# fish_command_not_found - what to do when a command wasn't found

## Metadata

| Field | Value |
|---|---|
| Institute | Molodetz Institute — Research & Retrieval Division |
| Subject | shell-multiplexer-auto-attach |
| Document type | Source Dossier |
| Tier | 1 |
| Source title | fish_command_not_found - what to do when a command wasn't found |
| Source URL | https://fishshell.com/docs/4.8/cmds/fish_command_not_found.html |
| Source publisher / author | fish-shell project (fishshell.com) |
| Source type | vendor docs |
| Source date (published) | Unknown — not stated |
| Date researched | 2026-10-10 |
| Researched by | tai (agent), model: Claude Sonnet 5, on behalf of retoor |
| Research method | web search synthesis |
| Confidence | High |
| Status | Active |

## Summary

fish's analogue of bash's `command_not_found_handle`/zsh's `command_not_found_handler` is a plain user-definable function named `fish_command_not_found`, called whenever fish fails to find a command. It receives the attempted command line as `$argv` (one token per arg, so `$argv[1]` is the missing command name), was introduced in fish 3.2.0, and falls back to fish's own default error message if left undefined or if the function explicitly calls `__fish_default_command_not_found_handler`.

## Key findings

- Defined with ordinary fish function syntax: `function fish_command_not_found; ...; end`, no special registration step required beyond defining the function (typically in `config.fish` or an autoloaded `~/.config/fish/functions/fish_command_not_found.fish` file).
- `$argv` holds the full attempted command line as separate tokens, directly analogous to bash's `"$@"` inside `command_not_found_handle`.
- Introduced in fish 3.2.0; versions before that used a `fish_command_not_found` *event* instead of a function, so a handler meant to support pre-3.2 fish needs a dual event+function definition — not a concern for this project, which only needs to support a function since its own `tai.py` requires modern Python 3 anyway and fish 3.2 (2020) predates any realistic target.
- Leaving the function undefined, or having it call `__fish_default_command_not_found_handler`, reproduces fish's normal plain error message — useful as the explicit fallback behavior if tai's own hook is ever uninstalled.

## Pros / strengths

- Official, current vendor documentation, consistent across every mirrored version from 3.2 through 4.x found in this search — a stable, non-moving target to build on.
- Directly structurally parallel to bash/zsh's own mechanism (one function, one argv), which kept the tai.py implementation simple: same body shape (`"$HOME/.local/bin/tai.py" $argv`) as the bash/zsh blocks, just fish function syntax instead of a shell function definition.

## Cons / limitations / caveats

- Unlike bash's `command_not_found_handle`/zsh's `command_not_found_handler`, which are ordinary shell functions sourced from any rc file read at the right time, fish's function must specifically be named `fish_command_not_found` — fish does not look for an alternately-named function for this purpose.

## Relevance to tai

This is the exact mechanism `FISH_NOTFOUND_BLOCK` in `tai.py` implements: a `function fish_command_not_found; "$HOME/.local/bin/tai.py" $argv; end` block written into `config.fish` by `upsert_fish_hook_block`/`op_fish_hook`, directly parallel to the existing bash/zsh blocks.

## Changelog

| Date | Change |
|---|---|
| 2026-10-10 | Initial dossier created. |