From ec55fe64f12f3431e0349def955898dc2a72fbeb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?T=C3=82CHES?= <afromanguy@me.com>
Date: Fri, 20 Mar 2026 10:39:15 -0600
Subject: [PATCH] fix: resolve ${VAR} env references in MCP client .mcp.json
 configs (#1609)

The MCP client passed raw "${VAR}" strings to child processes instead of
resolving them against process.env, breaking MCP servers that expect
resolved environment variable values.

Adds a resolveEnv() helper that interpolates ${VAR} patterns in env
config values before passing them to StdioClientTransport.

Closes #1599

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
---
 src/resources/extensions/mcp-client/index.ts | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/src/resources/extensions/mcp-client/index.ts b/src/resources/extensions/mcp-client/index.ts
index bdb60c2ed..904fbbcb4 100644
--- a/src/resources/extensions/mcp-client/index.ts
+++ b/src/resources/extensions/mcp-client/index.ts
@@ -114,6 +114,22 @@ function getServerConfig(name: string): McpServerConfig | undefined {
 	return readConfigs().find((s) => s.name === name);
 }
 
+/** Resolve ${VAR} references in env values against process.env. */
+function resolveEnv(env: Record<string, string>): Record<string, string> {
+	const resolved: Record<string, string> = {};
+	for (const [key, value] of Object.entries(env)) {
+		if (typeof value === "string") {
+			resolved[key] = value.replace(
+				/\$\{([^}]+)\}/g,
+				(_match, varName) => process.env[varName] ?? "",
+			);
+		} else {
+			resolved[key] = value;
+		}
+	}
+	return resolved;
+}
+
 async function getOrConnect(name: string, signal?: AbortSignal): Promise<Client> {
 	const existing = connections.get(name);
 	if (existing) return existing.client;
@@ -128,7 +144,7 @@ async function getOrConnect(name: string, signal?: AbortSignal): Promise<Client>
 		transport = new StdioClientTransport({
 			command: config.command,
 			args: config.args,
-			env: config.env ? { ...process.env, ...config.env } as Record<string, string> : undefined,
+			env: config.env ? { ...process.env, ...resolveEnv(config.env) } as Record<string, string> : undefined,
 			cwd: config.cwd,
 			stderr: "pipe",
 		});