tune-detection

---

name: tune-detection description: Investigate a noisy Scanner detection rule against real _detections data, classify recent firings as false-positive vs true-positive vs unknown, and produce a concrete tuning patch — extra filter clauses, post-aggregation thresholds, dedup windows, or severity downgrades — along with regression tests seeded from the FP events. Validates the tuned rule with scanner-cli validate + run-tests and backtests the new firing rate. For out-of-the-box rules (which can't be edited in place), recommends the fork-and-disable workflow: clone the OOB YAML into the user's private repo, tune it there, and disable the original in the Scanner UI. Use when the user types /tune-detection <rule-id-or-name>, says "this rule is noisy", "reduce false positives on…", or "tune ". Requires Scanner MCP, SCANNER_API_URL + SCANNER_API_KEY for scanner-cli, and SCANNER_DETECTIONS_DIR (comma-separated paths to local rule repos).

tune-detection

When to invoke

Trigger on any of:

• The user types /tune-detection <rule-id-or-name>.
• The user says "this rule is noisy", "reduce false positives on…", "tune <name>".
• The user shows a flood of recent detections from a single rule and asks for help.

If the user wants to write a brand-new rule, route to /write-detection. If they want a correlation rule, route to /write-correlation.

Required environment

• Scanner MCP configured.
• SCANNER_API_URL, SCANNER_API_KEY — for scanner-cli.
• SCANNER_DETECTIONS_DIR (recommended) — comma-separated absolute paths to the user's local rule repos. Falls back to asking per-invocation.
• scanner-cli on PATH.

Workflow

Follow the full procedure in references/methodology.md. The short version:

1. Locate the rule YAML. Search in order: every path in SCANNER_DETECTIONS_DIR → OOB packs (default: curl the matching scanner-inc/detection-rules-<source> pack on GitHub — see recommend-detections/references/oob_packs.md for the canonical list of packs and their GitHub URLs; OR, if the user has cloned individual packs locally and gives you a path, read from there) → the Scanner Detection Rule API (via scripts/fetch_detection_rule.sh --name "<name>" or --id <uuid>). The API path covers rules created in the Scanner UI that aren't in any local repo. See references/methodology.md Phase 1 for the four resolution branches: user repo → OOB → UI-created → git-synced from missing repo. OOB rules use fork-and-disable; UI-created rules use the UI-tune workflow.

2. Pull recent firings. Query _detections for the last 14d (extend to 30d if sparse). Group by the rule's natural pivot keys (the stats … by … columns in its query_text). Show the top 10 distinct values + counts.

3. Sample underlying events. For the noisiest groups, pivot back from _detections to the source index using detected_in_time_range + the rule's pivot keys. Pull 3–5 raw events per loud group via Scanner MCP.

4. Classify FP/TP/UNKNOWN. Render a small markdown table: group key → verdict → 1-line reason. Be concrete — "this fires because the user agent contains CloudShell, which is normal"; not "this looks fine".

5. Propose targeted tuning. Pick the smallest change that removes the FPs without losing TP coverage:

   • First choice: extra filter clause that narrowly excludes the FP signature. (and not userAgent: "CloudShell", and not requestParameters.maxResults > 1000.)
   • Second choice: post-aggregation threshold via | where @q.count > N.
   • Third choice: dedup_window_s to suppress repeats from the same dedup key.
   • Last resort: severity downgrade (Medium → Low; the rule becomes a signal for correlation rather than an alert). Avoid blanket exclude-lists when a narrower filter works.

6. Add regression tests. Append at least one new negative test to the rule's tests: array, seeded with a sanitised FP event from step 3. Update existing positive tests if the tuning changed expected behaviour.

7. Validate. Run scanner-cli validate -f <path> and scanner-cli run-tests -f <path>. Surface failures verbatim and iterate.

8. Backtest the tuned query. Re-run the full tuned query (filter + aggregations + threshold) via Scanner MCP over the same window as step 2. Report old vs new fire rate. Optionally also confirm the TP groups still fire — if any disappear, the tuning is too aggressive.

9. Hand off via the GitHub-app sync flow — see "Sync flow" below.

OOB rules — fork and disable

If the rule comes from a scanner-inc/detection-rules-* pack (each pack is its own GitHub repo — see recommend-detections/references/oob_packs.md), it's read-only. The user cannot edit it in place. Switch to the fork-and-disable workflow:

1. Obtain the OOB YAML. Default: curl -sSL "https://raw.githubusercontent.com/scanner-inc/detection-rules-<source>/main/rules/<file>.yml". (Or, if the user has cloned the pack locally and points you at a path, read it from there.) Copy the YAML into one of the paths in SCANNER_DETECTIONS_DIR. Suggested path: rules/<source>/<orig_filename>.
2. Tell the user explicitly: "I'm tuning a clone at <path>. After you push, you'll need to disable the original OOB rule in the Scanner UI (Settings → Detection Rules → search for <name> → toggle off). The UI is the only place the OOB toggle exists."
3. Continue tuning the clone in all subsequent steps.

The cloned-and-tuned rule overrides the (disabled) OOB rule once the GitHub app syncs.

Sync flow (always use this hand-off)

The Scanner GitHub app watches main and syncs on green. Never recommend scanner-cli sync. Hand-off:

1. Commit the modified YAML (or the cloned-and-tuned YAML for OOB rules).
2. Push to main (or open a PR and merge).
3. For OOB-cloned rules: separately disable the original in the Scanner UI.
4. The GitHub app validates and syncs.

If the rule wasn't already in Staging, do not auto-flip it to Staging during tuning — that's a deliberate severity-level change and the user should decide. Just tune and re-deploy at the existing enabled: level.

Output

Respond to the user with, in order:

1. The FP/TP classification table from step 4.
2. The YAML diff — show only the changed lines (or sections of query_text), not the whole file.
3. The validation result. scanner-cli prints <path>: OK from validate and <test name>: OK from run-tests — relay those literally, not paraphrased.
4. The before/after backtest — one line: Backtest: <window>, was <N>/day → now <M>/day (-X%).
5. TP coverage check — explicit confirmation that the previously-TP groups still fire under the new query.
6. Hand-off — the GitHub-app sync flow steps. For OOB-cloned rules, the explicit UI disable step.

Keep it tight. The diff is the deliverable.

Layout

tune-detection/
├── SKILL.md                          # this file
├── references/
│   └── methodology.md                # full procedure, FP/TP rubric, tuning toolbox, UI-tune workflow
└── scripts/
    ├── list_detection_rules.sh       # page through GET /v1/detection_rule
    └── fetch_detection_rule.sh       # find one rule by --name or --id, emit JSON

Pre-flight briefing

Before the first tool call, emit 2-3 lines telling the user what's about to happen. Include the destination path if the rule is going to be modified. Example:

Tuning <rule name>. I'll locate the YAML (local repo / OOB pack / Scanner UI), pull recent firings from _detections, sample underlying events, classify FP/TP, propose the smallest filter or threshold change that removes the FPs, add a regression test, validate, and backtest the new fire-rate. Writes to: <path>.

If the rule turns out to be an OOB rule, mention the fork-and-disable workflow up front so the user knows the destination changes from "edit in place" to "clone into your private repo + disable in UI".