Overview
FastContext-1.0-4B-RL is a 4B-parameter model from Microsoft, built on the Qwen3-4B-Instruct-2507 backbone and refined with reinforcement learning. It is a repository-exploration subagent: instead of solving a coding task itself, it scouts a codebase and hands back compact file paths and line ranges so a larger coding agent receives clean, grounded context instead of a long trail of exploratory reads.
Because it is only 4B parameters, it runs comfortably on local hardware. In Atomic Chat you load the weights once and run everything on-device, so your repository and your queries stay on your own machine, offline, with no API calls leaving your computer.
What it is good at
The model exposes three read-only tools (READ, GLOB, GREP) and can fire several of them in parallel within a single turn, then use the results to guide the next search. That design maps to a few concrete jobs:
- Repository exploration — given a natural-language query, it locates the relevant code and returns file paths with line ranges as focused evidence rather than dumping whole files.
- Tool calling — it issues independent READ/GLOB/GREP calls to cover several hypotheses at once, which is what lets it cut a main agent's token use by as much as 60% in the FastContext paper's tests.
- Code grounding for agents — wired into a coding agent such as Mini-SWE-Agent, it acts as a delegate the main model invokes on demand, without retraining the main model.
Running it locally
At 4B parameters the model is small enough for modest GPUs. A 4-bit quantized Qwen3-4B build needs roughly 2.5 GB of VRAM, an 8-bit build around 4 GB, and full FP16 about 8 GB, so a 6 GB card or a recent Mac handles it. Its context window is 262,144 tokens, though long contexts grow the KV cache and raise memory use. Pull the weights from Hugging Face:
huggingface-cli download microsoft/FastContext-1.0-4B-RL
From there you can serve it with Transformers or vLLM, or load it in Atomic Chat with one click and start querying a repository fully on-device.
License
FastContext-1.0-4B-RL is released under the MIT license. That permits commercial use, modification, redistribution, and private use, as long as the copyright and license notice are kept with the software.