What causes "MCP attach host is empty" in astrid-runtime/astrid?

Pass an explicit host: `aos mcp attach --host `

MCP attach host is empty

Error message

MCP attach host is empty

What it means

build_registration_with_context validates the host string before constructing the AttachRegistration. An all-whitespace or empty host would yield a registration the gateway cannot route, so it is rejected with this error.

Source

Thrown at crates/astrid-cli/src/commands/mcp/attach.rs:113

    let host_session_id = std::env::var("ASTRID_SESSION_ID")
        .or_else(|_| std::env::var("AOS_MCP_SESSION_ID"))
        .or_else(|_| std::env::var("MCP_SESSION_ID"))
        .context("MCP attach requires the host session key in ASTRID_SESSION_ID")?;
    if host_session_id.trim().is_empty() {
        anyhow::bail!("MCP attach host session key is empty");
    }
    build_registration_with_context(caller, workspace, ready, &host, &host_session_id)
}

fn build_registration_with_context(
    caller: &astrid_core::PrincipalId,
    workspace: Option<&Path>,
    ready: &super::lifecycle::GatewayReady,
    host: &str,
    host_session_id: &str,
) -> Result<AttachRegistration> {
    if host.trim().is_empty() {
        anyhow::bail!("MCP attach host is empty");
    }
    if host_session_id.trim().is_empty() {
        anyhow::bail!("MCP attach host session key is empty");
    }
    let workspace_abs = absolute_workspace(workspace)?;
    Ok(AttachRegistration {
        version: ATTACH_REGISTRATION_VERSION,
        principal: caller.to_string(),
        host: host.to_owned(),
        workspace_abs: workspace_abs.to_string_lossy().into_owned(),
        host_session_id: host_session_id.to_owned(),
        hook_token: ready.hook_token.clone(),
    })
}

/// Copy both directions until either side closes.
async fn proxy_stdio(stream: UnixStream) -> Result<()> {
    let (mut gateway_read, mut gateway_write) = tokio::io::split(stream);

View on GitHub (pinned to affd8760f4)

Solutions

Pass an explicit host: `aos mcp attach --host <host>`
Fix the config/env that supplies an empty host default
Trim whitespace from the value before invoking

Example fix

// before
$ aos mcp attach --host "$HOST"   # HOST=""
// after
$ aos mcp attach --host "localhost:8443"

Defensive patterns

Strategy: validation

Validate before calling

let host = cli_host.or(env_host).unwrap_or_default();
if host.trim().is_empty() { return Err("MCP attach host must be a non-empty value"); }

Type guard

fn valid_host(h: &str) -> bool { !h.trim().is_empty() }

Try / catch

match build_registration(&caller, &ws, &ready, &host).await {
    Err(e) if e.to_string().contains("host is empty") => eprintln!("supply --host"),
    other => other,
}

Prevention

Always pass --host explicitly in scripts
Validate config files for empty host fields
Default host values should never resolve to empty strings

When it happens

Trigger: `aos mcp attach` where the resolved --host value (or its default, e.g. from env/config) is "" or whitespace.

Common situations: Config file with host=""; a script passing an empty variable as --host; template expansion that dropped the host value.

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/9cc27e0befa3e009. Report an issue: GitHub.