golang/go · error

internal error: build ID length mismatch %q vs %q

Error message

internal error: build ID length mismatch %q vs %q

What it means

In updateBuildID, after computing a content-hash-based new ID by replacing the suffix of a.buildID, the code asserts len(newID)==len(a.buildID) because build IDs are rewritten in-place inside the binary (the old ID is overwritten byte-for-byte). A length mismatch is an internal invariant violation — it should never happen with well-formed inputs.

Source

Thrown at src/cmd/go/internal/work/buildid.go:722

				cache.PutBytes(c, cache.Subkey(a1.actionID, "link-stdout"), a.output)
				break
			}
		}
	}

	// Find occurrences of old ID and compute new content-based ID.
	r, err := os.Open(target)
	if err != nil {
		return err
	}
	matches, hash, err := buildid.FindAndHash(r, a.buildID, 0)
	r.Close()
	if err != nil {
		return err
	}
	newID := a.buildID[:strings.LastIndex(a.buildID, buildIDSeparator)] + buildIDSeparator + buildid.HashToString(hash)
	if len(newID) != len(a.buildID) {
		return fmt.Errorf("internal error: build ID length mismatch %q vs %q", a.buildID, newID)
	}

	// Replace with new content-based ID.
	a.buildID = newID
	if a.json != nil {
		a.json.BuildID = a.buildID
	}
	if len(matches) == 0 {
		// Assume the user specified -buildid= to override what we were going to choose.
		return nil
	}

	// Replace the build id in the file with the content-based ID.
	w, err := os.OpenFile(target, os.O_RDWR, 0)
	if err != nil {
		return err
	}
	err = buildid.Rewrite(w, matches, newID)

View on GitHub (pinned to b6b368adc5)

Solutions

1. Run 'go clean -cache' — a stale/corrupt cache entry can carry a malformed build ID.
2. Rebuild from scratch with a released Go toolchain.
3. If reproducing on a released toolchain, file a Go bug with the binary and build ID values from the error.
4. Avoid manually editing build IDs in compiled artifacts.

Example fix

// before — internal error after cache corruption
$ go build ./...
// error: internal error: build ID length mismatch "abc-X/Y" vs "abc-Z"

// after
$ go clean -cache
$ go build ./...

When it happens

Trigger: Effectively unreachable in normal use. It would fire if buildid.HashToString returned a string of unexpected length, or if a.buildID lacked the buildIDSeparator. Indicates memory corruption, a malformed build ID, or a regression in buildid.HashToString.

Common situations: A custom toolchain that injects a non-standard build ID format; a Go toolchain regression; a tampered binary where the build ID was edited to change its length.

Related errors

• loading compiled Go files from cache: %w
• too long
• file is empty
• entry file incomplete
• invalid header

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/aa8ce9163ac07490. Report an issue: GitHub.