# harness
A coding agent harness in Go. Point it at a repository, tell it to make the
failing test pass, and measure how often it actually does.
The code is the byproduct. The deliverable is [METRICS.md](METRICS.md).
[PLAN.md](PLAN.md) is the build plan this follows.
## Quick start
```bash
go build -o harness ./cmd/harness
./harness -C /path/to/a/throwaway/clone # full-screen TUI
./harness -C /path/to/repo -p "why does TestFoo fail?" # one prompt
```
If you are logged into Claude Code, that is all the setup there is: the harness
borrows the OAuth credential from `~/.claude/.credentials.json` when nothing
else supplied one. Otherwise, or to use a different account:
```bash
export ANTHROPIC_API_KEY=sk-ant-...
```
A different endpoint — a gateway, a proxy, a local server speaking the Messages
API:
```bash
./harness -base-url http://127.0.0.1:4000 -api-key ... -C .
# or ANTHROPIC_BASE_URL / ANTHROPIC_API_KEY / ANTHROPIC_AUTH_TOKEN
```
Flags beat the saved config, which beats the built-in defaults; the config is
what [the setup wizard](#setup) writes on a first run. `-auth-token` sends
`Authorization: Bearer`, which is what OAuth tokens and most gateways want;
`-api-key` sends `x-api-key`. A non-default endpoint announces itself on
stderr, because your prompt and every file the agent reads go wherever that URL
points.
The Claude Code login is the last resort, tried only when no flag and no
environment variable supplied a credential, and only against Anthropic's own
endpoint — a credential found on disk is not one you chose to send, so it is
never forwarded to a `-base-url` you pointed the harness at. `-no-claude-code-auth`
skips it entirely. Note that the credential is issued to Claude Code; using it
from another client is outside Anthropic's terms for it, which API keys and
gateway tokens do not carry.
On macOS Claude Code keeps this in the Keychain rather than a file, so there is
nothing to borrow there. An expired access token is refreshed automatically; the
credentials file is left alone unless the server rotates the refresh token, in
which case the replacement is written back — leaving the dead one there is what
would break your Claude Code login.
Useful flags: `-model`, `-effort low|medium|high|xhigh|max`, `-max-turns`,
`-resume latest`, `-no-stream`, `-no-cache`, `-v`, `-yolo`, `-no-tui`, `-no-skills`,
`-no-claude-code-auth`.
## The TUI
On a terminal, the interactive mode is a full-screen [Bubble Tea](https://github.com/charmbracelet/bubbletea)
app. It opens on a welcome block naming the model, the endpoint and the root,
and settles into a transcript above a prompt:
```
⏺ Let me see what the test expects before changing anything.
⏺ ReadFile(path=internal/agent/loop_test.go)
⎿ 1: package agent
…
⏺ EditFile(path=internal/agent/compact.go)
4 additions, 1 removal
@@ -50,7 +50,9 @@
+ if window == 0 {
✢ Noodling… (14s · turn 3 · ↓ 1.3k tokens · esc to interrupt) ● high · /effort
╭──────────────────────────────────────────────────────────────────────────────╮
│ › _ │
╰──────────────────────────────────────────────────────────────────────────────╯
joni ✳ Opus 5 ◐ 62.2k (6%) ↑ 4.2k ⟳ 58.0k ↓ 1.3k $0.0835
⏵ approvals on (shift+tab to cycle) · /help for commands
```
Every event hangs off a dot in the left gutter, and everything belonging to it
is indented under that dot, so a screen of tool output still reads as a
sequence of things that happened. Diffs are shown in full and in colour with a
count of what changed; other output is elided after six lines.
What the model says is Markdown, and it is rendered as Markdown — headings,
weight, lists, block quotes, rules, and tables laid out in columns:
```
⏺ Findings
The refresh path drops the rotated token. Three things are going on:
1. refresh() returns the new pair but never writes it back
2. the old refresh token is already dead by then
File │ Line │ Change
────────────────────────┼───────────────────────┼───────────────────────
refresh.go │ 84 │ write back
func refresh(tok string) (Creds, error) {
// TODO: persist the rotation
}
```
Fenced code is syntax-highlighted by [chroma](https://github.com/alecthomas/chroma),
and every colour — prose, headings, keywords, strings — comes from the theme in
force, so `/setup` repaints Markdown along with everything else. Headings drop
their hashes: the levels are told apart by colour and weight instead. It
renders as it streams, so a table fills in row by row.
Only what the model says goes through this. Tool results and the output of your
own `!` and `$` commands are program output and are shown as they came, and
`-no-tui` prints raw text so `-p` stays pipeable.
The status bar reports the context gauge — the last turn's input size against
the window, which drops when the conversation is compacted — and separates
fresh input tokens from cache reads, because the gap between the two is what
[METRICS.md](METRICS.md) is about. The prompt box grows with what you type, up
to six lines.
| Key | Does |
| --- | --- |
| `enter` | send |
| `/` | open the command menu — see below |
| `!` | run a shell command yourself — see below |
| `$` | the same, through `python3` |
| `ctrl+j` | newline without sending |
| `shift+tab` | cycle the approval mode |
| `ctrl+r` | expand elided tool output |
| `pgup`/`pgdn` | scroll the transcript |
| `esc` | interrupt the turn |
| `ctrl+c` | interrupt the turn; again when idle to exit |
Commands: `/allowed`, `/clear`, `/effort`, `/exit`, `/help`, `/model`,
`/setup`, `/skills`, and one per discovered skill.
A slash at the start of an empty prompt opens the command menu, which filters
as you type — a name that starts with what you typed before one that merely
contains it:
```
╭──────────────────────────────────────────────────────────────────────────────╮
│ › /s │
╰──────────────────────────────────────────────────────────────────────────────╯
❯ /skills the instruction sets found at startup
/systematic-debugging find the cause before changing code (project)
/brainstorming explore an idea before building it (personal)
```
`↑`/`↓` move, `tab` or `enter` completes the highlighted name into the prompt —
completing is not sending, so there is room to type arguments for a skill or a
level for `/effort` — and `esc` dismisses the menu without touching what you
typed. The menu offers exactly the commands the TUI answers, so nothing it
suggests can come back as "unknown command".
### Running something yourself
`!` on an empty prompt points the box at a shell instead of at the model; `$`
points it at `python3`. The marker replaces the `›`, the frame changes colour,
and `enter` runs the line where the agent's own `bash` tool would have run it —
from the working root, with the same timeout and the same output cap.
```
╭──────────────────────────────────────────────────────────────────────────────╮
│ ! go test ./internal/agent/ │
╰──────────────────────────────────────────────────────────────────────────────╯
```
```
⏺ ! go test ./internal/agent/
⎿ exit code: 1 (2.3s)
--- stdout ---
--- FAIL: TestCompactKeepsTheLastExchange (0.00s)
compact_test.go:88: history has 2 messages, want 4
… +9 more lines (ctrl+r)
```
**The agent is shown what you saw.** The command and its output are appended to
the conversation as a user message, so the next thing you type can be "fix
that" and the model already has the failure. Nothing is sent at that moment —
it waits in the history and goes up with your next real turn, so several
commands can be run before saying anything about them.
What reaches the model is the capped report, not the raw firehose: a noisy
command cannot cost more context from the prompt box than it would through the
tool. The transcript collapses it further to six lines, and `ctrl+r` expands to
exactly what the model was given, so the two never drift.
`esc` leaves the mode, as does `backspace` over the marker. The mode is
per-command — after a run the prompt returns to normal, the way a shell prompt
comes back. One command runs at a time; while it does, `esc` interrupts it and
a message you type waits rather than jumping ahead of the output it is about.
A `!` typed mid-sentence is an exclamation mark.
These bypass the approval policy by design: you typed the command, so there is
nobody left to ask. They are also the one part of the TUI that runs something
without the model's involvement — worth remembering when the root is not a
throwaway clone.
`/model` asks the endpoint what it serves — `GET /v1/models`, paginated — and
opens a picker on the answer:
```
Select model
from anthropic · ↑↓ move · enter apply · esc cancel
1. Claude Fable 5.1 claude-fable-5-1 · 1.0M in · $10 / $50 per Mtok
2. Claude Opus 5 claude-opus-5 · 1.0M in · $5 / $25 per Mtok ✓
❯ 3. Claude Sonnet 5 claude-sonnet-5 · 1.0M in · $2 / $10 per Mtok
4. Claude Haiku 4.5 claude-haiku-4-5-20251001 · 200.0k in · $1 / $5 per Mtok
```
The cursor starts on the model in force, which is also the one marked. The id,
the context window and the list price come from the endpoint and from the
pricing table; a model neither knows about simply says less. The switch applies
from the next turn — the conversation so far is replayed to the new model — and
costs already incurred stay priced at the model that incurred them, because a
session that switched models was not all billed at the model it ended on.
Not every endpoint serves that route. When the fetch fails the picker says so,
names the host and the reason, and closes; `/model <id>` and the `-model` flag
still switch without asking anyone's permission.
`/effort` opens a slider in place of the prompt, on the level in force — an
unset level is the API's own default, `high`, so the marker starts there rather
than at an end:
```
Faster Smarter
──────────────────────────────────────▲───────────────────────────────────
low medium high xhigh max
←→ move · 1-5 jump · enter apply · esc cancel
```
Moving the marker changes nothing until `enter`; `esc` leaves the level alone.
`/effort xhigh` still applies a level directly for anyone who can type it, and
either way it is the `-effort` flag reached from the keyboard. The level in
force shows above the prompt on the right. A terminal too narrow for five
labels gets `◀ high ▶` and the same keys rather than a track with the words
sliced in half.
`/model` and `/effort` change the session. `/setup` changes what the next
session starts with — see [Setup](#setup).
`shift+tab` cycles between asking before every mutating call and accepting them
all, which is `-yolo` reached from the keyboard. The mode line says which is in
force at all times, and the switch is recorded in the transcript, because a
mode that silently lets the agent write to disk is not one to keep in a corner.
Approvals appear inline as a bordered prompt, answered by number or by the key
in brackets — `y`, `n`, `a` for this exact command, `t` for the whole tool. The
agent goroutine blocks until you answer, which is the point: the tool has not
run yet. Interrupting while that prompt is open denies the call and cancels the
turn rather than leaving the agent waiting on a channel nobody will read.
The TUI is skipped automatically when stdin or stdout is not a terminal, and by
`-p` and `-no-tui`, which use the line-based renderer instead. Both front ends
consume the same `agent.Event` stream, so neither can show something the other
cannot.
## Setup
The first run on a terminal opens a full-screen wizard instead of the chat: the
mark assembles, the name lights up, and it asks four questions one at a time.
```
harness setup step 1 of 4
◆━━━━━━━━━━━━━━━◇───────────────◇───────────────◇
Which model should the agent use?
▌ ◆ Claude Opus 5 $5 / $25 per Mtok
◇ Claude Sonnet 5 $2 / $10 per Mtok
◇ Claude Haiku 4.5 $1 / $5 per Mtok
◇ custom…
claude-opus-5 · 1.0M token context
↑↓ move enter next esc back ctrl+c cancel
```
| Question | Answers |
| --- | --- |
| Model | whatever `GET /v1/models` returns, plus `custom…` for any id |
| Effort | the API's own `llm.EffortLevels`: `low` `medium` `high` `xhigh` `max` |
| Approvals | ask before every mutating call, or accept them all |
| Theme | `ember` `mint` `iris` `mono` |
It is the same catalogue and the same pricing table `/model` uses, through the
same `GET /v1/models` call, so the two cannot disagree about what a model is
called or what it costs. Where they differ is failure: `/model` reports the
error and closes, because there is a session to go back to. The wizard cannot
do that on a first run, so it falls back to the models this build can price and
says so. `custom…` is there because a gateway may serve something no build has
heard of.
The theme step previews by applying the palette for real — reading a swatch
tells you nothing about what a transcript looks like in it — and it recolours
the whole TUI, pickers and slider included. Cancelling puts the old one back.
Answers land in `~/.config/harness/config.json` (`XDG_CONFIG_HOME` is
honoured):
```json
{
"version": 1,
"model": "claude-opus-5",
"effort": "high",
"approvals": "ask",
"theme": "ember"
}
```
Nothing in that file is required, and none of it wins an argument with a flag:
precedence is **flag, then config, then the built-in default**, so
`-model claude-sonnet-5` is still a one-run override. `accept-all` saved as a
default is honoured, but `-yolo` can only turn approvals off, never on — a file
should not be able to quietly grant the agent more than the command line did.
A value this build does not recognise is dropped rather than forwarded to the
API, and effort is checked against `llm.EffortLevels` rather than a second copy
of the same five strings.
`/setup` reopens the wizard at any time. Finishing applies the answers through
the same setters `/model` and `/effort` use, so a setting chosen in the wizard
and the same setting chosen from the prompt cannot come out different — the
wizard just reports them in one line instead of three. It is refused mid-turn:
changing the model while a turn is in flight would finish it on a different
model than it started. A config file that cannot be parsed is reported and then
ignored, and is never overwritten until you finish the wizard.
`/model` and `/effort` change the running session and say "from the next turn
on"; they do not write to the file. The wizard is the thing that decides what
the *next* session starts with, which is why it is a separate screen rather
than a fourth picker.
The whole thing is `↑↓` and `enter`, `esc` to go back. Animations collapse to
their final frame under `NO_COLOR`, a `dumb` terminal, or a window under 60×20,
and any key skips the one playing.
## Tools
| Tool | Approval | Notes |
| --- | --- | --- |
| `read_file` | automatic | Numbered lines, `offset`/`limit` for paging |
| `glob` | automatic | Newest first; `*.go` matches at any depth |
| `grep` | automatic | ripgrep when present, pure Go otherwise, identical output either way |
| `edit_file` | prompts | Requires a prior read; refuses ambiguous matches; returns a diff |
| `write_file` | prompts | Whole files only; separate from `edit_file` so the metrics stay honest |
| `bash` | prompts | Own process group, killed whole on timeout; output capped head and tail |
## Skills
A skill is a directory of instructions the agent pulls in when it is relevant:
```
~/.agents/skills/commit-helper/SKILL.md every repo
./.agents/skills/commit-helper/SKILL.md this one, checked in
```
```markdown
---
name: commit-helper
description: Use when writing commit messages for this repo
---
Read the last twenty commits for tone before writing anything.
Conventions are in references/conventions.md.
```
The directory name is the skill's name, and a `name:` that disagrees with it is
a load error. Only the description reaches the model until the skill is
invoked, so it is what decides whether the skill fires at all; the body can be
as long as you like.
Two ways in, one way to load. The agent calls the `skill` tool when a
description matches what it is doing; you can force one with `/commit-helper
fix the flaky test`, which the model answers by making the same call. `/skills`
lists what was found, project shadowing personal on a name collision.
Skill directories are readable by `read_file`, `glob` and `grep` even though
they sit outside the working root, so a skill can ship reference files and
point at them. They are not writable by anything: `edit_file`, `write_file` and
`bash` keep the working root as their only root. A skill is input, not a work
product.
Relative paths still anchor to the working root, so a skill pointing at its own
files is read with the directory the `skill` tool prints joined on. If the model
forgets, the "no such file" it gets back names the skill directories, which is
usually enough to recover on the next turn. A shadowed skill's directory is not
readable — only the skills actually in play widen anything.
Discovery happens at startup. The name-and-description listing goes in the
cached prompt prefix, so re-reading the directory each turn would cost a cache
miss on every turn for a file that never changes. Restart to pick up a new
skill. `-no-skills` turns the whole thing off; the eval runner never discovers
skills at all, so METRICS.md is unaffected by what is in your home directory.
## Evals
```bash
./evals/selftest.sh # no API calls: checks the tasks measure something
go run ./evals -n 3 # the real thing
```
Ten tasks, a fresh temp directory per run, full transcripts under
`evals/runs/<timestamp>/`. See [METRICS.md](METRICS.md).
## Safety
Three layers, in increasing order of how much they actually protect you.
**Path jail.** Every path a tool is given is resolved with
`filepath.EvalSymlinks` and refused if it lands outside the working root,
including through a symlink that points out of the tree. Cheap, and it catches
a lot.
**Approval prompts.** Read-only tools run unattended. `bash`, `edit_file` and
`write_file` ask, with "yes / no / always this exact one / always this tool" and
session memory. `/allowed` lists what you have agreed to.
Shell commands are parsed with `mvdan.cc/sh/syntax` so the prompt can tell you
what a command will actually run — `git status; rm -rf ~` is shown as running
both `git` and `rm`. **This is not a security boundary**, and the harness does
not pretend otherwise: a command containing `$(...)`, backticks, or an
interpreter with `-c` cannot be analysed at all, so it is marked opaque and
"always allow this exact command" is refused for it. String-matching
allowlists for shell commands do not work; this exists to make the prompt
specific, not to make the decision for you.
**A sandbox.** This is the layer that actually contains a mistake, and the
harness cannot provide it for you. The honest recipe on Linux:
```bash
podman run --rm -it \
-v "$PWD":/work:Z -w /work \
-e ANTHROPIC_API_KEY \
docker.io/library/golang:1.26 \
/work/harness -C /work -yolo
```
**Decide about egress explicitly.** The container above has network access,
because the harness needs to reach the API — which means the agent's `bash`
has it too. That is the line between "broke my repo" and "read my `.env` and
made an HTTP request". If that matters for the tree you are pointing it at:
- Treat every secret reachable from inside the container as exposed.
- Use a token scoped to nothing else, and a clone rather than your working tree.
- For real isolation, run the harness on the host and put only `bash` in a
network-less namespace. That is not implemented here.
`-yolo` skips every prompt. It is correct inside a container or against a
throwaway clone, and wrong everywhere else. The eval runner sets it, because it
is headless and operates on a temp directory it created.
## Layout
```
cmd/harness flag parsing, wiring, REPL
internal/llm Messages API: types, HTTP, SSE, pricing
internal/agent turn loop, conversation state, compaction, sessions
internal/tools the tool registry and the tools
internal/skills discovery and parsing of SKILL.md
internal/config saved defaults: model, effort, approvals, theme
internal/approval allow / deny / ask, with shell analysis
internal/metrics JSONL event log
internal/ui line-based terminal rendering
internal/tui full-screen Bubble Tea front end, and the setup wizard
evals task suite and runner
```
Two rules hold the shape: `agent` never imports `ui` or `tui`, so the eval
runner can drive the loop headless; and every tool returns `(string, error)`
where the error is text the model reads, not an exception the harness swallows.
A failed edit is a normal event in the transcript.
The TUI obeys the first rule through two one-way hatches. Events reach it via
`agent.Observe`, forwarded into the Bubble Tea loop with `Program.Send`, which
is safe from the agent's goroutine. Approvals go out the same way and then
block the agent on a reply channel until a key arrives. That channel is
buffered and the wait honours the turn's context, so neither side can strand
the other.