//! Builds a subagent's own filtered `--mcp-config`: only
//! `hyperhive.extraMcpServers` entries with `availableToSubagents = true`
//! (see `hive_agent_sock::extra_mcp::ExtraMcpServer::available_to_subagents`) ever reach
//! a subagent's claude invocation. Everything else — the built-in hyperhive
//! surface (todos/messaging), the automatically injected `bash` and `subagent`
//! entries — stays unreachable by construction: none of those default to
//! opted in, and the built-in surface isn't an `extraMcpServers` entry at
//! all, so there's no name for an operator to opt it in under even if they
//! wanted to.

use std::path::PathBuf;

/// Filename the rendered config lives at, under [`crate::paths::harness_dir`].
const CONFIG_FILE: &str = "subagent-mcp-config.json";

/// Render the subagent-eligible extra-MCP servers to a `--mcp-config` file,
/// returning its path — or `None` when no entry opts in (or the render/write
/// fails), so [`hive_claude::Config::mcp_config`] stays unset and the
/// subagent gets literally zero MCP servers, the same default as before this
/// toggle existed. Re-rendered on every call (cheap: a filter plus a small
/// file write) rather than cached once at daemon startup, so a config change
/// takes effect on this subagent's next `start`/`continue` without needing
/// the daemon itself restarted.
#[must_use]
pub fn build() -> Option<PathBuf> {
    let state_dir = crate::paths::state_dir();
    let servers: serde_json::Map<String, serde_json::Value> =
        hive_agent_sock::extra_mcp::load_extra_mcp()
            .into_iter()
            .filter(|(_, spec)| spec.available_to_subagents())
            .map(|(name, spec)| (name, spec.to_json_entry(&state_dir)))
            .collect();
    if servers.is_empty() {
        return None;
    }
    let body = serde_json::to_string_pretty(&serde_json::json!({ "mcpServers": servers }))
        .unwrap_or_else(|_| "{}".into());
    let dir = crate::paths::harness_dir();
    if let Err(e) = std::fs::create_dir_all(&dir) {
        tracing::warn!(
            error = ?e,
            dir = %dir.display(),
            "subagent mcp-config: harness dir create failed; subagent gets zero extra MCP servers this turn",
        );
        return None;
    }
    let path = dir.join(CONFIG_FILE);
    if let Err(e) = std::fs::write(&path, body) {
        tracing::warn!(
            error = ?e,
            path = %path.display(),
            "subagent mcp-config: write failed; subagent gets zero extra MCP servers this turn",
        );
        return None;
    }
    Some(path)
}