AI agents use add_field_value to create or update resources in Tana MCP Server — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Tana MCP Server environment.
This tool creates or modifies data reversibly—it adds a field value to an existing node. This is a classic Write operation. The modification is reversible (the field value can be changed or removed), and the blast radius is limited to the specific node and field being modified. It does not execute arbitrary code, delete data irreversibly, move money, or expose read-only operations.
From the tool's definition Tool description states it will 'Add a field value to an existing node in Tana'. The verb 'add' indicates creation or modification of data.
Documented attack patterns abuse exactly the kind of access add_field_value gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Tana MCP Server, and nothing reaches the server without passing your rules. This is the rule we recommend for add_field_value:
{
"version": "1",
"default": "deny",
"tools": {
"add_field_value": {
"limits": [
{
"counter": "add_field_value_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}add_field_value 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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Add a field value to an existing node in Tana. It is categorised as a Write tool in the Tana MCP Server MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Tana MCP Server MCP server in PolicyLayer and add a rule for add_field_value: 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 Tana MCP Server. Nothing to install.
add_field_value 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 add_field_value 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 add_field_value. 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.
add_field_value is provided by the Tana MCP Server MCP server (tim-mcdonnell/tana-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Deterministic rules across all 12 Tana MCP Server tools. Per-identity grants. Full audit log. Live in minutes. Nothing to install.
Free to start. No card required.
12 Tana MCP Server tools catalogued and risk-classified — across an index of 42,500+ MCP servers.