Alibi

We check whether your agent has an alibi for what it did.

Alibi finds where a long AI-agent run went wrong. It borrows the playbook of automotive driver-assistance systems: treat the run as a time series, filter it forward, detect the fault, then smooth backward to the moment it began. The only sensor is Jev, TypeSafe AI's System One model, which answers with calibrated probabilities instead of text.

It is built for long traces, where reading everything at once breaks down: on real coding failures of 57K to 120K tokens, a single whole-trace read found the root-cause step in 0% of cases; Alibi found it in 16% and 7.8%, and points to the right neighbourhood (within 3 steps) in 21% and 17%.

How it works

flowchart LR
  A[Trace] --> B["Chapters of N tokens<br/>(default N = 10,000)"]
  B --> C[Forward filter:<br/>Jev rates each chapter]
  C -->|memory card| C
  C --> D[CUSUM alarm<br/>on health]
  D --> E[Look back from the alarm:<br/>chapter + per-step evidence]
  E --> F[Top 3 steps to read,<br/>earliest strong suspect first]

What you get

A short trace is refused, without spending a call:

$ alibi diagnose examples/sample_trace.json
Trace is 219 tokens, under the 50,000-token threshold: a single direct read is
enough; Alibi adds value on long traces.

A long one comes back as three steps to read, in order. This is a real run from the locked TrajErrBench set (a Claude Opus coding agent failing on a qutebrowser issue), replayed from its recorded result:

$ alibi diagnose trace.json
Read these 3 steps first, in order.
80,778 tokens, 10 chapters, alarm at chapter 6; 17 Jev calls, 35 s, $0.0096
1. step 74 (assistant), chapter 6, score 0.277
   'Now I see the full picture. The test on line 458 expects
    `str(proc.outcome) == 'Testprocess crashed.'` for SIGSEGV...'
2. step 76 (assistant), chapter 6, score 0.202
   '## Phase 5: FIX ANALYSIS\n\nNow I have a clear understanding. Let me
    implement the changes to `guiprocess.py`...'
3. step 78 (assistant), chapter 6, score 0.178
   'Now let me implement all the changes:\nTool calls:\nstr_replace_editor(...'

Step 74 is the labelled root cause: the agent reads the test wrong and every later edit builds on that reading. Being right at rank 1 happens on 16% of these traces; the honest claim is that three steps out of 118 is a much smaller haystack.

Results

Locked test sets, each run once against a pass bar written down beforehand:

Against published methods on the LongRCA leaderboard (all on DeepSeek-V4-Flash; exact root step, same 128 SWE-bench Pro failures):

Alibi ties ECHO (Fisher p = 0.66) and trails RCTA (p < 0.000001), which traces each suspect back to the handoff instruction between agents.

When to use it, and when not to

• Use it for traces of tens of thousands of tokens or more (default gate: 50K tokens).
• Do not use it for short traces: a single direct read by any capable model is enough, and Alibi says so without spending a call.
• Treat the output as "read these 3 steps first", not as a verdict.

Speed and cost

• Each chapter is one Jev call, and chapters are judged in parallel on the way back. A 38K-token trace is 4.6 chapters and 15 seconds of judge time; a 120K-token trace is 17 chapters and about 48 seconds. No reasoning tokens are generated.
• Cost scales with trace length: about $0.08 per million trace tokens at Jev's price of $0.042 per million input tokens, since every token is read about twice (forward, then the look-back). That is roughly $0.01 for a 100K-token trace, and $0.02 for a 250K one.

Install and run

In Claude Code (the plugin)

/plugin marketplace add ahmedezz26/alibi
/plugin install alibi@alibi

Then set one variable in the shell you start Claude Code from:

export TYPESAFE_API_KEY=...          # a key from TypeSafe AI

That is the whole setup. The plugin launches the MCP server itself with uvx, and sets the other two variables (ALIBI_JUDGE_BACKEND=typesafe, ALIBI_ALLOW_PAID_MODELS=1) for you. Ask Claude Code "why did my last session go wrong?" and it finds the session file, calls the tool, and reads the suspect steps back to you.

On the command line

The PyPI distribution is agent-alibi; the import package is alibi.

1. Install it.

uv tool install agent-alibi

Or run it without installing anything: uvx --from agent-alibi alibi diagnose ...

2. Check it runs, before any key and before any spending. Any short trace is answered locally, so this costs nothing and needs nothing configured:

$ printf '[{"type":"user","inputs":{"q":"hi"}}]' > /tmp/t.json
$ alibi diagnose /tmp/t.json
Trace is 9 tokens, under the 50,000-token threshold: a single direct read is enough;
Alibi adds value on long traces.

3. Set three variables. Jev is a paid API, and all three are required before Alibi will call it:

export ALIBI_JUDGE_BACKEND=typesafe   # Jev is the only sensor the method was measured with
export ALIBI_ALLOW_PAID_MODELS=1      # the spend guard, off by default
export TYPESAFE_API_KEY=...           # your key from TypeSafe AI

Miss one and the run stops at exit code 2, naming what is missing, with nothing sent and nothing spent:

$ alibi diagnose long-trace.json          # with none of them set
Alibi needs the Jev backend: set ALIBI_JUDGE_BACKEND=typesafe, TYPESAFE_API_KEY and
ALIBI_ALLOW_PAID_MODELS=1. Nothing was sent.

$ alibi diagnose long-trace.json          # with the key missing
TYPESAFE_API_KEY is not set (see .env.example). Nothing was sent.

These variables can also live in a .env file beside the project you run from. A shell variable wins over the file, but a variable you never set in the shell will be taken from it - so if a run spends when you expected it to refuse, check for a .env.

4. Diagnose one trace. One run, one trace:

alibi diagnose path/to/trace.json

What you can point it at

One trace per run. Three kinds are understood, and --source auto (the default) picks by file extension.

A JSON trace file (.json) - a list of steps, oldest first. Every field is optional, but include outputs and error where you have them: the judge reads the whole rendered step, and a misread observation or an unfixed error is most of the signal the method looks for.

[
  {"type": "llm",  "name": "plan",           "inputs": {"task": "Book the cheapest direct flight."},
                                             "outputs": {"text": "Plan: search, filter, pick."}},
  {"type": "tool", "name": "search_flights", "inputs": {"from": "CAI", "to": "BER"},
                                             "outputs": {"flights": []}},
  {"type": "tool", "name": "book",           "inputs": {"flight": "TK33"}, "error": "not direct"}
]

Recognised keys: type (free text: llm, tool, assistant, user, ...), name, inputs, outputs, error, step_id, timestamp (ISO 8601 if present - a value datetime.fromisoformat cannot parse is rejected with exit code 2). Steps are numbered by position, so step 74 in the output is the 75th entry in the file. examples/sample_trace.json is a working example. If your agent writes some other format, convert it to this shape or add a TraceSource adapter (see CONTRIBUTING.md).

A Claude Code session (.jsonl) - the transcripts under ~/.claude/projects/<project>/<session-id>.jsonl, where <project> is your project's path with slashes, underscores and dots turned into dashes (/Users/me/LLM_projects/app becomes -Users-me-LLM-projects-app). To diagnose the most recent session of the project you are in:

alibi diagnose ~/.claude/projects/-Users-me-LLM-projects-app/<session-id>.jsonl

To pick the most recent session of the project you are standing in, without looking the name up (the substitution turns /, _ and . into -, which is the same rule):

alibi diagnose "$(ls -t ~/.claude/projects/"${PWD//[\/_.]/-}"/*.jsonl | head -1)"

A working session is often longer than the 250,000-token ceiling, and is then refused with its cost rather than analysed:

Trace is 333,562 tokens, over the 250,000-token ceiling: analysing it would cost about
$0.03 of Jev. Raise ALIBI_MAX_TRACE_TOKENS to analyse it anyway.

That is the ceiling doing its job. If the estimate is acceptable, say so explicitly:

alibi diagnose <session>.jsonl --max-trace-tokens 400000

Parsing is best effort: the format is internal to Claude Code and may change. Assistant text, tool calls and tool results become steps; thinking blocks are skipped.

A LangSmith trace - pass the trace id and the project, with LANGSMITH_API_KEY set:

alibi diagnose <trace-id> --source langsmith --project "my-project"

There is no folder mode: Alibi diagnoses one run at a time, because the method is a filter over a single time series. To sweep a directory, loop:

for f in traces/*.json; do alibi diagnose "$f" --json > "${f%.json}.diagnosis.json"; done

Reading the result

Read these 3 steps first, in order.
80,778 tokens, 10 chapters, alarm at chapter 6; 17 Jev calls, 35 s, $0.0096
1. step 74 (assistant), chapter 6, score 0.277
   '...'

• chapters - how many windows the trace was split into. Each is ALIBI_JUDGE_WINDOW_TOKENS tokens (default 10,000), so a 80K-token trace is about 10.
• alarm at chapter N - where CUSUM first saw the agent's health break down. The cause is usually at or before it, which is why the look-back starts there. alarm at chapter None means no alarm fired and the run's ending was used as the anchor instead.
• score - fused evidence for that step, not a probability. Only the ordering is meaningful.
• steps are 0-based positions in the trace you passed in.

--json prints the same thing as a JSON object (gated, message, trace_tokens, n_steps, n_chapters, alarm_chapter, anchor, suspects[], judge_calls, judge_seconds, cost_usd) for piping into something else. The three cost fields are null, not zero, if the judge in use keeps no call log - unknown rather than free.

Exit codes: 0 on success, including a gated trace. 2 if the trace file is missing or unreadable, the Jev backend is not configured, or a setting is rejected - 2 always means no judge was built, so nothing was sent and nothing was spent. 3 if the run failed once the judge had started reading the trace. The message says how many Jev calls completed and are billed; at zero, nothing is billed and the first request may never have been sent.

The two gates, and what they cost

Nothing is sent anywhere, and nothing is spent, unless the trace falls between them:

The MCP tool

The server (alibi-mcp, stdio) exposes exactly one tool for any MCP client, not just Claude Code:

diagnose_trace(trace: str, source: str = "auto", project: str | None = None) -> Diagnosis

trace is the same path or id the CLI takes. To wire it up by hand:

{ "mcpServers": { "alibi": {
  "command": "uvx",
  "args": ["--from", "agent-alibi>=0.1,<0.2", "alibi-mcp"],
  "env": { "TYPESAFE_API_KEY": "...", "ALIBI_JUDGE_BACKEND": "typesafe",
           "ALIBI_ALLOW_PAID_MODELS": "1" } } } }

Privacy

Traces above the length gate are sent to TypeSafe's API; below it, nothing leaves your machine. There is no telemetry. Claude Code session parsing is best effort: the transcript format is internal to Claude Code and may change. Session transcripts often contain source code and secrets, so read SECURITY.md before diagnosing one.

Contributing

See CONTRIBUTING.md. Plumbing, adapters, bug fixes and docs are welcome as ordinary pull requests; changes to the estimation method need a pre-registered measurement, for the reason the research log makes obvious.

Background

Alibi started as an experiment by an ADAS engineer: can the tracking filters used in cars work on AI-agent traces? The research log with every pre-registered test, including the ones that failed, is in docs/research-log.md.

License

MIT