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. PLAN.md is the build plan this follows.
export ANTHROPIC_API_KEY=sk-ant-...
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
A different endpoint — a gateway, a proxy, a local server speaking the Messages API:
./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 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.
Useful flags: -model, -effort low|medium|high|xhigh|max, -max-turns,
-resume latest, -no-stream, -no-cache, -v, -yolo, -no-tui, -no-skills.
On a terminal, the interactive mode is a full-screen Bubble Tea 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.
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 is about. The prompt box grows with what you type, up to six lines.
| Key | Does |
|---|---|
enter | send |
/ | open the command menu — see below |
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".
/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.
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.
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):
{
"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.
| 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 |
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
---
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/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.
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 $(...), backticks, or an
interpreter with -c cannot be analysed at all, so it is marked opaque and
"always allow this exact command" is refused for it. String-matching
allowlists for shell commands do not work; this exists to make the prompt
specific, not to make the decision for you.
A sandbox. This is the layer that actually contains a mistake, and the harness cannot provide it for you. The honest recipe on Linux:
podman run --rm -it \
-v "$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:
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.
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.