feat(llm): cache-policy auto-placement (#26786)

packages/llm/src/schema/messages.ts

@@ -1,6 +1,6 @@
 import { Schema } from "effect"
 import { JsonSchema, MessageRole, ProviderMetadata } from "./ids"
-import { CacheHint, GenerationOptions, HttpOptions, ModelRef, ProviderOptions } from "./options"
+import { CacheHint, CachePolicy, GenerationOptions, HttpOptions, ModelRef, ProviderOptions } from "./options"
 
 const isRecord = (value: unknown): value is Record<string, unknown> =>
   typeof value === "object" && value !== null && !Array.isArray(value)
@@ -206,6 +206,7 @@ export class LLMRequest extends Schema.Class<LLMRequest>("LLM.Request")({
   providerOptions: Schema.optional(ProviderOptions),
   http: Schema.optional(HttpOptions),
   responseFormat: Schema.optional(ResponseFormat),
+  cache: Schema.optional(CachePolicy),
   metadata: Schema.optional(Schema.Record(Schema.String, Schema.Unknown)),
 }) {}
 
@@ -223,6 +224,7 @@ export namespace LLMRequest {
     providerOptions: request.providerOptions,
     http: request.http,
     responseFormat: request.responseFormat,
+    cache: request.cache,
     metadata: request.metadata,
   })
 

packages/llm/src/schema/options.ts

@@ -200,3 +200,35 @@ export class CacheHint extends Schema.Class<CacheHint>("LLM.CacheHint")({
   type: Schema.Literals(["ephemeral", "persistent"]),
   ttlSeconds: Schema.optional(Schema.Number),
 }) {}
+
+// Auto-placement policy for prompt caching. The protocol-neutral lowering step
+// reads this and injects `CacheHint`s at the configured boundaries; the
+// per-protocol body builders then translate those hints into wire markers as
+// usual. `"auto"` is the recommended default for agent loops — it places one
+// breakpoint at the last tool definition, one at the last system part, and one
+// at the latest user message. The combination of provider invalidation
+// hierarchy (tools → system → messages) and Anthropic/Bedrock's 20-block
+// lookback means three trailing breakpoints reliably cover the static prefix.
+//
+// Pass `"none"` to opt out entirely (the legacy behavior). Pass the granular
+// object form to override individual choices.
+export const CachePolicyObject = Schema.Struct({
+  tools: Schema.optional(Schema.Boolean),
+  system: Schema.optional(Schema.Boolean),
+  messages: Schema.optional(
+    Schema.Union([
+      Schema.Literal("latest-user-message"),
+      Schema.Literal("latest-assistant"),
+      Schema.Struct({ tail: Schema.Number }),
+    ]),
+  ),
+  ttlSeconds: Schema.optional(Schema.Number),
+})
+export type CachePolicyObject = Schema.Schema.Type<typeof CachePolicyObject>
+
+export const CachePolicy = Schema.Union([
+  Schema.Literal("auto"),
+  Schema.Literal("none"),
+  CachePolicyObject,
+])
+export type CachePolicy = Schema.Schema.Type<typeof CachePolicy>