fix: activity detection — fix path encoding, use tail -1 for JSONL

Two tightly coupled infrastructure fixes:

- Fix toClaudeProjectPath(): leading `/` becomes `-` (not stripped),
  matching Claude Code's actual project directory naming convention.
- Replace manual 4KB buffer read in readLastJsonlEntry() with
  `tail -1` + JSON.parse — handles any file size, any line length,
  and eliminates the truncated-line edge case entirely.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/core/src/utils.ts

@@ -2,7 +2,11 @@
  * Shared utility functions for agent-orchestrator plugins.
  */
 
-import { open } from "node:fs/promises";
+import { execFile } from "node:child_process";
+import { stat } from "node:fs/promises";
+import { promisify } from "node:util";
+
+const execFileAsync = promisify(execFile);
 
 /**
  * POSIX-safe shell escaping: wraps value in single quotes,
@@ -33,60 +37,34 @@ export function validateUrl(url: string, label: string): void {
 }
 
 /**
- * Read only the last 4KB of JSONL files to find the last entry.
- * Avoids loading entire (potentially large) files into memory.
- */
-const TAIL_READ_BYTES = 4096;
-
-/**
- * Read the last entry from a JSONL file efficiently.
- * Only reads the last TAIL_READ_BYTES (4KB) from the file to avoid loading
- * entire potentially large files into memory.
+ * Read the last entry from a JSONL file.
+ * Uses `tail -1` to get the last line — handles any file size, any line length.
  *
  * @param filePath - Path to the JSONL file
- * @returns Object containing the last entry's type and file mtime, or null if file is empty/invalid
+ * @returns Object containing the last entry's type and file mtime, or null if empty/invalid
  */
 export async function readLastJsonlEntry(
   filePath: string,
 ): Promise<{ lastType: string | null; modifiedAt: Date } | null> {
-  let fh;
   try {
-    fh = await open(filePath, "r");
-    const fileStat = await fh.stat();
-    const size = fileStat.size;
-    if (size === 0) return null;
+    const [{ stdout }, fileStat] = await Promise.all([
+      execFileAsync("tail", ["-1", filePath], { timeout: 5_000 }),
+      stat(filePath),
+    ]);
 
-    // Read only the last TAIL_READ_BYTES (4KB) from the file
-    const readSize = Math.min(TAIL_READ_BYTES, size);
-    const buffer = Buffer.alloc(readSize);
-    const { bytesRead } = await fh.read(buffer, 0, readSize, size - readSize);
+    const line = stdout.trim();
+    if (!line) return null;
 
-    const chunk = buffer.toString("utf-8", 0, bytesRead);
-    // Walk backwards through lines to find the last valid JSON object with a type
-    const lines = chunk.split("\n");
-    for (let i = lines.length - 1; i >= 0; i--) {
-      const line = lines[i];
-      if (!line) continue;
-      const trimmed = line.trim();
-      if (!trimmed) continue;
-      try {
-        const parsed: unknown = JSON.parse(trimmed);
-        if (typeof parsed === "object" && parsed !== null && !Array.isArray(parsed)) {
-          const obj = parsed as Record<string, unknown>;
-          if (typeof obj.type === "string") {
-            return { lastType: obj.type, modifiedAt: fileStat.mtime };
-          }
-        }
-      } catch {
-        // Skip malformed lines (possibly truncated first line in our chunk)
-      }
+    // JSONL entries have deterministic structure: {"type":"<value>",...}
+    const parsed: unknown = JSON.parse(line);
+    if (typeof parsed === "object" && parsed !== null && !Array.isArray(parsed)) {
+      const obj = parsed as Record<string, unknown>;
+      const lastType = typeof obj.type === "string" ? obj.type : null;
+      return { lastType, modifiedAt: fileStat.mtime };
     }
 
-    // No entry with a type field found
     return { lastType: null, modifiedAt: fileStat.mtime };
   } catch {
     return null;
-  } finally {
-    await fh?.close();
   }
 }

packages/plugins/agent-claude-code/src/index.test.ts

@@ -437,7 +437,7 @@ describe("getSessionInfo", () => {
       mockJsonlFiles('{"type":"user","message":{"content":"hello"}}');
       await agent.getSessionInfo(makeSession({ workspacePath: "/Users/dev/.worktrees/ao/ao-3" }));
       expect(mockReaddir).toHaveBeenCalledWith(
-        "/mock/home/.claude/projects/Users-dev--worktrees-ao-ao-3",
+        "/mock/home/.claude/projects/-Users-dev--worktrees-ao-ao-3",
       );
     });
   });

packages/plugins/agent-claude-code/src/index.ts

@@ -195,7 +195,8 @@ export const manifest = {
 export function toClaudeProjectPath(workspacePath: string): string {
   // Handle Windows drive letters (C:\Users\... → C-Users-...)
   const normalized = workspacePath.replace(/\\/g, "/");
-  return normalized.replace(/^\//, "").replace(/:/g, "").replace(/[/.]/g, "-");
+  // Claude Code replaces / and . with - (keeping the leading slash as a leading -)
+  return normalized.replace(/:/g, "").replace(/[/.]/g, "-");
 }
 
 /** Find the most recently modified .jsonl session file in a directory */