On March 21, Cursor said it “should have mentioned” that Composer 2 started from the base of Kimi K2.5. A day earlier, Moonshot AI had described the division of labor: Moonshot supplied the foundation, Cursor continued pretraining and applied high-compute reinforcement learning, and Fireworks AI served inference. Moonshot later released Kimi K3’s 2.8T-parameter weights for developers to download, modify, and host. Cursor, the editor owner, had crossed into model development.

Key takeaways

  • On March 21, 2026, Cursor acknowledged that Composer 2 started from Moonshot AI’s Kimi K2.5 base.
  • Fireworks AI served inference for Composer 2, while Cursor performed continued pretraining and reinforcement learning.
  • Kimi K3 scored 88.3 on Terminal Bench 2.1, 0.5 points below GPT-5.6 Sol’s 88.8.
  • Moonshot released Kimi K3’s 2.8-trillion-parameter weights under the Kimi K3 License for public download, modification, and hosting.
  • Bloomberg reported a $3.5 billion Moonshot funding round at a $35 billion valuation on July 29, 2026; Moonshot had not confirmed it.

Read together, the March 20–21 accounts name three companies across four operational layers: Moonshot supplied the foundation; Cursor handled post-training and the interface; Fireworks served inference. Neither account disclosed a routing share, revenue split, or switching terms. Cursor controls five product surfaces a developer encounters—repository context, workflow, tools, permissions, and the product name—while the price of each layer remains private.

Z.ai’s integrated stack has a visible customer price

Cursor continued pretraining Kimi K2.5, applied high-compute reinforcement learning, named the result Composer 2, and embedded it in its editor. The March 21 correction restored Kimi K2.5’s provenance while developers still encountered Cursor’s product name, project context, and permission system.

Z.ai supplies the useful counterfactual. It kept GLM-5.2 and ZCode inside one company and priced coding plans from $16.20 to $144 per month. Its subscription makes the model-to-product payment route visible. Composer 2’s public accounts stop before the transfer prices among Moonshot, Cursor, and Fireworks.

K3 gives Moonshot leverage below Cursor’s interface

On July 17, VentureBeat reported that Frontend Code Arena ranked Kimi K3 first and Terminal Bench 2.1 scored it 88.3, just 0.5 points behind GPT-5.6 Sol’s 88.8.

Those model-level results explain why Kimi can qualify as a coding base. They cannot isolate the value created by Cursor’s later training, repository context, or tool policy. K3’s downloadable weights give downstream developers another base to test, modify, and host.

Moonshot also open-sourced FlashKDA, saying its implementation produced a 1.72× to 2.22× prefill speedup over a flash-linear-attention baseline on H20. Moonshot can improve Kimi’s prefill performance even when another company owns the editor or inference route.

Cursor’s workflow control carries security duties

Cursor Automations can launch agents from codebase changes, Slack messages, or timers. After Cursor receives a trigger, its agent inspects files, invokes tools, maintains state, and frames the task before a model sees it. Cursor also decides whether an action may proceed.

Security researchers identified sandbox escapes or boundary bypasses in Cursor, Codex, Gemini CLI, and Antigravity through files later consumed by trusted tools. A model could write a file, another tool could trust it, and the boundary could fail during the handoff. Neither Frontend Code Arena nor Terminal Bench tests that chain.

OpenAI added native sandboxing and an in-distribution harness to its Agents SDK for long-horizon deployment and testing. That design places permissions, evaluation, and containment beside the agent. Cursor bears the corresponding duty wherever it defines triggers, tools, and permissions.

Moonshot’s valuation cannot price the Composer 2 split

On May 7, Bloomberg reported that Moonshot had reached $200 million in annual recurring revenue in April while raising about $2 billion at a valuation above $20 billion. On July 29, Bloomberg reported that Moonshot closed a $3.5 billion round at a $35 billion valuation, above its initial target of $1 billion to $2 billion. Moonshot had not confirmed the financing.

reported July valuation divided by reported April ARR

Bloomberg’s July valuation divided by its April ARR produces 175×, but the numerator and denominator describe Moonshot at different dates. Bloomberg did not assign any part of the valuation or ARR to Composer 2, API sales, license payments, or Moonshot’s own products. The calculation lines up two companywide reports; it cannot divide value among Moonshot, Cursor, and Fireworks.

Frequently asked questions

Can Cursor users choose Kimi K3 directly instead of Composer 2?

The piece does not establish that K3 is available as a selectable model inside Cursor. It only confirms that Composer 2 began from Kimi K2.5 and that K3’s weights can be downloaded and hosted elsewhere.

How different is Composer 2 from the original Kimi K2.5 model?

No controlled comparison or technical breakdown is disclosed. Cursor says it continued pretraining and applied high-compute reinforcement learning, but the resulting capability gain is not quantified.

Are Composer 2’s weights publicly downloadable?

The piece identifies Kimi K3—not Composer 2—as having downloadable weights. It provides no indication that Cursor released Composer 2’s weights.

What does the Kimi K3 License permit?

The available evidence says it permits public download, modification, and hosting. Any additional conditions or commercial restrictions would require consulting the license itself.

Who is contractually responsible if a Composer 2 tool handoff causes a security failure?

That cannot be determined from the public accounts cited. The answer would depend on agreements among Cursor, Moonshot, Fireworks AI, and the customer, none of which are disclosed here.

Kimi K3’s path from announcement to reported financing

  • July 17, 2026 — Kimi announced K3 as a 2.8-trillion-parameter model and said full weights were planned by July 27.
  • July 27–28, 2026 — Moonshot released Kimi K3 weights under the Kimi K3 License for public download, modification, and hosting.
  • July 29, 2026 — Sources reported that Moonshot closed a $3.5 billion round at a $35 billion valuation, above its earlier $1 billion–$2 billion target; the financing remained unconfirmed by Moonshot.

Cursor’s March 21 correction restored Moonshot’s credit for the foundation. K3’s 2.8T weights can travel across hosts and products. Composer 2 shows what happens at the destination: Cursor adds training and developer context, sets permissions, and puts its own name in the window, while the price of that journey remains undisclosed.