error An unexpected error occurred: "http://registry.npmjs.org/@pepperhq/hmac-http-authentication/-/hmac-http-authentication-0.1.2.tgz: Request failed \"404 Not Found\"".
As I stated previously, the other private repositories (all within the same @ scope) download and install as intended. This leads me to think this isn't an authentication issue.
My .npmrc which is in the root of my project and contains my _authToken looks like this:
//registry.npmjs.org/:_authToken=TOKEN_HERE
I have already completed the following activities:
Logging in using npm login again to get a fresh accessToken
Ensuring the package name is correct (I copied it from the npm website just to be sure)
Running rm -rf node_modules && rm yarn.lock && yarn cache clean
Removing the offending package from my package.json and attempting to re yarn add it
Attempting to run the equivalent npm install --save command. This worked.
If the current behavior is a bug, please provide the steps to reproduce.

As I am unsure of the cause I am unsure how to reproduce. Am willing to discuss further on Discord or in comments on this Issue.
What is the expected behavior?

I expect Yarn to install all of my private repositories from npmjs
Please mention your node.js, yarn and operating system version.

Taken from yarn-error.log
Yarn version: 
  0.20.3
Node version: 
  6.9.5
Platform: 
  darwin x64
          
Having ran some more tests, I've found that the same package fails to install from other repositories that require it when using Yarn. However, it works fine when using npm.
This leads me to think it isn't my repository specific config. Could it be something to do with the name of the package tripping Yarn up, or the url encoding or something?
Clutching at straws here, but hopefully this might bump someone in the right direction
          
I fixed this: I simply re-published the offending package with a new version number.
I have no idea why that fixed it.
I will leave this here in case it aids debugging.
          
We just experienced the same issue. Some more context here in the hopes that it might help
We were running yarn in our CI (Drone), which runs in ephemeral Docker containers [Alpine Linux]
The issue occurred in only ONE of our services building, whereas nothing went wrong with the other services, even though they have the exact same dependency tree (!)
a manual install on my Mac for the same repo yielded no issues at all (!)
the issue was limited to a single private package
As mentioned above as workaround, publishing a new version of the package resolved the issue.
Unfortunately, I was unable to obtain a detailed yarn-error.log as the containers are destroyed immediately after a failed build. If anyone else has this issue, could you try it again with a verbose install and throw the log in here?
          
I am seeing the same issue.  I'm using a docker image in circleci 2.0 and having some unrelated issues that I'm debugging locally (compilation difference on cci vs local).
when I yarn on the host OSX machine, everything works.
when I yarn on the container (Ubuntu), I get the error.
when I yarn on the ^^container (Ubuntu) running on circleci 2.0 infrastructure, it works.
I have setup .npmrc etc.  It's really a head-scratcher that it doesn't work for me on a locally running container.
Verbose didn't provide me with any information that I can spot:
root@2adc7c09ff9a:~/af/spec/dummy# yarn --pure-lockfile --ignore-optional
yarn install v0.21.3
[1/4] Resolving packages...
[2/4] Fetching packages...
error An unexpected error occurred: "https://registry.yarnpkg.com/@alienfast/build/-/build-2.0.38.tgz: Request failed \"404 Not Found\"".
info If you think this is a bug, please open a bug report with the information provided in "/root/af/spec/dummy/yarn-error.log".
info Visit https://yarnpkg.com/en/docs/cli/install for documentation about this command.
root@2adc7c09ff9a:~/af/spec/dummy# yarn --pure-lockfile --ignore-optional --verbose
yarn install v0.21.3
verbose 0.178 current time: 2017-03-01T20:22:52.715Z
[1/4] Resolving packages...
[2/4] Fetching packages...
verbose 0.798 Performing "GET" request to "https://registry.yarnpkg.com/@alienfast/build/-/build-2.0.38.tgz".
verbose 0.861 Performing "GET" request to "https://registry.yarnpkg.com/@alienfast/ui/-/ui-1.0.34.tgz".
verbose 0.894 Performing "GET" request to "https://registry.yarnpkg.com/react-relay/-/react-relay-0.10.0.tgz".
verbose 0.895 Performing "GET" request to "https://registry.yarnpkg.com/react-dom/-/react-dom-15.4.2.tgz".
verbose 0.919 Performing "GET" request to "https://registry.yarnpkg.com/eslint/-/eslint-3.15.0.tgz".
verbose 0.941 Performing "GET" request to "https://registry.yarnpkg.com/flow-bin/-/flow-bin-0.39.0.tgz".
verbose 0.951 Performing "GET" request to "https://registry.yarnpkg.com/@alienfast/material-ui/-/material-ui-0.16.87.tgz".
verbose 0.966 Performing "GET" request to "https://registry.yarnpkg.com/@alienfast/react-formal/-/react-formal-0.24.7.tgz".
verbose 1.003 Performing "GET" request to "https://registry.yarnpkg.com/babel-runtime/-/babel-runtime-6.23.0.tgz".
verbose 1.103 Performing "GET" request to "https://registry.yarnpkg.com/graphql/-/graphql-0.9.1.tgz".
verbose 1.178 Performing "GET" request to "https://registry.yarnpkg.com/lodash/-/lodash-4.17.4.tgz".
verbose 1.206 Performing "GET" request to "https://registry.yarnpkg.com/material-ui/-/material-ui-0.16.7.tgz".
verbose 1.249 Performing "GET" request to "https://registry.yarnpkg.com/react-i18next/-/react-i18next-2.2.0.tgz".
verbose 1.261 Performing "GET" request to "https://registry.yarnpkg.com/react-relay-network-layer/-/react-relay-network-layer-1.4.0.tgz".
verbose 1.272 Performing "GET" request to "https://registry.yarnpkg.com/react-router/-/react-router-3.0.2.tgz".
verbose 1.696 Performing "GET" request to "https://registry.yarnpkg.com/react-router-relay/-/react-router-relay-0.13.5.tgz".
verbose 1.764 Performing "GET" request to "https://registry.yarnpkg.com/react-sizeme/-/react-sizeme-2.2.0.tgz".
verbose 2.063 Performing "GET" request to "https://registry.yarnpkg.com/recompose/-/recompose-0.22.0.tgz".
verbose 2.144 Error: https://registry.yarnpkg.com/@alienfast/build/-/build-2.0.38.tgz: Request failed "404 Not Found"
    at Request.handleRequestError (/root/.yarn/lib/fetchers/tarball-fetcher.js:231:20)
    at emitOne (events.js:96:13)
    at Request.emit (events.js:189:7)
    at Request.onRequestResponse (/root/.yarn/node_modules/request/request.js:986:10)
    at emitOne (events.js:96:13)
    at ClientRequest.emit (events.js:189:7)
    at HTTPParser.parserOnIncomingClient (_http_client.js:522:21)
    at HTTPParser.parserOnHeadersComplete (_http_common.js:99:23)
    at TLSSocket.socketOnData (_http_client.js:411:20)
    at emitOne (events.js:96:13)
    at TLSSocket.emit (events.js:189:7)
error An unexpected error occurred: "https://registry.yarnpkg.com/@alienfast/build/-/build-2.0.38.tgz: Request failed \"404 Not Found\"".
info If you think this is a bug, please open a bug report with the information provided in "/root/af/spec/dummy/yarn-error.log".
info Visit https://yarnpkg.com/en/docs/cli/install for documentation about this command.




    

root@2adc7c09ff9a:~/af/spec/dummy# cat ~/.npmrc
registry=https://registry.npmjs.org/
//registry.npmjs.org/:_authToken=${NPM_TOKEN}
root@2adc7c09ff9a:~/af/spec/dummy# echo $NPM_TOKEN
xxxxx-xxxx-xxxx-xxxx-xxxxxxxx
          
Same here, it works locally on OSX but not on the CircleCI container (circleci 1.0).
I tried to yarn cache clean - afterwards a different package failed installation.
It happens with yarn 0.18.1 as well as 0.21.3 - so maybe something changed in the yarnpkg registry configuration? Or, since I think everyone in the comments here is using CircleCI in the problem-scenario, it's an issue with CircleCI configuration?
          
So, we think we've managed to solve the problem; would be keen to see if it works for others:

We had to make a slight change to .npmrc which makes little sense why this works:
registry=http://registry.npmjs.org/
//registry.npmjs.org/:_authToken=${NPM_TOKEN}
@ourscope:registry=https://registry.npmjs.org/
Thing to note is that we required both normal registry AND one for our private scoped registry. Just make sure you change @ourscope with your own.
Let me know if this helps anyone.
          
@rogchap i just tried your solution but still see the same problem as before :(
we have a hyphen in our scope name, maybe that could be the culprit?
registry=http://registry.npmjs.org/
//registry.npmjs.org/:_authToken=${NPM_TOKEN}
@our-scope:registry=https://registry.npmjs.org/
          
@rogchap this workaround didn't have any effect on our situation.
root@f41305331cb8:~/af/spec/dummy# cat ~/.npmrc
registry=https://registry.npmjs.org/
//registry.npmjs.org/:_authToken=${NPM_TOKEN}
@alienfast:registry=https://registry.npmjs.org/
I just deployed new versions of the 'offending' scoped modules and it made no difference, same error.
I ran npm install and have no problems fetching the modules in the problem environment
          
So, not fixed for me...again.
I had another private module give me the 404 message, even though it seems to resolve the correct version (must be reading something correctly somewhere), and seems to try and download the same tar url as NPM.
This time I REMOVED the registries (except the auth one) in .npmrc to get it to work!
There is no consistency at all, randomly "playing" with .npmrc seems to temporarily work.
I'm trying to promote Yarn, but every engineer in our company (rightly) says that Yarn is "not stable", "un usable", "not reliable", "doesn't work with private modules", "fails on the CI server" etc etc.
I wonder if the Private NPM registry is doing something "funky" with the user-agent!?? <- Wild guess.
          
We don't use private packages much so this feature has less eyes on.

Afaik most of the issues were fixed by the community.

This is a great opportunity to try to fix it.

I don't think npm registry would do something with the response based on UA, most likely the token is not passed at all for some reason.
          
Okay, so I forked yarn and fiddled around a bit. It seems like it loses the token before it fetches the tarball or rather after the first request is sent...
yarn add v0.23.0-0
verbose 0.227 current time: 2017-03-06T20:25:18.877Z
info No lockfile found.
[1/4] Resolving packages...
⠁ { authorization: 'Bearer MY_TOKEN_IS_HERE' } <<<<<----- here it uses my token for the request
verbose 0.409 Performing "GET" request to "https://registry.npmjs.org/@ovos-media%2flib-director".
verbose 1.306 Request "https://registry.npmjs.org/@ovos-media%2flib-director" finished with status code 200.
{} <--- token is gone
verbose 1.316 Performing "GET" request to "https://registry.npmjs.org/bluebird".
{} <--- token is gone
verbose 1.318 Performing "GET" request to "https://registry.npmjs.org/jwt-decode".
verbose 1.531 Request "https://registry.npmjs.org/bluebird" finished with status code 200.
verbose 1.572 Request "https://registry.npmjs.org/jwt-decode" finished with status code 200.
[2/4] Fetching packages...
https: /@ovos-media/lib-director/-/lib-director-0.4.1.tgz https://registry.yarnpkg.com/@ovos-media/lib-director/-/lib-director-0.4.1.tgz
{} <--- token is gone
verbose 1.597 Performing "GET" request to "https://registry.yarnpkg.com/@ovos-media/lib-director/-/lib-director-0.4.1.tgz".
verbose 2.663 Error: https://registry.yarnpkg.com/@ovos-media/lib-director/-/lib-director-0.4.1.tgz: Request failed "404 Not Found"
    at Request.res (/home/ubuntu/yarn/lib/fetchers/tarball-fetcher.js:232:20)
    at emitOne (events.js:90:13)
    at Request.emit (events.js:182:7)
    at Request.onRequestResponse (/home/ubuntu/yarn/node_modules/request/request.js:986:10)
    at emitOne (events.js:90:13)
    at ClientRequest.emit (events.js:182:7)
    at HTTPParser.parserOnIncomingClient (_http_client.js:469:21)
    at HTTPParser.parserOnHeadersComplete (_http_common.js:103:23)
    at TLSSocket.socketOnData (_http_client.js:359:20)
    at emitOne (events.js:90:13)
error An unexpected error occurred: "https://registry.yarnpkg.com/@ovos-media/lib-director/-/lib-director-0.4.1.tgz: Request failed \"404 Not Found\"".
When I explicitly set the authorization header in the npm-registry.js before the request is sent, everything works fine. So there seems to be some bug in yarn and not on the server side.
EDIT:
Okay, so after further inspection, this problem occurs because the check whether the authorization header should be set returns false in npm-registry.js ->  https://github.com/yarnpkg/yarn/pull/2598/files#diff-b053bee294c216269844e5874039b6caR62

And this, in turn, happens because it compares the yarnpkg registry proxy with the npmjs registry.
https://registry.yarnpkg.com/@ovos-media/lib-director/-/lib-director-0.4.1.tgz https://registry.npmjs.org/
          
Okay, so i figured out how to fix this issue for us without a change to yarns source code.

It's basically what @rogchap posted but (in our case) in the other direction:
registry=https://registry.npmjs.org/
@ovos-media:registry=https://registry.yarnpkg.com/
//registry.npmjs.org/:_authToken=your-auth-token
OR alternatively completely getting rid of registry:
//registry.npmjs.org/:_authToken=your-auth-token
This seems to work because then yarn does not use different registries for the package in question to fetch info and tarballs. However, I'm unsure as to why yarn mixes these two and I think that maybe the check for the auth token could be changed/improved somehow.
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Ah, removing .yarnrc worked for me.  Contents were:
# THIS IS AN AUTOGENERATED FILE. DO NOT EDIT THIS FILE DIRECTLY.
# yarn lockfile v1
registry "https://registry.npmjs.org/"
email <email>
lastUpdateCheck 1489181224804
username <username>
note, the npmjs.org registry was something I had added when troubleshooting why it wasn't working, so it wasn't the offending config.
          
None of these workarounds worked for me.
Actually it works if I put this in my docker file:
RUN echo -e "registry=https://registry.npmjs.org/\n//registry.npmjs.org/:_authToken=\${NPM_AUTH_TOKEN}\nsatoshipay:registry=https://registry.npmjs.org/" > /usr/src/app/.npmrc
RUN yarn
The scope in question for me is @satoshipay
          
Seeing this error on our project. It is bizarre, since I don't get a 404 when hitting the URL manually:
Trace:
  Error: https://codeload.github.com/types/npm-v8flags/tar.gz/de224ae1cd5fd7dbb4e7158a6cc7a29e5315930d: Request failed "404 Not Found"
      at Request.handleRequestError (C:\Users\stewx\AppData\Roaming\nvm\v6.10.0\node_modules\yarn\lib\fetchers\tarball-fetcher.js:203:20)
      at emitOne (events.js:96:13)
      at Request.emit (events.js:188:7)
      at Request.onRequestResponse (C:\Users\stewx\AppData\Roaming\nvm\v6.10.0\node_modules\yarn\node_modules\request\request.js:1068:10)
      at emitOne (events.js:96:13)
      at ClientRequest.emit (events.js:188:7)
      at HTTPParser.parserOnIncomingClient (_http_client.js:474:21)
      at HTTPParser.parserOnHeadersComplete (_http_common.js:99:23)
      at TLSSocket.socketOnData (_http_client.js:363:20)
      at emitOne (events.js:96:13)
          
We have solved all errors like these by switching our URLs a little. Avoid tar/gz and use tarball, i.e.:
"package_name":"https://github.com/somebody/package_name/tarball/master"
          
I'm exhibiting this today

error An unexpected error occurred: “https://registry.yarnpkg.com/@types/webpack/-/webpack-3.8.10.tgz: Request failed \“404 Not Found\“”.

Not a private repo :(
          
In my case I had a local .npmrc that defined a private registry for a specific scope, all I had to add was registry=https://registry.npmjs.org/ at the top of the file. Resulting in:
# .npmrc
registry=https://registry.npmjs.org/
@acme:registry=https://acme.jfrog.io/acme/api/npm/npm/
//acme.jfrog.io/acme/api/npm/npm/:_auth=YYYYYYYYYYYYYYYYYYYYYYYYYY
//acme.jfrog.io/acme/api/npm/npm/:username=username
//acme.jfrog.io/acme/api/npm/npm/:[email protected]
//acme.jfrog.io/acme/api/npm/npm/:always-auth=true
removing yarn.lock
This worked for me. Digging some more, discovered there were bad urls in the yarn lock.
The yarn lock was using registry.yarnpkg.com that would 404'd on CircleCI (no clue why it worked locally).
The resulting config that did the trick:
- run: echo "//registry.npmjs.org/:_authToken=$NPM_AUTH_TOKEN" > ~/.npmrc
- run: yarn config set registry https://registry.npmjs.org/
- run: yarn
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Update nps and nps-utils dependencies because those version depend on event-stream, which is unsafe and was removed from npm.
yarnpkg/yarn#2738
      
..that is often seen on Windows like this:
An unexpected error occurred: "https://registry.yarnpkg.com/@private/ngffwd-node-processes/-/ngffwd-node-processes-1.0.123.tgz: Request failed "404 Not Found".
re yarnpkg/yarn#2738
          
have the same issue with yarn + CI + material
yarn install v1.22.4
[1/4] Resolving packages...
[2/4] Fetching packages...
error An unexpected error occurred: "https://registry.yarnpkg.com/@material-ui/core/-/core-4.11.0.tgz: Request failed \"404 Not Found\"".
info If you think this is a bug, please open a bug report with the information provided in "/builds/springs-apps/flopanda/flopanda-front-end/yarn-error.log".
info Visit https://yarnpkg.com/en/docs/cli/install for documentation about this command.
          
was facing the same issue, fixed it with  yarn config set registry https://registry.npmjs.org + yarn cache clean + yarn install
my .npmrc (repository) looks like this:
@example:registry=https://registry.yarnpkg.com/ //registry.npmjs.org/:_authToken=${TOKEN}
Moreover, I noticed that yarn config list was showing an _authToken in the info npm config that did not match the one I intended to use (different NPM account). So in /C/Users/PC/.npmrc I replaced the default token with the correct one and it worked
          
We had a gitlab private package registry, and was constantly getting this error.

npm i @placeholder/package-name works fine, but yarn add @placeholder/package-name was giving 404 errors.
I tried all possible solutions listed on this thread. At last, there was a section in gitlab on troubleshooting yarn installation [1].

It said,
try adding this to your .npmrc file (and replace <your_token> with your personal access token or deploy token):
//gitlab.example.com/api/v4/projects/:_authToken=<your_token>
Our previous .npmrc looked like this
save-exact=true
unsafe-perm=true
@placeholder:registry=https://gitlab.com/api/v4/packages/npm/
//gitlab.com/api/v4/packages/npm/:_authToken=${PLACEHOLDER_TOKEN}
With the above new line addition, it looked like this,
save-exact=true
unsafe-perm=true
@placeholder:registry=https://gitlab.com/api/v4/packages/npm/
//gitlab.com/api/v4/packages/npm/:_authToken=${PLACEHOLDER_TOKEN}
//gitlab.com/api/v4/projects/:_authToken=${PLACEHOLDER_TOKEN}
With that addition, yarn add @placeholder/package-name worked fine. 😇
[1] https://docs.gitlab.com/ee/user/packages/npm_registry/#error-running-yarn-with-the-package-registry-for-npm-registry
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