Tuning Fork

Tune agent skills to your own model.

See it work tells the story behind the figure.

Tuning Fork (tuningfork) sits between your agent and your LLM and records every call the agent makes. If the LLM is the CPU and a skill is source code written in natural language, tuningfork is the debugger you have been missing: it finds the step where a run goes wrong, shows why the model went that way, down to the odds of each word, and tests your edit to the skill to tell a real fix from luck.

It works with Hermes Agent and Claude Code on llama.cpp, from a terminal, a web UI, or an agent over MCP. Any model that llama.cpp serves can be recorded and tested. Seeing the odds of each word works for Gemma 4, Qwen3.6, Qwen3.5, Qwen3-Coder, Qwen3 and Qwen2.5 so far, and the list will grow as the repo evolves (how to serve them).

Tuning Fork is pre-alpha: commands and their output may still change.

Why

You have a skill (agentskills.io-style SKILL.md) that works three times out of five. So you reword it, run the task again, and squint at the session history to see whether the edit worked; then again at the next failure, in a never-ending whack-a-mole. And when you change your LLM, most edits seem to lose their effect. This problem happens because:

• Wording sensitivity is real, and model-specific. Open models swing hard on meaning-preserving format changes, and the best format does not transfer between models (Sclar et al., ICLR 2024).
• Luck looks like progress. A run that passes after your edit may owe it to one lucky token, not to the edit.
• No automated tool. Reading agent session histories is like doing desk check in software development.

Quickstart

tuningfork runs on Linux, or on Windows inside WSL 2 (macOS support planned), with Python 3.12 or later.

Install it with uv, which fetches Python itself if you need it.

Try it without a GPU (five-minute UI walkthrough)

uvx --from tuningfork-lab tuningfork demo

Open the web UI at the address it prints. The runs are made up and the model is a script, but tuningfork's own proxy serves them, so every button works. The guide says what to click, and its Part 1 installs tuningfork and tours runs like these from the terminal too.

Install Tuning Fork (tuningfork)

uv tool install "tuningfork-lab[mcp]"

The command is tuningfork, and [mcp] brings its MCP server. To follow the guides, which use the repository's examples, clone it instead:

git clone https://github.com/hhuang91/tuningfork.git
cd tuningfork
uv sync
source .venv/bin/activate

The last line activates the environment, so that tuningfork and python are the environment's own.

On your own agent

You need a GPU that fits your model, llama.cpp's llama-server, and a harness: Hermes Agent or Claude Code (the versions we use). On Linux, install bubblewrap too (sudo apt install bubblewrap), so that every run sees the same folders: the model reads them (why it matters).

1. Start the model server, with the flags that make its numbers repeat (why). This is the guide's, Gemma 4 31B in about 24 GiB:

   llama-server -hf lmstudio-community/gemma-4-31B-it-GGUF:Q4_K_M --no-mmproj \
     --alias gemma-4-31b --host 127.0.0.1 --port 8081 \
     -ngl 999 -c 131072 -np 1 -ctk q4_0 -ctv q4_0 --jinja --ctx-checkpoints 0 --no-cache-prompt
   

2. Start tuningfork in front of it. It records to ~/.tuningfork and prints the web UI's address:

   tuningfork serve --upstream http://127.0.0.1:8081
   

   --upstream is the model server it forwards each call to. The harnesses tuningfork launches are pointed at tuningfork's own proxy for you. To record running agents as you use them, connect them by setting their base URL (where they send their model calls) to tuningfork's proxy: http://127.0.0.1:8787/v1 for agents that use an OpenAI-style API, such as Hermes Agent, or http://127.0.0.1:8787 for those that use Anthropic's, such as Claude Code. Recording an agent as you use it says how to move the proxy to another port, and how to reach it from another machine.

3. Check the server, in a second terminal (in a clone, with the venv activated): can it do what the harnesses need, and does it process every prompt whole?

   tuningfork connections check
   

4. Record runs of a task — a prompt, its input files and a pass/fail check (the format). This one is the repository's example, in a clone:

   tuningfork batch examples/tally-report.evals.json -n 8 --harness hermes    # or --harness claude-code
   

   Or start from what your agent did: a conversation it already had imports as a run (tuningfork import, from a Hermes session file or export), and any run, imported or recorded as you used it, becomes a task of its own (tuningfork scenario <run> <name>).

5. Look at them in the web UI, at the address serve printed, or in a terminal: what you can do.

docs/setup.md has the rest: other servers, the harnesses, the sandbox, a pinned clock, and protecting a server that matters.

How it works

tuningfork is a recording proxy at the seam between the harness and the model server. It speaks both OpenAI chat completions and Anthropic Messages, so a harness that takes a custom base URL goes through it unchanged.

See the diagram on GitHub.

Model requests are stateless: every request carries the model's whole context. So the proxy sees the full state of the program at every step without touching the harness's code, and any step can be sent again — as recorded, or with the skill's text swapped for an edit. A fork goes further: it relaunches the harness on the same task, answers its calls from the recording up to the step, and lets it run live from there to the end, where the task's check decides.

The work goes round one loop, from a skill to its next version, and the commands below follow it:

See the diagram on GitHub.

What you can do

Each row is a question, with one command that answers it. The question leads to its part of docs/commands.md, which lists its other commands with their main options, and every command takes --help. The web UI does nearly all of it with a click, and an agent can do the reading, measuring and refining itself, over MCP (docs/agents.md).

Question	For example	What it does
Does my skill work?	tuningfork batch spec.json -n 8	Runs a task N times, each run judged by the task's check
What happened in a run?	tuningfork timeline <run>	The run as one conversation, each step's answer where it happened
Where do the runs go wrong?	tuningfork divergence --task <id>	The first step where a task's runs stop agreeing, and how the runs that did each thing fared
Bad luck, or the wording?	tuningfork ab <run> <step> --variant SKILL.v2.md --target-match RE	Your edit against the original at that step, its history held fixed, toward a target declared before sampling
Which token decided it?	tuningfork forking-tokens <run> <step>	Each likely alternative forced at the uncertain tokens: which ones change what the model does
What is the model doing right now?	tuningfork break add --tool write_file	Stops a live call after the model answers, before the harness acts
How do I fix the skill?	tuningfork refine start <run> <step> --target-rules '[…]'	Starts a refinement, declaring what the step should do; each edit is then tested at the step, and a survivor is tested in full runs and written into the skill only when you approve it
Setting up, and keeping things tidy	tuningfork serve --upstream URL	The recording proxy, with its web UI, its jobs and its MCP route

The web UI

tuningfork serve serves a web UI at the address it prints, and tuningfork ui serves the same pages, read-only, over recordings with no proxy running. Everything above is in it, and it can start the work as well as show it: a batch of runs, a resample, an A/B, a fork, a refinement. Here, on the demo's scripted model, an A/B is a few clicks from a run's step:

A step's answer can be read as the model's own tokens, shaded by how unsure it was. This is the figure's fork, in a run that went on to search: the model wrote "Wait" at 7%, where "According" was at 80%.

More of its pages, pictured.

Let an agent drive it

Every operation is also a tool for an agent, over the Model Context Protocol (tuningfork mcp). Hermes Agent, Claude Code or OpenJarvis can count what the failed runs' checks said, find where a task's runs split, read that step, measure it, and screen edits of its own. No agent is offered anything that approves an edit, sends your recordings to another model, or shows it a held-out task's verdicts: those stay with you.

docs/agents.md sets it up for each client. examples/hermes-architecture-diagram/ is a case study: Hermes improving one of its own skills with tuningfork's tools, beside Hermes's own self-improvement loop, on the same local model.

See it work

The toy skill in examples/find-the-file/ summarizes a CSV into report.json, and says that if the file is not there, the agent should say so and stop. The request names data.csv, but the file is in a subfolder. Ten runs through Hermes Agent, on Gemma 4 31B:

$ tuningfork batch examples/find-the-file/evals.json -n 10 --harness hermes --model gemma-4-31b
...
4/10  40%  [17%-69%] passed

Every run first read data.csv where the request names it, and found nothing. Then it either searched for the file and passed, or gave up and failed. Bad luck, or the wording? Sent to the model twenty more times with its history held fixed, that step gives up 16 times. Scored, its answer shows where the reasoning forks. After "…but it was not found.", the next word is "According" at 80%, which goes on "…I should say so and stop", or "Wait" at 7%, which goes on "…search for it first?". The model wavers there, so one line of the skill can tip it. Tested at that step, toward a target declared before the first sample:

$ tuningfork ab <run> 1 --variant examples/find-the-file/fix/SKILL.md \
    --target-rules '[{"if": "calls", "value": "search_files"}]'
...
target (declared before sampling): a behaviour that calls search_files
  original  4/20  20% [8%-42%]
  variant   20/20 100% [84%-100%]
  Fisher exact p = 1.5e-07 → significant at α=0.05, variant higher

Full runs agree: with the edit in every run, 10/10 pass [72%-100%], against 4/10 before (Fisher p = 0.011). Every rate comes with its 95% interval, and an A/B without a declared target is labelled exploratory and corrected for every comparison it makes. The example shows each command with its output.

Guides

• Tuning Fork, step by step — every part of the tool, on a toy skill made to fail one way: a tour with no GPU, the real experiment with Claude Code and Hermes Agent, a look inside a step, and a fix proposed, screened, validated and approved.
• A step where the model wavers — the runs behind the figure at the top: where the runs part ways, how often the model goes each way, where its reasoning forks, and the line that tips it.
• A real skill, fixed at the step that fails — Hermes Agent's own design-md skill, from the failing step to an approved edit and the runs after it.
• The same model, two ways to improve a skill — Hermes's own self-improvement loop against Hermes with tuningfork's tools.
• Reference: setup · commands · the web UI · agents over MCP · writing a harness adapter · tuningfork glossary for the words.

Works with

	Today
Harnesses	Hermes Agent and Claude Code, which tuningfork launches, forks and validates. Any other harness that takes a custom base URL is recorded as it runs. Hermes Agent's session files and exports import as runs.
Inference servers	llama.cpp's llama-server, for everything. LM Studio, and other servers that speak OpenAI chat completions or Anthropic Messages, for recording and experiments at a step; tuningfork connections check measures what yours supports. A hosted model's API, for recording only: tuningfork sends no API key of its own (cloud models).
Token-level work	llama.cpp, with a model whose raw output tuningfork reads: Gemma 4, Qwen2.5, Qwen3, Qwen3.5, Qwen3.6 or Qwen3-Coder.
Systems	Linux, natively or under WSL 2 on Windows, with Python 3.12, 3.13 or 3.14. macOS is untested, and the sandbox needs Linux.

What is particular to a harness or a server lives behind an adapter, never in the core: docs/adapters/TEMPLATE.md is where a new harness starts, and tuningfork probe <url> writes a profile of a server that you can contribute back.

Contributing

The test suite runs against a scriptable mock server, with no GPU and no model: uv run pytest. CONTRIBUTING.md says how to set up and what review looks for; SECURITY.md says how to report a vulnerability, and what tuningfork does and does not guard. What changed in each version is in CHANGELOG.md.

License

Apache-2.0: see LICENSE, and NOTICE for the work it follows.