Updating a Compact Map key on Midnight Undeployed shows up as RPC 1010, not as a map error

A developer building a token circuit on Midnight Local Undeployed reported losing an afternoon to `SubmissionError` and `FiberFailure` wrapping `RpcError 1010: Invalid Transaction: Custom error: 117`, plus a WASM panic that surfaced as `Unreachable` in `feesWithMargin` / `transaction_merge` ^[1]. The circuit compiled, the proof server was up and the genesis wallet held tDUST; the defect was in how the code used a Compact `Map` ^[2]. That gap between symptom and cause is the whole story, because none of those messages name the ledger operation that caused them.

The rule the author lands on is blunt: on Local Undeployed, do not update an existing `Map` key, insert a new one ^[3]. He says he verified this against a running local stack rather than reasoning about it ^[4]. The EVM-shaped version - `lookup` a balance, then `insert` the same key with the decremented value - compiles as valid Compact but does not survive `callTx` on this stack, and updating an existing key is what trips the merge path ^[11]. He hit the same class of failure twice: first on an NFT `owners` map, then again on a token `balances` map after believing the NFT case was fixed, with the same ledger shape and the same panic ^[12].

The second half of the post is about privacy plumbing that developers coming from Solidity will read as ceremony. A Compact circuit has three tiers: witnesses such as `localSecretKey()`, circuit-local values including hashes, asserts and arithmetic, and the public ledger; only `disclose(...)` moves a value from private to public ^[9]. So `credits.insert(pk, amount)` without `disclose` is an attempt to write a private value into public state, and the compiler or runtime rejects it ^[10]. The signer key `fromPk` is a `persistentHash` over a padded domain tag and the witness secret, which means it is private until disclosed ^[17].

That is also where the access-control mistake lives. Hashing a witness and then checking the result is not empty proves the hash function ran, not that the caller is entitled to spend; the shipped circuit gates transfers on `faucet_claimed.member(fromPk)` instead ^[18]. The rewritten `MidnightUSDC.compact` under Compact 0.23 keeps four ledger fields - `credits`, `credit_to`, `faucet_claimed`, `spent_nonces` ^[14]. The faucet inserts once per signer key and grants 10000000 units ^[15]. Transfer asserts the nonce is unspent, the amount is non-zero and the signer has claimed, then writes rows keyed by the fresh nonce ^[16]. The NFT flow follows the same discipline: `owners` insert on mint only, sales appended under a fresh random id, and cancellation writing a new listing id rather than overwriting the old price key ^[19]. The circuit is called `listSale` because `list` is a Compact keyword ^[20].

The cost is not hidden. Each transfer writes three new ledger entries, so state grows with transaction count rather than holder count ^[21], and the author's own advice is to treat the ledger as an append-only journal and reconstruct display balances off-chain ^[13].

Watch the environment pins, because they change which failure you get. Local Undeployed writes are server-append and Lace does not sign them ^[5]; `setNetworkId("undeployed")` is a string at midnight-js 4.1.1 ^[6]; the standalone chain funds the genesis seed ending 0002, not 0001 ^[7]. Drift there produces a different class of error, often 196 for verifier mismatch, and mixing the public-network proof server 8.1.0 with this indexer is called out specifically ^[8]. This is one developer's account of one local stack, so the open question is whether the merge-path failure is a bug in Undeployed or the intended semantics of key updates.