diff --git a/circuitforge_core/preferences/__init__.py b/circuitforge_core/preferences/__init__.py
new file mode 100644
index 0000000..5b3e1f7
--- /dev/null
+++ b/circuitforge_core/preferences/__init__.py
@@ -0,0 +1,3 @@
+from .paths import get_path, set_path
+
+__all__ = ["get_path", "set_path"]
diff --git a/circuitforge_core/preferences/paths.py b/circuitforge_core/preferences/paths.py
new file mode 100644
index 0000000..86a1627
--- /dev/null
+++ b/circuitforge_core/preferences/paths.py
@@ -0,0 +1,64 @@
+"""Dot-path utilities for reading and writing nested preference dicts.
+
+All operations are immutable: set_path returns a new dict rather than
+mutating the input.
+
+Path format: dot-separated keys, e.g. "affiliate.byok_ids.ebay"
+"""
+from __future__ import annotations
+
+from typing import Any
+
+
+def get_path(data: dict, path: str, default: Any = None) -> Any:
+    """Return the value at *path* inside *data*, or *default* if missing.
+
+    Example::
+
+        prefs = {"affiliate": {"opt_out": False, "byok_ids": {"ebay": "my-id"}}}
+        get_path(prefs, "affiliate.byok_ids.ebay")  # "my-id"
+        get_path(prefs, "affiliate.missing", default="x")  # "x"
+    """
+    keys = path.split(".")
+    node: Any = data
+    for key in keys:
+        if not isinstance(node, dict):
+            return default
+        node = node.get(key, _SENTINEL)
+        if node is _SENTINEL:
+            return default
+    return node
+
+
+def set_path(data: dict, path: str, value: Any) -> dict:
+    """Return a new dict with *value* written at *path*.
+
+    Intermediate dicts are created as needed; existing values at other paths
+    are preserved.  The original *data* dict is never mutated.
+
+    Example::
+
+        prefs = {}
+        updated = set_path(prefs, "affiliate.opt_out", True)
+        # {"affiliate": {"opt_out": True}}
+    """
+    keys = path.split(".")
+    return _set_recursive(data, keys, value)
+
+
+# ---------------------------------------------------------------------------
+# Internal helpers
+# ---------------------------------------------------------------------------
+
+_SENTINEL = object()
+
+
+def _set_recursive(node: Any, keys: list[str], value: Any) -> dict:
+    if not isinstance(node, dict):
+        node = {}
+    key, rest = keys[0], keys[1:]
+    if rest:
+        child = _set_recursive(node.get(key, {}), rest, value)
+    else:
+        child = value
+    return {**node, key: child}
diff --git a/tests/test_preferences.py b/tests/test_preferences.py
new file mode 100644
index 0000000..6e6c25e
--- /dev/null
+++ b/tests/test_preferences.py
@@ -0,0 +1,75 @@
+"""Tests for circuitforge_core.preferences path utilities."""
+import pytest
+from circuitforge_core.preferences import get_path, set_path
+
+
+class TestGetPath:
+    def test_top_level_key(self):
+        assert get_path({"a": 1}, "a") == 1
+
+    def test_nested_key(self):
+        data = {"affiliate": {"opt_out": False}}
+        assert get_path(data, "affiliate.opt_out") is False
+
+    def test_deeply_nested(self):
+        data = {"affiliate": {"byok_ids": {"ebay": "my-tag"}}}
+        assert get_path(data, "affiliate.byok_ids.ebay") == "my-tag"
+
+    def test_missing_key_returns_default(self):
+        assert get_path({}, "missing", default="x") == "x"
+
+    def test_missing_nested_returns_default(self):
+        assert get_path({"a": {}}, "a.b.c", default=42) == 42
+
+    def test_default_is_none_when_omitted(self):
+        assert get_path({}, "nope") is None
+
+    def test_non_dict_intermediate_returns_default(self):
+        assert get_path({"a": "string"}, "a.b", default="d") == "d"
+
+
+class TestSetPath:
+    def test_top_level_key(self):
+        result = set_path({}, "opt_out", True)
+        assert result == {"opt_out": True}
+
+    def test_nested_key_created(self):
+        result = set_path({}, "affiliate.opt_out", True)
+        assert result == {"affiliate": {"opt_out": True}}
+
+    def test_deeply_nested(self):
+        result = set_path({}, "affiliate.byok_ids.ebay", "my-tag")
+        assert result == {"affiliate": {"byok_ids": {"ebay": "my-tag"}}}
+
+    def test_preserves_sibling_keys(self):
+        data = {"affiliate": {"opt_out": False, "byok_ids": {}}}
+        result = set_path(data, "affiliate.opt_out", True)
+        assert result["affiliate"]["opt_out"] is True
+        assert result["affiliate"]["byok_ids"] == {}
+
+    def test_preserves_unrelated_top_level_keys(self):
+        data = {"other": "value", "affiliate": {"opt_out": False}}
+        result = set_path(data, "affiliate.opt_out", True)
+        assert result["other"] == "value"
+
+    def test_does_not_mutate_original(self):
+        data = {"affiliate": {"opt_out": False}}
+        set_path(data, "affiliate.opt_out", True)
+        assert data["affiliate"]["opt_out"] is False
+
+    def test_overwrites_existing_value(self):
+        data = {"affiliate": {"byok_ids": {"ebay": "old-tag"}}}
+        result = set_path(data, "affiliate.byok_ids.ebay", "new-tag")
+        assert result["affiliate"]["byok_ids"]["ebay"] == "new-tag"
+
+    def test_non_dict_intermediate_replaced(self):
+        data = {"affiliate": "not-a-dict"}
+        result = set_path(data, "affiliate.opt_out", True)
+        assert result == {"affiliate": {"opt_out": True}}
+
+    def test_roundtrip_get_after_set(self):
+        prefs = {}
+        prefs = set_path(prefs, "affiliate.opt_out", True)
+        prefs = set_path(prefs, "affiliate.byok_ids.ebay", "tag-123")
+        assert get_path(prefs, "affiliate.opt_out") is True
+        assert get_path(prefs, "affiliate.byok_ids.ebay") == "tag-123"