AI agents use biome-format to create or update resources in Python — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Python environment.
This tool modifies files on the filesystem by reformatting them, which is a reversible write operation. While formatting changes can be undone (e.g., via version control or undo), the tool directly alters file contents. The severity is medium because accidental reformatting of critical files could cause workflow disruption, but changes are typically reversible and non-destructive in nature.
From the tool's definition Tool description states 'Formats files with Biome (format --write)' and 'returns a structured list of changed files.' The '--write' flag indicates files are modified in place.
Attacks that exploit this kind of access
Formats files with Biome (format --write) and returns a structured list of changed files. It is categorised as a Write tool in the Python MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Python MCP server in PolicyLayer and add a rule for biome-format: 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 Python. Nothing to install.
biome-format is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.
Yes. Add a rate_limit block to the biome-format 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 biome-format. 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.
biome-format is provided by the Python MCP server (Dave-London/Pare). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
biome-format is one line of Python's registry record.
The record carries the whole server: verified identity, auth posture, risk grade, every tool classified, recommended policy — re-checked continuously.
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