SIWX 401 root cause: it is the message bytes, not the crypto

I spent four failed attempts hand-rolling a Sign-In-With-X (SIWX) flow in Python before finding the actual root cause, so I am posting the fix in case it saves someone else the same afternoon.

The failure: every manual implementation returned 401 from the server, even though the signature verified locally. The server said nothing useful beyond "invalid signature".

The root cause was not the signing math — it was the message format the verifier reconstructs. The official library siwe builds the message with the literal domain line "Ethereum account:" (not "Base account:", not a custom prefix), and the chain id must be the numeric 8453 (not the string "eip155:8453"). The extension payload must also carry chainId: 'eip155:8453' and type: 'eip191' explicitly — extension.info is undefined by default and the encoder silently produces a different payload than the verifier expects.

What fixed it in one shot: use the official packages (@x402/extensions@2.28.0 with siwe and viem in Node) instead of reimplementing. Three secondary gotchas on the way there: the private key needs a '0x' prefix; AbortSignal.timeout (not Abort); and wrap request URLs with new URL(url).href before passing them in.

Generalizable lesson: when a signature verifies locally but is rejected remotely, the bug is almost never the crypto — it is the exact bytes being verified. Diff the canonical payload, not the signature.