`accept = "<Tab>"` does not reliably pass through when `trigger_on_accept = false`

[A-mess]

Description

When using copilot.lua inline suggestions with:

• accept = "<Tab>"
• trigger_on_accept = false

I expect <Tab> to behave like a normal Tab key when no Copilot suggestion is visible.

However, in practice, when no suggestion is visible, <Tab> still does not reliably behave like native Tab input/indentation.

This does not reproduce with other accept keys such as <C-l>.

Minimal config

vim.o.expandtab = true


vim.pack.add({
  { src = "https://github.com/zbirenbaum/copilot.lua" },
})

require("copilot").setup({
  suggestion = {
    enabled = true,
    auto_trigger = true,
    hide_during_completion = true,
    debounce = 75,
    trigger_on_accept = false,
    keymap = {
      accept = "<Tab>",
      accept_word = false,
      accept_line = false,
      next = false,
      prev = false,
      dismiss = "<C-]>",
      toggle_auto_trigger = false,
    },
  },
  panel = { enabled = false },
  filetypes = {
    ["*"] = true,
  },
})

Steps to reproduce

1. Start Neovim with the minimal config
2. Open a normal buffer
3. Wait for Copilot suggestion to appear at least once
4. Move to a place where no suggestion is visible
5. Press <Tab>

Expected behavior

When no suggestion is visible and trigger_on_accept = false, <Tab> should behave exactly like a normal Tab key.

Actual behavior

<Tab> is still intercepted by Copilot's accept mapping and does not reliably behave like native Tab input.

Additional observations

• Reproduces with accept = "<Tab>"
• Does not reproduce with accept = "<C-l>"
• Still reproduces after setting expandtab = false, so it does not seem to be caused by expandtab
• This suggests the issue is specific to the <Tab> key path, not the general accept/passthrough logic

Mapping info

:lua vim.print(vim.fn.maparg("<Tab>", "i", false, true)) shows:

{
  abbr = 0,
  buffer = 1,
  callback = <function 1>,
  desc = "[copilot] accept suggestion",
  expr = 1,
  lhs = "<Tab>",
  lhsraw = "\t",
  lnum = 0,
  mode = "i",
  mode_bits = 16,
  noremap = 1,
  nowait = 0,
  replace_keycodes = 1,
  script = 0,
  scriptversion = 1,
  sid = -8,
  silent = 1
}

Notes

From testing, this looks like a <Tab>-specific passthrough issue rather than a general trigger_on_accept = false issue.

Workaround

As a workaround, I disabled Copilot's built-in <Tab> accept mapping and implemented a custom expr mapping instead:

require("copilot").setup({
  suggestion = {
    keymap = {
      accept = false,
    },
  },
})

vim.keymap.set("i", "<Tab>", function()
  if require("copilot.suggestion").is_visible() then
    require("copilot.suggestion").accept()
    return "<Ignore>"
  end
  return "<Tab>"
end, { expr = true, desc = "Copilot accept or tab" })

This works correctly:

• If a Copilot suggestion is visible, <Tab> accepts it
• If no suggestion is visible, <Tab> behaves like a normal Tab key

nvim version

NVIM v0.12.0-nightly+3a7ade8
Build type: Release
LuaJIT 2.1.1741730670

[AntoineGS]

I think I got it, let me know if it still happens!