diff --git a/lib/inventory/build-inventory/interface.nix b/lib/inventory/build-inventory/interface.nix
index ea3e45422..2cf922197 100644
--- a/lib/inventory/build-inventory/interface.nix
+++ b/lib/inventory/build-inventory/interface.nix
@@ -362,7 +362,7 @@ in
                       services.borgbackup."instance_1" = {
                         roles.client.machines = ["machineA"];
 
-                        machineA.config = {
+                        machines.machineA.config = {
                           # Additional specific config for the machine
                           # This is merged with all other config places
                         };
diff --git a/pkgs/clan-cli/clan_cli/api/serde.py b/pkgs/clan-cli/clan_cli/api/serde.py
index 78de6a533..85c5f796a 100644
--- a/pkgs/clan-cli/clan_cli/api/serde.py
+++ b/pkgs/clan-cli/clan_cli/api/serde.py
@@ -319,5 +319,4 @@ def from_dict(
             msg = f"{data} is not a dict. Expected {t}"
             raise ClanError(msg)
         return construct_dataclass(t, data, path)  # type: ignore
-    # breakpoint()
     return construct_value(t, data, path)
diff --git a/pkgs/clan-cli/clan_cli/inventory/__init__.py b/pkgs/clan-cli/clan_cli/inventory/__init__.py
index 5903aeec1..1aa634d72 100644
--- a/pkgs/clan-cli/clan_cli/inventory/__init__.py
+++ b/pkgs/clan-cli/clan_cli/inventory/__init__.py
@@ -14,6 +14,7 @@ Operate on the returned inventory to make changes
 
 import contextlib
 import json
+from collections import Counter
 from pathlib import Path
 from typing import Any
 
@@ -94,7 +95,6 @@ def load_inventory_eval(flake_dir: str | Path) -> Inventory:
 def flatten_data(data: dict, parent_key: str = "", separator: str = ".") -> dict:
     """
     Recursively flattens a nested dictionary structure where keys are joined by the separator.
-    The flattened dictionary contains only entries with "__prio".
 
     Args:
         data (dict): The nested dictionary structure.
@@ -102,26 +102,110 @@ def flatten_data(data: dict, parent_key: str = "", separator: str = ".") -> dict
         separator (str): The string to use for joining keys.
 
     Returns:
-        dict: A flattened dictionary with "__prio" values.
+        dict: A flattened dictionary with all values. Directly in the root.
     """
     flattened = {}
 
     for key, value in data.items():
         new_key = f"{parent_key}{separator}{key}" if parent_key else key
 
-        if isinstance(value, dict) and "__prio" in value:
-            flattened[new_key] = {"__prio": value["__prio"]}
-
         if isinstance(value, dict):
             # Recursively flatten the nested dictionary
             flattened.update(flatten_data(value, new_key, separator))
+        else:
+            flattened[new_key] = value
 
     return flattened
 
 
+def unmerge_lists(all_items: list, filter_items: list) -> list:
+    """
+    Unmerge the current list. Given a previous list.
+
+    Returns:
+        The other list.
+    """
+    # Unmerge the lists
+    res = []
+    for value in all_items:
+        if value not in filter_items:
+            res.append(value)
+
+    return res
+
+
+def find_duplicates(string_list: list[str]) -> list[str]:
+    count = Counter(string_list)
+    duplicates = [item for item, freq in count.items() if freq > 1]
+    return duplicates
+
+
+def calc_patches(
+    persisted: dict, update: dict, all_values: dict, writeables: dict
+) -> dict[str, Any]:
+    """
+    Calculate the patches to apply to the inventory.
+
+    Given its current state and the update to apply.
+
+    Filters out nix-values so it doesnt matter if the anyone sends them.
+
+    : param persisted: The current state of the inventory.
+    : param update: The update to apply.
+    : param writeable: The writeable keys. Use 'determine_writeability'.
+        Example: {'writeable': {'foo', 'foo.bar'}, 'non_writeable': {'foo.nix'}}
+    : param all_values: All values in the inventory retrieved from the flake evaluation.
+    """
+    persisted_flat = flatten_data(persisted)
+    update_flat = flatten_data(update)
+    all_values_flat = flatten_data(all_values)
+
+    patchset = {}
+    for update_key, update_data in update_flat.items():
+        if update_key in writeables["non_writeable"]:
+            if update_data != all_values_flat.get(update_key):
+                msg = f"Key '{update_key}' is not writeable."
+                raise ClanError(msg)
+            continue
+
+        if update_key in writeables["writeable"]:
+            if type(update_data) is not type(all_values_flat.get(update_key)):
+                msg = f"Type mismatch for key '{update_key}'. Cannot update {type(all_values_flat.get(update_key))} with {type(update_data)}"
+                raise ClanError(msg)
+
+            # Handle list seperation
+            if isinstance(update_data, list):
+                duplicates = find_duplicates(update_data)
+                if duplicates:
+                    msg = f"Key '{update_key}' contains duplicates: {duplicates}. This not supported yet."
+                    raise ClanError(msg)
+                # List of current values
+                persisted_data = persisted_flat.get(update_key, [])
+                # List including nix values
+                all_list = all_values_flat.get(update_key, [])
+                nix_list = unmerge_lists(all_list, persisted_data)
+                if update_data != all_list:
+                    patchset[update_key] = unmerge_lists(update_data, nix_list)
+
+            elif update_data != persisted_flat.get(update_key, None):
+                patchset[update_key] = update_data
+
+            continue
+
+        if update_key not in all_values_flat:
+            msg = f"Key '{update_key}' cannot be set. It does not exist."
+            raise ClanError(msg)
+
+        msg = f"Cannot determine writeability for key '{update_key}'"
+        raise ClanError(msg)
+
+    return patchset
+
+
 def determine_writeability(
-    data: dict,
-    correlated: dict,
+    priorities: dict,
+    defaults: dict,
+    persisted: dict,
     parent_key: str = "",
     parent_prio: int | None = None,
     results: dict | None = None,
@@ -130,7 +214,7 @@ def determine_writeability(
     if results is None:
         results = {"writeable": set({}), "non_writeable": set({})}
 
-    for key, value in data.items():
+    for key, value in priorities.items():
         if key == "__prio":
             continue
 
@@ -149,6 +233,7 @@ def determine_writeability(
             if isinstance(value, dict):
                 determine_writeability(
                     value,
+                    defaults,
                     {},  # Children won't be writeable, so correlation doesn't matter here
                     full_key,
                     prio,  # Pass the same priority down
@@ -159,13 +244,22 @@ def determine_writeability(
             continue
 
         # Check if the key is writeable otherwise
-        key_in_correlated = key in correlated
+        key_in_correlated = key in persisted
         if prio is None:
             msg = f"Priority for key '{full_key}' is not defined. Cannot determine if it is writeable."
             raise ClanError(msg)
 
-        has_children = any(k != "__prio" for k in value)
-        is_writeable = prio > 100 or key_in_correlated or has_children
+        is_mergeable = False
+        if prio == 100:
+            default = defaults.get(key)
+            if isinstance(default, dict):
+                is_mergeable = True
+            if isinstance(default, list):
+                is_mergeable = True
+            if key_in_correlated:
+                is_mergeable = True
+
+        is_writeable = prio > 100 or is_mergeable
 
         # Append the result
         if is_writeable:
@@ -177,7 +271,8 @@ def determine_writeability(
         if isinstance(value, dict):
             determine_writeability(
                 value,
-                correlated.get(key, {}),
+                defaults.get(key, {}),
+                persisted.get(key, {}),
                 full_key,
                 prio,  # Pass down current priority
                 results,
diff --git a/pkgs/clan-cli/tests/test_patch_inventory.py b/pkgs/clan-cli/tests/test_patch_inventory.py
index 3a10088b0..969ecfc36 100644
--- a/pkgs/clan-cli/tests/test_patch_inventory.py
+++ b/pkgs/clan-cli/tests/test_patch_inventory.py
@@ -1,5 +1,12 @@
 # Functions to test
-from clan_cli.inventory import determine_writeability, patch
+import pytest
+from clan_cli.errors import ClanError
+from clan_cli.inventory import (
+    calc_patches,
+    determine_writeability,
+    patch,
+    unmerge_lists,
+)
 
 
 # --------- Patching tests ---------
@@ -49,8 +56,9 @@ def test_write_simple() -> None:
         },
     }
 
+    default: dict = {"foo": {}}
     data: dict = {}
-    res = determine_writeability(prios, data)
+    res = determine_writeability(prios, default, data)
 
     assert res == {"writeable": {"foo", "foo.bar"}, "non_writeable": set({})}
 
@@ -67,7 +75,7 @@ def test_write_inherited() -> None:
     }
 
     data: dict = {}
-    res = determine_writeability(prios, data)
+    res = determine_writeability(prios, {"foo": {"bar": {}}}, data)
     assert res == {
         "writeable": {"foo", "foo.bar", "foo.bar.baz"},
         "non_writeable": set(),
@@ -86,13 +94,34 @@ def test_non_write_inherited() -> None:
     }
 
     data: dict = {}
-    res = determine_writeability(prios, data)
+    res = determine_writeability(prios, {}, data)
     assert res == {
         "writeable": set(),
         "non_writeable": {"foo", "foo.bar", "foo.bar.baz"},
     }
 
 
+def test_write_list() -> None:
+    prios = {
+        "foo": {
+            "__prio": 100,
+        },
+    }
+
+    data: dict = {}
+    default: dict = {
+        "foo": [
+            "a",
+            "b",
+        ]  # <- writeable: because lists are merged. Filtering out nix-values comes later
+    }
+    res = determine_writeability(prios, default, data)
+    assert res == {
+        "writeable": {"foo"},
+        "non_writeable": set(),
+    }
+
+
 def test_write_because_written() -> None:
     prios = {
         "foo": {
@@ -107,7 +136,7 @@ def test_write_because_written() -> None:
 
     # Given the following data. {}
     # Check that the non-writeable paths are correct.
-    res = determine_writeability(prios, {})
+    res = determine_writeability(prios, {"foo": {"bar": {}}}, {})
     assert res == {
         "writeable": {"foo", "foo.bar"},
         "non_writeable": {"foo.bar.baz", "foo.bar.foobar"},
@@ -120,8 +149,247 @@ def test_write_because_written() -> None:
             }
         }
     }
-    res = determine_writeability(prios, data)
+    res = determine_writeability(prios, {}, data)
     assert res == {
         "writeable": {"foo", "foo.bar", "foo.bar.baz"},
         "non_writeable": {"foo.bar.foobar"},
     }
+
+
+# --------- List unmerge tests ---------
+
+
+def test_list_unmerge() -> None:
+    all_machines = ["machineA", "machineB"]
+    inventory = ["machineB"]
+
+    nix_machines = unmerge_lists(all_machines, inventory)
+    assert nix_machines == ["machineA"]
+
+
+# --------- Write tests ---------
+
+
+def test_update_simple() -> None:
+    prios = {
+        "foo": {
+            "__prio": 100,  # <- writeable: "foo"
+            "bar": {"__prio": 1000},  # <- writeable: mkDefault "foo.bar"
+            "nix": {"__prio": 100},  # <- non writeable: "foo.bar" (defined in nix)
+        },
+    }
+
+    data_eval = {"foo": {"bar": "baz", "nix": "this is set in nix"}}
+
+    data_disk: dict = {}
+
+    writeables = determine_writeability(prios, data_eval, data_disk)
+
+    assert writeables == {"writeable": {"foo", "foo.bar"}, "non_writeable": {"foo.nix"}}
+
+    update = {
+        "foo": {
+            "bar": "new value",  # <- user sets this value
+            "nix": "this is set in nix",  # <- user didnt touch this value
+            # If the user would have set this value, it would trigger an error
+        }
+    }
+    patchset = calc_patches(
+        data_disk, update, all_values=data_eval, writeables=writeables
+    )
+
+    assert patchset == {"foo.bar": "new value"}
+
+
+def test_update_many() -> None:
+    prios = {
+        "foo": {
+            "__prio": 100,  # <- writeable: "foo"
+            "bar": {"__prio": 100},  # <-
+            "nix": {"__prio": 100},  # <- non writeable: "foo.bar" (defined in nix)
+            "nested": {
+                "__prio": 100,
+                "x": {"__prio": 100},  # <- writeable: "foo.nested.x"
+                "y": {"__prio": 100},  # <- non-writeable: "foo.nested.y"
+            },
+        },
+    }
+
+    data_eval = {
+        "foo": {
+            "bar": "baz",
+            "nix": "this is set in nix",
+            "nested": {"x": "x", "y": "y"},
+        }
+    }
+
+    data_disk = {"foo": {"bar": "baz", "nested": {"x": "x"}}}
+
+    writeables = determine_writeability(prios, data_eval, data_disk)
+
+    assert writeables == {
+        "writeable": {"foo.nested", "foo", "foo.bar", "foo.nested.x"},
+        "non_writeable": {"foo.nix", "foo.nested.y"},
+    }
+
+    update = {
+        "foo": {
+            "bar": "new value for bar",  # <- user sets this value
+            "nix": "this is set in nix",  # <- user cannot set this value
+            "nested": {
+                "x": "new value for x",  # <- user sets this value
+                "y": "y",  # <- user cannot set this value
+            },
+        }
+    }
+    patchset = calc_patches(
+        data_disk, update, all_values=data_eval, writeables=writeables
+    )
+
+    assert patchset == {
+        "foo.bar": "new value for bar",
+        "foo.nested.x": "new value for x",
+    }
+
+
+def test_update_parent_non_writeable() -> None:
+    prios = {
+        "foo": {
+            "__prio": 50,  # <- non-writeable: "foo"
+            "bar": {"__prio": 1000},  # <- writeable: mkDefault "foo.bar"
+        },
+    }
+
+    data_eval = {
+        "foo": {
+            "bar": "baz",
+        }
+    }
+
+    data_disk = {
+        "foo": {
+            "bar": "baz",
+        }
+    }
+
+    writeables = determine_writeability(prios, data_eval, data_disk)
+
+    assert writeables == {"writeable": set(), "non_writeable": {"foo", "foo.bar"}}
+
+    update = {
+        "foo": {
+            "bar": "new value",  # <- user sets this value
+        }
+    }
+    with pytest.raises(ClanError) as error:
+        calc_patches(data_disk, update, all_values=data_eval, writeables=writeables)
+
+    assert str(error.value) == "Key 'foo.bar' is not writeable."
+
+
+def test_update_list() -> None:
+    prios = {
+        "foo": {
+            "__prio": 100,  # <- writeable: "foo"
+        },
+    }
+
+    data_eval = {
+        #  [ "A" ]  is defined in nix.
+        "foo": ["A", "B"]
+    }
+
+    data_disk = {"foo": ["B"]}
+
+    writeables = determine_writeability(prios, data_eval, data_disk)
+
+    assert writeables == {"writeable": {"foo"}, "non_writeable": set()}
+
+    # Add "C" to the list
+    update = {
+        "foo": ["A", "B", "C"]  # User wants to add "C"
+    }
+
+    patchset = calc_patches(
+        data_disk, update, all_values=data_eval, writeables=writeables
+    )
+
+    assert patchset == {"foo": ["B", "C"]}
+
+    # Remove "B" from the list
+    update = {
+        "foo": ["A"]  # User wants to remove "B"
+    }
+
+    patchset = calc_patches(
+        data_disk, update, all_values=data_eval, writeables=writeables
+    )
+
+    assert patchset == {"foo": []}
+
+
+def test_update_list_duplicates() -> None:
+    prios = {
+        "foo": {
+            "__prio": 100,  # <- writeable: "foo"
+        },
+    }
+
+    data_eval = {
+        #  [ "A" ]  is defined in nix.
+        "foo": ["A", "B"]
+    }
+
+    data_disk = {"foo": ["B"]}
+
+    writeables = determine_writeability(prios, data_eval, data_disk)
+
+    assert writeables == {"writeable": {"foo"}, "non_writeable": set()}
+
+    # Add "A" to the list
+    update = {
+        "foo": ["A", "B", "A"]  # User wants to add duplicate "A"
+    }
+
+    with pytest.raises(ClanError) as error:
+        calc_patches(data_disk, update, all_values=data_eval, writeables=writeables)
+
+    assert (
+        str(error.value)
+        == "Key 'foo' contains duplicates: ['A']. This not supported yet."
+    )
+
+
+def test_update_mismatching_update_type() -> None:
+    prios = {
+        "foo": {
+            "__prio": 100,  # <- writeable: "foo"
+        },
+    }
+
+    data_eval = {"foo": ["A", "B"]}
+
+    data_disk: dict = {}
+
+    writeables = determine_writeability(prios, data_eval, data_disk)
+
+    assert writeables == {"writeable": {"foo"}, "non_writeable": set()}
+
+    # set foo.A which doesnt exist
+    update_1 = {"foo": {"A": "B"}}
+
+    with pytest.raises(ClanError) as error:
+        calc_patches(data_disk, update_1, all_values=data_eval, writeables=writeables)
+
+    assert str(error.value) == "Key 'foo.A' cannot be set. It does not exist."
+
+    # set foo to an int but it is a list
+    update_2: dict = {"foo": 1}
+
+    with pytest.raises(ClanError) as error:
+        calc_patches(data_disk, update_2, all_values=data_eval, writeables=writeables)
+
+    assert (
+        str(error.value)
+        == "Type mismatch for key 'foo'. Cannot update <class 'list'> with <class 'int'>"
+    )