diff --git a/README.md b/README.md
index c57811a0..1fd89bc5 100644
--- a/README.md
+++ b/README.md
@@ -269,7 +269,7 @@ Active every session, with a handful of commands (see [Commands](#commands)). `/
 
 Set the level for every new session with the `PONYTAIL_DEFAULT_MODE` env var (`lite`/`full`/`ultra`/`off`), or a `defaultMode` field in `~/.config/ponytail/config.json` (`%APPDATA%\ponytail\config.json` on Windows). The default is `full`.
 
-While active, the ruleset is also injected into every subagent spawned via the Agent tool. To scope that to specific agent types (say, keep it off read-only search agents), set the `PONYTAIL_SUBAGENT_MATCHER` env var to a regex tested against the subagent's `agent_type`. It is unanchored and case-insensitive: `explore|general` matches either, `^general$` is exact, and plugin agent types look like `plugin:name`. Unset means inject into every subagent (the default); an invalid regex, or a subagent whose type the platform doesn't report, also falls back to injecting.
+While active, the ruleset is also injected into every subagent spawned via the Agent tool. To scope that to specific agent types (say, keep it off read-only search agents), set the `PONYTAIL_SUBAGENT_MATCHER` env var to a regex tested against the subagent's `agent_type`. It is unanchored and case-insensitive: `explore|general` matches either, `^general$` is exact, and plugin agent types look like `plugin:name`. Unset means inject into every subagent (the default); an invalid regex, a pattern over 256 characters, or a subagent whose type the platform doesn't report, also falls back to injecting, with a warning on stderr.
 
 Cursor, Windsurf, Cline, GitHub Copilot Chat (the VS Code, JetBrains, and Visual Studio editor extension, not the standalone Copilot CLI covered under [Install](#install)), Aider, Kiro, Zed, CodeWhale, Swival, Qoder: copy the matching rules file from this repo ([`.cursor/rules/`](.cursor/rules/), [`.windsurf/rules/`](.windsurf/rules/), [`.clinerules/`](.clinerules/), [`.github/copilot-instructions.md`](.github/copilot-instructions.md), [`AGENTS.md`](AGENTS.md), [`.kiro/steering/`](.kiro/steering/), [`.qoder/rules/`](.qoder/rules/)).
 
diff --git a/hooks/ponytail-subagent.js b/hooks/ponytail-subagent.js
index f2a7c776..7b0ad4a7 100644
--- a/hooks/ponytail-subagent.js
+++ b/hooks/ponytail-subagent.js
@@ -29,12 +29,25 @@ function inject() {
 }
 
 // A bad regex must never crash the hook; treat it as "no matcher" and inject.
+// The pattern is operator-controlled, so also cap it and warn on stderr
+// (issue #658); a typo should be visible, not silently ignored.
+const MAX_MATCHER_LEN = 256;
+function warn(msg) {
+  // Stderr only; stdout is the hook payload and must stay valid JSON.
+  try { process.stderr.write('ponytail-subagent: ' + msg + '\n'); } catch (e) {}
+}
 let matcherRe = null;
 try {
-  if (process.env.PONYTAIL_SUBAGENT_MATCHER) {
-    matcherRe = new RegExp(process.env.PONYTAIL_SUBAGENT_MATCHER, 'i');
+  const pattern = process.env.PONYTAIL_SUBAGENT_MATCHER;
+  if (pattern) {
+    if (pattern.length > MAX_MATCHER_LEN) {
+      warn('PONYTAIL_SUBAGENT_MATCHER exceeds ' + MAX_MATCHER_LEN + ' chars; ignoring');
+    } else {
+      matcherRe = new RegExp(pattern, 'i');
+    }
   }
 } catch (e) {
+  warn('PONYTAIL_SUBAGENT_MATCHER is invalid (' + (e && e.message) + '); ignoring');
   matcherRe = null;
 }
 
diff --git a/tests/hooks.test.js b/tests/hooks.test.js
index 44ee9222..470e52cf 100644
--- a/tests/hooks.test.js
+++ b/tests/hooks.test.js
@@ -335,13 +335,47 @@ assert.equal(result.status, 0, result.stderr);
 output = JSON.parse(result.stdout);
 assert.match(output.hookSpecificOutput.additionalContext, /PONYTAIL MODE ACTIVE — level: full/);
 
-// Invalid regex → must not crash; fall back to injecting everywhere.
+// Invalid regex → must not crash; fall back to injecting everywhere, with a
+// warning on stderr (issue #658).
 result = run(
   'ponytail-subagent.js',
   { ...scopeEnv, PONYTAIL_SUBAGENT_MATCHER: '(' },
   JSON.stringify({ agent_type: 'anything' }),
 );
 assert.equal(result.status, 0, result.stderr);
+assert.ok(/PONYTAIL_SUBAGENT_MATCHER is invalid/.test(result.stderr), 'bad pattern must warn on stderr');
+output = JSON.parse(result.stdout);
+assert.equal(output.hookSpecificOutput.hookEventName, 'SubagentStart');
+
+// Overlong pattern → rejected with a warning; falls back to injecting.
+result = run(
+  'ponytail-subagent.js',
+  { ...scopeEnv, PONYTAIL_SUBAGENT_MATCHER: 'a'.repeat(300) },
+  JSON.stringify({ agent_type: 'anything' }),
+);
+assert.equal(result.status, 0, result.stderr);
+assert.ok(/exceeds 256 chars/.test(result.stderr), 'overlong pattern must warn on stderr');
+output = JSON.parse(result.stdout);
+assert.equal(output.hookSpecificOutput.hookEventName, 'SubagentStart');
+
+// The length cap is inclusive at 256 characters.
+result = run(
+  'ponytail-subagent.js',
+  { ...scopeEnv, PONYTAIL_SUBAGENT_MATCHER: 'a'.repeat(256) },
+  JSON.stringify({ agent_type: 'a'.repeat(256) }),
+);
+assert.equal(result.status, 0, result.stderr);
+assert.equal(result.stderr, '', '256-char matcher is valid and must not warn');
+output = JSON.parse(result.stdout);
+assert.equal(output.hookSpecificOutput.hookEventName, 'SubagentStart');
+
+result = run(
+  'ponytail-subagent.js',
+  { ...scopeEnv, PONYTAIL_SUBAGENT_MATCHER: 'a'.repeat(257) },
+  JSON.stringify({ agent_type: 'a'.repeat(257) }),
+);
+assert.equal(result.status, 0, result.stderr);
+assert.ok(/exceeds 256 chars/.test(result.stderr), '257-char matcher must warn');
 output = JSON.parse(result.stdout);
 assert.equal(output.hookSpecificOutput.hookEventName, 'SubagentStart');