What causes "valid hex prefix" in gitbutlerapp/gitbutler?

Pass a real abbreviated commit id, e.g. the output of 'git rev-parse --short HEAD'

valid hex prefix

Error message

valid hex prefix

What it means

Panic in the 'but debug graph' command: a value passed via --limit-extension failed gix::hash::Prefix::from_hex, which returns None for strings containing characters outside 0-9a-f/A-F, for empty strings, or for input longer than the hash length. The debug command asserts validity with expect instead of reporting a parse error, so malformed input aborts the process.

Source

Thrown at crates/but-debug/src/command/graph.rs:64

    let extra_target = graph_args
        .extra_target
        .as_deref()
        .map(|rev_spec| repo.rev_parse_single(rev_spec))
        .transpose()?
        .map(|id| id.detach());
    let opts = but_graph::init::Options {
        extra_target_commit_id: extra_target,
        collect_tags: true,
        hard_limit: graph_args.hard_limit,
        commits_limit_hint: graph_args.limit.flatten(),
        commits_limit_recharge_location: graph_args
            .limit_extension
            .iter()
            .map(|short_hash| {
                repo.objects
                    .lookup_prefix(
                        gix::hash::Prefix::from_hex(short_hash).expect("valid hex prefix"),
                        None,
                    )
                    .unwrap()
                    .expect("object for prefix exists")
                    .expect("the prefix is unambiguous")
            })
            .collect(),
        dangerously_skip_postprocessing_for_debugging: graph_args.no_post,
        worktree_tips: vec![],
    };

    let graph = match graph_args.ref_name.as_deref() {
        None => but_graph::Graph::from_head(
            &repo,
            &meta,
            but_core::ref_metadata::ProjectMeta::default(),
            opts,
        ),

View on GitHub (pinned to 2497b8007a)

Solutions

Pass a real abbreviated commit id, e.g. the output of 'git rev-parse --short HEAD'
Strip non-hex characters (branch names, 'g' prefixes) before passing the value
Maintainer: replace the expect with a bad_input error echoing the offending value

Example fix

// before
.lookup_prefix(gix::hash::Prefix::from_hex(short_hash).expect("valid hex prefix"), None)

// after
let prefix = gix::hash::Prefix::from_hex(short_hash).ok_or_else(|| {
    anyhow::anyhow!("--limit-extension expects a hex object id prefix, got '{short_hash}'")
})?;

Defensive patterns

Strategy: validation

Validate before calling

fn is_valid_hex_prefix(s: &str) -> bool {
    !s.is_empty()
        && s.len() <= 40 // SHA-1 length; use 64 for SHA-256 repos
        && s.bytes().all(|b| b.is_ascii_hexdigit())
}

if !is_valid_hex_prefix(&short_hash) {
    return Err(anyhow::anyhow!("--limit-extension expects hex, got '{short_hash}'"));
}

Prevention

Feed the flag from 'git rev-parse --short HEAD' output rather than hand-typed values
Reject obvious non-hex values (branch names, 'g'-prefixed describe output) at the CLI boundary
Remember this is a debug command: panics here are loud by design; validate inputs in wrappers

When it happens

Trigger: Running 'but debug graph --limit-extension main' (a ref name, not hex); a short hash containing a 'g' (as in git describe output) or any non-hex character; an empty string; a value longer than 40 hex chars on a SHA-1 repository.

Common situations: Confusing branch names with object-id prefixes; pasting from 'git describe' output that embeds a 'g' before the hash; scripts forwarding unvalidated user input into the debug command.

Related errors

AI-assisted analysis of gitbutlerapp/gitbutler@2497b8007a (2026-08-17). Data as JSON: /api/errors/e474edc4c7a25e88. Report an issue: GitHub.