JetBrains has rolled out version 0.4.0 of Koog, its Kotlin-based framework for creating AI-driven agents. The update emphasizes production-grade reliability, broader platform reach, and deeper model integration.

Key improvements in Koog 0.4.0

• Structured output at the core: One of the standout changes is the introduction of native structured output. Large language models often generate the right type of data but stumble on consistency. Koog now provides a built-in mechanism to enforce strict formatting, with guardrails such as automated retries and error-correction strategies. If the LLM supports structured output natively, Koog uses it directly; if not, the framework falls back on a specialized parser and prompt-retry loop until the result meets specifications.
• iOS platform support: With this release, Apple’s mobile operating system joins Android and JVM as supported targets. Thanks to Kotlin Multiplatform, developers can design an agent once and deploy it seamlessly across these environments. JetBrains notes, however, that iOS builds require the upcoming Koog 0.4.1 for full functionality.
• Next-generation model support: Koog 0.4.0 introduces compatibility with GPT-5 and allows fine-tuning through custom LLM parameters. Developers can control reasoning depth via settings like reasoningEffort, balancing precision, speed, and cost on a per-request basis.
• Observability and monitoring: To make agents more transparent, the update integrates OpenTelemetry, with out-of-the-box support for tools such as W&B Weave and Langfuse. Developers gain visibility into token usage, request costs, and detailed event breakdowns.

Handling failures and reliability

Recognizing the challenges of unstable networks or long-running model calls, JetBrains has also introduced a RetryingLLMClient. It comes with three operational modes—Conservative, Production, and Aggressive—giving developers control over how resilient their agents should be. In addition, the framework now supports DeepSeek models for broader flexibility.

Availability

Koog 0.4.0 was officially announced on August 28, with code hosted openly on GitHub. JetBrains describes the release as a step toward making AI agents not only easier to build but also more predictable, portable, and production-ready across multiple platforms.