Download plugins/dotnet-diag/skills/clr-activation-debugging/SKILL.md from the publisher, or copy the path and open the repository. PlugMCP lists the skill and does not host the file.
Where does clr-activation-debugging live?
The publisher keeps it at https://github.com/dotnet/skills.
Which agents can use clr-activation-debugging?
Claude, Cursor, and Codex can read a SKILL.md you add from the publisher's repository.
>-
name: clr-activation-debugging
description: >-
Diagnoses .NET Framework CLR activation issues using CLR activation logs
(CLRLoad logs) produced by mscoree.dll. Use when: the shim picks the wrong
runtime, fails to load any runtime, shows unexpected .NET 3.5 Feature-on-Demand
(FOD) dialogs, unexpectedly does NOT show FOD dialogs, loads both v2 and v4
into the same process causing failures, or any time someone is wondering
"what is happening with .NET Framework activation?"
license: MIT
CLR Activation Debugging
Diagnose .NET Framework runtime activation issues by analyzing CLR activation logs (CLRLoad logs) produced by the shim (mscoree.dll). These logs record every decision the shim makes when selecting and loading a CLR version.
When to Use
A process fails to load the CLR at all ("Unable to find a version of the runtime to use")
The shim picks the wrong CLR version (e.g., v2.0 instead of v4.0)
Both CLR v2 and CLR v4 load into the same process, causing failures
A COM object fails to activate because the shim can't resolve the runtime
Legacy hosting APIs (CorBindToRuntime) bind to an unexpected version
When Not to Use
Modern .NET (CoreCLR / .NET 5+) — this skill covers .NET Framework only (the mscoree.dll shim)
Assembly binding failures — use Fusion logs (fuslogvw.exe), not CLR activation logs
Runtime crashes after the CLR has loaded — activation succeeded; the problem is elsewhere
Background
The Shim Architecture
The .NET Framework shim has two layers:
mscoree.dll (the "shell shim") — the public-facing DLL that is the registered InprocServer32 for CLR-hosted COM objects and the entry point for _CorExeMain, legacy APIs, etc.
mscoreei.dll — the actual shim implementation where the runtime selection logic, logging, and activation decisions live. mscoree.dll forwards into mscoreei.dll.
When reading logs, the caller-name:mscoreei.dll in FOD command lines reflects this — it's mscoreei.dll doing the work.
.NET 3.5 / v2.0.50727 Version Mapping
.NET 2.0, 3.0, and 3.5 all share the same CLR runtime version: v2.0.50727. The "3.0" and "3.5" releases were library additions on top of CLR v2.0. For activation purposes, they are all "v2.0.50727." When the shim resolves to v2.0.50727 or FOD offers to install "NetFx3", it's installing the CLR v2.0 runtime (plus the 3.0/3.5 libraries). Similarly, CLR v4.0 (v4.0.30319) covers all .NET Framework versions from 4.0 through 4.8.x.
.NET 3.5 Availability on Recent Windows
On recent Windows versions (Windows 11 Insider Preview Build 27965 and future platform releases), .NET Framework 3.5 is no longer available as a Windows optional component (Feature-on-Demand). It must be installed from a standalone MSI. This means the FOD dialog (fondue.exe /enable-feature:NetFx3) will not succeed on these systems even if it fires. On Windows 10 and Windows 11 through 25H2, FOD remains available. .NET Framework 3.5 reaches end of support on January 9, 2029.
Shim HRESULT Codes
When the shim fails, it returns specific HRESULTs in the 0x8013xxxx range. These are the errors you'll see from callers (not in the activation logs themselves, which log human-readable messages):
HRESULT
Symbol
Meaning
0x80131700
CLR_E_SHIM_RUNTIMELOAD
Cannot find or load a suitable runtime version. This is the most common shim error — it's what callers see when capped legacy activation fails on a v4-only machine.
0x80131701
CLR_E_SHIM_RUNTIMEEXPORT
Found a runtime but failed to get a required export or interface from it.
0x80131702
CLR_E_SHIM_INSTALLROOT
The .NET Framework install root is missing or invalid in the registry.
0x80131703
CLR_E_SHIM_INSTALLCOMP
A required component of the installation is missing.
0x80131704
CLR_E_SHIM_LEGACYRUNTIMEALREADYBOUND
A different runtime is already bound as the legacy runtime. A legacy API tried to bind to a version that conflicts with the one already chosen.
0x80131705
CLR_E_SHIM_SHUTDOWNINPROGRESS
The shim is shutting down and cannot service the request.
If a user reports one of these HRESULTs (especially 0x80131700), CLR activation logs are the right diagnostic tool.
Prerequisites
CLR activation logging must be enabled to produce log files. If the user doesn't have logs yet, instruct them to enable logging:
Via environment variable (recommended — scoped to current session):
set COMPLUS_CLRLoadLogDir=C:\CLRLoadLogs
Via registry (machine-wide — affects all .NET Framework processes):
On 64-bit systems, also set under Wow6432Node if 32-bit processes are involved.
⚠️ The log directory must already exist. The shim will not create it. If it doesn't exist, no logs will be written and there will be no error or indication of failure.
Logs are written as {ProcessName}.CLRLoad{NN}.log (NN = 00–99, one per process instance). Logs cannot be read until the process exits — the file is held open.
After capturing, remove the env var or registry key to stop logging.
Inputs
Input
Required
Description
CLR activation log files
Yes
One or more .CLRLoad*.log files
Symptom description
Recommended
What the user observed (FOD dialog, wrong runtime, failure, etc.)
Expected behavior
Recommended
What the user expected to happen
Workflow
Step 1: Load Reference Material
Resolve bundled paths from the directory that contains this SKILL.md, not from the user's workspace. Load the reference files in this order — they contain the detailed log format, decision flow, and CLSID registry documentation:
references/log-format.md — Log line format, fields, and all known log message types
references/activation-flow.md — The shim's decision tree for runtime selection
references/com-activation.md — COM (DllGetClassObject) activation specifics, CLSID registry layout
If a direct read fails, list this skill's references/ directory once and retry only when the listing shows the expected file. Do not use workspace file or text search to locate the skill installation.
If one or more expected reference files remain unavailable, use the loaded references where possible and the inline knowledge below for the missing coverage. Include Reference coverage: reduced; unavailable: <paths>; used inline guidance for missing references. in the final diagnosis, with <paths> replaced by the missing relative paths.
Step 2: Survey the Log Files
Get the big picture before diving into any single log:
List all log files and group by process name — this shows which executables triggered CLR activation
For each process, scan for outcome lines:
Decided on runtime: vX.Y.Z — successful resolution
ERROR: — failed resolution
Launching feature-on-demand — FOD dialog was shown
Could have launched feature-on-demand — FOD would have fired but was suppressed
V2.0 Capping is preventing consideration — v4+ was skipped due to capping
For each log file with an unexpected outcome, trace the full activation flow. Read the log top-to-bottom and identify:
⚠️ Nested log entries: The shim's own internal calls can trigger additional log entries within an activation sequence that is already being logged. For example, a DllGetClassObject call may internally call ComputeVersionString, which calls FindLatestVersion, each generating log lines. When the FOD check runs ("Checking if feature-on-demand installation would help"), it re-runs the entire version computation — producing a second ComputeVersionString block within the same activation. Don't mistake these nested/re-entrant entries for separate activation attempts.
3a. Entry Point
The first FunctionCall: or MethodCall: line tells you how activation was triggered:
Entry Point
Meaning
_CorExeMain
Managed EXE launch — the binary IS a .NET assembly
DllGetClassObject. Clsid: {guid}
COM activation — something CoCreated a COM class routed through mscoree.dll
ClrCreateInstance
Modern (v4+) hosting API
CorBindToRuntimeEx
Legacy (v1/v2) hosting API — binds the process to one runtime
ICLRMetaHostPolicy::GetRequestedRuntime
Policy-based hosting API (often called internally after other entry points)
LoadLibraryShim
Legacy API to load a framework DLL by name
3b. Input Parameters
Immediately after the entry point, the log dumps the version computation inputs:
IsLegacyBind: Is this a legacy (pre-v4) activation path? If 1, the shim uses the single-runtime "legacy" view of the world. Legacy APIs (CorBindToRuntimeEx, DllGetClassObject for legacy COM, LoadLibraryShim, etc.) set this.
IsCapped: If 1, the shim's roll-forward semantics are capped at Whidbey (v2.0.50727) — it will NOT consider v4.0+ when enumerating installed runtimes. This is the mechanism that makes v4 installation non-impactful: legacy codepaths continue to behave as if v4 doesn't exist. On a v4-only machine with no .NET 3.5, a capped enumeration sees no runtimes at all. Capping does NOT prevent loading v4+ if a specific v4 version string is explicitly provided (e.g., via CorBindToRuntimeEx("v4.0.30319", ...) or via config with useLegacyV2RuntimeActivationPolicy).
ShouldEmulateExeLaunch: Whether to pretend this is an EXE launch for policy purposes.
LegacyBindRequired: Whether a legacy bind is strictly required.
3c. Config File Processing
Look for config file parsing results:
Parsing config file: {path} — the shim is looking for a .config file
Config File (Open). Result:00000000 — config file found and opened successfully
Config File (Open). Result:80070002 — config file not found (HRESULT for ERROR_FILE_NOT_FOUND)
Found config file: {path} — config was successfully read
UseLegacyV2RuntimeActivationPolicy is set to {0|1} — whether <startup useLegacyV2RuntimeActivationPolicy="true"> is present. When 1, all runtimes are treated as candidates for legacy codepaths — meaning legacy shim APIs can enumerate and choose v4+. This can be used with multiple <supportedRuntime> entries, with other config options, or even with no <supportedRuntime> entries at all (in which case legacy APIs can simply enumerate v4). Side effect: turns off in-proc SxS with pre-v4 runtimes — locks them out of the process.
Config file includes SupportedRuntime entry. Version: vX.Y.Z, SKU: {sku} — each <supportedRuntime> found in config
Key insight: If a process has no config file AND is doing a capped legacy bind, the shim has nothing to direct it to v4.0. It will enumerate installed runtimes (capped to ≤v2.0), find nothing if 3.5 isn't installed, and fail. This is by design — v4 is intentionally invisible to these codepaths to keep v4 installation non-impactful.
3d. Version Resolution
Installed Runtime: vX.Y.Z. VERSION_ARCHITECTURE: N — what's installed on the machine
{exe} was built with version: vX.Y.Z — version from the binary's PE header (managed assemblies only; native EXEs won't have this)
Using supportedRuntime: vX.Y.Z — the shim picked a version from the config's <supportedRuntime> list
FindLatestVersion is returning the following version: vX.Y.Z ... V2.0 Capped: {0|1} — result of policy-based latest-version search
Default version of the runtime on the machine: vX.Y.Z or (null) — what the shim settled on; (null) means nothing was found
Decided on runtime: vX.Y.Z — final decision — this is the version that will be loaded
3e. Failure and FOD Path
If version resolution fails:
ERROR: Unable to find a version of the runtime to use — the shim found no suitable runtime
SEM_FAILCRITICALERRORS is set to {value} — checks the process error mode:
Value 0: Error dialogs and FOD are ALLOWED
Nonzero (any bit set, commonly 0x8001): Error dialogs and FOD are SUPPRESSED. The SEM_FAILCRITICALERRORS flag (0x0001) is inherited from the parent process.
Checking if feature-on-demand installation would help — the shim re-runs version computation to see if installing .NET 3.5 would resolve the request
Then either:
Launching feature-on-demand installation. CmdLine: "...\fondue.exe" /enable-feature:NetFx3 — FOD dialog shown
Could have launched feature-on-demand installation if was not opted out. — FOD suppressed because SEM_FAILCRITICALERRORS was set
3f. Multiple Activations in One Process
A single log can contain multiple activation sequences. Each begins with a new FunctionCall: or MethodCall: entry. A common pattern:
First activation via ClrCreateInstance / GetRequestedRuntime → succeeds (loads v4.0 via config)
Second activation via DllGetClassObject (COM) → legacy bind, capped → fails
This happens when a native EXE (like link.exe or mt.exe) loads the CLR successfully for its primary work, then a secondary COM activation request (e.g., for diasymreader) triggers a separate legacy resolution that can't find v2.0.
Step 4: Check System State (if needed)
When log analysis points to a registration or configuration issue, check:
Process error mode (why FOD did/didn't fire):
The SEM_FAILCRITICALERRORS flag is inherited from the parent process. If a build system or script sets it (or calls SetErrorMode), all child processes inherit it.
Step 5: Diagnose and Report
Produce a clear diagnosis covering:
What happened — which process(es) had activation issues and what the symptom was
Why it happened — trace through the specific decision path in the shim that led to the outcome
What controls the behavior — identify the specific inputs (config file presence, error mode, CLSID registration, capping state) that determined the outcome
What changed (if applicable) — if the user says behavior changed, identify which input could have changed (error mode from parent process, config file, CLSID registration, installed runtimes)
Common Scenarios
Unexpected FOD Dialogs
Pattern:DllGetClassObject → IsCapped: 1 → no config file → (null) → SEM_FAILCRITICALERRORS: 0 → FOD launched
Root cause: A native EXE is doing COM activation of a CLSID registered under mscoree.dll. This takes the legacy codepath, which is capped at v2.0. With no config file (and no useLegacyV2RuntimeActivationPolicy), v4 is invisible to this codepath. On a machine without .NET 3.5, there are no runtimes visible, and with SEM_FAILCRITICALERRORS not set, the FOD dialog fires.
Key question: Why did SEM_FAILCRITICALERRORS change? It's inherited from the parent. Different launch methods (script vs. direct invocation, different build systems) produce different error modes. The underlying capped-legacy-bind-on-v4-only-machine failure is always there — it's just that SEM_FAILCRITICALERRORS controls whether it manifests as a visible dialog or a silent failure.
Wrong Runtime Selected
Pattern:supportedRuntime entries in config list multiple versions; the shim picks the first one that's installed. If v2.0 is listed first and .NET 3.5 is installed, v2.0 wins even though v4.0 is also available.
Key insight: Config <supportedRuntime> entries are evaluated in order. First installed match wins.
Both v2 and v4 Loaded
Pattern: Multiple activation sequences in the same process log — one binds v4, another binds v2 (or vice versa). Side-by-side loading of CLR v2 and v4 in the same process IS supported but can cause issues with shared state.
Key insight: Look for separate Decided on runtime lines with different versions in the same log file.
Legacy Runtime Already Bound
Pattern: A legacy codepath succeeds early in the process (e.g., CorBindToRuntimeEx with an explicit v4 version, or config with useLegacyV2RuntimeActivationPolicy). This sets the legacy runtime to v4.0. All subsequent legacy activations — including capped COM activations that would otherwise fail — silently succeed by reusing the already-bound legacy runtime.
Key insight: The ORDER of activations within a process matters. If v4.0 is bound as the legacy runtime first, capped COM activations work. If the capped COM activation happens first (before any legacy runtime is bound), it fails. This means behavior can depend on which component activates first — a race condition in concurrent code can change the outcome.
Common Pitfalls
Pitfall
Correct Approach
Assuming IsCapped: 1 means v4.0 can never load
Capping only restricts roll-forward enumeration. v4.0 can still be loaded if: a specific version string is passed explicitly, config has useLegacyV2RuntimeActivationPolicy="true" with <supportedRuntime version="v4.0"/>, or the legacy runtime is already bound to v4+.
Thinking capping is broken or a bug
Capping is intentional — it makes v4 installation non-impactful. On a v4-only machine, legacy codepaths correctly see no runtimes. This is working as designed.
Assuming FOD is controlled per-process
SEM_FAILCRITICALERRORS is inherited from the parent process. A change in the parent (build system, script, shell) changes behavior for all children.
Looking only at the first activation in a log
A single log can contain multiple independent activation sequences. The problematic one is often a secondary COM activation, not the initial CLR load.
Assuming a missing config file is benign
For native EXEs doing COM activation with legacy/capped bind, the config file (with useLegacyV2RuntimeActivationPolicy) is the primary way to make legacy codepaths see v4.0. No config = capped = v4 invisible.
Adding <supportedRuntime> without useLegacyV2RuntimeActivationPolicy
Without useLegacyV2RuntimeActivationPolicy="true", rolling forward to v4 via config works for the primary EXE load, but legacy codepaths (COM activation, P/Invoke to mscoree.h APIs) remain capped at v2.0. Both are needed for legacy codepaths.
Setting useLegacyV2RuntimeActivationPolicy without understanding the trade-off
This attribute turns off in-proc SxS — it locks pre-v4 runtimes out of the process. This is usually fine for build tools but should be considered for apps that need to host both v2 and v4.
Validation
Before delivering a diagnosis, verify:
All log files with errors or FOD triggers were analyzed (not just the first one)
The entry point for each problematic activation was identified
The capping and legacy bind state was noted for each activation sequence
Config file presence/absence was checked
SEM_FAILCRITICALERRORS state was noted for FOD-related issues
Multiple activations within a single log were individually traced
The diagnosis explains the specific decision path, not just the outcome
How do I install clr-activation-debugging?
Download plugins/dotnet-diag/skills/clr-activation-debugging/SKILL.md from the publisher, or copy the path and open the repository. PlugMCP lists the skill and does not host the file.
Where does clr-activation-debugging live?
The publisher keeps it at https://github.com/dotnet/skills.
Which agents can use clr-activation-debugging?
Claude, Cursor, and Codex can read a SKILL.md you add from the publisher's repository.