diff --git a/packages/mcp-manager/src/types.ts b/packages/mcp-manager/src/types.ts
index 8d142d8..84571c7 100644
--- a/packages/mcp-manager/src/types.ts
+++ b/packages/mcp-manager/src/types.ts
@@ -33,6 +33,12 @@ export interface McpRuntimeCapabilitiesSnapshot {
 	readonly subscriptions: boolean;
 }
 
+/**
+ * Normalized immutable projection of one managed runtime's lifecycle state.
+ *
+ * Optional properties are absent rather than present with an `undefined` value.
+ * The snapshot and its nested capability projection are frozen.
+ */
 export interface McpRuntimeStateSnapshot {
 	readonly phase: McpRuntimePhase;
 	readonly lastTransitionAt: string;
diff --git a/packages/mcp-manager/tests/runtime-manager.test.ts b/packages/mcp-manager/tests/runtime-manager.test.ts
index c1d4bff..9062af4 100644
--- a/packages/mcp-manager/tests/runtime-manager.test.ts
+++ b/packages/mcp-manager/tests/runtime-manager.test.ts
@@ -466,6 +466,12 @@ describe("McpRuntimeManager", () => {
 		expect(mcpRuntimeStateSnapshotSchema["~standard"].validate(offline)).toEqual({
 			value: offline,
 		});
+
+		for (const state of [online, untracked, degraded, offline]) {
+			expect(Object.isFrozen(state)).toBe(true);
+			expect(Object.values(state)).not.toContain(undefined);
+		}
+		expect(Object.isFrozen(online.capabilities)).toBe(true);
 		await manager.close();
 	});