test(python): cover bridge version mismatch diagnostic

[sxlijin]

Summary

• add Python integration coverage for generated-bytecode toolchain mismatches
• verify the complete bridge identity diagnostic survives the PyO3 boundary
• verify compatibility validation runs before bytecode deserialization

Root cause and relation to #4315

The B-1473 report used generator 0.15.1-nightly.20260731.a with baml_bridge==0.15.0. That generator predates #4315 and emitted only raw bytecode, so the bridge had no independently readable generator identity and surfaced Unexpected variant tag: 7.

#4315 already fixed the supported regenerated-SDK path by embedding generation metadata and validating it before deserialization. Current coverage tests metadata emission and core bridge_cffi diagnostics separately; this PR closes the remaining Python host-boundary gap.

Legacy SDKs generated before #4315 must be regenerated to gain the compatibility preflight because their raw-bytecode-only payload has no toolchain identity to inspect.

Controlled reproduction

State	Generator	Installed Python bridge	Result
Before #4315	0.15.1-nightly.20260731.a	0.15.0	Reproduces BamlPanic: Failed to deserialize BAML bytecode: Unexpected variant tag: 7
With #4315	0.16.0	0.15.1.dev2026080700	Raises an actionable version-skew RuntimeError before deserialization with generated, installed, and required versions plus repair guidance

Validation

• uv run pytest -n 0 tests/test_engine.py::TestBasics -q — 4 passed, 1 expected xfail
• cargo test -p bridge_cffi generated_metadata_tests — 5 passed
• uv run ruff check tests/test_engine.py
• git diff --check

Linear: B-1473

Summary by CodeRabbit

• Bug Fixes
  • Improved runtime handling of bytecode generated with an incompatible toolchain.
  • Displays a clear version-mismatch error with installed and expected versions instead of an initialization failure.

[linear]

B-1473

[vercel]

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Project	Deployment	Actions	Updated (UTC)
beps	Error		Aug 12, 2026 8:25pm
promptfiddle2	Ready	Preview	Aug 12, 2026 8:25pm

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 8c32556a-cec8-4239-8340-3d7f71b2d84a

📥 Commits

Reviewing files that changed from the base of the PR and between f6c01b1 and da2bba3.

📒 Files selected for processing (1)

• baml_language/sdks/python/tests/test_engine.py

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📝 Walkthrough

Walkthrough

The Python engine tests now import bridge and toolchain version accessors. They verify that incompatible bytecode raises a detailed RuntimeError with version information and regeneration guidance instead of exposing a deserialization panic.

Changes

Runtime version validation

Layer / File(s)	Summary
Version-skew initialization test baml_language/sdks/python/tests/test_engine.py	The test imports runtime version accessors and validates the error raised when generated bytecode uses incompatible toolchain metadata. It checks installed and expected versions, regeneration guidance, and exclusion of the lower-level deserialization error.

Estimated code review effort: 2 (Simple) | ~10 minutes

Suggested reviewers: hellovai

Poem

A rabbit checked the bytecode with care,
Found mismatched versions hiding there.
“Regenerate,” said the test,
“Clear errors are best!”
No panic bounced through the software air.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies the Python test coverage for bridge version mismatch diagnostics, which is the main change.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch agent/b-1473-version-mismatch

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[github-actions]

⏭️ Performance benchmarks were skipped

Perf benchmarks (CodSpeed) are opt-in on pull requests — they no longer run on every push. They always run automatically after merge to canary/main.

To run them on this PR, do any of the following, then push a commit (or re-run CI):

• Add RUN_CODSPEED=1 to the PR description, or
• Include run-perf or /perf in the PR title or any commit message.

[github-actions]

Binary size checks passed

✅ 7 passed

	Artifact	Platform	File	Gzip	Gated on	Baseline	Delta	Status
✅	baml-cli	Linux	🔒 26.2 MB	11.1 MB	file	27.4 MB	-1.2 MB (-4.4%)	OK
✅	packed-program	Linux	🔒 16.9 MB	6.8 MB	file	18.6 MB	-1.7 MB (-9.1%)	OK
✅	baml-cli	macOS	🔒 20.4 MB	9.7 MB	file	21.3 MB	-913.9 KB (-4.3%)	OK
✅	packed-program	macOS	🔒 13.3 MB	6.0 MB	file	14.5 MB	-1.2 MB (-8.2%)	OK
✅	baml-cli	Windows	🔒 21.9 MB	9.9 MB	file	23.0 MB	-1.1 MB (-4.7%)	OK
✅	packed-program	Windows	🔒 14.1 MB	6.1 MB	file	15.5 MB	-1.4 MB (-9.3%)	OK
✅	bridge_wasm	WASM	15.5 MB	🔒 4.2 MB	gzip	4.6 MB	-440.8 KB (-9.6%)	OK

🔒 = the size this artifact is GATED on (ceiling + delta). Binaries gate on file size (installed binary); WASM gates on gzip (download size). The other size is shown for information only.

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Generated by cargo size-gate · workflow run