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, use troubleshoot-binlog-mcp to diagnose startup when repair is wanted. If repair is blocked, declined, or unnecessary for this task, continue with 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

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

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)

dotnet build /bl:build.binlog