Goal: make an agent run reproducible by recording every nondeterministic input in the event log, then reconstruct the exact committed state at any past point.

Prerequisites: the strata binary on your PATH, and jq for readable event output. Commands write to a durable directory (./replay) that each invocation reopens.

1. Record every external input

Before the agent acts on a clock reading, an API response, or a random draw, write it to the event log. The log is append-only and hash-linked, so it is the source of truth for the run. Derived decisions go in KV.

strata ./replay event append input '{"source":"clock","epoch":1706900000}'
strata ./replay event append input '{"source":"weather_api","temp_c":12}'
strata ./replay event append input '{"source":"rng","value":2}'
strata ./replay kv put decision "option 2"

created applied=true
created applied=true
created applied=true
created decision applied=true

2. Walk the recorded inputs in order

Read the log back by sequence. Because the decision was a pure function of these inputs, re-running the same logic over them reproduces the same result.

strata ./replay event range 0 --json | jq -c '.data.items[] | {seq: .event.sequence, type: .event.event_type, payload: .event.payload}'

{"seq":0,"type":"input","payload":{"epoch":1706900000,"source":"clock"}}
{"seq":1,"type":"input","payload":{"source":"weather_api","temp_c":12}}
{"seq":2,"type":"input","payload":{"source":"rng","value":2}}

3. Verify the log was not tampered with

Every event links to the previous one by hash. verify-chain confirms the sequence is dense and the linkage is intact.

strata ./replay event verify-chain

{
  "error": null,
  "first_invalid": null,
  "length": 3,
  "valid": true
}

4. Reconstruct an exact past state

Suppose the agent later overwrites its decision. You can still recover the earlier committed state: fork a branch at the version where option 2 was written. The decision write committed at version 6, so fork there.

strata ./replay kv put decision "option 9"
strata ./replay branch fork default replay --version 6 --json | jq -c '{name: .data.name, forked_from: .data.parent.name, at_version: .data.parent.fork_version}'
strata ./replay --raw kv get decision --branch replay
strata ./replay --raw kv get decision

updated decision applied=true
{"name":"replay","forked_from":"default","at_version":6}
option 2
option 9

The replay branch reads the historical option 2; default keeps the current option 9.

Why this works

Nondeterminism only breaks reproducibility if it is thrown away. Recording each input in the event log turns the run into a replayable transcript, and hash linkage lets you prove it is unaltered. Every write also gets a monotonic commit version, so forking a branch at that version rebuilds the exact state the agent saw — no snapshots to manage. See the branch management guide for the fork variants (at-version and at-timestamp).