[antoniosarosi]

Summary

The reflect.class.Instance / reflect.class.instance_from design was superseded and has been completely removed from this branch. This PR now retains only two independent fixes discovered during that work:

• restore E0007 for unresolved method calls on unknown receivers, preventing invalid calls such as unknown_value.get_field<T>("x") from reaching emission and failing inside the VM
• route the pre-existing reflect.class.get_field non-instance E0001 through the shared runtime diagnostic factory without changing its text or behavior

There is no replacement reflection API in this PR. The separately ratified reflect.AnyClass direction is out of scope.

Commits

1. fix: restore unknown member diagnostics
2. refactor: share class reflection mismatch diagnostic

Validation

• CARGO_BUILD_JOBS=8 CARGO_INCREMENTAL=0 rustup run 1.93.0 cargo insta test --test-runner nextest -p baml_tests -p baml_cli -p baml_lsp2_actions -p baml_lsp2_actions_tests -p baml_surface --all-features --unreferenced=reject — 3,753 passed, 24 skipped; doctests clean; no unreferenced or pending snapshots
• focused E0007 compile regression passed
• shared diagnostic constructor oracle passed
• commit hooks passed, including workspace Clippy and formatting

Superseded-design cleanup verification

All commands were run after the final commits. Empty results are shown explicitly.

$ rg -n --hidden -g '!target/**' -g '!.git/**' 'instance_from|reflect\.class\.Instance|baml\.reflect\.class\.Instance|copy::reflect::class::Instance' baml_language CHANGELOG.md
<empty>

$ rg -n --hidden -g '!target/**' -g '!.git/**' 'class[[:space:]]+Instance' baml_language
<empty>

$ git diff --unified=0 origin/canary...HEAD | rg '^\+.*(instance_from|reflect\.class\.Instance|class[[:space:]]+Instance|cannot_construct_reflection_instance|is_reflect_class_instance)'
<empty>

$ rg --files -g '*.snap.new'
<empty>

The final diff is limited to five files: the inference guard, its regression and changelog entry, the shared diagnostic factory, and the existing reflect.class.get_field implementation. There are no stdlib declarations, generated files, docs, completions, snapshots, or codegen changes.

Summary by CodeRabbit

• Bug Fixes

  • Restored compile-time diagnostics when accessing unresolved members on unknown values.
  • Improved error messages when class-only operations receive non-class values.
  • Prevented invalid unresolved calls from reaching an internal runtime error.

• Documentation

  • Added a changelog entry describing the restored diagnostics.

[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.

[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: b5756029-aa38-49bc-8dc8-83f3d447b14d

📥 Commits

Reviewing files that changed from the base of the PR and between 98591e1 and 25d0afb.

📒 Files selected for processing (5)

• baml_language/CHANGELOG.md
• baml_language/crates/baml_compiler2_hir_ty/src/infer.rs
• baml_language/crates/baml_compiler_diagnostics/src/runtime_type.rs
• baml_language/crates/baml_tests/tests/runtime_classes_and_composites.rs
• baml_language/crates/bex_vm/src/package_baml/type_kinds.rs

💤 Files with no reviewable changes (1)

• baml_language/crates/baml_compiler2_hir_ty/src/infer.rs

Included review availability: Your plan includes up to 8 reviews per rolling hour; 6 remain after this review.

---

📝 Walkthrough

Walkthrough

This change restores unresolved-member diagnostics for unknown receivers and adds a dedicated diagnostic when get_field receives a non-class-instance value. Tests cover both compiler and runtime diagnostic behavior.

Changes

Reflection diagnostics

Layer / File(s)	Summary
Unknown member diagnostic restoration baml_language/crates/baml_compiler2_hir_ty/src/infer.rs, baml_language/crates/baml_tests/tests/runtime_classes_and_composites.rs, baml_language/CHANGELOG.md	Unknown receivers now produce unresolved-member diagnostics. Tests verify the get_field and unknown details.
Class instance validation diagnostics baml_language/crates/baml_compiler_diagnostics/src/runtime_type.rs, baml_language/crates/bex_vm/src/package_baml/type_kinds.rs	get_field now reports a dedicated expected-class-instance diagnostic for invalid inputs. Diagnostic tests verify its code and message.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to 25d0a

The PR restores the intended runtime-class reflection surface and reports successful targeted and broad validation; no actionable merge-blocking risk remains beyond normal checks and review.

Possibly related PRs

• BoundaryML/baml#3836: Modifies diagnostics for unknown member access and adds related tests.
• BoundaryML/baml#4464: Introduces the related unknown and get_field behavior.

Poem

A rabbit checks the fields with care,
No unknown calls slip through the air.
Class instances pass the gate,
Clear diagnostics name their state.
Hop, hop—the tests now glow!

🚥 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 accurately describes the restored E0007 diagnostic for member access on unknown receivers, which is a documented change in the pull request.
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

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch agent/reflect-class-instance

---

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[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated (UTC)
beps	Ready	Preview	Aug 17, 2026 10:50pm
promptfiddle2	Ready	Preview	Aug 17, 2026 10:50pm

[github-actions]

Binary size checks passed

✅ 7 passed

	Artifact	Platform	File	Gzip	Gated on	Baseline	Delta	Status
✅	baml-cli	Linux	🔒 31.7 MB	12.6 MB	file	31.7 MB	+29.1 KB (+0.1%)	OK
✅	packed-program	Linux	🔒 24.9 MB	9.1 MB	file	24.9 MB	+27.9 KB (+0.1%)	OK
✅	baml-cli	macOS	🔒 25.4 MB	11.1 MB	file	25.4 MB	+70.9 KB (+0.3%)	OK
✅	packed-program	macOS	🔒 20.6 MB	8.2 MB	file	20.6 MB	+41.7 KB (+0.2%)	OK
✅	baml-cli	Windows	🔒 27.2 MB	11.3 MB	file	27.2 MB	+37.8 KB (+0.1%)	OK
✅	packed-program	Windows	🔒 21.7 MB	8.2 MB	file	21.7 MB	+12.4 KB (+0.1%)	OK
✅	bridge_wasm	WASM	21.3 MB	🔒 5.4 MB	gzip	5.3 MB	+33.6 KB (+0.6%)	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.

---

Generated by cargo size-gate · workflow run

[antoniosarosi]

Applied the requested one-round SHIP-WITH-FIXES follow-up in d7c8724.

• Restored committed unknown-member diagnostics, so unknown_value.get_field<T>(...) is rejected as E0007 before emission/VM execution.
• Sealed baml.reflect.class.Instance object literals with a constructor-local check and recovery guidance to reflect.class.instance_from; the type-kind classifier remains unchanged.
• Hoisted class-instance mismatch diagnostics into runtime_type, preserved existing wording, and made instance_from(null) report null.
• Added static-instance/type-identity coverage plus updated unknown docs, describe coverage, and Instance completions.

Validation:

• Pinned full gate: 3,756/3,756 passed, 24 skipped.
• Snapshot audit: no unreferenced or pending snapshots.
• Pre-commit workspace clippy, format, YAML, and hygiene hooks passed.
• No files under sdks/typescript/bridge_typescript/ were staged or committed.

[antoniosarosi]

Scope change per Antonio: this PR's revert half was split out to #4469, which merges on its own. This PR now carries ONLY the replacement value-side API (reflect.class.instance_from / reflect.class.Instance + the review fix round in d7c8724). It will be driven to green but must not be merged without Antonio's explicit sign-off. After #4469 lands, this branch will be rebased onto canary to drop the duplicated revert commit.

[antoniosarosi]

Force-pushed the human-ratified stripped scope in 25d0afb1a.

• Removed the superseded reflect.class.Instance / reflect.class.instance_from design in full.
• Retained only the independent unknown-receiver E0007 fix and the separable pre-existing reflect.class.get_field diagnostic-factory hoist.
• Rebased the two clean commits onto current canary a90dbec68.
• Full pinned gate is green: 3,753/3,753 passed, 24 skipped; doctests clean; no pending or unreferenced snapshots.
• Cleanup greps in the updated PR body are empty, and the final diff is limited to five intended files.

The previous do-not-merge design hold no longer applies. This ready PR is back in the normal coordinator review/queue pipeline.