diff --git a/packages/host/app/services/card-service.ts b/packages/host/app/services/card-service.ts
index ce3ea6bc9e2..66c69152a7b 100644
--- a/packages/host/app/services/card-service.ts
+++ b/packages/host/app/services/card-service.ts
@@ -185,7 +185,7 @@ export default class CardService extends Service {
         typeof requestInit.body === 'string'
           ? requestInit.body
           : JSON.stringify(requestInit.body, null, 2);
-      this.validateSizeLimit(urlString, jsonString, 'card');
+      this.validateCardWriteSize(urlString, jsonString);
     }
 
     let response = await this.network.authedFetch(url, requestInit);
@@ -429,6 +429,52 @@ export default class CardService extends Service {
     return response.json();
   }
 
+  // The 512 KB ceiling is a per-card-*file* limit, but a card write POSTs a
+  // document whose `included[]` inlines every linked card the tab happens to
+  // have resident — and the realm discards every included member that has no
+  // `lid` (it keeps only the primary card plus any brand-new, unsaved links it
+  // is being asked to create in the same request; see the realm's card POST
+  // handler). Measuring the concatenated body would therefore fail a tiny card
+  // because of cards it merely links to, so validate each resource that will
+  // actually become a file on its own — mirroring the realm's own per-file
+  // `assertWriteSize` — rather than the whole request body.
+  //
+  // The client is deliberately the stricter of the two on one edge: the realm
+  // additionally skips (without error) any resource whose `meta.realmURL`
+  // names a different realm, so a foreign-realm `lid` side-load is measured
+  // here but silently dropped there. That shape is a request defect the realm
+  // currently swallows — a co-create the write will never perform — so
+  // refusing it client-side over size is acceptable, and matching the skip
+  // would mean reproducing a silent-drop behavior rather than a contract.
+  private validateCardWriteSize(url: string, body: string) {
+    let doc: LooseSingleCardDocument | undefined;
+    try {
+      doc = JSON.parse(body);
+    } catch {
+      // Not a JSON document we can split into resources; fall back to holding
+      // the whole body to the limit rather than letting an unmeasured write by.
+    }
+    if (!doc || typeof doc !== 'object' || !doc.data) {
+      this.validateSizeLimit(url, body, 'card');
+      return;
+    }
+    // The primary card is always written; an included member is written only
+    // when it carries a `lid` (an unsaved link created alongside this card).
+    let resources = [
+      doc.data,
+      ...(doc.included ?? []).filter(
+        (resource) => typeof (resource as { lid?: unknown }).lid === 'string',
+      ),
+    ];
+    for (let resource of resources) {
+      this.validateSizeLimit(
+        url,
+        JSON.stringify({ data: resource }, null, 2),
+        'card',
+      );
+    }
+  }
+
   private validateSizeLimit(
     url: string,
     content: string,
diff --git a/packages/host/tests/integration/store-test.gts b/packages/host/tests/integration/store-test.gts
index 797e8d57f39..116ffd9a1ef 100644
--- a/packages/host/tests/integration/store-test.gts
+++ b/packages/host/tests/integration/store-test.gts
@@ -2201,6 +2201,76 @@ module('Integration | Store', function (hooks) {
     );
   });
 
+  test('a small new card saves even when its resident linked graph would overflow the size limit if inlined', async function (assert) {
+    // Regression: the client size check used to measure the whole POST body,
+    // `included[]` and all. A new card that links to already-saved cards the
+    // tab has loaded serialises those cards into `included` — which the realm
+    // discards on write — so a tiny card could fail "Card size exceeds maximum"
+    // purely because of how much of the realm the tab happened to have resident.
+    let environmentService = getService('environment-service') as any;
+    let originalMaxSize = environmentService.cardSizeLimitBytes;
+    try {
+      // A saved, resident linked card large enough that inlining it into
+      // `included` would blow the limit, while the new card's own document is
+      // tiny. Saved under the realm's default ceiling before the client limit
+      // is lowered; the realm keeps its own (unchanged) ceiling throughout.
+      let bigFriend = new PersonDef({ name: 'x'.repeat(6000) });
+      let savedFriend = await (storeService as any).persistAndUpdate(bigFriend);
+      assert.true(isCardInstance(savedFriend), 'the large linked card saved');
+
+      environmentService.cardSizeLimitBytes = 2500;
+
+      let instance = new PersonDef({ name: 'Small' });
+      (instance as any).bestFriend = bigFriend;
+
+      let result = await (storeService as any).persistAndUpdate(instance);
+      assert.true(
+        isCardInstance(result),
+        "a new card whose own document is under the limit saves regardless of how large a graph it links to — the check measures what the realm stores, not the tab's loaded `included`",
+      );
+      let cardPath = `${(instance as any).id.substring(
+        testRealmURL.length,
+      )}.json`;
+      assert.ok(
+        await testRealmAdapter.openFile(cardPath),
+        'the realm holds the created card',
+      );
+    } finally {
+      environmentService.cardSizeLimitBytes = originalMaxSize;
+    }
+  });
+
+  test('an oversized unsaved link created alongside a card still fails the size check', async function (assert) {
+    // The counterpart that makes the per-resource filter load-bearing: a
+    // `lid`-bearing side-load becomes its own file on the realm, so it must
+    // keep being measured. If the check stopped measuring included members
+    // entirely, this save would sail through client-side and 413 on the
+    // realm instead.
+    let environmentService = getService('environment-service') as any;
+    let originalMaxSize = environmentService.cardSizeLimitBytes;
+    try {
+      environmentService.cardSizeLimitBytes = 2500;
+
+      let bigUnsaved = new PersonDef({ name: 'x'.repeat(6000) });
+      let instance = new PersonDef({ name: 'Small' });
+      (instance as any).bestFriend = bigUnsaved;
+
+      let result = await (storeService as any).persistAndUpdate(instance);
+      assert.false(
+        isCardInstance(result),
+        'the save is refused: the unsaved link is co-created as its own file and is over the ceiling',
+      );
+      assert.ok(
+        String((result as any)?.message).includes(
+          'exceeds maximum allowed size',
+        ),
+        `the error names the size limit (got: ${(result as any)?.message})`,
+      );
+    } finally {
+      environmentService.cardSizeLimitBytes = originalMaxSize;
+    }
+  });
+
   test('a save overlapping a create PATCHes instead of issuing a second POST', async function (assert) {
     // Driven through `persistAndUpdate` rather than `save`, because the
     // autosave queue awaits the in-flight mutation before it saves at all —