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:

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:

{
  "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:

{
  "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.

{
  "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:

{
  "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. 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:

{
  "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:

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.