Binlog Failure Analysis (.NET)

---
name: binlog-failure-analysis
description: "Analyze MSBuild binary logs to diagnose build failures. USE FOR: build errors that are unclear from console output, diagnosing cascading failures across multi-project builds, tracing MSBuild target execution order, and generally any MSBuild build issues. Requires an existing .binlog file. DO NOT USE FOR: generating binlogs (use binlog-generation), non-MSBuild build systems."
license: MIT
metadata:
  origin: .NET
---

# Analyzing MSBuild Failures with Binary Logs

This skill diagnoses MSBuild build failures from a `.binlog` file. The preferred
path uses the **binlog MCP server** (`Microsoft.AITools.BinlogMcp`, exposed under the
`binlog` MCP namespace) which is bundled with this plugin. If the MCP server is
not available, fall back to the **binlog replay** workflow at the bottom.

## Primary workflow — binlog MCP

The MCP server exposes structured tools for inspecting a `.binlog` without
parsing text logs. Call them directly instead of replaying the binlog to a text
file. Call `tools/list` for the MCP first if you are unsure which tools are available.

**Important constraints:**
- The `.binlog` file is a **binary format** — do NOT try to `cat`, `head`, `strings`, or read it directly. Use only the MCP tools to query it.
- The **original source/project files might or might NOT be available on disk**. Project files (.csproj, .props, .targets, App.config, etc.) - if you cannot locate them on disk, they can only be read from within the binlog via MCP tools (e.g., embedded/source file retrieval).
- **Synthesize findings as you go.** Do not spend all available time investigating — once you have enough evidence, present your conclusions. A partial answer with clear reasoning is better than timing out mid-investigation.

Use the available MCP server tools to query the binary log for:
- Build errors and warnings
- MSBuild properties and their values
- MSBuild items (PackageReference, ProjectReference, etc.)
- Project evaluation data
- Target execution details
- File contents embedded in the binlog

## Fallback workflow — text-log replay (when MCP is unavailable)

Use this only when the MCP server cannot be started (for example, on an older
SDK or in an offline environment).

### Replay the binlog to text logs

```bash
dotnet msbuild build.binlog -noconlog \
  -fl  "-flp:v=diag;logfile=full.log;performancesummary" \
  -fl1 "-flp1:errorsonly;logfile=errors.log" \
  -fl2 "-flp2:warningsonly;logfile=warnings.log"
```

> **PowerShell note:** Use `-flp:"v=diag;logfile=full.log;performancesummary"`
> (quoted semicolons).

### Search the text logs

```bash
cat errors.log
grep -n -B2 -A2 "CS0246" full.log
grep -i "CoreCompile.*FAILED\|Build FAILED\|error MSB" full.log
grep 'Target "CoreCompile"' full.log | grep -oP 'project "[^"]*"'
```

## Generating a binlog (only if none exists)

```bash
dotnet build /bl:build.binlog
```