Preserve system prompt when changing adapter

[simsa-st]

Pre-submission checklist

• I have read the documentation
• I have updated the plugin and all dependencies to the latest versions
• I have searched for existing issues and discussions
• My issue is not a minor or cosmetic quirk (e.g., formatting, spacing, or other non-functional details)

Neovim version (nvim -v)

v0.11.5 LuaJIT 2.1.1765228720

Operating system/version

macOS 15.4

Adapter and model

any

Describe the bug

Changing adapter replaces my system prompt from a custom prompt by the default system prompt.

Steps to reproduce

1. Store the markdown below as prompts/test.md directory next to the minimal.lua.
2. Run :CodeCompanionActions and select the prompt
3. Use gd to confirm the system prompt that will be replaced is among the messages. It is the first system message.
4. Use ga and change adapter to anything.
5. Use gd and confirm the custom system prompt was replaced by the default prompt. The default system prompt is there twice now.

---
name: Test Prompt
interaction: chat
description: Reproduce bug
---

## system

system prompt that will be replaced

## user

Test

Expected behavior

I expected the custom system prompt to be kept when changing adapters.

I also expected this custom system prompt to be the last system prompt (most custom -> highest priority -> last) -- after the default system prompt and after system prompts from rules when they are used.

Moving the custom system prompt behind the default system prompt would also fix the bug because the change_adapter implementation assumes the default system prompt is the first message.

As a workaround it is possible to use opts: ignore_system_prompt: true in the frontmatter (this blocks change_adapter from replacing the system prompt) and copy the default system prompt directly to the file (optionally).

Screenshots or recordings (optional)

No response

minimal.lua file

---@diagnostic disable: missing-fields

vim.env.LAZY_STDPATH = ".repro"
load(vim.fn.system("curl -s https://raw.githubusercontent.com/folke/lazy.nvim/main/bootstrap.lua"))()

-- Your CodeCompanion setup
local plugins = {
	{
		"olimorris/codecompanion.nvim",
		dependencies = {
			{ "nvim-lua/plenary.nvim" },
			{
				"nvim-treesitter/nvim-treesitter",
				lazy = false,
				--[[
				Without master branch, the prompts were not loading and these warnings showed up:
				
				[Prompt Library] Missing frontmatter, name or interaction in `/private/tmp/nvim_issue_repro/try2/.repro/data/nvim/lazy/codecompanion.nvim/lua/codecompanion/actions/builtins/code_workflow.md`
				...
				[Prompt Library] Missing frontmatter, name or interaction in `/private/tmp/nvim_issue_repro/try2/.repro/data/nvim/lazy/codecompanion.nvim/lua/codecompanion/actions/builtins/unit_tests.md`
				[Prompt Library] Missing frontmatter, name or interaction in `/private/tmp/nvim_issue_repro/try2/prompts/test.md`
				
				--]]
				branch = "master",
				build = ":TSUpdate",
			},
		},
		opts = {
			--Refer to: https://github.com/olimorris/codecompanion.nvim/blob/main/lua/codecompanion/config.lua
			interactions = {
				chat = { adapter = "ollama" },
			},
			opts = {
				log_level = "DEBUG",
			},
			prompt_library = {
				markdown = {
					dirs = {
						vim.fn.getcwd() .. "/prompts",
					},
				},
			},
		},
	},
}

require("lazy.minit").repro({ spec = plugins })

-- CONFIGURE PLUGINS HERE -----------------------------------------------------

-- Setup Tree-sitter
require("nvim-treesitter.configs").setup({
	ensure_installed = { "lua", "markdown", "markdown_inline", "yaml" },
})

Log output (optional)

[INFO] 2026-01-01 03:53:44
[Interaction] Chat Initiated

Minimal reproduction confirmation

• Yes, I have tested and provided a minimal.lua file that reproduces the issue