diff --git a/.changeset/wasted-recompute-undefined-output.md b/.changeset/wasted-recompute-undefined-output.md
new file mode 100644
index 000000000..123972443
--- /dev/null
+++ b/.changeset/wasted-recompute-undefined-output.md
@@ -0,0 +1,5 @@
+---
+"@solidjs/signals": patch
+---
+
+A re-run that commits `undefined` is no longer counted as waste. A projection that mutates its draft or reconciles a returned value, or a memo that does its work by writing a signal, has no output to compare, so its re-runs were reported as pure cost. `RerunEvent.changed` now reports `true` for these runs (as it already did for side-effect-only effects), so `WASTED_RECOMPUTE` no longer fires for them and `costs().wastedMs`, `expectNoWaste` and the performance tracks stop counting them as wasted.
diff --git a/packages/signals/src/core/attribution.ts b/packages/signals/src/core/attribution.ts
index ea75cc811..d5733cc55 100644
--- a/packages/signals/src/core/attribution.ts
+++ b/packages/signals/src/core/attribution.ts
@@ -169,8 +169,9 @@ export interface RerunEvent {
    * effect phase re-fires on every recompute), so the engine derives this
    * fact itself: an effect run whose compute output is identical to the
    * previous run's reports `changed: false` — the phase re-fired with the
-   * same input, pure waste. Side-effect-only computes (`undefined` output)
-   * are exempt: identity of `undefined` proves nothing about their work.
+   * same input, pure waste. Side-effect-only computes (`undefined` output —
+   * effects, projections, memos that work by writing) report `true`:
+   * identity of `undefined` proves nothing about their work.
    * Summed as `wastedMs` in costs() (plain, non-held runs only — see
    * `phase`).
    */
@@ -1394,7 +1395,10 @@ function checkHotTime(el: Computed<any>, selfMs: number, causes: ChangeRecord[])
  * names it while it happens, with the input that keeps triggering it. The
  * fix is upstream: an equality boundary on the part of the input the scope
  * depends on, or a narrower read. Plain runs only — a held or overlay run
- * may be replayed and is never blamed as waste.
+ * may be replayed and is never blamed as waste. A run that committed
+ * `undefined` arrives here as changed (see `recomputeEnd`): its work is a
+ * side effect (a projection's draft writes, a write from inside a memo), not
+ * its output.
  */
 function checkWastedRecompute(
   el: Computed<any>,
@@ -4165,6 +4169,12 @@ const engineHooks: AttributionHooks = {
         frame.prevValue,
         el._pendingValue !== NOT_PENDING ? el._pendingValue : el._value
       );
+    // The same exemption for memos: a run that committed `undefined` did its
+    // work as a side effect (a projection's draft writes or reconcile, a
+    // write from inside a memo), so core's `undefined === undefined` gate
+    // proves nothing. Reported as changed, so no consumer counts it as waste.
+    if (!changed && (el._pendingValue !== NOT_PENDING ? el._pendingValue : el._value) === undefined)
+      changed = true;
     if (frame.causes !== null)
       recordRerun(
         el,
diff --git a/packages/signals/tests/attribution-wasted-recompute.test.ts b/packages/signals/tests/attribution-wasted-recompute.test.ts
index 6d36f56f9..1179250c5 100644
--- a/packages/signals/tests/attribution-wasted-recompute.test.ts
+++ b/packages/signals/tests/attribution-wasted-recompute.test.ts
@@ -8,11 +8,12 @@
  * whose waste is cheap are not reported. `checks: false` folds it off.
  */
 import { afterEach, describe, expect, it, vi } from "vitest";
-import { attribution } from "../src/attribution.js";
+import { attribution, costs } from "../src/attribution.js";
 import type { AttributionOptions } from "../src/attribution.js";
 import {
   createEffect,
   createMemo,
+  createProjection,
   createRoot,
   createSignal,
   flush,
@@ -158,4 +159,72 @@ describe("WASTED_RECOMPUTE", () => {
     }
     expect(disabled.events).toHaveLength(0);
   });
+
+  describe("computeds whose value is undefined", () => {
+    function run(make: (x: () => number) => () => unknown) {
+      const [x, setX] = createSignal(0, { name: "x" });
+      let read!: () => unknown;
+      createRoot(() => {
+        read = make(x);
+      });
+      flush();
+      const { events } = capture();
+      for (let i = 1; i <= 10; i++) {
+        setX(i);
+        flush();
+        read();
+      }
+      return events;
+    }
+
+    it("a projection that mutates its draft is not waste", () => {
+      const events = run(x => {
+        const s = createProjection<{ v: number }>(
+          d => {
+            d.v = x();
+          },
+          { v: 0 },
+          { name: "draft projection" }
+        );
+        return () => s.v;
+      });
+      expect(events).toHaveLength(0);
+      // Every consumer reads the same fact: the records and costs() agree.
+      const reruns = attribution.history("rerun").filter(e => e.nodeName === "draft projection");
+      expect(reruns).toHaveLength(10);
+      expect(reruns.every(e => e.changed)).toBe(true);
+      const scope = costs().scopes.find(s => s.name === "draft projection");
+      expect(scope?.runs).toBe(10);
+      expect(scope?.wastedMs).toBe(0);
+    });
+
+    it("a projection that returns a value to reconcile is not waste", () => {
+      const events = run(x => {
+        const s = createProjection(() => ({ v: x() }), { v: 0 }, { name: "returning projection" });
+        return () => s.v;
+      });
+      expect(events).toHaveLength(0);
+    });
+
+    it("a memo that returns undefined and does its work by writing is not waste", () => {
+      const [y, setY] = createSignal(0, { name: "y", ownedWrite: true });
+      const events = run(x => {
+        createMemo(
+          () => {
+            setY(x() * 2);
+          },
+          { name: "sweep memo" }
+        );
+        return y;
+      });
+      expect(y()).toBe(20);
+      expect(events).toHaveLength(0);
+    });
+
+    it("a memo that keeps returning the same defined value is still waste", () => {
+      const events = run(x => createMemo(() => (x(), 7), { name: "constant memo" }));
+      expect(events).toHaveLength(1);
+      expect(events[0]).toMatchObject({ nodeName: "constant memo" });
+    });
+  });
 });