diff --git a/src/lib/model/fixtures/dimensionlab.ts b/src/lib/model/fixtures/dimensionlab.ts
index 572067f..be23b02 100644
--- a/src/lib/model/fixtures/dimensionlab.ts
+++ b/src/lib/model/fixtures/dimensionlab.ts
@@ -140,16 +140,17 @@ function service(
   icon: string,
   href?: string,
 ) {
-  return {
+  const entry = {
     id,
     label,
     description,
     icon,
-    link: href ? { href, external: true } : undefined,
     severity: "ok" as const,
     detail: "ready",
     datasource: { type: "placeholder" as const, reason: "service health adapter pending" },
   };
+
+  return href ? { ...entry, link: { href, external: true } } : entry;
 }
 
 function group(id: string, title: string, services: ReturnType<typeof service>[]) {
diff --git a/src/lib/model/schema.test.ts b/src/lib/model/schema.test.ts
index 0f3153b..ab9e27a 100644
--- a/src/lib/model/schema.test.ts
+++ b/src/lib/model/schema.test.ts
@@ -125,6 +125,31 @@ describe("dashboard model validation", () => {
     }
   });
 
+  it("rejects thresholds on text metric values", () => {
+    const invalid = JSON.parse(JSON.stringify(genericDashboardFixture));
+    invalid.telemetry[0].value = { kind: "text", value: "available" };
+    invalid.telemetry[0].thresholds = { warning: 10 };
+
+    const result = validateDashboardDocument(invalid);
+
+    expect(result.valid).toBe(false);
+    if (!result.valid) {
+      expect(result.errors.join(" ")).toContain("text metric values");
+    }
+  });
+
+  it("rejects undefined properties because they are not JSON values", () => {
+    const invalid = JSON.parse(JSON.stringify(genericDashboardFixture));
+    invalid.serviceGroups[0].services[0].link = undefined;
+
+    const result = validateDashboardDocument(invalid);
+
+    expect(result.valid).toBe(false);
+    if (!result.valid) {
+      expect(result.errors.join(" ")).toContain("must be omitted instead of undefined");
+    }
+  });
+
   it("exports JSON Schema for external tool contracts", () => {
     expect(dashboardDocumentJsonSchema.$id).toContain("dashboard-document.v1");
     expect(dashboardDocumentJsonSchema.properties).toHaveProperty("schemaVersion");
diff --git a/src/lib/model/validation.ts b/src/lib/model/validation.ts
index 3ed22ab..7a79b54 100644
--- a/src/lib/model/validation.ts
+++ b/src/lib/model/validation.ts
@@ -55,11 +55,14 @@ export function validateDashboardDocument(
   value: unknown,
 ): DashboardValidationResult {
   const finiteNumberIssues: SemanticValidationIssue[] = [];
+  const undefinedIssues: SemanticValidationIssue[] = [];
   collectFiniteNumberIssues(value, "", finiteNumberIssues);
+  collectUndefinedIssues(value, "", undefinedIssues);
 
   if (validateDashboard(value)) {
     const semanticIssues = [
       ...finiteNumberIssues,
+      ...undefinedIssues,
       ...validateSemanticRules(value),
     ];
     if (semanticIssues.length === 0) {
@@ -73,7 +76,11 @@ export function validateDashboardDocument(
     };
   }
 
-  const details = [...(validateDashboard.errors || []), ...finiteNumberIssues];
+  const details = [
+    ...(validateDashboard.errors || []),
+    ...finiteNumberIssues,
+    ...undefinedIssues,
+  ];
   return {
     valid: false,
     errors: formatValidationErrors(details),
@@ -154,6 +161,16 @@ function validateSemanticRules(document: DashboardDocument): SemanticValidationI
   );
 
   document.telemetry.forEach((card, index) => {
+    if (card.value.kind === "text" && card.thresholds !== undefined) {
+      issues.push(
+        semanticIssue(
+          `/telemetry/${index}/thresholds`,
+          "must not be set for text metric values",
+          { id: card.id },
+        ),
+      );
+    }
+
     const warning = card.thresholds?.warning;
     const danger = card.thresholds?.danger;
     if (warning !== undefined && danger !== undefined && warning > danger) {
@@ -170,6 +187,32 @@ function validateSemanticRules(document: DashboardDocument): SemanticValidationI
   return issues;
 }
 
+function collectUndefinedIssues(
+  value: unknown,
+  path: string,
+  issues: SemanticValidationIssue[],
+) {
+  if (value === undefined) {
+    issues.push(
+      semanticIssue(path || "/", "must be omitted instead of undefined", {}),
+    );
+    return;
+  }
+
+  if (Array.isArray(value)) {
+    value.forEach((item, index) => {
+      collectUndefinedIssues(item, `${path}/${index}`, issues);
+    });
+    return;
+  }
+
+  if (value && typeof value === "object") {
+    Object.entries(value).forEach(([key, item]) => {
+      collectUndefinedIssues(item, `${path}/${escapeJsonPointer(key)}`, issues);
+    });
+  }
+}
+
 function collectFiniteNumberIssues(
   value: unknown,
   path: string,