How do you troubleshoot composition errors when publishing a new subgraph schema?

Composition errors are usually specific and actionable if you read them closely rather than guessing.

1. Read the exact error message — Rover/GraphOS typically names the specific field or type and the reason, such as a field having a mismatched type across two subgraphs.
2. If the conflict is a field intentionally defined in more than one subgraph, check whether it needs to be marked @shareable.
3. Check @key consistency — the key fields for a given entity need to be spelled and typed identically everywhere that entity is extended.
4. Verify that any @requires or @provides field selections actually exist, spelled correctly, in the subgraph they reference.
5. Reproduce locally with rover supergraph compose against a static supergraph config file before pushing again.
6. Run rover subgraph check to preview the composition impact against the currently published supergraph before publishing for real.

Most composition errors trace back to one of two root causes: an unmarked duplicate field definition, or inconsistent @key fields between subgraphs contributing to the same entity.