diff --git a/coworker/engine.py b/coworker/engine.py
index c87db216..71739639 100644
--- a/coworker/engine.py
+++ b/coworker/engine.py
@@ -1132,6 +1132,23 @@ class TurnEngine:
                     "your report which checks were degraded."
                 ),
             }
+        elif _toolchain.describe(name) is None:
+            # Not in the pinned catalog: no card at all (owner-hit 2026-08-20 — agents
+            # routed ordinary brew/pip installs through the install card, which could
+            # only fail after approval). The agent has a shell with its own approval
+            # flow; steer it there instead of at the user.
+            catalog = ", ".join(sorted(_toolchain.MANAGED))
+            result = {
+                "installed": False,
+                "error": (
+                    f"'{name}' is not in the pinned tool catalog ({catalog})."
+                ),
+                "guidance": (
+                    "Install it yourself with the shell (brew/pip/…, subject to the "
+                    "normal command approval), or continue without it and say in your "
+                    "report which checks were degraded."
+                ),
+            }
         else:
             # The prompt must say up front whether WE can install this (pinned build for
             # this platform) — a card that offers Install for a tool we can't fetch turns
diff --git a/coworker/server/app.py b/coworker/server/app.py
index 0e475423..ec62a10b 100644
--- a/coworker/server/app.py
+++ b/coworker/server/app.py
@@ -2073,6 +2073,20 @@ def create_app(manager: SessionManager) -> FastAPI:
             """
             name = str(args.get("name", "")).strip()
             info = toolchain.describe(name)
+            if not info:
+                # Not in the pinned catalog: never show an install card that can only
+                # end in "no pinned build" AFTER approval (owner-hit 2026-08-20 — agents
+                # routed ordinary brew/pip installs through the card). Steer to the
+                # shell, which has its own approval flow.
+                return {
+                    "installed": False,
+                    "error": (
+                        f"'{name}' is not in the pinned tool catalog "
+                        f"({', '.join(sorted(toolchain.MANAGED))}). Install it yourself "
+                        "with the shell (brew/pip/…, subject to the normal command "
+                        "approval), or proceed without it and note the gap."
+                    ),
+                }
             item = manager.inbox.add_tool_request(
                 session_id,
                 f"Install {name}?" if name else "Install a tool?",
diff --git a/coworker/tools/toolreq.py b/coworker/tools/toolreq.py
index a0ad4040..2137cd70 100644
--- a/coworker/tools/toolreq.py
+++ b/coworker/tools/toolreq.py
@@ -17,8 +17,13 @@ from aisuite.agents import ToolMetadata, tool
 
 def request_tool_tool() -> object:
     def request_tool(name: str, reason: str) -> dict:
-        """Ask the user to install a command-line tool you need but can't find on this
-        machine (e.g. `gitleaks`, `osv-scanner`, `semgrep`).
+        """Ask the user to install one of the PINNED catalog tools you need but can't find
+        on this machine. The catalog is a small closed set — currently `gitleaks`,
+        `trivy`, `osv-scanner` — installed at a pinned, checksum-verified version.
+
+        For ANY other missing CLI (semgrep, jq, kubectl, …) do NOT use this tool: install
+        it yourself with the shell (brew/pip/…), which goes through the normal command
+        approval, or proceed without it.
 
         Keep `reason` to ONE sentence: which check needs the tool. The prompt the user
         sees already explains what the install is (pinned version, publisher, checksum)
diff --git a/tests/test_tool_request.py b/tests/test_tool_request.py
index 6ca8e59b..1d58a822 100644
--- a/tests/test_tool_request.py
+++ b/tests/test_tool_request.py
@@ -118,7 +118,7 @@ async def test_decline_recheck_finds_a_copy_the_user_installed_themselves(tmp_pa
 async def test_event_tells_the_truth_about_installability(tmp_path, monkeypatch):
     """Owner-hit 2026-08-14: the card offered Install for a tool with no pinned build —
     the surface guessed because the event said nothing. The event must carry the
-    registry's verdict, and no metadata means NO."""
+    registry's verdict for catalog tools."""
     from coworker import toolchain
 
     monkeypatch.setattr(toolchain, "_platform_key", lambda: "darwin_arm64")
@@ -133,10 +133,29 @@ async def test_event_tells_the_truth_about_installability(tmp_path, monkeypatch)
     assert data["summary"]
     assert data["source"] == "github.com/gitleaks"
 
-    events = await _run(_engine(tmp_path, requester, tool="not-a-managed-tool"))
-    data = [e for e in events if e.type is EventType.TOOL_REQUESTED][0].data
-    assert data["installable"] is False
-    assert data["version"] == "" and data["summary"] == ""
+
+@pytest.mark.asyncio
+async def test_non_catalog_tool_gets_no_card_and_a_shell_steer(tmp_path, monkeypatch):
+    """Owner-hit 2026-08-20: agents routed ordinary brew/pip installs through the
+    install card, which could only fail AFTER the user approved. A non-catalog name
+    must produce NO prompt at all — just a result steering the agent to the shell."""
+    from coworker import toolchain
+
+    monkeypatch.setattr(toolchain, "_platform_key", lambda: "darwin_arm64")
+    called = []
+
+    async def requester(args, tool_call_id=None):
+        called.append(args)
+        return {"installed": False, "reason": "declined"}
+
+    engine = _engine(tmp_path, requester, tool="semgrep")
+    events = await _run(engine)
+    assert not [e for e in events if e.type is EventType.TOOL_REQUESTED]
+    assert called == []  # the user was never asked
+    tool_msg = [m for m in engine.messages if m.get("role") == "tool"][-1]
+    body = str(tool_msg["content"])
+    assert "not in the pinned tool catalog" in body
+    assert "shell" in body and "gitleaks" in body  # the catalog is named
 
 
 @pytest.mark.asyncio