rea/docs/process-capture.md

# Process capture

Process Capture runs one caller-selected command and records bounded evidence
about its terminal behavior, scheduled interactions, process lifetime, and
selected filesystem state. Use it to compare two direct runs of the same
scenario, such as an authority build and a reconstruction. It does not replay a
previous execution or emulate dependencies.

The command runs with the current user's permissions and inherits the host
environment before applying the scenario's explicit overrides. This is not a
sandbox. Filesystem observation paths select what REA snapshots; they do not
restrict what the process can read or write. The inherited environment is not
recorded, so its influence may remain unknown.

## Host support

Capturing a new scenario currently requires Linux or macOS and a working
native PTY backend. Native Windows capture is unavailable because the PTY
adapter does not yet verify descendant cleanup. The native Job Object controls
used by other REA providers do not establish PTY capture support. Reinstalling
the Windows PTY binary does not enable this workflow.

For Linux commands, use Linux REA inside WSL. Adapt the scenario to that host;
this does not establish capture of a native Windows process tree. Comparing
existing capture Evidence through the CLI or MCP remains available on Windows
and does not launch a PTY or the captured target.

On supported capture hosts, a missing or incompatible native PTY binary has a
different recovery: reinstall REA for the active platform, architecture, and
Node.js version with optional dependencies enabled. Capability diagnostics
distinguish this from the Windows capture-adapter limitation.

## Capture a command

Write a JSON scenario and pass its path to the CLI:

```sh
rea capture-process ./scenario.json --json > capture.json
```

`--json` is required when saving input for JSON consumers; the default terminal
format is TOON. The file contains the complete capture Evidence record.
Choose an output file distinct from the scenario input: shell redirection opens
and truncates the output before REA reads the scenario.

For example:

```json
{
  "executable": "node",
  "arguments": ["./signup.mjs"],
  "working_directory": ".",
  "environment": { "APP_MODE": "test" },
  "filesystem_observation_paths": ["./state"],
  "terminal": { "columns": 80, "rows": 24, "scrollback": 1000 },
  "events": [
    {
      "type": "input",
      "at_ms": 250,
      "data": "[email protected]\r",
      "sensitive": true
    },
    { "type": "resize", "at_ms": 500, "columns": 100, "rows": 30 },
    { "type": "signal", "at_ms": 1000, "signal": "SIGINT" }
  ],
  "timeout_ms": 30000,
  "idle_timeout_ms": 30000,
  "settle_ms": 100
}
```

`executable` is required. `arguments`, `working_directory`, `environment`,
`filesystem_observation_paths`, terminal settings, timed `events`, timeouts,
resource limits, and normalization settings have defaults; see
`processScenarioSchema` for the exact contract. Event times are milliseconds
from launch, must be ordered, and must fall within `timeout_ms`. Inputs marked
`sensitive` are sent to the process but persisted as a byte-count placeholder.
Environment overrides are recorded in the scenario commitment; inherited
values are not copied into Evidence.

The capture contains raw PTY output chunks and rendered terminal states,
interaction dispatch outcomes, exit reason, sampled process-tree observations,
and settlement status. When filesystem paths are selected, REA records
`files_before` and `files_after`, then classifies observed entries as created,
deleted, modified, or unchanged. These are bounded snapshots, not a syscall
trace; short-lived changes between snapshots may be missed. With no selected
paths, filesystem effects remain unknown.

Output, file count, file size, process sampling, filesystem depth, total
runtime, idle time, and post-exit settlement are bounded by the scenario's
limits. The result marks truncated observations and residual unknowns rather
than treating missing data as proof of equivalence. Cancellation and timeout
run the same owned-process cleanup path. Settlement reports whether the
sampled process group quiesced or whether cleanup was needed or unverifiable;
sampling cannot prove that every short-lived or detached descendant was seen.

## Compare two captures

Compare saved capture Evidence with:

```sh
rea compare-process-captures authority.json reconstruction.json
```

The comparison checks terminal, interaction, exit, filesystem, and
process observations under a shared comparison contract. It reports observed
differences with their locations, while residual unknowns or truncated
observations prevent a complete-equivalence claim. Matching scenario
commitments do not reveal whether redacted sensitive inputs were equal.

For scenarios where independent scheduling can change event order, the CLI
also accepts an optional trace-specification JSON file as the third argument.
That specification must state the exact events and ordering constraints to
accept; it does not discard unmatched observations or make timestamps into
causal evidence. Trace comparison returns `unknown` when a capture lacks the
required complete event journal or contains relevant residual unknowns.

Each capture carries commitments for the selected scenario, executable,
comparison contract, and normalization rules. Comparison rejects captures
whose comparison contracts differ. Captures are local Evidence files; keep
their source artifacts and invocation context available when interpreting a
difference.