Insert new content after a specific line number. Use line 0 to insert at the beginning of the file. Only works within allowed directories.
AI agents use insert_lines 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.
This tool inserts lines into files, which is a reversible modification operation. It does not delete or overwrite data irreversibly (Write, not Destructive), nor does it execute arbitrary code (not Execute). The medium severity reflects that an agent misusing this could inject malicious content into configuration or code files within allowed directories, but the effect is revertible through deletion or editing.
From the tool's definition Tool description explicitly states 'Insert new content after a specific line number', which creates or modifies file content. The phrase 'Only works within allowed directories' confirms the modification is reversible and scoped, not destructive.
Attacks that exploit this kind of access
Insert new content after a specific line number. Use line 0 to insert at the beginning of the file. Only works within allowed directories. 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 insert_lines: 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.
insert_lines 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 insert_lines 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 insert_lines. 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.
insert_lines 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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