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Thanks for the careful read. The "what breaks if I rename this column" question is exactly what column lineage from the compiler is meant to answer, and you said it better than I did in the post.
On the schema-grounded AI angle: agreed. The failure mode you describe — structurally valid SQL that joins on the wrong key or aggregates at the wrong grain because the model hallucinated a relationship — is exactly what the compiler is positioned to catch. AI-generated SQL runs through the type checker before it can land, so suggestions that don't validate against the actual DAG never reach the user. NL-to-SQL tools that integrate a compile step would close exactly the gap you're pointing at.
On your two questions:
1. Branch isolation for stateful models — mixed answer, and worth being honest about:
- Incremental: isolated. The watermark `state_key` includes the resolved schema, and `rocky branch create` swaps the schema prefix. So a branch run reads/writes a different redb key than main and they don't advance each other.
- Snapshot: not yet. Today `rocky branch create` only writes a branch record; it doesn't copy warehouse tables. A snapshot model on a branch starts with an empty table (CREATE TABLE IF NOT EXISTS in the branch schema) and accumulates from the first branch run, with no inherited history from main. That's the gap. The fix is the next wave: native Delta SHALLOW CLONE / Snowflake zero-copy at branch creation, which gives point-in-time snapshot semantics without copy-on-write overhead.
To your Snowflake point: the warehouse-size × duration credit model and the scan volume tell genuinely different stories, so they're tracked separately rather than rolled into a single number.