diff --git a/scripts/api-sync/__tests__/apply.test.ts b/scripts/api-sync/__tests__/apply.test.ts
index a1f6237..59302e6 100644
--- a/scripts/api-sync/__tests__/apply.test.ts
+++ b/scripts/api-sync/__tests__/apply.test.ts
@@ -35,6 +35,27 @@ describe('applyResourcesField', () => {
     expect(patched).toContain('externalId?: string')
     expect(patched).toContain('if (options.externalId !== undefined) body.external_id = options.externalId')
   })
+
+  // Regression: createQuote's body starts as a plain object literal
+  // (`const body = {...}`), not `Record<string, any>`. Assigning a new key
+  // to it after the fact doesn't typecheck (TS2339), which is exactly what
+  // broke CI when this generator added refund_wallet_address to it.
+  test('widens an untyped "const body = {" to Record<string, any> so the new field assignment typechecks', () => {
+    const change: ApplicableChange = { resource: 'quotes', op: 'create', fn: 'createQuote', field: 'refund_wallet_address', type: 'string', required: false }
+    const patched = applyResourcesField(real('src/commands/resources.ts'), change)
+    const fnStart = patched.indexOf('export async function createQuote(')
+    const fnRegion = patched.slice(fnStart, patched.indexOf('export async function', fnStart + 1))
+    expect(fnRegion).toContain('const body: Record<string, any> = {')
+    expect(fnRegion).toContain('if (options.refundWalletAddress !== undefined) body.refund_wallet_address = options.refundWalletAddress')
+  })
+
+  test('leaves an already-typed "const body: Record<string, any> =" untouched', () => {
+    const change: ApplicableChange = { resource: 'bank_accounts', op: 'create', fn: 'createBankAccount', field: 'clabe', type: 'string', required: false }
+    const patched = applyResourcesField(real('src/commands/resources.ts'), change)
+    const fnStart = patched.indexOf('export async function createBankAccount(')
+    const fnRegion = patched.slice(fnStart, patched.indexOf('export async function', fnStart + 1))
+    expect(fnRegion).not.toContain('Record<string, any> = Record<string, any>')
+  })
 })
 
 describe('applyIndexOption', () => {
diff --git a/scripts/api-sync/apply.ts b/scripts/api-sync/apply.ts
index 8ab09c4..b4d4b56 100644
--- a/scripts/api-sync/apply.ts
+++ b/scripts/api-sync/apply.ts
@@ -76,6 +76,15 @@ export function applyResourcesField(src: string, change: ApplicableChange): stri
   const requestIndent = leadingWhitespace(lines[requestIdx])
   lines.splice(requestIdx, 0, `${requestIndent}if (options.${fieldCamel} !== undefined) body.${change.field} = options.${fieldCamel}`)
 
+  // The pass-through line above assigns a key that isn't in body's initial
+  // literal, so body needs a permissive type or TS rejects the assignment
+  // (an untyped `const body = {...}` infers a closed object type). Widen it
+  // the same way hand-written conditional-field functions already do.
+  const bodyDeclRe = /^(\s*)const body = /
+  const bodyDeclIdx = lines.findIndex(l => bodyDeclRe.test(l))
+  if (bodyDeclIdx !== -1)
+    lines[bodyDeclIdx] = lines[bodyDeclIdx].replace('const body = ', 'const body: Record<string, any> = ')
+
   const patchedFn = lines.join('\n')
   return src.slice(0, declIdx) + patchedFn + src.slice(fnEnd)
 }