feat: support "draft" as a first-class spec version target

- Add LATEST_SPEC_VERSION and DATED_SPEC_VERSIONS constants so the next
  spec release is a one-line change in types.ts.
- Accept "draft" as a valid protocolVersion in the initialize check and
  mock-server response.
- --spec-version draft now selects latest-dated scenarios plus draft-tagged
  ones, so SEP authors can run the full suite against an SDK tracking the
  in-progress spec without retagging core scenarios.
- Forward --spec-version to the client process via
  MCP_CONFORMANCE_SPEC_VERSION so SDK examples can pick the matching
  protocolVersion.

Closes #253

Author: pcarleton

README.md

@@ -64,11 +64,12 @@ npx @modelcontextprotocol/conformance client --command "<client-command>" --scen
 - `--command` - The command to run your MCP client (can include flags)
 - `--scenario` - The test scenario to run (e.g., "initialize")
 - `--suite` - Run a suite of tests in parallel (e.g., "auth")
+- `--spec-version <version>` - Filter scenarios by spec version (e.g., `2025-11-25`, `draft`). `draft` runs the latest dated release plus any draft-only scenarios
 - `--expected-failures <path>` - Path to YAML baseline file of known failures (see [Expected Failures](#expected-failures))
 - `--timeout` - Timeout in milliseconds (default: 30000)
 - `--verbose` - Show verbose output
 
-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.
+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. When `--spec-version` is passed, its value is also forwarded as `MCP_CONFORMANCE_SPEC_VERSION`; example clients that target multiple spec versions can read this to select a `protocolVersion` at runtime. SDKs that hard-code their protocol version can ignore it — the harness accepts any draft-prefixed `protocolVersion` (e.g., `DRAFT-2026-v1`) regardless.
 
 ### Server Testing
 

src/checks/checks.test.ts

@@ -68,6 +68,23 @@ describe('createClientInitializationCheck', () => {
     expect(check.errorMessage).toContain('Client version missing');
   });
 
+  it.each(['DRAFT-2026-v1', 'draft'])(
+    'should accept draft protocol version %s',
+    (protocolVersion) => {
+      const request = {
+        protocolVersion,
+        clientInfo: {
+          name: 'TestClient',
+          version: '1.0.0'
+        }
+      };
+
+      const check = createClientInitializationCheck(request);
+      expect(check.status).toBe('SUCCESS');
+      expect(check.errorMessage).toBeUndefined();
+    }
+  );
+
   it('should support custom expected spec version', () => {
     const request = {
       protocolVersion: '2024-11-05',

src/checks/client.ts

@@ -1,4 +1,4 @@
-import { ConformanceCheck, CheckStatus } from '../types';
+import { ConformanceCheck, CheckStatus, LATEST_SPEC_VERSION } from '../types';
 
 export function createServerInfoCheck(serverInfo: {
   name: string;
@@ -23,20 +23,30 @@ export function createServerInfoCheck(serverInfo: {
   };
 }
 
-// Valid MCP protocol versions
-const VALID_PROTOCOL_VERSIONS = ['2025-06-18', '2025-11-25'];
+// Dated protocol versions the mock server will accept on initialize.
+const VALID_PROTOCOL_VERSIONS = ['2025-06-18', LATEST_SPEC_VERSION];
+
+// The spec repo's schema/draft/schema.ts uses a moving identifier of the form
+// DRAFT-YYYY-vN (e.g. "DRAFT-2026-v1"). Accept any draft-prefixed value so SEP
+// authors can run the suite against an SDK tracking the in-progress spec
+// without pinning the exact draft revision here.
+export function isDraftProtocolVersion(version: unknown): boolean {
+  return typeof version === 'string' && /^draft/i.test(version);
+}
 
 export function createClientInitializationCheck(
   initializeRequest: any,
-  expectedSpecVersion: string = '2025-11-25'
+  expectedSpecVersion: string = LATEST_SPEC_VERSION
 ): ConformanceCheck {
   const protocolVersionSent = initializeRequest?.protocolVersion;
 
   // Accept known valid versions OR custom expected version (for backward compatibility)
   const validVersions = VALID_PROTOCOL_VERSIONS.includes(expectedSpecVersion)
     ? VALID_PROTOCOL_VERSIONS
     : [...VALID_PROTOCOL_VERSIONS, expectedSpecVersion];
-  const versionMatch = validVersions.includes(protocolVersionSent);
+  const versionMatch =
+    validVersions.includes(protocolVersionSent) ||
+    isDraftProtocolVersion(protocolVersionSent);
 
   const errors: string[] = [];
   if (!protocolVersionSent) errors.push('Protocol version not provided');

src/index.ts

@@ -152,7 +152,8 @@ program
                 options.command,
                 scenarioName,
                 timeout,
-                outputDir
+                outputDir,
+                specVersionFilter
               );
               return {
                 scenario: scenarioName,
@@ -259,7 +260,8 @@ program
         validated.command,
         validated.scenario,
         timeout,
-        outputDir
+        outputDir,
+        specVersionFilter
       );
 
       const { overallFailure } = printClientResults(

src/runner/client.ts

@@ -1,7 +1,7 @@
 import { spawn } from 'child_process';
 import { promises as fs } from 'fs';
 import path from 'path';
-import { ConformanceCheck } from '../types';
+import { ConformanceCheck, SpecVersion } from '../types';
 import { getScenario } from '../scenarios';
 import { createResultDir, formatPrettyChecks } from './utils';
 
@@ -17,7 +17,8 @@ async function executeClient(
   scenarioName: string,
   serverUrl: string,
   timeout: number = 30000,
-  context?: Record<string, unknown>
+  context?: Record<string, unknown>,
+  specVersion?: SpecVersion
 ): Promise<ClientExecutionResult> {
   const commandParts = command.split(' ');
   const executable = commandParts[0];
@@ -34,6 +35,9 @@ async function executeClient(
   // 3. Semantic separation: scenario identifies "which test", context provides "test data"
   const env = { ...process.env };
   env.MCP_CONFORMANCE_SCENARIO = scenarioName;
+  if (specVersion) {
+    env.MCP_CONFORMANCE_SPEC_VERSION = specVersion;
+  }
   if (context) {
     // Include scenario name in context for discriminated union parsing
     env.MCP_CONFORMANCE_CONTEXT = JSON.stringify({
@@ -92,7 +96,8 @@ export async function runConformanceTest(
   clientCommand: string,
   scenarioName: string,
   timeout: number = 30000,
-  outputDir?: string
+  outputDir?: string,
+  specVersion?: SpecVersion
 ): Promise<{
   checks: ConformanceCheck[];
   clientOutput: ClientExecutionResult;
@@ -123,7 +128,8 @@ export async function runConformanceTest(
       scenarioName,
       urls.serverUrl,
       timeout,
-      urls.context
+      urls.context,
+      specVersion
     );
 
     // Print stdout/stderr if client exited with nonzero code

src/scenarios/client/initialize.ts

@@ -3,9 +3,11 @@ import {
   Scenario,
   ScenarioUrls,
   ConformanceCheck,
-  SpecVersion
+  SpecVersion,
+  LATEST_SPEC_VERSION
 } from '../../types';
 import { clientChecks } from '../../checks/index';
+import { isDraftProtocolVersion } from '../../checks/client';
 
 export class InitializeScenario implements Scenario {
   name = 'initialize';
@@ -117,11 +119,13 @@ export class InitializeScenario implements Scenario {
     this.checks.push(clientChecks.createServerInfoCheck(serverInfo));
 
     // Echo back client's version if valid, otherwise use latest
-    const VALID_VERSIONS = ['2025-06-18', '2025-11-25'];
+    const VALID_VERSIONS = ['2025-06-18', LATEST_SPEC_VERSION];
     const clientVersion = initializeRequest?.protocolVersion;
-    const responseVersion = VALID_VERSIONS.includes(clientVersion)
-      ? clientVersion
-      : '2025-11-25';
+    const responseVersion =
+      VALID_VERSIONS.includes(clientVersion) ||
+      isDraftProtocolVersion(clientVersion)
+        ? clientVersion
+        : LATEST_SPEC_VERSION;
 
     const response = {
       jsonrpc: '2.0',

src/scenarios/index.ts

@@ -2,7 +2,9 @@ import {
   Scenario,
   ClientScenario,
   ClientScenarioForAuthorizationServer,
-  SpecVersion
+  SpecVersion,
+  DATED_SPEC_VERSIONS,
+  LATEST_SPEC_VERSION
 } from '../types';
 import { InitializeScenario } from './client/initialize';
 import { ToolsCallScenario } from './client/tools_call';
@@ -257,9 +259,7 @@ export { listMetadataScenarios };
 
 // All valid spec versions, used by the CLI to validate --spec-version input.
 export const ALL_SPEC_VERSIONS: SpecVersion[] = [
-  '2025-03-26',
-  '2025-06-18',
-  '2025-11-25',
+  ...DATED_SPEC_VERSIONS,
   'draft',
   'extension'
 ];
@@ -273,23 +273,39 @@ export function resolveSpecVersion(value: string): SpecVersion {
   process.exit(1);
 }
 
+// `draft` selects everything in the latest dated release plus scenarios tagged
+// draft-only, so SEP authors can run the full suite against an SDK tracking the
+// in-progress spec without retagging core scenarios.
+function matchesSpecVersion(
+  scenario: { specVersions: SpecVersion[] },
+  version: SpecVersion
+): boolean {
+  if (version === 'draft') {
+    return (
+      scenario.specVersions.includes('draft') ||
+      scenario.specVersions.includes(LATEST_SPEC_VERSION)
+    );
+  }
+  return scenario.specVersions.includes(version);
+}
+
 export function listScenariosForSpec(version: SpecVersion): string[] {
   return scenariosList
-    .filter((s) => s.specVersions.includes(version))
+    .filter((s) => matchesSpecVersion(s, version))
     .map((s) => s.name);
 }
 
 export function listClientScenariosForSpec(version: SpecVersion): string[] {
   return allClientScenariosList
-    .filter((s) => s.specVersions.includes(version))
+    .filter((s) => matchesSpecVersion(s, version))
     .map((s) => s.name);
 }
 
 export function listClientScenariosForAuthorizationServerForSpec(
   version: SpecVersion
 ): string[] {
   return allClientScenariosListForAuthorizationServer
-    .filter((s) => s.specVersions.includes(version))
+    .filter((s) => matchesSpecVersion(s, version))
     .map((s) => s.name);
 }
 

src/scenarios/spec-version.test.ts

@@ -3,9 +3,11 @@ import {
   listScenarios,
   listClientScenarios,
   listScenariosForSpec,
+  listDraftScenarios,
   getScenarioSpecVersions,
   ALL_SPEC_VERSIONS
 } from './index';
+import { DATED_SPEC_VERSIONS, LATEST_SPEC_VERSION } from '../types';
 
 describe('specVersions helpers', () => {
   it('every Scenario has specVersions', () => {
@@ -69,26 +71,33 @@ describe('specVersions helpers', () => {
     }
   });
 
-  it('draft and extension scenarios are isolated', () => {
-    const draft = listScenariosForSpec('draft');
-    for (const name of draft) {
-      expect(getScenarioSpecVersions(name)).toContain('draft');
+  it('--spec-version draft is a superset of the latest dated release', () => {
+    const latest = new Set(listScenariosForSpec(LATEST_SPEC_VERSION));
+    const draft = new Set(listScenariosForSpec('draft'));
+    for (const name of latest) {
+      expect(draft.has(name)).toBe(true);
     }
-    const ext = listScenariosForSpec('extension');
-    for (const name of ext) {
-      expect(getScenarioSpecVersions(name)).toContain('extension');
+    for (const name of listDraftScenarios()) {
+      expect(draft.has(name)).toBe(true);
     }
   });
 
-  it('draft scenarios are not in dated versions', () => {
-    const draft = listScenariosForSpec('draft');
-    const dated = new Set([
-      ...listScenariosForSpec('2025-03-26'),
-      ...listScenariosForSpec('2025-06-18'),
-      ...listScenariosForSpec('2025-11-25')
-    ]);
-    for (const name of draft) {
-      expect(dated.has(name)).toBe(false);
+  it('draft-tagged scenarios are not also tagged with a dated version', () => {
+    for (const name of listDraftScenarios()) {
+      const versions = getScenarioSpecVersions(name)!;
+      for (const dated of DATED_SPEC_VERSIONS) {
+        expect(
+          versions,
+          `scenario "${name}" is tagged with both 'draft' and '${dated}'`
+        ).not.toContain(dated);
+      }
+    }
+  });
+
+  it('extension scenarios are isolated', () => {
+    const ext = listScenariosForSpec('extension');
+    for (const name of ext) {
+      expect(getScenarioSpecVersions(name)).toContain('extension');
     }
   });
 });

src/types.ts

@@ -23,12 +23,17 @@ export interface ConformanceCheck {
   logs?: string[];
 }
 
-export type SpecVersion =
-  | '2025-03-26'
-  | '2025-06-18'
-  | '2025-11-25'
-  | 'draft'
-  | 'extension';
+export const DATED_SPEC_VERSIONS = [
+  '2025-03-26',
+  '2025-06-18',
+  '2025-11-25'
+] as const;
+
+export type DatedSpecVersion = (typeof DATED_SPEC_VERSIONS)[number];
+
+export const LATEST_SPEC_VERSION: DatedSpecVersion = '2025-11-25';
+
+export type SpecVersion = DatedSpecVersion | 'draft' | 'extension';
 
 export interface ScenarioUrls {
   serverUrl: string;