diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip13Pow/miner/PoWRankEvaluator.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip13Pow/miner/PoWRankEvaluator.kt
index 5c0b3987f..a95a77f2d 100644
--- a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip13Pow/miner/PoWRankEvaluator.kt
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip13Pow/miner/PoWRankEvaluator.kt
@@ -53,7 +53,7 @@ class PoWRankEvaluator {
 
         fun calculatePowRankOf(id: HexKey): Int {
             var rank = 0
-            for (i in 0..id.length) {
+            for (i in id.indices) {
                 if (id[i] == '0') {
                     rank += 4
                 } else if (id[i] in '4'..'7') {
diff --git a/quartz/src/commonTest/kotlin/com/vitorpamplona/quartz/nip13Pow/PoWRankEvaluatorBoundaryTest.kt b/quartz/src/commonTest/kotlin/com/vitorpamplona/quartz/nip13Pow/PoWRankEvaluatorBoundaryTest.kt
new file mode 100644
index 000000000..e1fc01d05
--- /dev/null
+++ b/quartz/src/commonTest/kotlin/com/vitorpamplona/quartz/nip13Pow/PoWRankEvaluatorBoundaryTest.kt
@@ -0,0 +1,102 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.nip13Pow
+
+import com.vitorpamplona.quartz.nip13Pow.miner.PoWRankEvaluator
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class PoWRankEvaluatorBoundaryTest {
+    @Test
+    fun allZeroHexIdReturnsFullRankWithoutThrowing() {
+        val id = "0".repeat(64)
+        assertEquals(256, PoWRankEvaluator.calculatePowRankOf(id))
+    }
+
+    @Test
+    fun emptyStringReturnsZero() {
+        assertEquals(0, PoWRankEvaluator.calculatePowRankOf(""))
+    }
+
+    @Test
+    fun shortAllZeroIdsReturnFourBitsPerNibble() {
+        for (len in 1..16) {
+            val id = "0".repeat(len)
+            assertEquals(len * 4, PoWRankEvaluator.calculatePowRankOf(id), "len=$len")
+        }
+    }
+
+    @Test
+    fun leadingOneNibbleAddsThreeBitsThenBreaks() {
+        // '1' = 0001b → 3 leading zero bits; loop must break after, not keep scanning.
+        assertEquals(3, PoWRankEvaluator.calculatePowRankOf("1" + "0".repeat(63)))
+    }
+
+    @Test
+    fun leadingTwoOrThreeNibbleAddsTwoBitsThenBreaks() {
+        // '2' = 0010b, '3' = 0011b → 2 leading zero bits.
+        assertEquals(2, PoWRankEvaluator.calculatePowRankOf("2" + "0".repeat(63)))
+        assertEquals(2, PoWRankEvaluator.calculatePowRankOf("3" + "0".repeat(63)))
+    }
+
+    @Test
+    fun leadingFourThroughSevenNibbleAddsOneBitThenBreaks() {
+        // '4'..'7' all start with bit pattern 01xx → exactly 1 leading zero bit.
+        for (c in '4'..'7') {
+            assertEquals(1, PoWRankEvaluator.calculatePowRankOf(c + "0".repeat(63)), "leading=$c")
+        }
+    }
+
+    @Test
+    fun zerosThenLowNibbleAccumulatesThenBreaks() {
+        // Two zero nibbles (8 bits) + a '1' nibble (+3) = 11; remainder must not be scanned.
+        assertEquals(11, PoWRankEvaluator.calculatePowRankOf("001" + "f".repeat(61)))
+        // Three zero nibbles (12 bits) + a '4' nibble (+1) = 13.
+        assertEquals(13, PoWRankEvaluator.calculatePowRankOf("0004" + "f".repeat(60)))
+    }
+
+    @Test
+    fun leadingHighNibbleReturnsZero() {
+        // '8'..'f' start with bit 1 → no leading zero bits, immediate break with rank 0.
+        for (c in listOf('8', '9', 'a', 'c', 'f')) {
+            assertEquals(0, PoWRankEvaluator.calculatePowRankOf(c + "0".repeat(63)), "leading=$c")
+        }
+    }
+
+    @Test
+    fun knownNonZeroHashesRegression() {
+        // Fixtures mirrored from the existing androidDeviceTest so the commonTest
+        // target also covers the well-known cases.
+        val cases =
+            mapOf(
+                "000006d8c378af1779d2feebc7603a125d99eca0ccf1085959b307f64e5dd358" to 21,
+                "6bf5b4f434813c64b523d2b0e6efe18f3bd0cbbd0a5effd8ece9e00fd2531996" to 1,
+                "00003479309ecdb46b1c04ce129d2709378518588bed6776e60474ebde3159ae" to 18,
+                "01a76167d41add96be4959d9e618b7a35f26551d62c43c11e5e64094c6b53c83" to 7,
+                "ac4f44bae06a45ebe88cfbd3c66358750159650a26c0d79e8ccaa92457fca4f6" to 0,
+                "0000000000000000006cfbd3c66358750159650a26c0d79e8ccaa92457fca4f6" to 73,
+                "00000026c91e9fc75fdb95b367776e2594b931cebda6d5ca3622501006669c9e" to 26,
+            )
+        cases.forEach { (id, expected) ->
+            assertEquals(expected, PoWRankEvaluator.calculatePowRankOf(id), id)
+        }
+    }
+}