test(core): assert prompt cache prefix stability

packages/core/test/session-runner.test.ts

@@ -41,6 +41,7 @@ import { SessionRunCoordinator } from "@opencode-ai/core/session/run-coordinator
 import { SessionRunner } from "@opencode-ai/core/session/runner"
 import * as SessionRunnerLLM from "@opencode-ai/core/session/runner/llm"
 import { SessionRunnerModel } from "@opencode-ai/core/session/runner/model"
+import { PromptCacheDiagnostics } from "@opencode-ai/core/session/prompt-cache-diagnostics"
 import { SessionUsage } from "@opencode-ai/core/session/usage"
 import { ToolRegistry } from "@opencode-ai/core/tool/registry"
 import { CodeMode } from "@opencode-ai/core/codemode"
@@ -1366,6 +1367,12 @@ describe("SessionRunnerLLM", () => {
       yield* admit(session, "Second")
       yield* session.resume(sessionID)
 
+      expect(
+        PromptCacheDiagnostics.compare(
+          PromptCacheDiagnostics.snapshot(requests[0]!),
+          PromptCacheDiagnostics.snapshot(requests[1]!),
+        ),
+      ).toEqual({ status: "append-only", previousMessages: 1, currentMessages: 3 })
       expect(requests.map((request) => request.system.map((part) => part.text))).toEqual([
         [defaultSystem, "Initial context"],
         [defaultSystem, "Initial context"],