[krisoshea-eth]

What

• Add a working design draft for extending the Beacon API block event from Gloas onward with builder_index and block_hash.
• Record the current API-02 evidence, exact fork and self-build contract, mixed-client fallback behavior, versioning rationale, alternatives, and completion gates.
• Add a source-level feasibility audit for Lodestar, Lighthouse, Nimbus, Prysm, Teku, and Grandine.
• Define a concrete cross-client outreach sequence and track current overlap with Beacon APIs PRs #585 and #490.
• Link the draft from the repository README and source map.

Why

Beacon APIs issue #599 identifies that the existing block event does not expose enough information for a Builder to cheaply determine whether its bid was selected. The current Lodestar Builder implementation uses block plus getBlockV2 as a compatible fallback. This draft provides a place to refine the proposed additive event extension before opening a Beacon APIs specification PR.

Research basis

• Beacon APIs master at ba859dbca9076ba19dedc095c59afa7b05a66819.
• Gloas consensus self-build and ExecutionPayloadBid semantics.
• Recent Beacon APIs event precedents in #587, #590, and #621.
• Current event construction paths in the six client repositories named above.
• Current Lodestar Builder PRs and the Builder implementation plan and Living Technical Note.
• Linear LOD-48 and its synchronized GitHub issue #49.

Status

This is a non-normative working draft. It does not represent an accepted Beacon API change. Nico supports the proposed direction, but Lodestar PoC evidence and cross-client review are still pending. The document now separates the threshold for opening an upstream draft from the evidence required before that PR can leave draft.

Tracking

• Linear: LOD-48
• GitHub mirror: krisoshea-eth/lodestar#49
• Beacon APIs discussion: ethereum/beacon-APIs#599
• Lodestar API-02 draft: krisoshea-eth/lodestar#48

Validation

• Manual Markdown and link review
• git diff --check
• Verified that the Linear tracker and GitHub issue mirror carry the corrected #599 links and current audit tasks

AI assistance was used during codebase research, drafting, and review. The submitted documentation and pull request text were reviewed and revised by the author, including manual edits.

[linear-code]

LOD-48

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