Customize the prompt and tool descriptions

Two flags let an operator override the text mcp-v8 presents to MCP clients, without rebuilding the server:

  • --instructions — overrides the server instructions (the "system prompt" returned during initialize).
  • --run-js-description — overrides the description advertised for the run_js tool in tools/list.

Both apply in stateful and stateless mode. The run_js override is selective — all other built-in tools keep their default descriptions.

Value syntax: inline text or @file

Each flag's value is used verbatim as inline text, unless it begins with @:

Value Meaning
"some text" Used literally as the override
@./prompt.txt Read the override from the file ./prompt.txt
@@text Literal leading @ — produces @text

Override the server instructions

# Inline
mcp-v8 --http-port 8080 --instructions "You can run JavaScript for me via run_js."

# From a file
mcp-v8 --http-port 8080 --instructions @./prompts/instructions.md

The text is returned to the client in the initialize response, replacing the default instructions.

Override the run_js tool description

# Inline
mcp-v8 --http-port 8080 --run-js-description "Execute JavaScript/TypeScript in a sandbox."

# From a file (handy for long, multi-paragraph guidance)
mcp-v8 --http-port 8080 --run-js-description @./prompts/run_js.md

The text replaces the run_js entry's description in tools/list; other tools are unaffected.

See also