diff --git a/scripts/hooks/cost-tracker.js b/scripts/hooks/cost-tracker.js
index 1eb9c13f8..c43d0e6dc 100755
--- a/scripts/hooks/cost-tracker.js
+++ b/scripts/hooks/cost-tracker.js
@@ -174,7 +174,11 @@ function sumUsageFromTranscript(transcriptPath) {
     const key = (typeof msg.id === 'string' && msg.id)
       ? msg.id
       : `__line_${++syntheticKey}`;
-    usageById.set(key, { usage: msg.usage, model: msg.model });
+    const previous = usageById.get(key);
+    const requestModel = msg.model && msg.model !== 'unknown'
+      ? msg.model
+      : previous && previous.model;
+    usageById.set(key, { usage: msg.usage, model: requestModel });
 
     if (msg.model && msg.model !== 'unknown') model = msg.model;
   }
diff --git a/tests/hooks/cost-tracker.test.js b/tests/hooks/cost-tracker.test.js
index f27acef42..b64815aee 100644
--- a/tests/hooks/cost-tracker.test.js
+++ b/tests/hooks/cost-tracker.test.js
@@ -712,6 +712,20 @@ function runTests() {
       model: 'claude-haiku-5-5',
       usage: { input_tokens: 50000, output_tokens: 1000, cache_read_input_tokens: 50001 }
     }]).estimated_cost_usd, 0.03, 'cached prompt tokens count toward the long-prompt threshold');
+    assert.strictEqual(priceMessages([{
+      model: 'claude-haiku-5-5',
+      usage: { input_tokens: 50000, cache_creation_input_tokens: 50001 }
+    }]).estimated_cost_usd, 0.056251, 'cache writes count toward and use the long-prompt tier');
+    for (const laterModel of [undefined, 'unknown']) {
+      const original = message('claude-haiku-5-5', 500, 500, 'duplicate');
+      const later = message(laterModel, 1000, 1000, 'duplicate');
+      const mixed = priceMessages([
+        original, later, message('claude-opus-5-5', 1000, 1000, 'other')
+      ]);
+      assert.strictEqual(mixed.input_tokens, 2000, 'the latest duplicate usage replaces earlier usage');
+      assert.strictEqual(mixed.estimated_cost_usd, 0.0246,
+        'a duplicate without a known model retains its earlier request model');
+    }
     const first = message('claude-haiku-5-5', 60000, 1000, 'first');
     const second = message('claude-haiku-5-5', 60000, 1000, 'second');
     const row = priceMessages([first, first, second]);