Create, deploy, update, and manage custom detection rules in Microsoft Defender XDR via the Graph API (/beta/security/rules/detectionRules). Covers query adaptation from Sentinel KQL to custom detection format, deployment via PowerShell (Invoke-MgGraphRequest), manifest-driven batch deployment, and lifecycle management (list, enable/disable, delete). Companion script: Deploy-CustomDetections.ps1.
Create, deploy, update, and manage custom detection rules in Microsoft Defender XDR via the Graph API (/beta/security/rules/detectionRules). Covers query adaptation from Sentinel KQL to custom detection format, deployment via PowerShell (Invoke-MgGraphRequest), manifest-driven batch deployment, and lifecycle management (list, enable/disable, delete). Companion script: Deploy-CustomDetections.ps1.
Custom Detection Authoring — Instructions
Purpose
This skill deploys custom detection rules to Microsoft Defender XDR via the Microsoft Graph API (/beta/security/rules/detectionRules). It handles:
Query adaptation — Converting Sentinel KQL queries into custom detection format
Single-rule deployment — Creating one rule via Graph API
Batch deployment — Deploying multiple rules from a JSON manifest
Lifecycle management — Listing, updating, enabling/disabling, and deleting rules
Validation — Dry-run queries in Advanced Hunting before deployment
Writing new detection queries from scratch? This skill focuses on deploying and managing detection rules — not query creation. If you need to write detection KQL from scratch (schema validation, community examples, performance optimization), use the kql-query-authoring skill first with CD intent markers (say "create custom detection queries for [scenario]"). It will produce Sentinel-format queries with cd-metadata blocks ready for this skill to adapt and deploy.
Why Invoke-MgGraphRequest? The Graph MCP server and az rest both return 403 for custom detection endpoints — they lack the CustomDetection.* scopes. Invoke-MgGraphRequest uses interactive delegated auth with consent, which works.
Custom detection queries have strict requirements that differ from Sentinel analytic rules:
Requirement
Detail
🔴 Author-only by default
The default behavior is to author, validate, and write the manifest only — do NOT call the Graph API to deploy rules unless the user explicitly says "deploy", "create the rule", "push to Defender", or similar deployment-intent language. If deployment intent is ambiguous, ask before calling the API.
Timestamp column must be projected as-is
The query MUST project the timestamp column exactly as it appears in the source table — TimeGenerated for Sentinel/LA tables, Timestamp for XDR-native tables. Do not alias one to the other (e.g., Timestamp = TimeGenerated causes 400 Bad Request). See Pitfall 1.
Event-unique columns (per table type)
Required columns that uniquely identify the event differ by table family. A bare summarize count() or make_set() loses these columns and fails. summarize with arg_max IS allowed — see Pitfall 3. See table below for per-type requirements.
Impacted asset identifier column
The query must project at least one column whose name matches a valid impactedAssets identifier (e.g., AccountUpn, DeviceName, DeviceId). See Impacted Asset Types and Pitfall 9. Queries without project or summarize typically return these columns automatically.
🔴 entityMappings binds the column — send it explicitly
impactedAssets only declares the asset type. The actual query-column→entity binding lives in a separate entityMappings structure. The API auto-derives entityMappings from impactedAssets for recognized tables, but this silently fails for some connector tables (e.g. GSA NetworkAccess*), dropping impactedAssets to [] with no error. Always send entityMappings alongside impactedAssets. See Pitfall 19.
impactedAssets must be non-empty
The impactedAssets array must contain at least 1 element. An empty array ([]) is rejected with 400 BadRequest: "The field ImpactedAssets must be a string or array type with a minimum length of '1'." Every detection must declare which entity it impacts. See Pitfall 13.
No let statements (NRT)
NRT rules (schedule: "0") reject let entirely — the API returns a generic 400 Bad Request. This is not documented by Microsoft (empirically discovered Feb 2026) but consistently reproducible. Inline all dynamic arrays/lists directly in where clauses. Non-NRT rules (1H+) tolerate let.
Unique displayName AND title
Both the rule displayName and the alert title must be unique across all custom detections. Duplicate displayName returns 409 Conflict. Duplicate title returns 400 Bad Request.
🔴 Naming convention for displayName
Follow the standardized naming convention documented in Naming Convention below. No schedule prefixes, no MITRE IDs, no tactic labels — the portal columns already display these. Use clean, descriptive title-case names with colon (:) as the only sub-separator.
150 alerts per run
Each rule generates a maximum of 150 alerts per execution. Tune the query to avoid alerting on normal day-to-day activity.
🔴 No response actions
All rules deployed by this skill MUST use "responseActions": []. Automated response actions (isolate device, disable user, block file, etc.) are PROHIBITED — they must only be configured manually by a human operator in the Defender portal after the rule is validated. Never populate responseActions in manifests or API calls.
First run = 30-day backfill
When a new rule is saved, it immediately runs against the past 30 days of data. Expect a burst of initial alerts if the query has broad coverage.
Required event-unique columns by table type (MS Learn source):
ReportId recommended (use proxy: CorrelationId, OfficeObjectId, CallerProcessId) but not strictly mandated by the docs
Query Adaptation Checklist
When converting a Sentinel query to custom detection format:
✅ Remove bare summarize — project raw rows instead. Exception: summarize with arg_max is allowed for threshold-based detections (see Pitfall 3)
✅ Project the timestamp column as-is: TimeGenerated = TimeGenerated for Sentinel/LA tables, Timestamp for XDR tables. Never alias one to the other.
✅ Project the impacted asset identifier column — the column name must match a valid identifier from Impacted Asset Types. Examples: DeviceName = Computer for device-focused detections, AccountUpn = UserId for user-focused. See Pitfall 9. Also populate entityMappings binding that column to the entity (see Entity Mappings and Pitfall 19) — impactedAssets alone is silently dropped on some connector tables.
✅ Project event-unique columns per table type — DeviceId + ReportId for MDE tables; ReportId for other XDR tables; recommended proxy ReportId for Sentinel tables (e.g., ReportId = CorrelationId). Caveat: proxy columns may contain empty strings for some events — acceptable but means those rows won't be individually identifiable in alert details.
✅ Add a time filter as the first where clause — prefer ingestion_time() > ago(1h) over Timestamp > ago(1h) (see tip below). NRT exception: For NRT rules (schedule: "0"), omit all time filters — ingestion_time() causes 400 Bad Request in NRT mode (see Pitfall 17). Timestamp > ago(...) is accepted but unnecessary.
✅ Remove let variables for NRT rules — NRT rejects let entirely (generic 400 error, undocumented). Inline all dynamic arrays directly in where clauses. Non-NRT rules tolerate let.
✅ Validate via Advanced Hunting dry-run before deployment
✅ For NRT rules: avoid tostring() on dynamic columns — use native string columns instead (e.g., Properties instead of tostring(Properties_d)). See Pitfall 11.
✅ For NRT rules: verify the table's ingestion lag justifies NRT. See Pitfall 12.
✅ Count unique {{Column}} references across title AND description combined — max 3 unique columns total (shared across both fields, not per-field). Exceeding this returns 400 Bad Request: "Dynamic properties in alertTitle and alertDescription must not exceed 3 fields". See Pitfall 14.
Performance tip (from MS Learn): "Avoid filtering custom detections by using the Timestamp column. The data used for custom detections is prefiltered based on the detection frequency." Use ingestion_time() instead — it aligns with the platform's pre-filtering for better performance. For scheduled rules, match the time filter to the run frequency (ingestion_time() > ago(1h) for 1H rules). For NRT rules, no time filter is needed. ⚠️ PowerShell note: When building queryText containing backslashes (file paths, regex), always use single-quoted here-strings (@'...'@) to avoid escape sequence mangling — see Pitfall 15.
Example Adaptation
Before (Sentinel KQL — uses summarize):
let _Lookback = 7d;
SecurityEvent
| where TimeGenerated > ago(_Lookback)
| where EventID == 4799
| where TargetSid == "S-1-5-32-544"
| where SubjectUserSid != "S-1-5-18"
| where AccountType != "Machine"
| where not(SubjectUserSid endswith "-500")
| project TimeGenerated, Computer, Actor = SubjectUserName, ...
| summarize EnumerationCount = count(), Processes = make_set(CallerProcess)
by Actor, ActorDomain, ActorSID
After (Custom Detection — row-level, mandatory columns):
SecurityEvent
| where TimeGenerated > ago(1h)
| where EventID == 4799
| where TargetSid == "S-1-5-32-544"
| where SubjectUserSid != "S-1-5-18"
| where AccountType != "Machine"
| where not(SubjectUserSid endswith "-500")
| project
TimeGenerated = TimeGenerated,
DeviceName = Computer,
AccountName = SubjectUserName,
AccountDomain = SubjectDomainName,
AccountSid = SubjectUserSid,
CallerProcess = CallerProcessName,
ReportId = CallerProcessId
The displayName should be a clean, title-case description of what the detection finds. The portal columns already show Scheduling Type, Tactics, and Techniques — don't repeat them in the name.
Rule
Example
Use colon (:) for sub-detail
Event Log Clearing: Security or System Log Wiped
Threat actor/family in parentheses at end
Credential Dumping Tool Execution (Storm-2885)
TI rules: Threat Intelligence: {IoC} Match on {Table}
Threat Intelligence: IP Match on CloudAppEvents
No schedule prefixes (NRT —, 1H —)
Portal Scheduling Type column covers this
No MITRE IDs (T1036 —)
Portal Techniques column covers this
No tactic labels ((Collection), (Exfiltration))
Portal Tactics column covers this
No em dash (—) separator
Use colon (:) instead
API Reference
Endpoint
POST /beta/security/rules/detectionRules — Create
GET /beta/security/rules/detectionRules — List all
GET /beta/security/rules/detectionRules/{id} — Get by ID
PATCH /beta/security/rules/detectionRules/{id} — Update
DELETE /beta/security/rules/detectionRules/{id} — Delete
Non-tactic threat categories: Malware, Ransomware, SuspiciousActivity, UnwantedSoftware — these hide the MITRE techniques field in the portal (MS Learn); prefer a tactic when you want techniques to display.
Portal label note: The portal labels this control "Tactic" (UX rename, 2026), but the API field stays category — single-value, automation unaffected. Validation details in Pitfall 18.
impactedAssets declares the asset type; entityMappings binds the query column to the entity (both under detectionAction.alertTemplate). The API usually auto-derives entityMappings from impactedAssets, but that silently fails on some connector tables (e.g. GSA NetworkAccess*) — dropping impactedAssets to [] with no error. See Pitfall 19.
You rarely write this by hand — Deploy-CustomDetections.ps1 auto-generates entityMappings from each manifest impactedAssets entry (identifier→column lookup lives in the script) and merges any explicit entityMappings you add for related evidence. Only for manual Invoke-MgGraphRequest POSTs do you send it yourself:
The column value is the query output column name (PascalCase per the Pitfall 9 convention: accountUpn → AccountUpn). Send only populated entity types — the API fills the rest. Related-evidence types include ips, urls, cloudApplications, dns, oAuthApplications.
Minimal Valid POST Body
{"displayName":"Rule Name","isEnabled":true,"queryCondition":{"queryText":"SecurityEvent\r\n| where TimeGenerated > ago(1h)\r\n| ..."},"schedule":{"period":"1H"},"detectionAction":{"alertTemplate":{"title":"Alert Title","description":"Alert description text.","severity":"medium","category":"Discovery","recommendedActions":null,"mitreTechniques":["T1069.001"],"impactedAssets":[{"@odata.type":"#microsoft.graph.security.impactedDeviceAsset","identifier":"deviceName"}],"entityMappings":{"hosts":[{"nameColumn":"DeviceName"}]}},"responseActions":[]}}
impactedAssets: Must contain at least 1 element — an empty array causes 400 BadRequest. Every detection must map to at least one impacted entity (device, user, or mailbox). See Pitfall 13.
recommendedActions: Can be null or a string. The portal sets it to null by default.
responseActions: Must always be [] — response actions are prohibited in LLM-authored detections (see Critical Rules). Must be [], not null — sending null causes 400 Bad Request. See Pitfall 10.
organizationalScope: Omit this field entirely for tenant-wide rules (the API default). Including "organizationalScope": null explicitly may cause 400 Bad Request in some API versions.
Custom details (not shown above): The API also supports a customDetails array of key-value pairs surfaced in the alert side panel. Each rule supports up to 20 KVPs with a combined 4KB size limit. Keys are display labels; values are query column names. See MS Learn.
Related evidence (not shown above): Beyond impactedAssets, the entity mapping also supports linking related evidence entities (Process, File, Registry value, IP, OAuth application, DNS, Security group, URL, Mail cluster, Mail message). These provide correlation context but are not impacted assets. See MS Learn.
Dynamic Alert Titles and Descriptions
Alert titles and descriptions can reference query result columns using {{ColumnName}} syntax, making alerts self-descriptive:
{"title":"Admin Group Enumeration by {{AccountName}} on {{DeviceName}}","description":"User {{AccountName}} enumerated group {{TargetGroupName}} on the device."}
Constraint
Limit
Max unique dynamic columns
3 unique {{Column}} references TOTAL across title AND description combined — NOT per field. E.g., the example above uses AccountName + DeviceName in title and AccountName + TargetGroupName in description = 3 unique columns (AccountName is reused). Exceeding this returns 400 Bad Request with "Dynamic properties in alertTitle and alertDescription must not exceed 3 fields".
⚠️ Discrepancy with MS Learn docs: The official documentation states "The number of columns you can reference in each field is limited to three" (i.e., 3 per field). However, the Graph API empirically enforces 3 unique columns total across both fields combined (confirmed Feb 2026). The portal UI may enforce the per-field limit differently than the API. Use 3 unique total as the safe limit for Graph API deployments.
Format
{{ExactColumnName}} — must match a column in query output
Markup
Plain text only — HTML, Markdown, and code are sanitized
URLs
Must use percent-encoding format
Frequency & Lookback
Lookback Windows by Frequency
Each frequency has a built-in lookback window. Results outside this window are ignored even if the query requests them:
Frequency
Lookback Period
Query Filter Recommendation
NRT (Continuous)
Streaming
No time filter needed — events processed as collected
Every 1 hour
Past 4 hours
ago(4h) or ago(1h)
Every 3 hours
Past 12 hours
ago(12h) or ago(3h)
Every 12 hours
Past 48 hours
ago(48h) or ago(12h)
Every 24 hours
Past 30 days
ago(30d) or ago(24h)
Custom (Sentinel only)
4× frequency (<daily) or 30d (≥daily)
Match lookback
Tip: Match the query time filter to the run frequency (ago(1h) for 1H rules), not the full lookback window. The lookback ensures late-arriving data is caught, but your filter should target the detection window.
NRT Constraints
NRT (Continuous, period: "0") rules have stricter requirements than scheduled rules:
Constraint
Detail
Single table only
Query must reference exactly one table — no joins or unions
No let statements
let variables are silently rejected — the API returns a generic 400 Bad Request with no useful error message. Always inline dynamic arrays/lists directly in where clauses. This constraint is not listed in the official NRT docs (which list only 4 constraints) but is consistently reproducible via Graph API (empirically confirmed Feb 2026).
No externaldata
Cannot use the externaldata operator
No comments
Query text must not contain any comment lines (//)
Supported operators only
Limited to supported KQL features. tostring() on dynamic columns is rejected — use native string columns instead (e.g., Properties instead of tostring(Properties_d)). See Pitfall 11.
No time filter needed
NRT processes events as they stream in. The platform pre-filters automatically. Timestamp > ago(1h) is unnecessary but harmless. However, ingestion_time() is rejected — the API returns 400 Bad Request. See Pitfall 17.
NRT-Supported Tables
Not all tables support NRT frequency. Use NRT only with these tables:
Important:SecurityEvent and SigninLogs support NRT — our Event ID 4799/4702 queries can run as NRT if they meet the single-table/no-joins constraint.
Ingestion Lag Consideration — NRT Suitability
A table being NRT-supported means the API accepts NRT rules — not that NRT is the right choice. If a table's ingestion lag exceeds the detection frequency benefit, NRT adds overhead with no detection speed improvement. See Pitfall 12 for a per-table ingestion lag assessment and recommendation matrix. Rule of thumb: if ingestion lag > 30 min, use 1H scheduled instead.
Custom Frequency (Sentinel Data Only)
For rules based entirely on Sentinel-ingested data, a custom frequency is available (Preview):
Range: 5 minutes to 14 days
Lookback: Automatically calculated — 4× frequency for sub-daily, 30 days for daily or longer
Requirement: Data must be available in Microsoft Sentinel (not XDR-only tables)
Deployment Workflow
🔴 DEPLOYMENT GATE: Only proceed to Steps 2-3 (API calls) when the user has explicitly requested deployment. Trigger phrases: "deploy", "create the rule", "push", "POST it", "make it live". If the user asked to "author", "write", "create a manifest", "prepare", or "draft" a detection — stop after validation (Step 1) and manifest generation. Present the manifest JSON for review and wait for explicit deployment confirmation.
Single Rule Deployment
Step 1: Validate the query in Advanced Hunting
Run the adapted query with a 1h lookback to validate schema:
Use RunAdvancedHuntingQuery with the adapted KQL query.
Confirm: 0 or more results, correct column schema (TimeGenerated, DeviceName, AccountName, etc.)
Then run with 30d lookback to confirm it returns real data:
Change ago(1h) to ago(30d) for the validation run.
Verify results contain expected columns and realistic data.
Step 2: Check for duplicates, then build and POST the rule
Manifest storage: Save manifest JSON files in the temp/ folder (gitignored). Manifests are deployment artifacts, not versioned query definitions.
Manifest Format
See example-manifest.json for a complete 2-rule reference covering NRT and scheduled (with summarize/arg_max) patterns.
The script reads a JSON file containing an array of rule definitions:
[{"displayName":"Admin Group Enumeration by Non-Admin User","title":"Admin Group Enumeration by {{AccountName}} on {{DeviceName}}","queryText":"SecurityEvent\r\n| where TimeGenerated > ago(1h)\r\n| ...","schedule":"0","severity":"medium","category":"Discovery","mitreTechniques":["T1069.001","T1087.001"],"description":"User {{AccountName}} enumerated the local Administrators group.","recommendedActions":"Verify whether the user has a legitimate reason to enumerate admin group membership.","impactedAssets":[{"type":"device","identifier":"deviceName"},{"type":"user","identifier":"accountSid"}],"responseActions":[]}]
Usage
# Dry-run — validate all queries in Advanced Hunting without creating rules
.\Deploy-CustomDetections.ps1 -ManifestPath .\temp\4799_4702.json -DryRun
# Deploy all rules from manifest (skips existing rules by default)
.\Deploy-CustomDetections.ps1 -ManifestPath .\temp\4799_4702.json
# Deploy and overwrite — attempt POST even if rule name exists (may cause 409)
.\Deploy-CustomDetections.ps1 -ManifestPath .\temp\4799_4702.json -Force
⚠️ Deletion propagation delay: After deleting a rule, the name remains reserved for ~30-60 seconds. Creating a rule with the same displayName during this window returns 409 Conflict — but the rule may still be created despite the error. Always verify with a GET after creation.
Before authoring new custom detections, check what Analytic Rules (Sentinel) and Custom Detection rules (Defender XDR) already exist for the same table, EventID, or keyword. This avoids duplicating coverage and helps identify gaps.
Step 0: Construct the Analytic Rules URL (once per session)
All patterns below reuse $arUrl. The Sentinel REST API returns the full KQL query text for every rule — there is no server-side content filtering, so we pull all rules in one call and filter client-side with JMESPath contains().
Search Analytic Rules by Table Name or Keyword
# Which rules reference a specific table? (e.g., SecurityEvent)
az rest --method get --url $arUrl `
--query "value[?properties.query && contains(properties.query, 'SecurityEvent')].{name: properties.displayName, severity: properties.severity, enabled: properties.enabled}" `
-o table 2>$null
Search Analytic Rules by EventID
# Which rules reference a specific EventID?
az rest --method get --url $arUrl `
--query "value[?properties.query && contains(properties.query, '<EventID>')].{name: properties.displayName, severity: properties.severity, enabled: properties.enabled}" `
-o table 2>$null
The MS Learn docs confirm: "Timestamp or TimeGenerated — This column sets the timestamp for generated alerts. The query shouldn't manipulate this column and should return it exactly as it appears in the raw event." Aliasing across types causes 400 Bad Request.
Pitfall 2: Silent Rule Creation on Error Responses (400 AND 409)
The API can silently create a rule even when it returns an error. This applies to both 400 Bad Request and 409 Conflict responses.
Cause A — 400 with partial validation: A POST may pass structural validation (creating the rule) but fail a secondary check (e.g., let variable in NRT query, >3 dynamic fields). The API returns 400 Bad Request — but the rule was already created. A subsequent retry with a fixed query then hits 409 Conflict because the rule exists.
Cause B — Deletion propagation delay: Deleting a rule leaves a name reservation for ~30-60 seconds. POSTing a rule with the same displayName in this window returns 409 Conflict — but the API may still create the rule.
Cause C — Silent success + accidental retry: When running Invoke-MgGraphRequest in a terminal, the POST may succeed but the output buffer splits across calls, making it look like nothing happened. Re-running the same POST produces a 409 because the rule was already created seconds earlier.
Prevention:
Always run a GET before POST to check if the rule name already exists (see Step 2)
Always verify with GET after ANY error response (400 or 409) — the rule may have been created despite the error
Never re-run a POST without first checking via GET whether the previous attempt succeeded
If a rule was silently created with a bad query, use PATCH to update the queryCondition.queryText rather than deleting and re-creating
Pitfall 3: summarize — Allowed Only With Row-Level Output
Custom detection queries must return row-level results with required columns (TimeGenerated, DeviceName, ReportId). A bare summarize count() or make_set() as the final operator fails validation because the output lacks these columns.
However, summarize with arg_max IS allowed when used to return the required columns alongside aggregation:
// ✅ ALLOWED — uses arg_max to preserve row-level columns
DeviceEvents
| where ingestion_time() > ago(1d)
| where ActionType == "AntivirusDetection"
| summarize (Timestamp, ReportId)=arg_max(Timestamp, ReportId), count() by DeviceId
| where count_ > 5
This pattern counts by entity but still returns Timestamp, ReportId, and DeviceId per row — satisfying the requirement. Use this for threshold-based detections ("alert when count > N").
Pitfall 4: Graph MCP and az rest Cannot Access This API
Both the Graph MCP server and az rest lack the CustomDetection.ReadWrite.All scope. Only Invoke-MgGraphRequest with interactive delegated auth works.
Pitfall 5: recommendedActions Type
The recommendedActions field is a String (not an array). Set to null if not needed. The portal always sets it to null.
Pitfall 6: Query Newlines in JSON
The queryText JSON field requires \r\n (CRLF) line breaks on the wire. When using ConvertTo-Json on a PowerShell hashtable (the recommended approach), this is handled automatically — multiline here-string content in the hashtable value is serialized with correct CRLF encoding. No manual newline insertion is needed.
If manually constructing a raw JSON string body (not recommended), use PowerShell backtick escapes `r`n to produce CRLF in the output.
Pitfall 7: Duplicate Name AND Title Check
The API enforces unique displayName AND unique title (alert title) across all custom detections. Duplicate displayName returns 409 Conflict. Duplicate title returns 400 Bad Request. The batch deployment script checks for displayName duplicates by default — use -Force to override. The MS Learn docs state both should be unique: "Detection name... make it unique" and "Alert title... make it unique".
Pitfall 8: Alert Deduplication
Custom detections automatically deduplicate alerts. If a detection fires twice on events with the same entities, custom details, and dynamic details, only one alert is created. This can happen when the lookback period is longer than the run frequency (e.g., 1H frequency with 4H lookback means 3 hours of overlap). Different events on the same entity produce separate alert entries under the same alert.
Pitfall 9: impactedAssets Identifier Must Be a Predefined API Value
The identifier field in impactedAssets must use one of the predefined values from the Impacted Asset Types section — NOT arbitrary query column names. Using a custom column name (e.g., "identifier": "TargetComputer" or "identifier": "Actor") causes a silent 400 InvalidInput with an empty error message.
This aligns with the MS Learn docs which list specific "strong identifier" columns for impacted assets. The portal wizard enforces this via a dropdown; the Graph API rejects non-matching values silently.
Identifier values must use camelCase as listed in the Impacted Asset Types section (e.g., recipientEmailAddress, not RecipientEmailAddress). The API treats identifier values as case-sensitive when matching to the predefined list.
Additionally, the query MUST project a column whose name matches the chosen identifier. If you use "identifier": "accountUpn", the query must project an AccountUpn column (alias if needed: AccountUpn = UserId). The column name match is case-insensitive — AccountUpn in the query matches accountUpn in the identifier.
⚠️ InitiatingProcess* column trap (Apr 2026): Device* tables project many InitiatingProcess* columns (e.g., InitiatingProcessAccountName, InitiatingProcessAccountSid, InitiatingProcessAccountUpn, InitiatingProcessAccountObjectId). Only three of these are valid user identifiers: initiatingProcessAccountUpn, and the initiatingAccount* variants (initiatingAccountSid, initiatingAccountName, initiatingAccountDomain). Notably, initiatingProcessAccountName is NOT valid — it looks correct because the column exists, but the API enum uses accountName instead. The API rejects invalid identifiers with a silent 400 InvalidInput (empty error message), making this very hard to debug. Always alias the column:AccountName = InitiatingProcessAccountName.
DeviceId requirement: For XDR-native tables (Device*, Email*, CloudAppEvents) with a device-type impactedAsset, the query must project DeviceId (not just DeviceName). Sentinel/LA tables (SecurityEvent, AuditLogs) do not require DeviceId.
Root cause (Feb 2026): When using PowerShell if/else expressions to assign empty arrays, PowerShell swallows @() and produces $null instead:
# ❌ BUG — $x becomes $null, NOT an empty array
$x = if ($false) { @($items) } else { @() }
# Result: $null
# ✅ CORRECT — assign first, then overwrite conditionally
$x = @()
if ($condition) { $x = @($items) }
# Result: empty Object[] (serializes to [])
This caused array fields like responseActions and mitreTechniques to serialize as null instead of [], which the API rejects with 400 Bad Request.
Combined with organizationalScope: null — including this field explicitly (even as null) was also rejected. The fix: omit organizationalScope entirely and use direct assignment for array fields.
Symptoms: All rules in a batch return 400 Bad Request, but some may be silently created (see Pitfall 2). Manual deployment of the same rule body (without the null fields) succeeds.
Fixed in:Deploy-CustomDetections.ps1 — array fields now use direct assignment, organizationalScope removed from body.
Pitfall 11: tostring() on Dynamic Columns Rejected in NRT Mode
Root cause (Feb 2026): NRT rules (schedule: "0") reject tostring() wrapping dynamic-typed columns. The API returns a generic 400 Bad Request with no useful error message — similar to the let rejection described in NRT Constraints. The same query deploys successfully as a scheduled rule (1H+).
Example — AzureActivity table:
// ❌ FAILS in NRT mode — tostring() on dynamic column
AzureActivity
| where OperationNameValue =~ "MICROSOFT.SECURITY/PRICINGS/WRITE"
| where tostring(Properties_d.pricings_pricingTier) == "Free"
// ✅ WORKS — use the native string column instead
AzureActivity
| where OperationNameValue =~ "MICROSOFT.SECURITY/PRICINGS/WRITE"
| where Properties has '"pricingTier":"Free"'
Workarounds:
Prefer native string columns — many Sentinel tables have both a dynamic column (e.g., Properties_d) and a string column (e.g., Properties). Use the string column with has or contains for NRT.
Switch to 1H schedule — if tostring() is required for precise extraction, use a scheduled rule where it works reliably.
Ingestion lag consideration: Even when a table is NRT-supported, check whether ingestion lag makes NRT impractical — see Ingestion Lag Consideration.
Pitfall 12: NRT-Supported ≠ NRT-Practical — Check Ingestion Lag
A table appearing in the NRT-Supported Tables list means the API accepts NRT rules for that table — it does NOT mean NRT adds value. Tables with significant ingestion lag negate the benefit of continuous detection.
Rule of thumb: If the table's ingestion lag exceeds 30 minutes, use a 1H scheduled rule instead of NRT. The detection latency is dominated by ingestion lag, not rule frequency.
Pitfall 13: impactedAssets Must Be Non-Empty
Root cause (Feb 2026): The Graph API requires impactedAssets to contain at least 1 element. Sending an empty array ("impactedAssets": []) returns 400 BadRequest with InvalidInput code and the message: "The field ImpactedAssets must be a string or array type with a minimum length of '1'."
This error is particularly difficult to diagnose because:
The error message only appears in some response formats — when using Invoke-MgGraphRequest with raw JSON strings, the "message" field is often empty ("")
The actual error text only surfaced when using ConvertTo-Json on a PowerShell hashtable body
All other fields in the payload may be valid, making it seem like a server-side issue
Every custom detection must declare at least one impacted entity. Choose the most relevant asset type for the detection:
Detection Focus
Asset Type
Example Identifier
Email-based threats
impactedMailboxAsset
recipientEmailAddress, senderFromAddress
User activity
impactedUserAsset
accountUpn, accountObjectId
Endpoint/device
impactedDeviceAsset
deviceId, deviceName
Prevention:
Always include at least one impactedAssets entry in manifests and API payloads
The companion script Deploy-CustomDetections.ps1 validates this at manifest load time and rejects rules with empty impactedAssets before calling the API
Review the Impacted Asset Types section for the full list of valid identifiers per asset type
Pitfall 14: Max 3 Unique Dynamic Columns Across Title + Description
The Graph API enforces 3 unique {{Column}} references across title and descriptioncombined (not per field). Exceeding this returns 400 Bad Request — often with an empty error message via Invoke-MgGraphRequest.
⚠️ MS Learn discrepancy: Docs say 3 per field; the API empirically enforces 3 unique total across both fields (confirmed Mar 2026).
Scenario
Unique Columns
Result
title: {{A}} {{B}}, description: {{A}} {{C}}
A, B, C = 3
✅ Accepted
title: {{A}} {{B}}, description: {{C}} {{D}}
A, B, C, D = 4
❌ 400 Bad Request
Counting: Reuse across fields is free ({{A}} in both = 1). Count distinct names, not occurrences.
Workaround: Replace excess {{Column}} refs with static text, or use customDetails (up to 20 KVPs) to surface extra columns in the alert side panel. Deploy-CustomDetections.ps1 validates this at manifest load time.
When building queryText in PowerShell, always use single-quoted here-strings (@'...'@), NEVER double-quoted (@"..."@). Two distinct failure modes make double-quoted here-strings unreliable for KQL:
Risk 1 — $variable interpolation: PowerShell double-quoted strings interpolate $var references. KQL uses $left and $right in join syntax and $ as a dynamic property prefix. Inside @"..."@, PowerShell replaces these with empty strings (undefined variables → $null → empty), silently producing broken KQL with no compile-time warning.
Risk 2 — LLM/human escaping confusion (confirmed Mar 2026): When writing KQL inside a double-quoted context, an LLM (or human) instinctively adapts backslash escaping — writing \skills (single backslash) instead of \\skills (double backslash), because most languages interpret \\ → \ in double-quoted strings. PowerShell does NOT do this (backtick ` is the escape character, not backslash), so the single \ passes through literally to the KQL parser, which rejects \s as an invalid escape sequence → 400 Bad Request: "syntax errors".
Byte-level proof (Mar 2026): When identical \\skills content is deliberately placed in both here-string types, PowerShell produces identical bytes — confirming PowerShell itself does not mangle backslashes. The difference arises from what gets written into the string (by the LLM or human), not from PowerShell processing it. This makes the bug extremely hard to diagnose: the query looks correct in terminal output, and the root cause is an invisible content difference between attempts.
Here-String Type
$left / $right
Backslash Content
Practical Safety
@'...'@ (single-quoted)
Literal $left ✅
What you write is what you get
✅ Safe — no interpretation
@"..."@ (double-quoted)
Interpolated → empty ❌
What you write is what you get — but LLMs write different content
❌ Fragile — two failure modes
Rule: For ANY queryText, always use @'...'@. This eliminates both $ interpolation bugs and escaping confusion. Applies to inline PowerShell, the batch deployment script, and any LLM-generated deployment commands.
Additional validated finding:ingestion_time() IS accepted by the CD API for scheduled (non-NRT) rules (tested and confirmed Mar 2026). However, NRT rules reject ingestion_time() with 400 Bad Request — empirically confirmed Apr 2026, reproducible on retry. See Pitfall 17.
Pitfall 16: StrictMode .Count on Pipeline Scalars
PowerShell pipelines returning exactly 1 result unwrap to a scalar. Under Set-StrictMode -Version Latest, .Count on a scalar throws a terminating error. Always wrap in @() when .Count will be accessed — applies to pipelines, ConvertFrom-Json (single-element JSON arrays), and Get-* cmdlets.
# ❌ $x is scalar string when 1 result → .Count fails
$x = ... | Sort-Object -Unique
# ✅ $x is always Object[]
$x = @(... | Sort-Object -Unique)
Fixed inDeploy-CustomDetections.ps1 (Mar 2026): dynamic column validation, manifest load, and existing rule fetch all wrapped in @().
Pitfall 17: ingestion_time() Rejected in NRT Rules
NRT rules (schedule: "0") reject ingestion_time() with 400 Bad Request. The NRT Constraints table notes that Timestamp > ago(...) is "unnecessary but harmless" — however, ingestion_time() is NOT harmless in NRT mode. It is a function call (not a column filter), and the NRT streaming pipeline rejects it outright.
Empirically confirmed (Apr 2026): Four-attempt A/B test on the same query (DeviceProcessEvents with vssadmin/bcdedit destructive command detection):
Attempt
ingestion_time() present
Result
1st deploy
Yes
❌ 400 Bad Request
2nd deploy
Removed
✅ Created
Delete + redeploy
Restored
❌ 400 Bad Request
Delete + redeploy
Removed
✅ Created
Root cause hypothesis: NRT rules process events via streaming ingestion — ingestion_time() likely depends on a materialized ingestion timestamp that isn't available (or isn't filterable) in the NRT streaming pipeline.
Fix: For NRT rules, omit ingestion_time() entirely. If you need a time filter, use Timestamp > ago(...) instead (accepted but unnecessary since NRT pre-filters automatically).
Rule type
ingestion_time()
Timestamp > ago(...)
Scheduled (1H+)
✅ Accepted (preferred)
✅ Accepted
NRT ("0")
❌ 400 Bad Request
✅ Accepted (unnecessary)
Pitfall 18: category and mitreTechniques Are Server-Side Validated
Both fields are validated against fixed allowlists — invalid values return 400 Bad Request (with descriptive messages, unlike Pitfall 13), not silently accepted.
category — title-case and case-sensitive (defenseevasion → "Invalid alert category."), single-value (arrays rejected). Full accepted set in Alert Category Values.
mitreTechniques — invalid IDs → "Mitre techniques (...) are invalid." The accepted set tracks the tenant's ATT&CK version: legacy IDs (e.g. T1003.001) always work; newer sub-techniques (e.g. T1556.009, T1659) work only on refreshed tenants (expanded set rolling out via preview as of June 2026).
Fallback rule: If a newer sub-technique returns "Mitre techniques ... are invalid", fall back to the parent technique (T1556.009 → T1556) or a legacy ID. Don't assume the newest ATT&CK values are available everywhere.
Pitfall 19: impactedAssets Alone Is Silently Dropped — entityMappings Is the Real Binding
Entity extraction uses two parallel structures under detectionAction.alertTemplate: impactedAssets (the asset type) and entityMappings (the query-column→entity binding). The API auto-derives entityMappings from impactedAssets for recognized tables — so impactedAssets alone works on Device*, CloudAppEvents, Email*, SigninLogs. But auto-derivation silently fails on some connector tables (confirmed on GSA NetworkAccessGenerativeAIInsights / NetworkAccessTraffic, even with a single-table query):
POST/PATCH succeeds with no error, but a follow-up GET shows impactedAssets: [] → the alert gets no impacted assets. That empty GET is the diagnosis signature.
Fix: send entityMappings explicitly (what the portal does). Deploy-CustomDetections.ps1 does this automatically from the manifest — so this only bites manual Invoke-MgGraphRequest POSTs, which must include entityMappings themselves (see Entity Mappings):
Not an API change — entityMappings was always the binding; it only surfaces when auto-derivation fails on an unrecognized table.
CD Metadata Contract
Query files in queries/ can include per-query cd-metadata blocks that provide structured data for the detection authoring skill. This is the producer/consumer contract between the KQL Query Authoring skill (producer) and the Detection Authoring skill (consumer).
When cd-metadata is present
When a query in queries/ includes a cd-metadata block, the detection authoring skill uses it to:
Skip manual CD-readiness assessment — the block declares readiness explicitly
Generate the adapted CD query by applying the Query Adaptation Checklist to the Sentinel query in the same section
Schema
The cd-metadata block is an HTML comment with YAML content, placed immediately after the per-query metadata fields (Severity, MITRE, Tuning Notes) and before the KQL code block:
### Query N: [Title]**Purpose:** ...
**Severity:** High
**MITRE:** T1053.005, T1059.001
<!-- cd-metadata
cd_ready: true
schedule: "1H"
category: "Persistence"
title: "Encoded PowerShell in Scheduled Task on {{DeviceName}}"
impactedAssets:
- type: device
identifier: deviceName
recommendedActions: "Investigate the scheduled task XML. Decode the base64 payload and check for malicious content."
adaptation_notes: "Straightforward — already row-level, add mandatory columns"
-->
```kql
// Query code...
### Field Reference
| Field | Required | Type | Description |
|-------|----------|------|-------------|
| `cd_ready` | Yes | `true` / `false` | Whether this query can be adapted for custom detection deployment |
| `schedule` | If cd_ready | `"0"` / `"1H"` / `"3H"` / `"12H"` / `"24H"` | Detection frequency. `"0"` = NRT (single-table, no joins/unions) |
| `category` | If cd_ready | string | Alert category (see [API Reference](#api-reference) for valid values) |
| `title` | No | string | Dynamic alert title with `{{ColumnName}}` placeholders. Falls back to query heading if omitted. **Limit: max 3 unique `{{Column}}` references across `title` AND `description` combined** (see [Pitfall 14](#pitfall-14-max-3-unique-dynamic-columns-across-title--description)) |
| `impactedAssets` | If cd_ready | array | Asset entities to extract. Each entry: `type` (`device`/`user`/`mailbox`) + `identifier` (predefined API value, e.g., `accountUpn`, `deviceName` — see [Impacted Asset Types](#impacted-asset-types)). The deploy script derives the required `entityMappings` column binding from these automatically (see [Entity Mappings](#entity-mappings--required-column-binding) / [Pitfall 19](#pitfall-19-impactedassets-alone-is-silently-dropped--entitymappings-is-the-real-binding)). Optional per-entry `column` overrides the derived (PascalCase) column name |
| `recommendedActions` | No | string | Triage guidance shown in the alert. Omit if not needed |
| `responseActions` | No | array | **PROHIBITED** — must always be omitted or empty `[]`. Response actions must only be configured manually in the Defender portal |
| `adaptation_notes` | No | string | Human-readable notes on what adaptation is needed (for the summary table) |
### Queries NOT suitable for CD
For queries that cannot be adapted (baseline queries, statistical aggregations), use:
```markdown
<!-- cd-metadata
cd_ready: false
adaptation_notes: "Statistical baseline query — requires summarize with dcount, not suitable for CD"
-->
This explicitly documents the assessment so the detection skill doesn't re-evaluate it each time.
How the detection skill consumes cd-metadata
User says "deploy query 8 as a custom detection" → Skill reads the query file, finds the cd-metadata block for Query 8
