diff --git a/CHANGELOG.md b/CHANGELOG.md
index cc8a9982..cd7660dd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -16,6 +16,7 @@ Chart-experience release: chart authoring, editing, rule-based styling and click
 
 ### Fixed
 
+- A parameter widget's **Default value** was never applied, so every dashboard relying on defaults rendered empty on arrival. `extractParamDefaults` walked the layout correctly, had its own unit test, and had **zero production callers** — the editor's field wrote a value into the saved layout that was read only by a test. The seeded Chart Playground carries 21 configured defaults across 8 pages and showed `Waiting for parameters… $param_cat_dimension $param_cat_metric …` on every chart until the user set each knob by hand. Defaults are now seeded on load, filling only parameters that are not already set — which gives the precedence `URL > restored session > default` without any ordering machinery, since both of those are applied before the layout finishes loading. A once-per-dashboard guard stops a cleared parameter snapping straight back. This was the third instance of the same shape after #1234 and #1388, so a guard now asserts this helper has a production caller; the general form, a dead-export check across `lib/`, is #1477 (#1421)
 - Every dashboard page you had ever visited kept auto-refreshing, forever, so query load scaled with browsing history rather than with what was on screen. Measured on Chart Reference sitting on the same page with the same 18 visible widgets: 16 `/api/query` POSTs in 40s having opened only page 1, against 77 after touring six pages and returning — **4.81x the database traffic for an identical view**, ~68 hidden widgets polling for pages nobody was looking at. Tour all 20 pages and leave the tab open and roughly 230 widgets keep polling. All of it is refresh-tier work against the customer's database, which is exactly the load the query scheduler exists to shed, so one user exploring a large dashboard could crowd out interactive queries for everyone on that connector. `isActive` was already computed and already used to hide the page and to gate `onLayoutChange` — it just never gated the refresh. Visited pages still stay mounted, so tab switching remains instant and does not re-query; they simply stop polling while hidden (#1419)
 - Two options were advertised in the widget editor, implemented in the component library, and silently discarded in between: `trendEnabled` (single-value) and `thresholdZones` (gauge). Setting either did nothing, with no error, and the seeded reference dashboard shipped tiles demonstrating both — the `thresholdZones (3 bands)` tile was pixel-identical to `default`. The cause was not the Zod schema, which uses `.passthrough()` and preserved the keys intact; it was the plugin's explicit prop mapping, which never passed them to the chart. The worse half was the KPI: `transformToValueData` took the first column **positionally**, so following the editor's own "requires 2 rows" instruction on a `label, value` query rendered the date as the headline metric — `$2026-03`. It now picks the first numeric column and carries the previous row so a trend can be computed. A ratchet asserts that every option offered for a chart type is actually read by its plugin; it found the same bug in five more plugins, allowlisted and tracked in #1472 (#1397)
 - A long-format result — `category, series, value`, what `GROUP BY a, b` naturally produces — rendered a bar or line chart that looked plausible and was wrong. `resolveValueKeys` treats every non-label column as a value series, so the `series` column became a series of its own whose cells all coerced to null: 12 bars with duplicated x labels, a ghost legend swatch with no bars, and **no stacking at all** despite `stackMode` being set. Line was worse — a single revenue line drawn across duplicated x values, interleaving delivered -> shipped -> delivered, which reads as a violently spiky time series but is pure artifact. Nothing marked either as an error, and the seeded reference dashboard itself demonstrated two `stackMode` options with this exact query shape, which is how easy it is to fall into. Bar and line now reject a result whose plotted value columns contain no numeric values, naming the offending column and showing the pivot that fixes it. A column that is entirely null stays legal — that is a sparse series, what a LEFT JOIN produces. `validate` now receives the same column mapping as the transform, so a result whose text columns the user already mapped away is not wrongly rejected. The 11 affected reference tiles are rewritten to wide format, so the stacking options finally demonstrate stacking (#1400)
diff --git a/app/e2e/param-defaults.spec.ts b/app/e2e/param-defaults.spec.ts
new file mode 100644
index 00000000..c86fd1b6
--- /dev/null
+++ b/app/e2e/param-defaults.spec.ts
@@ -0,0 +1,90 @@
+import { test, expect, ALICE, createTestDashboard } from "./fixtures";
+
+/**
+ * #1421 — a parameter widget's **Default value** was never applied, because
+ * `extractParamDefaults` had zero production callers. Every dashboard relying
+ * on defaults rendered empty on arrival: the seeded Chart Playground (8 pages,
+ * 21 configured defaults) showed "Waiting for parameters…" on every chart until
+ * the user set each knob by hand.
+ *
+ * Built as a fixture rather than driving the seeded Playground, which is a demo
+ * showcase and is not present in the E2E database.
+ */
+test.describe("Parameter defaults are applied on load (#1421)", () => {
+  test.beforeEach(async ({ authPage }) => {
+    await authPage.login(ALICE.email, ALICE.password);
+  });
+
+  test("a configured default renders the chart instead of 'Waiting for parameters'", async ({
+    page,
+  }) => {
+    test.setTimeout(90_000);
+
+    const { id, cleanup } = await createTestDashboard(
+      page.request,
+      `Param defaults ${Date.now()}`,
+    );
+
+    try {
+      await page.request.put(`/api/dashboards/${id}`, {
+        data: {
+          layoutJson: {
+            version: 2,
+            pages: [
+              {
+                id: "p1",
+                title: "Page 1",
+                widgets: [
+                  {
+                    id: "sel",
+                    chartType: "parameter-select",
+                    connectionId: "conn-neo4j-001",
+                    query: "",
+                    settings: {
+                      title: "Title filter",
+                      chartOptions: {
+                        parameterName: "title_filter",
+                        parameterType: "text",
+                        defaultValue: "The",
+                      },
+                    },
+                  },
+                  {
+                    // Consumes the parameter. Neo4j binds `$param_` natively,
+                    // so this is a real bound parameter, not string splicing.
+                    id: "tbl",
+                    chartType: "table",
+                    connectionId: "conn-neo4j-001",
+                    query:
+                      "MATCH (m:Movie) WHERE m.title CONTAINS $param_title_filter RETURN m.title AS title LIMIT 3",
+                    settings: { title: "Filtered movies" },
+                  },
+                ],
+                gridLayout: [
+                  { i: "sel", x: 0, y: 0, w: 4, h: 3 },
+                  { i: "tbl", x: 4, y: 0, w: 8, h: 4 },
+                ],
+              },
+            ],
+          },
+        },
+      });
+
+      await page.goto(`/${id}`);
+
+      // The symptom: the consuming widget stalls, naming the token it lacks.
+      await expect(page.getByText(/Waiting for parameters/)).toHaveCount(0, {
+        timeout: 20_000,
+      });
+      await expect(page.getByText("$param_title_filter")).toHaveCount(0);
+
+      // And the chart actually renders, which only happens once the parameter
+      // resolves and the query runs.
+      await expect(page.locator("table").first()).toBeVisible({
+        timeout: 20_000,
+      });
+    } finally {
+      await cleanup();
+    }
+  });
+});
diff --git a/app/src/components/__tests__/dashboard-workspace.test.tsx b/app/src/components/__tests__/dashboard-workspace.test.tsx
index 115c9e85..96702b0a 100644
--- a/app/src/components/__tests__/dashboard-workspace.test.tsx
+++ b/app/src/components/__tests__/dashboard-workspace.test.tsx
@@ -1124,6 +1124,128 @@ describe("DashboardWorkspace", () => {
     expect(useDashboardStore.getState().layout.pages).toHaveLength(3);
   });
 
+  // ── Parameter defaults (#1421) ──────────────────────────────────────
+  // `extractParamDefaults` walked the layout correctly and had its own unit
+  // test, but zero production callers — so the editor's "Default value" field
+  // wrote into the saved layout and was read only by a test. The seeded Chart
+  // Playground carries 21 defaults and rendered "Waiting for parameters…" on
+  // every chart until the user configured each knob by hand.
+  describe("parameter defaults are applied on load (#1421)", () => {
+    function withDefaults() {
+      const d = makeDashboard(1);
+      d.layoutJson.pages[0].widgets.push({
+        id: "pw1",
+        chartType: "parameter-select",
+        connectionId: "c1",
+        query: "",
+        settings: {
+          chartOptions: {
+            parameterName: "dimension",
+            parameterType: "select",
+            defaultValue: "category",
+          },
+        },
+      } as unknown as (typeof d.layoutJson.pages)[0]["widgets"][0]);
+      return d;
+    }
+
+    function renderWithDefaults() {
+      mockUseDashboard.mockReturnValue({
+        data: withDefaults(),
+        isLoading: false,
+        isFetching: false,
+      });
+      return render(<DashboardWorkspace id="d1" editMode={false} />);
+    }
+
+    it("seeds the store from a widget's configured default", () => {
+      renderWithDefaults();
+      const entry = useParameterStore.getState().parameters.dimension;
+      expect(entry?.value).toBe("category");
+      // Provenance matters: without this, seeding could label the value as a
+      // user selection and the assertion above would not notice.
+      expect(entry?.sourceType).toBe("default");
+    });
+
+    it("a URL parameter beats the default", () => {
+      searchParams = new URLSearchParams("param_dimension=revenue");
+      renderWithDefaults();
+      expect(useParameterStore.getState().parameters.dimension?.value).toBe(
+        "revenue",
+      );
+    });
+
+    it("a restored session value beats the default", () => {
+      // Must go through localStorage, not the store: `restoreFromDashboard`
+      // runs on mount and *replaces* the store wholesale, so anything set
+      // beforehand is wiped before the defaults effect ever runs.
+      // Shape matters: `restoreFromDashboard` drops any entry missing a string
+      // `source`, `field` or `type`, so a near-miss fixture would restore
+      // nothing and this test would "pass" against a broken implementation.
+      localStorage.setItem(
+        "nb-params:d1",
+        JSON.stringify({
+          dimension: {
+            value: "region",
+            source: "Dimension",
+            field: "dimension",
+            type: "text",
+            sourceType: "selector-widget",
+            sourceWidgetId: "pw1",
+          },
+        }),
+      );
+      renderWithDefaults();
+      expect(useParameterStore.getState().parameters.dimension?.value).toBe(
+        "region",
+      );
+    });
+
+    // The dangerous regression: re-seeding on every render would make a
+    // parameter impossible to clear — it would snap back instantly.
+    it("does not re-apply the default after the user clears it", () => {
+      const { rerender } = renderWithDefaults();
+      expect(useParameterStore.getState().parameters.dimension?.value).toBe(
+        "category",
+      );
+
+      useParameterStore.getState().clearParameter("dimension");
+
+      // A plain rerender is not enough: it reuses the same dashboard object, so
+      // `serverLayout` keeps its identity, the effect's deps never change, and
+      // the effect does not re-run — the test would pass with the ref guard
+      // deleted. Handing back a fresh object forces the effect to fire again,
+      // which is the only thing that actually exercises the guard.
+      mockUseDashboard.mockReturnValue({
+        data: withDefaults(),
+        isLoading: false,
+        isFetching: false,
+      });
+      rerender(<DashboardWorkspace id="d1" editMode={false} />);
+
+      expect(useParameterStore.getState().parameters.dimension).toBeUndefined();
+    });
+
+    it("leaves a parameter-select with no default alone", () => {
+      const d = makeDashboard(1);
+      d.layoutJson.pages[0].widgets.push({
+        id: "pw2",
+        chartType: "parameter-select",
+        connectionId: "c1",
+        query: "",
+        settings: { chartOptions: { parameterName: "unset" } },
+      } as unknown as (typeof d.layoutJson.pages)[0]["widgets"][0]);
+      mockUseDashboard.mockReturnValue({
+        data: d,
+        isLoading: false,
+        isFetching: false,
+      });
+
+      render(<DashboardWorkspace id="d1" editMode={false} />);
+      expect(useParameterStore.getState().parameters.unset).toBeUndefined();
+    });
+  });
+
   // ── Parameters ↔ URL ────────────────────────────────────────────────
   it("applies param_ values from the URL on mount", () => {
     searchParams = new URLSearchParams("param_year=1999");
diff --git a/app/src/components/dashboard-workspace.tsx b/app/src/components/dashboard-workspace.tsx
index d35cde6d..7fb5c0b3 100644
--- a/app/src/components/dashboard-workspace.tsx
+++ b/app/src/components/dashboard-workspace.tsx
@@ -28,6 +28,7 @@ import {
   buildParamsUrl,
   extractSyncParams,
 } from "@/lib/shared/url-params";
+import { extractParamDefaults } from "@/lib/parameter/apply-param-defaults";
 import { migrateLayout } from "@/lib/dashboard/migrate-layout";
 import { getRefetchInterval } from "@/lib/dashboard/dashboard-settings";
 import { classifySaveError } from "@/lib/dashboard/save-error";
@@ -186,6 +187,33 @@ export function DashboardWorkspace({
     [serverLayout],
   );
 
+  // Widget "Default value" settings, applied once per dashboard (#1421).
+  //
+  // `extractParamDefaults` was written, tested, and never called — so the
+  // editor's Default value field wrote into the saved layout and was read only
+  // by its own unit test. The seeded Chart Playground carries 21 defaults and
+  // showed "Waiting for parameters…" on every chart until each knob was set by
+  // hand.
+  //
+  // Only fills parameters that are not already set, which is what gives the
+  // precedence `URL > restored session > default` without any ordering
+  // machinery: the restore and URL effects above both run before the layout has
+  // loaded, so whatever they put in the store is already there by the time this
+  // can run. The ref guard is what stops a cleared parameter snapping back —
+  // without it, clearing a knob would be impossible.
+  const defaultsAppliedFor = useRef<string | null>(null);
+  useEffect(() => {
+    if (!serverLayout || defaultsAppliedFor.current === id) return;
+    defaultsAppliedFor.current = id;
+
+    const defaults = extractParamDefaults(serverLayout);
+    const store = useParameterStore.getState();
+    for (const [name, value] of Object.entries(defaults)) {
+      if (store.parameters[name] !== undefined) continue;
+      store.setParameter(name, value, value, "", "text", "default", "");
+    }
+  }, [id, serverLayout]);
+
   // Seeded from the URL we arrived on, so the first sync is a no-op unless it
   // actually has something to strip.
   const lastSyncedUrlRef = useRef<string | null>(null);
diff --git a/app/src/lib/__tests__/parameter/apply-param-defaults.test.ts b/app/src/lib/__tests__/parameter/apply-param-defaults.test.ts
index 61d2f786..eb5e6df0 100644
--- a/app/src/lib/__tests__/parameter/apply-param-defaults.test.ts
+++ b/app/src/lib/__tests__/parameter/apply-param-defaults.test.ts
@@ -1,4 +1,7 @@
 import { describe, it, expect } from "vitest";
+import { execFileSync } from "node:child_process";
+import { readFileSync } from "node:fs";
+import { join } from "node:path";
 import { extractParamDefaults } from "@/lib/parameter/apply-param-defaults";
 import type { DashboardLayoutV2 } from "@/lib/db/schema";
 
@@ -104,3 +107,72 @@ describe("extractParamDefaults", () => {
     });
   });
 });
+
+// ---------------------------------------------------------------------------
+// #1421 — the helper below was written, unit-tested, and never called.
+// ---------------------------------------------------------------------------
+
+describe("extractParamDefaults has a production caller (#1421)", () => {
+  /**
+   * This function was correct and fully covered while doing nothing, because
+   * nothing invoked it: the editor's "Default value" field wrote into the saved
+   * layout and was read only by this test file. The seeded Chart Playground
+   * carries 21 defaults and showed "Waiting for parameters…" on every chart.
+   *
+   * A unit test proving a function works is not evidence that anything calls
+   * it. This is the third instance of that shape — #1388 (`extractNoSyncParams`)
+   * and #1234 (the audit trail) were the first two.
+   *
+   * Deliberately narrow. The general form — "any `lib/` export reachable only
+   * from `__tests__` fails the build" — was measured at ~69 current matches,
+   * the great majority legitimate (`_reset*` test hooks, Zod fragments composed
+   * in-file, Drizzle enums). Shipping that would mean shipping an allowlist
+   * bigger than the signal, so it is filed separately as a tooling change.
+   */
+  it("is imported and called by non-test source", () => {
+    const appSrc = join(__dirname, "../../..");
+    let matched: string[];
+    try {
+      matched = execFileSync(
+        "grep",
+        [
+          "-rl",
+          "extractParamDefaults",
+          appSrc,
+          "--include=*.ts",
+          "--include=*.tsx",
+        ],
+        { encoding: "utf8" },
+      )
+        .trim()
+        .split("\n")
+        .filter(Boolean);
+    } catch {
+      // grep exits 1 on no matches, which would otherwise throw before the
+      // assertion and hide the message explaining what broke.
+      matched = [];
+    }
+
+    // A raw text match is not enough: this file's own explanatory comment names
+    // the helper, so a comment alone would satisfy it even with the import and
+    // the call deleted. Strip comments, then require both.
+    const callers = matched
+      .filter((f) => !f.includes("apply-param-defaults.ts"))
+      .filter((f) => !f.includes("__tests__") && !f.includes(".test."))
+      .filter((f) => {
+        const code = readFileSync(f, "utf8")
+          .replace(/\/\*[\s\S]*?\*\//g, "")
+          .replace(/^\s*\/\/.*$/gm, "");
+        const imported = /import\s*\{[^}]*\bextractParamDefaults\b[^}]*\}/.test(
+          code,
+        );
+        const called = /\bextractParamDefaults\s*\(/.test(code);
+        return imported && called;
+      });
+
+    expect(
+      callers,
+      "extractParamDefaults has no production caller — the Default value field would silently do nothing",
+    ).not.toEqual([]);
+  });
+});
diff --git a/app/src/stores/parameter-store.ts b/app/src/stores/parameter-store.ts
index bf41b072..33c33e55 100644
--- a/app/src/stores/parameter-store.ts
+++ b/app/src/stores/parameter-store.ts
@@ -29,7 +29,10 @@ export type ParameterSource =
   | "click-action"
   | "selector-widget"
   | "url"
-  | "cross-dashboard";
+  | "cross-dashboard"
+  // Seeded from a parameter widget's configured Default value on load (#1421).
+  // Distinct from "selector-widget": the user never picked this.
+  | "default";
 
 export interface ParameterEntry {
   value: unknown;