Increment field value in hash. Args: key: The name of the key field: The field name amount: Amount to increment by (default: 1) Returns: New value or error message
AI agents use hash_increment to create or update resources in AWS — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your AWS environment.
This tool modifies an existing value in a hash data structure by incrementing it. This is a write operation (reversible modification) — the field value is updated but not permanently destroyed. Misuse could corrupt data or cause unintended numeric drift in stored values, warranting medium severity.
From the tool's definition Increment field value in hash
Documented attack patterns abuse exactly the kind of access hash_increment gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and AWS, and nothing reaches the server without passing your rules. This is the rule we recommend for hash_increment:
{
"version": "1",
"default": "deny",
"tools": {
"hash_increment": {
"limits": [
{
"counter": "hash_increment_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}hash_increment stays usable, but capped — an agent stuck in a loop can't make hundreds of changes a minute. Everything else on the server is denied unless you say otherwise.
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Increment field value in hash. Args: key: The name of the key field: The field name amount: Amount to increment by (default: 1) Returns: New value or error message. It is categorised as a Write tool in the AWS MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the AWS MCP server in PolicyLayer and add a rule for hash_increment: 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 AWS. Nothing to install.
hash_increment 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 hash_increment 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 hash_increment. 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.
hash_increment is provided by the AWS MCP server (@awslabs/mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from AWS, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
Free to start. No card required.
300 AWS tools catalogued and risk-classified — across an index of 43,000+ MCP servers.