Add Custom System Prompt 🐑

AddCustomSystemPrompt lets you register your own system prompt into the MieNodes prompt library under a name you choose. Once it's registered, the pack's prompt-generation nodes can use your instructions for how to rewrite a prompt, instead of only the built-in styles. It's the customization hook for people who've outgrown the default simple/advanced behavior.

It's part of ComfyUI-MieNodes, MieMieeeee's utility pack, in the prompt-generator group.

Why you'd bother

The prompt generators in this pack ship with preset system prompts - the hidden instructions that tell the LLM how to enhance your input. Those defaults are fine, but they're someone else's taste. Maybe you always want cinematic lighting language, or you want rewrites kept under a certain length, or you're building for a niche style that the generic enhancer keeps flattening. Rather than fighting the default every time, you write the system prompt once, name it, and register it with this node. The prompt-engineering KB is clear that the system-level framing is what really steers an LLM's output - so owning that framing is the difference between nudging results and actually controlling them.

Think of it as saving a reusable preset for the behavior of your prompt enhancement, not for a single prompt.

How it works

Two inputs, both required:

• prompt_name (STRING) - the name you'll refer to this system prompt by. Pick something you'll recognize later.
• system_prompt (STRING, multiline) - the actual instructions to the LLM: how to rewrite, what to emphasize, what to avoid, length, tone, structure.

Run the node once and your system prompt is registered. Outputs are success (BOOLEAN - did it register?) and log (STRING - the details). Wire success or log into a ShowAnything node to confirm it took.

This is a registration action - you run it to install the preset, then the prompt generators can draw on it. It's not a node you leave in the hot path of every render.

Installing it

Install the pack. ComfyUI Manager → search ComfyUI-MieNodes (ComfyUI_MieNodes) → install → restart. Or:

cd ComfyUI/custom_nodes
git clone https://github.com/MieMieeeee/ComfyUI-MieNodes

then restart. No model download. Nodes appear under the sheep-emoji 🐑 MieNodes menu.

Common issues

success came back false? Read the log output - that's exactly what it's there for. Usually it's an empty prompt_name or system_prompt, or a name collision with an existing preset.

Write the system prompt like a real instruction. Vague framing ("make it good") gives you vague behavior. The value here is in a specific, well-structured system prompt - spell out the style, the constraints, and the format you want, the same way you'd brief a person. A half-hearted system prompt is worse than the default it replaced.

It's a companion, not a generator. This node doesn't produce a prompt on its own - it defines how the generators behave. Pair it with the general PromptGenerator (or the model-specific ones) to actually see your custom style in action.