Create a new file with the given content. NEVER overwrites an existing file — if the path already exists, the content is automatically saved instead to an incremented filename (e.g.
AI agents use write_file to create or update resources in FilesystemMCP — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your FilesystemMCP environment.
The tool creates new files or saves content to alternate filenames if the target exists, making it a Write operation rather than Destructive (since no data is irreversibly lost). The severity is medium because uncontrolled file creation could fill disk space, create unwanted files, or overwrite application state if used against unexpected paths, though the tool's safety mechanism mitigates immediate data loss risk.
From the tool's definition Tool description states 'Create a new file with the given content' and explicitly does not overwrite existing files, instead saving to an incremented filename. This is a write operation that creates or modifies data reversibly.
Attacks that exploit this kind of access
Create a new file with the given content. NEVER overwrites an existing file — if the path already exists, the content is automatically saved instead to an incremented filename (e.g. It is categorised as a Write tool in the FilesystemMCP MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Filesystem MCP server in PolicyLayer and add a rule for write_file: 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 FilesystemMCP. Nothing to install.
write_file 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 write_file 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 write_file. 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.
write_file is provided by the Filesystem MCP server (rijadalisic/filesystemmcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Every MCP server has a record like this.
Type a name, get the same breakdown: verified identity, auth posture, risk grade, capabilities, recommended policy.
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