Cardano's Leios Emergency Kill Switch Collides With Constitution

Cardano governance is wrestling with an unresolved contradiction: a proposed emergency shutdown route for the Leios scaling upgrade would set a parameter to zero, while draft constitutional text expressly prohibits exactly that. Version 5 of $CAP-12, which remained open on Nov. 8, forbids zero-valued Leios parameters, and the shutdown mechanism proposed by Hammond would use one.

The switch, as designed, would disable Leios's supplementary transaction-processing path while leaving ordinary block production intact. It would set maxEndorserBlockTxsSize — the ceiling on transaction payload carried by a supplementary block — to zero.

How the fast path would work

Endorser blocks reference additional transactions that enter the chain only after certification and inclusion of that certificate in a later ranking block. Under Hammond's proposal, a Severity 1 or Severity 2 network issue would waive the PARAM-04a notice requirement and permit the transition to zero in any epoch.

Switching Leios back on would follow the normal process and remain subject to the network-group frequency limit. Hammond said ratification and enactment would still take at least one epoch boundary, describing that boundary as a threshold the action must clear.

The $CAP-12 analysis states that Leios parameter changes require approval from delegated representatives, the Constitutional Committee and a stake-pool-operator vote. Hammond separately distinguished that governance-controlled protocol switch from an operational switch built on ad-hoc operator participation.

What does this change for operators?

For node operators and stake pools, the practical question is whether an emergency off switch can exist at all under the current constitutional draft. Hammond made his protocol route conditional on written confirmation from the Linear Leios team about what a zero setting actually does:

• No endorser blocks should be announced
• No votes cast or diffused
• No certificates included in ranking blocks

Node behavior should be equivalent to Praos, Cardano's existing block-production system. Hammond also required switch-off and switch-back-on exercises on the Preview and Preprod test networks, with block production measured against the Praos baseline, plus conformance tests for the zero setting across all node implementations.

Constitutional permission to set a value would still need a matching implementation. Clients must interpret zero consistently, and switching back on must work as intended.

The constitutional conflict

The obstacle is textual. Version 5's MEBTS-09 rule expressly states that the parameter must not be zero. Intersect's Nov. 6 update confirms that the off switch was removed from the constitutional text, with deactivation left to the hard-fork plan and another draft expected.

Adopting Hammond's route would require reconciling that ban with the proposed exception — either by amending MEBTS-09 or by carving out an emergency carve-out that the current draft does not contain.

Testing and the timetable

The timeline is already tight. The Nov. 3 working-group minutes say the test network entered the Dijkstra era on Nov. 2 using node 11.2, without full Leios implementation. Intersect said on Nov. 6 that DijkstraNet supports BLS key registration but not full Linear Leios. Full Leios is available for early integration testing on Musashi, while the combined node 11.3 release candidate is estimated to be at least two to three weeks away.

The Nov. 3 consensus update reports progress on endorser-block forging, certificate checks and vote relaying. Essential Cardano's Nov. 6 summary describes those main-branch changes as development work rather than a mainnet release — a distinction that matters for anyone tracking when Leios components reach production.

The working-group minutes record an aim to submit the constitution and guardrails by Nov. 25. Hammond also said on Nov. 6 that the Parameter Committee had not yet seen the pending numerical values and would need time to review them.

That leaves a narrow window. The Nov. 25 submission target, a node 11.3 release candidate that remains weeks out, and a constitutional committee that has not yet reviewed the numbers together define the working schedule. Whether the zero-value ban survives into the next constitutional draft will determine whether Cardano's Leios deployment ships with an emergency brake or without one.