circleci config validate fails with "config compilation request returned an error: Not Found" on CircleCI Server

TL;DR

What actually fixes it (verified in the thread): The cause is a version mismatch: newer CLI versions validate by calling a config compilation endpoint that older CircleCI Server releases do not expose, so the server answers Not Found.

Steps

1. Running circleci config validate against a GitHub Enterprise / CircleCI Server project failed with: Error: config compilation request returned an error: Not Found. The config itself was fine; the CLI (newer versions) calls a config-compilation service endpoint on the server, and the reporter's self-hosted server version did not have that endpoint exposed yet, so the request 404'd with an unhelpful message.
2. What actually fixes it (verified in the thread):
3. The cause is a version mismatch: newer CLI versions validate by calling a config compilation endpoint that older CircleCI Server releases do not expose, so the server answers Not Found. Either update the CircleCI Server instance to a version that includes the endpoint, or pin the CLI to an older release that validates without it (a maintainer confirmed this diagnosis with the reporter). The CLI was also updated to surface the underlying reason explicitly instead of the bare Not Found error.

Expected: Either update the CircleCI Server instance to a version that includes the endpoint, or pin the CLI to an older release that validates without it (a maintainer confirmed this diagnosis with the reporter).

When to use

You are seeing this: The situation: Running circleci config validate against a GitHub Enterprise / CircleCI Server project failed with: Error: config compilation request returned an error: Not Found. Use this skill when you run into "circleci config validate fails with "config compilation request returned an error: Not Found" on CircleCI Server".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

Someone asked: Running circleci config validate against a GitHub Enterprise / CircleCI Server project failed with: Error: config compilation request returned an error: Not Found. The config itself was fine; the CLI (newer versions) calls a config-compilation service endpoint on the server, and the reporter's self-hosted server version did not have that endpoint exposed yet, so the request 404'd with an unhelpful message. The verified answer: The cause is a version mismatch: newer CLI versions validate by calling a config compilation endpoint that older CircleCI Server releases do not expose, so the server answers Not Found. Either update the CircleCI Server instance to a version that includes the endpoint, or pin the CLI to an older release that validates without it (a maintainer confirmed this diagnosis with the reporter). The CLI was also updated to surface the underlying reason explicitly instead of the bare Not Found error. Full thread: https://vectle.com/threads/thrlM0E2KcALR2hfYLxopDEw