Download an output file by filename (no prompt_id needed). Uses GET /view. Use when you have filename from get_history or get_last_output. If return_base64 is true, file is not written to disk; returns base64 data. When size > 800 KB, automatically converts to WebP to stay under 1MB base64 limits...
Part of the ComfyUI Builder server.
Free to start. No card required.
AI agents call download_by_filename to retrieve information from ComfyUI Builder without modifying any data. This is common in research, monitoring, and reporting workflows where the agent needs context before taking action. Because read operations don't change state, they are generally safe to allow without restrictions -- but you may still want rate limits to control API costs.
Even though download_by_filename only reads data, uncontrolled read access can leak sensitive information or rack up API costs. An agent caught in a retry loop could make thousands of calls per minute. A rate limit gives you a safety net without blocking legitimate use.
Read-only tools are safe to allow by default. No rate limit needed unless you want to control costs.
{
"version": "1",
"default": "deny",
"tools": {
"download_by_filename": {}
}
}See the full ComfyUI Builder policy for all 60 tools.
These attack patterns abuse exactly the kind of access download_by_filename gives an agent. Each links to the full case and the policy that stops it:
Other read tools across the catalogue. The same approach applies to each: allow, with a rate cap to control cost.
Download an output file by filename (no prompt_id needed). Uses GET /view. Use when you have filename from get_history or get_last_output. If return_base64 is true, file is not written to disk; returns base64 data. When size > 800 KB, automatically converts to WebP to stay under 1MB base64 limits. Requires COMFYUI_HOST.. It is categorised as a Read tool in the ComfyUI Builder MCP Server, which means it retrieves data without modifying state.
Register the ComfyUI Builder MCP server in PolicyLayer and add a rule for download_by_filename: 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 ComfyUI Builder. Nothing to install.
download_by_filename is a Read tool with low risk. Read-only tools are generally safe to allow by default.
Yes. Add a rate_limit block to the download_by_filename 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 download_by_filename. 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.
download_by_filename is provided by the ComfyUI Builder MCP server (mcp-comfy-ui-builder). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Deterministic rules across all 60 ComfyUI Builder tools. Per-identity grants. Full audit log. Live in minutes. Nothing to install.
Free to start. No card required.
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