diff --git a/src/asset.ts b/src/asset.ts
index 65210168..f7e09d4e 100644
--- a/src/asset.ts
+++ b/src/asset.ts
@@ -192,10 +192,10 @@ export class Asset {
    *
    *  - `native`,
    *  - `credit_alphanum4`,
-   *  - `credit_alphanum12`, or
-   *  - `unknown` as the error case (which should never occur)
+   *  - `credit_alphanum12`
+   * @throws {Error} Throws `Error` if asset type is unsupported.
    */
-  getAssetType(): AssetType | "unknown" {
+  getAssetType(): AssetType {
     switch (this.getRawAssetType().value) {
       case xdr.AssetType.assetTypeNative().value:
         return AssetType.native;
@@ -204,7 +204,9 @@ export class Asset {
       case xdr.AssetType.assetTypeCreditAlphanum12().value:
         return AssetType.credit12;
       default:
-        return "unknown";
+        throw new Error(
+          "Supported asset types are: native, credit_alphanum4, credit_alphanum12",
+        );
     }
   }
 
diff --git a/src/fee_bump_transaction.ts b/src/fee_bump_transaction.ts
index 7432e24c..bd02da5f 100644
--- a/src/fee_bump_transaction.ts
+++ b/src/fee_bump_transaction.ts
@@ -15,8 +15,8 @@ import { encodeMuxedAccountToAddress } from "./util/decode_encode_muxed_account.
  * submitting to the network or forwarding on to additional signers.
  */
 export class FeeBumpTransaction extends TransactionBase<xdr.FeeBumpTransaction> {
-  _feeSource: string;
-  _innerTransaction: Transaction;
+  private _feeSource: string;
+  private _innerTransaction: Transaction;
 
   /**
    * @param envelope - transaction envelope object or base64 encoded string.
diff --git a/src/numbers/index.ts b/src/numbers/index.ts
index 01f4c730..23fbca25 100644
--- a/src/numbers/index.ts
+++ b/src/numbers/index.ts
@@ -44,15 +44,21 @@ export function scValToBigInt(scv: xdr.ScVal): bigint {
     case "scvI64":
     case "scvTimepoint":
     case "scvDuration":
+      if (scIntType === undefined) {
+        throw TypeError(`invalid integer type for ${switchName}`);
+      }
       return new XdrLargeInt(
-        scIntType as ScIntType,
+        scIntType,
         value as xdr.Int64 | xdr.Uint64,
       ).toBigInt();
 
     case "scvU128":
     case "scvI128": {
+      if (scIntType === undefined) {
+        throw TypeError(`invalid integer type for ${switchName}`);
+      }
       const int128Value = value as xdr.Int128Parts | xdr.UInt128Parts;
-      return new XdrLargeInt(scIntType as ScIntType, [
+      return new XdrLargeInt(scIntType, [
         int128Value.lo(),
         int128Value.hi(),
       ]).toBigInt();
@@ -60,8 +66,11 @@ export function scValToBigInt(scv: xdr.ScVal): bigint {
 
     case "scvU256":
     case "scvI256": {
+      if (scIntType === undefined) {
+        throw TypeError(`invalid integer type for ${switchName}`);
+      }
       const int256Value = value as xdr.Int256Parts | xdr.UInt256Parts;
-      return new XdrLargeInt(scIntType as ScIntType, [
+      return new XdrLargeInt(scIntType, [
         int256Value.loLo(),
         int256Value.loHi(),
         int256Value.hiLo(),
diff --git a/src/numbers/xdr_large_int.ts b/src/numbers/xdr_large_int.ts
index c068cc0f..80d57bc8 100644
--- a/src/numbers/xdr_large_int.ts
+++ b/src/numbers/xdr_large_int.ts
@@ -34,7 +34,7 @@ export type ScIntType =
  */
 export class XdrLargeInt {
   int: LargeInt;
-  type: string;
+  type: ScIntType;
 
   /**
    * @param type - specifies a data type to use to represent the integer, one
@@ -257,7 +257,10 @@ export class XdrLargeInt {
       case "duration":
         return this.toDuration();
       default:
-        throw TypeError(`invalid type: ${this.type}`);
+        // This should be unreachable if the compiler enforces valid types
+        // This serves as a runtime check if the type is somehow invalid
+        // (e.g. from user input or a future extension)
+        throw TypeError(`invalid type: ${this.type as string}`);
     }
   }
 
@@ -286,7 +289,7 @@ export class XdrLargeInt {
   }
 
   /** Returns true if the given string is a valid XDR large integer type name. */
-  static isType(type: string): boolean {
+  static isType(type: string): type is ScIntType {
     switch (type) {
       case "i64":
       case "i128":
@@ -307,8 +310,15 @@ export class XdrLargeInt {
    * to a type description for {@link XdrLargeInt} construction (e.g. 'i128')
    *
    * @param scvType - the `xdr.ScValType` as a string
+   * @returns the corresponding {@link ScIntType} if it's an integer type, or
+   *    `undefined` if it's not an integer type
    */
-  static getType(scvType: string): string {
-    return scvType.slice(3).toLowerCase();
+  static getType(scvType: string): ScIntType | undefined {
+    const type = scvType.slice(3).toLowerCase();
+    if (this.isType(type)) {
+      return type;
+    }
+
+    return undefined;
   }
 }
diff --git a/src/operations/set_options.ts b/src/operations/set_options.ts
index 1b9fd8f1..32fb75e1 100644
--- a/src/operations/set_options.ts
+++ b/src/operations/set_options.ts
@@ -165,11 +165,16 @@ export function setOptions(
 
     if (setValues !== 1) {
       throw new Error(
-        "Signer object must contain exactly one of signer.ed25519PublicKey, signer.sha256Hash, signer.preAuthTx.",
+        "Signer object must contain exactly one of signer.ed25519PublicKey, signer.sha256Hash, signer.preAuthTx, or signer.ed25519SignedPayload.",
       );
     }
-
-    signer = new xdr.Signer({ key: key!, weight: weight! });
+    if (weight === undefined) {
+      throw new Error("signer weight is required.");
+    }
+    if (key === undefined) {
+      throw new Error("signer key is required.");
+    }
+    signer = new xdr.Signer({ key: key, weight: weight });
   }
 
   const setOptionsOp = new xdr.SetOptionsOp({
diff --git a/src/operations/types.ts b/src/operations/types.ts
index f5358a2f..32a07267 100644
--- a/src/operations/types.ts
+++ b/src/operations/types.ts
@@ -73,7 +73,7 @@ export interface LiquidityPoolWithdrawOpts {
 export interface AllowTrustOpts {
   trustor: string;
   assetCode: string;
-  authorize?: boolean | number;
+  authorize?: TrustLineFlag | boolean;
   source?: string;
 }
 
@@ -105,8 +105,8 @@ export interface SignerOpts {
 
 export interface SetOptionsOpts {
   inflationDest?: string;
-  clearFlags?: number | string;
-  setFlags?: number | string;
+  clearFlags?: AuthFlags;
+  setFlags?: AuthFlags;
   masterWeight?: number | string;
   lowThreshold?: number | string;
   medThreshold?: number | string;
@@ -411,20 +411,18 @@ export type OperationType =
 
 // Literal types matching the AuthRequiredFlag/AuthRevocableFlag/AuthImmutableFlag/AuthClawbackEnabledFlag
 // constants exported from src/operation.ts.
-// TODO: Once src/index.js is migrated to src/index.ts, replace these literals with
-// `typeof AuthRequiredFlag` etc. to avoid duplication with the runtime constants.
-export namespace AuthFlag {
-  export type required = 1;
-  export type revocable = 2;
-  export type immutable = 4;
-  export type clawbackEnabled = 8;
-}
-export type AuthFlag =
-  | AuthFlag.clawbackEnabled
-  | AuthFlag.immutable
-  | AuthFlag.required
-  | AuthFlag.revocable;
 
+export const AuthFlag = {
+  required: 1,
+  revocable: 2,
+  immutable: 4,
+  clawbackEnabled: 8,
+} as const;
+export type AuthFlag = (typeof AuthFlag)[keyof typeof AuthFlag];
+/**
+ * A single {@link AuthFlag} or multiple flags combined with `|` (e.g. `AuthRequiredFlag | AuthRevocableFlag`).
+ */
+export type AuthFlags = AuthFlag | (number & {});
 export namespace TrustLineFlag {
   export type deauthorize = 0;
   export type authorize = 1;
@@ -525,8 +523,8 @@ export interface SetOptionsResult<
   // hold raw uint32 bitmasks, so combined values (e.g. AuthRequired | AuthRevocable = 3)
   // are valid but not expressible as AuthFlag. Use bitwise AND to test individual flags:
   //   if (result.clearFlags & AuthRequiredFlag) { ... }
-  clearFlags?: AuthFlag;
-  setFlags?: AuthFlag;
+  clearFlags?: AuthFlags;
+  setFlags?: AuthFlags;
   masterWeight?: number;
   lowThreshold?: number;
   medThreshold?: number;
diff --git a/src/scval.ts b/src/scval.ts
index 7f703ec7..e3df1ef9 100644
--- a/src/scval.ts
+++ b/src/scval.ts
@@ -299,7 +299,7 @@ export function nativeToScVal(
 
         default:
           if (XdrLargeInt.isType(optType)) {
-            return new XdrLargeInt(optType as ScIntType, val).toScVal();
+            return new XdrLargeInt(optType, val).toScVal();
           }
 
           throw new TypeError(
diff --git a/src/transaction.ts b/src/transaction.ts
index b80099dc..ea3fc933 100644
--- a/src/transaction.ts
+++ b/src/transaction.ts
@@ -25,20 +25,20 @@ import { OperationRecord } from "./operations/types.js";
 export class Transaction extends TransactionBase<
   xdr.Transaction | xdr.TransactionV0
 > {
-  _envelopeType: xdr.EnvelopeType;
-  _source: string = "";
-  _memo: xdr.Memo;
-  _sequence: string;
-  _operations: OperationRecord[];
-  _timeBounds?: { minTime: string; maxTime: string };
-  _ledgerBounds?: { minLedger: number; maxLedger: number };
-  _minAccountSequence?: string;
+  private _envelopeType: xdr.EnvelopeType;
+  private _source: string = "";
+  private _memo: xdr.Memo;
+  private _sequence: string;
+  private _operations: OperationRecord[];
+  private _timeBounds?: { minTime: string; maxTime: string };
+  private _ledgerBounds?: { minLedger: number; maxLedger: number };
+  private _minAccountSequence?: string;
   // TODO: types/index.d.ts declares this as `number`, but the XDR type is
   // Duration = Uint64 (64-bit), which can exceed Number.MAX_SAFE_INTEGER.
   // This is a breaking type change that should be surfaced when types/index.d.ts is updated.
-  _minAccountSequenceAge?: xdr.Uint64;
-  _minAccountSequenceLedgerGap?: number;
-  _extraSigners?: xdr.SignerKey[];
+  private _minAccountSequenceAge?: xdr.Uint64;
+  private _minAccountSequenceLedgerGap?: number;
+  private _extraSigners?: xdr.SignerKey[];
 
   /**
    * @param envelope - transaction envelope object or base64 encoded string
diff --git a/src/transaction_base.ts b/src/transaction_base.ts
index 380d5dbd..abaea4d3 100644
--- a/src/transaction_base.ts
+++ b/src/transaction_base.ts
@@ -8,10 +8,10 @@ import { Keypair } from "./keypair.js";
 export class TransactionBase<
   TTx extends xdr.FeeBumpTransaction | xdr.Transaction | xdr.TransactionV0,
 > {
-  _tx: TTx;
-  _signatures: xdr.DecoratedSignature[];
-  _fee: string;
-  _networkPassphrase: string;
+  private _tx: TTx;
+  private _signatures: xdr.DecoratedSignature[];
+  private _fee: string;
+  private _networkPassphrase: string;
 
   constructor(
     tx: TTx,
diff --git a/src/transaction_builder.ts b/src/transaction_builder.ts
index ea3d4c57..d751866b 100644
--- a/src/transaction_builder.ts
+++ b/src/transaction_builder.ts
@@ -275,7 +275,7 @@ export class TransactionBuilder {
 
     const builder = new TransactionBuilder(source, builderOpts);
 
-    tx._tx.operations().forEach((op) => builder.addOperation(op));
+    tx.tx.operations().forEach((op) => builder.addOperation(op));
 
     return builder;
   }