diff --git a/packages/core/src/session/model-request.ts b/packages/core/src/session/model-request.ts
index 4cf3e4f8de7..3cd0a264203 100644
--- a/packages/core/src/session/model-request.ts
+++ b/packages/core/src/session/model-request.ts
@@ -3,7 +3,7 @@ export * as SessionModelRequest from "./model-request"
 import { LLM, Message, SystemPart, type LLMRequest } from "@opencode-ai/ai"
 import type { Content } from "@opencode-ai/schema/tool"
 import { SessionError } from "@opencode-ai/schema/session-error"
-import { Cause, Context, Effect, Layer, Result } from "effect"
+import { Cause, Config, Context, Effect, Layer, Result } from "effect"
 import { makeLocationNode } from "@opencode-ai/util/effect/app-node"
 import { App } from "../app"
 import { Model } from "../model"
@@ -13,6 +13,7 @@ import { QuestionTool } from "../tool/plugin/question"
 import { Tool } from "../tool"
 import { SessionContext } from "./context"
 import { SessionModelHeaders } from "./model-headers"
+import { PromptCacheDiagnostics } from "./prompt-cache-diagnostics"
 import { MAX_STEPS_PROMPT } from "./runner/max-steps"
 import PROMPT_DEFAULT from "./runner/prompt/base.txt"
 import { toLLMMessages } from "./runner/to-llm-message"
@@ -109,6 +110,11 @@ export const layer = Layer.effect(
   Effect.gen(function* () {
     const hooks = yield* PluginHooks.Service
     const app = yield* App.Metadata
+    const diagnostics = yield* Config.boolean("OPENCODE_PROMPT_CACHE_DIAGNOSTICS").pipe(
+      Config.withDefault(false),
+      Effect.orDie,
+    )
+    const promptCacheSnapshots = diagnostics ? new Map<string, PromptCacheDiagnostics.Snapshot>() : undefined
 
     const prepare = Effect.fn("SessionModelRequest.prepare")(function* (input: PrepareInput) {
       const session = input.context.session
@@ -156,6 +162,23 @@ export const layer = Layer.effect(
         tools: hookedTools,
         toolChoice: stepLimitReached ? "none" : undefined,
       })
+      if (promptCacheSnapshots) {
+        const current = PromptCacheDiagnostics.snapshot(request)
+        const comparison = PromptCacheDiagnostics.compare(promptCacheSnapshots.get(session.id), current)
+        promptCacheSnapshots.delete(session.id)
+        promptCacheSnapshots.set(session.id, current)
+        const oldest = promptCacheSnapshots.keys().next().value
+        if (promptCacheSnapshots.size > 100 && oldest !== undefined) promptCacheSnapshots.delete(oldest)
+        yield* Effect.logInfo("prompt cache prefix").pipe(
+          Effect.annotateLogs({
+            sessionID: session.id,
+            toolCount: current.tools.length,
+            systemParts: current.system.length,
+            messageCount: current.messages.length,
+            ...comparison,
+          }),
+        )
+      }
       const executeTool: Prepared["executeTool"] = (executeInput) => {
         if (stepLimitReached)
           return new Tool.Error({ message: "Tools are disabled after the maximum agent steps" })
diff --git a/packages/core/src/session/prompt-cache-diagnostics.ts b/packages/core/src/session/prompt-cache-diagnostics.ts
new file mode 100644
index 00000000000..99ab54576a1
--- /dev/null
+++ b/packages/core/src/session/prompt-cache-diagnostics.ts
@@ -0,0 +1,95 @@
+export * as PromptCacheDiagnostics from "./prompt-cache-diagnostics"
+
+import type { LLMRequest } from "@opencode-ai/ai"
+import { Hash } from "@opencode-ai/util/hash"
+
+interface Entry {
+  readonly label: string
+  readonly hash: string
+}
+
+export interface Snapshot {
+  readonly settings: string
+  readonly tools: ReadonlyArray<Entry>
+  readonly system: ReadonlyArray<Entry>
+  readonly messages: ReadonlyArray<Entry>
+}
+
+export type Comparison =
+  | { readonly status: "initial" }
+  | { readonly status: "stable"; readonly messages: number }
+  | { readonly status: "append-only"; readonly previousMessages: number; readonly currentMessages: number }
+  | {
+      readonly status: "changed"
+      readonly component: "settings" | "tools" | "system" | "messages"
+      readonly index: number
+      readonly label: string
+    }
+
+const hash = (value: unknown) => Hash.sha256(JSON.stringify(value)).slice(0, 16)
+
+export function snapshot(request: LLMRequest): Snapshot {
+  return {
+    settings: hash({
+      route: request.model.route.id,
+      provider: request.model.provider,
+      model: request.model.id,
+      modelDefaults: request.model.defaults,
+      compatibility: request.model.compatibility,
+      routeDefaults: {
+        generation: request.model.route.defaults.generation,
+        providerOptions: request.model.route.defaults.providerOptions,
+        http: request.model.route.defaults.http,
+      },
+      generation: request.generation,
+      providerOptions: request.providerOptions,
+      http: request.http,
+      toolChoice: request.toolChoice,
+      cache: request.cache,
+    }),
+    tools: request.tools.map((tool) => ({ label: tool.name, hash: hash(tool) })),
+    system: request.system.map((part, index) => ({ label: `system[${index}]`, hash: hash(part) })),
+    messages: request.messages.map((message, index) => ({
+      label: message.id ?? `${message.role}[${index}]`,
+      hash: hash(message),
+    })),
+  }
+}
+
+export function compare(previous: Snapshot | undefined, current: Snapshot): Comparison {
+  if (!previous) return { status: "initial" }
+  if (previous.settings !== current.settings)
+    return {
+      status: "changed",
+      component: "settings",
+      index: 0,
+      label: "model settings",
+    }
+  const tools = firstChange(previous.tools, current.tools, false)
+  if (tools) return { status: "changed", component: "tools", ...tools }
+  const system = firstChange(previous.system, current.system, false)
+  if (system) return { status: "changed", component: "system", ...system }
+  const messages = firstChange(previous.messages, current.messages, true)
+  if (messages) return { status: "changed", component: "messages", ...messages }
+  if (previous.messages.length === current.messages.length)
+    return { status: "stable", messages: current.messages.length }
+  return {
+    status: "append-only",
+    previousMessages: previous.messages.length,
+    currentMessages: current.messages.length,
+  }
+}
+
+function firstChange(previous: ReadonlyArray<Entry>, current: ReadonlyArray<Entry>, allowAppend: boolean) {
+  const index = previous.findIndex((entry, index) => entry.hash !== current[index]?.hash)
+  if (index >= 0)
+    return {
+      index,
+      label: current[index]?.label ?? previous[index]?.label ?? `entry[${index}]`,
+    }
+  if (current.length === previous.length || (allowAppend && current.length > previous.length)) return
+  return {
+    index: previous.length,
+    label: current[previous.length]?.label ?? `entry[${previous.length}]`,
+  }
+}
diff --git a/packages/core/test/prompt-cache-diagnostics.test.ts b/packages/core/test/prompt-cache-diagnostics.test.ts
new file mode 100644
index 00000000000..4049c99465f
--- /dev/null
+++ b/packages/core/test/prompt-cache-diagnostics.test.ts
@@ -0,0 +1,54 @@
+import { describe, expect, test } from "bun:test"
+import { GenerationOptions, LLM, LLMRequest, Message, Model, ToolDefinition } from "@opencode-ai/ai"
+import { OpenAIChat } from "@opencode-ai/ai/protocols"
+import { PromptCacheDiagnostics } from "@opencode-ai/core/session/prompt-cache-diagnostics"
+
+const model = Model.make({ id: "test", provider: "test", route: OpenAIChat.route })
+const tool = ToolDefinition.make({
+  name: "read",
+  description: "Read a file",
+  inputSchema: { type: "object", properties: {} },
+})
+
+const request = LLM.request({
+  model,
+  system: "System",
+  prompt: "First",
+  tools: [tool],
+})
+const compare = (current: LLMRequest) =>
+  PromptCacheDiagnostics.compare(PromptCacheDiagnostics.snapshot(request), PromptCacheDiagnostics.snapshot(current))
+
+describe("PromptCacheDiagnostics", () => {
+  test("distinguishes initial and stable requests", () => {
+    const snapshot = PromptCacheDiagnostics.snapshot(request)
+    expect(PromptCacheDiagnostics.compare(undefined, snapshot)).toEqual({ status: "initial" })
+    expect(PromptCacheDiagnostics.compare(snapshot, snapshot)).toEqual({ status: "stable", messages: 1 })
+  })
+
+  test("recognizes append-only history", () => {
+    const current = LLMRequest.update(request, { messages: [...request.messages, Message.assistant("Second")] })
+    expect(compare(current)).toEqual({ status: "append-only", previousMessages: 1, currentMessages: 2 })
+  })
+
+  test("detects cache-sensitive setting changes", () => {
+    const current = LLMRequest.update(request, { generation: GenerationOptions.make({ temperature: 0.5 }) })
+    expect(compare(current)).toEqual({ status: "changed", component: "settings", index: 0, label: "model settings" })
+  })
+
+  test("finds the first changed prefix component", () => {
+    const changedTool = ToolDefinition.make({ ...tool, description: "Read one file" })
+    const current = LLMRequest.update(request, { tools: [changedTool] })
+    expect(compare(current)).toEqual({ status: "changed", component: "tools", index: 0, label: "read" })
+  })
+
+  test("treats appended tools as a prefix change", () => {
+    const write = ToolDefinition.make({
+      name: "write",
+      description: "Write a file",
+      inputSchema: { type: "object", properties: {} },
+    })
+    const current = LLMRequest.update(request, { tools: [...request.tools, write] })
+    expect(compare(current)).toEqual({ status: "changed", component: "tools", index: 1, label: "write" })
+  })
+})
diff --git a/packages/core/test/session-runner.test.ts b/packages/core/test/session-runner.test.ts
index 7be6e0cc5ef..e7c8e057835 100644
--- a/packages/core/test/session-runner.test.ts
+++ b/packages/core/test/session-runner.test.ts
@@ -42,6 +42,7 @@ 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 { PluginSupervisor } from "@opencode-ai/core/plugin/supervisor"
 import { PluginHooks } from "@opencode-ai/core/plugin/hooks"
@@ -1315,6 +1316,12 @@ 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"],