diff --git a/packages/ai/src/protocols/openai-chat.ts b/packages/ai/src/protocols/openai-chat.ts
index 545adb3f7cf..c883a3011d6 100644
--- a/packages/ai/src/protocols/openai-chat.ts
+++ b/packages/ai/src/protocols/openai-chat.ts
@@ -715,7 +715,10 @@ const detectZaiToolStream = (provider: string, baseURL: string | undefined, mode
 
 const lowerOptions = (request: LLMRequest, supportsStore: boolean) => {
   const options = OpenAIOptions.resolve(request)
-  const cacheKey = ProviderShared.promptCacheKey(request)
+  // Default off: strict providers 400 on unknown body fields, so only send
+  // the key where compatibility explicitly allows it. Header-based affinity
+  // (x-session-affinity, x-grok-conv-id, ...) is unaffected.
+  const cacheKey = (request.model.compatibility?.supportsPromptCacheKey ?? false) ? ProviderShared.promptCacheKey(request) : undefined
   return {
     ...(supportsStore && options.store !== undefined ? { store: options.store } : {}),
     // For providers that support `store`, ensure stateless `store:false` is sent
diff --git a/packages/ai/src/providers/azure.ts b/packages/ai/src/providers/azure.ts
index 1c438fa1b77..aa2ee472830 100644
--- a/packages/ai/src/providers/azure.ts
+++ b/packages/ai/src/providers/azure.ts
@@ -135,7 +135,7 @@ export const configure = (input: Config) => {
   const chat = (modelID: string | ModelID) =>
     configuredRoute(chatRoute, input, modelID)
       .with(withOpenAIOptions(modelID, modelDefaults))
-      .model<OpenAIProviderOptionsInput>({ id: modelID })
+      .model<OpenAIProviderOptionsInput>({ id: modelID, compatibility: { supportsPromptCacheKey: true } })
 
   return {
     id,
diff --git a/packages/ai/src/providers/cerebras.ts b/packages/ai/src/providers/cerebras.ts
index f9a83f04548..456c3a0ee82 100644
--- a/packages/ai/src/providers/cerebras.ts
+++ b/packages/ai/src/providers/cerebras.ts
@@ -44,7 +44,7 @@ export const configure = (input: LanguageModelOptions = {}) => {
     model: (modelID: string | ModelID) =>
       configured.model<OpenAIProviderOptionsInput>({
         id: modelID,
-        compatibility: { maxTokensField: "max_tokens", reasoningField: "reasoning", supportsStore: false },
+        compatibility: { maxTokensField: "max_tokens", reasoningField: "reasoning", supportsStore: false, supportsPromptCacheKey: true },
       }),
     configure,
   }
diff --git a/packages/ai/src/providers/deepinfra.ts b/packages/ai/src/providers/deepinfra.ts
index 184b2554731..821a4a92128 100644
--- a/packages/ai/src/providers/deepinfra.ts
+++ b/packages/ai/src/providers/deepinfra.ts
@@ -47,7 +47,7 @@ export const configure = (input: LanguageModelOptions = {}) => {
     model: (modelID: string | ModelID) =>
       configured.model<OpenAIProviderOptionsInput>({
         id: modelID,
-        compatibility: { maxTokensField: "max_tokens", reasoningField: "reasoning_content", supportsStore: false },
+        compatibility: { maxTokensField: "max_tokens", reasoningField: "reasoning_content", supportsStore: false, supportsPromptCacheKey: true },
       }),
     configure,
   }
diff --git a/packages/ai/src/providers/fireworks.ts b/packages/ai/src/providers/fireworks.ts
index ff3c28720d6..68e4d98c17a 100644
--- a/packages/ai/src/providers/fireworks.ts
+++ b/packages/ai/src/providers/fireworks.ts
@@ -41,7 +41,8 @@ export const configure = (input: LanguageModelOptions = {}) => {
   })
   return {
     id,
-    model: (modelID: string | ModelID) => configured.model<OpenAIProviderOptionsInput>({ id: modelID }),
+    model: (modelID: string | ModelID) =>
+      configured.model<OpenAIProviderOptionsInput>({ id: modelID, compatibility: { supportsPromptCacheKey: true } }),
     configure,
   }
 }
diff --git a/packages/ai/src/providers/meta.ts b/packages/ai/src/providers/meta.ts
index 297de7a3452..693d511ce81 100644
--- a/packages/ai/src/providers/meta.ts
+++ b/packages/ai/src/providers/meta.ts
@@ -132,7 +132,7 @@ export const configure = (input: LanguageModelOptions = {}) => {
   const chat = (modelID: string | ModelID) =>
     configuredChat.model<OpenResponsesProviderOptionsInput>({
       id: modelID,
-      compatibility: { maxTokensField: "max_completion_tokens", supportsStore: false },
+      compatibility: { maxTokensField: "max_completion_tokens", supportsStore: false, supportsPromptCacheKey: true },
     })
   const messages = (modelID: string | ModelID) =>
     configuredMessages.model<MessagesOptionsInput>({
diff --git a/packages/ai/src/providers/moonshot.ts b/packages/ai/src/providers/moonshot.ts
index df77c2129e6..99b3bbdcc02 100644
--- a/packages/ai/src/providers/moonshot.ts
+++ b/packages/ai/src/providers/moonshot.ts
@@ -112,6 +112,7 @@ export const configure = (input: Config = {}) => {
         maxTokensField: "max_tokens",
         supportsStore: false,
         supportsStrictMode: false,
+        supportsPromptCacheKey: true,
         toolSchema: "moonshot",
         reasoningField: "reasoning_content",
       },
diff --git a/packages/ai/src/providers/openai.ts b/packages/ai/src/providers/openai.ts
index 613429fac35..0416f554a03 100644
--- a/packages/ai/src/providers/openai.ts
+++ b/packages/ai/src/providers/openai.ts
@@ -91,7 +91,10 @@ export const configure = (input: Config = {}) => {
       .with(withOpenAIOptions(id, modelDefaults, { textVerbosity: true }))
       .model<OpenAIProviderOptionsInput>({ id })
   const chat = (id: string | ModelID) =>
-    chatRoute.with(withOpenAIOptions(id, modelDefaults)).model<OpenAIProviderOptionsInput>({ id })
+    chatRoute.with(withOpenAIOptions(id, modelDefaults)).model<OpenAIProviderOptionsInput>({
+      id,
+      compatibility: { supportsPromptCacheKey: true },
+    })
   const image = (modelID: string | ModelID) =>
     OpenAIImages.model({
       id: modelID,
diff --git a/packages/ai/src/providers/openrouter.ts b/packages/ai/src/providers/openrouter.ts
index 765d8524d68..4bec70e3601 100644
--- a/packages/ai/src/providers/openrouter.ts
+++ b/packages/ai/src/providers/openrouter.ts
@@ -183,7 +183,8 @@ export const configure = (input: LanguageModelOptions = {}) => {
   const route = configuredRoute(input)
   return {
     id,
-    model: (modelID: string | ModelID) => route.model<OpenRouterProviderOptionsInput>({ id: modelID }),
+    model: (modelID: string | ModelID) =>
+      route.model<OpenRouterProviderOptionsInput>({ id: modelID, compatibility: { supportsPromptCacheKey: true } }),
     configure,
   }
 }
diff --git a/packages/ai/src/schema/options.ts b/packages/ai/src/schema/options.ts
index 15a30cb6d5b..736f741c20b 100644
--- a/packages/ai/src/schema/options.ts
+++ b/packages/ai/src/schema/options.ts
@@ -168,6 +168,9 @@ export class LanguageModelCompatibility extends Schema.Class<LanguageModelCompat
   supportsStore: Schema.optional(Schema.Boolean),
   supportsUsageInStreaming: Schema.optional(Schema.Boolean),
   supportsStrictMode: Schema.optional(Schema.Boolean),
+  // Accepts `prompt_cache_key` in the Chat Completions body. Chat omits the
+  // key unless this is set; session-affinity headers still flow regardless.
+  supportsPromptCacheKey: Schema.optional(Schema.Boolean),
   zaiToolStream: Schema.optional(Schema.Boolean),
   requireSignature: Schema.optional(Schema.Boolean),
   /** Supports Anthropic's thinking-prefix mismatch controls. Overrides model-ID detection. */
diff --git a/packages/ai/test/provider/openai-chat.test.ts b/packages/ai/test/provider/openai-chat.test.ts
index d45613f5990..91c44b57d44 100644
--- a/packages/ai/test/provider/openai-chat.test.ts
+++ b/packages/ai/test/provider/openai-chat.test.ts
@@ -248,7 +248,23 @@ describe("OpenAI Chat route", () => {
     }),
   )
 
-  it.effect("maps the request prompt cache key", () =>
+  it.effect("maps the request prompt cache key when the compatibility flag is set", () =>
+    Effect.gen(function* () {
+      const prepared = yield* compileRequest(
+        LLM.request({
+          model: OpenAIChat.route
+            .with({ endpoint: { baseURL: "https://api.compatible.test/v1" }, auth: Auth.bearer("test") })
+            .model({ id: "compatible-model", compatibility: { supportsPromptCacheKey: true } }),
+          prompt: "Hello",
+          promptCacheKey: "session_123",
+        }),
+      )
+
+      expect(prepared.body.prompt_cache_key).toBe("session_123")
+    }),
+  )
+
+  it.effect("omits the prompt cache key without the compatibility flag", () =>
     Effect.gen(function* () {
       const prepared = yield* compileRequest(
         LLM.request({
@@ -261,7 +277,7 @@ describe("OpenAI Chat route", () => {
         }),
       )
 
-      expect(prepared.body.prompt_cache_key).toBe("session_123")
+      expect(prepared.body).not.toHaveProperty("prompt_cache_key")
     }),
   )
 
@@ -280,7 +296,7 @@ describe("OpenAI Chat route", () => {
     }),
   )
 
-  it.effect("maps the xAI Chat prompt cache key to conversation affinity", () =>
+  it.effect("maps the xAI Chat prompt cache key to conversation affinity header only", () =>
     LLMClient.generate(
       LLM.request({
         model: XAI.configure({ apiKey: "test", baseURL: "https://api.x.ai/v1" }).chat("grok-4.5"),
@@ -294,7 +310,8 @@ describe("OpenAI Chat route", () => {
             const web = yield* HttpClientRequest.toWeb(input.request).pipe(Effect.orDie)
             expect(web.headers.get("x-grok-conv-id")).toBe("session_123")
             const body = decodeJson(yield* Effect.promise(() => web.text()))
-            expect(ProviderShared.isRecord(body) ? body.prompt_cache_key : undefined).toBe("session_123")
+            // Chat uses the header; prompt_cache_key is Responses-only.
+            expect(ProviderShared.isRecord(body) ? body.prompt_cache_key : undefined).toBeUndefined()
             return input.respond(sseEvents(deltaChunk({}, "stop")), {
               headers: { "content-type": "text/event-stream" },
             })
diff --git a/packages/core/src/plugin/provider.ts b/packages/core/src/plugin/provider.ts
index d91e920fbe7..fd415abf83f 100644
--- a/packages/core/src/plugin/provider.ts
+++ b/packages/core/src/plugin/provider.ts
@@ -22,6 +22,7 @@ import { OpencodePlugin } from "./provider/opencode.js"
 import { OpenRouterPlugin } from "./provider/openrouter.js"
 import { PerplexityPlugin } from "./provider/perplexity.js"
 import { PoePlugin } from "./provider/poe.js"
+import { PromptCacheKeyPlugin } from "./provider/prompt-cache-key.js"
 import { SapAICorePlugin } from "./provider/sap-ai-core.js"
 import { VercelPlugin } from "./provider/vercel.js"
 import { VenicePlugin } from "./provider/venice.js"
@@ -54,6 +55,7 @@ export const ProviderPlugins: PluginInternal.InternalPlugin[] = [
   OpenRouterPlugin,
   PerplexityPlugin,
   PoePlugin,
+  PromptCacheKeyPlugin,
   SapAICorePlugin,
   VercelPlugin,
   VenicePlugin,
diff --git a/packages/core/src/plugin/provider/prompt-cache-key.ts b/packages/core/src/plugin/provider/prompt-cache-key.ts
new file mode 100644
index 00000000000..48046248f13
--- /dev/null
+++ b/packages/core/src/plugin/provider/prompt-cache-key.ts
@@ -0,0 +1,39 @@
+import { Effect } from "effect"
+import { define } from "@opencode/plugin/effect/plugin"
+import { Provider } from "../../provider.js"
+
+// Chat-completions `prompt_cache_key` body support for catalog providers
+// without a dedicated AI package. Audited per provider (docs + OpenAPI +
+// SDK request types); only unscoped body-key support is listed. Scoped-only
+// support is left off because the boolean cannot express route scope:
+// fastrouter (GPT-5.6 routes only), frogbot (Google/Vertex routes only).
+const providerIDs = [
+  "abliteration-ai",
+  "aihubmix",
+  "anyapi",
+  "auriko",
+  "edenai",
+  "jalapeno",
+  "llama",
+  "morph",
+  "snowflake-cortex",
+  "tencent-tokenhub",
+  "upstage",
+  "venice",
+] as const
+
+export const PromptCacheKeyPlugin = define({
+  id: "opencode.provider.prompt-cache-key",
+  effect: Effect.fn(function* (ctx) {
+    yield* ctx.model.transform((models) => {
+      for (const id of providerIDs) {
+        const providerID = Provider.ID.make(id)
+        for (const model of models.list(providerID)) {
+          models.update(model.providerID, model.id, (draft) => {
+            draft.compatibility = { ...draft.compatibility, supportsPromptCacheKey: true }
+          })
+        }
+      }
+    })
+  }),
+})
diff --git a/packages/core/test/session-runner-recorded.test.ts b/packages/core/test/session-runner-recorded.test.ts
index 9a0273be094..f28db2b0da2 100644
--- a/packages/core/test/session-runner-recorded.test.ts
+++ b/packages/core/test/session-runner-recorded.test.ts
@@ -62,7 +62,7 @@ const model = OpenAIChat.route
     auth: Auth.bearer(process.env.OPENAI_API_KEY ?? "fixture"),
     generation: { maxTokens: 20, temperature: 0 },
   })
-  .model({ id: "gpt-4o-mini" })
+  .model({ id: "gpt-4o-mini", compatibility: { supportsPromptCacheKey: true } })
 const models = Layer.mock(SessionRunnerModel.Service)({
   resolve: () =>
     Effect.succeed(
diff --git a/packages/schema/src/model.ts b/packages/schema/src/model.ts
index 5a7e6b687fc..49045ac6169 100644
--- a/packages/schema/src/model.ts
+++ b/packages/schema/src/model.ts
@@ -64,6 +64,7 @@ export const Compatibility = Schema.Struct({
   maxTokensField: MaxTokensField.pipe(optional),
   requireFinishReason: Schema.Boolean.pipe(optional),
   requireAssistantAfterTool: Schema.Boolean.pipe(optional),
+  supportsPromptCacheKey: Schema.Boolean.pipe(optional),
 }).annotate({ identifier: "Model.Compatibility" })
 
 export interface Capabilities extends Schema.Schema.Type<typeof Capabilities> {}