From 58a27b8b215fd4d85ca20c4f43a427bffedeb7de Mon Sep 17 00:00:00 2001
From: Vishwak Thatikonda <vishwakt@gmail.com>
Date: Thu, 7 May 2026 06:55:56 -0700
Subject: [PATCH] fix(metrics): account for all dimensions in
 setDefaultDimensions() limit check

The setDefaultDimensions() method previously only checked default dimension
count against the limit, ignoring regular dimensions added via addDimension()
and dimension sets from addDimensions(). This allowed silently exceeding
CloudWatch's 29-dimension limit.

Now uses getDimensionCount() which correctly sums all dimension types (default
+ regular + dimension sets), and only counts genuinely new keys to avoid
double-counting when overriding existing defaults.

Closes #5206
---
 packages/metrics/src/Metrics.ts               | 10 ++++--
 .../metrics/tests/unit/dimensions.test.ts     | 36 +++++++++++++++++++
 2 files changed, 43 insertions(+), 3 deletions(-)

diff --git a/packages/metrics/src/Metrics.ts b/packages/metrics/src/Metrics.ts
index 94d615c108..64f1540945 100644
--- a/packages/metrics/src/Metrics.ts
+++ b/packages/metrics/src/Metrics.ts
@@ -798,9 +798,13 @@ class Metrics extends Utility implements MetricsInterface {
     const newDimensions = this.#sanitizeDimensions(dimensions);
     const currentDefaultDimensions =
       this.#dimensionsStore.getDefaultDimensions();
-    const currentCount = Object.keys(currentDefaultDimensions).length;
-    const newSetCount = Object.keys(newDimensions).length;
-    if (currentCount + newSetCount >= MAX_DIMENSION_COUNT) {
+    const newKeysCount = Object.keys(newDimensions).filter(
+      (key) => !Object.hasOwn(currentDefaultDimensions, key)
+    ).length;
+    if (
+      this.#dimensionsStore.getDimensionCount() + newKeysCount >=
+      MAX_DIMENSION_COUNT
+    ) {
       throw new RangeError(
         `The number of metric dimensions must be lower than ${MAX_DIMENSION_COUNT}`
       );
diff --git a/packages/metrics/tests/unit/dimensions.test.ts b/packages/metrics/tests/unit/dimensions.test.ts
index d7c6febd3c..774a748795 100644
--- a/packages/metrics/tests/unit/dimensions.test.ts
+++ b/packages/metrics/tests/unit/dimensions.test.ts
@@ -361,6 +361,42 @@ describe('Working with dimensions', () => {
     );
   });
 
+  it('throws when setDefaultDimensions would exceed the limit with existing regular dimensions', () => {
+    // Prepare
+    const metrics = new Metrics({
+      singleMetric: true,
+    });
+
+    // Act
+    for (let i = 1; i < MAX_DIMENSION_COUNT - 1; i++) {
+      metrics.addDimension(`regular-${i}`, 'test');
+    }
+
+    // Assess
+    expect(() =>
+      metrics.setDefaultDimensions({ 'new-default': 'test' })
+    ).toThrow(
+      `The number of metric dimensions must be lower than ${MAX_DIMENSION_COUNT}`
+    );
+  });
+
+  it('allows overriding existing default dimension keys without triggering the limit', () => {
+    // Prepare
+    const metrics = new Metrics({
+      singleMetric: true,
+    });
+
+    // Act
+    for (let i = 1; i < MAX_DIMENSION_COUNT - 1; i++) {
+      metrics.setDefaultDimensions({ [`dimension-${i}`]: 'test' });
+    }
+
+    // Assess
+    expect(() =>
+      metrics.setDefaultDimensions({ 'dimension-1': 'updated' })
+    ).not.toThrow();
+  });
+
   it('throws when adding dimension sets would exceed the limit', () => {
     // Prepare
     const metrics = new Metrics({