diff --git a/test/test_api_v3_optional_body.py b/test/test_api_v3_optional_body.py
index c03db5a8..4b7eb133 100644
--- a/test/test_api_v3_optional_body.py
+++ b/test/test_api_v3_optional_body.py
@@ -18,6 +18,7 @@ This file covers the endpoints whose bodyless behaviour is not already
 tested in their own suite.
 """
 
+import re
 import sys
 from pathlib import Path
 from unittest.mock import MagicMock
@@ -70,21 +71,66 @@ class TestPluginLimits:
         assert api_v3_client.post(self.URL).status_code != 500
 
 
-class TestNoToleratedBodyReadIsUnguarded:
-    def test_every_or_default_body_read_uses_silent(self):
+class TestMissingBodyGivesTheDeclaredError:
+    """Handlers that answer "No data provided" must actually be able to.
+
+    A second group of handlers reads `data = request.get_json()` and then
+    guards with `if not data: return 400`. That guard is unreachable for a
+    request with no JSON body, because get_json() raises first — so the
+    caller got a 500 "an error occurred; see logs for details" instead of
+    the 400 the handler plainly intends to send.
+    """
+
+    @pytest.mark.parametrize("url", [
+        "/api/v3/plugins/install",
+        "/api/v3/plugins/install-from-url",
+        "/api/v3/plugins/registry-from-url",
+        "/api/v3/config/raw/main",
+        "/api/v3/config/raw/secrets",
+        "/api/v3/cache/delete",
+    ])
+    def test_bodyless_post_gets_a_400_not_a_500(self, api_v3_client, api_v3_module, url):
+        response = api_v3_client.post(url)
+        assert response.status_code == 400, (
+            f"{url} answered {response.status_code}: "
+            f"{response.get_data(as_text=True)[:200]}")
+
+    @pytest.mark.parametrize("url", [
+        "/api/v3/plugins/install",
+        "/api/v3/config/raw/main",
+    ])
+    def test_malformed_json_gets_a_400_not_a_500(self, api_v3_client, api_v3_module, url):
+        response = api_v3_client.post(
+            url, data="{not json", content_type="application/json")
+        assert response.status_code == 400
+
+
+class TestNoBodyReadContradictsItsOwnGuard:
+    SOURCE = Path(__file__).parent.parent / "web_interface/blueprints/api_v3.py"
+
+    def test_no_or_default_read_is_unguarded(self):
         """`get_json() or <default>` is a contradiction without silent=True.
 
         Writing `or {}` declares the body optional; omitting silent=True
-        means the call raises before the default can apply. Catch the
-        combination here rather than waiting for each endpoint to be
-        exercised by hand.
+        means the call raises before the default can apply.
         """
-        source = Path(__file__).parent.parent.joinpath(
-            "web_interface/blueprints/api_v3.py").read_text()
         offenders = [
-            line.strip() for line in source.splitlines()
+            line.strip() for line in self.SOURCE.read_text().splitlines()
             if "request.get_json()" in line and " or " in line
         ]
         assert offenders == [], (
             "these reads declare a default but raise before reaching it; "
             f"use get_json(silent=True): {offenders}")
+
+    def test_no_not_data_guard_is_unreachable(self):
+        """A `if not data:` guard needs a read that can actually return None."""
+        lines = self.SOURCE.read_text().splitlines()
+        offenders = []
+        for i, line in enumerate(lines):
+            if re.search(r"=\s*request\.get_json\(\)\s*$", line):
+                window = "\n".join(lines[i + 1:i + 3])
+                if re.search(r"if\s+(not\s+data\b|data\s+is\s+None)", window):
+                    offenders.append(f"line {i + 1}: {line.strip()}")
+        assert offenders == [], (
+            "these handlers guard on a missing body but raise before the "
+            f"guard runs; use get_json(silent=True): {offenders}")
diff --git a/test/test_api_v3_plugin_install_endpoints.py b/test/test_api_v3_plugin_install_endpoints.py
new file mode 100644
index 00000000..29f7f7dc
--- /dev/null
+++ b/test/test_api_v3_plugin_install_endpoints.py
@@ -0,0 +1,302 @@
+"""
+Endpoint tests for POST /plugins/install and POST /plugins/install-from-url.
+
+Both were only ever tested at the PluginStoreManager layer, so the route
+logic — the queue-vs-direct branch, schema invalidation, plugin discovery,
+state and history recording — was unexercised.
+
+/plugins/install carries the same install logic twice: once inside the
+operation-queue callback and once in the direct fallback. The paired
+tests below assert both branches produce the same side effects, so the
+duplication cannot quietly drift.
+"""
+
+import sys
+from pathlib import Path
+from unittest.mock import MagicMock
+
+import pytest
+
+sys.path.insert(0, str(Path(__file__).parent.parent))
+
+from test._api_v3_test_helpers import api_v3_client, api_v3_module  # noqa: F401,E402
+
+INSTALL = "/api/v3/plugins/install"
+FROM_URL = "/api/v3/plugins/install-from-url"
+
+
+@pytest.fixture
+def queued(api_v3_module):
+    """Enable the operation queue and run its callback synchronously."""
+    queue = MagicMock()
+
+    def enqueue(operation_type, plugin_id, operation_callback=None):
+        queue.callback_result = operation_callback(MagicMock())
+        return "op-123"
+
+    queue.enqueue_operation.side_effect = enqueue
+    api_v3_module.api_v3.operation_queue = queue
+    return queue
+
+
+def side_effects(module):
+    """The manager calls a successful install is expected to make."""
+    api = module.api_v3
+    return {
+        "schema_invalidated": api.schema_manager.invalidate_cache.call_args_list,
+        "discovered": api.plugin_manager.discover_plugins.call_count,
+        "loaded": api.plugin_manager.load_plugin.call_args_list,
+        "state_set": api.plugin_state_manager.set_plugin_installed.call_args_list,
+        "history": api.operation_history.record_operation.call_args_list,
+    }
+
+
+class TestInstallValidation:
+    def test_uninitialized_store_manager_is_a_500(self, api_v3_client, api_v3_module):
+        api_v3_module.api_v3.plugin_store_manager = None
+        response = api_v3_client.post(INSTALL, json={"plugin_id": "clock"})
+        assert response.status_code == 500
+        assert "not initialized" in response.get_json()["message"]
+
+    def test_missing_plugin_id_is_a_400(self, api_v3_client, api_v3_module):
+        response = api_v3_client.post(INSTALL, json={})
+        assert response.status_code == 400
+        assert "plugin_id required" in response.get_json()["message"]
+        api_v3_module.api_v3.plugin_store_manager.install_plugin.assert_not_called()
+
+    def test_empty_body_is_a_400(self, api_v3_client, api_v3_module):
+        assert api_v3_client.post(INSTALL, json=None).status_code == 400
+
+
+class TestInstallDirectPath:
+    """operation_queue is None — the fallback branch."""
+
+    def test_success(self, api_v3_client, api_v3_module):
+        api_v3_module.api_v3.plugin_store_manager.install_plugin.return_value = True
+        response = api_v3_client.post(INSTALL, json={"plugin_id": "clock"})
+        assert response.status_code == 200
+        assert response.get_json()["status"] == "success"
+
+    def test_success_side_effects(self, api_v3_client, api_v3_module):
+        api_v3_module.api_v3.plugin_store_manager.install_plugin.return_value = True
+        api_v3_client.post(INSTALL, json={"plugin_id": "clock"})
+        effects = side_effects(api_v3_module)
+        assert effects["schema_invalidated"] == [(("clock",), {})]
+        assert effects["discovered"] == 1
+        assert effects["loaded"] == [(("clock",), {})]
+        assert effects["state_set"] == [(("clock",), {})]
+        assert effects["history"][0].kwargs["status"] == "success"
+
+    def test_branch_forwarded_to_the_manager(self, api_v3_client, api_v3_module):
+        manager = api_v3_module.api_v3.plugin_store_manager
+        manager.install_plugin.return_value = True
+        api_v3_client.post(INSTALL, json={"plugin_id": "clock", "branch": "dev"})
+        manager.install_plugin.assert_called_once_with("clock", branch="dev")
+
+    def test_branch_named_in_the_message(self, api_v3_client, api_v3_module):
+        api_v3_module.api_v3.plugin_store_manager.install_plugin.return_value = True
+        response = api_v3_client.post(INSTALL, json={"plugin_id": "clock", "branch": "dev"})
+        assert "(branch: dev)" in response.get_json()["message"]
+
+    def test_failure_is_a_500(self, api_v3_client, api_v3_module):
+        api_v3_module.api_v3.plugin_store_manager.install_plugin.return_value = False
+        response = api_v3_client.post(INSTALL, json={"plugin_id": "clock"})
+        assert response.status_code == 500
+        assert "Failed to install" in response.get_json()["message"]
+
+    def test_failure_mentions_missing_registry_entry(self, api_v3_client, api_v3_module):
+        manager = api_v3_module.api_v3.plugin_store_manager
+        manager.install_plugin.return_value = False
+        manager.get_plugin_info.return_value = None
+        response = api_v3_client.post(INSTALL, json={"plugin_id": "ghost"})
+        assert "not found in registry" in response.get_json()["message"]
+
+    def test_failure_omits_registry_note_when_plugin_is_known(
+            self, api_v3_client, api_v3_module):
+        manager = api_v3_module.api_v3.plugin_store_manager
+        manager.install_plugin.return_value = False
+        manager.get_plugin_info.return_value = {"id": "clock"}
+        response = api_v3_client.post(INSTALL, json={"plugin_id": "clock"})
+        assert "not found in registry" not in response.get_json()["message"]
+
+    def test_failure_recorded_in_history(self, api_v3_client, api_v3_module):
+        api_v3_module.api_v3.plugin_store_manager.install_plugin.return_value = False
+        api_v3_client.post(INSTALL, json={"plugin_id": "clock"})
+        record = api_v3_module.api_v3.operation_history.record_operation.call_args
+        assert record.kwargs["status"] == "failed"
+
+    def test_no_side_effects_on_failure(self, api_v3_client, api_v3_module):
+        api_v3_module.api_v3.plugin_store_manager.install_plugin.return_value = False
+        api_v3_client.post(INSTALL, json={"plugin_id": "clock"})
+        effects = side_effects(api_v3_module)
+        assert effects["schema_invalidated"] == []
+        assert effects["loaded"] == []
+        assert effects["state_set"] == []
+
+
+class TestInstallQueuedPath:
+    """operation_queue present — the callback branch."""
+
+    def test_returns_an_operation_id(self, api_v3_client, api_v3_module, queued):
+        api_v3_module.api_v3.plugin_store_manager.install_plugin.return_value = True
+        response = api_v3_client.post(INSTALL, json={"plugin_id": "clock"})
+        assert response.status_code == 200
+        assert response.get_json()["data"]["operation_id"] == "op-123"
+
+    def test_message_says_queued(self, api_v3_client, api_v3_module, queued):
+        api_v3_module.api_v3.plugin_store_manager.install_plugin.return_value = True
+        response = api_v3_client.post(INSTALL, json={"plugin_id": "clock"})
+        assert "queued" in response.get_json()["message"]
+
+    def test_callback_success_side_effects(self, api_v3_client, api_v3_module, queued):
+        api_v3_module.api_v3.plugin_store_manager.install_plugin.return_value = True
+        api_v3_client.post(INSTALL, json={"plugin_id": "clock"})
+        effects = side_effects(api_v3_module)
+        assert effects["schema_invalidated"] == [(("clock",), {})]
+        assert effects["discovered"] == 1
+        assert effects["loaded"] == [(("clock",), {})]
+        assert effects["state_set"] == [(("clock",), {})]
+        assert effects["history"][0].kwargs["status"] == "success"
+
+    def test_callback_reports_success(self, api_v3_client, api_v3_module, queued):
+        api_v3_module.api_v3.plugin_store_manager.install_plugin.return_value = True
+        api_v3_client.post(INSTALL, json={"plugin_id": "clock"})
+        assert queued.callback_result["success"] is True
+
+    def test_callback_failure_raises_for_the_queue(self, api_v3_client, api_v3_module, queued):
+        api_v3_module.api_v3.plugin_store_manager.install_plugin.return_value = False
+        # The callback signals failure by raising, so the queue can mark the
+        # operation failed; the route's catch-all turns it into a 500.
+        response = api_v3_client.post(INSTALL, json={"plugin_id": "clock"})
+        assert response.status_code == 500
+
+    def test_callback_failure_recorded_in_history(self, api_v3_client, api_v3_module, queued):
+        api_v3_module.api_v3.plugin_store_manager.install_plugin.return_value = False
+        api_v3_client.post(INSTALL, json={"plugin_id": "clock"})
+        record = api_v3_module.api_v3.operation_history.record_operation.call_args
+        assert record.kwargs["status"] == "failed"
+
+    def test_branch_forwarded_from_the_callback(self, api_v3_client, api_v3_module, queued):
+        manager = api_v3_module.api_v3.plugin_store_manager
+        manager.install_plugin.return_value = True
+        api_v3_client.post(INSTALL, json={"plugin_id": "clock", "branch": "dev"})
+        manager.install_plugin.assert_called_once_with("clock", branch="dev")
+
+
+class TestInstallPathsAgree:
+    """The queue callback and the direct fallback duplicate the same logic."""
+
+    def _run(self, client, module, install_ok, queue):
+        module.api_v3.plugin_store_manager.install_plugin.return_value = install_ok
+        client.post(INSTALL, json={"plugin_id": "clock", "branch": "dev"})
+        return side_effects(module)
+
+    def test_success_side_effects_match(self, api_v3_client, api_v3_module):
+        direct = self._run(api_v3_client, api_v3_module, True, None)
+
+        # Reset and re-run through the queue.
+        for mock in (api_v3_module.api_v3.schema_manager,
+                     api_v3_module.api_v3.plugin_manager,
+                     api_v3_module.api_v3.plugin_state_manager,
+                     api_v3_module.api_v3.operation_history):
+            mock.reset_mock()
+        queue = MagicMock()
+        queue.enqueue_operation.side_effect = (
+            lambda t, p, operation_callback=None: operation_callback(MagicMock()) and "op")
+        api_v3_module.api_v3.operation_queue = queue
+        queued = self._run(api_v3_client, api_v3_module, True, queue)
+
+        assert direct["schema_invalidated"] == queued["schema_invalidated"]
+        assert direct["discovered"] == queued["discovered"]
+        assert direct["loaded"] == queued["loaded"]
+        assert direct["state_set"] == queued["state_set"]
+        assert (direct["history"][0].kwargs["status"]
+                == queued["history"][0].kwargs["status"])
+        assert (direct["history"][0].kwargs["details"]
+                == queued["history"][0].kwargs["details"])
+
+    def test_only_the_message_wording_differs(self, api_v3_client, api_v3_module):
+        # Characterized: the direct path says "Plugin installed
+        # successfully" while the queue callback says "Plugin clock
+        # installed successfully". Cosmetic, and the queue's text is
+        # internal to the operation record rather than the HTTP response.
+        api_v3_module.api_v3.plugin_store_manager.install_plugin.return_value = True
+        direct = api_v3_client.post(INSTALL, json={"plugin_id": "clock"}).get_json()
+        assert direct["message"] == "Plugin installed successfully"
+
+
+class TestInstallFromUrl:
+    def test_uninitialized_store_manager_is_a_500(self, api_v3_client, api_v3_module):
+        api_v3_module.api_v3.plugin_store_manager = None
+        assert api_v3_client.post(FROM_URL, json={"repo_url": "http://x"}).status_code == 500
+
+    def test_missing_repo_url_is_a_400(self, api_v3_client, api_v3_module):
+        response = api_v3_client.post(FROM_URL, json={})
+        assert response.status_code == 400
+        assert "repo_url required" in response.get_json()["message"]
+
+    def test_success(self, api_v3_client, api_v3_module):
+        api_v3_module.api_v3.plugin_store_manager.install_from_url.return_value = {
+            "success": True, "plugin_id": "clock", "name": "Clock"}
+        response = api_v3_client.post(FROM_URL, json={"repo_url": "https://github.com/o/r"})
+        assert response.status_code == 200
+        body = response.get_json()
+        assert body["plugin_id"] == "clock"
+        assert body["name"] == "Clock"
+
+    def test_all_optional_arguments_forwarded(self, api_v3_client, api_v3_module):
+        manager = api_v3_module.api_v3.plugin_store_manager
+        manager.install_from_url.return_value = {"success": True, "plugin_id": "clock"}
+        api_v3_client.post(FROM_URL, json={
+            "repo_url": "  https://github.com/o/r  ",
+            "plugin_id": "clock",
+            "plugin_path": "plugins/clock",
+            "branch": "dev",
+        })
+        manager.install_from_url.assert_called_once_with(
+            repo_url="https://github.com/o/r",
+            plugin_id="clock",
+            plugin_path="plugins/clock",
+            branch="dev",
+        )
+
+    def test_success_invalidates_schema_and_loads_plugin(self, api_v3_client, api_v3_module):
+        api_v3_module.api_v3.plugin_store_manager.install_from_url.return_value = {
+            "success": True, "plugin_id": "clock"}
+        api_v3_client.post(FROM_URL, json={"repo_url": "http://x"})
+        api_v3_module.api_v3.schema_manager.invalidate_cache.assert_called_once_with("clock")
+        api_v3_module.api_v3.plugin_manager.load_plugin.assert_called_once_with("clock")
+
+    def test_success_without_plugin_id_skips_discovery(self, api_v3_client, api_v3_module):
+        # install_from_url can succeed without naming the plugin; there is
+        # then nothing to invalidate or load.
+        api_v3_module.api_v3.plugin_store_manager.install_from_url.return_value = {
+            "success": True, "plugin_id": None}
+        api_v3_client.post(FROM_URL, json={"repo_url": "http://x"})
+        api_v3_module.api_v3.schema_manager.invalidate_cache.assert_not_called()
+        api_v3_module.api_v3.plugin_manager.load_plugin.assert_not_called()
+
+    def test_branch_from_result_included(self, api_v3_client, api_v3_module):
+        api_v3_module.api_v3.plugin_store_manager.install_from_url.return_value = {
+            "success": True, "plugin_id": "clock", "branch": "dev"}
+        body = api_v3_client.post(FROM_URL, json={"repo_url": "http://x"}).get_json()
+        assert body["branch"] == "dev"
+        assert "(branch: dev)" in body["message"]
+
+    def test_failure_reports_the_managers_error(self, api_v3_client, api_v3_module):
+        api_v3_module.api_v3.plugin_store_manager.install_from_url.return_value = {
+            "success": False, "error": "repo not found"}
+        response = api_v3_client.post(FROM_URL, json={"repo_url": "http://x"})
+        assert response.status_code == 500
+        assert response.get_json()["message"] == "repo not found"
+
+    def test_failure_without_error_uses_fallback_text(self, api_v3_client, api_v3_module):
+        api_v3_module.api_v3.plugin_store_manager.install_from_url.return_value = {
+            "success": False}
+        response = api_v3_client.post(FROM_URL, json={"repo_url": "http://x"})
+        assert "Failed to install plugin from URL" in response.get_json()["message"]
+
+    def test_manager_exception_is_a_500(self, api_v3_client, api_v3_module):
+        api_v3_module.api_v3.plugin_store_manager.install_from_url.side_effect = (
+            RuntimeError("boom"))
+        assert api_v3_client.post(FROM_URL, json={"repo_url": "http://x"}).status_code == 500
diff --git a/web_interface/blueprints/api_v3.py b/web_interface/blueprints/api_v3.py
index 48db77cf..e887e36f 100644
--- a/web_interface/blueprints/api_v3.py
+++ b/web_interface/blueprints/api_v3.py
@@ -328,7 +328,7 @@ def save_schedule_config():
         if not api_v3.config_manager:
             return jsonify({'status': 'error', 'message': 'Config manager not initialized'}), 500
 
-        data = request.get_json()
+        data = request.get_json(silent=True)
         if not data:
             return jsonify({'status': 'error', 'message': 'No data provided'}), 400
 
@@ -536,7 +536,7 @@ def save_dim_schedule_config():
         if not api_v3.config_manager:
             return jsonify({'status': 'error', 'message': 'Config manager not initialized'}), 500
 
-        data = request.get_json()
+        data = request.get_json(silent=True)
         if not data:
             return jsonify({'status': 'error', 'message': 'No data provided'}), 400
 
@@ -1345,7 +1345,7 @@ def save_raw_main_config():
         if not api_v3.config_manager:
             return jsonify({'status': 'error', 'message': 'Config manager not initialized'}), 500
 
-        data = request.get_json()
+        data = request.get_json(silent=True)
         if not data:
             return jsonify({'status': 'error', 'message': 'No data provided'}), 400
 
@@ -1391,7 +1391,7 @@ def save_raw_secrets_config():
         if not api_v3.config_manager:
             return jsonify({'status': 'error', 'message': 'Config manager not initialized'}), 500
 
-        data = request.get_json()
+        data = request.get_json(silent=True)
         if not data:
             return jsonify({'status': 'error', 'message': 'No data provided'}), 400
 
@@ -2966,7 +2966,7 @@ def toggle_plugin():
         content_type = request.content_type or ''
 
         if 'application/json' in content_type:
-            data = request.get_json()
+            data = request.get_json(silent=True)
             if not data or 'plugin_id' not in data or 'enabled' not in data:
                 return jsonify({'status': 'error', 'message': 'plugin_id and enabled required'}), 400
             plugin_id = data['plugin_id']
@@ -3837,7 +3837,7 @@ def install_plugin():
         if not api_v3.plugin_store_manager:
             return jsonify({'status': 'error', 'message': 'Plugin store manager not initialized'}), 500
 
-        data = request.get_json()
+        data = request.get_json(silent=True)
         if not data or 'plugin_id' not in data:
             return jsonify({'status': 'error', 'message': 'plugin_id required'}), 400
 
@@ -3971,7 +3971,7 @@ def install_plugin_from_url():
         if not api_v3.plugin_store_manager:
             return jsonify({'status': 'error', 'message': 'Plugin store manager not initialized'}), 500
 
-        data = request.get_json()
+        data = request.get_json(silent=True)
         if not data or 'repo_url' not in data:
             return jsonify({'status': 'error', 'message': 'repo_url required'}), 400
 
@@ -4026,7 +4026,7 @@ def get_registry_from_url():
         if not api_v3.plugin_store_manager:
             return jsonify({'status': 'error', 'message': 'Plugin store manager not initialized'}), 500
 
-        data = request.get_json()
+        data = request.get_json(silent=True)
         if not data or 'repo_url' not in data:
             return jsonify({'status': 'error', 'message': 'repo_url required'}), 400
 
@@ -4071,7 +4071,7 @@ def add_saved_repository():
         if not api_v3.saved_repositories_manager:
             return jsonify({'status': 'error', 'message': 'Saved repositories manager not initialized'}), 500
 
-        data = request.get_json()
+        data = request.get_json(silent=True)
         if not data or 'repo_url' not in data:
             return jsonify({'status': 'error', 'message': 'repo_url required'}), 400
 
@@ -4102,7 +4102,7 @@ def remove_saved_repository():
         if not api_v3.saved_repositories_manager:
             return jsonify({'status': 'error', 'message': 'Saved repositories manager not initialized'}), 500
 
-        data = request.get_json()
+        data = request.get_json(silent=True)
         if not data or 'repo_url' not in data:
             return jsonify({'status': 'error', 'message': 'repo_url required'}), 400
 
@@ -6529,7 +6529,7 @@ def get_fonts_overrides():
 def save_fonts_overrides():
     """Save font overrides"""
     try:
-        data = request.get_json()
+        data = request.get_json(silent=True)
         if not data:
             return jsonify({'status': 'error', 'message': 'No data provided'}), 400
 
@@ -7635,7 +7635,7 @@ def connect_wifi():
     try:
         from src.wifi_manager import WiFiManager
 
-        data = request.get_json()
+        data = request.get_json(silent=True)
         if not data:
             return jsonify({
                 'status': 'error',
@@ -7789,7 +7789,7 @@ def set_auto_enable_ap_mode():
     try:
         from src.wifi_manager import WiFiManager
 
-        data = request.get_json()
+        data = request.get_json(silent=True)
         if data is None or 'auto_enable_ap_mode' not in data:
             return jsonify({
                 'status': 'error',
@@ -7918,7 +7918,7 @@ def delete_cache_file():
             from src.cache_manager import CacheManager
             api_v3.cache_manager = CacheManager()
 
-        data = request.get_json()
+        data = request.get_json(silent=True)
         if not data or 'key' not in data:
             return jsonify({'status': 'error', 'message': 'cache key is required'}), 400