Transform strict structured-output schemas on the OpenAI request path

[shibayan]

What this changes

Fixes #102. Structured output defaults strict to true, but the OpenAI request path sent the
JSON Schema unchanged, so a raw schema that is a valid framework input could reach the Responses
API in a form strict mode rejects — commonly an object node without additionalProperties: false
or without a complete required.

A strict format's schema now goes through a recursive transform on a deep clone before it is sent.
Object nodes gain additionalProperties: false and a completed required, default moves into
the description, and a schema strict mode cannot express fails locally with its path instead of as
a service 400. A strict: false format is passed through structurally unchanged, and the caller's
schema object is never written to. An unnamed raw schema takes its format name from a string root
title; an explicit name wins.

Parity

• Reference checked: Go's provider/openaiprovider/strict_schema.go, branch for branch — the root
  validation, the 27 unsupported keywords, the container-shape errors, the required ordering
  (caller's order, then the remaining names sorted), the default-into-description rule and the
  strict JSON schema at <path>: … message shape. Python's _chat_client.py supplies the
  title-as-format-name behaviour, which Go lacks. Python's raw-schema path patches
  additionalProperties at the root only, which is the shallow form this deliberately does not
  follow.
• Wire format affected: yes
• Public API affected: no
• Breaking change: yes

A strict request now carries additionalProperties: false and a completed required on object
nodes, default becomes description text, and text.format.name may come from a schema title.
Requests that worked before are unchanged in shape, since the ones this affects were rejected by
the service. What breaks: strict: true with a schema strict mode cannot express now fails locally
with a ChatClientError naming the path, where it previously reached the service and returned a
400.

The transform is package-internal — exported from neither the root entry nor ./internal — so
there is no public API addition.

Two deliberate divergences: Go's transform cache is not ported, because the clone here is a
plain-object round trip and a content-keyed cache would only reintroduce the shared-mutable-result
hazard Go spends three of its tests guarding. And unlike Python, title is not popped off the
schema: Go's 27-keyword unsupported list does not include it, so Go preserves it, and popping would
change what the caller declared.

Checklist

• pnpm check passes (lint, typecheck, build, test)
• Behaviour changes are covered by a test that fails without the change

[Copilot]

🟢 Approval recommended

Pull request overview

This PR fixes OpenAI structured-output failures when strict is effectively true by transforming JSON Schemas into the strict-compatible “closed” form before sending them to the Responses API, while preserving the caller’s original schema object.

Changes:

• Add a strict-schema transformer (toStrictJsonSchema) that deep-clones, recursively closes object schemas (additionalProperties: false), completes required, moves default into description, and rejects unsupported strict-mode shapes with path-aware errors.
• Apply the transformer on the OpenAI request mapping path only when strict resolves to true; pass schemas through unchanged when strict: false.
• Improve default response-format naming by deriving the format name from a string root title when the caller didn’t provide a name, and add/adjust tests + changelog.

File summaries

File	Description
packages/openai/src/to-openai.ts	Applies strict-schema transformation on the OpenAI Responses text.format path when strict is true.
packages/openai/src/strict-schema.ts	Implements deep-clone + recursive strict-schema transform and path-aware local validation errors.
packages/openai/src/strict-schema.test.ts	Adds comprehensive transformer tests plus toResponsesTextFormat strict/non-strict behavioral tests.
packages/openai/src/chat-client.test.ts	Extends request-mapping tests to cover strict transformation, pass-through for strict:false, immutability, and title-based naming.
packages/openai/src/to-openai.wire-fallbacks.test.ts	Updates an existing wire test to use a strict-compatible object schema shape.
packages/core/src/client/structured-output.ts	Derives default response-format names from a usable root title instead of always "response".
packages/core/src/client/structured-output.test.ts	Adds tests for strict defaulting and title-based naming behavior in resolveResponseFormat.
CHANGELOG.md	Documents the breaking behavior change and its rationale in the OpenAI package.

Review details

• Files reviewed: 8/8 changed files
• Comments generated: 0
• Review effort level: Lite

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[Copilot]

🔵 Needs a closer look

Review details

• Files reviewed: 8/8 changed files
• Comments generated: 0 new
• Review effort level: Lite