Non-deterministic IDs in Static Export HTML/TXT Files

[avernikoz]

Link to the code that reproduces this issue

https://github.com/avernikoz/nextjs-non-deterministic-ids

To Reproduce

1. Clone the reproduction repository
2. Install dependencies: npm install
3. Run the build: npm run build
4. Check any HTML file in the out directory (e.g., out/index.html or out/blog.html) - note the 21-character ID
5. Run the build again: npm run build
6. Compare the same HTML file - notice the ID has changed despite no code changes

Note: Setting generateBuildId in next.config.js to return a static value doesn't affect this behavior - the IDs still change between builds.

Current vs. Expected behavior

Current Behavior:
Each build generates new random-looking 21-character IDs (containing alphanumeric characters plus - and _) that appear in both:

• The HTML version of each page (/out/index.html)
• The TXT version of the same page (/out/index.txt)

<!-- /out/index.html -->
    <script>
      self.__next_f.push([
        1,
        '0:{"P":null,"b":"static-build-id-v1","p":"","c":["",""],"i":false,"f":[[["",{"children":["__PAGE__",{}]},"$undefined","$undefined",true],["",["$","$1","c",{"children":[[["$","link","0",{"rel":"stylesheet","href":"/_next/static/css/b3cbcd051438d1d5.css","precedence":"next","crossOrigin":"$undefined","nonce":"$undefined"}]],["$","html",null,{"lang":"en","children":["$","body",null,{"className":"__className_d65c78","children":["$","$L2",null,{"parallelRouterKey":"children","error":"$undefined","errorStyles":"$undefined","errorScripts":"$undefined","template":["$","$L3",null,{}],"templateStyles":"$undefined","templateScripts":"$undefined","notFound":[[["$","title",null,{"children":"404: This page could not be found."}],["$","div",null,{"style":{"fontFamily":"system-ui,\\"Segoe UI\\",Roboto,Helvetica,Arial,sans-serif,\\"Apple Color Emoji\\",\\"Segoe UI Emoji\\"","height":"100vh","textAlign":"center","display":"flex","flexDirection":"column","alignItems":"center","justifyContent":"center"},"children":["$","div",null,{"children":[["$","style",null,{"dangerouslySetInnerHTML":{"__html":"body{color:#000;background:#fff;margin:0}.next-error-h1{border-right:1px solid rgba(0,0,0,.3)}@media (prefers-color-scheme:dark){body{color:#fff;background:#000}.next-error-h1{border-right:1px solid rgba(255,255,255,.3)}}"}}],["$","h1",null,{"className":"next-error-h1","style":{"display":"inline-block","margin":"0 20px 0 0","padding":"0 23px 0 0","fontSize":24,"fontWeight":500,"verticalAlign":"top","lineHeight":"49px"},"children":404}],["$","div",null,{"style":{"display":"inline-block"},"children":["$","h2",null,{"style":{"fontSize":14,"fontWeight":400,"lineHeight":"49px","margin":0},"children":"This page could not be found."}]}]]}]}]],[]],"forbidden":"$undefined","unauthorized":"$undefined"}]}]}]]}],{"children":["__PAGE__",["$","$1","c",{"children":[["$","div",null,{"children":["$","h1",null,{"children":"Home page"}]}],"$undefined",null,["$","$L4",null,{"children":["$L5","$L6",null]}]]}],{},null,false]},null,false],["$","$1","h",{"children":[null,["$","$1","8did5CzdK-_FUXgUnI31p",{"children":[["$","$L7",null,{"children":"$L8"}],["$","meta",null,{"name":"next-size-adjust","content":""}]]}],["$","$L9",null,{"children":"$La"}]]}],false]],"m":"$undefined","G":["$b","$undefined"],"s":false,"S":true}\n',
      ]);
    </script>
<!-- /out/index.txt -->
// Same ID appears here: 8did5CzdK-_FUXgUnI31p

Example from second build (same code):

<!-- /out/index.html -->
    <script>
      self.__next_f.push([
        1,
        '0:{"P":null,"b":"static-build-id-v1","p":"","c":["",""],"i":false,"f":[[["",{"children":["__PAGE__",{}]},"$undefined","$undefined",true],["",["$","$1","c",{"children":[[["$","link","0",{"rel":"stylesheet","href":"/_next/static/css/b3cbcd051438d1d5.css","precedence":"next","crossOrigin":"$undefined","nonce":"$undefined"}]],["$","html",null,{"lang":"en","children":["$","body",null,{"className":"__className_d65c78","children":["$","$L2",null,{"parallelRouterKey":"children","error":"$undefined","errorStyles":"$undefined","errorScripts":"$undefined","template":["$","$L3",null,{}],"templateStyles":"$undefined","templateScripts":"$undefined","notFound":[[["$","title",null,{"children":"404: This page could not be found."}],["$","div",null,{"style":{"fontFamily":"system-ui,\\"Segoe UI\\",Roboto,Helvetica,Arial,sans-serif,\\"Apple Color Emoji\\",\\"Segoe UI Emoji\\"","height":"100vh","textAlign":"center","display":"flex","flexDirection":"column","alignItems":"center","justifyContent":"center"},"children":["$","div",null,{"children":[["$","style",null,{"dangerouslySetInnerHTML":{"__html":"body{color:#000;background:#fff;margin:0}.next-error-h1{border-right:1px solid rgba(0,0,0,.3)}@media (prefers-color-scheme:dark){body{color:#fff;background:#000}.next-error-h1{border-right:1px solid rgba(255,255,255,.3)}}"}}],["$","h1",null,{"className":"next-error-h1","style":{"display":"inline-block","margin":"0 20px 0 0","padding":"0 23px 0 0","fontSize":24,"fontWeight":500,"verticalAlign":"top","lineHeight":"49px"},"children":404}],["$","div",null,{"style":{"display":"inline-block"},"children":["$","h2",null,{"style":{"fontSize":14,"fontWeight":400,"lineHeight":"49px","margin":0},"children":"This page could not be found."}]}]]}]}]],[]],"forbidden":"$undefined","unauthorized":"$undefined"}]}]}]]}],{"children":["__PAGE__",["$","$1","c",{"children":[["$","div",null,{"children":["$","h1",null,{"children":"Home page"}]}],"$undefined",null,["$","$L4",null,{"children":["$L5","$L6",null]}]]}],{},null,false]},null,false],["$","$1","h",{"children":[null,["$","$1","Wo-j9eG_HP1r23sgh1j2d",{"children":[["$","$L7",null,{"children":"$L8"}],["$","meta",null,{"name":"next-size-adjust","content":""}]]}],["$","$L9",null,{"children":"$La"}]]}],false]],"m":"$undefined","G":["$b","$undefined"],"s":false,"S":true}\n',
      ]);
    </script>
<!-- /out/index.txt -->
// Same ID appears here: Wo-j9eG_HP1r23sgh1j2d

Note how the ID changes (8did5CzdK-_FUXgUnI31p → Wo-j9eG_HP1r23sgh1j2d) between builds despite identical source code and static build ID.

Expected Behavior:
Either:

1. These IDs should be deterministic between builds when the source code hasn't changed, similar to how generateBuildId works for the build ID
2. Or Next.js should provide a configuration option to make these IDs deterministic in cases where it doesn't break the intended design

Provide environment information

Operating System:
  Platform: darwin
  Arch: x64
  Version: Darwin Kernel Version 23.6.0: Thu Dec 19 20:44:50 PST 2024; root:xnu-10063.141.1.703.2~1/RELEASE_X86_64
  Available memory (MB): 65536
  Available CPU cores: 16
Binaries:
  Node: 20.11.0
  npm: 10.2.4
  Yarn: 1.22.4
  pnpm: 9.6.0
Relevant Packages:
  next: 15.2.4
  eslint-config-next: N/A
  react: 19.1.0
  react-dom: 19.1.0
  typescript: 5.7.3
Next.js Config:
  output: export

Which area(s) are affected? (Select all that apply)

Webpack

Which stage(s) are affected? (Select all that apply)

next build (local), Other (Deployed)

Additional context

• The IDs appear in script tags in the HTML and corresponding locations in TXT files
• The purpose of these IDs is not immediately clear from the build output or documentation
• The IDs are 21 characters long and contain alphanumeric characters plus - and _
• Each page gets its own unique ID, but that ID is shared between its HTML and TXT versions
• While IDs are consistent between HTML and TXT files within the same build, they change completely on subsequent builds
• This affects build reproducibility, cache invalidation, and deployment comparisons

I've spent several days trying to track down the source of these non-deterministic IDs in Next.js/React RSC source code without success. If the Next.js team could point to where this ID generation happens, I'd be happy to work on a fix and submit a PR.

Questions

1. What is the purpose of these IDs?
2. Is their non-deterministic nature intentional?
3. If they need to be unique per build, could this be documented?
4. Is there a way to make these IDs static/deterministic between builds?
5. If not, what's the technical reason preventing deterministic ID generation, given that even generateBuildId doesn't affect these IDs?