diff --git a/src/shared/decodeCalldata.js b/src/shared/decodeCalldata.js
new file mode 100644
index 0000000..e93c058
--- /dev/null
+++ b/src/shared/decodeCalldata.js
@@ -0,0 +1,103 @@
+// Decode transaction calldata into human-readable details.
+// Shared between the approval screen and transaction history views.
+// Returns { name, description, details } or null.
+
+const { Interface, formatUnits } = require("ethers");
+const { ERC20_ABI } = require("./constants");
+const { TOKEN_BY_ADDRESS } = require("./tokenList");
+const uniswap = require("./uniswap");
+
+const erc20Iface = new Interface(ERC20_ABI);
+
+function formatTxValue(val) {
+    const parts = val.split(".");
+    if (parts.length === 1) return val + ".0000";
+    const dec = (parts[1] + "0000").slice(0, 4);
+    return parts[0] + "." + dec;
+}
+
+function decodeCalldata(data, toAddress) {
+    if (!data || data === "0x" || data.length < 10) return null;
+
+    // Try ERC-20 (approve / transfer)
+    try {
+        const parsed = erc20Iface.parseTransaction({ data });
+        if (parsed) {
+            const token = TOKEN_BY_ADDRESS.get(toAddress.toLowerCase());
+            const tokenSymbol = token ? token.symbol : null;
+            const tokenDecimals = token ? token.decimals : 18;
+            const contractLabel = tokenSymbol
+                ? tokenSymbol + " (" + toAddress + ")"
+                : toAddress;
+
+            if (parsed.name === "approve") {
+                const spender = parsed.args[0];
+                const rawAmount = parsed.args[1];
+                const maxUint = BigInt(
+                    "0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff",
+                );
+                const isUnlimited = rawAmount === maxUint;
+                const amountStr = isUnlimited
+                    ? "Unlimited"
+                    : formatTxValue(formatUnits(rawAmount, tokenDecimals)) +
+                      (tokenSymbol ? " " + tokenSymbol : "");
+
+                return {
+                    name: "Token Approval",
+                    description: tokenSymbol
+                        ? "Approve spending of your " + tokenSymbol
+                        : "Approve spending of an ERC-20 token",
+                    details: [
+                        {
+                            label: "Token",
+                            value: contractLabel,
+                            address: toAddress,
+                            isToken: true,
+                        },
+                        {
+                            label: "Spender",
+                            value: spender,
+                            address: spender,
+                        },
+                        { label: "Amount", value: amountStr },
+                    ],
+                };
+            }
+
+            if (parsed.name === "transfer") {
+                const to = parsed.args[0];
+                const rawAmount = parsed.args[1];
+                const amountStr =
+                    formatTxValue(formatUnits(rawAmount, tokenDecimals)) +
+                    (tokenSymbol ? " " + tokenSymbol : "");
+
+                return {
+                    name: "Token Transfer",
+                    description: tokenSymbol
+                        ? "Transfer " + tokenSymbol
+                        : "Transfer ERC-20 token",
+                    details: [
+                        {
+                            label: "Token",
+                            value: contractLabel,
+                            address: toAddress,
+                            isToken: true,
+                        },
+                        { label: "Recipient", value: to, address: to },
+                        { label: "Amount", value: amountStr },
+                    ],
+                };
+            }
+        }
+    } catch {
+        // Not ERC-20 — fall through
+    }
+
+    // Try Uniswap Universal Router
+    const routerResult = uniswap.decode(data, toAddress);
+    if (routerResult) return routerResult;
+
+    return null;
+}
+
+module.exports = { decodeCalldata };