diff --git a/test/FluxPair.t.sol b/test/FluxPair.t.sol
new file mode 100644
index 0000000..4d6d80a
--- /dev/null
+++ b/test/FluxPair.t.sol
@@ -0,0 +1,135 @@
+// SPDX-License-Identifier: MIT
+pragma solidity 0.8.36;
+
+import {Test} from "forge-std/Test.sol";
+import {FluxFactory} from "../src/FluxFactory.sol";
+import {FluxPair} from "../src/FluxPair.sol";
+import {TestErc20} from "../src/TestErc20.sol";
+import {IFluxCallee} from "../src/interfaces/IFluxCallee.sol";
+
+contract ReentrantSwapper is IFluxCallee {
+    FluxPair private pair;
+
+    constructor(FluxPair pairAddress) {
+        pair = pairAddress;
+    }
+
+    function fluxCall(address, uint256, uint256, bytes calldata) external {
+        pair.swap(0, 1, address(this), "");
+    }
+}
+
+contract FluxPairTest is Test {
+    address private constant receiver = address(0xBEEF);
+
+    FluxFactory private factory;
+    FluxPair private pair;
+    TestErc20 private token0;
+    TestErc20 private token1;
+
+    uint256 private initialLp;
+
+    function setUp() public {
+        factory = new FluxFactory(address(this));
+        TestErc20 tokenA = new TestErc20("Alpha", "ALPHA", 1_000_000e18);
+        TestErc20 tokenB = new TestErc20("Beta", "BETA", 1_000_000e18);
+        address pairAddress = factory.createPair(address(tokenA), address(tokenB));
+        pair = FluxPair(pairAddress);
+        token0 = TestErc20(pair.token0());
+        token1 = TestErc20(pair.token1());
+
+        token0.transfer(pairAddress, 100e18);
+        token1.transfer(pairAddress, 400e18);
+        initialLp = pair.mint(address(this));
+    }
+
+    function testInitialMint() public view {
+        assertEq(initialLp, 199999999999999999000);
+        assertEq(pair.balanceOf(address(this)), 199999999999999999000);
+        assertEq(pair.totalSupply(), 200e18);
+        assertEq(pair.balanceOf(address(0)), 1000);
+        (uint112 reserve0, uint112 reserve1,) = pair.getReserves();
+        assertEq(reserve0, 100e18);
+        assertEq(reserve1, 400e18);
+    }
+
+    function testProportionalMint() public {
+        token0.transfer(address(pair), 50e18);
+        token1.transfer(address(pair), 200e18);
+        uint256 minted = pair.mint(receiver);
+        assertEq(minted, 100e18);
+        assertEq(pair.balanceOf(receiver), 100e18);
+        assertEq(pair.totalSupply(), 300e18);
+        (uint112 reserve0, uint112 reserve1,) = pair.getReserves();
+        assertEq(reserve0, 150e18);
+        assertEq(reserve1, 600e18);
+    }
+
+    function testSwapMovesReservesAndKeepsInvariant() public {
+        uint256 expectedOut = 36264435755205965263;
+        uint256 kBefore = uint256(100e18) * 400e18;
+
+        token0.transfer(address(pair), 10e18);
+        pair.swap(0, expectedOut, receiver, "");
+
+        assertEq(token1.balanceOf(receiver), expectedOut);
+        (uint112 reserve0, uint112 reserve1,) = pair.getReserves();
+        assertEq(reserve0, 110e18);
+        assertEq(reserve1, 363735564244794034737);
+        assertGt(uint256(reserve0) * uint256(reserve1), kBefore);
+    }
+
+    function testSwapRevertsWhenInvariantBroken() public {
+        token0.transfer(address(pair), 10e18);
+        vm.expectRevert(bytes("Flux: K"));
+        pair.swap(0, 36264435755205965263 + 1e15, receiver, "");
+    }
+
+    function testSwapRevertsWithoutInput() public {
+        vm.expectRevert(bytes("Flux: INSUFFICIENT_INPUT_AMOUNT"));
+        pair.swap(0, 1e18, receiver, "");
+    }
+
+    function testBurnReturnsProportional() public {
+        uint256 lp = pair.balanceOf(address(this));
+        pair.transfer(address(pair), lp);
+        (uint256 amount0, uint256 amount1) = pair.burn(receiver);
+
+        assertEq(amount0, 99999999999999999500);
+        assertEq(amount1, 399999999999999998000);
+        assertEq(token0.balanceOf(receiver), 99999999999999999500);
+        assertEq(token1.balanceOf(receiver), 399999999999999998000);
+        assertEq(pair.totalSupply(), 1000);
+    }
+
+    function testSkimSendsExcessAndKeepsReserves() public {
+        token0.transfer(address(pair), 5e18);
+        pair.skim(receiver);
+        assertEq(token0.balanceOf(receiver), 5e18);
+        (uint112 reserve0,,) = pair.getReserves();
+        assertEq(reserve0, 100e18);
+    }
+
+    function testSyncAdoptsBalances() public {
+        token0.transfer(address(pair), 5e18);
+        pair.sync();
+        (uint112 reserve0, uint112 reserve1,) = pair.getReserves();
+        assertEq(reserve0, 105e18);
+        assertEq(reserve1, 400e18);
+    }
+
+    function testPriceAccumulatorGrowsOverTime() public {
+        assertEq(pair.price0CumulativeLast(), 0);
+        vm.warp(block.timestamp + 3600);
+        pair.sync();
+        assertGt(pair.price0CumulativeLast(), 0);
+        assertGt(pair.price1CumulativeLast(), 0);
+    }
+
+    function testReentrancyLockBlocksSwap() public {
+        ReentrantSwapper attacker = new ReentrantSwapper(pair);
+        token0.transfer(address(pair), 10e18);
+        vm.expectRevert(bytes("Flux: LOCKED"));
+        pair.swap(0, 1e18, address(attacker), "0x01");
+    }
+}