diff --git a/packages/ai/src/protocols/open-responses.ts b/packages/ai/src/protocols/open-responses.ts
index 97ad084614..c9a86a915b 100644
--- a/packages/ai/src/protocols/open-responses.ts
+++ b/packages/ai/src/protocols/open-responses.ts
@@ -542,7 +542,7 @@ const lowerOptions = (request: LLMRequest) => {
   return {
     ...(options.instructions ? { instructions: options.instructions } : {}),
     ...(options.store !== undefined ? { store: options.store } : {}),
-    ...(options.promptCacheKey ? { prompt_cache_key: options.promptCacheKey } : {}),
+    ...(request.promptCacheKey ? { prompt_cache_key: request.promptCacheKey } : {}),
     ...(options.include ? { include: options.include } : {}),
     ...(options.reasoningEffort || options.reasoningSummary
       ? { reasoning: { effort: options.reasoningEffort, summary: options.reasoningSummary } }
diff --git a/packages/ai/src/protocols/utils/open-responses-options.ts b/packages/ai/src/protocols/utils/open-responses-options.ts
index 80288ca5cb..38a0e7063f 100644
--- a/packages/ai/src/protocols/utils/open-responses-options.ts
+++ b/packages/ai/src/protocols/utils/open-responses-options.ts
@@ -33,7 +33,6 @@ export const ServiceTierSchema = Schema.Literals(ServiceTiers)
 export interface Resolved {
   readonly instructions?: string
   readonly store?: boolean
-  readonly promptCacheKey?: string
   readonly reasoningEffort?: string
   readonly reasoningSummary?: "auto" | "concise" | "detailed"
   readonly include?: ReadonlyArray<ResponseIncludable>
@@ -50,7 +49,6 @@ export const resolve = (request: LLMRequest): Resolved => {
   return {
     instructions: typeof input?.instructions === "string" ? input.instructions : undefined,
     store: typeof input?.store === "boolean" ? input.store : undefined,
-    promptCacheKey: typeof input?.promptCacheKey === "string" ? input.promptCacheKey : undefined,
     reasoningEffort: typeof input?.reasoningEffort === "string" ? input.reasoningEffort : undefined,
     reasoningSummary:
       reasoningSummary === "auto" || reasoningSummary === "concise" || reasoningSummary === "detailed"
diff --git a/packages/ai/src/providers/open-responses-options.ts b/packages/ai/src/providers/open-responses-options.ts
index 686cdcb808..b0194da2a4 100644
--- a/packages/ai/src/providers/open-responses-options.ts
+++ b/packages/ai/src/providers/open-responses-options.ts
@@ -5,7 +5,6 @@ export interface OpenResponsesOptionsInput {
   readonly [key: string]: unknown
   readonly instructions?: string
   readonly store?: boolean
-  readonly promptCacheKey?: string
   readonly reasoningEffort?: ReasoningEffort
   readonly reasoningSummary?: "auto" | "concise" | "detailed"
   readonly include?: ReadonlyArray<ResponseIncludable>
diff --git a/packages/ai/src/providers/openai-options.ts b/packages/ai/src/providers/openai-options.ts
index 86c7c2e334..c2336b5968 100644
--- a/packages/ai/src/providers/openai-options.ts
+++ b/packages/ai/src/providers/openai-options.ts
@@ -17,7 +17,6 @@ const openAIProviderOptions = (options: OpenAIOptionsInput | undefined): Provide
   const openai = Object.fromEntries(
     definedEntries({
       store: options?.store,
-      promptCacheKey: options?.promptCacheKey,
       reasoningEffort: options?.reasoningEffort,
       reasoningSummary: options?.reasoningSummary,
       include: options?.include,
diff --git a/packages/ai/src/providers/openrouter.ts b/packages/ai/src/providers/openrouter.ts
index bacb355e1a..935a6f05b4 100644
--- a/packages/ai/src/providers/openrouter.ts
+++ b/packages/ai/src/providers/openrouter.ts
@@ -55,7 +55,6 @@ export interface OpenRouterOptions {
   readonly debug?: Readonly<{ echo_upstream_body?: boolean }>
   readonly models?: ReadonlyArray<string>
   readonly plugins?: ReadonlyArray<OpenRouterPlugin>
-  readonly promptCacheKey?: string
   readonly provider?: OpenRouterProviderRouting
   readonly reasoning?: Readonly<{
     enabled?: boolean
@@ -122,6 +121,7 @@ export const protocol = Protocol.make({
             ...body,
             messages,
             ...bodyOptions(request.providerOptions?.openrouter),
+            ...(request.promptCacheKey ? { prompt_cache_key: request.promptCacheKey } : {}),
           } as OpenRouterBody
         }),
       ),
@@ -161,7 +161,6 @@ const bodyOptions = (input: unknown) => {
     ...(isRecord(debug) ? { debug } : {}),
     ...(typeof user === "string" ? { user } : {}),
     ...(isRecord(reasoning) ? { reasoning } : {}),
-    ...(typeof promptCacheKey === "string" ? { prompt_cache_key: promptCacheKey } : {}),
   }
 }
 
diff --git a/packages/ai/src/schema/messages.ts b/packages/ai/src/schema/messages.ts
index d3689dc0e3..bf8ffba3f0 100644
--- a/packages/ai/src/schema/messages.ts
+++ b/packages/ai/src/schema/messages.ts
@@ -272,6 +272,8 @@ export class LLMRequest extends Schema.Class<LLMRequest>("LLM.Request")({
   providerOptions: Schema.optional(ProviderOptions),
   http: Schema.optional(HttpOptions),
   cache: Schema.optional(CachePolicy),
+  // Stable cache affinity for protocols that support provider-managed prompt caching.
+  promptCacheKey: Schema.optional(Schema.String),
   metadata: Schema.optional(Schema.Record(Schema.String, Schema.Unknown)),
 }) {}
 
@@ -289,6 +291,7 @@ export namespace LLMRequest {
     providerOptions: request.providerOptions,
     http: request.http,
     cache: request.cache,
+    promptCacheKey: request.promptCacheKey,
     metadata: request.metadata,
   })
 
diff --git a/packages/ai/test/provider-options/cloudflare.types.ts b/packages/ai/test/provider-options/cloudflare.types.ts
index 6e2af83bae..e98d5153ca 100644
--- a/packages/ai/test/provider-options/cloudflare.types.ts
+++ b/packages/ai/test/provider-options/cloudflare.types.ts
@@ -3,11 +3,11 @@ import { CloudflareWorkersAI } from "../../src/providers"
 
 const model = CloudflareWorkersAI.configure({ accountId: "account", apiKey: "test" }).model("model")
 
-LLM.request({ model, prompt: "Hello", providerOptions: { openai: { promptCacheKey: "cache" } } })
+LLM.request({ model, prompt: "Hello", promptCacheKey: "cache" })
 
 LLM.request({
   model,
   prompt: "Hello",
-  // @ts-expect-error Cloudflare's OpenAI-compatible prompt cache key must be a string.
-  providerOptions: { openai: { promptCacheKey: 1 } },
+  // @ts-expect-error Prompt cache keys must be strings.
+  promptCacheKey: 1,
 })
diff --git a/packages/ai/test/provider/openai-responses-cache.recorded.test.ts b/packages/ai/test/provider/openai-responses-cache.recorded.test.ts
index 638c30e667..fa764f3535 100644
--- a/packages/ai/test/provider/openai-responses-cache.recorded.test.ts
+++ b/packages/ai/test/provider/openai-responses-cache.recorded.test.ts
@@ -20,7 +20,7 @@ const cacheRequest = LLM.request({
   system: LARGE_CACHEABLE_SYSTEM,
   prompt: "Say hi.",
   generation: { maxTokens: 16, temperature: 0 },
-  providerOptions: { openai: { promptCacheKey: "recorded-cache-test" } },
+  promptCacheKey: "recorded-cache-test",
 })
 
 const recorded = recordedTests({
diff --git a/packages/ai/test/provider/openai-responses.test.ts b/packages/ai/test/provider/openai-responses.test.ts
index 3b9a0b76aa..d42c972045 100644
--- a/packages/ai/test/provider/openai-responses.test.ts
+++ b/packages/ai/test/provider/openai-responses.test.ts
@@ -680,9 +680,9 @@ describe("OpenAI Responses route", () => {
         LLM.request({
           model: OpenAI.configure({ baseURL: "https://api.openai.test/v1/", apiKey: "test" }).model("gpt-5.2"),
           prompt: "think",
+          promptCacheKey: "session_123",
           providerOptions: {
             openai: {
-              promptCacheKey: "session_123",
               reasoningEffort: "high",
               reasoningSummary: "auto",
               include: ["reasoning.encrypted_content"],
@@ -801,17 +801,16 @@ describe("OpenAI Responses route", () => {
     }),
   )
 
-  it.effect("request OpenAI provider options override route defaults", () =>
+  it.effect("maps the request prompt cache key", () =>
     Effect.gen(function* () {
       const prepared = yield* compileRequest(
         LLM.request({
           model: OpenAI.configure({
             baseURL: "https://api.openai.test/v1/",
             apiKey: "test",
-            providerOptions: { openai: { promptCacheKey: "model_cache" } },
           }).model("gpt-4.1-mini"),
           prompt: "no cache",
-          providerOptions: { openai: { promptCacheKey: "request_cache" } },
+          promptCacheKey: "request_cache",
         }),
       )
 
diff --git a/packages/ai/test/provider/openrouter.test.ts b/packages/ai/test/provider/openrouter.test.ts
index 9f6efe5bfc..6077e5c797 100644
--- a/packages/ai/test/provider/openrouter.test.ts
+++ b/packages/ai/test/provider/openrouter.test.ts
@@ -162,7 +162,6 @@ describe("OpenRouter", () => {
               openrouter: {
                 usage: true,
                 reasoning: { effort: "high" },
-                promptCacheKey: "session_123",
                 models: ["anthropic/claude-sonnet-4.6", "google/gemini-3.1-pro"],
                 provider: { order: ["anthropic", "google"], require_parameters: true },
                 plugins: [{ id: "response-healing" }],
@@ -174,6 +173,7 @@ describe("OpenRouter", () => {
             },
           }).model("anthropic/claude-3.7-sonnet:thinking"),
           prompt: "Think briefly.",
+          promptCacheKey: "session_123",
         }),
       )
 
diff --git a/packages/core/src/aisdk-native.ts b/packages/core/src/aisdk-native.ts
index 0d888bfc1b..32f00f1181 100644
--- a/packages/core/src/aisdk-native.ts
+++ b/packages/core/src/aisdk-native.ts
@@ -108,6 +108,7 @@ function mapOpenRouterOptions(settings: Readonly<Record<string, unknown>>) {
           "extraBody",
           "fetch",
           "headers",
+          "promptCacheKey",
           "timeout",
         ].includes(key),
     ),
@@ -124,7 +125,6 @@ function mapXAIOptions(settings: Readonly<Record<string, unknown>>) {
   const options = {
     ...(typeof settings.reasoningEffort === "string" ? { reasoningEffort: settings.reasoningEffort } : {}),
     ...(typeof settings.store === "boolean" ? { store: settings.store } : {}),
-    ...(typeof settings.promptCacheKey === "string" ? { promptCacheKey: settings.promptCacheKey } : {}),
   }
   if (Object.keys(options).length === 0) return {}
   return { providerOptions: { xai: options } }
diff --git a/packages/core/src/session/compaction.ts b/packages/core/src/session/compaction.ts
index b36d997727..3c47a2b3e8 100644
--- a/packages/core/src/session/compaction.ts
+++ b/packages/core/src/session/compaction.ts
@@ -10,6 +10,7 @@ import { llmClient } from "../effect/app-node-platform"
 import { SessionEvent } from "./event"
 import type { SessionMessage } from "./message"
 import { SessionModelHeaders } from "./model-headers"
+import { SessionPromptCacheKey } from "./prompt-cache-key"
 import { App } from "../app"
 import { SessionRunnerModel } from "./runner/model"
 import { SessionSchema } from "./schema"
@@ -257,6 +258,7 @@ const make = (dependencies: Dependencies) => {
       .stream(
         LLM.request({
           model: plan.model,
+          promptCacheKey: SessionPromptCacheKey.make(plan.session.id),
           http: { headers: SessionModelHeaders.make(plan.session, dependencies.app) },
           messages: [Message.user(plan.prompt)],
           tools: [],
diff --git a/packages/core/src/session/generate-node.ts b/packages/core/src/session/generate-node.ts
index eed4d9763d..a36582ce38 100644
--- a/packages/core/src/session/generate-node.ts
+++ b/packages/core/src/session/generate-node.ts
@@ -11,6 +11,7 @@ import { SessionContext } from "./context"
 import { SessionGenerate } from "./generate"
 import { SessionHistory } from "./history"
 import { SessionModelHeaders } from "./model-headers"
+import { SessionPromptCacheKey } from "./prompt-cache-key"
 import { SessionRunnerModel } from "./runner/model"
 import PROMPT_DEFAULT from "./runner/prompt/base.txt"
 import { toLLMMessages } from "./runner/to-llm-message"
@@ -31,9 +32,6 @@ export const layer = Layer.effect(
         const model = yield* models.resolve(selection.session)
         const history = yield* SessionHistory.preview(database.db, selection.session.id, selection.instructions)
         const providerMetadataKey = model.model.route.providerMetadataKey ?? model.model.provider
-        const promptCacheKey = /^ses_[0-9a-f]{64}$/.test(selection.session.id)
-          ? selection.session.id.slice(4)
-          : selection.session.id
         const tools = selection.tools
         const toolDefinitions = tools.definitions
         const toolsByName = new Map(toolDefinitions.map((tool) => [tool.name, tool]))
@@ -71,7 +69,7 @@ export const layer = Layer.effect(
           LLM.request({
             model: model.model,
             http: { headers: SessionModelHeaders.make(selection.session, app) },
-            providerOptions: { [providerMetadataKey]: { promptCacheKey } },
+            promptCacheKey: SessionPromptCacheKey.make(selection.session.id),
             system: contextEvent.system,
             messages: contextEvent.messages,
             tools: hookedTools,
diff --git a/packages/core/src/session/model-request.ts b/packages/core/src/session/model-request.ts
index fab9e4c0e4..6e8c9306d5 100644
--- a/packages/core/src/session/model-request.ts
+++ b/packages/core/src/session/model-request.ts
@@ -14,6 +14,7 @@ import { QuestionTool } from "../tool/plugin/question"
 import { Tool } from "../tool"
 import { SessionContext } from "./context"
 import { SessionModelHeaders } from "./model-headers"
+import { SessionPromptCacheKey } from "./prompt-cache-key"
 import { PromptCacheDiagnostics } from "./prompt-cache-diagnostics"
 import { MAX_STEPS_PROMPT } from "./runner/max-steps"
 import PROMPT_DEFAULT from "./runner/prompt/base.txt"
@@ -183,7 +184,6 @@ export const layer = Layer.effect(
       // The final Step keeps definitions available to protocols with native "none",
       // preserving their prompt cache prefix. Calls are still rejected at execution.
       const tools = input.context.tools
-      const promptCacheKey = /^ses_[0-9a-f]{64}$/.test(session.id) ? session.id.slice(4) : session.id
       const system = [agent.info.system ? agent.info.system : PROMPT_DEFAULT, input.context.initial]
         .filter((part) => part.length > 0)
         .map(SystemPart.make)
@@ -213,7 +213,7 @@ export const layer = Layer.effect(
         http: {
           headers: SessionModelHeaders.make(session, app),
         },
-        providerOptions: { [providerMetadataKey]: { promptCacheKey } },
+        promptCacheKey: SessionPromptCacheKey.make(session.id),
         system: contextEvent.system,
         messages: boundImages(unsupportedParts(contextEvent.messages, resolved.capabilities)),
         tools: hookedTools,
diff --git a/packages/core/src/session/prompt-cache-key.ts b/packages/core/src/session/prompt-cache-key.ts
new file mode 100644
index 0000000000..6c8ec873b2
--- /dev/null
+++ b/packages/core/src/session/prompt-cache-key.ts
@@ -0,0 +1,6 @@
+export * as SessionPromptCacheKey from "./prompt-cache-key"
+
+import { SessionSchema } from "./schema"
+
+export const make = (sessionID: SessionSchema.ID) =>
+  /^ses_[0-9a-f]{64}$/.test(sessionID) ? sessionID.slice(4) : sessionID
diff --git a/packages/core/src/session/title.ts b/packages/core/src/session/title.ts
index 0f2d611f3d..d4d134cf78 100644
--- a/packages/core/src/session/title.ts
+++ b/packages/core/src/session/title.ts
@@ -12,6 +12,7 @@ import { llmClient } from "../effect/app-node-platform"
 import { SessionEvent } from "./event"
 import { SessionHistory } from "./history"
 import { SessionModelHeaders } from "./model-headers"
+import { SessionPromptCacheKey } from "./prompt-cache-key"
 import { SessionRunnerModel } from "./runner/model"
 import { SessionSchema } from "./schema"
 import { SessionUsage } from "./usage"
@@ -80,6 +81,7 @@ const make = (dependencies: Dependencies) => {
       .stream(
         LLM.request({
           model: resolved.model,
+          promptCacheKey: SessionPromptCacheKey.make(session.id),
           http: { headers: SessionModelHeaders.make(session, dependencies.app) },
           system: agent.system,
           messages: [Message.user(firstUser.text)],
diff --git a/packages/core/test/aisdk-native.test.ts b/packages/core/test/aisdk-native.test.ts
index 6e243852af..bfbd5006b1 100644
--- a/packages/core/test/aisdk-native.test.ts
+++ b/packages/core/test/aisdk-native.test.ts
@@ -13,7 +13,6 @@ describe("AISDKNative", () => {
         models: ["anthropic/claude-sonnet-4.6"],
         provider: { only: ["anthropic"], require_parameters: true },
         reasoning: { effort: "high" },
-        promptCacheKey: "session_123",
         future_option: { enabled: true },
       }),
     ).toEqual({
@@ -24,7 +23,6 @@ describe("AISDKNative", () => {
             models: ["anthropic/claude-sonnet-4.6"],
             provider: { only: ["anthropic"], require_parameters: true },
             reasoning: { effort: "high" },
-            promptCacheKey: "session_123",
             future_option: { enabled: true },
           },
         },
@@ -105,7 +103,6 @@ describe("AISDKNative", () => {
         baseURL: "https://xai.example/v1",
         reasoningEffort: "custom",
         store: true,
-        promptCacheKey: "cache-key",
       }),
     ).toEqual({
       package: "@opencode-ai/ai/providers/xai",
@@ -116,7 +113,6 @@ describe("AISDKNative", () => {
           xai: {
             reasoningEffort: "custom",
             store: true,
-            promptCacheKey: "cache-key",
           },
         },
       },
diff --git a/packages/core/test/session-compaction.test.ts b/packages/core/test/session-compaction.test.ts
index b32f453285..3dc66f549e 100644
--- a/packages/core/test/session-compaction.test.ts
+++ b/packages/core/test/session-compaction.test.ts
@@ -235,6 +235,7 @@ it.effect("manual compaction summarizes short context instead of no-op", () =>
     expect(Array.from(yield* Fiber.join(delta)).map((event) => event.data.text)).toEqual(["manual summary"])
 
     expect(requests).toHaveLength(1)
+    expect(requests[0]?.promptCacheKey).toBe(sessionID)
     expect(requests[0]?.http?.headers).toEqual({
       "x-session-affinity": sessionID,
       "X-Session-Id": sessionID,
diff --git a/packages/core/test/session-generate.test.ts b/packages/core/test/session-generate.test.ts
index 9a0d1efa3f..f98438e5af 100644
--- a/packages/core/test/session-generate.test.ts
+++ b/packages/core/test/session-generate.test.ts
@@ -287,7 +287,7 @@ it.effect("generates from fresh settled Session context without durable mutation
     expect(requests[0]?.system[0]?.text).toBe("Hooked system")
     expect(requests[0]?.system.map((part) => part.text)).toContain("Initial context")
     expect(requests[0]?.http?.headers).toMatchObject({ "X-Session-Id": sessionID })
-    expect(requests[0]?.providerOptions).toMatchObject({ openai: { promptCacheKey: sessionID } })
+    expect(requests[0]?.promptCacheKey).toBe(sessionID)
     const instructionUpdates = requests[0]?.messages.flatMap((message) =>
       message.role === "system"
         ? message.content.flatMap((content) => (content.type === "text" ? [content.text] : []))
diff --git a/packages/core/test/session-runner.test.ts b/packages/core/test/session-runner.test.ts
index 110d1505a0..052048e7c9 100644
--- a/packages/core/test/session-runner.test.ts
+++ b/packages/core/test/session-runner.test.ts
@@ -3210,7 +3210,7 @@ describe("SessionRunnerLLM", () => {
       yield* stream.started
 
       expect(requests).toHaveLength(2)
-      expect(requests.map((request) => request.providerOptions?.openai?.promptCacheKey)).toEqual([
+      expect(requests.map((request) => request.promptCacheKey)).toEqual([
         sessionID,
         otherSessionID,
       ])
@@ -3241,7 +3241,7 @@ describe("SessionRunnerLLM", () => {
       yield* session.resume(longSessionID)
       yield* session.resume(otherLongSessionID)
 
-      const keys = requests.map((request) => request.providerOptions?.openai?.promptCacheKey)
+      const keys = requests.map((request) => request.promptCacheKey)
       expect(keys).toEqual([longSessionID.slice(4), otherLongSessionID.slice(4)])
       expect(keys.every((key) => typeof key === "string" && key.length === 64)).toBe(true)
       expect(keys[0]).not.toBe(keys[1])
diff --git a/packages/core/test/session-title.test.ts b/packages/core/test/session-title.test.ts
index ac4e4f6097..d0cfc78b49 100644
--- a/packages/core/test/session-title.test.ts
+++ b/packages/core/test/session-title.test.ts
@@ -153,6 +153,7 @@ it.effect("generates a title from the sole user message and renames the session"
     yield* title.generateForFirstPrompt(sessionID)
 
     expect(requests).toHaveLength(1)
+    expect(requests[0]?.promptCacheKey).toBe(sessionID)
     expect(requests[0]?.http?.headers).toEqual({
       "x-session-affinity": sessionID,
       "X-Session-Id": sessionID,