Merge JSON objects with conflict resolution strategies
AI agents use json_merge 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.
Merging JSON objects is a reversible write operation that combines or updates data structures. It does not execute arbitrary code, delete data irreversibly, or move funds. Severity is medium because unintended merges could corrupt application data or configuration if an AI agent applies it to the wrong JSON payload, but the operation can typically be undone via backup or transaction rollback.
From the tool's definition Tool description states it "Merge[s] JSON objects" — this is a data modification operation. The phrase "with conflict resolution strategies" indicates it modifies existing JSON structures rather than merely reading them.
Documented attack patterns abuse exactly the kind of access json_merge 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_merge:
{
"version": "1",
"default": "deny",
"tools": {
"json_merge": {
"limits": [
{
"counter": "json_merge_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}json_merge 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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Merge JSON objects with conflict resolution strategies. 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_merge: 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_merge 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_merge 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_merge. 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_merge 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.
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60 Postgres Mcp Legacy tools catalogued and risk-classified — across an index of 43,000+ MCP servers.