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.

macOS also requires Apple's Swift compiler through `xcrun` for process
ownership inspection. REA compiles its packaged, narrow process-inspection
helper into a private temporary directory before a capture or owned provider
process starts; the same prerequisite applies to owned provider-process
supervision on macOS. REA removes that directory when it exits. REA does not
install Xcode, Command Line Tools, or other software. Capability checks prepare
this helper before reporting macOS capture as available.

## 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.

While the capture runs, REA writes live status to stderr as one JSON object per
line. Stdout stays the final Evidence document:

```json
{
  "rea_progress": {
    "phase": "running",
    "completed": 3,
    "total": null,
    "message": "elapsed_ms=1200 frames=1 samples=2 interactions=0",
    "sequence": 4
  }
}
```

`phase` is the observed lifecycle stage: `prepare`, `running`, `settling`, or
`cleanup`. `completed` counts collected terminal frames, process samples, and
interaction events. `total` stays null; REA does not invent a unit total or a
percentage. `message` reports elapsed time and those counts. The final line uses
`phase` `cleanup`, sets `terminal` to true, and adds `disposition` (`exited`,
`timeout`, `idle_timeout`, `cancelled`, or `failed`) plus the owned-process,
renderer, and temporary-root cleanup states. Updates use the shared progress
boundary and are limited to one intermediate line per 100 ms; the terminal line
is always emitted. If a receiver is slow, intermediate observations are merged
into the latest status, and stale live observations are discarded before final
cleanup status. Progress never copies child output. Child output remains in the
capture Evidence on stdout.

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": "user@example.test\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.

Terminal dimensions must satisfy `columns × (rows + scrollback) ≤ 1,000,000`.
Every scheduled resize uses the same selected scrollback and must fit this
working-buffer budget. REA rejects oversized scenarios before launching the
command; it never clamps the requested dimensions. This limit is independent
of `limits.output_bytes`, which bounds retained terminal observations.

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.

A missing entry proves creation or deletion only when the corresponding
observation root's path enumeration was exhausted. Otherwise its effect has
`status: "unknown"`, the available before/after state, and a reason; a null
state in this variant means unobserved. Paths below symlinks that REA did not
follow also remain unknown. Partial enumeration of one root does not erase
known effects in another, and unavailable content hashes do not by themselves
make path absence unknown.

Terminal frame `data` is the comparison text after the selected normalization.
New captures also preserve `raw_data` when normalization changes a PTY chunk;
otherwise `data` is already the original text. Older captures may lack the
original text for changed chunks. Both values participate in Evidence identity,
while comparisons and trace assertions use the normalized text. Retaining the
original adds at most `limits.output_bytes` of text, because it comes from the
same admitted PTY chunks rather than a second unbounded stream.

Rendered frame `lines` hold the visible rows after normalization, with trailing
U+0020 spaces removed. Blank rows and internal spaces remain. `columns` records
terminal cell width; normalization and Unicode can make string lengths differ
from that width. `serialized_state` holds terminal serialization after the
selected normalization. Original admitted PTY chunks are available through
`raw_data ?? data`, as described above. Captures commit this line format in their
comparison contract, so a capture whose lines were padded to full width does not
compare as the same contract.

Custom `normalization.patterns` replace literal text with the caller's literal
replacement, including dollar sequences such as `$&` and `$$`. An empty pattern
also matches the first and last string positions. Captures with custom patterns
commit `pattern_normalization_version: "literal-replacements-v1"`. Legacy captures
remain readable. Cross-version comparison is rejected; capture both sides with
the same replacement semantics. Captures without custom patterns keep their
existing comparison contract.

Port normalization recognizes explicit `port`, `tcp_port`, `udp_port`, and
`listen` fields, URL authorities, IP endpoints, and `localhost` endpoints, with
ports from 0 through 65535. A port may end a sentence, as in `port: 8080.`;
a period followed by a letter or digit continues a decimal, version, or host
name instead. Each PTY chunk is normalized on its own, so a port whose period
ends the chunk is left unchanged because its continuation is not yet known. It preserves ordinary counters, dimensions, file line numbers,
and ambiguous bare host labels. Use explicit literal `patterns`
for a different application-specific spelling. Set `ports: false` to preserve
all endpoint numbers in comparison text. Captures commit the port-normalization
version so older broad numeric normalization cannot silently compare as the
same contract.

Output retention, file count, aggregate file-hashing bytes, 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.

### Finalization interval

By default a deadline sends `SIGKILL` at once. Set `finalization_ms` to give the
target time to finish: when `timeout_ms` or `idle_timeout_ms` fires, REA sends
`SIGTERM`, keeps capturing output and selected files, and sends `SIGKILL` once
if the interval ends first. Cancellation is not delayed: it sends `SIGKILL`
immediately, also during finalization, and ends the run as cancelled.

```json
{
  "executable": "node",
  "arguments": ["./job.mjs"],
  "filesystem_observation_paths": ["./reports"],
  "timeout_ms": 30000,
  "finalization_ms": 1500
}
```

REA binds the captured root's first start identity to its ownership token,
checking the identity before and after reading that token. A missing or
mismatched token, or an identity change during validation, leaves the identity
unverified rather than adopting a replacement process. Both finalization
signals recheck that retained identity immediately before the call; the check
and signal remain separate operating-system operations. REA does not fall back
to a bare PID kill for either finalization signal; failed identity or ownership
validation is recorded as `unverified` with its reason. Cancellation before any
deadline keeps the existing terminal kill. Descendants are not signalled during
finalization; owned-process cleanup handles them afterwards.

`exit.reason` keeps the initiating deadline (`timeout` or `idle_timeout`).
`exit.code` is `null` for every deadline exit; the code of an exit observed
during finalization is `exit.finalization.exit_code`, and the observed signal
stays in `exit.signal`. `exit.finalization` records what was observed and is
absent when no finalization was attempted:

```json
{
  "code": null,
  "signal": 0,
  "reason": "timeout",
  "finalization": {
    "requested_ms": 1500,
    "elapsed_ms": 12,
    "exit_code": 0,
    "signals": [
      { "signal": "SIGTERM", "sent_at_ms": 0, "delivery": "signaled" }
    ]
  }
}
```

- `signals` lists each attempt in order: `SIGTERM` first, at most one `SIGKILL`.
  `sent_at_ms` is the attempt time on a monotonic clock, relative to the start of
  finalization, also for an attempt that could not be delivered.
- `delivery` is `signaled` (the identity matched and the signal call returned),
  `gone` (no live process matched), `identity-changed` (the PID now belongs to a
  different process, which was not signalled) or `unverified` (the identity could
  not be established or the call failed). None of them proves the process
  exited.
  An `unverified` attempt may carry a `reason` string naming why: an unavailable
  identity and its reason, an inspection or signal error, or delivery that did
  not settle within the bound (1000 ms). An attempt recorded `unverified` after
  that bound can still complete later; it stays identity-checked, and its late
  result is not added to the record.
- `elapsed_ms` is when REA observed the exit, relative to the start of
  finalization, or `null` when no exit was observed. The PTY layer can deliver the
  exit slightly after the operating system ended the process.
- `exit_code` is the code the PTY reported when the exit was observed during
  finalization and was not caused by a signal (`exit.signal` is `0`); a
  finalizer that exits with `3` records `3`, one that exits cleanly records `0`.
  It is `null` when no exit was observed or the target ended on a signal, and it
  is never inferred. A target that exited on its own near the deadline is
  recorded the same way.
- The observed signal stays in `exit.signal`; `exit.code` is `null` for every
  deadline exit. The record does not attribute an exit to a particular signal,
  and a `SIGKILL` from a scenario event next to the escalation cannot be told
  apart from it.
- If the `SIGKILL` cannot be delivered, REA stops waiting for an exit, keeps every
  observation collected so far with the delivery results, and releases the run
  through owned-process cleanup.
- The `SIGKILL` is not sequenced after the `SIGTERM` delivery result: the two
  deliveries are independent, because cancellation must not wait for a slow
  identity check.

`timeout_ms + finalization_ms + settle_ms` must stay at or below
`Number.MAX_SAFE_INTEGER` (9007199254740991), the largest total exactly
representable as a JavaScript number; the scenario is rejected otherwise. This is
a representability limit, not a timer limit. The nominal lifecycle budget is that
sum; polling, snapshotting and cleanup lie outside it. Scenario `events` keep
being dispatched during finalization and are recorded as usual. `finalization_ms`
of `0`, the default, keeps today's behaviour and the committed identity of every
existing scenario. `elapsed_ms` and `sent_at_ms` are timing data and take part in
exit comparison like `settlement.elapsed_ms`.

Every capture requires `truncation_details`, with separate accounting for:

- `raw_terminal`: original UTF-8 PTY chunk bytes and observed/retained chunk
  counts. A chunk that does not fit `limits.output_bytes` is omitted whole;
  a later smaller chunk can still fit.
- `rendered_terminal`: cumulative UTF-8 serialized-state and visible-line
  bytes, plus observed/retained frame counts. This is independent of the raw
  chunk budget and does not count JSON encoding. Repeated screen snapshots can
  exhaust it even when all raw output fits. Rendered states use retained PTY
  input, so raw omissions also limit rendered coverage.
- `filesystem_before` and `filesystem_after`: file-count/depth limits,
  enumeration failures, whole-file hash budget and bytes successfully hashed.
  Directory names are read incrementally within the remaining entry capacity.
  When a directory exceeds that capacity, the retained subset follows filesystem
  enumeration order and is then sorted; it is explicitly incomplete and cannot
  establish that an omitted path is absent.
  Each retained regular file without a digest has an aliased path, size,
  remaining budget and reason: `file_bytes_budget`,
  `file_changed_or_short_read`, or `file_unavailable`. Its `system_code` is
  null unless an OS file operation failed, in which case it preserves the
  reported error code. A file too large for the remaining budget is
  skipped; a later smaller file can still be hashed. Hash omissions do not
  imply incomplete path enumeration.
- `process`: sampling limit and whether sampling ended partially. Coverage
  remains `sampled`, including when that limit was not exhausted.

The aggregate `truncated` flag summarizes these observations. Comparisons retain
results for unaffected dimensions and mark affected dimensions unknown; an
incomplete capture cannot locate the first divergence across all dimensions.
Trace assertions use coverage for the sources they select, so an assertion
about complete raw terminal output need not fail because rendered snapshots
were omitted. Captures missing `truncation_details` are rejected on import;
preserve them as historical files and recapture for current comparisons.
`limits.file_bytes` is already the cumulative whole-file hashing budget,
independent of `limits.output_bytes`, which bounds retained terminal content.
Each filesystem checkpoint starts with a fresh `file_bytes` budget across all
selected roots; it is not a per-file allowance. File contents are read in
64 KiB chunks for hashing and are not embedded in capture Evidence.

When the host withholds an unrelated process’s ownership token, REA leaves that
process untouched and records its PID and reason in `cleanup.unverified_processes`
and process residual unknowns. Successful cleanup verifies the owned group;
it does not attribute those unrelated processes. Related or otherwise unexplained
unreadable processes still prevent successful cleanup.

On macOS, changing a Node process's `process.title` can make its run token
unreadable. Node documents that [setting the title overwrites argv memory](https://nodejs.org/download/release/v24.18.0/docs/api/process.html#processtitle).
With the pinned toolchain, a live child retained a readable start identity
while its token became unavailable after this change; npm changes its title
as well. A newly started unreadable process can prevent verified cleanup even
after the selected command exits. REA preserves this uncertainty and leaves
that process untouched.

## Hash a report independently of terminal output

Select the producer's report directory and give `file_bytes` enough room for
the complete files you need, including other files in those roots. For example,
save this scenario and run `rea capture-process ./scenario.json --json > capture.json`:

```json
{
  "executable": "node",
  "arguments": [
    "-e",
    "const fs=require('node:fs');fs.mkdirSync('reports',{recursive:true});fs.writeFileSync('reports/final.json',JSON.stringify({status:'complete',data:'X'.repeat(131072)}));console.log('x'.repeat(8192));"
  ],
  "working_directory": ".",
  "filesystem_observation_paths": ["./reports"],
  "limits": { "output_bytes": 1024, "file_bytes": 262144 }
}
```

The report exceeds the terminal budget but fits the independent 256 KiB hash
budget. Its complete SHA-256 appears in `normalized_result.files_after` at
`root_0:final.json`; check `truncation_details.filesystem_after` for omissions
and actual bytes hashed. The verbose terminal output is truncated without
preventing the report digest. The same scenario object is the MCP
`capture_process_scenario` input. REA never returns a prefix hash as a full-file
digest: an insufficient remaining budget produces a null digest and an
explicit omission reason.

A digest binds the complete bytes observed at that checkpoint, not the
producer's notion of a finished report. A stable periodic or running report
can also have a complete digest. Choose the final file and verify the
producer's documented completion marker, exit outcome and settlement
observations before calling it final. `after_settlement` names a filesystem
checkpoint, not a guarantee that an application finished successfully or
that an unobserved writer will never change the file. REA checks opened-file
identity/state around hashing and reports observed changes or unavailable
files; cancellation stops the hash. If an external sealer produces a manifest,
keep its completion claims and digest separate from REA's checkpoint facts.

## 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. The comparison contract covers
the working directory, explicit environment, filesystem observation paths,
terminal size, scripted events, timeouts, limits, and normalization; the
executable and its arguments may differ. Comparison rejects captures whose
comparison contracts differ and names the differing fields, so run both
scenarios from the same absolute working directory and observation paths.
Captures are local Evidence files; keep
their source artifacts and invocation context available when interpreting a
difference.