diff --git a/test/fixtures/agent-schema/invalid/persona/string-principles.agent.yaml b/test/fixtures/agent-schema/invalid/persona/string-principles.agent.yaml
new file mode 100644
index 000000000..e03fb3cd5
--- /dev/null
+++ b/test/fixtures/agent-schema/invalid/persona/string-principles.agent.yaml
@@ -0,0 +1,26 @@
+# Test: Principles as block scalar string instead of array
+# Expected: FAIL
+# Error path: agent.persona.principles
+# Error expected: array
+# Error received: string
+
+agent:
+  metadata:
+    id: string-principles
+    name: String Principles
+    title: String Principles
+    icon: ❌
+    hasSidecar: false
+
+  persona:
+    role: Test agent
+    identity: Test identity
+    communication_style: Test style
+    principles: |
+      - This is a string, not an array
+      - Each line is part of one string
+
+  menu:
+    - trigger: help
+      description: Show help
+      action: display_help
diff --git a/tools/schema/agent.js b/tools/schema/agent.js
index b6a36a985..c373286db 100644
--- a/tools/schema/agent.js
+++ b/tools/schema/agent.js
@@ -240,12 +240,9 @@ function buildPersonaSchema() {
       role: createNonEmptyString('agent.persona.role'),
       identity: createNonEmptyString('agent.persona.identity'),
       communication_style: createNonEmptyString('agent.persona.communication_style'),
-      principles: z.union([
-        createNonEmptyString('agent.persona.principles'),
-        z
-          .array(createNonEmptyString('agent.persona.principles[]'))
-          .min(1, { message: 'agent.persona.principles must include at least one entry' }),
-      ]),
+      principles: z
+        .array(createNonEmptyString('agent.persona.principles[]'))
+        .min(1, { message: 'agent.persona.principles must include at least one entry' }),
     })
     .strict();
 }