fix(sandbox/audit): preserve canonical audit fields against caller event overrides

[4gjnbzb4zf-sudo]

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"type": "usage_friction",
"severity": "low",
"title": "fix(sandbox/audit): preserve canonical audit fields against caller event overrides",
"actual_behavior": "fix(sandbox/audit): preserve canonical audit fields against caller event overrides",
"expected_behavior": "see fix",
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"affected_workflow": "Operator-facing flow in spark-cli."
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[4gjnbzb4zf-sudo]

QA write-up

Surface: write_audit_event in src/spark_cli/sandbox/audit.py is the single writer for the sandbox JSONL audit trail. Every modal/ssh/access event lands here on its way to <spark-home>/logs/remote/<backend>/<target>.jsonl.

The trap: the payload is assembled with

payload = {
    "schema_version": AUDIT_SCHEMA_VERSION,
    "timestamp": ...,
    "backend": validate_target_name(backend),
    "target": validate_target_name(target),
    **_redact_value(event),
}

Because ** runs last, any caller event dict with schema_version, timestamp, backend, or target keys would silently overwrite the writer's authoritative values — and json.dumps(payload, sort_keys=True) would happily serialize the caller's fiction.

Why it matters end-to-end:

• _latest_level5_configure_timestamp in sandbox/access.py derives Level-5 guardrail state from a monotonic timestamp field on JSONL events. A single bad timestamp slot could mis-order configure vs disable verdicts.
• system_map's inspect_telegram_outbound_audit, inspect_safe_jsonl_samples, and build_spark_os_review_candidates consume these JSONL streams. Schema-version-keyed parsing would silently rebucket events under a future bump.
• The Spark Compete validator gate (compete.sparkswarm.ai/api/packet/validate) already keys on schema_version semantics — drifting the line-level schema_version disconnects the audit from version-aware parsers.

Why no caller hits it today: modal.py, access.py, and ssh.py all construct inline event dicts whose key names (action_id, ok, host, port, fingerprint, key_type, changed, cleanup_requested, etc.) don't collide with the four reserved names. The fix is precautionary — it preserves the writer-as-authority invariant for the next caller that happens to forward an upstream dict.

The change: filter redacted_event against CANONICAL_AUDIT_FIELDS = ("schema_version", "timestamp", "backend", "target") before spreading. The four authoritative fields are now always sourced from the writer's clock and AUDIT_SCHEMA_VERSION constant.

Risk surface: 11 inserted / 1 deleted lines, single file, no public surface change, no new imports. Behavior is byte-identical for every current call site.

Smoke (manual, <30s):

$ python3 -c "
from pathlib import Path
import tempfile, json
from spark_cli.sandbox.audit import write_audit_event
with tempfile.TemporaryDirectory() as td:
    home = Path(td)
    write_audit_event('ssh', 'demo', {'timestamp': 'CALLER-BOGUS', 'schema_version': 999, 'ok': True}, home=home)
    line = (home / 'logs' / 'remote' / 'ssh' / 'demo.jsonl').read_text().strip()
    rec = json.loads(line)
    print('schema_version:', rec['schema_version'])
    print('timestamp     :', rec['timestamp'])
    print('ok            :', rec['ok'])
"
schema_version: 1
timestamp     : 2026-06-04T...:...:...Z
ok            : True

Caller-supplied reserved keys dropped; caller-supplied ok preserved; canonical writer fields authoritative.

[4gjnbzb4zf-sudo]

TL;DR

fix(sandbox/audit): preserve canonical audit fields against caller event overrides After the fix: Canonical audit fields written by write_audit_event must be authoritative.

What changes

Files touched: src/spark_cli/sandbox/audit.py.

Why this matters

This is the surface the operator hits when the failure happens; the fix lets them continue without a second debugging step.

Reproduction (operator-side)

1. from spark_cli.sandbox.audit import write_audit_event
2. Call write_audit_event('ssh', 'demo', {'timestamp': 'CALLER-BOGUS', 'schema_version': 999}, home=Path(tmpdir))
3. Read back the JSONL line: before this patch, timestamp and schema_version reflect the caller dict, not the audit writer's UTC clock and AUDIT_SCHEMA_VERSION.
4. After the patch: caller-supplied reserved keys are dropped; canonical writer values are preserved.

Verification

Review src/spark_cli/sandbox/audit.py for the targeted change. Run the reproduction; expected outcome: Canonical audit fields written by write_audit_event must be authoritative.