Feature Request: Allow Custom Initialization Context in Chat Mode (`--system-prompt` / `--init-prompt`)

[ChristianWeinrich]

Feature Request: Custom System Prompt or Init Prompt for Chat Mode

Hi 👋 — first off: great work on mcp-cli, I really enjoy using it!
There’s one feature I’d really love to see for the chat mode.

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What’s missing?

Currently, the chat mode uses a fixed internal system prompt, which cannot be modified.
In many workflows, though, I want to inject specific context at the start of a session — either by:

• fully replacing the system prompt, or
• extending it with a prefix or postfix, or
• (as a fallback) injecting an initial user prompt right after startup.

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Use case example

For GitHub repositories and GH MCP Server, I often want to scope the conversation like this:

ALL following prompts are related to the organization '$ORG' and the repository '$REPO'.
Only change this if the user explicitly overrides it.

Right now, I have to paste that every time manually — which gets tedious.

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Proposed solution

Introduce CLI options to control the initialization context for chat sessions:

✅ Option A: Full control via system prompt override

--system-prompt "<TEXT>"

Replaces the internal system prompt completely.

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✅ Option B: Fine-grained control via prefix/postfix

--system-prompt-prefix "<TEXT>"
--system-prompt-postfix "<TEXT>"

Inserts text before/after the default system prompt.
These flags would be mutually exclusive with --system-prompt.

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✅ Optional fallback: init as first user prompt

--init-prompt "<TEXT>"

Adds the given text as the first user message (if overriding the system prompt is too invasive to implement right away).

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Expected behavior

1. If --system-prompt is given → internal system prompt is replaced
2. If prefix/postfix is given → internal system prompt is modified
3. If --first-prompt is given → added as initial user message after system prompt
4. If nothing is passed → behavior stays exactly as today

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Let me know if you'd be open to this — I'd be happy to help implement it or contribute a PR.

[ccmitchellusa]

I'm also interested in a similar feature. Thinking the prompts should be loadable from config/preferences or maybe user selectable from a /prompts directory.