feat: add ruby-sdk to the known SDKs registry (#457)

Registers modelcontextprotocol/ruby-sdk in KNOWN_SDKS so `tier-check --sdk-path` and `sdk ruby-sdk` manage
the build and per-revision runs themselves, instead of requiring the --conformance-server-url/--client-cmd
fallback for unknown SDKs. This is the registry path the Ruby SDK's SEP-1730 Tier 1 application
(modelcontextprotocol/modelcontextprotocol#3247) was scored through: with the entry in place, reviewers can
reproduce the submission with a single `tier-check --sdk-path <checkout> --requirements 2025-11-25,2026-07-28`,
the same way every other registered SDK is checked.

The entry drives the same fixtures the SDK's own CI runs through `rake conformance`: `conformance/server.rb`
behind the `conformance:server` rake task (PORT from the environment; PORT=3000 matches the registry's port convention)
and `conformance/client.rb`, which reads MCP_CONFORMANCE_SCENARIO from the environment and the server URL from argv,
per the runner's contract. The baseline is `conformance/expected_failures.yml`. One dual-era server serves every revision
from the same /mcp endpoint (stateful dated-spec handshake and the SEP-2575 stateless lifecycle side by side),
so the entry needs no specOverrides.

Testing: `npm test` (525 passing, including a new known-sdks entry test), `npm run check`, and `npm run build` pass.
The entry has already carried four full end-to-end audit runs: the 2026-08-14 and 2026-08-15 tier-check runs against
a local ruby-sdk checkout (`--sdk-path` with `--requirements 2025-11-25,2026-07-28`) each built the SDK, started the
server on PORT=3000, and completed both legs at both revisions, producing the 67/67 server and 50/50 client results
submitted in modelcontextprotocol/modelcontextprotocol#3247. Individually, `sdk ruby-sdk --path <checkout> --mode
server --requirements 2025-11-25` exits 0 with 78/78 checks, and `--mode client --requirements 2025-11-25` exits 0
with every scored scenario passing (failures confined to not_scored extensions).

.claude/skills/mcp-sdk-tier-audit/SKILL.md

@@ -26,7 +26,7 @@ If this fails (exit code non-zero or shows "not logged in"), stop immediately an
 
 Do NOT proceed to any other step if this check fails.
 
-**Known SDKs need no server from you.** When the SDK is in `src/sdk-runner/known-sdks.ts` (typescript-sdk, python-sdk, go-sdk, rust-sdk, csharp-sdk, and variants), skip the server preflight entirely: Step 2's `--sdk-path` mode builds the SDK and starts the right server per revision itself. The reachability check below applies only to the URL fallback for unknown SDKs.
+**Known SDKs need no server from you.** When the SDK is in `src/sdk-runner/known-sdks.ts` (typescript-sdk, python-sdk, go-sdk, rust-sdk, csharp-sdk, ruby-sdk, and variants), skip the server preflight entirely: Step 2's `--sdk-path` mode builds the SDK and starts the right server per revision itself. The reachability check below applies only to the URL fallback for unknown SDKs.
 
 After parsing arguments (Step 1), for the URL fallback only, verify the conformance server is reachable:
 

src/sdk-runner/known-sdks.ts

@@ -159,6 +159,25 @@ export const KNOWN_SDKS: Record<string, SdkConfig> = {
         server: { url: 'http://localhost:3000/stateless' }
       }
     }
+  },
+  // Fixtures live under conformance/ (server.rb + client.rb, the same files
+  // the SDK's own `rake conformance` CI drives). The client reads
+  // MCP_CONFORMANCE_SCENARIO from the environment and the server URL from argv.
+  // One dual-era server serves every revision from the same /mcp endpoint —
+  // the stateful (dated-spec) handshake and the SEP-2575 stateless lifecycle
+  // side by side — so no specOverrides are needed. The rake task reads PORT
+  // from the environment (the SDK's own default is 9292); PORT=3000 matches
+  // the 3000 convention used above.
+  'ruby-sdk': {
+    build: 'bundle install',
+    client: {
+      command: 'bundle exec ruby conformance/client.rb'
+    },
+    server: {
+      command: 'PORT=3000 bundle exec rake conformance:server',
+      url: 'http://localhost:3000/mcp'
+    },
+    expectedFailures: 'conformance/expected_failures.yml'
   }
 };
 

src/sdk-runner/sdk-runner.test.ts

@@ -141,6 +141,19 @@ describe('lookupBuiltinConfig', () => {
     expect(rs?.server?.url).toBe('http://localhost:3000/mcp');
   });
 
+  it('exposes ruby-sdk with the conformance/ fixtures and a baseline', () => {
+    const rb = lookupBuiltinConfig('ruby-sdk');
+    expect(rb?.build).toBe('bundle install');
+    expect(rb?.client?.command).toBe('bundle exec ruby conformance/client.rb');
+    expect(rb?.server?.command).toBe(
+      'PORT=3000 bundle exec rake conformance:server'
+    );
+    expect(rb?.server?.url).toBe('http://localhost:3000/mcp');
+    expect(rb?.expectedFailures).toBe('conformance/expected_failures.yml');
+    // One dual-era server serves every revision, so no per-spec overrides.
+    expect(rb?.specOverrides).toBeUndefined();
+  });
+
   it('every built-in entry validates against SdkConfigSchema', () => {
     for (const [name, cfg] of Object.entries(KNOWN_SDKS)) {
       expect(() => SdkConfigSchema.parse(cfg), name).not.toThrow();