[bug]: Structured output fails on optional Zod fields in native JSON-schema mode

[EricLiu2795]

Summary

Context

While validating the Founder Weekly Review quality follow-up in #333, the realistic E2E reached generation and failed during structured-output schema preparation before any provider request was sent.

The failure is caused by an existing optional field in the Founder Weekly Review schema:

rationale: z.string().min(1).max(2000).optional()

The problem appears at the shared structured-output boundary when the configured model declares native JSON-schema support.

Current behavior

Founder Weekly Review generation uses the shared invokeStructured(...) path through the configured reasoning route.

The current flow is approximately:

Zod schema
→ invokeStructured(...)
→ model declares native structured output
→ native json-schema mode selected
→ LangChain withStructuredOutput(...)
→ schema preparation
→ failure on optional Zod field

The failure occurs before the model/provider request is made.

In the observed realistic E2E:

computed evidence collection
→ Snapshot v2 validation
→ document-change audit/group validation
→ isolated DB setup
→ generation start
→ structured-output schema preparation
→ failure

No generation provider call was completed.

Why this was not previously visible

The historical Founder Weekly Review implementation used a different structured-output path for Kimi/Moonshot.

That path used JSON-object generation followed by local Zod validation:

Zod schema
→ schema guidance in prompt
→ provider returns JSON object
→ local schema.parse(...)

With that approach, an omitted field such as rationale is valid when the Zod schema marks it as .optional().

The current shared architecture instead prefers native JSON-schema when the configured model advertises that capability, exposing a compatibility mismatch between otherwise-valid Zod schemas and the native structured-output path.

Reproduction

A minimal reproduction should use the shared structured-output adapter with a schema containing an optional field, for example:

const ExampleSchema = z.object({
required: z.string(),
optional: z.string().optional(),
});

Then invoke it through a configured model whose behavior advertises native json-schema.

Expected behavior

The schema is accepted and structured generation proceeds.

Actual behavior

Structured-output schema preparation fails before the provider request.

@launchstack/core version (if applicable)

No response

Node.js version

No response

Affected area

@launchstack/features

Additional context

No response