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How to write a custom guardrail or PII masking plugin with Respan Gateway

Use the public plugin API to add masking or policy checks to every call.

Who it is forA Rust developer on a platform team who must enforce a rule, such as masking personal data, on all model traffic.

The problemModel traffic may contain personal data or policy-breaking content that must be filtered before it leaves the company.

The outcomeA custom plugin built on the gateway's one-trait, four-hook API runs on every request.

What you need first

  • Python with pip, or a Rust toolchain with cargo, or Docker
  • An API key for at least one model vendor, such as OPENAI_API_KEY
  • An existing app or SDK that accepts a custom base URL
  • Optional: a Respan API key for tracing and a tool vendor key for MCP tools

Steps

  1. Read the plugin guide at docs/plugins.md.
  2. Open the examples index at examples/README.md and pick the example closest to your rule, such as guardrails or PII masking.
  3. Implement the plugin trait's four hooks in Rust, following the example.
  4. Build the gateway with your plugin mounted, using the embed option in docs/plugins.md, then run the check command against your config. (inferred, check the README)
  5. Send a request containing test personal data and confirm the masked output in the live log.

Prompt for your agent

In the respan-gateway repo, read docs/plugins.md and examples/README.md, then write a Rust plugin that implements the four hooks for <rule>. Return the plugin file and the test request result, and verify that the masked value does not appear in the live log.

Caveat. The README describes the plugin API only at a high level, so the exact hook names and signatures must be taken from docs/plugins.md rather than assumed.

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