What's Changed

• fix(openai): make gpt-5 family reasoning models by @hntrl in #8646
• feat(openai): custom tools by @hntrl in #8645
• feat(openai): add prompt cache key support by @hntrl in #8649
• chore(openai): add gpt-5 parameter tests by @hntrl in #8650

Full Changelog: https://github.com/langchain-ai/langchainjs/compare/@langchain/openai==0.6.5...@langchain/openai==0.6.6