diff --git a/src/vs/server/node/serverConnectionToken.ts b/src/vs/server/node/serverConnectionToken.ts
index 163597daca3ff..769ca56285aeb 100644
--- a/src/vs/server/node/serverConnectionToken.ts
+++ b/src/vs/server/node/serverConnectionToken.ts
@@ -85,7 +85,7 @@ export async function parseServerConnectionToken(args: ServerParsedArgs, default
 		}
 
 		if (!connectionTokenRegex.test(rawConnectionToken)) {
-			return new ServerConnectionTokenParseError(`The connection token defined in '${connectionTokenFile} does not adhere to the characters 0-9, a-z, A-Z or -.`);
+			return new ServerConnectionTokenParseError(`The connection token defined in '${connectionTokenFile} does not adhere to the characters 0-9, a-z, A-Z, _, or -.`);
 		}
 
 		return new MandatoryServerConnectionToken(rawConnectionToken);
@@ -93,7 +93,7 @@ export async function parseServerConnectionToken(args: ServerParsedArgs, default
 
 	if (typeof connectionToken !== 'undefined') {
 		if (!connectionTokenRegex.test(connectionToken)) {
-			return new ServerConnectionTokenParseError(`The connection token '${connectionToken} does not adhere to the characters 0-9, a-z, A-Z or -.`);
+			return new ServerConnectionTokenParseError(`The connection token '${connectionToken} does not adhere to the characters 0-9, a-z, A-Z, _, or -.`);
 		}
 
 		if (compatibility) {
diff --git a/src/vs/server/node/serverEnvironmentService.ts b/src/vs/server/node/serverEnvironmentService.ts
index 432344f146a1e..45b04ba2a0e01 100644
--- a/src/vs/server/node/serverEnvironmentService.ts
+++ b/src/vs/server/node/serverEnvironmentService.ts
@@ -100,7 +100,7 @@ export interface ServerParsedArgs {
 
 	/**
 	 * A secret token that must be provided by the web client with all requests.
-	 * Use only `[0-9A-Za-z\-]`.
+	 * Use only `[0-9A-Za-z_-]`.
 	 *
 	 * By default, a UUID will be generated every time the server starts up.
 	 *
@@ -113,7 +113,7 @@ export interface ServerParsedArgs {
 	 * A path to a filename which will be read on startup.
 	 * Consider placing this file in a folder readable only by the same user (a `chmod 0700` directory).
 	 *
-	 * The contents of the file will be used as the connection token. Use only `[0-9A-Z\-]` as contents in the file.
+	 * The contents of the file will be used as the connection token. Use only `[0-9A-Za-z_-]` as contents in the file.
 	 * The file can optionally end in a `\n` which will be ignored.
 	 *
 	 * This secret must be communicated to any vscode instance via the resolver or embedder API.
diff --git a/src/vs/workbench/api/common/extHostTypes.ts b/src/vs/workbench/api/common/extHostTypes.ts
index f045831327431..5f41e8b8b9c96 100644
--- a/src/vs/workbench/api/common/extHostTypes.ts
+++ b/src/vs/workbench/api/common/extHostTypes.ts
@@ -492,7 +492,7 @@ export class ResolvedAuthority {
 			throw illegalArgument('port');
 		}
 		if (typeof connectionToken !== 'undefined') {
-			if (typeof connectionToken !== 'string' || connectionToken.length === 0 || !/^[0-9A-Za-z\-]+$/.test(connectionToken)) {
+			if (typeof connectionToken !== 'string' || connectionToken.length === 0 || !/^[0-9A-Za-z_\-]+$/.test(connectionToken)) {
 				throw illegalArgument('connectionToken');
 			}
 		}