What causes "object for prefix exists" in gitbutlerapp/gitbutler?

Confirm the object exists: `git rev-parse ` in the same repository; use its output as the value.

object for prefix exists

Error message

object for prefix exists

What it means

A panic (expect) in but-debug's graph command: gix's lookup_prefix() returned Ok(None) for a --limit-extension prefix, meaning no object in the object database starts with that hex prefix. The debug tool assumes passed prefixes exist and aborts otherwise.

Solutions

Confirm the object exists: `git rev-parse <prefix>` in the same repository; use its output as the value.
Run the debug command from the correct repository/worktree.
If the repo is shallow, fetch more history (`git fetch --unshallow` or deepen) so the object is present.

Example fix

# before
but debug graph --limit-extension 4c1f0ab  # no such object here

# after
git rev-parse --verify 4c1f0ab && but debug graph --limit-extension $(git rev-parse --short 4c1f0ab)

Defensive patterns

Strategy: validation

Validate before calling

# verify the prefix resolves to an object before the debug run
git rev-parse --verify --quiet "${EXT}^{commit}" >/dev/null || { echo "no object for prefix $EXT" >&2; exit 2; }

Prevention

Only pass prefixes you obtained from this repository (rev-parse, log output).
Deepen shallow clones before debugging old history.
Run debug commands from the intended worktree.

When it happens

Trigger: Running `but debug graph --limit-extension <hex>` where <hex> is valid hex but matches no object: wrong repository, truncated/garbled hash, or an object that only exists in a different clone.

Common situations: Copying a hash from a different repo/PR/fork; a hash from before a shallow-clone boundary; case-sensitive typos that still form valid hex; running the command in the wrong worktree.

Related errors

AI-assisted analysis of gitbutlerapp/gitbutler@caf1f223d3 (2026-08-20). Data as JSON: /api/errors/b653560c029083f9. Report an issue: GitHub.

Appendix: source

Thrown at crates/but-debug/src/command/graph.rs:68

        .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,
        worktrees: false,
    };

    let graph = match graph_args.ref_name.as_deref() {
        None => but_graph::Graph::from_head(
            &repo,
            &meta,
            but_core::ref_metadata::ProjectMeta::default(),
            &mut setup::debug_db()?,
            opts,
        ),
        Some(ref_name) => {
            let mut reference = repo.find_reference(ref_name)?;
            let id = reference.peel_to_id()?;

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