diff --git a/IMPLEMENTATION_PLAN.md b/IMPLEMENTATION_PLAN.md
index 2ed1741..0cb88f4 100644
--- a/IMPLEMENTATION_PLAN.md
+++ b/IMPLEMENTATION_PLAN.md
@@ -163,12 +163,12 @@ Drawer selection → update_drug_selection → app-state store → load_pathway_
 ## Phase 3: Core Callbacks
 
 ### 3.1 Reference data loading + filter state management
-- [ ] Create `dash_app/callbacks/filters.py`:
+- [x] Create `dash_app/callbacks/filters.py`:
   - `load_reference_data` callback: fires on page load, calls `queries.load_initial_data()`, populates `reference-data` store + header indicators
   - `update_app_state` callback: fires when chart-type toggle or date dropdowns change, computes `date_filter_id` (e.g., `"all_6mo"`), updates `app-state` store
   - Chart type toggle: use `callback_context` to determine which button was clicked, set active class via `className`
-- [ ] Create `dash_app/callbacks/__init__.py` with `register_callbacks(app)` that imports and registers all callback modules
-- [ ] Wire `register_callbacks(app)` in `app.py`
+- [x] Create `dash_app/callbacks/__init__.py` with `register_callbacks(app)` that imports and registers all callback modules
+- [x] Wire `register_callbacks(app)` in `app.py`
 - **Checkpoint**: Page loads reference data, filter dropdowns update app-state store (verify via browser dev tools → dcc.Store)
 
 ### 3.2 Pathway data loading callback
diff --git a/dash_app/app.py b/dash_app/app.py
index 9eaaca3..9f32a3a 100644
--- a/dash_app/app.py
+++ b/dash_app/app.py
@@ -27,6 +27,7 @@ app.layout = dmc.MantineProvider(
         }),
         dcc.Store(id="chart-data", storage_type="memory"),
         dcc.Store(id="reference-data", storage_type="session"),
+        dcc.Location(id="url", refresh=False),
 
         # Page structure
         make_header(),
@@ -43,4 +44,8 @@ app.layout = dmc.MantineProvider(
     ],
 )
 
+from dash_app.callbacks import register_callbacks
+
+register_callbacks(app)
+
 server = app.server
diff --git a/dash_app/callbacks/__init__.py b/dash_app/callbacks/__init__.py
index e69de29..e3255c5 100644
--- a/dash_app/callbacks/__init__.py
+++ b/dash_app/callbacks/__init__.py
@@ -0,0 +1,8 @@
+"""Callback registration — imports all callback modules and wires them to the app."""
+
+
+def register_callbacks(app):
+    """Register all Dash callbacks with the app instance."""
+    from dash_app.callbacks.filters import register_filter_callbacks
+
+    register_filter_callbacks(app)
diff --git a/dash_app/callbacks/filters.py b/dash_app/callbacks/filters.py
new file mode 100644
index 0000000..0f89199
--- /dev/null
+++ b/dash_app/callbacks/filters.py
@@ -0,0 +1,77 @@
+"""Callbacks for reference data loading and filter state management."""
+from dash import Input, Output, State, callback, ctx, no_update
+
+
+def register_filter_callbacks(app):
+    """Register reference data loading and filter state callbacks."""
+
+    @app.callback(
+        Output("reference-data", "data"),
+        Output("header-record-count", "children"),
+        Output("header-last-updated", "children"),
+        Input("url", "pathname"),  # fires once on page load
+    )
+    def load_reference_data(_pathname):
+        """Load reference data once on page load."""
+        from dash_app.data.queries import load_initial_data
+
+        ref = load_initial_data()
+        total = ref.get("total_records", 0)
+        record_text = f"{total:,} records" if total else "Data loaded"
+        last_updated = ref.get("last_updated", "")
+        updated_text = last_updated[:10] if last_updated else "Unknown"
+
+        return ref, record_text, updated_text
+
+    @app.callback(
+        Output("app-state", "data"),
+        Output("chart-type-directory", "className"),
+        Output("chart-type-indication", "className"),
+        Input("chart-type-directory", "n_clicks"),
+        Input("chart-type-indication", "n_clicks"),
+        Input("filter-initiated", "value"),
+        Input("filter-last-seen", "value"),
+        State("app-state", "data"),
+    )
+    def update_app_state(
+        _dir_clicks, _ind_clicks, initiated, last_seen, current_state
+    ):
+        """Update app-state when chart type toggle or date filters change."""
+        if not current_state:
+            current_state = {
+                "chart_type": "directory",
+                "initiated": "all",
+                "last_seen": "6mo",
+                "date_filter_id": "all_6mo",
+                "selected_drugs": [],
+                "selected_directorates": [],
+            }
+
+        triggered_id = ctx.triggered_id
+
+        # Determine chart type from toggle pills
+        chart_type = current_state.get("chart_type", "directory")
+        if triggered_id == "chart-type-directory":
+            chart_type = "directory"
+        elif triggered_id == "chart-type-indication":
+            chart_type = "indication"
+
+        # Compute date_filter_id from dropdown values
+        date_filter_id = f"{initiated}_{last_seen}"
+
+        # Build updated state
+        updated_state = {
+            **current_state,
+            "chart_type": chart_type,
+            "initiated": initiated,
+            "last_seen": last_seen,
+            "date_filter_id": date_filter_id,
+        }
+
+        # Toggle pill CSS classes
+        base = "toggle-pill"
+        active = f"{base} toggle-pill--active"
+        dir_class = active if chart_type == "directory" else base
+        ind_class = active if chart_type == "indication" else base
+
+        return updated_state, dir_class, ind_class