Create a custom Semgrep scanning rule
Risk signalsCreates new security scanning rule
Part of the Semgrep server.
Free to start. No card required.
AI agents use write_custom_semgrep_rule to create or modify resources in Semgrep. Write operations carry medium risk because an autonomous agent could trigger bulk unintended modifications. Rate limits prevent a single agent session from making hundreds of changes in rapid succession. Argument validation ensures the agent passes expected values.
Without a policy, an AI agent could call write_custom_semgrep_rule repeatedly, creating or modifying resources faster than any human could review. PolicyLayer's rate limiting ensures write operations happen at a controlled pace, and argument validation catches malformed or unexpected inputs before they reach Semgrep.
Write tools can modify data. A rate limit prevents runaway bulk operations from AI agents.
{
"version": "1",
"default": "deny",
"tools": {
"write_custom_semgrep_rule": {
"limits": [
{
"counter": "write_custom_semgrep_rule_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}See the full Semgrep policy for all 8 tools.
These attack patterns abuse exactly the kind of access write_custom_semgrep_rule gives an agent. Each links to the full case and the policy that stops it:
Other write tools across the catalogue. The same approach applies to each: rate-limit and validate the arguments.
Create a custom Semgrep scanning rule. It is categorised as a Write tool in the Semgrep MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Semgrep MCP server in PolicyLayer and add a rule for write_custom_semgrep_rule: 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 Semgrep. Nothing to install.
write_custom_semgrep_rule 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_custom_semgrep_rule 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_custom_semgrep_rule. 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_custom_semgrep_rule is provided by the Semgrep MCP server (@semgrep/mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Deterministic rules across all 8 Semgrep tools. Per-identity grants. Full audit log. Live in minutes. Nothing to install.
Free to start. No card required.
4,600+ MCP servers and 31,000+ tools scanned and risk-classified.