diff --git a/hooks/hooks.json b/hooks/hooks.json
index d034f7f..7f62c8b 100644
--- a/hooks/hooks.json
+++ b/hooks/hooks.json
@@ -5,7 +5,7 @@
         "hooks": [
           {
             "type": "command",
-            "command": "node -e \"var r=process.env.CLAUDE_PLUGIN_ROOT;if(r){var f=require('fs'),p=require('path');var t=f.readFileSync(p.join(r,'hooks','system-prompt.md'),'utf8');process.stdout.write(JSON.stringify({hookSpecificOutput:{hookEventName:'SessionStart',additionalContext:t}}))}\"",
+            "command": "node -e \"var r=process.env.CLAUDE_PLUGIN_ROOT;if(r){require(require('path').join(r,'hooks','print-prompt.js'))}\"",
             "bash": "node \"${CLAUDE_PLUGIN_ROOT}/hooks/print-prompt.js\"",
             "powershell": "node \"${CLAUDE_PLUGIN_ROOT}/hooks/print-prompt.js\""
           },
diff --git a/hooks/print-prompt.js b/hooks/print-prompt.js
index 9b46674..e65d3b1 100644
--- a/hooks/print-prompt.js
+++ b/hooks/print-prompt.js
@@ -1,6 +1,50 @@
 const path = require('path');
 const fs = require('fs');
 
+// Only inject the Copilot Studio system prompt when this session is actually a
+// Copilot Studio project — i.e. an `agent.mcs.yml` exists in the working
+// directory or a subdirectory. This is the plugin's own definition of a CS
+// project (see system-prompt.md: "already inside a Copilot Studio project ...
+// there is an agent.mcs.yml file in the current directory or any subdirectory").
+//
+// A SessionStart hook runs before any user message, so it cannot read user
+// intent; the project marker is the only reliable signal. Without this gate the
+// hook fires on every session and injects the ~11KB prompt into unrelated work.
+function hasCopilotStudioProject(root, maxDepth, maxDirs) {
+  const skip = new Set([
+    'node_modules', '.git', '.svn', '.hg', 'dist', 'build', 'out',
+    '.next', '.cache', '.turbo', 'coverage', 'vendor', 'target',
+  ]);
+  const stack = [[root, 0]];
+  let visited = 0;
+  while (stack.length) {
+    const [dir, depth] = stack.pop();
+    if (++visited > maxDirs) return false; // bounded — never a long scan
+    let entries;
+    try {
+      entries = fs.readdirSync(dir, { withFileTypes: true });
+    } catch {
+      continue;
+    }
+    for (const ent of entries) {
+      if (ent.isFile() && ent.name === 'agent.mcs.yml') return true;
+      if (
+        ent.isDirectory() &&
+        depth < maxDepth &&
+        !skip.has(ent.name) &&
+        !ent.name.startsWith('.')
+      ) {
+        stack.push([path.join(dir, ent.name), depth + 1]);
+      }
+    }
+  }
+  return false;
+}
+
+if (!hasCopilotStudioProject(process.cwd(), 4, 2000)) {
+  process.exit(0); // not a Copilot Studio session — inject nothing
+}
+
 const text = fs.readFileSync(path.join(__dirname, 'system-prompt.md'), 'utf8');
 
 // Copilot CLI (v1.0.11+) injects additionalContext from JSON output.