codegen: emit guards for every anyOf variant to fix mypy union-attr on array-containing unions

[kush-replit]

Why

The encoder generator for non-discriminated anyOf unions emits a chain of ternary expressions, with the last variant historically rendered as the unguarded else branch. That works for simple unions like object | str | list (mypy can negative-narrow x to list in the final branch), but it breaks for deeper unions where the array variant is last, e.g.

str | float | bool | list[scalar] | None

When mypy fails to fully narrow x to list[...] through the prior isinstance checks (isinstance(x, (int, float)) plus bool subclassing int make this tricky), it complains that scalar items of the union have no __iter__ attribute:

error: Item "float" of "str | float | bool | None | list[...]"
    has no attribute "__iter__" (not iterable)  [union-attr]
error: Item "bool"  of ... has no attribute "__iter__"  [union-attr]
error: Item "object" of ... has no attribute "__iter__"  [union-attr]

This is the exact failure that has been blocking ai-infra's codegen-latest-pid2-schema.yml auto-update workflow since 2026-05-04, when repl-it-web#78355 widened agentToolPostgreSQL.executeSqlCommand.params from a flat scalar union to array<scalar | array<scalar>>. Every run since has failed on the regenerated executeSqlCommand.py at the for y in x iteration inside encode_ExecutesqlcommandInputParams. The committed pid2 client in ai-infra has been kept current by hand (see replit/ai-infra#12813), but the bot has been red for ~2.5 weeks.

What changed

src/replit_river/codegen/client.py: in the non-discriminated-anyOf branch of encode_type, emit an explicit isinstance / is None guard for every entry in encoder_parts — including the last one — and append a cast(Any, x) fallback. mypy no longer has to negative-narrow into the iterating branch, so deep unions with an array variant lint cleanly. Any and cast are already part of FILE_HEADER so no import bookkeeping changes.

Concretely, for the failing executeSqlCommand schema, the encoder now ends with:

return (
    x if isinstance(x, str)
    else x if isinstance(x, (int, float))
    else x if isinstance(x, bool)
    else None if x is None
    else [encode_..._AnyOf_4(y) for y in x]
        if isinstance(x, list)
        else cast(Any, x)
)

Test plan

• Existing tests/v1/codegen/snapshot/test_anyof_mixed.py snapshot updated to show the new if isinstance(x, list) else cast(Any, x) tail on its obj | str | list[str] encoder (the change is additive — the runtime behavior is unchanged).
• New snapshot test tests/v1/codegen/snapshot/test_anyof_array_in_union.py added with a schema that mirrors executeSqlCommand.params (array<scalar | array<scalar>>) and locks in the fixed output. This is the regression test for ai-infra's CI failure.
• uv run pytest is green (67 passed, including all v1 and v2 codegen tests).
• make lint is clean apart from a pre-existing pyright grpc import error in tests/v1/test_communication.py that also fails on main (unrelated).
• End-to-end verification against ai-infra: pointed ai-infra's ./pkgs/pid2_client/scripts/generate.sh at this branch via RIVER_CODEGEN_PATH=/tmp/opencode/river-python and reran the full lint pipeline that the auto-update workflow runs in CI; [mypy] completed in 15.19s and the script exited OK. instead of the historical union-attr failure.

Once this is released (e.g. v0.17.20) ai-infra can bump replit-river in pkgs/pid2_client/pyproject.toml and the auto-update workflow will start producing green PRs again.

~ written by Zerg 👾 (ascendant-goliath-6d2f)