flake-check
Checks a Nix flake for errors and returns structured check results, warnings, and errors.
This record as markdown: /tools/io-github-dave-london-build/flake-check.md
What flake-check does on Build
AI agents invoke flake-check to trigger actions in Build. What it does depends on the arguments the agent supplies, and its effects often reach beyond the immediate call: builds kicked off, notifications sent, workflows started.
Why flake-check is rated High
This tool runs a Nix flake check, which executes the Nix evaluator and potentially builds derivations. It is not a passive read — it triggers an external build system operation. The blast radius is medium: it can consume resources and invoke arbitrary Nix build code, but typically does not directly delete data or move money.
From the tool's definition 'Checks a Nix flake' — running 'nix flake check' executes the Nix toolchain against a flake definition, which evaluates and builds flake outputs; 'returns structured check results, warnings, and errors' confirms it triggers external build/evaluation operations
Attacks that exploit this kind of access
The rule that runs flake-check safely
PolicyLayer is an MCP gateway: it sits between your AI agents and Build, and checks every tool call against a rule you set before the call runs. Nothing changes on the server itself. For flake-check, this is the rule to start with:
flake-check stays usable, but rate-capped: a runaway agent can't fire it dozens of times a minute. Everything else on the server is denied unless you say otherwise.
The button opens the PolicyLayer dashboard: create your workspace, connect Build, apply this rule, and every flake-check call is checked against it from then on.
Questions about flake-check
Checks a Nix flake for errors and returns structured check results, warnings, and errors. It is categorised as a Execute tool in the Build MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.
Register the Build MCP server in PolicyLayer and add a rule for flake-check: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches Build. Nothing to install.
flake-check is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.
Yes. Add a rate_limit block to the flake-check rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.
Set action: deny in the PolicyLayer policy for flake-check. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.
flake-check is provided by the Build MCP server (Dave-London/Pare). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
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