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.
- License: Mit
- Context length: 256K tokens
- Languages: en
- Minimum hardware: ~3 GB VRAM
- Strengths: reasoning, coding and on-device inference
Highlights
- 4B-parameter model sized for local hardware.
- Runs on ~3 GB VRAM.
- Handles long inputs with a 256K-token context window.
- Strong at repository exploration tasks.
- Also handles tool calling out of the box.
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.