EIP-7843 — SLOTNUM opcode

EIP-7843SLOTNUM opcode

Runnable

Live on the hosted MCP — free on Amsterdam. Connect an agent and ask.

Amsterdam (Glamsterdam) vs FusakaRuns withrun_blockrun_bytecode

Website twin: exploration on feelyourprotocol.org

What became possible

How do you read the beacon slot on-chain without hardcoding 12-second slots?

EIP-7843 adds SLOTNUM (0x4b): the consensus layer writes the slot on the execution header, and the opcode pushes it for 2 gas.

Dividing the block timestamp by 12 works only while slots stay 12 seconds. The opcode abstracts that away.

The MCP server runs your bytecode in a lab block with a chosen header.slotNumber and returns the per-tx result (return data, gas) plus a header snapshot. It does not ship demo programs.

Try asking your agent

The hosted MCP is live (Connect). Copy a prompt into your agent — each quote is self-contained.

Job

Amsterdam

Run a lab block on Amsterdam with header slot 42 — one transaction calls a contract whose runtime runs SLOTNUM and returns the slot number. What value is returned?

Look forreturn data equal to 42 (0x2a)

Agent will likely call run_block

Lesson

Fusaka vs Amsterdam

On Fusaka and on Glamsterdam, run the same lab block with header slot 100 — each block has one transaction calling a contract whose runtime runs SLOTNUM and returns the slot. Does the opcode exist only on Glamsterdam?

Look forFusaka rejecting the opcode, Glamsterdam returning 100

Agent will likely call run_block

Check

Amsterdam

On Amsterdam, run a lab block with header slot 12345678 — one transaction calls a contract whose runtime runs SLOTNUM and returns the slot number. What value is returned?

Look forreturn data equal to 12345678

Agent will likely call run_block

What the server does

1. 1

   Discover support

   Confirm 7843 is runnable; tools include run_block; read opcode encoding.

2. 2

   Run block with a header slot

   Per-tx return data equals the slot on Glamsterdam; slotNumber is rejected on Fusaka.

Run bytecode still uses a mock header (slot 0). A chosen beacon slot needs run block.

Fork caveat

Glamsterdam already bundles EIP-7843 in EthereumJS v10. Passing eips: [7843] is accepted but is not a before/after toggle on Glamsterdam itself.

For 7843 off vs on, run the same program on baseline fork fusaka, then on preview fork glamsterdam. Opcode 0x4b is invalid on Fusaka; on Glamsterdam it pushes header.slotNumber (2 gas). You do not need Fusaka for Glamsterdam-only SLOTNUM runs.

Twins and spec

Website exploration

EIP-7843 on feelyourprotocol.org

Canonical spec

EIP-7843

Spec date

2026-01-20

Test release

tests-glamsterdam-devnet@v8.1.0

Coverage index

EIP catalogue

EIP-7843 MCP Catalogue Changelog

EIP-7843 MCP Catalogue Changelog

1. v0.32026-09-10Runnable catalog row — SLOTNUM via run_block header.slotNumber.
2. v0.22026-09-10run_block ships header.slotNumber; 7843 catalog row still Planned until the module.
3. v0.12026-09-10Planned twin — exploration live; runnable catalog after run_block / header slot.