MIP-009: Increase zkApp account update limit (verifier-only)

[richardpringle]

Summary

MIP-009
triples the zkApp account update limit by replacing the old weighted
cost formula with a simpler segment-count rule. This is a hard fork
change. As a verifier-only node, we must accept and validate
transactions/blocks that use the new higher limits.

Background

• MIP: MIP-009
• Status: Finalization (last call deadline 2025-10-29)
• Author: Yihang Liu (o1labs)
• OCaml PR: MIP Ref Impl: Increase zkApp account update limit MinaProtocol/mina#17386
• OCaml reference:
  src/lib/node_config/unconfigurable_constants/node_config_unconfigurable_constants.ml
• Old formula: 10.26*np + 10.08*n2 + 9.14*n1 < 69.45
• New formula: np + n2 + n1 <= 16
• Where np = proof segments, n2 = signed pair segments,
  n1 = signed single segments
• Max per transaction: 31 signed/no-auth updates (16 paired
  segments) or 15 proof updates + 1 fee payer segment
• Prerequisite: SNARK worker proof-level parallelization from
  the slot reduction MIP

Files requiring changes

1. Cost constants

• crates/ledger/src/scan_state.rs (lines 14-39)
  • GenesisConstant struct and GENESIS_CONSTANT values
  • zkapp_proof_update_cost: 10.26 -> 1.0
  • zkapp_signed_single_update_cost: 9.14 -> 1.0
  • zkapp_signed_pair_update_cost: 10.08 -> 1.0
  • zkapp_transaction_cost_limit: 69.45 -> 16.0
  • Alternatively, simplify the formula entirely to use integer
    comparison and remove the float fields

2. Cost comparison operator

• crates/ledger/src/scan_state/transaction_logic/zkapp_command.rs
  (line 2846)
  • Change Self::zkapp_cost(...) < cost_limit to
    Self::zkapp_cost(...) <= cost_limit
  • The MIP specifies <= (inclusive), the old code uses <
    (strict). This is the difference between allowing exactly
    16 segments vs only 15.
  • The zkapp_cost() function (line 2778) and valid_size()
    (line 2799) both consume GENESIS_CONSTANT; updating the
    constants + operator is sufficient

3. Test data

• tools/testing/data/daemon.json (lines 4-7)
  • Update cost values to match the new constants

Verification path

The valid_size() method (zkapp_command.rs:2799) is used in both
verification paths:

1. Transaction pool prevalidation --
   transaction_pool.rs:2108 calls check_well_formedness()
   which calls valid_size()
2. Block application -- staged_ledger.rs:1271 calls
   check_commands() during apply(), verifying transactions
   in incoming blocks

Not required for verifier-only

• Block production / diff creation (staged_ledger/resources.rs)
• Scan state capacity (transaction_capacity_log_2: 7 = 128 base
  jobs, already sufficient)
• zkapp_cmd_limit_hardcap: 128 (already accommodates new limit)
• SNARK verification pipeline (handles any valid proof tree)
• Consensus rules (unchanged by this MIP)

Scope

Minimal change: update 4 constants in one file, change one
comparison operator in another, and update test data. Optionally
simplify by replacing the float-based cost formula with a direct
integer segment count.