feat: Add "system.prompt.append" hook

packages/opencode/src/session/prompt.ts

@@ -256,6 +256,11 @@ export namespace SessionPrompt {
       await using _ = defer(async () => {
         await processor.end()
       })
+
+      // Dynamically append plugin prompts for this turn
+      const dynamicPrompts = await appendPluginPrompts(input.sessionID, agent, msgs)
+      const turnSystem = [...system, ...dynamicPrompts]
+
       const doStream = () =>
         streamText({
           onError(error) {
@@ -308,7 +313,7 @@ export namespace SessionPrompt {
           temperature: params.temperature,
           topP: params.topP,
           messages: [
-            ...system.map(
+            ...turnSystem.map(
               (x): ModelMessage => ({
                 role: "system",
                 content: x,
@@ -483,6 +488,38 @@ export namespace SessionPrompt {
     return Provider.defaultModel()
   }
 
+  async function appendPluginPrompts(
+    sessionID: string,
+    agent: Agent.Info,
+    history: MessageV2.WithParts[],
+  ): Promise<string[]> {
+    const appendedPrompts: string[] = []
+    try {
+      const output = { prompt: [] as string[] }
+      await Plugin.trigger(
+        "system.prompt.append",
+        {
+          sessionID,
+          agent,
+          // Convert internal MessageV2 types to SDK types for plugin boundary
+          history: history.map((m) => ({
+            info: m.info as any, // Cast to SDK message types
+            parts: m.parts,
+          })),
+        },
+        output,
+      )
+      // Basic validation
+      if (Array.isArray(output.prompt)) {
+        appendedPrompts.push(...output.prompt.filter((p) => typeof p === "string"))
+      }
+    } catch (error) {
+      console.error(`Error triggering 'system.prompt.append' hook:`, error)
+      // Gracefully continue without plugin content
+    }
+    return appendedPrompts
+  }
+
   async function resolveSystemPrompt(input: {
     system?: string
     agent: Agent.Info

packages/plugin/src/index.ts

@@ -6,16 +6,20 @@ import type {
   Provider,
   Permission,
   UserMessage,
+  AssistantMessage,
   Part,
   Auth,
   Config,
+  Agent,
 } from "@opencode-ai/sdk"
 
 import type { BunShell } from "./shell"
 import { type ToolDefinition } from "./tool"
 
 export * from "./tool"
 
+export type MessageWithParts = { info: UserMessage | AssistantMessage; parts: Part[] }
+
 export type PluginInput = {
   client: ReturnType<typeof createOpencodeClient>
   project: Project
@@ -167,4 +171,19 @@ export interface Hooks {
       metadata: any
     },
   ) => Promise<void>
+  /**
+   * Append text to the system prompt.
+   * This hook is called at the beginning of each step in a turn, allowing plugins
+   * to dynamically add context to the system prompt based on the latest history.
+   */
+  "system.prompt.append"?: (
+    context: {
+      sessionID: string
+      agent: Agent
+      history: MessageWithParts[]
+    },
+    output: {
+      prompt: string[]
+    },
+  ) => Promise<void>
 }