diff --git a/src/abi/calldata/string.ts b/src/abi/calldata/string.ts
index b87410c..6856003 100644
--- a/src/abi/calldata/string.ts
+++ b/src/abi/calldata/string.ts
@@ -7,7 +7,12 @@ function reportError(msg: string, data: CalldataEncodable): never {
 
 function toStringImplMap(data: Iterable<[string, CalldataEncodable]>, to: string[]) {
     to.push("{");
+    let first = true;
     for (const [k, v] of data) {
+      if (!first) {
+        to.push(",");
+      }
+      first = false;
       to.push(JSON.stringify(k));
       to.push(":");
       toStringImpl(v, to);
@@ -48,13 +53,17 @@ function toStringImplMap(data: Iterable<[string, CalldataEncodable]>, to: string
         if (data instanceof Uint8Array) {
           to.push("b#");
           for (const b of data) {
-            to.push(b.toString(16));
+            to.push(b.toString(16).padStart(2, "0"));
           }
         } else if (data instanceof Array) {
           to.push("[");
+          let first = true;
           for (const c of data) {
+            if (!first) {
+              to.push(",");
+            }
+            first = false;
             toStringImpl(c, to);
-            to.push(",");
           }
           to.push("]");
         } else if (data instanceof Map) {
@@ -62,7 +71,7 @@ function toStringImplMap(data: Iterable<[string, CalldataEncodable]>, to: string
         } else if (data instanceof CalldataAddress) {
           to.push("addr#");
           for (const c of data.bytes) {
-            to.push(c.toString(16));
+            to.push(c.toString(16).padStart(2, "0"));
           }
         } else if (Object.getPrototypeOf(data) === Object.prototype) {
           toStringImplMap(Object.entries(data), to);
diff --git a/tests/calldata-to-string.test.ts b/tests/calldata-to-string.test.ts
new file mode 100644
index 0000000..8b2af0b
--- /dev/null
+++ b/tests/calldata-to-string.test.ts
@@ -0,0 +1,41 @@
+import {describe, it, expect} from "vitest";
+import * as calldata from "@/abi/calldata";
+import {CalldataAddress} from "@/types/calldata";
+
+// Expected output matches the reference implementation in the genvm repo
+// (runners/genlayer-py-std/src/genlayer/calldata/__init__.py :: to_str)
+
+describe("calldata.toString", () => {
+  it("encodes bytes as zero-padded hex", () => {
+    expect(calldata.toString(new Uint8Array([0xab, 0xcd]))).toBe("b#abcd");
+    expect(calldata.toString(new Uint8Array([0x01, 0x02]))).toBe("b#0102");
+    expect(calldata.toString(new Uint8Array([0xff]))).toBe("b#ff");
+  });
+
+  it("does not collapse distinct byte arrays into the same string", () => {
+    const a = calldata.toString(new Uint8Array([0x01, 0x02]));
+    const b = calldata.toString(new Uint8Array([0x12]));
+    expect(a).not.toBe(b);
+  });
+
+  it("encodes an address as 40 hex characters", () => {
+    const addr = new CalldataAddress(new Uint8Array(20).fill(0x01));
+    expect(calldata.toString(addr)).toBe("addr#" + "01".repeat(20));
+  });
+
+  it("separates map entries with commas", () => {
+    expect(calldata.toString({a: 1})).toBe('{"a":1}');
+    expect(calldata.toString({x: true, y: null})).toBe('{"x":true,"y":null}');
+    expect(calldata.toString({})).toBe("{}");
+  });
+
+  it("separates array items with commas and emits no trailing comma", () => {
+    expect(calldata.toString([1, 2, 3])).toBe("[1,2,3]");
+    expect(calldata.toString([])).toBe("[]");
+    expect(calldata.toString([[1], [2, 3]])).toBe("[[1],[2,3]]");
+  });
+
+  it("handles nesting", () => {
+    expect(calldata.toString({items: [1, "two", null]})).toBe('{"items":[1,"two",null]}');
+  });
+});