diff --git a/src/domain/webCaptureDiff.resources.test.ts b/src/domain/webCaptureDiff.resources.test.ts
index f710f07279..f840b6308f 100644
--- a/src/domain/webCaptureDiff.resources.test.ts
+++ b/src/domain/webCaptureDiff.resources.test.ts
@@ -110,3 +110,47 @@ describe("web resource comparison identity and optional size", () => {
     });
   });
 });
+
+it.each([
+  [[128, 256], [128], [false, true], "unknown"],
+  [[128], [256], [true, true], "unknown"],
+  [[128], [128], [true, true], "unknown"],
+  [[128], [128, 256], [false, true], "changed"],
+  [[], [128], [false, true], "changed"],
+  [[128], [], [true, true], "unknown"],
+  [[128], [null], [false, true], "unknown"],
+] as const)(
+  "uses directional resource coverage for %j vs %j (limited=%j): %s",
+  (leftSizes, rightSizes, [leftLimited, rightLimited], status) => {
+    const before = capture(leftSizes.map((size) => resource(size)));
+    const after = capture(rightSizes.map((size) => resource(size)));
+    if (leftLimited) before.completeness.truncated_sections.push("resources");
+    if (rightLimited) after.completeness.truncated_sections.push("resources");
+    for (const [left, right] of [
+      [before, after],
+      [after, before],
+    ] as const)
+      expect(compareResources(left, right)).toMatchObject({
+        status,
+        total_changes: status === "changed" ? 1 : 0,
+        changes:
+          status === "changed"
+            ? [
+                {
+                  identity: expect.stringMatching(/^resg_/u),
+                  change:
+                    left.resources.length === 0
+                      ? "added"
+                      : right.resources.length === 0
+                        ? "removed"
+                        : "modified",
+                },
+              ]
+            : [],
+        reason:
+          status === "changed"
+            ? null
+            : expect.stringContaining("Resource inventory"),
+      });
+  },
+);
diff --git a/src/domain/webCaptureDiff.ts b/src/domain/webCaptureDiff.ts
index b1e799267e..781fb30345 100644
--- a/src/domain/webCaptureDiff.ts
+++ b/src/domain/webCaptureDiff.ts
@@ -213,9 +213,8 @@ const compareWebCaptureDimensions = (
     resources: compareResources(
       before.resources,
       after.resources,
-      sectionsComplete(before, ["resources"]) &&
-        sectionsComplete(after, ["resources"]),
-      "Resource inventory or content-size evidence was incomplete in at least one observation.",
+      sectionsComplete(before, ["resources"]),
+      sectionsComplete(after, ["resources"]),
     ),
     network: networkDimension(before, after),
     metadata: compareAggregate(
@@ -235,9 +234,12 @@ const compareWebCaptureDimensions = (
 const compareResources = (
   before: WebPageInspection["resources"],
   after: WebPageInspection["resources"],
-  complete: boolean,
-  reason: string,
+  beforeComplete: boolean,
+  afterComplete: boolean,
 ): Dimension => {
+  const reason =
+    "Resource inventory or content-size evidence was incomplete in at least one observation.";
+  const complete = beforeComplete && afterComplete;
   const left = resourceGroups(before);
   const right = resourceGroups(after);
   const changes: Change[] = [];
@@ -246,12 +248,14 @@ const compareResources = (
     const leftSizes = left.get(identity);
     const rightSizes = right.get(identity);
     if (leftSizes === undefined || rightSizes === undefined) {
-      if (!complete) unresolved = true;
-      else
+      const absenceProven =
+        leftSizes === undefined ? beforeComplete : afterComplete;
+      if (absenceProven)
         changes.push({
           identity,
           change: leftSizes === undefined ? "added" : "removed",
         });
+      else unresolved = true;
       continue;
     }
     // Stable inventories deduplicate identical observations. A null size also
@@ -261,7 +265,12 @@ const compareResources = (
       unresolved = true;
       continue;
     }
-    if (!sameResourceSizes(leftSizes, rightSizes))
+    // A partial set can omit any size variant. Only a complete peer can
+    // contradict a retained variant, including when the other side is partial.
+    if (
+      (beforeComplete && !resourceSizesContained(rightSizes, leftSizes)) ||
+      (afterComplete && !resourceSizesContained(leftSizes, rightSizes))
+    )
       changes.push({ identity, change: "modified" });
   }
   if (changes.length > 0)
@@ -292,11 +301,10 @@ const resourceGroups = (
   return groups;
 };
 
-const sameResourceSizes = (
-  left: ReadonlySet<number | null>,
-  right: ReadonlySet<number | null>,
-): boolean =>
-  left.size === right.size && [...left].every((size) => right.has(size));
+const resourceSizesContained = (
+  observed: ReadonlySet<number | null>,
+  complete: ReadonlySet<number | null>,
+): boolean => [...observed].every((size) => complete.has(size));
 
 type BodyShapeSources = {
   readonly request: boolean;