feat(plugin): add session options hook

packages/plugin/src/v2/effect/README.md

@@ -92,6 +92,20 @@ yield *
   )
 ```
 
+Resolved generation and provider options are mutable before provider lowering:
+
+```ts
+yield *
+  ctx.session.hook("options", (event) =>
+    Effect.sync(() => {
+      event.generation.temperature = 0.2
+      event.generation.topP = 0.9
+      event.providerOptions.openai ??= {}
+      event.providerOptions.openai.reasoningEffort = "high"
+    }),
+  )
+```
+
 ## Reloading A Domain
 
 When data captured by a transform changes, reload the affected domain:

packages/plugin/src/v2/effect/session.ts

@@ -1,5 +1,5 @@
 import type { SessionApi } from "@opencode-ai/client/effect/api"
-import type { Message, SystemPart } from "@opencode-ai/ai"
+import type { GenerationOptionsFields, Message, SystemPart } from "@opencode-ai/ai"
 import type { Agent } from "@opencode-ai/schema/agent"
 import type { Model } from "@opencode-ai/schema/model"
 import type { Session } from "@opencode-ai/schema/session"
@@ -15,8 +15,23 @@ export interface SessionContext {
   tools: Record<string, { description: string; input: JsonSchema.JsonSchema }>
 }
 
+export type SessionGenerationOptions = {
+  -readonly [Key in keyof GenerationOptionsFields]: GenerationOptionsFields[Key]
+}
+
+export type SessionProviderOptions = Record<string, Record<string, unknown>>
+
+export interface SessionOptions {
+  readonly sessionID: Session.ID
+  readonly agent: Agent.ID
+  readonly model: Model.Ref
+  generation: SessionGenerationOptions
+  providerOptions: SessionProviderOptions
+}
+
 export interface SessionHooks {
   readonly context: SessionContext
+  readonly options: SessionOptions
 }
 
 export type SessionDomain = Pick<

packages/plugin/src/v2/promise/README.md

@@ -94,6 +94,17 @@ await ctx.session.hook("context", (event) => {
 })
 ```
 
+Resolved generation and provider options are mutable before provider lowering:
+
+```ts
+await ctx.session.hook("options", (event) => {
+  event.generation.temperature = 0.2
+  event.generation.topP = 0.9
+  event.providerOptions.openai ??= {}
+  event.providerOptions.openai.reasoningEffort = "high"
+})
+```
+
 Promise tools use plain object declarations with async executors:
 
 ```ts

packages/plugin/src/v2/promise/session.ts

@@ -1,5 +1,5 @@
 import type { SessionApi } from "@opencode-ai/client/promise/api"
-import type { Message, SystemPart } from "@opencode-ai/ai"
+import type { GenerationOptionsFields, Message, SystemPart } from "@opencode-ai/ai"
 import type { Agent } from "@opencode-ai/schema/agent"
 import type { Model } from "@opencode-ai/schema/model"
 import type { Session } from "@opencode-ai/schema/session"
@@ -15,8 +15,23 @@ export interface SessionContext {
   tools: Record<string, { description: string; input: JsonSchema.JsonSchema }>
 }
 
+export type SessionGenerationOptions = {
+  -readonly [Key in keyof GenerationOptionsFields]: GenerationOptionsFields[Key]
+}
+
+export type SessionProviderOptions = Record<string, Record<string, unknown>>
+
+export interface SessionOptions {
+  readonly sessionID: Session.ID
+  readonly agent: Agent.ID
+  readonly model: Model.Ref
+  generation: SessionGenerationOptions
+  providerOptions: SessionProviderOptions
+}
+
 export interface SessionHooks {
   readonly context: SessionContext
+  readonly options: SessionOptions
 }
 
 export type SessionDomain = Pick<