diff --git a/packages/compiler-cli/src/ngtsc/typecheck/extended/checks/defer_trigger_misconfiguration/index.ts b/packages/compiler-cli/src/ngtsc/typecheck/extended/checks/defer_trigger_misconfiguration/index.ts
index e947f3af2bbb..9632a5c2496c 100644
--- a/packages/compiler-cli/src/ngtsc/typecheck/extended/checks/defer_trigger_misconfiguration/index.ts
+++ b/packages/compiler-cli/src/ngtsc/typecheck/extended/checks/defer_trigger_misconfiguration/index.ts
@@ -12,6 +12,7 @@ import {
   TmplAstDeferredBlock,
   TmplAstDeferredTrigger,
   TmplAstHoverDeferredTrigger,
+  TmplAstIdleDeferredTrigger,
   TmplAstImmediateDeferredTrigger,
   TmplAstInteractionDeferredTrigger,
   TmplAstTimerDeferredTrigger,
@@ -60,6 +61,26 @@ function areLiteralMapsEqual(a: LiteralMap | null, b: LiteralMap | null): boolea
   return true;
 }
 
+function getTimedTriggerValue(
+  trigger: TmplAstTimerDeferredTrigger | TmplAstIdleDeferredTrigger,
+): number | null {
+  if (trigger instanceof TmplAstTimerDeferredTrigger) {
+    return trigger.delay;
+  }
+
+  return trigger.timeout;
+}
+
+function formatTimedTrigger(
+  trigger: TmplAstTimerDeferredTrigger | TmplAstIdleDeferredTrigger,
+): string {
+  if (trigger instanceof TmplAstTimerDeferredTrigger) {
+    return `timer(${trigger.delay}ms)`;
+  }
+
+  return trigger.timeout === null ? 'idle' : `idle(${trigger.timeout}ms)`;
+}
+
 /**
  * This check implements warnings for unreachable or redundant @defer triggers.
  * Emits ErrorCode.DEFER_TRIGGER_MISCONFIGURATION with messages matching the project's
@@ -114,26 +135,54 @@ class DeferTriggerMisconfiguration extends TemplateCheckWithVisitor<ErrorCode.DE
       }
     }
 
+    // `prefetch` without an explicit main trigger defaults the main trigger to `idle`,
+    if (mains.length === 0 && prefetches.length > 0) {
+      const msg =
+        `Define a main trigger when using 'prefetch' triggers. ` +
+        `Without an explicit main trigger, @defer defaults to 'idle' and prefetch may have no effect.`;
+      diags.push(
+        ctx.makeTemplateDiagnostic(
+          node.sourceSpan,
+          formatExtendedError(ErrorCode.DEFER_TRIGGER_MISCONFIGURATION, msg),
+        ),
+      );
+    }
+
     // If there is exactly one main and at least one prefetch, compare them.
     if (mains.length === 1 && prefetches.length > 0) {
       const main = mains[0];
 
       for (const pre of prefetches) {
-        // Timer vs Timer: warn when prefetch delay >= main delay
-        const isTimerTriggger =
+        // Delay-based pairs (timer/idle): warn when prefetch fires no sooner than main.
+
+        const isTimerPair =
           main instanceof TmplAstTimerDeferredTrigger && pre instanceof TmplAstTimerDeferredTrigger;
-        if (isTimerTriggger) {
-          const mainDelay = main.delay;
-          const preDelay = pre.delay;
-          if (preDelay >= mainDelay) {
-            const msg = `The Prefetch 'timer(${preDelay}ms)' is not scheduled before the main 'timer(${mainDelay}ms)', so it won’t run prior to rendering. Lower the prefetch delay or remove it.`;
-            diags.push(
-              ctx.makeTemplateDiagnostic(
-                pre.sourceSpan ?? node.sourceSpan,
-                formatExtendedError(ErrorCode.DEFER_TRIGGER_MISCONFIGURATION, msg),
-              ),
-            );
+
+        const isIdlePair =
+          main instanceof TmplAstIdleDeferredTrigger && pre instanceof TmplAstIdleDeferredTrigger;
+
+        if (isTimerPair || isIdlePair) {
+          const mainVal = getTimedTriggerValue(main);
+          const preVal = getTimedTriggerValue(pre);
+          const sameUntimedIdle = mainVal == null && preVal == null;
+          const comparableTimedPair = mainVal != null && preVal != null && preVal >= mainVal;
+
+          if (!sameUntimedIdle && !comparableTimedPair) {
+            continue;
           }
+
+          const msg =
+            `The Prefetch '${formatTimedTrigger(pre)}' is not scheduled before the main '${formatTimedTrigger(main)}',` +
+            ` so it won't run prior to rendering. Lower the prefetch timing or remove it.`;
+
+          diags.push(
+            ctx.makeTemplateDiagnostic(
+              pre.sourceSpan ?? node.sourceSpan,
+              formatExtendedError(ErrorCode.DEFER_TRIGGER_MISCONFIGURATION, msg),
+            ),
+          );
+
+          continue;
         }
 
         // Reference-based triggers (hover/interaction/viewport): warn if both
diff --git a/packages/compiler-cli/src/ngtsc/typecheck/extended/test/checks/defer_trigger_misconfiguration/defer_trigger_misconfiguration_spec.ts b/packages/compiler-cli/src/ngtsc/typecheck/extended/test/checks/defer_trigger_misconfiguration/defer_trigger_misconfiguration_spec.ts
index 6b7c1ed97e91..df1ff63f5781 100644
--- a/packages/compiler-cli/src/ngtsc/typecheck/extended/test/checks/defer_trigger_misconfiguration/defer_trigger_misconfiguration_spec.ts
+++ b/packages/compiler-cli/src/ngtsc/typecheck/extended/test/checks/defer_trigger_misconfiguration/defer_trigger_misconfiguration_spec.ts
@@ -67,6 +67,35 @@ runInEachFileSystem(() => {
       expect(diags[0].code).toBe(ngErrorCode(ErrorCode.DEFER_TRIGGER_MISCONFIGURATION));
     });
 
+    it('should emit when prefetch idle timeout >= main idle timeout', () => {
+      const diags = getDiags(`@defer (on idle(1s); prefetch on idle(2s)) { <div></div> }`);
+      expect(diags.length).toBe(1);
+      expect(diags[0].category).toBe(ts.DiagnosticCategory.Warning);
+      expect(diags[0].code).toBe(ngErrorCode(ErrorCode.DEFER_TRIGGER_MISCONFIGURATION));
+    });
+
+    it('should not emit when prefetch idle timeout < main idle timeout', () => {
+      const diags = getDiags(`@defer (on idle(2s); prefetch on idle(1s)) { <div></div> }`);
+      expect(diags.length).toBe(0);
+    });
+
+    it('should emit when prefetch idle matches main idle with no timeout', () => {
+      const diags = getDiags(`@defer (on idle; prefetch on idle) { <div></div> }`);
+      expect(diags.length).toBe(1);
+      expect(diags[0].category).toBe(ts.DiagnosticCategory.Warning);
+      expect(diags[0].code).toBe(ngErrorCode(ErrorCode.DEFER_TRIGGER_MISCONFIGURATION));
+    });
+
+    it('should not emit when only main idle has timeout', () => {
+      const diags = getDiags(`@defer (on idle(1s); prefetch on idle) { <div></div> }`);
+      expect(diags.length).toBe(0);
+    });
+
+    it('should not emit when only prefetch idle has timeout', () => {
+      const diags = getDiags(`@defer (on idle; prefetch on idle(1s)) { <div></div> }`);
+      expect(diags.length).toBe(0);
+    });
+
     it('should emit when prefetch identical to main viewport/interaction/hover', () => {
       const diags = getDiags(
         `@defer (on viewport(ref); prefetch on viewport(ref)) { <div></div> }`,
@@ -151,5 +180,33 @@ runInEachFileSystem(() => {
       expect(diags[0].category).toBe(ts.DiagnosticCategory.Warning);
       expect(diags[0].code).toBe(ngErrorCode(ErrorCode.DEFER_TRIGGER_MISCONFIGURATION));
     });
+
+    it('should emit when prefetch trigger is configured without a main trigger', () => {
+      const diags = getDiags(`@defer (prefetch on viewport(ref)) { <div></div> }`);
+      expect(diags.length).toBe(1);
+      expect(diags[0].category).toBe(ts.DiagnosticCategory.Warning);
+      expect(diags[0].code).toBe(ngErrorCode(ErrorCode.DEFER_TRIGGER_MISCONFIGURATION));
+    });
+
+    it('should emit when prefetch timer is configured without a main trigger', () => {
+      const diags = getDiags(`@defer (prefetch on timer(500ms)) { <div></div> }`);
+      expect(diags.length).toBe(1);
+      expect(diags[0].category).toBe(ts.DiagnosticCategory.Warning);
+      expect(diags[0].code).toBe(ngErrorCode(ErrorCode.DEFER_TRIGGER_MISCONFIGURATION));
+    });
+
+    it('should emit when prefetch idle is configured without a main trigger', () => {
+      const diags = getDiags(`@defer (prefetch on idle(500)) { <div></div> }`);
+      expect(diags.length).toBe(1);
+      expect(diags[0].category).toBe(ts.DiagnosticCategory.Warning);
+      expect(diags[0].code).toBe(ngErrorCode(ErrorCode.DEFER_TRIGGER_MISCONFIGURATION));
+    });
+
+    it('should emit when prefetch when is configured without a main trigger', () => {
+      const diags = getDiags(`@defer (prefetch when true) { <div></div> }`);
+      expect(diags.length).toBe(1);
+      expect(diags[0].category).toBe(ts.DiagnosticCategory.Warning);
+      expect(diags[0].code).toBe(ngErrorCode(ErrorCode.DEFER_TRIGGER_MISCONFIGURATION));
+    });
   });
 });