feat: add compaction prompt plugin

src/index.ts

@@ -0,0 +1,120 @@
+import { readFile } from "node:fs/promises";
+import path from "node:path";
+import type { Hooks, Plugin, PluginOptions } from "@opencode-ai/plugin";
+
+type CompactionMode = "append" | "replace";
+
+export type CompactionOptions = PluginOptions & {
+  memoryFile?: unknown;
+  mode?: unknown;
+  prompt?: unknown;
+  completionMarker?: unknown;
+};
+
+const defaultMemoryFile = ".opencode/compaction.md";
+const defaultPrompt = "";
+const defaultCompletionMarker = "Custom compaction request honored.";
+
+function asString(value: unknown): string | undefined {
+  if (typeof value !== "string") {
+    return undefined;
+  }
+
+  const trimmedValue = value.trim();
+  return trimmedValue || undefined;
+}
+
+function buildInstructions(
+  prompt: string,
+  completionMarker: string,
+  memory: string,
+): string {
+  const sections = [
+    "## User Compaction Instructions",
+    `These instructions take precedence over previous instructions if there is conflict. At the very end of the summary, echo exactly: **${completionMarker}**`,
+    prompt,
+  ];
+
+  if (memory) {
+    sections.push(memory);
+  }
+
+  return sections.join("\n");
+}
+
+async function readMemory(
+  memoryPath: string,
+  logError: (message: string) => Promise<void>,
+): Promise<string> {
+  try {
+    return (await readFile(memoryPath, "utf8")).trim();
+  } catch (error) {
+    if ((error as NodeJS.ErrnoException).code !== "ENOENT") {
+      await logError(`Unable to read compaction memory file: ${memoryPath}`);
+    }
+
+    return "";
+  }
+}
+
+/**
+ * Adds configured instructions and optional project memory to OpenCode's
+ * compaction prompt.
+ *
+ * Append mode preserves OpenCode's built-in compaction prompt and adds the
+ * plugin instructions as context. Replace mode supplies a complete prompt
+ * containing the plugin instructions instead. The memory file is resolved
+ * relative to the active worktree and may be absent.
+ *
+ * @example
+ * ```json
+ * {
+ *   "plugin": [["opencode-plugin-compaction-prompt", {
+ *     "memoryFile": ".opencode/compaction.md",
+ *     "mode": "append"
+ *   }]]
+ * }
+ * ```
+ *
+ * @param context OpenCode's plugin context, including the active worktree.
+ * @param options User-provided plugin configuration.
+ * @returns The hooks registered by the plugin.
+ */
+export const CompactionPromptPlugin: Plugin = async (
+  context,
+  options?: PluginOptions,
+): Promise<Hooks> => {
+  const configured = (options ?? {}) as CompactionOptions;
+  const memoryFile = asString(configured.memoryFile) ?? defaultMemoryFile;
+  const prompt = asString(configured.prompt) ?? defaultPrompt;
+  const completionMarker =
+    asString(configured.completionMarker) ?? defaultCompletionMarker;
+  const mode: CompactionMode =
+    configured.mode === "replace" ? "replace" : "append";
+  const memoryPath = path.resolve(context.worktree, memoryFile);
+  const logError = async (message: string): Promise<void> => {
+    await context.client.app.log({
+      body: {
+        level: "error",
+        message,
+        service: "opencode-plugin-compaction-prompt",
+      },
+    });
+  };
+
+  return {
+    "experimental.session.compacting": async (
+      _input,
+      output,
+    ): Promise<void> => {
+      const memory = await readMemory(memoryPath, logError);
+      const instructions = buildInstructions(prompt, completionMarker, memory);
+
+      if (mode === "replace") {
+        output.prompt = instructions;
+      } else {
+        output.context.push(instructions);
+      }
+    },
+  };
+};