diff --git a/.changeset/kind-lynx-munch.md b/.changeset/kind-lynx-munch.md
new file mode 100644
index 000000000..d9190ea41
--- /dev/null
+++ b/.changeset/kind-lynx-munch.md
@@ -0,0 +1,7 @@
+---
+type: Changed
+pr: 1728
+---
+**Internal: derive the non-Claude runtime list from the capability registry** — `NON_CLAUDE_RUNTIMES` is now computed from the capability registry instead of a hand-maintained literal, so it can no longer drift from the per-runtime descriptors. No user-visible behavior change (the list is identical).
+
+<!-- docs-exempt: internal refactor, no user-facing surface -->
diff --git a/src/runtime-artifact-conversion.cts b/src/runtime-artifact-conversion.cts
index 77cb11e02..0ebfe5d13 100644
--- a/src/runtime-artifact-conversion.cts
+++ b/src/runtime-artifact-conversion.cts
@@ -23,6 +23,7 @@ import commandRoster = require('./command-roster.cjs');
 const { readGsdCommandNames, transformContentToHyphen } = commandRoster;
 import runtimeNamePolicy = require('./runtime-name-policy.cjs');
 const { getDirName } = runtimeNamePolicy;
+import capabilityRegistry = require('./capability-registry.cjs');
 
 // #1383: resolve GSD's version WITHOUT a top-level
 // `require('../../../package.json')`. That require ran at module load on every
@@ -2199,16 +2200,15 @@ function computePathPrefix({ isGlobal, isOpencode, isWindowsHost: _isWindowsHost
 
 /**
  * Canonical list of every non-Claude runtime that gsd-core emits artifacts for.
- * Exported so test files can import this single source of truth rather than
- * maintaining divergent hand-rolled arrays (#1521).
- *
- * Keep in sync with the runtime flags in bin/install.js and getDirName().
+ * DERIVED from the capability registry (ADR-1239 Phase B, #1679) — the registry's
+ * `runtimes` map is the single source of truth for runtime identity, so the
+ * non-Claude set is its key set minus 'claude'. This replaces a hand-maintained
+ * literal that had to be kept in sync with bin/install.js and getDirName(), and
+ * can no longer drift from the registry. Exported so tests import one source (#1521).
  */
-const NON_CLAUDE_RUNTIMES: string[] = [
-  'codex', 'opencode', 'kilo', 'gemini', 'copilot', 'antigravity',
-  'cursor', 'windsurf', 'augment', 'trae', 'qwen', 'hermes', 'kimi',
-  'codebuddy', 'cline',
-];
+const NON_CLAUDE_RUNTIMES: string[] = Object.keys(capabilityRegistry.runtimes)
+  .filter((id) => id !== 'claude')
+  .sort();
 
 /**
  * #1521: Every non-Claude runtime resolves its own runtime identity from a
diff --git a/tests/non-claude-runtimes-registry-derivation.test.cjs b/tests/non-claude-runtimes-registry-derivation.test.cjs
new file mode 100644
index 000000000..50293358b
--- /dev/null
+++ b/tests/non-claude-runtimes-registry-derivation.test.cjs
@@ -0,0 +1,81 @@
+'use strict';
+/**
+ * Drift-guard: NON_CLAUDE_RUNTIMES must always be derived from the capability
+ * registry. Verifies:
+ *   1. The exported constant equals a hardcoded golden expected list — a pinned
+ *      oracle that catches BOTH formula bugs (derived value diverges from golden)
+ *      AND unintended registry drift (adding/removing a runtime forces a
+ *      deliberate golden-list update).
+ *   2. Every registry entry with role === 'runtime' and id !== 'claude' appears
+ *      in NON_CLAUDE_RUNTIMES — a cross-check from a different angle than the
+ *      production derivation formula.
+ *   3. Every member of NON_CLAUDE_RUNTIMES has an explicit getDirName branch that
+ *      does not return '.claude' — guards against adding a runtime to the registry
+ *      without teaching getDirName about it (ADR-1239 Phase B, #1679).
+ *
+ * Behavioral tests only: assert on returned values, no source-grep.
+ */
+
+const { test } = require('node:test');
+const assert = require('node:assert/strict');
+const conversion = require('../gsd-core/bin/lib/runtime-artifact-conversion.cjs');
+const registry = require('../gsd-core/bin/lib/capability-registry.cjs');
+const runtimeNamePolicy = require('../gsd-core/bin/lib/runtime-name-policy.cjs');
+
+const { NON_CLAUDE_RUNTIMES } = conversion;
+const { getDirName } = runtimeNamePolicy;
+
+// Golden oracle: hardcoded sorted known-good list of all non-Claude runtimes.
+// A pinned expected value in a TEST is correct — the test IS the oracle.
+// Only PRODUCTION code should derive dynamically from the registry.
+// If this list diverges from NON_CLAUDE_RUNTIMES, either the formula is wrong
+// OR the registry changed — both require a deliberate golden-list update here.
+const EXPECTED = [
+  'antigravity', 'augment', 'cline', 'codebuddy', 'codex', 'copilot',
+  'cursor', 'gemini', 'hermes', 'kilo', 'kimi', 'opencode', 'qwen',
+  'trae', 'windsurf',
+];
+
+test('NON_CLAUDE_RUNTIMES matches the golden expected set (sorted)', () => {
+  assert.deepEqual(
+    [...NON_CLAUDE_RUNTIMES],
+    EXPECTED,
+    `NON_CLAUDE_RUNTIMES diverged from golden list.\n` +
+    `  actual:   [${[...NON_CLAUDE_RUNTIMES].join(', ')}]\n` +
+    `  expected: [${EXPECTED.join(', ')}]`,
+  );
+  // Explicit readability assertion: 'claude' must never appear.
+  assert.ok(
+    !NON_CLAUDE_RUNTIMES.includes('claude'),
+    'NON_CLAUDE_RUNTIMES must not contain "claude"',
+  );
+});
+
+test('every registry-declared runtime except claude is present in NON_CLAUDE_RUNTIMES', () => {
+  // Cross-check from a DIFFERENT angle than the production derivation formula:
+  // iterate registry entries by their role field rather than by Object.keys().filter().
+  // This catches a case where a runtime is added to the registry with role==='runtime'
+  // but is somehow excluded from NON_CLAUDE_RUNTIMES by a formula bug.
+  for (const [id, entry] of Object.entries(registry.runtimes)) {
+    if (entry.role === 'runtime' && id !== 'claude') {
+      assert.ok(
+        NON_CLAUDE_RUNTIMES.includes(id),
+        `Registry declares runtime '${id}' (role==='runtime') but it is missing from NON_CLAUDE_RUNTIMES`,
+      );
+    }
+  }
+});
+
+// Forward direction (registry → getDirName coverage) is the load-bearing guard:
+// the registry is the authoritative runtime source, so every member of
+// NON_CLAUDE_RUNTIMES must have an explicit getDirName branch.
+test('DRIFT GUARD: every registry-declared non-Claude runtime has an explicit getDirName branch (not .claude)', () => {
+  for (const rt of NON_CLAUDE_RUNTIMES) {
+    const dir = getDirName(rt);
+    assert.notEqual(
+      dir,
+      '.claude',
+      `getDirName('${rt}') returned '.claude' — runtime '${rt}' is in the registry but missing an explicit getDirName branch`,
+    );
+  }
+});