diff --git a/claude_bottle/backend/docker/cred_proxy_apply.py b/claude_bottle/backend/docker/cred_proxy_apply.py
new file mode 100644
index 0000000..baf9266
--- /dev/null
+++ b/claude_bottle/backend/docker/cred_proxy_apply.py
@@ -0,0 +1,123 @@
+"""Host-side helper to apply a routes.json change to a running
+cred-proxy sidecar (PRD 0014).
+
+Used by the supervise dashboard when the operator approves a
+cred-proxy-block proposal (or runs the operator-initiated `routes
+edit <bottle>` verb). Fetches the current routes.json via `docker
+exec cat`, validates the new JSON, writes it into the sidecar via
+`docker cp`, then `docker kill --signal HUP` to make the in-sidecar
+SIGHUP handler (PRD 0014 Phase 1) reload without dropping
+connections.
+
+Raises CredProxyApplyError on any failure — the dashboard surfaces
+the message and keeps the proposal pending so the operator can
+retry.
+"""
+
+from __future__ import annotations
+
+import json
+import os
+import subprocess
+import tempfile
+from pathlib import Path
+
+from .cred_proxy import (
+    CRED_PROXY_ROUTES_IN_CONTAINER,
+    cred_proxy_container_name,
+)
+
+
+class CredProxyApplyError(RuntimeError):
+    """Raised when fetch / apply fails. Caller renders to the
+    operator; does not crash the dashboard."""
+
+
+def fetch_current_routes(slug: str) -> str:
+    """Read the live routes.json from the running cred-proxy sidecar
+    for `slug`. Returns the file content as a string. Raises
+    CredProxyApplyError if the sidecar isn't reachable or the read
+    fails."""
+    container = cred_proxy_container_name(slug)
+    r = subprocess.run(
+        ["docker", "exec", container, "cat", CRED_PROXY_ROUTES_IN_CONTAINER],
+        capture_output=True, text=True, check=False,
+    )
+    if r.returncode != 0:
+        raise CredProxyApplyError(
+            f"could not read routes.json from {container}: "
+            f"{(r.stderr or '').strip() or 'container not running?'}"
+        )
+    return r.stdout
+
+
+def validate_routes_json(content: str) -> None:
+    """Syntactic check before SIGHUP — the sidecar's reload also
+    validates, but failing here keeps the old routes live and gives
+    the operator a clearer error than 'reload failed' in the
+    sidecar logs."""
+    try:
+        parsed = json.loads(content)
+    except json.JSONDecodeError as e:
+        raise CredProxyApplyError(
+            f"proposed routes.json is not valid JSON: {e}"
+        ) from e
+    if not isinstance(parsed, dict) or not isinstance(parsed.get("routes"), list):
+        raise CredProxyApplyError(
+            "proposed routes.json must be an object with a 'routes' array"
+        )
+
+
+def apply_routes_change(slug: str, new_content: str) -> tuple[str, str]:
+    """Apply `new_content` to the cred-proxy sidecar for `slug`:
+      1. Fetch current routes.json (for the before-diff).
+      2. Validate the new JSON.
+      3. Write to a temp file, `docker cp` into the sidecar.
+      4. `docker kill --signal HUP` so cred-proxy reloads.
+
+    Returns (before, after) where `after` == `new_content`. Raises
+    CredProxyApplyError on any step; the existing routes in the
+    sidecar are unchanged if the failure is before docker cp, and
+    are reverted in spirit if SIGHUP fails (cp landed but reload
+    didn't fire — caller's next attempt will SIGHUP again)."""
+    container = cred_proxy_container_name(slug)
+    before = fetch_current_routes(slug)
+    validate_routes_json(new_content)
+
+    fd, tmp_path = tempfile.mkstemp(prefix="cb-routes.", suffix=".json")
+    try:
+        with os.fdopen(fd, "w") as f:
+            f.write(new_content)
+        cp = subprocess.run(
+            ["docker", "cp", tmp_path, f"{container}:{CRED_PROXY_ROUTES_IN_CONTAINER}"],
+            capture_output=True, text=True, check=False,
+        )
+        if cp.returncode != 0:
+            raise CredProxyApplyError(
+                f"failed to copy routes.json into {container}: "
+                f"{(cp.stderr or '').strip()}"
+            )
+        sig = subprocess.run(
+            ["docker", "kill", "--signal", "HUP", container],
+            capture_output=True, text=True, check=False,
+        )
+        if sig.returncode != 0:
+            raise CredProxyApplyError(
+                f"failed to SIGHUP {container}: "
+                f"{(sig.stderr or '').strip()}"
+            )
+    finally:
+        try:
+            Path(tmp_path).unlink()
+        except OSError:
+            pass
+
+    return before, new_content
+
+
+__all__ = [
+    "CredProxyApplyError",
+    "apply_routes_change",
+    "fetch_current_routes",
+    "validate_routes_json",
+]
diff --git a/tests/unit/test_cred_proxy_apply.py b/tests/unit/test_cred_proxy_apply.py
new file mode 100644
index 0000000..b0877d2
--- /dev/null
+++ b/tests/unit/test_cred_proxy_apply.py
@@ -0,0 +1,39 @@
+"""Unit: validate_routes_json (PRD 0014 Phase 2). docker exec / cp /
+kill paths are covered by the integration test."""
+
+import unittest
+
+from claude_bottle.backend.docker.cred_proxy_apply import (
+    CredProxyApplyError,
+    validate_routes_json,
+)
+
+
+class TestValidateRoutesJson(unittest.TestCase):
+    def test_accepts_routes_array(self):
+        validate_routes_json('{"routes": []}')
+        validate_routes_json(
+            '{"routes": [{"path": "/a/", "upstream": "https://example.com",'
+            ' "auth_scheme": "Bearer", "token_env": "T0"}]}'
+        )
+
+    def test_rejects_bad_json(self):
+        with self.assertRaises(CredProxyApplyError) as cm:
+            validate_routes_json("{not json")
+        self.assertIn("not valid JSON", str(cm.exception))
+
+    def test_rejects_non_object_top_level(self):
+        with self.assertRaises(CredProxyApplyError):
+            validate_routes_json("[]")
+
+    def test_rejects_missing_routes_key(self):
+        with self.assertRaises(CredProxyApplyError):
+            validate_routes_json('{"other": []}')
+
+    def test_rejects_non_list_routes(self):
+        with self.assertRaises(CredProxyApplyError):
+            validate_routes_json('{"routes": "not a list"}')
+
+
+if __name__ == "__main__":
+    unittest.main()