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:

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:

{
  "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.

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.

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.

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. Each retained regular file without a digest has an aliased path, size, remaining budget and reason: file_bytes_budget or file_changed_or_short_read. 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. Older truncated captures without these details retain the conservative comparison behavior. These diagnostics use the existing scenario budgets; they do not introduce a separate file-hashing budget.

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.

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.