In reply to @h7x4:nani.wtf
No worries 😄 Ping me if you'd like the package file with the current failing setup.

I've bundled a patch, likely introducing IFD, but good enough for testing.

{ lib
, buildNpmPackage
, fetchNpmDeps
, fetchFromGitea
, git
, cacert

, writeText
}:
# Smaller version of the problem
# fetchNpmDeps {
#   name = "out-of-your-element-deps";
#   src = fetchFromGitea {
#     domain = "gitdab.com";
#     owner = "cadence";
#     repo = "out-of-your-element";
#     rev = "v1.3";
#     hash = "sha256-dLDm3xOMoprkGU7D9IFX1hjMfCR3YjVgXb3c5IEuUTQ";
#   };

#   patches = [ ./add-discord-markdown-dep-integrity.patch ];

  # npmDepsHash = "sha256-k2MeejRWteIzY1Q+iNLCLDLnE1z9+cBaAGYAIlQHa/4=";
# };
buildNpmPackage rec {
  pname = "out-of-your-element";
  version = "1.3";
  src = fetchFromGitea {
    domain = "gitdab.com";
    owner = "cadence";
    repo = "out-of-your-element";
    rev = "v${version}";
    hash = "sha256-dLDm3xOMoprkGU7D9IFX1hjMfCR3YjVgXb3c5IEuUTQ";
  };

  patches = [ (writeText "add-discord-markdown-dep-integrity.patch" ''
    diff --git i/package-lock.json w/package-lock.json
    index 3847ee1..79a3ac0 100644
    --- i/package-lock.json
    +++ w/package-lock.json
    @@ -1060,6 +1060,7 @@
         "node_modules/discord-markdown": {
           "version": "2.4.1",
           "resolved": "git+https://git.sr.ht/~cadence/nodejs-discord-markdown#abc56d544072a1dc5624adfea455b0e902adf7b3",
    +      "integrity": "sha512-mPjPXXNgL3prKagvj/guOzAEMcJdcr6S95u7MRlYZvsgeUiacQDQatYrJgYAf53vJw+U0JvEPUwRHZY02oX1og==",
           "license": "MIT",
           "dependencies": {
             "simple-markdown": "^0.7.2"
  '') ];

  dontNpmBuild = true;
  npmDepsHash = "sha256-fPTFyX2GYSdMeIzVKXBacZxhTTj8fvlMFEGOxN0/cm8=";

  meta = with lib; {
    homepage = "https://gitdab.com/cadence/out-of-your-element";
    description = "Modern Matrix-to-Discord appservice bridge";
    changelog = "https://gitdab.com/cadence/out-of-your-element/releases/tag/${src.rev}";
    license = licenses.agpl3Plus;
    mainProgram = "start.js";
  };
}