Mistral Medium 3.5: Open-Weights 128B Posts 77.6% SWE-Bench and Ships Cloud Coding Agents
Mistral today released Mistral Medium 3.5 in public preview, its first flagship merged model and the new default for both Le Chat and the Vibe coding agent CLI. The model ships as open weights under a modified MIT license, self-hostable on four GPUs.
The Model
Medium 3.5 is a 128B dense model with a 256k context window. Unlike Mistral’s previous specialist models — Devstral 2 for code, Pixtral for vision — Medium 3.5 merges instruction-following, reasoning, and coding into a single set of weights. Reasoning effort is configurable per request: the same model handles a quick chat reply or a long agentic run.
Mistral trained the vision encoder from scratch to handle variable image sizes and aspect ratios, extending the model into multimodal territory without a separate deployment.
Benchmark Numbers
| Benchmark | Score |
|---|---|
| SWE-Bench Verified | 77.6% |
| tau³-Telecom | 91.4% |
77.6% on SWE-Bench Verified beats Devstral 2 and Qwen3.5 397B. For context: Claude Opus 4.6 sits at 80.8%, GPT-5.4 at 77.2%, and Qwen3.6-27B at 77.2%. Medium 3.5 ties GPT-5.4 on SWE-Bench while running on a fraction of the infrastructure. Tau³-Telecom at 91.4% is strong in the agent reliability category — above the 91.9% reported for Claude Opus 4.6 on tau2-Telecom and within the range of purpose-built agentic models.
Vibe Remote Agents
The bigger operational shift is cloud-hosted coding agents. Vibe sessions now run remotely, multiple at once, notifying the user when work completes. Previously, Vibe ran locally.
Capabilities at launch:
- Start sessions from the Vibe CLI or from Le Chat
- Live inspection: file diffs, tool calls, progress states, approval prompts surfaced in real time
- Local CLI sessions can be teleported to the cloud mid-session, carrying history, state, and pending approvals
- GitHub integration for pull request creation, Linear and Jira for issue tracking, Sentry for incident response
A new Work mode in Le Chat runs on Medium 3.5 for multi-step research, analysis, and cross-tool tasks — the consumer-facing equivalent of the Vibe agent runtime.
Context
Mistral raised $830M in March 2026 at a $14B valuation targeting 13,800 Nvidia GB300 GPUs. Medium 3.5 is the first major model to come out of that compute expansion. API pricing for Medium 3.5 was not announced at launch. Medium 3 API pricing is $0.40/M input, $2.00/M output.
Open weights at this benchmark tier are materially significant. The previous generation of models at 77% SWE-Bench was firmly proprietary. Medium 3.5 gives enterprise teams a self-hosted path to GPT-5.4-equivalent coding performance without vendor lock-in.