diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index cb8e7e7f..81968bce 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -94,7 +94,6 @@ jobs:
         node-version:
           - 18
           - 20
-          - 21
           - 22
         os:
           - ubuntu-latest
diff --git a/README.md b/README.md
index d1f8e281..c3d39dcf 100644
--- a/README.md
+++ b/README.md
@@ -27,7 +27,7 @@ Run `smee --help` for usage.
 ### Node Client
 
 ```js
-const SmeeClient = require('smee-client')
+import SmeeClient from 'smee-client'
 
 const smee = new SmeeClient({
   source: 'https://smee.io/abc123',
diff --git a/index.ts b/index.ts
index b4442a2f..066c3e63 100644
--- a/index.ts
+++ b/index.ts
@@ -1,5 +1,5 @@
 import validator from "validator";
-import EventSource from "eventsource";
+import { EventSource } from "undici";
 import url from "url";
 import querystring from "querystring";
 
@@ -111,4 +111,4 @@ class Client {
   }
 }
 
-export = Client;
+export default Client;
diff --git a/package-lock.json b/package-lock.json
index 17e062e9..c7a8d181 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -10,7 +10,7 @@
       "license": "ISC",
       "dependencies": {
         "commander": "^12.0.0",
-        "eventsource": "^2.0.2",
+        "undici": "6.19.8",
         "validator": "^13.11.0"
       },
       "bin": {
@@ -18,8 +18,7 @@
       },
       "devDependencies": {
         "@octokit/tsconfig": "^3.0.0",
-        "@types/eventsource": "^1.1.15",
-        "@types/node": "^20.0.0",
+        "@types/node": "^18.19.14",
         "@types/validator": "^13.11.6",
         "@vitest/coverage-v8": "^2.0.0",
         "fastify": "^4.24.3",
@@ -63,13 +62,13 @@
       }
     },
     "node_modules/@babel/parser": {
-      "version": "7.25.3",
-      "resolved": "https://registry.npmjs.org/@babel/parser/-/parser-7.25.3.tgz",
-      "integrity": "sha512-iLTJKDbJ4hMvFPgQwwsVoxtHyWpKKPBrxkANrSYewDPaPpT5py5yeVkgPIJ7XYXhndxJpaA3PyALSXQ7u8e/Dw==",
+      "version": "7.25.4",
+      "resolved": "https://registry.npmjs.org/@babel/parser/-/parser-7.25.4.tgz",
+      "integrity": "sha512-nq+eWrOgdtu3jG5Os4TQP3x3cLA8hR8TvJNjD8vnPa20WGycimcparWnLK4jJhElTK6SDyuJo1weMKO/5LpmLA==",
       "dev": true,
       "license": "MIT",
       "dependencies": {
-        "@babel/types": "^7.25.2"
+        "@babel/types": "^7.25.4"
       },
       "bin": {
         "parser": "bin/babel-parser.js"
@@ -79,9 +78,9 @@
       }
     },
     "node_modules/@babel/types": {
-      "version": "7.25.2",
-      "resolved": "https://registry.npmjs.org/@babel/types/-/types-7.25.2.tgz",
-      "integrity": "sha512-YTnYtra7W9e6/oAZEHj0bJehPRUlLH9/fbpT5LfB0NhQXyALCRkRs3zH9v07IYhkgpqX6Z78FnuccZr/l4Fs4Q==",
+      "version": "7.25.4",
+      "resolved": "https://registry.npmjs.org/@babel/types/-/types-7.25.4.tgz",
+      "integrity": "sha512-zQ1ijeeCXVEh+aNL0RlmkPkG8HUiDcU2pzQQFjtbntgAczRASFzj4H+6+bV+dy1ntKR14I/DypeuRG1uma98iQ==",
       "dev": true,
       "license": "MIT",
       "dependencies": {
@@ -100,278 +99,6 @@
       "dev": true,
       "license": "MIT"
     },
-    "node_modules/@esbuild/aix-ppc64": {
-      "version": "0.21.5",
-      "resolved": "https://registry.npmjs.org/@esbuild/aix-ppc64/-/aix-ppc64-0.21.5.tgz",
-      "integrity": "sha512-1SDgH6ZSPTlggy1yI6+Dbkiz8xzpHJEVAlF/AM1tHPLsf5STom9rwtjE4hKAF20FfXXNTFqEYXyJNWh1GiZedQ==",
-      "cpu": [
-        "ppc64"
-      ],
-      "dev": true,
-      "license": "MIT",
-      "optional": true,
-      "os": [
-        "aix"
-      ],
-      "engines": {
-        "node": ">=12"
-      }
-    },
-    "node_modules/@esbuild/android-arm": {
-      "version": "0.21.5",
-      "resolved": "https://registry.npmjs.org/@esbuild/android-arm/-/android-arm-0.21.5.tgz",
-      "integrity": "sha512-vCPvzSjpPHEi1siZdlvAlsPxXl7WbOVUBBAowWug4rJHb68Ox8KualB+1ocNvT5fjv6wpkX6o/iEpbDrf68zcg==",
-      "cpu": [
-        "arm"
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-      "dev": true,
-      "license": "MIT",
-      "optional": true,
-      "os": [
-        "android"
-      ],
-      "engines": {
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-      }
-    },
-    "node_modules/@esbuild/android-arm64": {
-      "version": "0.21.5",
-      "resolved": "https://registry.npmjs.org/@esbuild/android-arm64/-/android-arm64-0.21.5.tgz",
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-        "ppc64"
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-      "optional": true,
-      "os": [
-        "linux"
-      ],
-      "engines": {
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-      }
-    },
-    "node_modules/@esbuild/linux-riscv64": {
-      "version": "0.21.5",
-      "resolved": "https://registry.npmjs.org/@esbuild/linux-riscv64/-/linux-riscv64-0.21.5.tgz",
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-      "cpu": [
-        "riscv64"
-      ],
-      "dev": true,
-      "license": "MIT",
-      "optional": true,
-      "os": [
-        "linux"
-      ],
-      "engines": {
-        "node": ">=12"
-      }
-    },
-    "node_modules/@esbuild/linux-s390x": {
-      "version": "0.21.5",
-      "resolved": "https://registry.npmjs.org/@esbuild/linux-s390x/-/linux-s390x-0.21.5.tgz",
-      "integrity": "sha512-zus5sxzqBJD3eXxwvjN1yQkRepANgxE9lgOW2qLnmr8ikMTphkjgXu1HR01K4FJg8h1kEEDAqDcZQtbrRnB41A==",
-      "cpu": [
-        "s390x"
-      ],
-      "dev": true,
-      "license": "MIT",
-      "optional": true,
-      "os": [
-        "linux"
-      ],
-      "engines": {
-        "node": ">=12"
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     "node_modules/@esbuild/linux-x64": {
       "version": "0.21.5",
       "resolved": "https://registry.npmjs.org/@esbuild/linux-x64/-/linux-x64-0.21.5.tgz",
@@ -389,108 +116,6 @@
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       }
     },
-    "node_modules/@esbuild/netbsd-x64": {
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-      "resolved": "https://registry.npmjs.org/@esbuild/netbsd-x64/-/netbsd-x64-0.21.5.tgz",
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-      "cpu": [
-        "x64"
-      ],
-      "dev": true,
-      "license": "MIT",
-      "optional": true,
-      "os": [
-        "netbsd"
-      ],
-      "engines": {
-        "node": ">=12"
-      }
-    },
-    "node_modules/@esbuild/openbsd-x64": {
-      "version": "0.21.5",
-      "resolved": "https://registry.npmjs.org/@esbuild/openbsd-x64/-/openbsd-x64-0.21.5.tgz",
-      "integrity": "sha512-HLNNw99xsvx12lFBUwoT8EVCsSvRNDVxNpjZ7bPn947b8gJPzeHWyNVhFsaerc0n3TsbOINvRP2byTZ5LKezow==",
-      "cpu": [
-        "x64"
-      ],
-      "dev": true,
-      "license": "MIT",
-      "optional": true,
-      "os": [
-        "openbsd"
-      ],
-      "engines": {
-        "node": ">=12"
-      }
-    },
-    "node_modules/@esbuild/sunos-x64": {
-      "version": "0.21.5",
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-    "node_modules/@esbuild/win32-arm64": {
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-    "node_modules/@esbuild/win32-ia32": {
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-      "integrity": "sha512-SWXFF1CL2RVNMaVs+BBClwtfZSvDgtL//G/smwAc5oVK/UPu2Gu9tIaRgFmYFFKrmg3SyAjSrElf0TiJ1v8fYA==",
-      "cpu": [
-        "ia32"
-      ],
-      "dev": true,
-      "license": "MIT",
-      "optional": true,
-      "os": [
-        "win32"
-      ],
-      "engines": {
-        "node": ">=12"
-      }
-    },
-    "node_modules/@esbuild/win32-x64": {
-      "version": "0.21.5",
-      "resolved": "https://registry.npmjs.org/@esbuild/win32-x64/-/win32-x64-0.21.5.tgz",
-      "integrity": "sha512-tQd/1efJuzPC6rCFwEvLtci/xNFcTZknmXs98FYDfGE4wP9ClFV98nyKrzJKVPMhdDnjzLhdUyMX4PsQAPjwIw==",
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-      "optional": true,
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       "version": "3.6.0",
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@@ -629,160 +254,6 @@
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-      "license": "MIT",
-      "optional": true,
-      "os": [
-        "linux"
-      ]
-    },
-    "node_modules/@rollup/rollup-linux-powerpc64le-gnu": {
-      "version": "4.21.0",
-      "resolved": "https://registry.npmjs.org/@rollup/rollup-linux-powerpc64le-gnu/-/rollup-linux-powerpc64le-gnu-4.21.0.tgz",
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-        "ppc64"
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-      "dev": true,
-      "license": "MIT",
-      "optional": true,
-      "os": [
-        "linux"
-      ]
-    },
-    "node_modules/@rollup/rollup-linux-riscv64-gnu": {
-      "version": "4.21.0",
-      "resolved": "https://registry.npmjs.org/@rollup/rollup-linux-riscv64-gnu/-/rollup-linux-riscv64-gnu-4.21.0.tgz",
-      "integrity": "sha512-H1eRaCwd5E8eS8leiS+o/NqMdljkcb1d6r2h4fKSsCXQilLKArq6WS7XBLDu80Yz+nMqHVFDquwcVrQmGr28rg==",
-      "cpu": [
-        "riscv64"
-      ],
-      "dev": true,
-      "license": "MIT",
-      "optional": true,
-      "os": [
-        "linux"
-      ]
-    },
-    "node_modules/@rollup/rollup-linux-s390x-gnu": {
-      "version": "4.21.0",
-      "resolved": "https://registry.npmjs.org/@rollup/rollup-linux-s390x-gnu/-/rollup-linux-s390x-gnu-4.21.0.tgz",
-      "integrity": "sha512-zJ4hA+3b5tu8u7L58CCSI0A9N1vkfwPhWd/puGXwtZlsB5bTkwDNW/+JCU84+3QYmKpLi+XvHdmrlwUwDA6kqw==",
-      "cpu": [
-        "s390x"
-      ],
-      "dev": true,
-      "license": "MIT",
-      "optional": true,
-      "os": [
-        "linux"
-      ]
-    },
     "node_modules/@rollup/rollup-linux-x64-gnu": {
       "version": "4.21.0",
       "resolved": "https://registry.npmjs.org/@rollup/rollup-linux-x64-gnu/-/rollup-linux-x64-gnu-4.21.0.tgz",
@@ -811,48 +282,6 @@
         "linux"
       ]
     },
-    "node_modules/@rollup/rollup-win32-arm64-msvc": {
-      "version": "4.21.0",
-      "resolved": "https://registry.npmjs.org/@rollup/rollup-win32-arm64-msvc/-/rollup-win32-arm64-msvc-4.21.0.tgz",
-      "integrity": "sha512-s5oFkZ/hFcrlAyBTONFY1TWndfyre1wOMwU+6KCpm/iatybvrRgmZVM+vCFwxmC5ZhdlgfE0N4XorsDpi7/4XQ==",
-      "cpu": [
-        "arm64"
-      ],
-      "dev": true,
-      "license": "MIT",
-      "optional": true,
-      "os": [
-        "win32"
-      ]
-    },
-    "node_modules/@rollup/rollup-win32-ia32-msvc": {
-      "version": "4.21.0",
-      "resolved": "https://registry.npmjs.org/@rollup/rollup-win32-ia32-msvc/-/rollup-win32-ia32-msvc-4.21.0.tgz",
-      "integrity": "sha512-G9+TEqRnAA6nbpqyUqgTiopmnfgnMkR3kMukFBDsiyy23LZvUCpiUwjTRx6ezYCjJODXrh52rBR9oXvm+Fp5wg==",
-      "cpu": [
-        "ia32"
-      ],
-      "dev": true,
-      "license": "MIT",
-      "optional": true,
-      "os": [
-        "win32"
-      ]
-    },
-    "node_modules/@rollup/rollup-win32-x64-msvc": {
-      "version": "4.21.0",
-      "resolved": "https://registry.npmjs.org/@rollup/rollup-win32-x64-msvc/-/rollup-win32-x64-msvc-4.21.0.tgz",
-      "integrity": "sha512-2jsCDZwtQvRhejHLfZ1JY6w6kEuEtfF9nzYsZxzSlNVKDX+DpsDJ+Rbjkm74nvg2rdx0gwBS+IMdvwJuq3S9pQ==",
-      "cpu": [
-        "x64"
-      ],
-      "dev": true,
-      "license": "MIT",
-      "optional": true,
-      "os": [
-        "win32"
-      ]
-    },
     "node_modules/@types/estree": {
       "version": "1.0.5",
       "resolved": "https://registry.npmjs.org/@types/estree/-/estree-1.0.5.tgz",
@@ -860,21 +289,14 @@
       "dev": true,
       "license": "MIT"
     },
-    "node_modules/@types/eventsource": {
-      "version": "1.1.15",
-      "resolved": "https://registry.npmjs.org/@types/eventsource/-/eventsource-1.1.15.tgz",
-      "integrity": "sha512-XQmGcbnxUNa06HR3VBVkc9+A2Vpi9ZyLJcdS5dwaQQ/4ZMWFO+5c90FnMUpbtMZwB/FChoYHwuVg8TvkECacTA==",
-      "dev": true,
-      "license": "MIT"
-    },
     "node_modules/@types/node": {
-      "version": "20.16.0",
-      "resolved": "https://registry.npmjs.org/@types/node/-/node-20.16.0.tgz",
-      "integrity": "sha512-vDxceJcoZhIVh67S568bm1UGZO0DX0hpplJZxzeXMKwIPLn190ec5RRxQ69BKhX44SUGIxxgMdDY557lGLKprQ==",
+      "version": "18.19.45",
+      "resolved": "https://registry.npmjs.org/@types/node/-/node-18.19.45.tgz",
+      "integrity": "sha512-VZxPKNNhjKmaC1SUYowuXSRSMGyQGmQjvvA1xE4QZ0xce2kLtEhPDS+kqpCPBZYgqblCLQ2DAjSzmgCM5auvhA==",
       "dev": true,
       "license": "MIT",
       "dependencies": {
-        "undici-types": "~6.19.2"
+        "undici-types": "~5.26.4"
       }
     },
     "node_modules/@types/validator": {
@@ -1382,15 +804,6 @@
         "node": ">=0.8.x"
       }
     },
-    "node_modules/eventsource": {
-      "version": "2.0.2",
-      "resolved": "https://registry.npmjs.org/eventsource/-/eventsource-2.0.2.tgz",
-      "integrity": "sha512-IzUmBGPR3+oUG9dUeXynyNmf91/3zUSJg1lCktzKw47OXuhco54U3r9B7O4XX+Rb1Itm9OZ2b0RkTs10bICOxA==",
-      "license": "MIT",
-      "engines": {
-        "node": ">=12.0.0"
-      }
-    },
     "node_modules/execa": {
       "version": "8.0.1",
       "resolved": "https://registry.npmjs.org/execa/-/execa-8.0.1.tgz",
@@ -1584,21 +997,6 @@
         "node": ">= 0.6"
       }
     },
-    "node_modules/fsevents": {
-      "version": "2.3.3",
-      "resolved": "https://registry.npmjs.org/fsevents/-/fsevents-2.3.3.tgz",
-      "integrity": "sha512-5xoDfX+fL7faATnagmWPpbFtwh/R77WmMMqqHGS65C3vvB0YHrgF+B1YmZ3441tMj5n63k0212XNoJwzlhffQw==",
-      "dev": true,
-      "hasInstallScript": true,
-      "license": "MIT",
-      "optional": true,
-      "os": [
-        "darwin"
-      ],
-      "engines": {
-        "node": "^8.16.0 || ^10.6.0 || >=11.0.0"
-      }
-    },
     "node_modules/get-func-name": {
       "version": "2.0.2",
       "resolved": "https://registry.npmjs.org/get-func-name/-/get-func-name-2.0.2.tgz",
@@ -2630,9 +2028,9 @@
       "license": "MIT"
     },
     "node_modules/tinypool": {
-      "version": "1.0.0",
-      "resolved": "https://registry.npmjs.org/tinypool/-/tinypool-1.0.0.tgz",
-      "integrity": "sha512-KIKExllK7jp3uvrNtvRBYBWBOAXSX8ZvoaD8T+7KB/QHIuoJW3Pmr60zucywjAlMb5TeXUkcs/MWeWLu0qvuAQ==",
+      "version": "1.0.1",
+      "resolved": "https://registry.npmjs.org/tinypool/-/tinypool-1.0.1.tgz",
+      "integrity": "sha512-URZYihUbRPcGv95En+sz6MfghfIc2OJ1sv/RmhWZLouPY0/8Vo80viwPvg3dlaS9fuq7fQMEfgRRK7BBZThBEA==",
       "dev": true,
       "license": "MIT",
       "engines": {
@@ -2693,10 +2091,19 @@
         "node": ">=14.17"
       }
     },
+    "node_modules/undici": {
+      "version": "6.19.8",
+      "resolved": "https://registry.npmjs.org/undici/-/undici-6.19.8.tgz",
+      "integrity": "sha512-U8uCCl2x9TK3WANvmBavymRzxbfFYG+tAu+fgx3zxQy3qdagQqBLwJVrdyO1TBfUXvfKveMKJZhpvUYoOjM+4g==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=18.17"
+      }
+    },
     "node_modules/undici-types": {
-      "version": "6.19.6",
-      "resolved": "https://registry.npmjs.org/undici-types/-/undici-types-6.19.6.tgz",
-      "integrity": "sha512-e/vggGopEfTKSvj4ihnOLTsqhrKRN3LeO6qSN/GxohhuRv8qH9bNQ4B8W7e/vFL+0XTnmHPB4/kegunZGA4Org==",
+      "version": "5.26.5",
+      "resolved": "https://registry.npmjs.org/undici-types/-/undici-types-5.26.5.tgz",
+      "integrity": "sha512-JlCMO+ehdEIKqlFxk6IfVoAUVmgz7cU7zD/h9XZ0qzeosSHmUJVOzSQvvYSYWXkFXC+IfLKSIffhv0sVZup6pA==",
       "dev": true,
       "license": "MIT"
     },
@@ -2710,15 +2117,15 @@
       }
     },
     "node_modules/vite": {
-      "version": "5.4.1",
-      "resolved": "https://registry.npmjs.org/vite/-/vite-5.4.1.tgz",
-      "integrity": "sha512-1oE6yuNXssjrZdblI9AfBbHCC41nnyoVoEZxQnID6yvQZAFBzxxkqoFLtHUMkYunL8hwOLEjgTuxpkRxvba3kA==",
+      "version": "5.4.2",
+      "resolved": "https://registry.npmjs.org/vite/-/vite-5.4.2.tgz",
+      "integrity": "sha512-dDrQTRHp5C1fTFzcSaMxjk6vdpKvT+2/mIdE07Gw2ykehT49O0z/VHS3zZ8iV/Gh8BJJKHWOe5RjaNrW5xf/GA==",
       "dev": true,
       "license": "MIT",
       "dependencies": {
         "esbuild": "^0.21.3",
         "postcss": "^8.4.41",
-        "rollup": "^4.13.0"
+        "rollup": "^4.20.0"
       },
       "bin": {
         "vite": "bin/vite.js"
diff --git a/package.json b/package.json
index bcc8f28d..c6e14758 100644
--- a/package.json
+++ b/package.json
@@ -3,6 +3,7 @@
   "version": "0.0.0-development",
   "description": "Client to proxy webhooks to localhost",
   "main": "index.js",
+  "type": "module",
   "bin": {
     "smee": "./bin/smee.js"
   },
@@ -24,13 +25,12 @@
   "license": "ISC",
   "dependencies": {
     "commander": "^12.0.0",
-    "eventsource": "^2.0.2",
+    "undici": "6.19.8",
     "validator": "^13.11.0"
   },
   "devDependencies": {
     "@octokit/tsconfig": "^3.0.0",
-    "@types/eventsource": "^1.1.15",
-    "@types/node": "^20.0.0",
+    "@types/node": "^18.19.14",
     "@types/validator": "^13.11.6",
     "@vitest/coverage-v8": "^2.0.0",
     "fastify": "^4.24.3",
diff --git a/test/index.test.ts b/test/index.test.ts
index 6dc66726..7147f4c8 100644
--- a/test/index.test.ts
+++ b/test/index.test.ts
@@ -1,4 +1,4 @@
-import Client from "../index";
+import Client from "../index.ts";
 import { describe, test, expect } from "vitest";
 import { fastify as Fastify } from "fastify";
 
diff --git a/tsconfig.json b/tsconfig.json
index 2b8c93d8..019a3011 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -2,6 +2,7 @@
   "extends": "@octokit/tsconfig",
 
   "compilerOptions": {
+    "module": "ES2020",
     "verbatimModuleSyntax": false,
     "noImplicitReturns": true,
     "noFallthroughCasesInSwitch": true,
@@ -13,7 +14,8 @@
     "esModuleInterop": true,
     "declaration": true,
     "allowJs": true,
-    "lib": ["es2023", "dom"]
+    "lib": ["es2023", "dom"],
+    "moduleResolution": "Node"
   },
   "include": ["./*"],
   "files": ["index.ts"]