astrid-runtime/astrid · error

MCP attach registration has an empty host

Error message

MCP attach registration has an empty host

What it means

validate_registration requires registration.host to be a non-empty, non-blank string. The gateway routes attach bytes per host/project, so an empty host makes the registration unusable and it is rejected before any slot is reserved.

Source

Thrown at crates/astrid-cli/src/commands/mcp/gateway.rs:928

            if control.pid == 0 || control.hook_token.trim().is_empty() {
                anyhow::bail!("MCP gateway control authority is incomplete");
            }
        },
        GatewayRequest::Attach(registration) => validate_registration(registration)?,
    }
    Ok(request)
}

fn validate_registration(registration: &AttachRegistration) -> Result<()> {
    if registration.version != ATTACH_REGISTRATION_VERSION {
        anyhow::bail!(
            "unsupported MCP attach registration version {}",
            registration.version
        );
    }
    super::lifecycle::resolve_principal(Some(&registration.principal))?;
    if registration.host.trim().is_empty() {
        anyhow::bail!("MCP attach registration has an empty host");
    }
    if registration.host_session_id.trim().is_empty() {
        anyhow::bail!("MCP attach registration has an empty host_session_id");
    }
    if registration.hook_token.trim().is_empty() {
        anyhow::bail!("MCP attach registration is missing hook_token");
    }
    validate_workspace(&registration.workspace_abs)?;
    Ok(())
}

fn authenticate_registration(
    registration: &AttachRegistration,
    state: &GatewayState,
) -> Result<astrid_core::PrincipalId> {
    let principal = super::lifecycle::resolve_principal(Some(&registration.principal))?;
    if principal != state.principal {
        anyhow::bail!(

View on GitHub (pinned to affd8760f4)

Solutions

  1. Ensure the attaching client sets host from a valid source (hostname / project root) before serializing the registration; trim it and check non-empty first.
  2. Run `mcp attach` inside a recognized workspace/project directory so host context is detected.
  3. Check that environment variables or config supplying the host value are present and non-blank in the attach client's environment.
  4. If host should default to the machine hostname, fix the client to fall back to hostname::get() instead of an empty literal.

Example fix

// before
let host = std::env::var("ASTRID_HOST").unwrap_or_default();
// after
let host = std::env::var("ASTRID_HOST")
    .ok()
    .filter(|h| !h.trim().is_empty())
    .or_else(|| hostname::get().ok().map(|h| h.to_string_lossy().into_owned()))
    .context("host is required")?;
Defensive patterns

Strategy: validation

Validate before calling

fn host_ok(host: &str) -> bool { !host.trim().is_empty() }

Type guard

fn non_blank(s: &str) -> Option<&str> {
    let t = s.trim();
    if t.is_empty() { None } else { Some(t) }
}

Try / catch

match result {
    Err(e) if e.to_string().contains("empty host") => eprintln!("registration.host must be a non-empty hostname/project identifier"),
    other => other?,
}

Prevention

When it happens

Trigger: An attach registration whose host field is "" or whitespace-only, checked in validate_registration right after resolve_principal.

Common situations: An integration layer that fails to detect the current project/host and passes an empty string; environment variables like hostname/project context missing when the client built the registration; CLI invocations run outside a recognized workspace so host resolves to empty.

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

AI-assisted analysis of astrid-runtime/astrid@affd8760f4 (2026-09-09). Data as JSON: /api/errors/a01aa11c886e72bd. Report an issue: GitHub.