Composer 2.5

What is Composer 2.5?

Composer 2.5 is a major update to the AI coding assistant available in Cursor. It represents a substantial leap in intelligence and behavior over its predecessor, Composer 2, with particular strength in long-horizon agentic tasks. The model is built on the same open-source checkpoint as Composer 2—Moonshot's Kimi K2.5—but benefits from scaled training, more complex reinforcement learning environments, and new learning methods that improve both capability and usability.

Who it's for

• Developers working on large codebases who need an AI assistant that can sustain focus across long, multi-step tasks without losing context or coherence.
• Engineering teams shipping complex features that require reliable instruction-following, precise tool usage, and consistent coding style over extended sessions.
• Power users of AI coding tools who value behavioral polish—clear communication, appropriate effort calibration, and a collaborative feel—alongside raw intelligence.

Key features

Targeted textual feedback for credit assignment

Composer 2.5 introduces a novel training technique that provides localized feedback at specific points in a rollout. Instead of relying solely on a final reward signal—which can be noisy over hundreds of thousands of tokens—the model receives hints inserted directly into the context where a behavior needs improvement. This allows Composer 2.5 to learn from mistakes like bad tool calls or confusing explanations without being penalized for the entire trajectory.

25x more synthetic training tasks

The model is trained on 25 times more synthetic tasks than Composer 2. These tasks are dynamically created and selected throughout the training run, ensuring the model continues to face harder problems as its coding ability improves. Approaches like feature deletion—where the agent must remove code while keeping a test suite passing—ground the synthetic data in real-world codebase challenges.

Improved communication and effort calibration

Beyond raw coding benchmarks, Composer 2.5 has been refined on behavioral dimensions that matter for real-world use. The model communicates more clearly, calibrates its effort appropriately to the task, and is generally more pleasant to collaborate with over long sessions.

What stands out

"Composer 2.5 is better at sustained work on long-running tasks, follows complex instructions more reliably, and is more pleasant to collaborate with."

This combination of sustained intelligence and behavioral polish is rare in AI coding assistants. While many models can handle short, well-defined tasks, Composer 2.5 excels at the kind of extended, multi-step work that defines real software development. The targeted feedback training method means it learns from specific mistakes rather than being punished for entire trajectories, making it both smarter and more adaptable in practice.

Worth checking out if…

You're a developer who regularly works on complex, long-running coding tasks and wants an AI assistant that maintains focus, follows nuanced instructions, and communicates clearly throughout the process. If you've found other coding assistants lose coherence on multi-step work or fail to learn from localized mistakes, Composer 2.5 offers a meaningful upgrade in both intelligence and collaboration quality.