diff --git a/.claude/skills/mcp-sdk-tier-audit/SKILL.md b/.claude/skills/mcp-sdk-tier-audit/SKILL.md
index 8033050b..509df1d8 100644
--- a/.claude/skills/mcp-sdk-tier-audit/SKILL.md
+++ b/.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:
 
diff --git a/src/sdk-runner/known-sdks.ts b/src/sdk-runner/known-sdks.ts
index cafd3373..443219d7 100644
--- a/src/sdk-runner/known-sdks.ts
+++ b/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'
   }
 };
 
diff --git a/src/sdk-runner/sdk-runner.test.ts b/src/sdk-runner/sdk-runner.test.ts
index 0eb051e2..be68faeb 100644
--- a/src/sdk-runner/sdk-runner.test.ts
+++ b/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();