# Diagnostics and failed builds

Run `diplodocus check` to validate configuration, declared sources, authored syntax, and links without executing cells or publishing output. Errors cause a nonzero exit status. Warnings identify information that Diplodocus retained but could not fully interpret; a command can succeed with warnings. 

Each diagnostic includes a stable code, severity, message, and, when known, a repository-relative source path and byte range. For example, `unresolved-document-reference` points to a missing local page, asset, or anchor. `unsupported-authored-syntax` marks syntax outside the selected GFM or QMD profile. `python-dynamic-export` means a computed Python export list cannot establish a certain public surface. A warning about Rd source attribution means the parsed documentation has file-level, rather than exact byte-level, location evidence. 

When a build fails, fix the first source error and rerun `check`. If the error only appears during `build` or `serve`, inspect the execution code and kernel configuration in the reported collection. `serve` keeps the last successful site available after a failed rebuild. Snapshot or output publication failures also preserve the previous completed artifact. 

See [authored pages](<authoring>) for supported syntax, [extraction](<extraction>) for static API limits, and [execution](<execution>) for kernel failures and permissions.