feat: add GPT-5.6 prompt cache retention

[smolpaws]

Summary

• add persisted promptCacheRetention / promptCacheKey profile fields
• default direct OpenAI GPT-5.6 Responses and Chat request bodies to prompt_cache_retention: "24h"
• gate the fields to known-safe direct OpenAI GPT-5.6 routes and strip them for LiteLLM/OpenRouter aliases, unsupported models, and ChatGPT subscription/Codex endpoints
• document official prompt-caching docs tension plus live OpenAI Responses/Chat probes, including the 5,412-token repeated-prefix cache proof

Live probe highlights

• Responses prompt_cache_retention: "24h": 200 and echoed "24h"
• Chat prompt_cache_retention: "24h": 200 with cache usage fields
• prompt_cache_options.ttl: "24h": 400 on both Responses and Chat; only "30m" was accepted
• Identical 5,412-token prefix + stable prompt_cache_key produced first-run cache write of 5,409 tokens and subsequent 5,409 cached tokens on both Responses and Chat

Verification

• npm test
• npm run typecheck
• npm run lint
• npm run build

Stacking

• Stacked on docs: investigate provider-native reasoning controls #4 (feature/provider-native-reasoning-docs) per requested sequence.

This PR was created by an AI agent (OpenHands) on behalf of Engel Nyst.

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 94755063-2a6b-4962-93d2-5d0a010a7b68

📥 Commits

Reviewing files that changed from the base of the PR and between aa91ef2 and f61c9f2.

📒 Files selected for processing (12)

• docs/PROMPT_CACHE_RETENTION.md
• docs/README.md
• docs/TRANSPILE_PLAN.md
• src/conversation/__tests__/event-log-parity.test.ts
• src/conversation/__tests__/event-log.test.ts
• src/conversation/__tests__/local-conversation.test.ts
• src/llm/__tests__/openai-client.test.ts
• src/llm/index.ts
• src/llm/openai.ts
• src/llm/provider-quirks.ts
• src/settings/__tests__/settings.test.ts
• src/settings/index.ts

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[smolpaws]

@coderabbitai review

This review trigger was posted by an AI agent (OpenHands) on behalf of Engel Nyst.

[coderabbitai]

@smolpaws purrs acknowledged 🐾 — hopping in to review the PR now. I'll take a close look at the new promptCacheRetention/promptCacheKey profile fields, the GPT-5.6 route gating logic, and the stacking on #4.

(=^･ω･^=)

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[gemini-code-assist]

Code Review

This pull request implements GPT-5.6 prompt-cache retention support in the TypeScript SDK, introducing new profile fields (promptCacheRetention and promptCacheKey) and integrating them into OpenAI request builders. The feedback suggests improving model version matching using a regular expression to prevent false positives like gpt-5.60, and broadening the base URL check to support provider-branded custom or internal proxy URLs.

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