diff --git a/docs/ja-JP/skills/cost-aware-llm-pipeline/SKILL.md b/docs/ja-JP/skills/cost-aware-llm-pipeline/SKILL.md
index adb4b532c..196361e1a 100644
--- a/docs/ja-JP/skills/cost-aware-llm-pipeline/SKILL.md
+++ b/docs/ja-JP/skills/cost-aware-llm-pipeline/SKILL.md
@@ -158,6 +158,7 @@ def process(text: str, config: Config, tracker: CostTracker) -> tuple[Result, Co
 | Haiku 4.5 | $1.00 | $5.00 | 1x |
 | Sonnet 4.6 | $3.00 | $15.00 | 3x |
 | Opus 4.5 | $5.00 | $25.00 | 5x |
+| Fable 5 / Mythos 5 | $10.00 | $50.00 | 10x |
 
 ## ベストプラクティス
 
diff --git a/docs/zh-CN/skills/cost-aware-llm-pipeline/SKILL.md b/docs/zh-CN/skills/cost-aware-llm-pipeline/SKILL.md
index 396171570..f54f31297 100644
--- a/docs/zh-CN/skills/cost-aware-llm-pipeline/SKILL.md
+++ b/docs/zh-CN/skills/cost-aware-llm-pipeline/SKILL.md
@@ -158,6 +158,7 @@ def process(text: str, config: Config, tracker: CostTracker) -> tuple[Result, Co
 | Haiku 4.5 | $1.00 | $5.00 | 1x |
 | Sonnet 4.6 | $3.00 | $15.00 | 3x |
 | Opus 4.5 | $5.00 | $25.00 | 5x |
+| Fable 5 / Mythos 5 | $10.00 | $50.00 | 10x |
 
 ## 最佳实践
 
diff --git a/scripts/lib/cost-estimate.js b/scripts/lib/cost-estimate.js
index 0dfa9c834..cc7d3fdd7 100644
--- a/scripts/lib/cost-estimate.js
+++ b/scripts/lib/cost-estimate.js
@@ -48,11 +48,24 @@ const LEGACY_HAIKU_RE = /3-5-haiku|haiku-3-5/;
  * Estimate USD cost from token counts.
  * @param {string} model - Model name (may contain "haiku", "sonnet", "opus",
  *   "fable" or "mythos"); anything else is priced at sonnet rates.
- * @param {number} inputTokens
- * @param {number} outputTokens
+ * @param {number} inputTokens - Finite, non-negative.
+ * @param {number} outputTokens - Finite, non-negative.
  * @returns {number} Estimated cost in USD (rounded to 6 decimal places)
+ * @throws {RangeError} If either token count is negative or non-finite.
  */
 function estimateCost(model, inputTokens, outputTokens) {
+  // Callers use this to decide whether a call fits a budget, and both bad
+  // inputs defeat that check silently rather than loudly: a negative count
+  // yields a negative cost, and a non-finite one yields NaN, for which every
+  // `cost > budget` comparison is false. An unpriceable model still falls
+  // back to sonnet rates on purpose — that degrades an estimate; this
+  // corrupts one.
+  for (const [name, value] of [['inputTokens', inputTokens], ['outputTokens', outputTokens]]) {
+    if (!Number.isFinite(value) || value < 0) {
+      throw new RangeError(`${name} must be a finite non-negative number, got ${value}`);
+    }
+  }
+
   const normalized = String(model || '').toLowerCase();
   let rates = RATE_TABLE.sonnet;
   if (normalized.includes('haiku')) {
diff --git a/skills/cost-aware-llm-pipeline/SKILL.md b/skills/cost-aware-llm-pipeline/SKILL.md
index 63b10f6b3..34945844c 100644
--- a/skills/cost-aware-llm-pipeline/SKILL.md
+++ b/skills/cost-aware-llm-pipeline/SKILL.md
@@ -159,6 +159,7 @@ def process(text: str, config: Config, tracker: CostTracker) -> tuple[Result, Co
 | Haiku 4.5 | $1.00 | $5.00 | 1x |
 | Sonnet 4.6 | $3.00 | $15.00 | 3x |
 | Opus 4.5 | $5.00 | $25.00 | 5x |
+| Fable 5 / Mythos 5 | $10.00 | $50.00 | 10x |
 
 ## Best Practices
 
diff --git a/tests/lib/cost-estimate.test.js b/tests/lib/cost-estimate.test.js
index 2aa60770f..ce5a08193 100644
--- a/tests/lib/cost-estimate.test.js
+++ b/tests/lib/cost-estimate.test.js
@@ -172,6 +172,36 @@ function runTests() {
     passed++;
   else failed++;
 
+  // A negative count yields a negative cost and a non-finite one yields NaN,
+  // and `NaN > budget` is false — so bad input defeats a budget check silently
+  // rather than loudly. Pin the throw so a regression fails here.
+  for (const bad of [-1, -0.5, NaN, Infinity, -Infinity, undefined, null, '100']) {
+    if (
+      test(`rejects ${String(bad)} as an input token count`, () => {
+        assert.throws(() => estimateCost('sonnet', bad, 100), RangeError);
+      })
+    )
+      passed++;
+    else failed++;
+
+    if (
+      test(`rejects ${String(bad)} as an output token count`, () => {
+        assert.throws(() => estimateCost('sonnet', 100, bad), RangeError);
+      })
+    )
+      passed++;
+    else failed++;
+  }
+
+  if (
+    test('accepts zero and fractional token counts', () => {
+      assert.strictEqual(estimateCost('sonnet', 0, 0), 0);
+      assert.strictEqual(estimateCost('sonnet', 500_000, 0), 1.5);
+    })
+  )
+    passed++;
+  else failed++;
+
   // Summary
   console.log(`\nResults: ${passed} passed, ${failed} failed\n`);
   return { passed, failed };