diff --git a/packages/wasm-utxo/src/wasm/fixed_script_wallet/mod.rs b/packages/wasm-utxo/src/wasm/fixed_script_wallet/mod.rs
index de46a06..854782c 100644
--- a/packages/wasm-utxo/src/wasm/fixed_script_wallet/mod.rs
+++ b/packages/wasm-utxo/src/wasm/fixed_script_wallet/mod.rs
@@ -421,10 +421,8 @@ impl BitGoPsbt {
                 as u32;
             let value_js = js_sys::Reflect::get(&item, &"value".into())
                 .map_err(|_| WasmUtxoError::new("Missing 'value' field on unspent"))?;
-            let value = js_sys::BigInt::from(value_js)
-                .as_f64()
-                .ok_or_else(|| WasmUtxoError::new("'value' must be a bigint"))?
-                as u64;
+            let value = u64::try_from(js_sys::BigInt::unchecked_from_js(value_js))
+                .map_err(|_| WasmUtxoError::new("'value' must be a bigint convertible to u64"))?;
             parsed_unspents.push(ScriptIdWithValue {
                 chain,
                 index,
diff --git a/packages/wasm-utxo/test/fixedScript/fromHalfSignedLegacyTransaction.ts b/packages/wasm-utxo/test/fixedScript/fromHalfSignedLegacyTransaction.ts
new file mode 100644
index 0000000..e1e30ea
--- /dev/null
+++ b/packages/wasm-utxo/test/fixedScript/fromHalfSignedLegacyTransaction.ts
@@ -0,0 +1,131 @@
+/**
+ * Tests for BitGoPsbt.fromHalfSignedLegacyTransaction()
+ *
+ * Bug: js_sys::BigInt::from(value_js).as_f64() does an unchecked wrap but then
+ * JsValue::as_f64() only works for JS Number type — not BigInt. Passing any proper
+ * JS BigInt value (e.g. 10000n) returned None, so the function always threw
+ * "'value' must be a bigint" even though the caller did exactly the right thing.
+ *
+ * Fix: u64::try_from(js_sys::BigInt::unchecked_from_js(value_js)) uses the
+ * BigInt-specific conversion path and then safely maps to u64.
+ */
+import { describe, it } from "mocha";
+import * as assert from "assert";
+import * as utxolib from "@bitgo/utxo-lib";
+import { BitGoPsbt, type HydrationUnspent } from "../../js/fixedScriptWallet/BitGoPsbt.js";
+import { ZcashBitGoPsbt } from "../../js/fixedScriptWallet/ZcashBitGoPsbt.js";
+import { ChainCode } from "../../js/fixedScriptWallet/chains.js";
+import { getDefaultWalletKeys, getKeyTriple } from "../../js/testutils/keys.js";
+import { getCoinNameForNetwork } from "../networks.js";
+
+const ZCASH_NU5_HEIGHT = 1687105;
+
+const p2msScriptTypes = ["p2sh", "p2shP2wsh", "p2wsh"] as const;
+
+function isSupportedNetwork(n: utxolib.Network): boolean {
+  return utxolib.isMainnet(n) && n !== utxolib.networks.bitcoinsv && n !== utxolib.networks.ecash;
+}
+
+function createHalfSignedP2msPsbt(
+  network: utxolib.Network,
+  valueOverride?: bigint,
+): { psbt: BitGoPsbt; unspents: HydrationUnspent[] } {
+  const coinName = getCoinNameForNetwork(network);
+  const rootWalletKeys = getDefaultWalletKeys();
+  const [userXprv] = getKeyTriple("default");
+
+  const supportedTypes = p2msScriptTypes.filter((scriptType) =>
+    utxolib.bitgo.outputScripts.isSupportedScriptType(network, scriptType),
+  );
+
+  const isZcash = utxolib.getMainnet(network) === utxolib.networks.zcash;
+  const psbt = isZcash
+    ? ZcashBitGoPsbt.createEmpty(coinName as "zec" | "tzec", rootWalletKeys, {
+        version: 4,
+        lockTime: 0,
+        blockHeight: ZCASH_NU5_HEIGHT,
+      })
+    : BitGoPsbt.createEmpty(coinName, rootWalletKeys, { version: 2, lockTime: 0 });
+
+  const unspents: HydrationUnspent[] = [];
+  supportedTypes.forEach((scriptType, index) => {
+    const chain = ChainCode.value(scriptType, "external");
+    const value = valueOverride ?? BigInt(10000 + index * 10000);
+    psbt.addWalletInput(
+      {
+        txid: `${"00".repeat(31)}${index.toString(16).padStart(2, "0")}`,
+        vout: 0,
+        value,
+        sequence: 0xfffffffd,
+      },
+      rootWalletKeys,
+      { scriptId: { chain, index } },
+    );
+    unspents.push({ chain, index, value });
+  });
+
+  psbt.addWalletOutput(rootWalletKeys, { chain: 0, index: 100, value: BigInt(5000) });
+  psbt.sign(userXprv);
+
+  return { psbt, unspents };
+}
+
+describe("BitGoPsbt.fromHalfSignedLegacyTransaction", function () {
+  describe("BigInt value conversion (regression for unchecked-from/as_f64 bug)", function () {
+    it("should not throw when unspent values are JS BigInt", function () {
+      // With the buggy Rust code this always threw "'value' must be a bigint"
+      // because BigInt::from(value_js).as_f64() calls JsValue::as_f64(), which
+      // returns None for JS BigInt (it only works for JS Number).
+      const rootWalletKeys = getDefaultWalletKeys();
+      const { psbt, unspents } = createHalfSignedP2msPsbt(utxolib.networks.bitcoin);
+      const txBytes = psbt.getHalfSignedLegacyFormat();
+
+      assert.doesNotThrow(() => {
+        BitGoPsbt.fromHalfSignedLegacyTransaction(txBytes, "btc", rootWalletKeys, unspents);
+      }, "fromHalfSignedLegacyTransaction must not throw for valid JS BigInt values");
+    });
+
+    it("should handle values larger than Number.MAX_SAFE_INTEGER", function () {
+      // Values beyond 2^53-1 would silently lose precision through f64; the fixed
+      // code converts directly via u64::try_from so precision is preserved.
+      const rootWalletKeys = getDefaultWalletKeys();
+      // 21 million BTC in satoshis — the maximum possible UTXO value
+      const maxSats = 21_000_000n * 100_000_000n;
+      const { psbt, unspents } = createHalfSignedP2msPsbt(utxolib.networks.bitcoin, maxSats);
+      const txBytes = psbt.getHalfSignedLegacyFormat();
+
+      assert.doesNotThrow(() => {
+        BitGoPsbt.fromHalfSignedLegacyTransaction(txBytes, "btc", rootWalletKeys, unspents);
+      }, "fromHalfSignedLegacyTransaction must handle large satoshi values");
+    });
+  });
+
+  describe("Round-trip: getHalfSignedLegacyFormat → fromHalfSignedLegacyTransaction", function () {
+    // Zcash uses a non-standard transaction format (version 4 overwintered) that
+    // fromHalfSignedLegacyTransaction does not support; skip it here.
+    const roundTripNetworks = utxolib
+      .getNetworkList()
+      .filter(isSupportedNetwork)
+      .filter((n) => utxolib.getMainnet(n) !== utxolib.networks.zcash);
+
+    for (const network of roundTripNetworks) {
+      const networkName = utxolib.getNetworkName(network);
+      it(`${networkName}: reconstructed PSBT serializes without error`, function () {
+        const rootWalletKeys = getDefaultWalletKeys();
+        const coinName = getCoinNameForNetwork(network);
+        const { psbt, unspents } = createHalfSignedP2msPsbt(network);
+        const txBytes = psbt.getHalfSignedLegacyFormat();
+
+        const reconstructed = BitGoPsbt.fromHalfSignedLegacyTransaction(
+          txBytes,
+          coinName,
+          rootWalletKeys,
+          unspents,
+        );
+
+        const serialized = reconstructed.serialize();
+        assert.ok(serialized.length > 0, "Reconstructed PSBT should serialize to non-empty bytes");
+      });
+    }
+  });
+});