[aaronvg]

Summary

Adds a render_null_as option to ctx.output_format(...) so nullable schema members can render a custom marker such as omit instead of null.

This is wired through both output-format implementations:

• engine/baml-lib/jinja-runtime for the existing engine runtime and Jinja type checker
• baml_language for the newer sys_llm renderer, legacy Jinja kwargs, typed ctx.output_format_with(...), and template type checking

Impact

Prompt authors can write:

{{ ctx.output_format(render_null_as='omit') }}

or, in prompt tags:

${ctx.output_format_with(render_null_as = "omit")}

and nullable fields render like string or omit, which gives models an explicit omit-key instruction surface.

Validation

• cargo nextest run -p sys_llm output_format
• cargo nextest run -p sys_jinja_types test_output_format
• cargo nextest run -p bex_engine test_output_format_can_render_null_as_omit
• cargo nextest run -p baml_tests prompt_output_format_with_can_render_null_as_omit
• cargo nextest run -p baml_tests __baml_std__::test_03_hir
• cargo nextest run -p baml_tests __baml_std__::test_06_codegen
• cargo nextest run -p internal-baml-jinja output_format
• cargo nextest run -p internal-baml-jinja-types test_output_format

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Note

Low Risk
Additive, optional prompt-schema rendering change with no auth or response-parsing behavior; risk is mainly inconsistent wording if one runtime path were missed, which tests cover.

Overview
Adds optional render_null_as on parameterized output-format rendering so nullable schema text can use a custom token (e.g. omit) instead of null.

Exposed on Context.output_format_with(...) and Jinja ctx.output_format(render_null_as='…'), with the option threaded through sys_llm / sys_ops and the legacy engine/baml-lib/jinja-runtime renderer. Nullable unions, bare null types, and related media instructions now substitute the chosen label in rendered prompts.

Jinja type checkers accept none | string for the kwarg; new engine, baml_tests, and sys_llm tests assert schemas like nickname: string or omit,.

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Summary by CodeRabbit

• New Features
  • Added optional render_null_as support to output-format schema rendering, letting you customize how null is worded for nullable fields (e.g., rendering as "omit" instead of the default).
  • Exposed render_null_as through template helpers and rendering APIs, with validation to accept only none/null or a string value.
• Tests
  • Added/extended coverage to confirm custom null wording appears in rendered schemas and prompt instructions, and that invalid render_null_as values are rejected.

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[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

Run ID: 1fa26735-4e7c-4c63-8c49-3c3ff6ab0fc1

📥 Commits

Reviewing files that changed from the base of the PR and between be2408c and 515f318.

📒 Files selected for processing (1)

• engine/baml-lib/jinja-runtime/src/output_format/types.rs

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📝 Walkthrough

Walkthrough

A new render_null_as optional parameter is added to the output-format schema rendering pipeline in both the engine and baml_language stacks. When set, it substitutes a caller-specified string (e.g., "omit") for the hardcoded "null" literal in generated output-format schemas. The change threads through type registration, Jinja kwarg parsing, RenderOptions struct, core rendering helpers, public APIs, and comprehensive tests across both implementations.

Changes

render_null_as option for output format rendering

Layer / File(s)	Summary
RenderOptions struct and core null rendering engine/baml-lib/jinja-runtime/src/output_format/types.rs, baml_language/crates/sys_llm/src/types/output_format.rs	RenderOptions gains render_null_as: RenderSetting<String> (defaulting to Auto) in both stacks. TypeValue::Null / RuntimeTy::Null rendering is redirected from the hardcoded "null" string to new render_null_type / rendered_null_type helpers that consult the option. media_output_instruction receives &RenderOptions to interpolate the configured null string in instruction text.
Engine null rendering in prompts and schemas engine/baml-lib/jinja-runtime/src/output_format/types.rs	The engine implementation updates multiple rendering paths (auto_prefix, prompt/prefix generation for union null branches, inner_type_render) to use the new render_null_type helper instead of hardcoding "null". A new private helper render_null_type implements the dispatch logic.
Jinja kwarg parsing and public API wiring engine/baml-lib/jinja-runtime/src/output_format/mod.rs, baml_language/crates/sys_llm/src/jinja/output_format_object.rs, baml_language/crates/sys_llm/src/lib.rs, baml_language/crates/sys_ops/src/lib.rs	Both OutputFormat::call implementations parse the new render_null_as kwarg (string or null), add it to the allowed-keywords list, and forward it into RenderOptions::new. A new helper parse_render_null_as_kwarg implements absent→Auto, null→Auto, string→Always semantics. The public render_output_format_content function and IoClassLlmContext::output_format_with gain a new Option<String> parameter.
Type system registration engine/baml-lib/jinja/src/evaluate_type/types.rs, baml_language/crates/sys_jinja_types/src/evaluate_type/types.rs, baml_language/crates/baml_builtins2/baml_std/baml/ns_llm/llm_types.baml	PredefinedTypes::default in both stacks registers render_null_as as a `String
Tests engine/baml-lib/jinja-runtime/src/output_format/types.rs, engine/baml-lib/jinja/src/evaluate_type/test_expr.rs, baml_language/crates/sys_llm/src/types/output_format.rs, baml_language/crates/sys_jinja_types/src/evaluate_type/test_expr.rs, baml_language/crates/bex_engine/tests/llm_render.rs, baml_language/crates/baml_tests/tests/prompt_tag_runtime.rs	Unit and integration tests verify that optional and media types render with the custom null token, that type-checking accepts render_null_as='omit' as Type::String and rejects integer values with a type error, and that end-to-end prompt assembly produces "string or omit" rather than "string or null". Existing tests are updated to pass the new parameter.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

• BoundaryML/baml#3699: Modifies the same RenderOptions/render_type_hoisted/media_output_instruction paths in sys_llm for nullable union handling, directly touching the code this PR extends.

Poem

🐇 A null by any other name
Would render just the same—
Unless you pass render_null_as,
Then "omit" joins the game!
No more hardcoded "null" in sight,
The rabbit hops with pure delight. ✨

🚥 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 PR title clearly and concisely describes the main change: adding a render_null_as output format option to customize null value rendering.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
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.

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✨ 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 codex/render-null-as

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