feat(plugin): allow compaction hook to replace prompt entirely (#5907)

packages/web/src/content/docs/plugins.mdx

@@ -233,3 +233,30 @@ Include any state that should persist across compaction:
 ```
 
 The `experimental.session.compacting` hook fires before the LLM generates a continuation summary. Use it to inject domain-specific context that the default compaction prompt would miss.
+
+You can also replace the compaction prompt entirely by setting `output.prompt`:
+
+```ts title=".opencode/plugin/custom-compaction.ts"
+import type { Plugin } from "@opencode-ai/plugin"
+
+export const CustomCompactionPlugin: Plugin = async (ctx) => {
+  return {
+    "experimental.session.compacting": async (input, output) => {
+      // Replace the entire compaction prompt
+      output.prompt = `
+You are generating a continuation prompt for a multi-agent swarm session.
+
+Summarize:
+1. The current task and its status
+2. Which files are being modified and by whom
+3. Any blockers or dependencies between agents
+4. The next steps to complete the work
+
+Format as a structured prompt that a new agent can use to resume work.
+`
+    },
+  }
+}
+```
+
+When `output.prompt` is set, it completely replaces the default compaction prompt. The `output.context` array is ignored in this case.