What causes "preview failed: %w" in alibaba/open-code-review?

Read the wrapped cause after `preview failed:` — it names the underlying git error

preview failed: %w

Error message

preview failed: %w

What it means

executeDelegatePreview calls dc.preview(ctx) to compute the delegate review preview (merge-base, diff scope, etc.) and wraps any failure as "preview failed: <cause>". It means the underlying git/preview computation errored; the real cause is preserved by %w.

Source

Thrown at cmd/opencodereview/delegate_cmd.go:166

		return "workspace"
	}
}

// resolver returns the rules Resolver, asserting DetailResolver if available.
func (dc *delegateContext) resolver() rules.Resolver {
	return dc.cc.Resolver
}

func executeDelegatePreview(opts delegateOptions) error {
	dc, err := loadDelegateContext(opts)
	if err != nil {
		return err
	}

	ctx := context.Background()
	preview, err := dc.preview(ctx)
	if err != nil {
		return fmt.Errorf("preview failed: %w", err)
	}
	mergeBase := dc.mergeBase(ctx)
	if opts.format == "json" {
		return writeDelegateJSON(delegatePreviewJSON{
			SchemaVersion:   delegateSchemaVersion,
			Mode:            dc.reviewMode(),
			Repository:      dc.cc.RepoDir,
			From:            dc.opts.from,
			To:              dc.opts.to,
			Commit:          dc.opts.commit,
			MergeBase:       mergeBase,
			Background:      dc.opts.background,
			TotalFiles:      preview.TotalFiles,
			ReviewableCount: preview.ReviewableCount,
			ExcludedCount:   preview.ExcludedCount,
			TotalInsertions: preview.TotalInsertions,
			TotalDeletions:  preview.TotalDeletions,
			ReviewableFiles: previewFiles(preview, true),

View on GitHub (pinned to 5cf97d0d15)

Solutions

Read the wrapped cause after `preview failed:` — it names the underlying git error
Run the command inside a git repository with at least one commit on both sides of the merge base
Fetch/unshallow the repo if the merge base commit is missing
Verify `git log`, `git merge-base` work manually in the same directory

Example fix

// before (empty repo)
$ ocr delegate --preview
preview failed: exit status 128 (bad revision 'HEAD')
// after
$ git commit --allow-empty -m init && ocr delegate --preview

Defensive patterns

Strategy: try-catch

Validate before calling

func repoHasCommits(dir string) bool {
    return runGitCmd(dir, "rev-parse", "--verify", "HEAD") == nil
}

Try / catch

preview, err := dc.preview(ctx)
if err != nil {
    if errors.Is(err, exec.ErrNotFound) { /* git missing */ }
    var ee *exec.ExitError
    if errors.As(err, &ee) { log.Fatalf("preview failed: %s", ee.Stderr) }
    return fmt.Errorf("preview failed: %w", err)
}

Prevention

Run delegate commands only inside initialized, committed git repos
Ensure the merge base exists (fetch before comparing across remotes)
Avoid shallow clones when diffing against remote branches

When it happens

Trigger: Running a delegate command in preview mode outside a git repository, with no commits reachable from the merge base, on an unborn branch (no HEAD), or with a corrupted git index.

Common situations: Running the command in a freshly `git init`-ed repo with no commits; detached/empty branch; git binary missing or misconfigured; shallow clone lacking the merge base.

Understand the failure class

Background: "git command failed": what it means when a tool shells out to git and git exits non-zero — this error's family across 21 libraries.

Related errors

AI-assisted analysis of alibaba/open-code-review@5cf97d0d15 (2026-09-02). Data as JSON: /api/errors/83959856611222e8. Report an issue: GitHub.