diff --git a/scripts/hooks/ecc-metrics-bridge.js b/scripts/hooks/ecc-metrics-bridge.js
index d7f97cfa..f446dd24 100644
--- a/scripts/hooks/ecc-metrics-bridge.js
+++ b/scripts/hooks/ecc-metrics-bridge.js
@@ -91,6 +91,13 @@ function readSessionCost(sessionId) {
       fs.readSync(fd, buf, 0, readSize, Math.max(0, stat.size - readSize));
       const lines = buf.toString('utf8').split('\n').filter(Boolean);
 
+      // Each row in costs.jsonl is *already* a cumulative session total — see
+      // scripts/hooks/cost-tracker.js: "Each row therefore represents the
+      // cumulative session total up to that point. To get per-session cost,
+      // take the last row per session_id." Summing every matching row
+      // therefore double-counts: for N rows of the same session it over-
+      // reports by roughly N(N+1)/2 / N = (N+1)/2 ×. Take the last matching
+      // row instead.
       let totalCost = 0;
       let totalIn = 0;
       let totalOut = 0;
@@ -98,9 +105,9 @@ function readSessionCost(sessionId) {
         try {
           const row = JSON.parse(line);
           if (row.session_id === sessionId) {
-            totalCost += toNumber(row.estimated_cost_usd);
-            totalIn += toNumber(row.input_tokens);
-            totalOut += toNumber(row.output_tokens);
+            totalCost = toNumber(row.estimated_cost_usd);
+            totalIn = toNumber(row.input_tokens);
+            totalOut = toNumber(row.output_tokens);
           }
         } catch {
           /* skip malformed lines */
diff --git a/tests/hooks/ecc-metrics-bridge.test.js b/tests/hooks/ecc-metrics-bridge.test.js
index ac7631ca..bf1cc72c 100644
--- a/tests/hooks/ecc-metrics-bridge.test.js
+++ b/tests/hooks/ecc-metrics-bridge.test.js
@@ -145,6 +145,45 @@ function runTests() {
     passed++;
   else failed++;
 
+  if (
+    test('readSessionCost returns the LAST cumulative row, not the sum (cost-tracker contract)', () => {
+      // cost-tracker.js writes one row per Stop event; each row is already
+      // a cumulative session total ("To get per-session cost, take the
+      // last row per session_id."). Summing across rows over-counts:
+      // 0.01 + 0.02 + 0.03 = 0.06, but the correct answer is 0.03.
+      const tmpHome = makeTempHome();
+      const originalHome = process.env.HOME;
+      const originalUserProfile = process.env.USERPROFILE;
+      try {
+        process.env.HOME = tmpHome;
+        process.env.USERPROFILE = tmpHome;
+        const metricsDir = path.join(tmpHome, '.claude', 'metrics');
+        fs.mkdirSync(metricsDir, { recursive: true });
+        fs.writeFileSync(
+          path.join(metricsDir, 'costs.jsonl'),
+          [
+            JSON.stringify({ session_id: 'S1', estimated_cost_usd: 0.01, input_tokens: 333, output_tokens: 166 }),
+            JSON.stringify({ session_id: 'S1', estimated_cost_usd: 0.02, input_tokens: 666, output_tokens: 333 }),
+            JSON.stringify({ session_id: 'S1', estimated_cost_usd: 0.03, input_tokens: 1000, output_tokens: 500 })
+          ].join('\n') + '\n',
+          'utf8'
+        );
+        const result = readSessionCost('S1');
+        assert.strictEqual(result.totalCost, 0.03, `expected last-row 0.03, got ${result.totalCost} (was the bug: 0.06)`);
+        assert.strictEqual(result.totalIn, 1000);
+        assert.strictEqual(result.totalOut, 500);
+      } finally {
+        if (originalHome === undefined) delete process.env.HOME;
+        else process.env.HOME = originalHome;
+        if (originalUserProfile === undefined) delete process.env.USERPROFILE;
+        else process.env.USERPROFILE = originalUserProfile;
+        fs.rmSync(tmpHome, { recursive: true, force: true });
+      }
+    })
+  )
+    passed++;
+  else failed++;
+
   if (
     test('readSessionCost does not include unrelated default-session rows', () => {
       const tmpHome = makeTempHome();