From b207c869ee35ddf5c9b4f85ae3707e899dc16712 Mon Sep 17 00:00:00 2001
From: Kyle Cox <kshot9000@gmail.com>
Date: Fri, 9 Oct 2026 06:40:56 +0000
Subject: [PATCH] fix(common): don't mutate the caller's options in
 decimalize()/undecimalize()

---
 .changeset/tidy-bigints-share.md         |  5 ++++
 packages/common/src/utils/bigInt.spec.ts | 27 ++++++++++++++++++++
 packages/common/src/utils/bigInt.ts      | 32 ++++++++++++------------
 3 files changed, 48 insertions(+), 16 deletions(-)
 create mode 100644 .changeset/tidy-bigints-share.md

diff --git a/.changeset/tidy-bigints-share.md b/.changeset/tidy-bigints-share.md
new file mode 100644
index 00000000..aa5484de
--- /dev/null
+++ b/.changeset/tidy-bigints-share.md
@@ -0,0 +1,5 @@
+---
+"@fleet-sdk/common": patch
+---
+
+Fix `decimalize()` and `undecimalize()` mutating the caller's options object: the resolved `decimals` and `decimalMark` defaults were assigned back into the passed object, so a caller's options came back with fields it never set (and a frozen options object threw a `TypeError`). The defaults are now resolved into local variables, leaving the caller's object untouched; outputs are unchanged.
diff --git a/packages/common/src/utils/bigInt.spec.ts b/packages/common/src/utils/bigInt.spec.ts
index 8f9d4fa0..b911dd6d 100644
--- a/packages/common/src/utils/bigInt.spec.ts
+++ b/packages/common/src/utils/bigInt.spec.ts
@@ -78,6 +78,17 @@ describe("decimalize()", () => {
     const options = { decimals: 9 };
     expect(decimalize(undecimalize("129.8379183", options), options)).toBe("129.8379183");
   });
+
+  it("Should not mutate the caller's options object", () => {
+    const options = { decimals: 2 };
+    expect(decimalize(1763874n, options)).toBe("17638.74");
+    expect(options).toEqual({ decimals: 2 });
+  });
+
+  it("Should accept a frozen options object", () => {
+    const options = Object.freeze({ decimals: 2 });
+    expect(decimalize(1763874n, options)).toBe("17638.74");
+  });
 });
 
 describe("undecimalize()", () => {
@@ -115,6 +126,22 @@ describe("undecimalize()", () => {
       undecimalize("12.23.1");
     }).toThrow();
   });
+
+  it("Should not mutate the caller's options object", () => {
+    const options = { decimals: 2 };
+    expect(undecimalize("1.23", options)).toBe(123n);
+    expect(options).toEqual({ decimals: 2 });
+  });
+
+  it("Should accept a frozen options object", () => {
+    const options = Object.freeze({ decimals: 2 });
+    expect(undecimalize("1.23", options)).toBe(123n);
+  });
+
+  it("Should accept frozen options relying on defaults", () => {
+    const options = Object.freeze({});
+    expect(undecimalize("1.23", options)).toBe(123n);
+  });
 });
 
 describe("ensureBigInt()", () => {
diff --git a/packages/common/src/utils/bigInt.ts b/packages/common/src/utils/bigInt.ts
index ee352ef0..fd3fb277 100644
--- a/packages/common/src/utils/bigInt.ts
+++ b/packages/common/src/utils/bigInt.ts
@@ -47,15 +47,11 @@ type ParsingOptions = {
 export function undecimalize(input: string, options?: ParsingOptions | number): bigint {
   if (!input) return _0n;
 
-  options = typeof options === "number" ? { decimals: options } : options;
-  if (isUndefined(options)) {
-    options = {};
-  }
-
-  options.decimals = options.decimals || 0;
-  options.decimalMark = options.decimalMark || ".";
+  const parsedOptions = typeof options === "number" ? { decimals: options } : options;
+  const decimals = parsedOptions?.decimals || 0;
+  const decimalMark = parsedOptions?.decimalMark || ".";
 
-  const fragments = input.split(options.decimalMark);
+  const fragments = input.split(decimalMark);
   if (fragments.length > 2) {
     throw new Error("Invalid numeric string.");
   }
@@ -65,9 +61,9 @@ export function undecimalize(input: string, options?: ParsingOptions | number):
   const negative = integer.startsWith("-") ? "-" : "";
 
   if (!decimal) {
-    decimal = "0".repeat(options.decimals);
-  } else if (decimal.length < options.decimals) {
-    decimal = decimal.padEnd(options.decimals, "0");
+    decimal = "0".repeat(decimals);
+  } else if (decimal.length < decimals) {
+    decimal = decimal.padEnd(decimals, "0");
   }
 
   return BigInt(negative + _stripNonDigits(integer + decimal));
@@ -114,15 +110,19 @@ export function decimalize(value: Amount, options?: FormattingOptions | number):
     return value.toString();
   }
 
-  options = typeof options === "number" ? { decimals: options } : options;
-  options.decimals = options.decimals || 0;
-  options.decimalMark = options.decimalMark || ".";
+  const parsedOptions = typeof options === "number" ? { decimals: options } : options;
+  const decimals = parsedOptions.decimals || 0;
+  const decimalMark = parsedOptions.decimalMark || ".";
 
-  const pow = _10n ** BigInt(options.decimals);
+  const pow = _10n ** BigInt(decimals);
   const integer = value / pow;
   const decimal = value - integer * pow;
 
-  return buildFormattedDecimal(integer.toString(10), decimal.toString(10), options);
+  return buildFormattedDecimal(integer.toString(10), decimal.toString(10), {
+    ...parsedOptions,
+    decimals,
+    decimalMark
+  });
 }
 
 /**