⚠️ Draft Standards Track: Core

EIP-7906: Transaction Assertions via State Diff Opcode

An opcode that provides a mechanism to restrict the outcomes of transaction execution

Authors	Alex Forshtat (@forshtat), Shahaf Nacson (@shahafn), Dror Tirosh (@drortirosh), Yoav Weiss (@yoavw), Fredrik Svantes (@fredrik0x)
Created	2025-02-21
Discussion Link	https://ethereum-magicians.org/t/eip-restricted-behavior-transaction-type/23130
Requires	EIP-2929, EIP-8141

Abstract

This proposal introduces a new opcode that allows contracts to inspect the transaction outcomes on-chain. This opcode will allow contract developers to define assertions for state changes that can be enforced on-chain. These can protect Ethereum users by restricting the behavior of the smart contracts they are interacting with.

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

Constants

Name	Value
TXTRACE_GAS_COST	TBD
EVENTDATACOPY_GAS_COST	TBD
POST_TX	3

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:

• The static constraint assert frame.mode < 3 on each frame is replaced by assert frame.mode < 4, admitting POST_TX as a valid mode value.
• 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. A POST_TX frame has no valid reason to call APPROVE, and its usage is forbidden.
• If a POST_TX frame reverts, the entire execution body of a transaction is reverted unconditionally. This overrides the atomic-batch unrolling behavior that would otherwise apply: a POST_TX revert always validates or reverts the whole transaction execution, up to the “validation prefix”, rather than merely unwinding an atomic batch.
• A POST_TX revert does not invalidate the transaction (unlike a VERIFY frame revert). The transaction remains valid, is included in the block, and generates a receipt with a failed status (status = 0).
• 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 revert.
• 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.

It can be used to retrieve the full state diff of the current transaction up to this point.

It accepts a (param, index) inputs similar to the FRAMEPARAM opcode from EIP-8141. The available parameters are listed in the table below.

param	in2	Return value
0x00	must be 0	balances_changed - the total number of changed balances
0x01	must be 0	slots_changed - the total number of changes storage slots
0x02	must be 0	contracts_deployed - the total number of newly deployed contracts
0x03	index in balances_changed	change_address - the address of the account with balance change
0x04	index in balances_changed	balance_before - the balance of the address at the start of the transaction
0x05	index in balances_changed	balance_after - the balance of the address as of this TXTRACE call
0x06	index in slots_changed	change_address - the address of the account with storage change
0x07	index in slots_changed	slot_key - the storage slot key that was changed
0x08	index in slots_changed	slot_value_before - the value of the slot at the start of the transaction
0x09	index in slots_changed	slot_value_after - the value of the slot as of this TXTRACE call
0x0A	index in contracts_deployed	deployed_address - the address of the newly deployed contract
0x0B	index in contracts_deployed	codehash_after - the codehash of the newly deployed contract
0x0C	must be 0	events_count - the total number of emitted events
0x0D	index in events_count	events_address - the address of the contract that emitted the event
0x0E	index in events_count	event_topic_count - the number of topics of the event (0–4)
0x0F	index in events_count	event_topic0 - the first topic of the event; exceptional halt if no topic
0x10	index in events_count	event_topic1 - the second topic of the event
0x11	index in events_count	event_topic2 - the third topic of the event
0x12	index in events_count	event_topic3 - the fourth topic of the event
0x13	index in events_count	event_data_len - the byte length of the event’s non-indexed data
0x14	must be 0	gas_pre_charge - the total amount deducted from the gas payer
0x15	must be 0	gas_payer_address - the address charged the gas pre-charge

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.

For transactions with blobs attached, the gas_pre_charge parameter includes the blob fees as gas_pre_charge = gas_limit × gas_price + blob_count × GAS_PER_BLOB × 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.

Transaction Diff Lookup Opcode

While TXTRACE enumerates the full state diff, it has no mechanism to directly query the diff for one specific account’s balance, codehash, or storage slot.

We introduce an additional TXDIFF opcode for this purpose, complementing TXTRACE’s enumeration model with direct, keyed access.

Params

param	in2	in3	Return value
0x00	address	slot_key value	slot_value_before
0x01	address	slot_key value	slot_value_after
0x02	address	must be 0	balance_before
0x03	address	must be 0	balance_after
0x04	address	must be 0	codehash_before
0x05	address	must be 0	codehash_after
0x06	address	must be 0	address_slots_count
0x07	address	index in address_slots_count	TXTRACE index for slots_changed
0x08	address	must be 0	address_events_count
0x09	address	index in address_events_count	TXTRACE index for events_count
0x0A	address	must be 0	account_change_flags

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.

Per-Address Remapping

Params queryalbe 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 recevied an invalid local index, i.e. 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):

Binary	Set when
0b0001	the account nonce differs from its transaction prestate value
0b0010	balance_after != balance_before
0b0100	any storage slot of the account differs from its prestate value (address_slots_count > 0)
0b1000	codehash_after != codehash_before

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 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:

• Storage slot params (0x00, 0x01): COLD_SLOAD_COST (2100) if the (address, slot) pair is not in the accessed storage list; WARM_STORAGE_READ_COST (100) otherwise.
• Balance and codehash params (0x02–0x05): COLD_ACCOUNT_ACCESS_COST (2600) if the address is not in the accessed addresses set; WARM_STORAGE_READ_COST (100) otherwise.
• Per-address view and flags params (0x06–0x0A): a flat cost of TXTRACE_GAS_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 respective access list after the call. Params 0x06–0x0A do not interact with the EIP-2929 access lists.

codehash_before is equal to the empty-code hash for undeployed contracts.

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.

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

EVENTDATACOPY opcode

This opcode copies event 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

Stack	Value
top - 0	event_index
top - 1	memOffset
top - 2	dataOffset
top - 3	length

No stack output value is produced.

Behavior

The operation semantics match CALLDATACOPY, copying length bytes from the event’s non-indexed data, starting at the given byte dataOffset, into a memory region starting at memOffset.

• If event_index >= events_count, an exceptional halt occurs.
• If dataOffset + length exceeds the event’s data length, an exceptional halt occurs.

Rationale

Selection Parameter Design

The TXTRACE opcode follows the same (param, index) two-argument pattern used by FRAMEPARAM in EIP-8141. 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.

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.

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 generates a standard 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.

Gas Pre-Charge Parameter

The gas pre-charge (gas_limit × gas_price) is deducted at transaction start 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 invalidates the entire transaction, including any gas payment already approved by an earlier frame, 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.

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). Because the wallet controls the validation prefix (e.g., ensuring the deploy frame points to a trusted factory), it is inherently safe to commit it to state. Malicious dApps cannot inject arbitrary state changes into the validation prefix.

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.

Copyright

Copyright and related rights waived via CC0.

Citation

Please cite this document as:

Alex Forshtat (@forshtat), Shahaf Nacson (@shahafn), Dror Tirosh (@drortirosh), Yoav Weiss (@yoavw), Fredrik Svantes (@fredrik0x), "EIP-7906: Transaction Assertions via State Diff Opcode [DRAFT]," Ethereum Improvement Proposals, no. 7906, February 2025. Available: https://eips.ethereum.org/EIPS/eip-7906.