When a config field is optional/nullable (or has multiple shapes), never rely on “default” or presence quirks. Make absence vs presence explicit, align schema rules with runtime parsing/normalization, and validate the structural inputs that runtime can actually interpret.

Apply these rules:

• Distinguish “key omitted” from “key present but empty/null”: if a branch should apply only when a field is explicitly empty, encode that with schema logic that won’t accidentally match omitted keys.
• Don’t assume JSON Schema default will populate runtime values: implement (and test) normalization during reconciliation/parsing so omitted fields resolve to the correct internal default, while explicit values like 0 are preserved.
• For object-typed variants, require the properties that runtime can’t resolve safely. If runtime parsing only succeeds inside a “value exists” gate, the schema should reject {}-like objects so you don’t fall through to unintended plaintext/empty literals.
• Keep “derived” fields out of the source-of-truth validation: if some fields are recomputed from anchors/other totals, enforce ordering/invariants at runtime where the derived value is computed.

Example (absence vs explicit empty mode):

• Avoid a plain const: "" if you want to reject cases where override_mode is omitted but other override fields are set.
• Instead, use explicit if/then/else/not logic that makes the empty-mode branch match only when the field is truly in the empty-state, not when it’s missing.

Example (defaults annotation vs runtime normalization):

• Treat schema default as documentation.
• Add reconciliation logic like:
  • if input == nil { resolved = 200 } else { resolved = input }
  • ensuring 0 stays 0 and omission becomes the intended default.