Update JSON value by path, optionally creating path if missing
AI agents use json_update to create or update resources in Postgres Mcp Legacy — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Postgres Mcp Legacy environment.
This tool modifies data (Update in name, explicit mention of updating JSON values) but the operation is reversible—updates can be undone by reverting to previous values or applying inverse updates. It does not irreversibly delete data (ruling out Destructive) or execute arbitrary code (ruling out Execute).
From the tool's definition Tool name 'json_update' and description 'Update JSON value by path, optionally creating path if missing' indicate modification of data. The capability to create missing paths expands the scope of potential changes.
Documented attack patterns abuse exactly the kind of access json_update gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Postgres Mcp Legacy, and nothing reaches the server without passing your rules. This is the rule we recommend for json_update:
{
"version": "1",
"default": "deny",
"tools": {
"json_update": {
"limits": [
{
"counter": "json_update_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}json_update 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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Update JSON value by path, optionally creating path if missing. It is categorised as a Write tool in the Postgres Mcp Legacy MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Postgres Mcp Legacy MCP server in PolicyLayer and add a rule for json_update: 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 Postgres Mcp Legacy. Nothing to install.
json_update 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 json_update 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 json_update. 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.
json_update is provided by the Postgres Mcp Legacy MCP server (neverinfamous/postgres-mcp-legacy). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Postgres Mcp Legacy, 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.
60 Postgres Mcp Legacy tools catalogued and risk-classified — across an index of 43,000+ MCP servers.