Nexus-Agent

A terminal coding assistant with a ReAct tool loop, local GGUF inference and optional cloud providers. It can inspect a workspace, make targeted edits, search documentation and help debug code. Model output can be wrong. Review changes before keeping them.

Python 3.11+ | MIT | Package: nexus-agent-ai | Commands: nexus-agent, agent

Release status

2.8.1 was released on PyPI on September 30, 2026. It includes the capability-enforcement, sensitive-file, packaging and local-runtime fixes described below. The older PyPI 2.7.2 release lacks the newer patch_file and run_tests tools and these safety fixes. Upgrade the correct package with python -m pip install --upgrade nexus-agent-ai and check nexus-agent --version.

pip install nexus-agent is an unrelated package. Use nexus-agent-ai.

Install

For the published release:

python -m pip install nexus-agent-ai
nexus-agent --version
nexus-agent --help

Windows: if PowerShell says nexus-agent is not recognized, the Python Scripts folder is not on PATH (common with pip install --user). Run python -m nexus_agent_ai repl -p local instead, or add the folder printed by nexus-agent doctor to PATH. Windows support is covered by unit tests that simulate Windows paths, line endings and permissions on Linux; install and startup were seen on one Windows PC, and there is no Windows CI.

For this checkout:

python -m venv .venv
# Linux/macOS
source .venv/bin/activate
# Windows: .venv\Scripts\activate
python -m pip install .

The default installation has the CLI, Hugging Face download support and web search. Cloud SDKs are optional:

python -m pip install 'nexus-agent-ai[cloud]'
# Or choose one: [openai], [anthropic], [gemini]

For an unpublished checkout, use python -m pip install '.[cloud]' instead. The [local] extra installs torch/transformers; [cpu] installs llama-cpp-python and can require a C++ compiler. Neither is required for the downloaded llama-server path on supported desktop platforms.

Use

Provider flags belong to the subcommand, not the root command:

nexus-agent chat -p local "What is a Python generator?"
nexus-agent chat -p local "Explain @calculator.py"
nexus-agent review calculator.py -p local
nexus-agent debug calculator.py -p local --error "Wrong calculation"
nexus-agent repl -p local
nexus-agent commit -p local
nexus-agent chat --help

Run inside the project you want Nexus to inspect. Regular chat/generate/debug sessions may write files in that workspace. Review sessions advertise only inspection tools, and the dispatcher rejects tools outside the session's allowed list, including REPL /review. Tool failures are appended to the final answer so a model cannot silently claim success after a failed operation. Commit-message generation is also read-only; the commit command asks for confirmation unless you deliberately pass --yes.

@file.py, REPL review/debug reads and read_file share workspace and sensitive-path checks. Credential-like filenames, .env files, keys and sensitive configuration directories are blocked, including resolved symlink targets. This is not content-based secret detection: an innocently named source file may still contain a secret. Do not ask Nexus to inspect such files.

Project execution and Git mutations

Pytest imports arbitrary project code. A working-directory check and allowed arguments do not sandbox that code. It can read/write outside the workspace, access the network and run commands with your account's permissions.

run_tests is disabled and omitted from the model's tools by default. For a project you trust, opt in before launching Nexus:

# Linux/macOS
NEXUS_ALLOW_PROJECT_EXECUTION=1 nexus-agent chat -p local "Run tests for calculator.py"
# PowerShell
$env:NEXUS_ALLOW_PROJECT_EXECUTION='1'
nexus-agent chat -p local "Run tests for calculator.py"

Once opted in, the model may run pytest without another prompt. Pytest has a 45-second timeout, restricted flags, disabled external plugin auto-loading and a reduced child environment. These are risk reductions, not filesystem/network isolation. Use a disposable container or VM for untrusted projects. Do not opt in merely because a model tells you to.

Autonomous git_commit is also disabled by default. Prefer the separate nexus-agent commit command to review the generated message. NEXUS_ALLOW_GIT_COMMIT=1 deliberately enables model-directed commits. Git hooks can execute code; use trusted repositories only.

run_code is for restricted pure-computation Python snippets with AST checks, reduced environment and a timeout. It is not a general Python interpreter or a security boundary for hostile code.

Local models

The built-in model family is Liquid AI LFM2.5. The initial run downloads a GGUF and, on supported platforms, a checksum-verified llama-server binary. Subsequent local inference works offline after downloads are complete. Web search still uses the network. Local inference has no provider API charge, but uses your CPU/GPU, memory, disk and power.

Official model information:

• LFM2.5-1.2B-Instruct: 32K model context and native tool calling.
• LFM2.5-2.6B: 128K model context and training for agent workloads.
• Tool-use format: Pythonic calls by default; JSON calls can be requested.

A model's advertised maximum context is not Nexus's configured context. Nexus defaults to 4096, with one llama-server slot. Use --context-size on chat or repl to change it for that run, or set NEXUS_CONTEXT_SIZE as a default; a larger context consumes more RAM. Local HTTP errors now retain the server detail, including context overflow. Shorten input/history or choose a suitable context/model if a request will not fit.

nexus-agent chat "Explain this project" -p local --context-size 8192
nexus-agent repl -p local --context-size 8192 --persist --session my-project

Priority: --context-size > NEXUS_CONTEXT_SIZE > 4096. Values must be integer token counts of at least 512. The setting covers the whole window, including instructions, tool schemas, conversation, and generated output. It does not enlarge a model's trained limit, compact history, or guarantee that your RAM can hold the requested window. Start with 4096 on low-memory machines and raise it only within your model and hardware limits.

This option currently applies to local llama-server and llama-cpp-python, not cloud providers, auto, Ollama, or Transformers. Unsupported routes fail clearly instead of silently ignoring the option. An already-running server must be stopped before using an explicit override because its window is not verified. In REPL, /context shows the configured window and restart guidance. Changing the window requires a restart; the history can be resumed using the same --persist --session ID.

Long sessions: history is trimmed by estimated tokens, not message count. Before each request Nexus drops the oldest whole turns, and cuts the largest file or tool outputs (keeping their start and end), so the request fits the window. If the server still rejects it as too large, Nexus retries once with a tighter budget. After each REPL turn a meter shows how full the window is. /compact replaces older turns with a short note (earlier requests, files and tools touched, built without calling the model) and keeps the last 2 turns. /clear removes all history, including the current persistent session. Token counts are estimates (about 3.5 characters per token), and the meter only applies to local models with a known window.

First run and model fit

On a first local run Nexus picks a model by free RAM (not total), keeping about 1 GB of headroom plus room for the context window. If the big model will not fit, it falls back to the 1.2B model (~0.7 GB) and says why. Before downloading it shows the file size, a measured download speed and an estimate, and asks [y]es / [s]mall instead / [n]o. Downloads resume if interrupted. Set NEXUS_AGENT_ASSUME_YES=1 to skip the question, or NEXUS_AGENT_MODEL_REPO and NEXUS_AGENT_MODEL_FILENAME to choose a model yourself.

nexus-agent and nexus-agent chat with no question both open the interactive REPL.

nexus-agent doctor checks Python version, free RAM and disk, download speed to Hugging Face, the inference engine, and which model fits. It downloads nothing. Use --no-network to skip the speed test.

You can select a smaller model explicitly:

export NEXUS_AGENT_MODEL_REPO=LiquidAI/LFM2.5-1.2B-Instruct-GGUF
export NEXUS_AGENT_MODEL_FILENAME=LFM2.5-1.2B-Instruct-Q4_0.gguf
nexus-agent chat -p local "What is 17 times 23?"

September 30 real-user smoke results

On a Linux x86_64 host with 2 CPUs and about 2GB RAM, genuine offline LFM2.5-1.2B-Instruct Q4_0 answered 17x23 correctly (391). One targeted model-directed patch and a read-only review succeeded before publication. After a fresh install from public PyPI, math passed again, but two file-repair attempts did not complete: one used a nonexistent filename, and one read the right file then stopped without editing. The failed tool call was reported and the original file stayed unchanged. Direct patch_file checks passed with a backup and independently verified output.

These are smoke tests, not a model benchmark or proof of unattended reliability. Review the actual diff and run your own trusted tests. LFM2.5-2.6B inference did not complete reliably on this low-memory host, so its quality on suitable hardware remains unverified.

Setup and response time depend on download size and hardware. A 2GB CPU test host is not a useful quality benchmark for 2.6B models. Small models can produce wrong answers and tool calls. Native tool support alone does not guarantee successful file repair in every harness.

The downloaded desktop binary supports Windows, macOS and Linux x86_64. ARM Linux/Android require a compatible external engine/build; Termux installation is not a promise of tested on-device inference. Windows and Android execution were not verified in this review.

Cloud providers

Install the matching extra, then configure the appropriate key in your own environment or the onboarding wizard. Never commit keys.

nexus-agent chat -p openai "Explain @calculator.py"
nexus-agent chat -p anthropic "Review this project"
nexus-agent chat -p gemini "Explain this function"

-p auto uses configured providers with fallback. Cloud calls can cost money and transmit your selected file context. Cost output is an estimate based on the supported pricing table, not an invoice; unknown model prices are not tracked. Local sessions show zero estimated provider cost. OpenAI streaming requests usage totals when supported by the endpoint.

Tools and persistence

Default tools: read_file, write_file, patch_file, list_directory, run_code, search_web, git_status, git_diff. Optional trusted-project tools: run_tests, git_commit.

Python writes and patches are syntax-checked before changing the file. This detects malformed Python, not logical mistakes.

patch_file creates a backup and requires a unique target unless multiple replacements are explicitly requested. Inspect the backup and diff. Workspace path checks resolve symlinks; they are not OS-level isolation or protection against concurrent filesystem changes.

Edit safety. write_file (over an existing file) and patch_file are refused unless the file was read with read_file (or an @file mention) in this run and has not changed on disk since. If a patch target is not found, Nexus tolerates CRLF line endings and trailing-whitespace differences, otherwise it returns the closest real text so the model can retry. nexus-agent chat exits with code 1 if a tool error was never fixed by the end of the run, or the iteration limit was hit; errors the model recovered from do not count.

API keys. Keys entered during setup are typed hidden (not echoed) and saved to ~/.nexus-agent/.env, created with mode 0600 inside a 0700 folder on Linux and macOS. On Windows the file sits in your user profile folder; POSIX permission bits do not apply there. nexus-agent doctor warns if the key file is readable by other users.

Sessions: the REPL now saves the conversation by default, one session per folder, in ~/.nexus-agent/history.db. Open the REPL again in the same folder and it resumes. nexus-agent repl --continue (or -c) resumes the most recently used session from any folder, --session NAME picks one, and --no-persist gives a throwaway session. One-shot nexus-agent chat "question" still does not save unless you pass --persist or --continue. File contents you read can be kept in the saved history, so use --no-persist for sensitive work. File context may be retained in that history. Both persistent and in-memory history preserve user-turn/tool-result boundaries when pruning; an active long turn can exceed the nominal message limit rather than lose its original request.

CodeForge-250M integration is planned, not a shipped inference backend.

Development and tests

python -m pip install -e '.[dev,cloud]'
python -m pytest -q
python -m pip install build
python -m build

CI runs Python 3.11, 3.12 and 3.13. The September 30 branch suite has 135 tests, including regression coverage for read-only capability bypass, credential attachment, pytest opt-in and environment stripping, Git mutation gating, SQLite history, local cost and detailed context errors. Passing tests are not proof of model accuracy or a hostile-code sandbox.

Use a clean environment to install the built wheel and test the CLI before publishing. Publication requires a separate maintainer approval and matching version; branch fixes do not update existing PyPI installations.

License

MIT. See LICENSE.