Status Codes

StagnantStandards Track: ERC
Created: 2018-05-05
Brooklyn Zelenka (@expede), Tom Carchrae (@carchrae), Gleb Naumenko (@naumenkogs)
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1 min read

ERC-1066 is a proposed standard for status codes in smart contracts on the Ethereum blockchain. It aims to provide a common set of signals that can be used to allow smart contracts to react to situations autonomously, expose localized error messages to users, and more. The standard is similar to HTTP status codes and is intended to make it easier for developers to understand which of the finite states their request produced. The standard includes a format for the codes, a code table, and example function changes and sequence diagrams. The motivation behind the standard is to provide semantic density, familiarity, and human readability to smart contract status codes. The standard is also extensible, allowing for application-specific statuses and the use of larger byte arrays if 256 codes become insufficient. The proposal also includes the use of empty space and the ability to relay information beyond simply pass or fail. Overall, ERC-1066 aims to make smart contract development easier and more efficient by providing a standard set of known codes.

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Simple Summary

Broadly applicable status codes for smart contracts.


This standard outlines a common set of status codes in a similar vein to HTTP statuses. This provides a shared set of signals to allow smart contracts to react to situations autonomously, expose localized error messages to users, and so on.

The current state of the art is to either revert on anything other than a clear success (ie: require human intervention), or return a low-context true or false. Status codes are similar-but-orthogonal to reverting with a reason, but aimed at automation, debugging, and end-user feedback (including translation). They are fully compatible with both revert and revert-with-reason.

As is the case with HTTP, having a standard set of known codes has many benefits for developers. They remove friction from needing to develop your own schemes for every contract, makes inter-contract automation easier, and makes it easier to broadly understand which of the finite states your request produced. Importantly, it makes it much easier to distinguish between expected errors states, truly exceptional conditions that require halting execution, normal state transitions, and various success cases.


Semantic Density

HTTP status codes are widely used for this purpose. BEAM languages use atoms and tagged tuples to signify much the same information. Both provide a lot of information both to the programmer (debugging for instance), and to the program that needs to decide what to do next.

Status codes convey a much richer set of information than Booleans, and are able to be reacted to autonomously unlike arbitrary strings.

User Experience (UX)

End users get little to no feedback, and there is no translation layer.

Since ERC1066 status codes are finite and known in advance, we can leverage ERC-1444 to provide global, human-readable sets of status messages. These may also be translated into any language, differing levels of technical detail, added as revert messages, natspecs, and so on.

Status codes convey a much richer set of information than Booleans, and are able to be reacted to autonomously unlike arbitrary strings.

Developer Experience (DX)

Developers currently have very little context exposed by their smart contracts.

At time of writing, other than stepping through EVM execution and inspecting memory dumps directly, it is very difficult to understand what is happening during smart contract execution. By returning more context, developers can write well-decomposed tests and assert certain codes are returned as an expression of where the smart contract got to. This includes status codes as bare values, events, and reverts.

Having a fixed set of codes also makes it possible to write common helper functions to react in common ways to certain signals. This can live off- or on-chain library, lowering the overhead in building smart contracts, and helping raise code quality with trusted shared components.

We also see a desire for this in transactions, and there's no reason that these status codes couldn't be used by the EVM itself.

Smart Contract Autonomy

Smart contracts don’t know much about the result of a request beyond pass/fail; they can be smarter with more context.

Smart contracts are largely intended to be autonomous. While each contract may define a specific interface, having a common set of semantic codes can help developers write code that can react appropriately to various situations.

While clearly related, status codes are complementary to revert-with-reason. Status codes are not limited to rolling back the transaction, and may represent known error states without halting execution. They may also represent off-chain conditions, supply a string to revert, signal time delays, and more.

All of this enables contracts to share a common vocabulary of state transitions, results, and internal changes, without having to deeply understand custom status enums or the internal business logic of collaborator contracts.



Codes are returned either on their own, or as the first value of a multiple return.

// Status only function isInt(uint num) public pure returns (byte status) { return hex"01"; } // Status and value uint8 private counter; function safeIncrement(uint8 interval) public returns (byte status, uint8 newCounter) { uint8 updated = counter + interval; if (updated >= counter) { counter = updated; return (hex"01", updated); } else { return (hex"00", counter); } }

Code Table

Codes break nicely into a 16x16 matrix, represented as a 2-digit hex number. The high nibble represents the code's kind or "category", and the low nibble contains the state or "reason". We present them below as separate tables per range for explanatory and layout reasons.

NB: Unspecified codes are not free for arbitrary use, but rather open for further specification.

0x0* Generic

General codes. These double as bare "reasons", since 0x01 == 1.

0x02Awaiting Others
0x04Lower Limit or Insufficient
0x05Receiver Action Requested
0x06Upper Limit
0x08Duplicate, Unnecessary, or Inapplicable
0x0FInformational or Metadata

0x1* Permission & Control

Also used for common state machine actions (ex. "stoplight" actions).

0x10Disallowed or Stop
0x11Allowed or Go
0x12Awaiting Other's Permission
0x13Permission Requested
0x14Too Open / Insecure
0x15Needs Your Permission or Request for Continuation
0x16Revoked or Banned
0x18Not Applicable to Current State
0x1FPermission Details or Control Conditions

0x2* Find, Inequalities & Range

This range is broadly intended for finding and matching. Data lookups and order matching are two common use cases.

0x20Not Found, Unequal, or Out of Range
0x21Found, Equal or In Range
0x22Awaiting Match
0x23Match Request Sent
0x24Below Range or Underflow
0x25Request for Match
0x26Above Range or Overflow
0x28Duplicate, Conflict, or Collision
0x2FMatching Meta or Info

0x3* Negotiation & Governance

Negotiation, and very broadly the flow of such transactions. Note that "other party" may be more than one actor (not necessarily the sender).

0x30Sender Disagrees or Nay
0x31Sender Agrees or Yea
0x32Awaiting Ratification
0x33Offer Sent or Voted
0x34Quorum Not Reached
0x35Receiver's Ratification Requested
0x36Offer or Vote Limit Reached
0x38Already Voted
0x3FNegotiation Rules or Participation Info

0x4* Availability & Time

Service or action availability.

0x44Not Available Yet
0x45Awaiting Your Availability
0x48Already Done
0x4FAvailability Rules or Info (ex. time since or until)

0x5* Tokens, Funds & Finance

Special token and financial concepts. Many related concepts are included in other ranges.

0x50Transfer Failed
0x51Transfer Successful
0x52Awaiting Payment From Others
0x53Hold or Escrow
0x54Insufficient Funds
0x55Funds Requested
0x56Transfer Volume Exceeded
0x58Funds Not Required
0x5FToken or Financial Information

0x6* TBD

Currently unspecified. (Full range reserved)

0x7* TBD

Currently unspecifie. (Full range reserved)

0x8* TBD

Currently unspecified. (Full range reserved)

0x9* TBD

Currently unspecified. (Full range reserved)

0xA* Application-Specific Codes

Contracts may have special states that they need to signal. This proposal only outlines the broadest meanings, but implementers may have very specific meanings for each, as long as they are coherent with the broader definition.

0xA0App-Specific Failure
0xA1App-Specific Success
0xA2App-Specific Awaiting Others
0xA3App-Specific Acceptance
0xA4App-Specific Below Condition
0xA5App-Specific Receiver Action Requested
0xA6App-Specific Expiry or Limit
0xA8App-Specific Inapplicable Condition
0xAFApp-Specific Meta or Info

0xB* TBD

Currently unspecified. (Full range reserved)

0xC* TBD

Currently unspecified. (Full range reserved)

0xD* TBD

Currently unspecified. (Full range reserved)

0xE* Encryption, Identity & Proofs

Actions around signatures, cryptography, signing, and application-level authentication.

The meta code 0xEF is often used to signal a payload describing the algorithm or process used.

0xE0Decrypt Failure
0xE1Decrypt Success
0xE2Awaiting Other Signatures or Keys
0xE4Unsigned or Untrusted
0xE5Signature Required
0xE6Known to be Compromised
0xE8Already Signed or Not Encrypted
0xEFCryptography, ID, or Proof Metadata

0xF* Off-Chain

For off-chain actions. Much like th 0x0*: Generic range, 0xF* is very general, and does little to modify the reason.

Among other things, the meta code 0xFF may be used to describe what the off-chain process is.

0xF0Off-Chain Failure
0xF1Off-Chain Success
0xF2Awaiting Off-Chain Process
0xF3Off-Chain Process Started
0xF4Off-Chain Service Unreachable
0xF5Off-Chain Action Required
0xF6Off-Chain Expiry or Limit Reached
0xF8Duplicate Off-Chain Request
0xFFOff-Chain Info or Meta

As a Grid

0x0* General0x1* Permission & Control0x2* Find, Inequalities & Range0x3* Negotiation & Governance0x4* Availability & Time0x5* Tokens, Funds & Finance0x6* TBD0x7* TBD0x8* TBD0x9* TBD0xA* Application-Specific Codes0xB* TBD0xC* TBD0xD* TBD0xE* Encryption, Identity & Proofs0xF* Off-Chain
0x*00x00 Failure0x10 Disallowed or Stop0x20 Not Found, Unequal, or Out of Range0x30 Sender Disagrees or Nay0x40 Unavailable0x50 Transfer Failed0x60 [reserved]0x70 [reserved]0x80 [reserved]0x90 [reserved]0xA0 App-Specific Failure0xB0 [reserved]0xC0 [reserved]0xD0 [reserved]0xE0 Decrypt Failure0xF0 Off-Chain Failure
0x*10x01 Success0x11 Allowed or Go0x21 Found, Equal or In Range0x31 Sender Agrees or Yea0x41 Available0x51 Transfer Successful0x61 [reserved]0x71 [reserved]0x81 [reserved]0x91 [reserved]0xA1 App-Specific Success0xB1 [reserved]0xC1 [reserved]0xD1 [reserved]0xE1 Decrypt Success0xF1 Off-Chain Success
0x*20x02 Awaiting Others0x12 Awaiting Other's Permission0x22 Awaiting Match0x32 Awaiting Ratification0x42 Paused0x52 Awaiting Payment From Others0x62 [reserved]0x72 [reserved]0x82 [reserved]0x92 [reserved]0xA2 App-Specific Awaiting Others0xB2 [reserved]0xC2 [reserved]0xD2 [reserved]0xE2 Awaiting Other Signatures or Keys0xF2 Awaiting Off-Chain Process
0x*30x03 Accepted0x13 Permission Requested0x23 Match Request Sent0x33 Offer Sent or Voted0x43 Queued0x53 Hold or Escrow0x63 [reserved]0x73 [reserved]0x83 [reserved]0x93 [reserved]0xA3 App-Specific Acceptance0xB3 [reserved]0xC3 [reserved]0xD3 [reserved]0xE3 Signed0xF3 Off-Chain Process Started
0x*40x04 Lower Limit or Insufficient0x14 Too Open / Insecure0x24 Below Range or Underflow0x34 Quorum Not Reached0x44 Not Available Yet0x54 Insufficient Funds0x64 [reserved]0x74 [reserved]0x84 [reserved]0x94 [reserved]0xA4 App-Specific Below Condition0xB4 [reserved]0xC4 [reserved]0xD4 [reserved]0xE4 Unsigned or Untrusted0xF4 Off-Chain Service Unreachable
0x*50x05 Receiver Action Required0x15 Needs Your Permission or Request for Continuation0x25 Request for Match0x35 Receiver's Ratification Requested0x45 Awaiting Your Availability0x55 Funds Requested0x65 [reserved]0x75 [reserved]0x85 [reserved]0x95 [reserved]0xA5 App-Specific Receiver Action Requested0xB5 [reserved]0xC5 [reserved]0xD5 [reserved]0xE5 Signature Required0xF5 Off-Chain Action Required
0x*60x06 Upper Limit0x16 Revoked or Banned0x26 Above Range or Overflow0x36 Offer or Vote Limit Reached0x46 Expired0x56 Transfer Volume Exceeded0x66 [reserved]0x76 [reserved]0x86 [reserved]0x96 [reserved]0xA6 App-Specific Expiry or Limit0xB6 [reserved]0xC6 [reserved]0xD6 [reserved]0xE6 Known to be Compromised0xF6 Off-Chain Expiry or Limit Reached
0x*70x07 [reserved]0x17 [reserved]0x27 [reserved]0x37 [reserved]0x47 [reserved]0x57 [reserved]0x67 [reserved]0x77 [reserved]0x87 [reserved]0x97 [reserved]0xA7 [reserved]0xB7 [reserved]0xC7 [reserved]0xD7 [reserved]0xE7 [reserved]0xF7 [reserved]
0x*80x08 Duplicate, Unnecessary, or Inapplicable0x18 Not Applicable to Current State0x28 Duplicate, Conflict, or Collision0x38 Already Voted0x48 Already Done0x58 Funds Not Required0x68 [reserved]0x78 [reserved]0x88 [reserved]0x98 [reserved]0xA8 App-Specific Inapplicable Condition0xB8 [reserved]0xC8 [reserved]0xD8 [reserved]0xE8 Already Signed or Not Encrypted0xF8 Duplicate Off-Chain Request
0x*90x09 [reserved]0x19 [reserved]0x29 [reserved]0x39 [reserved]0x49 [reserved]0x59 [reserved]0x69 [reserved]0x79 [reserved]0x89 [reserved]0x99 [reserved]0xA9 [reserved]0xB9 [reserved]0xC9 [reserved]0xD9 [reserved]0xE9 [reserved]0xF9 [reserved]
0x*A0x0A [reserved]0x1A [reserved]0x2A [reserved]0x3A [reserved]0x4A [reserved]0x5A [reserved]0x6A [reserved]0x7A [reserved]0x8A [reserved]0x9A [reserved]0xAA [reserved]0xBA [reserved]0xCA [reserved]0xDA [reserved]0xEA [reserved]0xFA [reserved]
0x*B0x0B [reserved]0x1B [reserved]0x2B [reserved]0x3B [reserved]0x4B [reserved]0x5B [reserved]0x6B [reserved]0x7B [reserved]0x8B [reserved]0x9B [reserved]0xAB [reserved]0xBB [reserved]0xCB [reserved]0xDB [reserved]0xEB [reserved]0xFB [reserved]
0x*C0x0C [reserved]0x1C [reserved]0x2C [reserved]0x3C [reserved]0x4C [reserved]0x5C [reserved]0x6C [reserved]0x7C [reserved]0x8C [reserved]0x9C [reserved]0xAC [reserved]0xBC [reserved]0xCC [reserved]0xDC [reserved]0xEC [reserved]0xFC [reserved]
0x*D0x0D [reserved]0x1D [reserved]0x2D [reserved]0x3D [reserved]0x4D [reserved]0x5D [reserved]0x6D [reserved]0x7D [reserved]0x8D [reserved]0x9D [reserved]0xAD [reserved]0xBD [reserved]0xCD [reserved]0xDD [reserved]0xED [reserved]0xFD [reserved]
0x*E0x0E [reserved]0x1E [reserved]0x2E [reserved]0x3E [reserved]0x4E [reserved]0x5E [reserved]0x6E [reserved]0x7E [reserved]0x8E [reserved]0x9E [reserved]0xAE [reserved]0xBE [reserved]0xCE [reserved]0xDE [reserved]0xEE [reserved]0xFE [reserved]
0x*F0x0F Informational or Metadata0x1F Permission Details or Control Conditions0x2F Matching Meta or Info0x3F Negotiation Rules or Participation Info0x4F Availability Rules or Info (ex. time since or until)0x5F Token or Financial Information0x6F [reserved]0x7F [reserved]0x8F [reserved]0x9F [reserved]0xAF App-Specific Meta or Info0xBF [reserved]0xCF [reserved]0xDF [reserved]0xEF Cryptography, ID, or Proof Metadata0xFF Off-Chain Info or Meta

Example Function Change

uint256 private startTime; mapping(address => uint) private counters; // Before function increase() public returns (bool _available) { if (now < startTime && counters[msg.sender] == 0) { return false; }; counters[msg.sender] += 1; return true; } // After function increase() public returns (byte _status) { if (now < start) { return hex"44"; } // Not yet available if (counters[msg.sender] == 0) { return hex"10"; } // Not authorized counters[msg.sender] += 1; return hex"01"; // Success }

Example Sequence Diagrams

0x03 = Waiting
0x31 = Other Party (ie: not you) Agreed
0x41 = Available
0x44 = Not Yet Available


AwesomeCoin                 DEX                     TraderBot
     +                       +                          +
     |                       |       buy(AwesomeCoin)   |
     |                       | <------------------------+
     |         buy()         |                          |
     | <---------------------+                          |
     |                       |                          |
     |     Status [0x44]     |                          |
     +---------------------> |       Status [0x44]      |
     |                       +------------------------> |
     |                       |                          |
     |                       |        isDoneYet()       |
     |                       | <------------------------+
     |                       |                          |
     |                       |       Status [0x44]      |
     |                       +------------------------> |
     |                       |                          |
     |                       |                          |
     |     Status [0x41]     |                          |
     +---------------------> |                          |
     |                       |                          |
     |       buy()           |                          |
     | <---------------------+                          |
     |                       |                          |
     |                       |                          |
     |     Status [0x31]     |                          |
     +---------------------> |      Status [0x31]       |
     |                       +------------------------> |
     |                       |                          |
     |                       |                          |
     |                       |                          |
     |                       |                          |
     +                       +                          +
0x01 = Generic Success
0x10 = Disallowed
0x11 = Allowed

                                              Token Validation

           Buyer                  RegulatedToken           TokenValidator               IDChecker          SpendLimiter
             +                          +                         +                         +                   +
             |        buy()             |                         |                         |                   |
             +------------------------> |          check()        |                         |                   |
             |                          +-----------------------> |          check()        |                   |
             |                          |                         +-----------------------> |                   |
             |                          |                         |                         |                   |
             |                          |                         |         Status [0x10]   |                   |
             |                          |       Status [0x10]     | <-----------------------+                   |
             |        revert()          | <-----------------------+                         |                   |
             | <------------------------+                         |                         |                   |
             |                          |                         |                         |                   |
+---------------------------+           |                         |                         |                   |
|                           |           |                         |                         |                   |
| Updates ID with provider  |           |                         |                         |                   |
|                           |           |                         |                         |                   |
+---------------------------+           |                         |                         |                   |
             |                          |                         |                         |                   |
             |         buy()            |                         |                         |                   |
             +------------------------> |        check()          |                         |                   |
             |                          +-----------------------> |         check()         |                   |
             |                          |                         +-----------------------> |                   |
             |                          |                         |                         |                   |
             |                          |                         |       Status [0x11]     |                   |
             |                          |                         | <-----------------------+                   |
             |                          |                         |                         |                   |
             |                          |                         |                         |   check()         |
             |                          |                         +-------------------------------------------> |
             |                          |                         |                         |                   |
             |                          |                         |                         |  Status [0x11]    |
             |                          |       Status [0x11]     | <-------------------------------------------+
             |        Status [0x01]     | <-----------------------+                         |                   |
             | <------------------------+                         |                         |                   |
             |                          |                         |                         |                   |
             |                          |                         |                         |                   |
             |                          |                         |                         |                   |
             +                          +                         +                         +                   +



Status codes are encoded as a byte. Hex values break nicely into high and low nibbles: category and reason. For instance, 0x01 stands for general success (ie: true) and 0x00 for general failure (ie: false).

As a general approach, all even numbers are blocking conditions (where the receiver does not have control), and odd numbers are nonblocking (the receiver is free to continue as they wish). This aligns both a simple bit check with the common encoding of Booleans.

bytes1 is very lightweight, portable, easily interoperable with uint8, cast from enums, and so on.


Alternate schemes include bytes32 and uint8. While these work reasonably well, they have drawbacks.

uint8 feels even more similar to HTTP status codes, and enums don't require as much casting. However does not break as evenly as a square table (256 doesn't look as nice in base 10).

Packing multiple codes into a single bytes32 is nice in theory, but poses additional challenges. Unused space may be interpreted as 0x00 Failure, you can only efficiently pack four codes at once, and there is a challenge in ensuring that code combinations are sensible. Forcing four codes into a packed representation encourages multiple status codes to be returned, which is often more information than strictly necessarily. This can lead to paradoxical results (ex 0x00 and 0x01 together), or greater resources allocated to interpreting 2564 (4.3 billion) permutations.

Multiple Returns

While there may be cases where packing a byte array of status codes may make sense, the simplest, most forwards-compatible method of transmission is as the first value of a multiple return.

Familiarity is also a motivating factor. A consistent position and encoding together follow the principle of least surprise. It is both viewable as a "header" in the HTTP analogy, or like the "tag" in BEAM tagged tuples.

Human Readable

Developers should not be required to memorize 256 codes. However, they break nicely into a table. Cognitive load is lowered by organizing the table into categories and reasons. 0x10 and 0x11 belong to the same category, and 0x04 shares a reason with 0x24

While this repository includes helper enums, we have found working directly in the hex values to be quite natural. Status code 0x10 is just as comfortable as HTTP 401, for example.


One commonly requested application of this spec is human-readable translations of codes. This has been moved to its own proposal: ERC-1444, primarily due to a desire to keep both specs focused.


The 0xA category is reserved for application-specific statuses. In the case that 256 codes become insufficient, bytes1 may be embedded in larger byte arrays.

EVM Codes

The EVM also returns a status code in transactions; specifically 0x00 and 0x01. This proposal both matches the meanings of those two codes, and could later be used at the EVM level.

Empty Space

Much like how HTTP status codes have large unused ranges, there are totally empty sections in this proposal. The intent is to not impose a complete set of codes up front, and to allow users to suggest uses for these spaces as time progresses.

Beyond Errors

This spec is intended to be much more than a set of common errors. One design goal is to enable easier contract-to-contract communication, protocols built on top of status codes, and flows that cross off-chain. Many of these cases include either expected kinds of exception state (as opposed to true errors), neutral states, time logic, and various successes.

Just like how HTTP 200 has a different meaning from HTTP 201, ERC-1066 status codes can relay information between contract beyond simply pass or fail. They can be thought of as the edges in a graph that has smart contracts as nodes.

Fully revertable

This spec is fully compatible with revert-with-reason and does not intend to supplant it in any way. Both by reverting with a common code, the developer can determine what went wrong from a set of known error states.

Further, by leveraging ERC-1066 and a translation table (such as in ERC-1444) in conjunction, developers and end users alike can receive fully automated human-readable error messages in the language and phrasing of their choice.

Nibble Order

Nibble order makes no difference to the machine, and is purely mnemonic. This design was originally in opposite order, but changed it for a few convenience factors. Since it's a different scheme from HTTP, it may feel strange initially, but becomes very natural after a couple hours of use.

Short Forms

Generic is 0x0*, general codes are consistent with their integer representations

hex"1" == hex"01" == 1 // with casting

Contract Categories

Many applications will always be part of the same category. For instance, validation will generally be in the 0x10 range.

contract Whitelist { mapping(address => bool) private whitelist; uint256 private deadline; byte constant private prefix = hex"10"; check(address _, address _user) returns (byte _status) { if (now >= deadline) { return prefix | 5; } if (whitelist[_user]) { return prefix | 1; } return prefix; } }


This above also means that working with app-specific enums is slightly easier, and also saves gas (fewer operations required).

enum Sleep { Awake, Asleep, BedOccupied, WindingDown } // From the helper library function appCode(Sleep _state) returns (byte code) { return byte(160 + _state); // 160 = 0xA0 } // Versus function appCode(Sleep _state) returns (byte code) { return byte((16 * _state) + 10); // 10 = 0xA }


Reference cases and helper libraries (Solidity and JS) can be found at:

Copyright and related rights waived via CC0.

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