diff --git a/packages/core/src/session/usage.ts b/packages/core/src/session/usage.ts
index e55f4802f1..518415122c 100644
--- a/packages/core/src/session/usage.ts
+++ b/packages/core/src/session/usage.ts
@@ -5,7 +5,8 @@ import { Money } from "@opencode-ai/schema/money"
 import type { TokenUsage } from "@opencode-ai/schema/token-usage"
 import type { Model } from "../model.js"
 
-const safe = (value: number | undefined) => Math.max(0, Number.isFinite(value) ? (value ?? 0) : 0)
+const finite = (value: number) => (Number.isFinite(value) ? value : 0)
+const safe = (value: number | undefined) => Math.max(0, finite(value ?? 0))
 
 export const tokens = (usage: Usage | undefined): TokenUsage.Info => ({
   input: safe(usage?.nonCachedInputTokens),
@@ -26,10 +27,10 @@ export function calculateCost(costs: Model.Info["cost"], usage: TokenUsage.Info)
   const cost = tier ?? costs.find((cost) => cost.tier === undefined)
   if (!cost) return Money.USD.zero
   return Money.USD.make(
-    (usage.input * cost.input +
-      (usage.output + usage.reasoning) * cost.output +
-      usage.cache.read * cost.cache.read +
-      usage.cache.write * cost.cache.write) /
+    (usage.input * finite(cost.input) +
+      (usage.output + usage.reasoning) * finite(cost.output) +
+      usage.cache.read * finite(cost.cache.read) +
+      usage.cache.write * finite(cost.cache.write)) /
       1_000_000,
   )
 }
diff --git a/packages/core/test/session-runner.test.ts b/packages/core/test/session-runner.test.ts
index e55f1d8e27..d0167a6cec 100644
--- a/packages/core/test/session-runner.test.ts
+++ b/packages/core/test/session-runner.test.ts
@@ -197,6 +197,29 @@ test("calculates step cost using the matching context tier", () => {
   ).toBeCloseTo(0.0002926)
 })
 
+test("ignores malformed model cost fields", () => {
+  const costs = [
+    {
+      input: Money.USDPerMillionTokens.make(3),
+      output: Money.USDPerMillionTokens.make(15),
+      cache: {
+        read: Money.USDPerMillionTokens.make(0.3),
+        write: Money.USDPerMillionTokens.make(3.75),
+      },
+    },
+  ]
+  Object.assign(costs[0], { input: {} })
+
+  expect(
+    SessionUsage.calculateCost(costs, {
+      input: 1_000_000,
+      output: 100_000,
+      reasoning: 0,
+      cache: { read: 0, write: 0 },
+    }),
+  ).toBe(Money.USD.make(1.5))
+})
+
 test("does not apply an ineligible tier without base pricing", () => {
   expect(
     SessionUsage.calculateCost(