fix(harness): keep MCP startup logs and forget failed commands on time

- Write server log messages on the service's fibers, so a failed handshake closing the
  connection no longer drops the messages that explain it.
- Forget a failed !command as soon as it fails, so the next connect runs it again even
  when every caller waiting on it gave up.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: sanchitrk

packages/harness/src/mcp/mcp.ts

@@ -233,7 +233,9 @@ export const layer = (options: { readonly hostCwd: string; readonly home: string
 							endpoint: target,
 							...(server.roots === undefined ? {} : { roots: server.roots }),
 							timeout: { request: Duration.seconds(config.timeout ?? DEFAULT_TIMEOUT_SECONDS) },
-							onLog: (entry) => log.write(server.name, entry),
+							// Written on the service's fibers: a failed handshake closes the connection's, and
+							// its last messages usually say why.
+							onLog: (entry) => Effect.sync(() => fork(log.write(server.name, entry))),
 						}).pipe(Scope.provide(scope));
 						const tools = yield* connection.tools();
 						return { connection, tools };

packages/harness/src/mcp/value.ts

@@ -101,15 +101,17 @@ const run = (command: string) =>
 const command = Effect.fnUntraced(function* (line: string, what: string) {
 	let running = commands.get(line);
 	if (running === undefined) {
-		// Detached, so a caller giving up does not stop it for the others waiting on it.
-		running = yield* Effect.forkDetach(run(line));
+		// Detached, so a caller giving up does not stop it for the others waiting on it. A failure is
+		// forgotten when it happens, whether or not anyone still waits.
+		const forget = (output: string | undefined) =>
+			Effect.sync(() => {
+				if (output === undefined && commands.get(line) === running) commands.delete(line);
+			});
+		running = yield* Effect.forkDetach(Effect.andThen(Effect.yieldNow, run(line).pipe(Effect.tap(forget))));
 		commands.set(line, running);
 	}
 	const output = yield* Fiber.join(running);
-	if (output === undefined) {
-		if (commands.get(line) === running) commands.delete(line);
-		return yield* Effect.fail(`Failed to resolve ${what} from shell command: ${line}`);
-	}
+	if (output === undefined) return yield* Effect.fail(`Failed to resolve ${what} from shell command: ${line}`);
 	return output;
 });