`uv` plugin produces architecture-mislabeled charm on cross-build (`build-on: amd64`, `build-for: arm64|ppc64el|s390x`)
Metadata
Current evaluation
uv plugin cross-build (build-on amd64, build-for arm64/ppc64el/s390x) silently packs a charm whose venv contains host-arch compiled wheels while manifest declares target arch. No maintainer response, no labels, 0 comments, 114 days old.
Suggested action: needs triage
Reason: No labels, no comments, no maintainer acknowledgement in 114 days, so it has not been triaged. The report is a well-evidenced real bug (x86-64 .so files inside an arm64-tagged charm), and the uv plugin code in craft-parts (uv sync with no --python-platform) confirms the mechanism, but the fix requires an architectural decision (fail-fast vs cross-resolve) and possibly a craft-parts change, so it needs maintainer triage rather than auto-closing.
Impact:
75
Quick Win:
22.5
Staleness:
55
Complexity:
70
Confidence:
70
Support Request:
5
Issue body
## Summary
`charmcraft pack` with the `uv` plugin and a cross-arch `platforms:` entry (host arch `!=` target arch) succeeds without error and emits a `.charm` whose `manifest.yaml` declares the target architecture, but whose Python venv contains compiled wheels (`.so` files) built for the **host** architecture. The artifact is silently corrupt and will fail at import on a host of the declared architecture.
## Environment
- charmcraft: 4.2.1 (snap, `latest/candidate`)
- LXD: 5.21.4-aee7e08 (rev 38767, `5.21/stable`)
- host arch: `x86_64`
## Reproduction
`charmcraft.yaml`:
```yaml
type: charm
platforms:
ubuntu-24.04-amd64:
build-on: [ubuntu@24.04:amd64]
build-for: [ubuntu@24.04:amd64]
ubuntu-24.04-arm64:
build-on: [ubuntu@24.04:amd64]
build-for: [ubuntu@24.04:arm64]
parts:
charm:
plugin: uv
source: .
build-snaps: [astral-uv]
build-packages: [git]
```
`pyproject.toml` pulls in any wheel-bearing dependency, e.g. `cryptography`, `pydantic`, `pyyaml`.
```
$ charmcraft pack --platform ubuntu-24.04-arm64 -v
...
Packed microceph_ubuntu-24.04-arm64.charm
```
No warning. No error.
## Observed
```
$ unzip -p microceph_ubuntu-24.04-arm64.charm manifest.yaml | head -7
charmcraft-version: 4.2.1
charmcraft-started-at: '2026-05-06T18:48:24.634836'
bases:
- name: ubuntu
channel: '24.04'
architectures:
- arm64
```
```
$ unzip -l microceph_ubuntu-24.04-arm64.charm | grep -E '\.so'
... venv/lib/python3.12/site-packages/cryptography/hazmat/bindings/_rust.abi3.so
... venv/lib/python3.12/site-packages/pydantic_core/_pydantic_core.cpython-312-x86_64-linux-gnu.so
... venv/lib/python3.12/site-packages/yaml/_yaml.cpython-312-x86_64-linux-gnu.so
... (etc.)
```
```
$ unzip -p microceph_ubuntu-24.04-arm64.charm \
venv/lib/python3.12/site-packages/cryptography/hazmat/bindings/_rust.abi3.so > /tmp/r.so
$ file /tmp/r.so
/tmp/r.so: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), ...
```
Same for `pydantic_core`: x86-64 ELF inside an artifact tagged `arm64`. Filenames carrying `*-x86_64-linux-gnu.so` survive in the venv.
## Expected
One of:
1. **Fail the pack.** `uv` plugin (or charmcraft itself) detects target arch `!=` build host arch with no functioning cross-resolve and aborts with a clear error directing the user to a native runner. This is the safe default — silent mislabeling is the worst failure mode.
2. **Cross-resolve wheels.** `uv` plugin invokes `uv` with `--python-platform <target>` so manylinux wheels for the target architecture are downloaded and staged. (Source-only deps would still need a real cross-toolchain, which is out of scope; but the common manylinux case is solvable.)
## Workaround in use
`charmcraft.yaml` is restricted to `build-on == build-for` entries. Non-amd64 publishes are gated on per-arch native self-hosted runners.
Evaluation history
| Date | Model | Scores | Action | Summary |
|---|---|---|---|---|
| qwen/qwen3.8-27b |
Impact:
75
Quick Win:
22.5
Staleness:
55
Complexity:
70
Confidence:
70
Support Request:
5
|
needs triage | uv plugin cross-build (build-on amd64, build-for arm64/ppc64el/s390x) silently packs a charm whose venv contains host-arch compiled wheels while manifest declares target arch. No maintainer response, no labels, 0 comments, 114 days old. | |
| qwen3.6-35b-a3b-mtp-q6 |
Staleness:
65
Complexity:
60
Confidence:
90
Support Request:
10
|
needs triage | uv plugin silently packages host-architecture binaries into charms targeting different architectures. Unlabeled, untriaged, and inactive for 76 days. | |
| qwen3.6-35b-a3b-mtp-q6 |
Staleness:
65
Complexity:
75
Confidence:
85
Support Request:
10
|
needs triage | uv plugin silently packages host-architecture binaries into charms targeting different architectures, causing runtime failures. Untriaged with no maintainer response after 62 days. | |
| qwen3.6-35b-moe-q4 |
Staleness:
60
Complexity:
60
Confidence:
90
Support Request:
10
|
needs triage | uv plugin silently packages host-architecture compiled wheels into cross-architecture charms, causing runtime failures. Untriaged, no maintainer response after 49 days. | |
| qwen3.6-35b-a3b-mtp-q6 |
Staleness:
45
Complexity:
55
Confidence:
90
Support Request:
5
|
needs triage | uv plugin silently packages host-arch wheels into cross-arch charms, causing runtime failures. Build succeeds but produces corrupt artifacts. Open, unassigned, no comments. Awaiting triage to fail on arch mismatch or implement cross-resolve. |
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