sqlclusterlog-review

name	sqlclusterlog-review
description	Analyzes Windows Server Failover Cluster (WSFC) CLUSTER.LOG files for Always On Availability Group root-cause diagnosis. Use this skill when an availability group has gone offline, a failover occurred unexpectedly, or a node was evicted, and you need to identify the WSFC-level cause that SQL Server DMVs cannot see. Applies 30 checks (L1–L30) covering lease timeouts, health check failures, quorum loss, node eviction, network partition, RHS crashes, AG resource transitions, Cloud Witness, Azure Arc, and Contained AG.
triggers	["/sqlclusterlog-review"]

WSFC Cluster Log Review Skill

Purpose

Analyze Windows Server Failover Cluster (WSFC) CLUSTER.LOG files to diagnose Always On Availability Group failures at the cluster level — the layer below SQL Server DMVs. Applies 30 checks (L1–L30) across five categories:

• L1–L8 — File-wide patterns: lease timeouts, health check failures, RHS crashes, error bursts, repeated failover cycling, quorum loss, node eviction, log time gaps
• L9–L17 — AG resource checks: offline transitions, SQL connectivity loss, forced failovers, long pending states, DLL init failures, API timeouts, cascade failures, primary role loss, replica disconnection
• L18–L22 — Network and node: partition/split-brain, NIC failure, heartbeat timeout, witness failure, node isolation
• L23–L25 — Configuration signals: VerboseLogging=0, SeparateMonitor absent, incomplete node coverage
• L26–L30 — Modern cluster features: Cloud Witness timeout, Azure Arc agent disconnect, Contained AG system database offline, cross-subnet probe failure, sp_server_diagnostics warning

Input

Accept any of:

• File path — path to CLUSTER.LOG (live log: C:\Windows\Cluster\cluster.log; log generated via Get-ClusterLog: C:\Windows\Cluster\Reports\CLUSTER.LOG)
• Inline paste — raw CLUSTER.LOG content pasted directly into chat
• Natural language description — describe symptoms ("the AG went offline at 14:32, SQL error log shows lease expiry")

For full analysis, the log should cover at least the 10 minutes before the incident and include entries from all cluster nodes. If only a partial extract is available, note which time range and nodes are covered and flag L25 if node coverage appears incomplete.

Log Entry Format

WSFC log entries follow this pattern:

<tid>.<pid>::<YYYY>/<MM>/<DD>-<HH>:<MM>:<SS>.<ms> <LEVEL> [<COMPONENT>] <message>

Key components:

• [RES] — Resource DLL host (hadrres.dll operations)
• [hadrag] — AG-specific resource agent inside RES
• [RHS] — Resource Hosting Subsystem (manages RES process lifecycle)
• [RCM] — Resource Control Manager (orchestrates state transitions)
• [NM] — Network Manager
• [NODE] — Node membership and heartbeat
• ERR / WARN / INFO — Severity prefixes in log lines

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Thresholds Reference

Threshold	Value	Used by
Error burst window	>10 ERR lines in 5 min → Critical; >5 → Warning	L4
Failover cycling	≥3 group moves in 30 min → Critical; ≥2 → Warning	L5
Log time gap	>30 min → Critical; >5 min → Warning	L8
Pending state duration	>120 sec → Critical; >30 sec → Warning	L12
Lease timeout	20 sec (SQL Server default LeaseTimeout — distinct from HealthCheckTimeout)	L1
Health check timeout	30 sec (SQL Server default HealthCheckTimeout for sp_server_diagnostics)	L2
Heartbeat timeout	Missed heartbeats reach the threshold — WSFC default SameSubnetThreshold/CrossSubnetThreshold is version-dependent: 20 (Windows Server 2019), 10 (WS2016), 5 (WS2008–2012 R2)	L20

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File-Wide Pattern Checks (L1–L8)

Evaluate these first — they reveal root causes that explain all downstream AG failures.

L1 — Lease Timeout

• Trigger: Log contains [hadrag] Lease Thread terminated, lease time expired, HealthCheckTimeout associated with a lease expiry message, or LeaseExpired in [RES] or [hadrag] context
• Severity: Critical — lease expiry causes an immediate AG resource failure with no grace period
• Fix: Lease timeout indicates the SQL Server health check thread did not respond within the lease window (see Thresholds Reference). Root causes: (1) SQL Server scheduler starvation — check for long-running queries blocking the health thread; (2) memory pressure causing paging — review sys.dm_os_memory_clerks; (3) storage I/O latency > 10 ms on the system drive — check Windows Performance Monitor; (4) if underlying cause is none of the above, increase sp_server_diagnostics timeout via CLUSTER_DIAGNOSTICS_TIMEOUT or raise HealthCheckTimeout in the AG resource properties.

L2 — Health Check Failure

• Trigger: Log contains IsAlive check failed, LooksAlive check failed, HealthCheckTimeout, or sp_server_diagnostics returning a failure state (STATE = 3 or STATE = 4) in [RES]/[hadrag] messages
• Severity: Critical — consecutive health check failures trigger resource restart or failover
• Fix: Identify whether LooksAlive or IsAlive failed. LooksAlive failures (process-level ping) indicate SQL Server process termination or severe hangs. IsAlive failures (sp_server_diagnostics query) indicate scheduler starvation, I/O hangs, or insufficient health check timeout. Capture sys.dm_os_ring_buffers for the incident time and review the SQL ERRORLOG for the matching SPID N error.

L3 — RHS Process Crash

• Trigger: Log contains RHS process terminated, RHS.EXE terminated unexpectedly, creating new RHS process, rhs.exe exit in [RHS] context, or RHS exiting / unhandled exception in RHS
• Severity: Critical — RHS crash causes all resources hosted in that process to go offline
• Fix: RHS crash is a Windows-level failure, not SQL Server. Capture the Windows Application and System event logs at the incident time. Look for Event ID 1146 (RHS terminated) and corresponding Dr. Watson / crash dump. Common causes: a resource DLL (hadrres.dll or another DLL) threw an unhandled exception, or a third-party DLL was loaded into RHS and faulted. Enable SeparateMonitor on the AG resource to isolate hadrres.dll in its own RHS process (see L24).

L4 — Error Burst Density

• Trigger: More than 5 ERR-level lines appear within any 5-minute window in the log
• Severity: Warning if >5 ERR lines in 5 min; Critical if >10 ERR lines in 5 min (see Thresholds Reference)
• Fix: An error burst signals that multiple subsystems are failing simultaneously — often the symptom of a single root cause (L1, L6, L18). Identify the first ERR line in the burst — that is the originating failure. All subsequent ERRs in the burst are usually cascades. Fix the root cause first; the cascade errors will stop.

L5 — Repeated Failover Cycling

• Trigger: The log contains ≥2 [RCM] or [hadrag] group Move, Online, or Offline events for the same AG resource within any 30-minute window
• Severity: Warning if ≥2 moves in 30 min; Critical if ≥3 moves in 30 min (see Thresholds Reference)
• Fix: Rapid cycling means the AG resource goes online, fails again, and attempts recovery repeatedly. This exhausts MaxRestarts and eventually leaves the AG permanently offline. Root cause is almost always L1, L2, or L18 — a recurring condition that fails every recovery attempt. Identify and fix the root cause before the next failover window. Consider temporarily suspending the AG resource to prevent further cycling while the root cause is resolved.

L6 — Quorum Loss

• Trigger: Log contains quorum loss, quorum not achieved, no quorum, lost quorum, or cluster service stopping — no quorum in any component
• Severity: Critical — quorum loss stops the cluster service on all nodes, taking all AG resources offline
• Fix: Identify how many votes were lost. Review which nodes are in the log — if a majority-node-set configuration lost too many nodes, quorum fails. Immediate action: if a node is temporarily isolated, restore network connectivity. If a witness (disk/FSW/cloud) is unavailable, fix the witness first (see L21). Long-term: review quorum configuration — avoid even-node clusters without a witness.

L7 — Node Eviction

• Trigger: Log contains node eviction, removing node, node was removed from membership, evicted from cluster, or NodeMembership showing a node leaving in [NODE] or [NM] context
• Severity: Critical — node eviction means the cluster forcibly removed a node from membership, ending all resources that were primary on that node
• Fix: Node eviction is caused by sustained communication failure (heartbeat timeout, network partition) or manual eviction. Check [NODE] entries immediately before the eviction for heartbeat failures (L20) or network partition signals (L18). If eviction was manual, the cluster is operating as expected. If unexpected, review NIC bonding configuration and cross-subnet latency.

L8 — Log Time Gap

• Trigger: Consecutive log entries have a timestamp gap larger than 5 minutes with no intervening entries from any component
• Severity: Warning if >5 min; Critical if >30 min (see Thresholds Reference)
• Fix: A large time gap means the cluster log was not recording events — either the cluster service stopped, the node was powered off, or VerboseLogging was too low to capture events at the right frequency (see L23). If the gap coincides with the incident time, critical diagnostic data is missing. Retrieve CLUSTER.LOG from all nodes — one node may have continued logging while another was silent. Enable verbose logging proactively on all cluster nodes.

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AG Resource Checks (L9–L17)

These checks fire on SQL Server AG-specific resource events within the WSFC log layer.

L9 — AG Offline Transition

• Trigger: Log contains TransitionToState ... Online-->Offline, TransitionToState ... OnlinePending-->Offline, OfflineCallIssued, or resource going offline in [RCM] or [hadrag] for an AG resource
• Severity: Critical if transition source is Online (unexpected); Warning if transition source is OnlinePending (resource never completed initialization)
• Fix: The AG resource transitioned to Offline. Identify whether the transition was initiated by WSFC (health check failure — see L2) or by SQL Server itself (the AG resource DLL called OfflineResource). If initiated by WSFC: fix L1 or L2. If initiated by the DLL: capture the [hadrag] log entries immediately before the transition — they explain the SQL-side reason (replica disconnect, data sync failure, or SQL Server error).

L10 — SQL Connectivity Loss

• Trigger: Log contains Disconnect from SQL Server, SQL Server connection failed, ODBC error, or SqlConnect failed in [hadrag] or [RES] context
• Severity: Critical — loss of connectivity between hadrres.dll and the local SQL Server instance means health checks cannot run and the AG resource will fail
• Fix: The resource DLL connects to SQL Server on the local loopback to run health checks. Failure means: (1) SQL Server service is stopped or starting — check Windows Service Control Manager; (2) SQL Server is overloaded and not accepting new connections — check for max connections reached (sys.dm_exec_sessions vs. max connections); (3) the dedicated admin connection (DAC) is in use — hadrres.dll uses a regular connection, not DAC, so this is not a DAC issue. Restart SQL Server if terminated unexpectedly.

L11 — Forced Failover

• Trigger: Log contains forced failover in [hadrag], [RCM], or [RES] context, or FAILOVER_MODE = MANUAL paired with an Online event on a formerly secondary node
• Severity: Warning — forced failover may result in data loss if the secondary was not synchronized
• Fix: Determine whether the forced failover was administrator-initiated or automatic. If automatic: this should not happen in a normal AG — a forced failover implies the primary failed and automatic failover was configured. Check whether data loss occurred by comparing last_commit_lsn on the former secondary (now primary) with the last confirmed last_hardened_lsn on the old primary. If administrator-initiated: document the reason and verify the replica is in synchronized state.

L12 — Long Pending State

• Trigger: An AG resource remains in OnlinePending or OfflinePending state for longer than the pending state thresholds (see Thresholds Reference) — calculated from the timestamp of the Pending entry to the next state transition entry
• Severity: Warning if >30 sec in pending; Critical if >120 sec (see Thresholds Reference)
• Fix: Pending states longer than expected indicate the resource DLL's Online or Offline call is not returning promptly. For OnlinePending: hadrres.dll is waiting for SQL Server to complete AG initialization — check SQL ERRORLOG for slow database recovery or role change. For OfflinePending: the DLL is waiting for the AG to gracefully suspend — if the wait is very long, a KILL or forced offline may be issued by WSFC, causing a dirty shutdown.

L13 — hadrres.dll Init Failure

• Trigger: Log contains DLL load failure, hadrres.dll initialization error, failed to initialize, or DLL could not be loaded in [RES] or [RHS] context
• Severity: Critical — the AG resource DLL cannot run, so the AG resource cannot come online on this node
• Fix: DLL init failure is usually caused by a missing dependency (Visual C++ runtime, Windows Server feature) or a corrupted hadrres.dll. Steps: (1) verify SQL Server is fully installed and the path in the cluster resource properties points to the correct hadrres.dll version; (2) check the Windows Application event log for DLL load errors; (3) run sfc /scannow to check for corrupted system files; (4) if after a SQL Server patch, the cluster resource DLL path may need to be updated manually.

L14 — Resource DLL API Timeout

• Trigger: Log contains API call timed out, Resource DLL returned ... after ... ms, or Dll timeout in [RCM] or [RHS] context for an AG resource
• Severity: Warning — if the DLL API timeout repeats, WSFC will declare the resource failed
• Fix: The resource DLL took longer than the configured DllWatchdogTimeout to respond to a WSFC API call (Online, Offline, LooksAlive, IsAlive). Caused by the same conditions as L1 and L2 — SQL Server scheduler starvation or I/O hangs. Correlate the timeout timestamp with SQL Server ERRORLOG. If timeouts recur, raise the cluster resource DllWatchdogTimeout value or fix the underlying SQL Server performance issue.

L15 — Cascade Across AGs

• Trigger: Multiple distinct AG resource names appear in ERR lines within the same 5-minute window — indicating more than one AG failed concurrently
• Severity: Warning — simultaneous multi-AG failure indicates a shared infrastructure failure (network partition, node failure) rather than an AG-specific issue
• Fix: Multiple AGs failing at the same time rules out AG-specific tuning as the solution. Focus on the shared infrastructure: network (L18, L19), node health (L7), or quorum (L6). Identify which AG failed first — that is the originating AG; the others are cascades caused by the same underlying event. Fix the root infrastructure issue.

L16 — Primary Role Loss

• Trigger: Log contains [hadrag] messages showing the primary replica transitioning to Resolving or Secondary role without a corresponding planned failover command
• Severity: Warning — unexpected primary role loss means the AG is momentarily without a primary; all write workloads will fail
• Fix: Unexpected primary role loss is caused by WSFC declaring the AG resource offline (see L1, L2, L9) or by a network split that caused the primary to lose quorum. Check whether a secondary promoted to primary simultaneously — if so, a failover completed. If no secondary promoted, the AG is in a Resolving state and requires manual intervention to bring a replica online. Review L6 for quorum loss.

L17 — Replica Disconnection

• Trigger: Log contains DISCONNECTED, replica disconnected, or connectivity failure messages in [hadrag] or [RES] context that refer to a remote replica endpoint
• Severity: Warning — a disconnected replica means data is not flowing to that secondary; during failover, the disconnected replica will have a stale copy
• Fix: Replica disconnection is a network-layer event — the AG mirroring endpoint (typically TCP 5022) cannot reach the remote replica. Check: (1) firewall rules on port 5022; (2) SQL Server Database Mirroring Endpoint is in STARTED state on both replicas; (3) network latency and packet loss between nodes; (4) DNS resolution for the AG listener and endpoint addresses. Run /sqlwait-review and check for HADR_SYNC_COMMIT and HADR_WORK_QUEUE waits that signal the send queue is backing up.

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Network and Node Checks (L18–L22)

L18 — Network Partition / Split-Brain

• Trigger: Log contains network partition, split brain, lost quorum due to network, or unable to communicate with a quorum of nodes in any component
• Severity: Critical — split-brain means two node subsets each believe they hold quorum; only the subset with a majority/witness actually holds it; the other loses all resources
• Fix: Network partition is a physical or virtual network failure. Immediate: identify which subnet was lost. Check NIC bonding/teaming configuration — a single physical NIC for cluster heartbeats is a single point of failure. Check switch VLAN configuration. Long-term: implement redundant NICs for the cluster network, configure multiple cluster networks, and ensure the heartbeat network is dedicated (not shared with SQL client traffic).

L19 — Cluster Network Interface Failure

• Trigger: Log contains cluster network offline, NIC failure, network interface, adapter, or NetworkInterface going to failed state in [NM] context
• Severity: Warning — a NIC failure degrades cluster network redundancy; if the remaining network also fails, it becomes L18
• Fix: A cluster network interface failed. Check Windows Device Manager and the System event log on the affected node for NIC driver errors (Event ID 27, 32). Common causes: cable failure, switch port failure, NIC driver bug, or power management putting the NIC in a low-power state. For VMs: check vSwitch configuration and the hypervisor's virtual NIC health. Replace or repair the hardware, then ensure the cluster network is verified healthy in Failover Cluster Manager before the next planned maintenance.

L20 — Heartbeat Timeout

• Trigger: Log contains missed heartbeats, heartbeat timeout, node is not responding, or connectivity timeout between nodes in [NODE] or [NM] context — particularly when the count of missed heartbeats reaches or exceeds the CrossSubnetThreshold or SameSubnetThreshold (see Thresholds Reference)
• Severity: Critical if the node is subsequently evicted; Warning if heartbeats resume before eviction
• Fix: Node heartbeats are the cluster's mechanism for detecting node failures. The default SameSubnetThreshold/CrossSubnetThreshold depends on the Windows Server version — 20 heartbeats on Windows Server 2019, 10 on WS2016, 5 on WS2008–2012 R2 (with default 1 s delay). Missed heartbeats are caused by: (1) network congestion or latency spike on the heartbeat network; (2) node CPU starvation (100% CPU prevents the heartbeat thread from running); (3) memory pressure causing paging on the heartbeat buffer. Tune CrossSubnetDelay and CrossSubnetThreshold for geographically distributed clusters (higher latency = higher threshold needed). Do not tune same-subnet thresholds unless explicitly recommended by Microsoft — reducing them increases false evictions.

L21 — Witness Access Failure

• Trigger: Log contains disk witness, file share witness, or cloud witness failure — phrases such as witness resource failed, disk witness offline, cannot access file share witness, or cloud witness errors in [RES] or [RCM]
• Severity: Critical — without the witness, an even-node cluster cannot achieve quorum after any single node failure
• Fix: For disk witness: check that the witness disk is online in Disk Management on all nodes; verify the disk's cluster resource is Online. For file share witness: verify the UNC path is accessible from all nodes and the cluster service account has write permissions. For cloud witness: verify Azure storage account connectivity (TCP 443 outbound to *.blob.core.windows.net), and that the storage account key in the cluster configuration matches the current key.

L22 — Node Isolation

• Trigger: Log contains node isolated, unable to communicate with followed by multiple node names, or all communication lost for a node in [NODE] or [NM] context
• Severity: Critical — an isolated node cannot vote in quorum, and all primary resources on it will fail over (or fail entirely if quorum is lost)
• Fix: Node isolation is the most severe form of L18/L20 — the node has lost communication with all peers simultaneously. Check all network adapters on the isolated node. If it is a VM, check the hypervisor host's network health. If a physical host, check switch port configuration. If the node recovers connectivity, WSFC should automatically re-admit it to membership. If the node is permanently isolated, evict it from the cluster and re-add after restoring connectivity.

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Configuration Signal Checks (L23–L25)

L23 — VerboseLogging = 0 (Sparse Events)

• Trigger: The log contains fewer than 20 entries per minute in the period surrounding the incident, or contains VerboseLogging = 0 or VerboseLogging disabled explicitly, or critical diagnostic context (API call durations, resource state details) is absent from entries that would normally include it
• Severity: Info — VerboseLogging=0 does not cause failures but reduces diagnostic detail
• Fix: Enable verbose logging before the next maintenance window: (Get-ClusterResource "AG Resource Name") | Set-ClusterParameter VerboseLogging 1. Verbose logging captures API call durations, state transition details, and health check results that are essential for post-incident diagnosis. Note that verbose logging increases disk I/O for the cluster log on busy clusters — test the disk impact before enabling in production.

L24 — SeparateMonitor Not Set

• Trigger: The log contains multiple resource DLL entries from the same RHS process (same thread ID prefix) — indicating hadrres.dll shares rhs.exe with other resource DLLs, and SeparateMonitor is not enabled for the AG resource
• Severity: Info — SeparateMonitor isolates hadrres.dll in its own RHS process; without it, a fault in any other DLL in the shared process can crash the AG resource (see L3)
• Fix: Enable SeparateMonitor on the AG resource: (Get-ClusterResource "AG Resource Name") | Set-ClusterParameter SeparateMonitor 1. This causes the AG resource DLL to run in a dedicated rhs.exe process. An RHS crash in another resource DLL will no longer affect the AG. This is a Microsoft best practice for SQL Server AG resources on Windows Server 2012 R2 and later.

L25 — Missing Node Coverage

• Trigger: Log entries reference nodes or IP addresses not seen in the file-wide node list, or the expected number of cluster nodes (from [NODE] membership entries) is greater than the number of distinct node identifiers that appear as log entry sources
• Severity: Info — incomplete node coverage means the analysis cannot rule out failures on uncovered nodes
• Fix: CLUSTER.LOG is per-node — each node writes its own log. Collect logs from all cluster nodes for the same time window: Get-ClusterLog -Destination C:\ClusterLogs -TimeSpan 60 (omit -Node to collect from all nodes by default). Without logs from all nodes, an isolated node failure or network partition seen only from the failing node's perspective may not be visible. State which nodes are covered in the analysis summary.

Modern Cluster Feature Checks (L26–L30)

L26 — Cloud Witness Repeated Timeout

• Trigger: Log contains CloudWitness entries with Timeout or Unable to reach repeated ≥ 3 times in any 10-minute window — Windows Server 2016+ (Cloud Witness requires WS2016 or later)
• Severity: Critical — repeated Cloud Witness timeouts indicate the Azure Blob Storage endpoint is intermittently or persistently unreachable; in a two-node cluster, witness unavailability means quorum is at risk
• Fix: Check outbound HTTPS to <storageaccount>.blob.core.windows.net:443. Verify no firewall or NSG change was made. Rotate or re-enter the access key in Failover Cluster Manager. If the witness is in a different Azure region than the cluster, consider a witness in the nearest region or fail over to a File Share Witness during the outage.

L27 — Azure Arc-Managed Cluster Agent Disconnect

• Trigger: Log contains ArcSqlExtension or HybridConnectivity entries with disconnected or heartbeat failure — any SQL Server version with Azure Arc agent installed on cluster nodes
• Severity: Warning — the Azure Arc agent on one or more cluster nodes has lost contact with the Azure control plane; Arc-based management features (policy, Defender, automated backups) are not functioning on those nodes
• Fix: On each cluster node check the Arc agent (himds) and the SQL Server extension service. The extension's Windows service display name is Microsoft SQL Server Extension Service and it runs as NT SERVICE\SqlServerExtension (on Linux the service is named SqlServerExtension) — there is no ArcSqlInstanceExtension service. Example: Get-Service -Name 'himds'; Get-Service -DisplayName 'Microsoft SQL Server Extension Service'. Check for service restarts in the Windows Event Log. Verify outbound connectivity to *.arc.azure.com:443 and *.<region>.arcdataservices.com:443. Re-run azcmagent connect if the managed-identity certificate has expired.

L28 — Contained AG: Contained System Database Offline

• Trigger: Log contains entries showing a Contained AG's contained system database resource (typically named <ag_name>_master) in FAILED or OFFLINE state — SQL 2022+ only
• Severity: Critical — a Contained AG system database offline means the contained logins, SQL Agent jobs, and linked servers for that AG are unavailable; applications depending on contained system objects will fail
• Fix: Check the resource state in Failover Cluster Manager. Attempt to bring the resource online: Start-ClusterResource -Name '<ag_name>_master'. If it fails, check SQL Server ERRORLOG for the contained system database for corruption or I/O errors. Correlate with /sqlhadr-review (H23) and /sqlerrorlog-review (E31).

L29 — Cross-Subnet Probe Failure

• Trigger: Log contains CrossSubnet probe entries with FAILED or No response — indicates cross-subnet heartbeat connectivity loss between cluster nodes in different subnets or sites
• Severity: Critical — cross-subnet probe failure means nodes cannot verify each other's health across a WAN or site boundary; this is a direct precursor to node isolation and quorum loss
• Fix: Verify UDP port 3343 (cluster communication) is open between subnets. Check network routing between sites. Review firewall rules for changes made near the incident time. For multisite clusters, confirm the RouteHistoryLength parameter is set appropriately and that multisite DNS resolution is functioning.

L30 — sp_server_diagnostics Component Warning

• Trigger: Log contains [RHS] Resource 'SQL Server' IsAlive check failed or sp_server_diagnostics output showing state=WARNING or state=ERROR for any component — SQL 2012+
• Severity: Warning (WARNING); Critical (ERROR) — sp_server_diagnostics is the health check procedure SQL Server uses to report its own health to the WSFC; component warnings often precede lease timeouts and AG failovers
• Fix: The component field identifies the failing subsystem: system (scheduler/I/O non-yielding), resource (memory pressure), query_processing (blocking/deadlock/spinlock), io_subsystem (I/O errors), or events (recent critical events). Each maps to a specific diagnosis path: query_processing warnings → /sqlwait-review; io_subsystem warnings → /sqlerrorlog-review (E15–E19); resource warnings → /sqlerrorlog-review (E9–E14). Capture the full sp_server_diagnostics output: EXEC sys.sp_server_diagnostics.

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Version-Aware Check Suppression

If the SQL Server version is stated by the user, read VERSION_COMPATIBILITY.md (~/.claude/skills/VERSION_COMPATIBILITY.md if installed, or skills/VERSION_COMPATIBILITY.md from the repo). If unavailable, skip silently. For checks whose minimum version exceeds the instance version: verbose mode → log as SKIP (version: requires SQL 20XX+, instance is SQL 20YY); standard report → omit entirely. Do not suppress NOT ASSESSED rows from missing input — only suppress version-inapplicable checks.

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Output Format

Structure the report as follows. The reference output in skills/sqlclusterlog-review/examples/cluster-analysis.md demonstrates the expected quality level.

## Cluster Log Analysis

### Summary
- X Critical, Y Warnings, Z Info
- Time range: [first timestamp] – [last timestamp]
- Nodes covered: [node list from log entries]
- Highest-risk finding: [check name and check ID]

### Critical Issues
### [C1 — L1] Lease Timeout — ag_primary (14:32:01)
- **Observed:** [specific log lines, timestamps, and component tags]
- **Impact:** [why this matters at runtime — what failed and what the user experienced]
- **Fix:** [concrete action referencing the check fix steps]

### Warnings
### [W1 — L4] Error Burst — 8 ERR lines in 3 min (14:31:58–14:34:47)
- **Observed:** ...
- **Impact:** ...
- **Fix:** ...

### Info
### [I1 — L23] VerboseLogging = 0 — sparse event density
- **Observed:** ...
- **Impact:** ...
- **Fix:** ...

### Passed Checks
| Check | Result |
|-------|--------|
| L6 — Quorum Loss | PASS — no quorum loss entries in log |
| L7 — Node Eviction | PASS — no eviction events found |

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*Analyzed by: [state the AI model and version you are running as, e.g. "Claude Sonnet 4.6", "DeepSeek R1", "GPT-4o"] · [current date and time in the user's local timezone, or UTC if timezone is unknown, e.g. "2026-05-16 20:15 NZST"]*

Labeling convention: Output labels use [C1], [W1], [I1] — not raw check IDs. Check IDs (L1, L9) appear in parentheses after the label in finding headers.

Each finding states Observed (exact log evidence) → Impact (runtime effect) → Fix (actionable step). The Passed Checks table explicitly lists every L-check that was evaluated and not triggered, to signal analysis confidence.

If fewer than two cluster nodes are represented in the log, note this in the Summary and flag L25. If the log covers less than 5 minutes, note the limited time window.

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Section: Output Filters (--brief / --critical-only)

--brief — Omit the Passed Checks table and attribution footer. Output the Summary, Findings, and Prioritized Fix Sequence sections only. Use when a quick scan of what fired is all that's needed.

--critical-only — Suppress Warning and Info findings. Show only Critical findings. The Passed Checks table is also omitted. Use when triaging an incident and only actionable blockers matter.

Both flags can be combined: --brief --critical-only produces the Summary section plus Critical findings only.

When neither flag is present, produce the full report as documented above.

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Section: Verbose Output (--verbose)

When the user's request includes --verbose, --trace, or the word verbose:

1. Append a ## Check Evaluation Log section after the Passed Checks table.

Include one row for every check in this skill's ruleset, in check-ID order:

Check	Evidence	Threshold	Result
[ID — Name]	[key attribute(s) and value found, or "absent"]	[threshold or condition]	PASS / FIRE → [severity] / NOT ASSESSED

Result conventions:

• PASS — attribute present, threshold not met
• **FIRE → Critical/Warning/Info** — threshold met; bold to distinguish from passes
• NOT ASSESSED — required attribute absent from input

2. Save both files to the current working directory using the Write tool:

output//-/analysis.md ← full report output//-/trace.md ← Check Evaluation Log

Derive <input-prefix>:

1. Filename stem if a file path was provided (e.g. horrible.sqlplan → horrible)
2. First meaningful identifier from the artifact (top wait type, first table name, procedure name, etc.)
3. Fallback: run Sanitize: alphanumeric + hyphens/underscores only, max 32 chars.

File headers: analysis.md → # Analysis — <skill-name> / # Input: <first 80 chars> / # Generated: <UTC timestamp> trace.md → # Check Evaluation Log — <skill-name> / # Input: <first 80 chars> / # Generated: <UTC timestamp>

Create directories as needed. When --verbose is not present, write nothing to disk.

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Companion Skills

• /sqlhadr-review — SQL-side AG state snapshot: replica sync health, redo/send queue sizes, estimated data loss — the complement to CLUSTER.LOG root-cause analysis

• /sqlerrorlog-review — SQL Server ERRORLOG timeline: AG failover events, lease expiry messages, memory pressure, and I/O warnings that correspond to WSFC events

• /sqlwait-review — correlate HADR_WORK_QUEUE, HADR_SYNC_COMMIT, and HADR_REPLICA_DDL_END waits with cluster log timestamps to connect the SQL-side wait signal to the WSFC-level event

• /sqlquerystore-review — after an AG failover identified in CLUSTER.LOG, use Query Store to detect plan regressions on the new primary

• /sqlplan-review — if scheduler starvation caused L1 or L2, analyze the long-running query that blocked the health check thread

• mssql-performance-review — Orchestrator that routes mixed artifacts to multiple specialised skills (this one included), runs an adversarial root-cause check, and produces a single consolidated report with evidence chain, risk-rated fixes, and rollback. Use when you have several artifact types together or describe a symptom without knowing which skill to run.