feat(opencode): V2 agent.pre-system waterfall alongside V1 transform

packages/opencode/src/agent/agent.ts

@@ -2,6 +2,8 @@ import { LayerNode } from "@opencode-ai/core/effect/layer-node"
 import { PermissionV1 } from "@opencode-ai/core/v1/permission"
 import { Config } from "@/config/config"
 import { serviceUse } from "@opencode-ai/core/effect/service-use"
+import { EventV2Bridge } from "@/event-v2-bridge"
+import { SessionEvent } from "@opencode-ai/core/session/event"
 import { Provider } from "@/provider/provider"
 
 import { generateObject, streamObject, type ModelMessage } from "ai"
@@ -75,7 +77,8 @@ export interface Interface {
       whenToUse: string
       systemPrompt: string
     },
-    Provider.DefaultModelError
+    Provider.DefaultModelError,
+    EventV2Bridge.Service
   >
 }
 
@@ -371,14 +374,24 @@ const layer = Layer.effect(
       }) {
         const cfg = yield* config.get()
         const model = input.model ?? (yield* provider.defaultModel())
+        const events = yield* EventV2Bridge.Service
         const resolved = yield* provider.getModel(model.providerID, model.modelID)
         const language = yield* provider.getLanguage(resolved)
         const tracer = cfg.experimental?.openTelemetry
           ? Option.getOrUndefined(yield* Effect.serviceOption(OtelTracer.OtelTracer))
           : undefined
 
-        const system = [PROMPT_GENERATE]
+        let system = [PROMPT_GENERATE]
         yield* plugin.trigger("experimental.chat.system.transform", { model: resolved }, { system })
+        const decidedSystem = yield* events.waterfall(SessionEvent.Agent.PreSystem, [
+          (_e, next) =>
+            Effect.gen(function* () {
+              const downstream = yield* next()
+              const downstreamSystem = (downstream as { system?: typeof system } | undefined)?.system
+              return { system: downstreamSystem ?? system }
+            }),
+        ])
+        system = (decidedSystem as { system: typeof system }).system
         const existing = yield* InstanceState.useEffect(state, (s) => s.list())
 
         // TODO: clean this up so provider specific logic doesnt bleed over
@@ -447,7 +460,7 @@ const locationServiceMapNode = LayerNode.make({
 export const node = LayerNode.make({
   service: Service,
   layer: layer,
-  deps: [Config.node, Auth.node, Plugin.node, Skill.node, Provider.node, locationServiceMapNode],
+  deps: [Config.node, Auth.node, Plugin.node, Skill.node, Provider.node, locationServiceMapNode, EventV2Bridge.node],
 })
 
 export * as Agent from "./agent"

packages/opencode/src/cli/cmd/agent.ts

@@ -10,6 +10,7 @@ import { EOL } from "os"
 import type { Argv } from "yargs"
 import { Effect } from "effect"
 import { effectCmd } from "../effect-cmd"
+import { EventV2Bridge } from "@/event-v2-bridge"
 
 type AgentMode = "all" | "primary" | "subagent"
 
@@ -66,8 +67,11 @@ const AgentCreateCommand = effectCmd({
     if (!maybeCtx) return yield* Effect.die("InstanceRef not provided")
     const ctx = maybeCtx
     const agentSvc = yield* Agent.Service
-    const runLocalEffect = <A, E>(effect: Effect.Effect<A, E>) =>
-      Effect.runPromise(effect.pipe(Effect.provideService(InstanceRef, ctx)))
+    const events = yield* EventV2Bridge.Service
+    const runLocalEffect = <A, E, R>(effect: Effect.Effect<A, E, R>) =>
+      Effect.runPromise(
+        effect.pipe(Effect.provideService(InstanceRef, ctx), Effect.provideService(EventV2Bridge.Service, events)) as unknown as Effect.Effect<Promise<A>, E>,
+      )
     yield* Effect.promise(async () => {
       const cliPath = args.path
       const cliDescription = args.description

packages/schema/src/session-event.ts

@@ -73,6 +73,22 @@ export namespace Agent {
     },
   })
   export type PreStep = typeof PreStep.Type
+
+  /**
+   * Live waterfall fired when assembling the system prompt for a model call.
+   * Listeners return `{ system }` to rewrite the system prompt that reaches
+   * the LLM, or throw to abort. This is the V2 replacement for the legacy V1
+   * `experimental.chat.system.transform` hook and gives plugins proper
+   * `next()`-based mutation semantics with short-circuit.
+   */
+  export const PreSystem = Event.define({
+    type: "session.next.agent.pre_system",
+    schema: {
+      ...AgentBase,
+      system: Schema.Array(Schema.Unknown),
+    },
+  })
+  export type PreSystem = typeof PreSystem.Type
 }
 
 export const AgentSwitched = Event.define({