feat: make /compact prompt configurable via compactPrompt config

- Add compactPrompt optional string field to config schema
- Users can customize session summary prompts without code changes
- SystemPrompt.summarize() now reads from config, falls back to default
- Maintains backward compatibility with default summarization prompt

Author: maxeonyx

packages/opencode/src/config/config.ts

@@ -568,6 +568,12 @@ export namespace Config {
           },
         ),
       instructions: z.array(z.string()).optional().describe("Additional instruction files or patterns to include"),
+      compactPrompt: z
+        .string()
+        .optional()
+        .describe(
+          "Custom prompt to use for the /compact command. If not specified, uses the default summarization prompt.",
+        ),
       layout: Layout.optional().describe("@deprecated Always uses stretch layout."),
       permission: z
         .object({

packages/opencode/src/session/compaction.ts

@@ -103,7 +103,7 @@ export namespace SessionCompaction {
     const toSummarize = await MessageV2.filterCompacted(MessageV2.stream(input.sessionID))
     const model = await Provider.getModel(input.providerID, input.modelID)
     const system = [
-      ...SystemPrompt.summarize(model.providerID),
+      ...(await SystemPrompt.summarize(model.providerID)),
       ...(await SystemPrompt.environment()),
       ...(await SystemPrompt.custom()),
     ]

packages/opencode/src/session/system.ts

@@ -116,12 +116,14 @@ export namespace SystemPrompt {
     return Promise.all(found).then((result) => result.filter(Boolean))
   }
 
-  export function summarize(providerID: string) {
+  export async function summarize(providerID: string) {
+    const config = await Config.get()
+    const prompt = config.compactPrompt || PROMPT_SUMMARIZE
     switch (providerID) {
       case "anthropic":
-        return [PROMPT_ANTHROPIC_SPOOF.trim(), PROMPT_SUMMARIZE]
+        return [PROMPT_ANTHROPIC_SPOOF.trim(), prompt]
       default:
-        return [PROMPT_SUMMARIZE]
+        return [prompt]
     }
   }
 

packages/sdk/js/src/gen/types.gen.ts

@@ -454,6 +454,10 @@ export type Config = {
    * Additional instruction files or patterns to include
    */
   instructions?: Array<string>
+  /**
+   * Custom prompt to use for the /compact command. If not specified, uses the default summarization prompt.
+   */
+  compactPrompt?: string
   layout?: LayoutConfig
   permission?: {
     edit?: "ask" | "allow" | "deny"