[nemo83]

Fixes #701.

Plutus 1.72.0.0 decodeTerm (UntypedPlutusCore/Core/Instance/Flat.hs, handleTerm 8 line 167 and handleTerm 9 line 181) rejects constr and case in a program below version 1.1.0. It reads the constr tag as a Word64 with dWord64 (PlutusCore/Flat/Decoder/Strict.hs line 165), which allows at most ten 7-bit chunks with a tenth chunk of 0 or 1. UPLC.ts checked neither, in either direction.

Changes

• decodeTerm and encodeTerm take the program version. Before reading or writing constr or case, they check it with Plutus's ordering (major, then minor, then patch) and fail with Plutus's message: 'constr' is not allowed before version 1.1.0, this program has version: 1.0.0.
• The constr tag is decoded with a new decodeWord64 that follows dWord64. The encoder refuses tags above 2^64-1. A negative tag was already refused by encodeNatural.
• decodeVersion and encodeVersion read and write each version component as a Flat natural, as Plutus's instance Flat Version does (PlutusCore/FlatInstances.hs), instead of one byte. Without this, the encoder's version check could be bypassed: 257.0.0 was truncated to 1.0.0. Versions with every component below 128 encode to the same bytes as before.
• The public API is unchanged. Only internal function signatures changed.
• Patch changeset for @evolution-sdk/evolution.

Tests

New tests in packages/evolution/test/UPLC.test.ts:

• Each of the conformance files constr/constr-06.flat, constr/constr-08.flat, constr/constr-09.flat and case/case-07.flat (bytes copied in) now fails to decode with a UPLCError. For constr-09, the error is the version one.
• Boundaries that still pass: constr/constr-01.flat, case/case-01.flat and constr/constr-07.flat (tag 2^64-1) decode and re-encode byte for byte. constr is accepted in a 2.0.0 program, and a tag spread over exactly ten chunks is accepted.
• Versions: 257.0.0 encodes as 810200008001 and decodes back, and (program 0.255.0 (constr 0)) (00ff01008001) is rejected with the version error.
• Also rejected: a tag spread over eleven chunks, constr nested under a lambda in a 1.0.0 program, and encoding constr/case at 1.0.0 or a tag of 2^64.

The new rejection tests fail on main and pass with the fix. With each guard reverted on its own (decode or encode version gate, Word64 decode, Word64 encode), at least one test fails.

Across all 908 .flat files in plutus-conformance/test-cases/uplc (Plutus 1.72.0.0), decode then re-encode changes outcome for exactly these four files, from accepted to rejected.

🤖 Generated with Claude Code

[solidsnakedev]

Thanks, this matches Plutus. The version gate, the Word64 tag and the version naturals all hold against Flat.hs:167-184, Strict.hs:165-184 and FlatInstances.hs:177-179 at 1.72.0.0. Every program in the repo re-encodes to the same bytes. Of the 908 conformance files, only constr-06, constr-08, constr-09 and case-07 change, and all four are now rejected, as their expected output says.

One small change: encodeProgram adds a second program.version as SemVer cast (UPLC.ts:1078). Taking the version as a string in encodeTerm and allowsConstrCase removes it.

A follow-up, not for this PR: a var index is still read as an unbounded natural, while Plutus reads it as a Word64 (FlatInstances.hs:363), so (program 1.1.0 (lam (var 2^64))) decodes here and fails in Plutus.

It merges cleanly with #667, so I'll merge it right after that one.

[solidsnakedev]

Merged, thanks. The new program.version cast is removed in #720.