If a service expects an EIP-712 signature as 0x followed by 130 hexadecimal characters, a dependency update can make the same bytes fail validation by changing how they are rendered as text. In the reported HexBytes tests, HexBytes.hex() included the prefix in version 0.3.1 but returned bare hexadecimal in later versions. Check the installed version and normalize the output at the point it leaves your code.
Why a valid signature can fail string validation
A signature has both underlying bytes and a textual representation. A receiving service may validate that representation, not merely whether the bytes encode a valid signature. If it requires a string beginning with 0x, a bare hex string can be rejected even when it represents the expected bytes.
For eth_signTypedData, the EIP-712 specification describes the result as a hex-encoded 65-byte signature beginning with 0x. With two hexadecimal characters per byte, that representation is 132 characters long: the prefix plus 130 hex characters. EIP-712 specifies the signature representation; it does not specify how Python’s HexBytes library formats the result of .hex(). EIP-712
The pattern re.fullmatch(r"0x[0-9a-fA-F]{130}", value) checks that particular shape. It is an example boundary contract, not a universal signature rule: use the format required by the service receiving your value.
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What the reported HexBytes version tests found
The DEV Community article by minia2a reports the following results for a 65-byte input. These are the author’s reported tests, not independently reproduced results here.
| HexBytes version | Reported .hex() output |
Reported length | to_0x_hex() |
|---|---|---|---|
| 0.3.1 | Begins with 0x |
132 characters | Not available, according to the author |
| 1.0.0 and 1.1.0 | Bare hexadecimal, no 0x |
130 characters | Not available, according to the author |
| 1.2.0 and 1.3.1 | Bare hexadecimal, no 0x |
130 characters | Available, according to the author |
| 2.0.0 | Not stated; the author inspected source rather than running a test | Not stated | Not stated |
The author attributes the change to HexBytes 0.3.x overriding .hex() to include the prefix, with that override removed in 1.0.0. Treat this as the article author’s account of the tested versions rather than a guarantee for every release or environment. The author put the compatibility lesson this way: “Any fix that requires knowing the version is a fix that will be wrong on the machine you didn’t test.” minia2a’s HexBytes version report
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Normalize the prefix without creating a double prefix
The tempting patch "0x" + h.hex() works only if .hex() returns bare hexadecimal. If it already returns 0x…, concatenation produces 0x0x…, which is overlong and fails the example validator.
Instead, add the prefix only when it is missing:
sig = h.hex()
sig = sig if sig.startswith("0x") else "0x" + sigThis prefix check avoids relying on to_0x_hex() being present. The article reports that accessor in HexBytes 1.2.0 and 1.3.1, but not in the earlier versions it tested.
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Validate the value at the service boundary
Normalize the value, then check that it matches the receiving service’s documented format before sending it. For the specific 65-byte EIP-712 representation shown above, Python can check the complete string with:
import re
assert re.fullmatch(r"0x[0-9a-fA-F]{130}", sig)This catches a missing prefix, an accidental duplicate prefix, a wrong length, or non-hexadecimal characters. If the service documents a different representation, adapt the assertion to that contract rather than assuming every API accepts the same signature shape.
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Place the check where your application serializes or sends the value. That tests the actual outgoing string and can reveal a dependency change before the receiving service rejects the request. The cited article offers this as a test pattern, not as evidence of a production deployment.
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