diff --git a/src/runtime/supervise/coordination-driver.test.ts b/src/runtime/supervise/coordination-driver.test.ts
new file mode 100644
index 00000000..31565144
--- /dev/null
+++ b/src/runtime/supervise/coordination-driver.test.ts
@@ -0,0 +1,70 @@
+import { describe, expect, it } from 'vitest'
+import { validateDriverPromptCache } from './coordination-driver'
+
+describe('validateDriverPromptCache', () => {
+  it('accepts fractional USD savings — the shape a real router reports', () => {
+    // The exact evidence that refused an otherwise healthy driver: tangle-router returns
+    // cache savings in dollars, which are fractional, and the count rule rejected them.
+    expect(
+      validateDriverPromptCache({ readTokens: 5888, writeTokens: 0, readSavingsUsd: 0.0034 }),
+    ).toBeUndefined()
+  })
+
+  it('still refuses a fractional TOKEN count', () => {
+    const error = validateDriverPromptCache({ readTokens: 12.5 })
+    expect(error?.message).toContain('"readTokens"')
+    expect(error?.message).toContain('non-negative safe integer')
+  })
+
+  it('refuses a negative USD amount and a non-finite one', () => {
+    expect(validateDriverPromptCache({ readSavingsUsd: -0.01 })?.message).toContain(
+      'non-negative finite number of dollars',
+    )
+    expect(validateDriverPromptCache({ writeCostUsd: Number.NaN })?.message).toContain(
+      'non-negative finite number of dollars',
+    )
+    expect(validateDriverPromptCache({ writeCostUsd: Number.POSITIVE_INFINITY })).toBeDefined()
+  })
+
+  it('accepts a whole-dollar USD amount and -0, and keeps an absurd one out', () => {
+    expect(validateDriverPromptCache({ readSavingsUsd: 5 })).toBeUndefined()
+    expect(validateDriverPromptCache({ readSavingsUsd: -0 })).toBeUndefined()
+    // The old all-integer rule rejected 1e308 as a side effect; the ceiling keeps that.
+    expect(validateDriverPromptCache({ readSavingsUsd: 1e308 })?.message).toContain('dollars')
+  })
+
+  it('accepts the full router-reported shape including missTokens', () => {
+    expect(
+      validateDriverPromptCache({
+        readTokens: 5888,
+        writeTokens: 0,
+        missTokens: 111,
+        readSavingsUsd: 0.0034,
+        status: 'hit',
+      }),
+    ).toBeUndefined()
+  })
+
+  it('refuses a negative token count and ignores string fields', () => {
+    expect(validateDriverPromptCache({ readTokens: -1 })?.message).toContain('safe integer')
+    expect(validateDriverPromptCache({ tier: 'ephemeral-1h' })).toBeUndefined()
+  })
+
+  it('treats absent evidence as acceptable rather than inventing zeroes', () => {
+    expect(validateDriverPromptCache(undefined)).toBeUndefined()
+    expect(validateDriverPromptCache({})).toBeUndefined()
+  })
+
+  it('classifies known schema members by what they are, not by their name', () => {
+    // readSavingsUsd is the schema's only dollar member; the token members stay counts even
+    // though nothing in their names says so.
+    expect(validateDriverPromptCache({ missTokens: 12.5 })?.message).toContain('safe integer')
+    expect(validateDriverPromptCache({ readSavingsUsd: 0.5 })).toBeUndefined()
+  })
+
+  it('falls back to the usd name-suffix convention for unknown pass-through fields', () => {
+    expect(validateDriverPromptCache({ savingsUSD: 1.25 })).toBeUndefined()
+    // `usdTokens` is a count despite carrying the substring — the suffix is what decides.
+    expect(validateDriverPromptCache({ usdTokens: 1.25 })?.message).toContain('safe integer')
+  })
+})
diff --git a/src/runtime/supervise/coordination-driver.ts b/src/runtime/supervise/coordination-driver.ts
index ecfdc79d..db340fc1 100644
--- a/src/runtime/supervise/coordination-driver.ts
+++ b/src/runtime/supervise/coordination-driver.ts
@@ -296,6 +296,53 @@ function providerAttemptEvidence(model: string | undefined): ProviderModelExecut
   )
 }
 
+/** The USD-denominated members of {@link PromptCacheUsage} — the schema, not a guess. Every
+ *  other known member (`readTokens`, `writeTokens`, `missTokens`) is a token COUNT. */
+const PROMPT_CACHE_USD_FIELDS: ReadonlySet<string> = new Set(['readSavingsUsd'])
+
+/** Dollar amounts above this are provider nonsense, not evidence. The old all-integer rule
+ *  rejected them as a side effect; keeping an explicit ceiling preserves that protection
+ *  without pretending a dollar amount is an integer. */
+const MAX_PROMPT_CACHE_USD = 1_000_000
+
+/**
+ * Validate provider-reported prompt-cache evidence.
+ *
+ * Prompt-cache carries two kinds of number and they obey different rules: token COUNTS are
+ * integers, and USD amounts are fractional by nature. Applying the count rule to a dollar
+ * field refuses every provider that reports cache savings in dollars — a healthy router
+ * response carrying `readSavingsUsd: 0.0034` failed the driver outright before this split.
+ *
+ * Classification is schema-first: a field named in {@link PromptCacheUsage} is validated by
+ * what that member IS. `promptCache` is an open record (the sandbox path forwards provider
+ * fields verbatim), so an unknown field falls back to the `usd` name-suffix convention —
+ * documented here as the contract a provider must follow to report dollars.
+ *
+ * Returns the refusal, or `undefined` when the evidence is acceptable.
+ */
+export function validateDriverPromptCache(
+  promptCache: Readonly<Record<string, number | string>> | undefined,
+): ValidationError | undefined {
+  for (const [field, value] of Object.entries(promptCache ?? {})) {
+    if (typeof value !== 'number') continue
+    const isUsdField = PROMPT_CACHE_USD_FIELDS.has(field) || /usd$/i.test(field)
+    if (isUsdField) {
+      if (!Number.isFinite(value) || value < 0 || value > MAX_PROMPT_CACHE_USD) {
+        return new ValidationError(
+          `driverAgent: prompt-cache field ${JSON.stringify(field)} must be a non-negative finite number of dollars`,
+        )
+      }
+      continue
+    }
+    if (!Number.isSafeInteger(value) || value < 0) {
+      return new ValidationError(
+        `driverAgent: prompt-cache field ${JSON.stringify(field)} must be a non-negative safe integer`,
+      )
+    }
+  }
+  return undefined
+}
+
 /**
  * Build the intelligent recursive driver. Its `act` is the LLM tool-loop; spawn it as a
  * `driverChild` (`driver-executor.ts`) to run it inside a nested scope, recursively.
@@ -496,14 +543,7 @@ export function driverAgent(opts: DriverAgentOptions): Agent<unknown, unknown> {
             'driverAgent: transportAttempts must be a positive safe integer when reported',
           )
         }
-        for (const [field, value] of Object.entries(res.promptCache ?? {})) {
-          if (typeof value === 'number' && (!Number.isSafeInteger(value) || value < 0)) {
-            evidenceError = new ValidationError(
-              `driverAgent: prompt-cache field ${JSON.stringify(field)} must be a non-negative safe integer`,
-            )
-            break
-          }
-        }
+        evidenceError = validateDriverPromptCache(res.promptCache) ?? evidenceError
         const trustedCost =
           res.costProvenance === 'provider-receipt' || res.costProvenance === 'billing-receipt'
         const cacheUsage = driverPromptCacheUsage(res.usage?.input, res.promptCache)