Gateway Rules Overview

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Every tool call that passes through an MCP Manager gateway can be inspected and acted on before it continues. Gateway rules are the content-level policies that do this. Each rule is attached to one gateway, runs on the tool traffic flowing through that gateway, and — when it matches — can block the message, modify its contents, or let it through while raising an alert. You create and manage rules on a gateway’s Rules tab: open a gateway from Gateways in the left-hand navigation and select Rules. A rule is built from two halves: a detection method (what to look for) and what to do about it (an action — or, for custom engines, the engine’s own verdict). MCP Manager ships two built-in detection methods, regular expressions and Microsoft Presidio, and lets you plug in your own custom rule engines — including the AWS Bedrock Guardrails and Lakera Guard templates, or build your own.

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Where gateway rules run in a tool call

A gateway rule fires at one of two points in a tool call’s round trip — on the request leg (the client’s tools/call heading to the MCP server) or the response leg (the server’s result heading back to the client). The diagram shows both hook points: On the request leg, a rule scans the tool’s arguments (params.arguments) and can stop the call before it ever reaches the server. On the response leg, a rule scans the tool’s result and can stop or rewrite it before it reaches the client. A rule that blocks a request means the call never reaches the server; a rule that blocks a response means the result never reaches the client.

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The detection methods

The Detection method dropdown in the rule editor is populated with both built-in engines and every custom rule engine your workspace has registered: The two built-in methods are always offered. Below them, the dropdown lists every engine registered in the Rule Engines section, so anything you add there becomes selectable as a detection method on any gateway rule.

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Choosing a detection method

The three methods aren’t ranked — they’re suited to different kinds of data, and most mature setups use more than one. Match the method to what you’re trying to catch: A few rules of thumb:

• Start with regex for anything that has a fixed shape. It’s free in latency terms and deterministic, so structured secrets and IDs are best caught here.
• Reach for Presidio when the value is a human artifact — a name or address won’t yield to a regex. Tune it against your own traffic; see Microsoft Presidio.
• Bring a custom engine when the judgment is hard — intent, context, or policy a pattern can’t express. These add the latency of an external call and are bounded by a 30-second timeout and a failure mode.
• Layer them. The methods compose: a regex rule that blocks prompt injection placed first, then a Presidio rule that replaces PII as a safety net, then a custom engine for nuanced policy. Because rule order matters, put your most decisive rules at the top.

For PII specifically, PII Filtering walks through this decision in the context of keeping customer data out of the model.

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Add a new rule

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Detection hook: when a rule fires

The Detection hook controls which leg of the tool call a rule runs on:

• Request — the rule fires before the tools/call reaches the MCP server, scanning the tool’s arguments. Use this to stop a call from ever being made.
• Response — the rule fires after the server returns its result, scanning the result before it reaches the client. This is the default and the most common choice.
• Both — a convenience that, on save, materializes a paired rule per direction: one request rule and one response rule, each independently sortable, editable, and toggleable. There is no single “both” rule at runtime — you end up with two rules. When editing an existing rule you can only set a single direction; to cover both, add a second rule.

A custom rule engine can advertise a preferred direction, which pre-selects this picker when you choose that engine — but you can always override it.

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Failure mode: what happens when a detection method fails

Some detection methods call an external system that can fail — Microsoft Presidio, an AWS Bedrock guardrail, Lakera Guard, or your own endpoint. “Fail” means the service is unreachable, too slow, returns an invalid response, or otherwise signals an error. For these methods the rule has a Failure mode that decides what happens in that case:

• Block — a failure blocks the message. A failed request never reaches the server; a failed response never reaches the client. This is the default for custom rule engines, so a misconfigured or unavailable engine fails closed rather than silently leaking data.
• Allow — a failure lets the original message through unchanged. This is the default for Microsoft Presidio rules.

Regular-expression rules run in-process and synchronously, so they have no failure mode — there is no external call to fail.

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Actions: what a matching rule does

For the built-in detection methods you choose an Action that applies when the rule matches. Custom rule engines do not show an action picker — their webhook response (pass, modify, block, or error) determines the outcome. Regular-expression rules support all five actions; Microsoft Presidio rules support Block and Replace only. For the modification actions (redact, replace, mask, hash), the message continues to the next enabled rule after being rewritten; a Block action stops rule processing immediately.

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Alerts

Every rule has an independent Alerts toggle. When it’s on, MCP Manager raises a real-time alert each time the rule acts — whether the action was block, a modification, or a custom engine’s verdict — and the alert appears in the Alerts section in the left-hand navigation. You can leave a rule on a non-destructive action (say, Replace) and still be alerted every time it fires, which is a good way to roll a rule out safely before switching it to Block.

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Rule order and the enable toggle

Rules are listed in the order they run, with a number in the leftmost column. Hover the number, grab the gripper, and drag to reorder. Ordering is compared within a detection hook: all request-hook rules run in their relative order, and all response-hook rules run in their relative order, independently of each other. Order matters because a Block action stops processing immediately — no later rule runs on that message — while modification actions chain, each operating on the text the previous rule already modified. Put your most decisive rules (such as prompt-injection blocking) first. Each rule also has an Enabled toggle. Flipping it takes effect almost immediately, so you can turn a rule off to investigate a false positive and back on without deleting and re-creating it.

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How rule activity appears in your logs

Every time a rule runs it is recorded in your logs — not only when it blocks. Rule-engine activity populates three log columns:

• rule_engine_id — which engine acted.
• rule_engine_type — how it classified the message: pass, modify, or block.
• rule_engine_comment — any comment the engine returned (for a custom engine, the comment field of its response).

Alongside these, a blocked message is logged with the type policy_enforced_abort and a modified message with policy_enforced_mutation, so you can tell from the log type alone whether a rule blocked, modified, or allowed a given message. Block entries also record the specific detection that triggered them — for example the Presidio entity or regex pattern that matched — not just the rule name. See Viewing Logs for the full column reference, and Alerts for the higher-level event feed.

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Further reading