Run a shell command.",
        "parameters": {
          "type": "object",
          "properties": { "command": { "type": "string" } },
          "required": ["command"]
        }
      }
    }],
    "plugins": [{
      "id": "firewall",
      "mode": "enforce",
      "on_error": "block",
      "classifier_context": {
        "v": 1,
        "auto_mode": {
          "hard_deny": [{
            "id": "force-push",
            "name": "Force push",
            "kind": "regex",
            "pattern": "git push .*(--force|-f)\\b"
          }]
        }
      }
    }]
  }'
```

If the model calls `bash` with a force push, the response still returns `200`, but the tool call is gone.
In its place, the assistant message says the call was blocked, `finish_reason` is `stop`, and `safeguard_results` reports the verdict:

```json theme={null}
{
  "choices": [{
    "message": {
      "role": "assistant",
      "content": "<ngrok_firewall_blocked tool=\"bash\" rule=\"[Force push]\">Tool call \"bash\" was blocked by ngrok.ai Firewall and was not executed. Do not retry it with the same arguments.</ngrok_firewall_blocked>"
    },
    "finish_reason": "stop"
  }],
  "safeguard_results": [{
    "type": "dangerous_tool_use",
    "status": {
      "type": "available",
      "tool_uses": {
        "call_abc123": { "type": "evaluated", "outcome": "flagged", "explanation": "[Force push]" }
      }
    }
  }]
}
```

The response is trimmed to the fields Firewall changes.
Change `mode` to `audit` to see what Dry run reports: the tool call comes back unchanged, and `safeguard_results` still shows `flagged`.

See the [Firewall reference](https://ngrok.com/docs/ai-gateway/reference/firewall.md) for the full `plugins` format and every response field.

## 4. Apply the policy to keys

Open the policy, find **Apply to**, and pick one:

* **All access keys**: the policy becomes your baseline and checks every key in your account, including keys you create later.
  Your account has one baseline, so this replaces the current one.
* **Only these configurations**: the policy checks keys that use the configurations you pick.

Click **Save changes**.
Saved changes reach the gateway within seconds.

You can also pick the policy from an access key configuration.
Open the configuration, go to its **Firewall** section, and select the policy under **Policy**.

> [!NOTE]
>
> Policies combine.
> A key whose configuration picks a policy is also checked by your baseline.
> Each policy keeps its own mode, so a Dry run policy only reports, even next to an enforced baseline.
> See [How policies combine](https://ngrok.com/docs/ai-gateway/concepts/firewall.md#how-policies-combine).

## 5. Watch, tune, and enforce

While the policy is in Dry run, check what it would block:

* Read the `Ngrok-AIG-Firewall-Request` header for request rules, such as `flagged; rules=prompt-injection`.
* Read `safeguard_results` in the response body for both request rules and tool calls.

When a rule flags calls it shouldn't, raise its confidence, narrow its pattern, or add an allow rule next to a soft block rule.
When the results look right, open the policy, select **Enforce** under **Mode**, and click **Save changes**.

> [!WARNING]
>
> Once a key has an enforced policy with **On error** set to **Block**, the gateway rejects that key's requests to endpoints Firewall can't check, such as the Gemini Interactions API.
> Use a separate key for those requests, or set **On error** to **Continue** to let them through unchecked.

## Use the API instead

You can do every step in this guide with the [AI Gateway API](https://ngrok.com/docs/ai-gateway/api-reference/overview.md).
Authorize each request with your [AI Gateway API key](https://ngrok.com/docs/ai-gateway/concepts/access-keys.md#credentials-overview).

Create a policy with the [Firewall Policies API](https://ngrok.com/docs/ai-gateway/api-reference/firewall-policies/create.md).
The API uses the same fields as the dashboard, and each rule sets its own `tier`:

```bash theme={null}
curl -X POST https://api.ngrok.ai/firewall-policies \
  -H "Authorization: Bearer $AI_GATEWAY_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "name": "Coding agent guard",
    "mode": "audit",
    "onError": "block",
    "rules": [
      {
        "id": "force-push",
        "name": "Force push",
        "tier": "hardDeny",
        "kind": "regex",
        "pattern": "git push .*(--force|-f)\\b"
      }
    ]
  }'
```

Attach the policy to an access key configuration:

```bash theme={null}
curl -X PATCH https://api.ngrok.ai/access-key-configurations/aigac_xxxxx \
  -H "Authorization: Bearer $AI_GATEWAY_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{ "firewallPolicyId": "aigfp_xxxxx" }'
```

Send `"firewallPolicyId": null` to detach it.

Or make it your baseline with the [gateway configuration API](https://ngrok.com/docs/ai-gateway/api-reference/gateway-config/update.md).
Send `expectedFirewallPolicyId` with the baseline you read, or `null` if you expect none, so the update fails with a `409` instead of replacing a baseline someone else just set:

```bash theme={null}
curl -X PATCH https://api.ngrok.ai/gateway-config \
  -H "Authorization: Bearer $AI_GATEWAY_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{ "firewallPolicyId": "aigfp_xxxxx", "expectedFirewallPolicyId": null }'
```

To enforce the policy, [update it](https://ngrok.com/docs/ai-gateway/api-reference/firewall-policies/update.md) with `"mode": "enforce"`.

## Next steps

* [Firewall](https://ngrok.com/docs/ai-gateway/concepts/firewall.md): How rules, tiers, and modes work.
* [Firewall reference](https://ngrok.com/docs/ai-gateway/reference/firewall.md): Limits, headers, and response formats.
* [Securing your gateway](https://ngrok.com/docs/ai-gateway/guides/securing-endpoints.md): Per-client keys and revocation.


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