How to Fix MSBuild ‘<’ Is an Invalid Start of a Value Error

How Did a Bizarre MSBuild Error Disrupt Our Enterprise SaaS System?

During a recent project involving a large-scale enterprise SaaS platform, our team was tasked with modernizing the backend authentication infrastructure. The application architecture relied on a complex set of .NET APIs and required integration with modern identity providers. While upgrading our core security libraries to enhance role-based access control, we encountered a baffling development environment issue that severely impacted engineering velocity.

One of our engineers suddenly could not build the solution. Visual Studio presented a cryptic error originating from Microsoft.WebTools.Aspire.targets: '<' is an invalid start of a value. LineNumber: 2 | BytePositionInLine: 0. To make matters worse, the local IDE environment degraded significantly. Standard syntax highlighting vanished and hovering over core C# types like IEnumerable yielded no IntelliSense descriptions.

Interestingly, this issue only affected two specific projects within a massive microservices solution. Furthermore, the application would still run if forced, but the local build step reported a hard failure. When companies look to hire dotnet developers for enterprise modernization, they expect teams to navigate exactly these types of opaque toolchain failures. This challenge inspired this article so other engineering teams can understand the deep interplay between NuGet dependencies and MSBuild build targets.

Why Did This Microsoft.WebTools.Aspire.targets Error Surface in Our Architecture?

The business use case for this phase of the project was strict enterprise compliance. We were migrating older authentication mechanisms to use the latest standards, which involved integrating modern identity SDKs. The specific projects affected were our web entry points—the API gateways that brokered requests between the front-end clients and our internal microservices.

In the .NET ecosystem, MSBuild relies on .targets files to execute build steps, such as code generation, dependency resolution and asset bundling. The error was pointing to an internal Visual Studio build task responsible for aggregating JSON schema files for application settings.

Local build consistency is a fundamental requirement for any high-performing engineering team. When one developer out of three is blocked by an environment-specific compiler error, it risks introducing “it works on my machine” syndrome, which can eventually bleed into continuous integration (CI) pipelines if left unchecked.

What Caused the Build Process to Fail with an XML Parsing Error?

When investigating the symptoms, the primary clue was the specific error message: '<' is an invalid start of a value. In software engineering, this signature almost universally indicates that a JSON parser is attempting to parse an XML or HTML document. The JSON parser expects a curly brace { or a square bracket [, but instead encounters the angle bracket < of an XML tag or an HTML declaration.

We examined the exact XML block highlighted by the build output:

<GenerateCombinedAppSettingsSchema
  AppSettingJsonSchemaFilePath="$(AppSettingJsonSchemaLoadFilePath)"
  ComponentJsonSchemaCombinedFilePath="$(JsonSchemaCombinedFilePath)"
  AppSettingsJsonSchemaCombinedFilePath="$(AppSettingsJsonSchemaCombinedFilePath)">
</GenerateCombinedAppSettingsSchema>

We validated the target file and it was perfectly valid MSBuild XML syntax. We knew the file itself wasn’t corrupted. The issue was that the custom build task Microsoft.WebTools.Aspire.MSBuild.GenerateCombinedAppSettingsSchema was failing internally. It was attempting to read a JSON schema from the intermediate obj/ directory, but something was injecting an XML response (or a default XML error page) into the file it was reading, causing the underlying System.Text.Json parser to crash.

The sudden degradation of IDE features (like the failure to recognize IEnumerable) indicated that the language server (OmniSharp/Roslyn) was failing to complete the design-time build. Design-time builds run in the background to provide IntelliSense and if a target fails, the language server loses its semantic understanding of the project.

How Did We Diagnose and Resolve the MSBuild JSON Conflict?

When you hire software developer teams with deep architectural experience, you expect a structured diagnostic approach rather than random trial and error. We mapped out several potential resolution paths.

Approach 1: Environment Rebuild and Cache Invalidation

Initially, the affected developer reformatted their workstation and reinstalled the operating system and IDE. This temporarily resolved the issue for about a week. This behavior confirmed that the issue was tied to dynamically generated files cached in the local user profile or intermediate project directories (like obj/ or bin/), which were eventually regenerating and causing the failure again.

Approach 2: XML and Network Validation

Since a JSON parser was reading XML, we considered whether a network call fetching a remote JSON schema was failing and returning a 403 or 404 HTML/XML error page. We verified network traffic during the build process, but all local schema generation was happening offline via local file paths, eliminating remote network failures.

Approach 3: Dependency Isolation and Version Pinning

By comparing commit histories from when the environment was stable to when it failed, we isolated the trigger: upgrading the authentication library (specifically Azure.Identity to version 1.20.0 or higher). This NuGet package update introduced a newer, transient version of System.Text.Json and Azure.Core. Because MSBuild tasks load assemblies into a shared application domain during the build process, the newer transient dependencies from our project were overriding or conflicting with the assemblies expected by Visual Studio’s internal Microsoft.WebTools.Aspire.MSBuild.dll.

What Was the Final Implementation to Fix the Build Dependency Error?

The root cause was an MSBuild assembly resolution conflict. The GenerateCombinedAppSettingsSchema build task relies on specific versions of JSON parsing libraries distributed with Visual Studio. When our web project referenced a newer identity package, MSBuild loaded incompatible DLLs into the build context, causing the schema generation task to silently fail and output corrupted XML into the intermediate JSON schema files. Upon the next build, the JSON parser read that cached XML and crashed.

Our final implementation involved decoupling the build-time requirements from our runtime dependencies:

• Dependency Encapsulation: We moved the Azure.Identity reference out of the primary web entry point and into a dedicated, heavily encapsulated class library project (e.g., Infrastructure.Security). We then ensured that the specific dependencies did not flow upward as build assets.
• Modifying Package References: We adjusted the MSBuild properties of the offending package reference in the project file to ensure its assets did not interfere with the build tooling. We utilized the ExcludeAssets="build" metadata to prevent the transient build targets of the package from executing during the parent project’s design-time build.
• Cache Purging Strategy: We implemented a pre-build script that aggressively cleans the $(IntermediateOutputPath) (typically the obj/ folder) specifically for the AppSettingsSchema.json if the design-time build is detected. This prevents corrupted XML from persisting across local IDE restarts.

Once implemented, we validated the fix by clearing all caches, updating to the latest security packages and running a complete rebuild. IntelliSense returned immediately and the design-time build completed without throwing the JSON parsing exception.

What Can Architectural Teams Learn From This Dependency Clash?

When organizations scale their platforms, toolchain stability is just as critical as application logic. Here are actionable insights technical leaders should apply:

• Trust the Semantic Signature of Errors: An error stating '<' is an invalid start of a value almost always means an unexpected XML/HTML document was fed into a JSON parser. Follow the data flow of the parser, not just the file where the error is reported.
• Understand Design-Time vs. Compile-Time Builds: Visual Studio runs background MSBuild tasks continuously to provide IntelliSense. If standard types like IEnumerable lose their color coding, it is a guaranteed sign that a background target has crashed.
• Beware of MSBuild Assembly Conflicts: Custom build tasks run in the same process space. Upgrading a NuGet package that shares dependencies with MSBuild (like JSON parsers or Core libraries) can destabilize the IDE.
• Isolate Infrastructure Concerns: Keep heavy dependencies like authentication, cloud identity and telemetry encapsulated in separate class libraries. Do not pollute your web entry points with overly broad NuGet packages.
• Purge Intermediate Caches: When dealing with code generation or schema merging tasks, a corrupt output file in the obj/ directory will cause cascading failures. Always test build fixes on a completely clean repository.

How Does Resolving This Inform Future Architecture Enhancements?

By identifying that an identity library update was conflicting with the internal JSON schema generation of the IDE’s build tools, our team stabilized the local development environments without compromising on our modernization goals. Engineering maturity is about tracing obscure symptoms—like failing syntax highlights—down to their architectural root causes, ensuring that the entire team can maintain high velocity.

If your organization is planning complex migrations and needs to avoid costly toolchain disruptions, contact us to learn how you can hire dedicated engineering teams capable of navigating deep enterprise integrations.

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