EIP-7906 - Transaction Assertions via State Diff Opcode

Abstract

This proposal introduces three opcodes, TXTRACE, TXDIFF, and EVENTDATACOPY, that let a contract inspect the outcome of the transaction executing it: the balance, storage, and code changes it produced, and the events it emitted. TXTRACE enumerates the full transaction state diff, TXDIFF reads a single entry by key, and EVENTDATACOPY copies event data into memory.

These opcodes are valid only inside a POST_TX frame, a new EIP-8141 frame mode also defined here. POST_TX frames execute read-only as a trailing suffix of the transaction's frames, so they observe the transaction's final outcome, and their revert rolls back the execution body while leaving the transaction valid and the gas paid.

Together these allow a wallet or dApp to attach an on-chain assertion to a transaction and have the transaction's effects discarded if the assertion fails, giving users a way to bound what a transaction can do to them and reducing the risk of blind signing.

Motivation

The total value of crypto assets that have been stolen to date exceeds the yearly GDP of a medium-sized nation. This level of loss and waste is indefensible and has a long list of negative consequences for everyone around the world.

The ability of an average user or a Wallet application to find, collect, review, and analyze the EVM code the transaction will execute is very limited.

This leaves the users with no mechanism to enforce any restrictions on what the transaction actually does once it is signed. This leads users to perform de-facto blind signing every time they interact with Ethereum, exposing themselves to significant risks.

By providing the Wallets and dApps with the ability to observe and restrict the possible outcomes of a transaction, we create a tool that users can apply to reduce their risk levels.

Specification

This EIP defines the following instructions:

Constants

The POST_TX Frame Mode

This EIP requires EIP-8141 and amends its frame transaction specification by adding a new frame mode value, POST_TX, alongside the DEFAULT, VERIFY, and SENDER modes already defined there.

The following rules apply to EIP-8141 frame transactions wherever this EIP is active:

• POST_TX is added to the set of mode values accepted by EIP-8141's static constraint on frame.mode.
• POST_TX frames must form a contiguous trailing suffix of tx.frames: once any frame has mode POST_TX, every subsequent frame in the transaction must also have mode POST_TX. A frame transaction violating this is invalid.
• As with DEFAULT and VERIFY frames, the caller of a POST_TX frame is ENTRY_POINT.
• A POST_TX frame is executed as a STATICCALL, disallowing all state manipulation. Unlike VERIFY frames, POST_TX frames are not granted the APPROVE exception defined in EIP-8141: calling APPROVE anywhere in a POST_TX frame's call subtree is an ordinary STATICCALL violation and causes an exceptional halt of that call frame, handled like any other POST_TX failure per the rule below.
• Approval-scope flags are valid on a POST_TX frame but have no effect, since APPROVE cannot succeed in it. The atomic batch flag is not valid on a POST_TX frame, as the EIP-8141 flag table allows it only with DEFAULT and SENDER, nor on a frame directly before a POST_TX frame. No atomic batch contains a POST_TX frame, so unrolling a failed batch never skips an assertion.
• If a POST_TX frame reverts or halts exceptionally, the entire execution body of a transaction is reverted unconditionally. This overrides the atomic-batch unrolling behavior that would otherwise apply: a POST_TX failure always validates or reverts the whole transaction execution, up to the "validation prefix", rather than merely unwinding an atomic batch.
• A POST_TX failure ends frame execution. The remaining POST_TX frames are not executed: their receipts have status 2 and no gas used, as for the skipped frames of a failed EIP-8141 atomic batch, and their gas is refunded at the end of the transaction.
• Neither a POST_TX revert nor an exceptional halt invalidates the transaction (unlike a VERIFY frame revert). The transaction remains valid and is included in the block. The failing POST_TX frame's receipt has status 0. The receipts of the reverted execution-body frames keep their status and execution gas, but their logs are discarded and the state gas recorded in them is removed, as for an unrolled EIP-8141 atomic batch.
• All state changes made within the validation prefix (such as gas payment via APPROVE and account creation in a deploy frame) are permanently committed to the state, and the payer is fully charged for the gas consumed up to the point of the failure.
• Under EIP-8141's default code, a POST_TX frame is handled the same way as SENDER or DEFAULT.

Transaction Trace Opcode

We introduce a new TXTRACE opcode (0xb6).

TXTRACE reads the state diff of the current transaction: the net difference between the transaction prestate and the state as of this call, spanning balance changes, storage changes, deployed contracts, and emitted events. Since POST_TX frames cannot write state and form a trailing suffix of the transaction, that diff is the transaction's final outcome. The exact semantics are given under State Difference Semantics.

A single call returns one 256-bit value. Two stack inputs select it: param chooses which piece of the diff to read, and in2 is an index into the table that param selects, or 0 for the params that take no index. An assertion reads the diff by calling TXTRACE repeatedly: a count param first (0x00, 0x01, 0x02, or 0x0C), then one call per field of each entry. This is the same selector-and-index idea as the FRAMEPARAM opcode from EIP-8141, but the operand order differs: TXTRACE takes param from the top of the stack, while FRAMEPARAM takes its index from the top.

The selected value is pushed onto the stack.

Each TXTRACE call costs the active fork's WARM_STORAGE_READ_COST, as defined in EIP-2929 and subject to the inherited gas schedule described under Gas Cost, for every param.

The available parameters are listed in the table below.

TXTRACE, EVENTDATACOPY, and TXDIFF are valid only for execution inside a POST_TX mode frame, as defined above. Executing any of these opcodes in any other context — including legacy transactions, EIP-1559 transactions, or any other EIP-8141 frame mode — results in an exceptional halt.

The gas_pre_charge parameter is the transaction's max_cost as defined in EIP-8141: the amount collected from the payer when payment is approved, equal to TXPARAM param 0x06. That is gas_pre_charge = max_gas × max_fee_per_gas + blob_gas × blob_base_fee, where max_gas is EIP-8141's maximum gas, including the calldata floor where it applies. For transactions with blobs attached, it therefore includes the blob fees, priced at the blob base fee.

The gas_payer_address is the target of whichever frame called APPROVE(APPROVE_PAYMENT) or APPROVE(APPROVE_EXECUTION_AND_PAYMENT), i.e. the EIP-8141 payer, which may or may not be the transaction sender.

State Difference Semantics

The before values reflect the transaction prestate values recorded before the start of entire transaction's execution, before any state writes made in relation to this transaction. The after values reflect the current state as of the TXTRACE opcode call. Intermediary writes between transaction start and the TXTRACE call are not observable separately.

An address will appear in balances_changed when its balance at the time of the TXTRACE call differs from its balance at transaction start. This includes the gas fee pre-charge applied to the gas payer address. Callers computing the net ETH transferred to or from an address can look up the gas payer via gas_payer_address (param 0x15) and subtract gas_pre_charge (param 0x14) from that address's balance delta.

A (address, slot) pair will appear in slots_changed when its value at the time of the TXTRACE call differs from its value at transaction start. Multiple writes to the same slot during the transaction collapse into this single entry.

A storage wipe can clear slots the transaction never wrote, for example a CREATE onto an address that holds only storage if the active fork's creation rules permit it, or the removal of an empty account that holds storage. Those slots are not reported: they are absent from slots_changed and the per-address view, and they do not set the storage bit of account_change_flags. Reporting them would require iterating the account's prestate storage, which is unbounded. A keyed TXDIFF lookup of such a slot still reports the wipe: slot_value_before (param 0x00) returns the slot's transaction prestate value, and slot_value_after (param 0x01) returns its current value, 0.

An address appears in contracts_deployed when its code hash changed during the transaction from the empty-code hash to a non-empty code hash that is not an EIP-7702 delegation designator. An account that did not exist at transaction start counts as having the empty-code hash, so a contract created at a fresh address is enumerated. An account that a CREATE/CREATE2 leaves with empty code is not enumerated, as it has no code change.

An account that EIP-6780 deletes at the end of the transaction appears as the execution body left it, since the deletion happens after the last frame. A contract created and destroyed in the same transaction therefore still appears in contracts_deployed.

Logs that the protocol emits on the transaction's behalf, such as EIP-7708 transfer logs, count as events like any other log, so the event tables also expose ETH transfers.

Transaction Diff Lookup Opcode

TXTRACE enumerates the state diff but cannot be asked about one specific entry, so reading a single known value means searching the enumeration. We introduce an additional TXDIFF opcode (0xb7) for this purpose.

A single call returns one 256-bit value. Three stack inputs select it: param chooses what to read, in2 is the address the query is keyed on, and in3 is a storage slot key, a per-address index, or 0, depending on param. Params 0x00 to 0x05 read one account's balance, codehash, or storage slot directly. Params 0x06 to 0x0A expose per-address views over the diff, mapping into TXTRACE's tables, and a bitmask summarizing an account's changes. For the address-keyed params 0x00 to 0x0A, only the low 20 bytes of in2 are used as the address, as with BALANCE, and the high 12 bytes are ignored. For the per-topic params, all 32 bytes of in2 are the topic value.

The selected value is pushed onto the stack.

Params

If the queried key address/(address, slot) was never modified during the transaction, TXDIFF (params 0x00–0x05) returns the current live value for both the before and after variant of that param. A slot cleared by a storage wipe counts as modified for this rule, so its before value is its transaction prestate value, as described in State Difference Semantics.

An account that does not exist reads as an empty account: balance 0 and the empty-code hash. This applies to codehash_before for an account that did not exist at transaction start, and to codehash_after for an account that does not exist at the time of the call. For a non-existent account, TXDIFF params 0x04 and 0x05 therefore return the empty-code hash where EXTCODEHASH would return 0.

Per-Address Remapping

Params queryable with TXDIFF 0x06, 0x07, 0x08, and 0x09 expose per-address filtered views over the storage slots and events exposed via enumeration by the TXTRACE opcode.

The count params (0x06, 0x08) return the size of this view, returning 0 for an address with no entries.

The index params (0x07, 0x09) map a per-address, local index, passed as in3, to the entry's global index in the corresponding table. The returned global index can be used directly with the per-entry TXTRACE params and with EVENTDATACOPY.

If TXDIFF received an invalid local index, i.e. value greater than or equal to the view's count, an exceptional halt occurs.

Per-Topic Views

Params 0x0B and 0x0C expose a per-topic-value filtered view over the events exposed via enumeration by the TXTRACE opcode, keyed by an indexed topic value rather than by the emitting address.

The count param (0x0B) returns the number of entries in events_count that carry in2 as an indexed topic in any of the positions topic1, topic2, or topic3, returning 0 when no event carries the value. The signature topic topic0 is excluded, as it identifies the event type rather than a participant.

The index param (0x0C) maps a per-value, local index, passed as in3, to the entry's global index in the events_count table, in ascending emission order. The returned global index can be used directly with the per-event TXTRACE params and with EVENTDATACOPY.

If TXDIFF received an invalid local index, i.e. a value greater than or equal to the view's count, an exceptional halt occurs.

Account Change Flags

Param 0x0A returns a bitmask summarizing all net changes to the account's state. The bits follow the field order of the account tuple (nonce, balance, storage_root, code_hash):

All higher bits are set to zero.

A set bit reflects a net difference between the transaction prestate and the state as of the opcode call. Values that were modified and later restored within the transaction do not set a bit.

account_change_flags == 0 if and only if the account's internal state, including its entire storage, is identical to the transaction prestate. The one exception is a storage wipe of slots the transaction never wrote, as described in State Difference Semantics.

The actual nonce value is not observable through TXTRACE or TXDIFF.

Gas Cost

TXDIFF params that may fall back to reading live state use the EIP-2929 access lists to determine their cost. All access costs in this EIP use the active fork's gas schedule, including any repricing of the referenced parameters. Where EIP-8038 is active, COLD_ACCOUNT_ACCESS_COST, COLD_SLOAD_COST, and WARM_STORAGE_READ_COST are named COLD_ACCOUNT_ACCESS, COLD_STORAGE_ACCESS, and WARM_ACCESS, respectively, and have values 3000, 2100, and 100. This also applies to TXTRACE's warm-read cost.

• Storage slot params (0x00, 0x01): COLD_SLOAD_COST if the (address, slot) pair is not in the accessed storage list; WARM_STORAGE_READ_COST otherwise. Only the (address, slot) pair is priced: the address itself is not charged COLD_ACCOUNT_ACCESS_COST and is not added to the accessed addresses set, so a later BALANCE or CALL of a cold address still pays the cold account cost.
• Balance and codehash params (0x02–0x05): COLD_ACCOUNT_ACCESS_COST if the address is not in the accessed addresses set; WARM_STORAGE_READ_COST otherwise.
• Per-address and per-topic view and flags params (0x06–0x0C): WARM_STORAGE_READ_COST. These params are answered entirely from the transaction-local state diff and never read the live state.

For params 0x00–0x05, the accessed slot or address is added to the EIP-2929 access list after the call, and — where EIP-7928 is active — recorded in the block-level access list, like any other state-reading opcode. For params 0x00 and 0x01 that is the (address, slot) pair only; for params 0x02–0x05 it is the address. The full gas cost is charged before state is read: if the charge fails, or the opcode halts on a non-zero reserved input or outside a POST_TX frame, the opcode adds no access to the block-level access list. TXTRACE and EVENTDATACOPY read only already-recorded diff and log data and therefore add no new accesses. Params 0x06–0x0C do not interact with the EIP-2929 access lists.

Event Data Copy Opcode

TXTRACE reports an event's topics and the byte length of its non-indexed data, but not the data itself, which is variable-length and so cannot be returned on the stack. We introduce an EVENTDATACOPY opcode (0xb8) to copy that data into memory.

The gas cost matches CALLDATACOPY, i.e. the operation has a fixed cost of 3 and a variable cost that accounts for the memory expansion and copying.

Stack

No stack output value is produced.

Behavior

The operation copies length bytes from the event's non-indexed data, starting at the given byte dataOffset, into a memory region starting at memOffset. Copying and memory expansion follow CALLDATACOPY, with the following source-range checks:

• If event_index >= events_count, an exceptional halt occurs.
• If dataOffset + length exceeds the event's data length, an exceptional halt occurs. This applies even when length is 0, unlike CALLDATACOPY, which never halts on its source range.

The addition dataOffset + length is evaluated without 256-bit wraparound.

Reserved Inputs

Any in2/in3 operand marked must be 0 in the parameter tables above MUST be zero; supplying a non-zero value causes an exceptional halt.

Undefined Params and Out-of-Range Indices

A param value not listed in the parameter tables above, that is any value above 0x15 for TXTRACE or above 0x0C for TXDIFF, causes an exceptional halt. Selectors, indices, and reserved-zero inputs are checked as full 256-bit values, without truncation.

For every TXTRACE param whose in2 is an index into a table (0x03–0x0B and 0x0D–0x13), an in2 greater than or equal to that table's count (balances_changed, slots_changed, contracts_deployed, or events_count) causes an exceptional halt.

Results Ordering

Balance and storage slot changes returned by the TXTRACE opcode are enumerated in ascending order sorted by the affected address as a numerical uint160 value.

Storage changes within a single address are sorted by the storage slot key as a numerical uint256 value.

Newly deployed contracts are enumerated in ascending order sorted by the deployed address as a numerical uint160 value.

Events are enumerated in the order they were emitted during transaction execution, matching their global log index within the transaction.

The per-address views of TXDIFF (params 0x07 and 0x09) and the per-topic view (param 0x0C) list their entries in ascending order of the global index each entry maps to. A per-address slot view is therefore in slot key order, and a per-address or per-topic event view is in emission order.

Rationale

Selection Parameter Design

The TXTRACE opcode follows the same selector-and-index idea used by FRAMEPARAM in EIP-8141, a param selecting the field and a second operand selecting the entry. Unlike FRAMEPARAM, it takes param from the top of the stack, as TXDIFF does, so both diff opcodes read their selector first. This keeps the interface consistent and avoids introducing a separate opcode for every piece of trace information.

Enumeration and Lookup

The TXTRACE opcode exposes transaction outcomes through index-based access over the full set of observable state changes.

The TXDIFF opcode complements it with direct, keyed access to one specific balance, codehash, or storage slot.

Typical transaction assertion costs are negligible compared to the gas cost of the storage modification itself.

Events are in emission order and require a linear scan.

Most assertion scripts are expected to enumerate the full set of allowed state changes and will not require a binary search.

Gas Cost

This EIP adds no gas constants. TXTRACE and the TXDIFF view and flags params read data that the client already holds for the current transaction: its state diff, its logs, and its gas payment. Every slot or account in the diff was written by the transaction, so it is already warm, and TXDIFF already charges WARM_STORAGE_READ_COST to read its before or after value. TXTRACE returns those same values by index and has the same price, so the cost of a changed value does not depend on which opcode reads it. The other TXTRACE params, such as counts, addresses, and event fields, could cost less, as FRAMEPARAM does in EIP-8141. They use the same price so that TXTRACE has one cost for every param.

A client can build an index over the diff and the logs when a POST_TX frame first reads them, and reuse it while the diff does not change. POST_TX frames cannot write state or emit logs, so the diff changes only if one of them fails and the execution body is reverted. The work to build the index grows with the accounts, storage slots, and logs that the transaction touched, and the transaction has already paid for each of them: at least a cold access for a slot or for an account that did not start warm, and the LOG cost or the value transfer cost for an event. A transaction with no POST_TX frame never needs the index.

Once the index is built, each TXTRACE call and each TXDIFF view or flags call does a constant amount of work, so the gas an assertion needs grows linearly with the number of entries it reads.

Direct Lookup via TXDIFF

TXTRACE's enumeration model has no way to directly check one specific value, such as "the value of usdc.balances[vitalik.eth] before this transaction". An assertion contract would have to implement its own search over the sorted enumeration output to look up such a value. TXDIFF addresses this gap directly.

A point lookup over storage needs both an address and a slot to be unambiguous, not fitting the TXTRACE's existing 2-argument (param, in2) shape.

Balance and codehash are keyed only by address.

Per-Address Views

Assertion contracts frequently need per-contract answers: "did this contract's storage change at all", "did this contract emit any event", "check every slot this contract changed".

For events, no efficient workaround exists: events are enumerated in emission order, so finding one contract's events requires a linear scan over every event in the transaction. The number of unrelated events is attacker-controlled — a malicious dApp can pad a transaction with cheap logs, inflating the assertion's gas cost until it exceeds its stipend. The per-address views make the cost proportional only to the activity of the contracts the assertion actually inspects.

Returning global indices keeps the parameter space small, as one conversion call plugs into all existing TXTRACE params and EVENTDATACOPY.

Per-Topic Views

The per-address event view answers "which events did this contract emit", keyed by the emitting address. An assertion protecting a user asks the dual question: "which events name this user as a participant" — for example, every Transfer whose from is the user, or every Approval whose owner is the user — regardless of which contract emitted them.

Under dynamic routing the set of emitting contracts is not known at signing time, so the emitting address is not a usable key: the assertion cannot enumerate the tokens a route will touch. The participant, by contrast, is a fixed value the wallet places in the assertion at signing time. Keying the event view on the indexed topic value makes such an assertion's cost proportional to the events that name the participant, rather than to the transaction's total event count, which is subject to the same padding argument that motivates the per-address views.

Keying on the raw 32-byte topic value, rather than on an address, keeps the view semantics-free and lets it serve any indexed identity an assertion cares about, including token ids and other opaque handles carried in indexed topics.

Account Change Flags as a Shield Assertion

A common assertion is expected to be a "shield": asserting that a given account was not affected by the transaction. Without the flags param this requires three separate lookups — balance, codehash, and storage count — and still leaves the nonce unobserved. account_change_flags collapses the entire check into a single opcode call: flags == 0 guarantees the account's state, including its full storage, is identical to the transaction prestate.

Events are deliberately excluded from the bitmask: an emitted event is not a change to the account's state. A "the contract stayed silent" check is available separately as address_events_count == 0.

Receipt Representation and Anti-DoS

If a POST_TX revert were to completely exclude the transaction from the block and roll back the gas payment, it would introduce a severe Denial-of-Service vector. Attackers could consume up to the block gas limit and then revert in the POST_TX frame for free.

Keeping the transaction valid and committing the validation prefix is strictly required to ensure block builders are compensated for the execution work performed. A POST_TX revert operates as an application-level execution revert, not a protocol-level invalidation, and thus gives the failing frame a status = 0 receipt while keeping the gas payment intact.

The POST_TX Frame Mode Requirement

The TXTRACE and EVENTDATACOPY opcodes provide significant introspection capabilities that may break code encapsulation.

By only allowing their execution inside a POST_TX frame we ensure this capability may only be used to determine the outcome validity and decide whether to revert the entire transaction.

Because POST_TX frames are required to be a trailing suffix of tx.frames, the diff TXTRACE observes is always the final outcome of the transaction, and because a POST_TX revert unconditionally invalidates the whole transaction, a failed assertion can never be partially bypassed by atomic-batch semantics or by frames that ran before it.

Allowing multiple POST_TX frames allows independent assertion providers to compose without a need for an active collaboration.

Each transaction assertion module can run its own assertion logic in its own frame and independently invalidate the transaction if its check fails.

Per-contract Usage

Individual contracts can use the TXTRACE opcode to inspect the state changes made internally, using a pattern similar to "reentrancy guard" modifier for their external functions. This applies to any contract called from within a POST_TX frame's call subtree.

Individual Topic Access

EVM events carry 0–4 topics, each a 32-byte word. Topic 0 is conventionally the event signature hash; topics 1–3 carry indexed parameters. Assertion contracts that verify which specific token was transferred, which address was approved, or which identifier was involved need to inspect these indexed values directly.

Accessing a topic slot at or beyond event_topic_count causes an exceptional halt, consistent with out-of-bounds behavior for all other indexed params.

EVENTDATACOPY as a Companion Opcode

Event non-indexed data is variable-length and cannot be returned as a single 32-byte stack word. A memory-copy opcode with the same semantics as CALLDATACOPY is the idiomatic EVM approach for variable-length data access.

Events as a Channel from Earlier Frames

While store and balance changes represent the actual source of truth for closed-ended assertions, events are the most accessible way to express an assertion restricting certain standardized actions. Their signatures and indexed topics are parts of the public standard and external interface from day one — see ERC-20, ERC-721, ERC-1155 and others. Events are routinely decoded by blockchain tooling like wallets, explorers, and indexers. An event-based assertion needs only the public signature: "this transaction emits no Transfer(address,address,uint256) with my account as from" - to achieve a useful safety check.

Additionally, EIP-8141 discards transient storage between frames and gives a later frame no access to an earlier frame's return data, so a POST_TX frame cannot receive a value measured before the transaction body through the frame mechanism. Events provide a channel to deliver pre-state snapshots to the assertions frame without writing anything to the persistent state.

Gas Pre-Charge Parameter

The gas pre-charge (EIP-8141's max_cost) is collected from the payer when payment is approved and appears in the gas payer's balance_after, making it hard to isolate actual ETH transfers. The pre-charge is also provisional: a refund for unused gas is issued after execution, so the bundled figure is not the final cost.

Exposing gas_pre_charge directly lets callers subtract it with a single opcode call. It covers all gas-related deductions including the blob fee for EIP-4844 transactions, so the same subtraction isolates pure ETH transfers. gas_payer_address completes the picture: the EIP-8141 gas payer may be a separate paymaster rather than the sender, and no existing opcode exposes that address. Together the two parameters let assertion contracts identify the right balances_changed entry and apply the subtraction correctly.

Deterministic Enumeration

State changes use address-sorted order because the state diff model collapses all intermediate writes into a single entry per (address, slot). Sequence of execution does not define a deterministic order for the collapsed state diff, as the same slot may be written multiple times across interleaved reentrant calls, yet produce exactly one entry. Sorting by address and slot key ensures a canonical, deterministic enumeration independent of execution flow.

Events can use emission order because each event is a distinct, non-collapsed entity with a canonical position corresponding to its log index. Assertion contracts that verify cross-contract event sequencing require this ordering.

Backwards Compatibility

TXTRACE, EVENTDATACOPY, and TXDIFF occupy previously unused opcode slots. No changes are made to existing opcodes, transaction types, or precompiles, so existing contracts and tooling are unaffected.

This proposal has a hard dependency on EIP-8141 (requires: 8141): TXTRACE, EVENTDATACOPY, and TXDIFF can only execute inside an EIP-8141 POST_TX frame, as defined under The POST_TX Frame Mode. Legacy transactions, EIP-1559 transactions, and any other EIP-8141 frame mode cannot use any of these opcodes.

Security Considerations

Insufficiently Restrictive Assertions

The main risk is a false sense of security: an assertion contract that checks too little may mislead users into believing a transaction is safe when it is not.

Wallets and dApps that build on TXTRACE must ensure their assertion logic covers all relevant state changes for the protected operation. It is critical that the ecosystem treats incomplete assertions as no better than no assertion at all.

The POST_TX frame mode requirement strengthens, but does not replace, this guarantee: it ensures a triggered assertion cleanly and unconditionally reverts the entire execution body, while the transaction stays valid and keeps the gas payment approved in the validation prefix, and that this cannot be partially bypassed via atomic-batch flags or frames ordered after the assertion. It does not, by itself, make any individual assertion more restrictive or correct.

Enforcing POST_TX Frame Inclusion

While POST_TX frames dictate how assertions are executed, there is no explicit protocol-level requirement that a transaction must include a POST_TX frame. Smart accounts aiming to use POST_TX frames must explicitly configure their VERIFY frame's validation logic to strictly require the presence of their specific POST_TX frame in every transaction they approve without any mechanist that could allow circumventing this requirement.

Because a transaction can contain multiple POST_TX frames, and they are guaranteed to form a contiguous suffix at the end of the transaction, the smart account can iterate backwards to locate its required assertion frame.

It is critical that the expected target of all POST_TX frames is an immutable, non-upgradeable contract. If the assertion contract were upgradeable, a malicious transaction could potentially upgrade it during execution to bypass the security checks. Only as long as the VERIFY frame executes before any state changes are made, checking the POST_TX` frame target during validation guarantees the transaction will be evaluated against the intended immutable policy.

This can be enforced as follows in Solidity (pseudocode):

function requirePostTx() internal view {
    bool found = false;
    for (uint256 i = tx.frames.length; i > 0; i--) {
        Frame memory f = tx.frames[i - 1];
        if (f.mode != 3) {
            break; // POST_TX frames are a contiguous suffix; no more left to check
        }
        if (f.target == this.postTxEnforcer) {
            // (Optionally check f.data matches the expected POST_TX method selector)
            found = true;
            break;
        }
    }
    require(found, "Missing required POST_TX frame for this account");
}

Assertion Inputs Controlled by the Transaction

The execution body runs before any POST_TX frame. Any state the body can write is therefore chosen by whoever built the execution frames at the moment the assertion reads it. An assertion that takes its reference values from such state can be steered to accept a harmful outcome, even though it reads the real post-execution state.

This applies to two sources of reference values:

• Calls from the POST_TX frame's call subtree to other contracts, such as a price oracle, a token's balanceOf, a registry, or a proxy. The execution body can move an oracle price, edit a registry entry, or upgrade a proxy before the assertion runs. A called contract can also detect that its caller is inside a POST_TX frame, for example by executing TXTRACE in a sub-call that halts in any other context, and answer the assertion differently from other callers.
• after values of accounts and slots that the transaction was able to modify, when they are used as a threshold or a price rather than as the outcome being checked. They report what the execution body left behind and carry no more trust than the body itself.

Assertions should take the reference values they compare the outcome against, such as minimum amounts received, permitted spenders, and prices, from data fixed at signing time, for example the POST_TX frame's data committed by the canonical signature hash, or from before values, which the transaction cannot modify. before values reflect the transaction prestate, so earlier transactions in the same block can still move them; a price read from the prestate keeps the usual oracle manipulation risk.

Assertion Gas Exhaustion

Assertion contracts that enumerate TXTRACE results may run out of gas. As stated previously, a transaction can produce up to ~42,600 events in a transaction in the current Ethereum configuration. Asserting over them will require a significant amount of gas in the worst-case.

Assertion contracts should defend against assertion gas related issues by reading the total entry counts and ensuring these are below a safe limit. The framework layer calling the assertion must forward a gas stipend proportional to the entry counts it expects to process.

Assertions concerned with specific contracts should use the per-address TXDIFF views instead of enumerating the global tables, making their gas cost independent of unrelated — and potentially attacker-controlled — entries.

An assertion that runs out of gas before completing its enumeration loop has not verified the full outcome. Any framework built on TXTRACE must ensure that assertion OOG is treated as an explicit assertion revert.

The POST_TX Frame Mode Not Reverting Validation Prefix

It is crucial that the transaction relying on a POST_TX frame to ensure the outcome validity does not contain untrusted execution in its validation prefix.

As it is not feasible to revert the entire transaction including the validation prefix without un-paying the block builder (which introduces a massive DoS vector), the validation prefix is NOT reverted by reverting POST_TX frames.

In a correctly constructed EIP-8141 transaction, the wallet software constructs the validation prefix, while untrusted dApp actions are placed strictly in the execution phase (SENDER frames). This is inherently safe as far as the wallet's own validation-prefix logic — including any deploy frame — is itself trustworthy. A SENDER frame placed before the frame that approves payment is part of that validation prefix, so a failed assertion does not revert it. Actions an assertion protects belong after the payment approval.

Therefore, committing the validation prefix on a POST_TX revert is not a security flaw for the user (provided they use standard wallet software), but it is a strict necessity for protecting the network from DoS attacks.

The deploy frame's restriction to installing tx.sender's code is a mempool-propagation rule (Structural Rules), not a protocol invariant — nothing prevents it from producing other side effects, which TXTRACE will surface but which POST_TX cannot undo. A wallet must never use the deploy frame for anything beyond deploying tx.sender; they cannot rely on POST_TX to catch or reverse it.

Copyright

Copyright and related rights waived via CC0.