feat(mcp): facade-only run mode — adapter-less config with [mcp] is valid (#1453)

Closes #1451. An adapter-less config with [mcp] present now runs just the
OAB MCP Facade listener in the foreground (one entrypoint, config-driven)
instead of bailing with 'no adapter configured'. The bail message now
mentions the [mcp] option. Removes the openab-agent mcp-facade
subcommand — serving surfaces do not belong on the coding-agent binary
(nothing shipped depends on it; #1449 established the openab-side home).

Co-authored-by: chaodu-agent <chaodu-agent@users.noreply.github.com>

Author: chaodu-agent

config.toml.example

@@ -387,6 +387,9 @@ error_hold_ms = 2500
 # ~/.openab/agent/mcp.json. Any coding CLI on this host (Kiro, Claude Code,
 # Codex, ...) can connect to http://127.0.0.1:8848/mcp.
 # Presence of [mcp] enables the listener; omit the section to disable.
+# With no chat adapter configured, [mcp] alone is a valid config: openab
+# runs in facade-only mode (serves just the MCP listener) — for
+# coding-CLI-only hosts, dev loops, and CI runners (#1451).
 # NOTE: the endpoint has no authentication — it must stay on loopback
 # (non-loopback addresses are refused at startup).
 # [mcp]

openab-agent/src/main.rs

@@ -29,17 +29,6 @@ enum Commands {
         #[command(subcommand)]
         action: McpAction,
     },
-    /// Serve the OAB MCP Facade over loopback Streamable HTTP: an inbound
-    /// MCP server exposing `search_capabilities` / `execute_capability`
-    /// backed by the configured downstream MCP servers (global + project
-    /// mcp.json). The OAB broker hosts the same facade in-process when
-    /// `[mcp]` is set in config.toml; this subcommand is the standalone way
-    /// to run it.
-    McpFacade {
-        /// Loopback address to listen on (non-loopback addresses are refused).
-        #[arg(long, default_value = "127.0.0.1:8848")]
-        listen: String,
-    },
 }
 
 #[derive(Subcommand)]
@@ -171,12 +160,6 @@ async fn main() {
                 }
             }
         },
-        Some(Commands::McpFacade { listen }) => {
-            if let Err(e) = mcp::facade::serve_http(&listen).await {
-                eprintln!("mcp-facade: {e:#}");
-                std::process::exit(1);
-            }
-        }
     }
 }
 

src/main.rs

@@ -384,8 +384,25 @@ async fn main() -> anyhow::Result<()> {
         && cfg.telegram.is_none()
         && !has_unified_platform(&cfg)
     {
+        // Facade-only run mode (#1451): an adapter-less config with `[mcp]`
+        // present is a valid deployment — the broker serves just the OAB MCP
+        // Facade listener. One entrypoint, config-driven: hosts that only
+        // need the capability surface (coding-CLI-only users, dev loops, CI
+        // runners, agent hosts with no chat platform) run the same
+        // `openab run` with a two-line config instead of a chat token.
+        if let Some(mcp_cfg) = cfg.mcp.clone() {
+            tracing::info!(
+                listen = %mcp_cfg.listen,
+                "no chat adapter configured — running in facade-only mode ([mcp] present)"
+            );
+            // Foreground, not spawned: the facade IS the workload. A bind
+            // failure or server exit terminates the process (fail fast).
+            return openab_mcp::mcp::facade::serve_http(&mcp_cfg.listen)
+                .await
+                .map_err(|e| anyhow::anyhow!("OAB MCP facade exited: {e:#}"));
+        }
         anyhow::bail!(
-            "no adapter configured — add [discord], [slack], [telegram], [wecom], [googlechat], or [gateway] to config, or set platform env vars (TELEGRAM_BOT_TOKEN, etc.)"
+            "no adapter configured — add [discord], [slack], [telegram], [wecom], [googlechat], or [gateway] to config (or [mcp] for facade-only mode), or set platform env vars (TELEGRAM_BOT_TOKEN, etc.)"
         );
     }