What causes "could not parse host as URL: %s" in github/github-mcp-server?

Trim whitespace and newlines from the env value before use

could not parse host as URL: %s

Error message

could not parse host as URL: %s

What it means

parseAPIHost feeds the configured host string (empty means github.com) to url.Parse and this is the raw parse failure. It only fires on syntactically broken URLs - control characters, spaces, or invalid percent-encoding - because almost any hostname-shaped string parses fine and fails the later scheme checks instead.

Source

Thrown at pkg/utils/api.go:239

	resp, err := client.Get(subdomainURL)
	if err != nil {
		return false
	}
	defer resp.Body.Close()

	return resp.StatusCode == http.StatusOK
}

// Note that this does not handle ports yet, so development environments are out.
func parseAPIHost(s string) (APIHost, error) {
	if s == "" {
		return newDotcomHost()
	}

	u, err := url.Parse(s)
	if err != nil {
		return APIHost{}, fmt.Errorf("could not parse host as URL: %s", s)
	}

	if u.Scheme == "" {
		return APIHost{}, fmt.Errorf("host must have a scheme (http or https): %s", s)
	}

	// Enforce HTTPS centrally so no deployment (GHES in particular) can build
	// authenticated REST/GraphQL/upload/raw URLs over cleartext http, which
	// would leak the bearer token/PAT to anyone on the network.
	if err := requireSecureScheme(u); err != nil {
		return APIHost{}, err
	}

	switch classifyHost(u) {
	case HostTypeDotcom:
		return newDotcomHost()
	case HostTypeGHEC:
		return newGHECHost(s)

View on GitHub (pinned to 0ea1f775a7)

Solutions

Trim whitespace and newlines from the env value before use
Re-enter the value without stray characters and confirm it starts with https://
Log the exact (non-secret) host value at startup to spot invisible characters

Example fix

# before (trailing space from env_file)
GITHUB_HOST='https://github.example.com '

# after
GITHUB_HOST=https://github.example.com

Defensive patterns

Strategy: validation

Validate before calling

func validAPIHost(s string) error {
	s = strings.TrimSpace(s)
	if s == "" {
		return nil // dotcom default
	}
	u, err := url.Parse(s)
	if err != nil {
		return fmt.Errorf("unparseable host %q: %w", s, err)
	}
	if u.Scheme == "" {
		return fmt.Errorf("host %q must include an https:// scheme", s)
	}
	return nil
}

Prevention

Trim env-sourced host strings before use
Validate GITHUB_HOST once at process start and fail fast showing the offending value (it is non-secret)
Quote values in env_file/compose files to avoid stray whitespace

When it happens

Trigger: GITHUB_HOST / --github-host containing a space ('https://github. example.com'), a trailing newline from a docker env_file or Kubernetes secret, or stray control characters.

Common situations: env_file entries with trailing whitespace; k8s secrets storing values with a trailing \n; shell quoting mistakes injecting spaces.

Related errors

AI-assisted analysis of github/github-mcp-server@0ea1f775a7 (2026-08-15). Data as JSON: /api/errors/6aa665042d548ebd. Report an issue: GitHub.