diff --git a/src/tier-check/checks/release.test.ts b/src/tier-check/checks/release.test.ts
new file mode 100644
index 00000000..f58da54a
--- /dev/null
+++ b/src/tier-check/checks/release.test.ts
@@ -0,0 +1,119 @@
+import { describe, it, expect } from 'vitest';
+import { Octokit } from '@octokit/rest';
+import { checkStableRelease } from './release';
+
+type Release = {
+  tag_name: string;
+  draft?: boolean;
+  prerelease?: boolean;
+};
+
+/** An Octokit stub that returns a fixed release list. */
+function octokitWith(releases: Release[]): Octokit {
+  return {
+    repos: {
+      listReleases: async () => ({
+        data: releases.map((r) => ({
+          draft: false,
+          prerelease: false,
+          ...r
+        }))
+      })
+    }
+  } as unknown as Octokit;
+}
+
+async function check(releases: Release[]) {
+  return checkStableRelease(
+    octokitWith(releases),
+    'modelcontextprotocol',
+    'sdk'
+  );
+}
+
+describe('checkStableRelease', () => {
+  it('accepts a bare semver tag', async () => {
+    expect(await check([{ tag_name: '1.2.3' }])).toMatchObject({
+      status: 'pass',
+      version: '1.2.3',
+      is_stable: true
+    });
+  });
+
+  it('accepts a v-prefixed tag', async () => {
+    expect(await check([{ tag_name: 'v1.2.3' }])).toMatchObject({
+      status: 'pass',
+      version: '1.2.3',
+      is_stable: true
+    });
+  });
+
+  // https://github.com/modelcontextprotocol/conformance/issues/425
+  it('accepts a monorepo tag carrying a package-name prefix', async () => {
+    expect(await check([{ tag_name: 'rmcp-v3.0.1' }])).toMatchObject({
+      status: 'pass',
+      version: '3.0.1',
+      is_stable: true
+    });
+  });
+
+  it('accepts a monorepo tag whose package name contains a hyphen', async () => {
+    expect(await check([{ tag_name: 'rmcp-macros-v3.0.1' }])).toMatchObject({
+      status: 'pass',
+      version: '3.0.1',
+      is_stable: true
+    });
+  });
+
+  it('rejects a pre-1.0 release', async () => {
+    expect(await check([{ tag_name: 'rmcp-v0.9.0' }])).toMatchObject({
+      status: 'fail',
+      version: '0.9.0',
+      is_stable: false
+    });
+  });
+
+  it('rejects a prerelease named in the tag', async () => {
+    expect(await check([{ tag_name: 'rmcp-v3.0.1-alpha.1' }])).toMatchObject({
+      status: 'fail',
+      version: '3.0.1-alpha.1',
+      is_stable: false,
+      is_prerelease: true
+    });
+  });
+
+  it('rejects a release GitHub flags as a prerelease', async () => {
+    expect(
+      await check([{ tag_name: 'rmcp-v3.0.1', prerelease: true }])
+    ).toMatchObject({
+      status: 'fail',
+      is_stable: false,
+      is_prerelease: true
+    });
+  });
+
+  it('skips drafts and scores the first published release', async () => {
+    expect(
+      await check([
+        { tag_name: 'rmcp-v4.0.0', draft: true },
+        { tag_name: 'rmcp-v3.0.1' }
+      ])
+    ).toMatchObject({ status: 'pass', version: '3.0.1' });
+  });
+
+  it('reports an unparseable tag unchanged rather than dropping it', async () => {
+    expect(await check([{ tag_name: 'nightly' }])).toMatchObject({
+      status: 'fail',
+      version: 'nightly',
+      is_stable: false
+    });
+  });
+
+  it('fails when the repo has no releases', async () => {
+    expect(await check([])).toMatchObject({
+      status: 'fail',
+      version: null,
+      is_stable: false
+    });
+  });
+});
diff --git a/src/tier-check/checks/release.ts b/src/tier-check/checks/release.ts
index 3eb53f1d..f47192aa 100644
--- a/src/tier-check/checks/release.ts
+++ b/src/tier-check/checks/release.ts
@@ -33,7 +33,7 @@ export async function checkStableRelease(
       };
     }
 
-    const version = latest.tag_name.replace(/^v/, '');
+    const version = versionFromTag(latest.tag_name);
     const isPrerelease =
       latest.prerelease ||
       /-(alpha|beta|rc|dev|preview|snapshot)/i.test(version);
@@ -57,3 +57,16 @@ export async function checkStableRelease(
     };
   }
 }
+
+/**
+ * Extract the semantic version from a release tag.
+ *
+ * Handles bare tags (`1.2.3`), `v`-prefixed tags (`v1.2.3`), and the
+ * package-prefixed tags monorepos publish (`rmcp-v3.0.1`,
+ * `rmcp-macros-v3.0.1`). Tags that carry no recognizable version are
+ * returned unchanged so they still surface in the check output.
+ */
+function versionFromTag(tag: string): string {
+  const match = tag.match(/(?:^|-)v?(\d+(?:\.\d+)*(?:[-+].*)?)$/);
+  return match ? match[1] : tag;
+}