Create or update a file. Use mode=create for new files, mode=overwrite to replace existing, mode=append to add to end.
AI agents use write_file to create or update resources in Obsidian HTTP MCP — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Obsidian HTTP MCP environment.
write_file creates or modifies file content reversibly. While 'overwrite' replaces existing data, it is not permanent destruction (the previous version could theoretically be recovered from backups or version control). This is a Write operation. It ranks below Destructive (which would be permanent, unrecoverable deletion) but above Read operations.
From the tool's definition Tool description states 'Create or update a file' with modes for 'create', 'overwrite', and 'append'. The 'overwrite' mode modifies existing data, and 'append' adds to files. These are reversible operations.
Documented attack patterns abuse exactly the kind of access write_file gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Obsidian HTTP MCP, and nothing reaches the server without passing your rules. This is the rule we recommend for write_file:
{
"version": "1",
"default": "deny",
"tools": {
"write_file": {
"limits": [
{
"counter": "write_file_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}write_file 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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Create or update a file. Use mode=create for new files, mode=overwrite to replace existing, mode=append to add to end. It is categorised as a Write tool in the Obsidian HTTP MCP MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Obsidian HTTP MCP server in PolicyLayer and add a rule for write_file: 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 Obsidian HTTP MCP. Nothing to install.
write_file 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_file 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_file. 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_file is provided by the Obsidian HTTP MCP server (nasandnora/obsidian-http-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Obsidian HTTP MCP, 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.
12 Obsidian HTTP MCP tools catalogued and risk-classified — across an index of 43,000+ MCP servers.