fix(session): warn on cache busts

packages/opencode/src/session/processor.ts

@@ -440,6 +440,14 @@ const layer = Layer.effect(
               usage: value.usage ?? new Usage({}),
               metadata: value.providerMetadata,
             })
+            const cacheWarning = Session.cacheBustWarning({
+              providerID: ctx.model.providerID,
+              modelID: ctx.model.id,
+              sessionID: ctx.sessionID,
+              finish: value.reason,
+              usage,
+            })
+            if (cacheWarning) yield* Effect.logWarning("possible Anthropic prompt cache bust", cacheWarning)
             ctx.assistantMessage.finish = value.reason
             ctx.assistantMessage.cost += usage.cost
             ctx.assistantMessage.tokens = usage.tokens

packages/opencode/src/session/session.ts

@@ -406,6 +406,33 @@ export const getUsage = (input: { model: Provider.Model; usage: Usage; metadata?
   }
 }
 
+export function cacheBustWarning(input: {
+  providerID: string
+  modelID: string
+  sessionID: string
+  finish: string
+  usage: ReturnType<typeof getUsage>
+}) {
+  const cacheWriteTokens = input.usage.tokens.cache.write
+  const cacheReadTokens = input.usage.tokens.cache.read
+  if (cacheWriteTokens < 50_000) return undefined
+  if (cacheReadTokens !== 0) return undefined
+
+  const provider = input.providerID.toLowerCase()
+  const model = input.modelID.toLowerCase()
+  if (!provider.includes("anthropic") && !model.includes("claude")) return undefined
+
+  return {
+    providerID: input.providerID,
+    modelID: input.modelID,
+    sessionID: input.sessionID,
+    finish: input.finish,
+    cost: input.usage.cost,
+    cacheWriteTokens,
+    cacheReadTokens,
+  }
+}
+
 export class BusyError extends Schema.TaggedErrorClass<BusyError>()("SessionBusyError", {
   sessionID: SessionID,
 }) {}

packages/opencode/test/session/compaction.test.ts

@@ -1579,6 +1579,44 @@ describe("SessionNs.getUsage", () => {
     expect(result.tokens.cache.write).toBe(300)
   })
 
+  test("flags likely Anthropic cache busts when a step rewrites cache without reads", () => {
+    const warning = SessionNs.cacheBustWarning({
+      providerID: "anthropic",
+      modelID: "claude-opus-5",
+      sessionID: "ses_test",
+      finish: "tool-calls",
+      usage: {
+        cost: 1.63,
+        tokens: { input: 3, output: 39, reasoning: 0, cache: { read: 0, write: 259_313 }, total: 259_355 },
+      },
+    })
+
+    expect(warning).toEqual({
+      providerID: "anthropic",
+      modelID: "claude-opus-5",
+      sessionID: "ses_test",
+      finish: "tool-calls",
+      cost: 1.63,
+      cacheWriteTokens: 259_313,
+      cacheReadTokens: 0,
+    })
+  })
+
+  test("does not flag ordinary cache hits", () => {
+    const warning = SessionNs.cacheBustWarning({
+      providerID: "anthropic",
+      modelID: "claude-opus-5",
+      sessionID: "ses_test",
+      finish: "stop",
+      usage: {
+        cost: 0.03,
+        tokens: { input: 3, output: 39, reasoning: 0, cache: { read: 200_000, write: 0 }, total: 200_042 },
+      },
+    })
+
+    expect(warning).toBeUndefined()
+  })
+
   test("subtracts cached tokens for anthropic provider", () => {
     const model = createModel({ context: 100_000, output: 32_000 })
     // AI SDK v6 normalizes inputTokens to include cached tokens for all providers