[sxlijin]

Summary

• keep required and defaulted argument inputs aligned in a shared column to the right of their argument names
• use a checkbox to toggle between the declared default and an explicitly entered value, with hover help explaining both modes and a bare SquarePen icon beside the input
• carry the exact unevaluated default expression from the source map to the playground and display it as the disabled control placeholder

Root cause

The typed args form represented a default only as hasDefault. When the argument was omitted, the form replaced its control with explanatory text, and no source expression was available for a useful placeholder.

Playground UI

Before

After

Validation

• cargo test -p baml_project param_schema --lib (22 passed)
• cargo test -p bridge_wasm schema_twins_serialize_identically_to_source_types --lib
• pnpm --dir typescript2 format:ci --since=origin/canary
• pnpm --dir typescript2 --filter @b/pkg-playground typecheck
• pnpm --dir typescript2 --filter @b/pkg-playground test (137 passed)
• browser validation of horizontal alignment, disabled exact-default placeholders, checkbox hover help, and explicit/default payload behavior

Linear: B-1054

Summary by CodeRabbit

• New Features

  • Parameter schemas now expose the exact source text for declared default values.
  • Playground forms clearly indicate when a parameter uses its default, with checkbox controls and tooltip guidance.
  • Inputs display helpful default-value placeholders and consistently support disabled states across field types.

• Bug Fixes

  • Improved parameter form rendering and validation behavior for optional, numeric, union, list, map, and JSON inputs.
  • Updated schema serialization to keep default-expression data consistent across supported environments.

[linear]

B-1054

[vercel]

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

Project	Deployment	Actions	Updated (UTC)
beps	Ready	Preview, Comment	Jul 29, 2026 6:42am
promptfiddle	Ready	Preview, Comment	Jul 29, 2026 6:42am
promptfiddle2	Ready	Preview, Comment	Jul 29, 2026 6:42am

[coderabbitai]

📝 Walkthrough

Walkthrough

The change adds exact unevaluated parameter default expressions to Rust, WASM, and TypeScript schemas, then uses them in the playground argument form as placeholders for omitted defaults with disabled-field and tooltip behavior.

Changes

Default argument UI

Layer / File(s)	Summary
Extract default expressions baml_language/crates/baml_project/src/param_schema.rs, baml_language/crates/baml_project/src/symbols.rs	Parameter schema generation receives function locations, extracts default source spans, populates defaultExpression, and validates structured JSON output.
Transport parameter metadata baml_language/crates/bridge_wasm/src/wasm_playground.rs, typescript2/pkg-playground/src/worker-protocol.ts, typescript2/pkg-playground/src/__fixtures__/param-schema-golden.json	WASM and TypeScript schemas serialize optional default expressions, while protocol shapes and golden schema data are updated.
Render defaulted arguments typescript2/pkg-playground/src/ArgsForm.tsx	The argument form detects native default placeholders, displays default controls and tooltips, and disables omitted parameter fields.
Propagate field state through widgets typescript2/pkg-playground/src/ArgsForm.tsx	Primitive, enum, class, list, map, optional, union, and raw JSON fields forward placeholder and disabled state through their editing controls.

Estimated code review effort: 4 (Complex) | ~45 minutes

Sequence Diagram(s)

sequenceDiagram
  participant BAMLFunction
  participant ParamSchema
  participant WASMBridge
  participant WorkerProtocol
  participant ArgsForm
  BAMLFunction->>ParamSchema: provide function source and default span
  ParamSchema->>WASMBridge: serialize defaultExpression
  WASMBridge->>WorkerProtocol: transport parameter schema
  WorkerProtocol->>ArgsForm: provide defaultExpression
  ArgsForm->>ArgsForm: render placeholder and disabled omitted field

Loading

Possibly related PRs

• BoundaryML/baml#3935: Extends the same schema-driven playground argument form and parameter schema infrastructure.

Suggested reviewers: rossirpaulo

Poem

A rabbit found defaults in code,
And carried their text along the road.
The fields now sleep, disabled and bright,
With tooltips guiding hops just right.
“Uncheck,” said Bun, “to edit with delight!”

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
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.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title is concise and accurately summarizes the main playground UX change around default argument inputs.

✨ 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 sam/b-1054

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}

[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.

[sxlijin]

@coderabbitai review

[coderabbitai]

✅ Action performed

Review finished.

Note: CodeRabbit is an incremental review system and does not re-review already reviewed commits. This command is applicable only when automatic reviews are paused.

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 3d50920668

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[github-actions]

Binary size checks passed

✅ 7 passed

	Artifact	Platform	File	Gzip	Gated on	Baseline	Delta	Status
✅	baml-cli	Linux	🔒 27.4 MB	11.7 MB	file	27.3 MB	+118.1 KB (+0.4%)	OK
✅	packed-program	Linux	🔒 17.8 MB	7.3 MB	file	17.7 MB	+30.6 KB (+0.2%)	OK
✅	baml-cli	macOS	🔒 21.3 MB	10.2 MB	file	21.2 MB	+99.3 KB (+0.5%)	OK
✅	packed-program	macOS	🔒 13.9 MB	6.4 MB	file	13.9 MB	+48 B (+0.0%)	OK
✅	baml-cli	Windows	🔒 23.0 MB	10.4 MB	file	22.9 MB	+116.7 KB (+0.5%)	OK
✅	packed-program	Windows	🔒 14.8 MB	6.5 MB	file	14.8 MB	+15.2 KB (+0.1%)	OK
✅	bridge_wasm	WASM	16.8 MB	🔒 4.6 MB	gzip	4.5 MB	+48.3 KB (+1.1%)	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