Code Review

✅ Strengths

1. Proper prerequisite: Correctly builds on #8417 which enables backfill to re-import the checkpoint sync block
2. Safety compliance: No .unwrap(), .expect(), or unsafe array indexing
3. Logic simplification: Removes complex is_before_fulu check, making code more straightforward
4. Clear documentation: Updated comment explains the change well

⚠️ Recommendations

1. Missing test coverage

The existing test at beacon_node/beacon_chain/tests/store_tests.rs:2874 still provides blobs via wss_blobs_opt.clone(). There's no test covering the new code path where blobs are NOT provided and backfill must fetch them.

Recommendation: Add a test case that passes None for anchor_blobs_bytes to verify backfill correctly handles this scenario.

2. Consider adding logging

When a block has blobs but the user doesn't provide them, there's no log message indicating that backfill will handle fetching them. This could aid debugging.

Suggestion: Add a log when blobs are omitted:

} else {
    if anchor_block.message().body().has_blobs() {
        info!("Checkpoint block has blobs/columns which will be fetched via backfill");
    }
    None
};

3. Documentation updates

Since this changes user-facing behavior (users no longer need --checkpoint-blobs), verify these don't need updates:

• book/src/advanced_checkpoint_sync.md
• book/src/advanced_blobs.md
• CLI help text for --checkpoint-blobs flag

🎯 Verdict

The change is sound and safe. The core functionality should work correctly given #8417's foundation. Test coverage for the None blobs path would provide additional confidence and prevent future regressions.

Suggestion for beacon_node/client/src/builder.rs around line 357:

Consider adding a log message when blobs are not provided but the block has them:

let anchor_blobs = if let Some(anchor_blobs_bytes) = anchor_blobs_bytes {
    let max_blobs_len = spec.max_blobs_per_block(anchor_block.epoch()) as usize;
    Some(
        BlobSidecarList::from_ssz_bytes(&anchor_blobs_bytes, max_blobs_len)
            .map_err(|e| format!("Unable to parse weak subj blobs SSZ: {e:?}"))?,
    )
} else {
    if anchor_block.message().body().has_blobs() {
        info!("Checkpoint block has blobs/columns which will be fetched via backfill");
    }
    None
};

This would help users understand what's happening when they omit --checkpoint-blobs and aid debugging if backfill encounters issues.

LGTM!