Use env var for scenario name instead of positional arg

Pass scenario name via MCP_CONFORMANCE_SCENARIO env var instead of as a
positional argument. This is non-breaking for existing clients that just
accept <server-url>.

The env var approach is consistent with how context (including private
keys for JWT auth) is already passed via MCP_CONFORMANCE_CONTEXT.

Author: pcarleton

README.md

@@ -67,7 +67,7 @@ npx @modelcontextprotocol/conformance client --command "<client-command>" --scen
 - `--timeout` - Timeout in milliseconds (default: 30000)
 - `--verbose` - Show verbose output
 
-The framework appends `<scenario-name> <server-url>` as arguments to your command. Your client should accept these two positional arguments.
+The framework appends `<server-url>` as an argument to your command and sets the `MCP_CONFORMANCE_SCENARIO` environment variable to the scenario name. For scenarios that require additional context (e.g., client credentials), the `MCP_CONFORMANCE_CONTEXT` environment variable contains a JSON object with scenario-specific data.
 
 ### Server Testing
 

examples/clients/typescript/everything-client.ts

@@ -3,7 +3,10 @@
 /**
  * Everything client - a single conformance test client that handles all scenarios.
  *
- * Usage: everything-client <scenario-name> <server-url>
+ * Usage: everything-client <server-url>
+ *
+ * The scenario name is read from the MCP_CONFORMANCE_SCENARIO environment variable,
+ * which is set by the conformance test runner.
  *
  * This client routes to the appropriate behavior based on the scenario name,
  * consolidating all the individual test clients into one.
@@ -177,11 +180,16 @@ registerScenario('elicitation-defaults', runElicitationDefaultsClient);
 // ============================================================================
 
 async function main(): Promise<void> {
-  const scenarioName = process.argv[2];
-  const serverUrl = process.argv[3];
+  const scenarioName = process.env.MCP_CONFORMANCE_SCENARIO;
+  const serverUrl = process.argv[2];
 
   if (!scenarioName || !serverUrl) {
-    console.error('Usage: everything-client <scenario-name> <server-url>');
+    console.error(
+      'Usage: MCP_CONFORMANCE_SCENARIO=<scenario> everything-client <server-url>'
+    );
+    console.error(
+      '\nThe MCP_CONFORMANCE_SCENARIO env var is set automatically by the conformance runner.'
+    );
     console.error('\nAvailable scenarios:');
     for (const name of Object.keys(scenarioHandlers).sort()) {
       console.error(`  - ${name}`);

src/runner/client.ts

@@ -21,14 +21,15 @@ async function executeClient(
 ): Promise<ClientExecutionResult> {
   const commandParts = command.split(' ');
   const executable = commandParts[0];
-  const args = [...commandParts.slice(1), scenarioName, serverUrl];
+  const args = [...commandParts.slice(1), serverUrl];
 
   let stdout = '';
   let stderr = '';
   let timedOut = false;
 
-  // Build environment with optional context
+  // Build environment with scenario name and optional context
   const env = { ...process.env };
+  env.MCP_CONFORMANCE_SCENARIO = scenarioName;
   if (context) {
     env.MCP_CONFORMANCE_CONTEXT = JSON.stringify(context);
   }
@@ -98,9 +99,7 @@ export async function runConformanceTest(
   console.error(`Starting scenario: ${scenarioName}`);
   const urls = await scenario.start();
 
-  console.error(
-    `Executing client: ${clientCommand} ${scenarioName} ${urls.serverUrl}`
-  );
+  console.error(`Executing client: ${clientCommand} ${urls.serverUrl}`);
   if (urls.context) {
     console.error(`With context: ${JSON.stringify(urls.context)}`);
   }