fix(session): omit undefined optional fields (#24676)

packages/opencode/src/util/schema.ts

@@ -1,4 +1,5 @@
-import { Schema } from "effect"
+import { Option, Schema, SchemaGetter } from "effect"
+import { zod, ZodOverride } from "./effect-zod"
 
 /**
  * Integer greater than zero.
@@ -10,6 +11,19 @@ export const PositiveInt = Schema.Int.check(Schema.isGreaterThan(0))
  */
 export const NonNegativeInt = Schema.Int.check(Schema.isGreaterThanOrEqualTo(0))
 
+/**
+ * Optional public JSON field that accepts explicit `undefined` internally but
+ * encodes it as an omitted key, matching `JSON.stringify` legacy responses.
+ */
+export const optionalOmitUndefined = <S extends Schema.Top>(schema: S) =>
+  Schema.optionalKey(schema).pipe(
+    Schema.decodeTo(Schema.optional(schema), {
+      decode: SchemaGetter.passthrough({ strict: false }),
+      encode: SchemaGetter.transformOptional(Option.filter((value) => value !== undefined)),
+    }),
+    Schema.annotate({ [ZodOverride]: zod(schema).optional() }),
+  )
+
 /**
  * Strip `readonly` from a nested type. Stand-in for `effect`'s `Types.DeepMutable`
  * until `effect:core/x228my` ("Types.DeepMutable widens unknown to `{}`") lands.