retrial

Git for agent trajectories. Branch, diff, and bisect LLM agent runs — backed by real re-execution, not static logs.

When an agent does something wrong, most debugging is scrolling through logs. retrial lets you go back to step N, change one fact — a tool result, a retrieved doc, an intermediate decision — and re-run your real agent from there to see what it actually would have done differently. With the real model, not a guess. Validated end to end against live claude-opus-4-8.

Why it's different

• Real re-execution, not log branching. A fork re-enters your live agent loop and makes real model calls from the fork point onward. It does not relabel stored JSON and call it a branch.
• Zero-export integration. A decorator on your loop, not a JSON schema to hand-build.
• Narrow and deep. One thing — branch/diff/bisect by real execution — done well.
• Python-native, local-first. No account, no telemetry, no dashboard. One SQLite file in .retrial/.

Install

pip install retrial

Integrate

retrial needs two things from your loop: the function that calls the model, and the function that runs tool calls. You pass both in — no monkey-patching of your SDK, so every recorded step traces back to a line you wrote.

from retrial import record

@record(session_name="booking-agent")
def run_agent(messages, tools=TOOLS, call_model=call_model, execute_tools=execute_tools):
    while True:
        response = call_model(messages, tools)
        messages.append({"role": "assistant", "content": response.content})
        if response.stop_reason != "tool_use":
            return response
        messages.append({"role": "user", "content": execute_tools(response)})

The one rule: messages must be a parameter, not a list created inside the function. That's what lets a fork seed your loop with edited history and get genuine re-execution. Give the other parameters defaults to fork from the CLI. Then just run it; steps log as the loop runs, with no export step. Async agents work too — decorate the async def and await it as usual; fork, bisect, ablate, sweep, and rerun re-execute async agents from the CLI or any synchronous caller.

A bundled example under examples/ forks, diffs, and bisects with no API key.

Example: fork, then diff

Every step gets a content-hash SHA, addressable by a short prefix like git:

$ retrial log s_a8d4f64945
  4f0c1e2  step 0  model_call  (312ms, 450 tok)
  a1b2c3d  step 1  tool_call   ran search_flight
  9e77b10  step 2  model_call  (288ms, 544 tok)

Fork step 1 with one fact changed — the edit is a JSON patch — and the real model decides again from there:

$ retrial fork a1b2c3d --agent examples.booking_agent:run_agent --edit-file edit.json
Forked into session s_3f9c02ab1e

$ retrial diff s_a8d4f64945 s_3f9c02ab1e
diverged at d66697c
  cause: replace /output/0/content = flight_price 1200

  - A  book_flight    model_call
  + B  check_budget   model_call

final answer
  A  Confirmed: AUS-SFO booked for $450.
  B  That's over the $600 limit. I need approval before booking.

The fork called check_budget, a tool the original never touched — the kind of divergence only real re-execution produces. The original is never mutated; a fork is a new session, so you can branch the same step as many times as you like.

Also

Same machinery — a fork plus a check — pointed at different questions:

• bisect — which step doomed a failed run? Binary search over resume points, about log2(steps) re-executions.
• ablate — which recorded facts did a good run actually need? Perturb each and see if the outcome flips. Causal, not heuristic.
• sweep — fork one step across many values to find a decision threshold in the model's behavior.
• rerun — re-execute every recorded run against your current code. Your traces are the regression suite; it exits non-zero on a regression, so CI fails without extra plumbing.
• cost — token and dollar breakdown per step. An unknown model prices as unpriced, never as a guess.

Bisect and ablate are duals and each refuses the other's job: bisect wants a run that failed, ablate a run that worked.

What retrial refuses to do

The whole product rests on the replay being exactly what happened, so retrial raises rather than guesses when it can't verify that:

• Your loop transforms a tool result before appending it — the patch would land on a value you never saw.
• An edit invents or drops a tool result the run never produced.
• A run crashed mid-loop — the message state after its trailing tool call was never observed.
• A sync agent is handed an async call_model or execute_tools — a sync loop can't await it, so retrial would record an un-awaited coroutine. (Async agents are fine; this is only the mismatch.)
• The database was written by a newer retrial, or an imported step's content doesn't match its SHA.

A wrong replay would be worse than no replay. Merge was cut for the same reason: two forks are competing hypotheses, and answers don't merge.

CLI

retrial init                    create .retrial/ + sqlite db
retrial list                    sessions, tree view
retrial log <session>           step-by-step history, SHA per step
retrial show <sha>              full detail on one step
retrial fork <sha> --agent M:F --edit-file e.json
retrial diff <a> <b>            --full to expand shared steps
retrial bisect <session> --check EXPR --agent M:F
retrial ablate <session> --check EXPR --agent M:F
retrial sweep <sha> --values-file v.json --agent M:F
retrial rerun --check EXPR --agent M:F
retrial cost <session>
retrial export <session> > trace.jsonl
retrial import trace.jsonl

SHA prefix matching applies throughout. The store is found by searching upward for .retrial/, the way git finds .git/; override with --db or RETRIAL_DB.

Python API

from retrial import record, fork, diff, bisect, ablate, sweep, rerun, trajectory, Store
from retrial import export, import_

Every function returns a plain dict, so results stay JSON-shaped and printable. The shapes are declared in retrial/types.py and shipped with py.typed, so a typo in a key is a type error, not a KeyError at 3am.

License

MIT