diff --git a/packages/views/onboarding/components/icon-option-card.tsx b/packages/views/onboarding/components/icon-option-card.tsx
index 4f3c6bf54..be8c6da6a 100644
--- a/packages/views/onboarding/components/icon-option-card.tsx
+++ b/packages/views/onboarding/components/icon-option-card.tsx
@@ -74,7 +74,6 @@ export function IconOtherOptionCard({
   onConfirm,
   placeholder,
   mode = "radio",
-  allowToggleOff = false,
 }: {
   icon: ReactNode;
   label: string;
@@ -85,34 +84,13 @@ export function IconOtherOptionCard({
   onConfirm: () => void;
   placeholder: string;
   mode?: "radio" | "checkbox";
-  /**
-   * When true, clicking the card while it is already selected fires
-   * `onSelect` again so the parent can toggle the pick off (multi-select
-   * pattern, used by the source-backfill modal). Default `false`
-   * preserves the original onboarding StepSource behaviour: a re-click
-   * on the selected Other card is a no-op so users can move focus
-   * into the text input without deselecting. Clicks that originate
-   * inside the input never toggle — the input stops their propagation.
-   */
-  allowToggleOff?: boolean;
 }) {
   return (
     <div
       role={mode}
       aria-checked={selected}
-      onClick={(e) => {
-        // In `allowToggleOff` mode, clicking the row again deselects.
-        // Skip when the click originated on the inner text input so
-        // typing / focusing it doesn't accidentally deselect — clicks
-        // on the icon, padding, or label area still toggle.
-        if (
-          allowToggleOff &&
-          selected &&
-          e.target instanceof HTMLInputElement
-        ) {
-          return;
-        }
-        if (allowToggleOff || !selected) onSelect();
+      onClick={() => {
+        if (!selected) onSelect();
       }}
       className={cn(
         "flex w-full items-center gap-3 rounded-xl border bg-card px-4 py-3 text-left transition-all",
diff --git a/packages/views/onboarding/source-backfill-modal.test.tsx b/packages/views/onboarding/source-backfill-modal.test.tsx
index c77e0037f..8ef72e0a2 100644
--- a/packages/views/onboarding/source-backfill-modal.test.tsx
+++ b/packages/views/onboarding/source-backfill-modal.test.tsx
@@ -216,14 +216,10 @@ describe("SourceBackfillModal", () => {
     expect(await screen.findByText("GitHub")).toBeInTheDocument();
   });
 
-  it("Other toggles off on the second click instead of getting stuck", async () => {
-    // Regression: an earlier version held a parallel `pendingOther`
-    // flag that flipped to true on every Other click. A second click
-    // then removed "other" from `source` but the row's onClick guard
-    // (`if (!selected) onSelect()`) ALSO swallowed re-clicks while
-    // selected, so the toggle-off never fired. Fixed by dropping
-    // pendingOther AND opting the modal's Other card into the new
-    // `allowToggleOff` prop on `IconOtherOptionCard`.
+  it("picking a second option replaces the first (single-select primary source)", async () => {
+    // The modal is now a single-select radio. Industry default for
+    // HDYHAU is to capture the primary acquisition source, so picking
+    // a second option must replace the first — never accumulate.
     setUser({
       id: "u1",
       onboarded_at: "2026-01-01T00:00:00Z",
@@ -231,37 +227,30 @@ describe("SourceBackfillModal", () => {
     });
     const user = userEvent.setup();
     renderModal();
-    // Wait for the modal to render. Pick the Other card by its
-    // checkbox role — `role="checkbox"` + matching `aria-checked`
-    // stays stable whether the label is showing or the input has
-    // replaced it.
     await screen.findByText("Friends or colleagues");
-    const checkboxes = screen.getAllByRole("checkbox");
-    // 12 options: 11 normal + 1 Other. Other is the last per the
-    // component's option array.
-    const other = checkboxes[checkboxes.length - 1]!;
-    const friends = checkboxes[0]!;
+    const radios = screen.getAllByRole("radio");
+    const friends = radios[0]!;
+    const search = radios[1]!;
 
-    // Tick Other → Submit disabled (no free-text yet)
-    await user.click(other);
-    expect(other).toHaveAttribute("aria-checked", "true");
-    expect(screen.getByRole("button", { name: "Submit" })).toBeDisabled();
-    // Add a real selection so we can test the un-tick path without
-    // the validator hiding the bug behind the empty-other-text branch.
     await user.click(friends);
-    expect(screen.getByRole("button", { name: "Submit" })).toBeEnabled();
-    // Un-tick Other → still enabled because friends is still picked.
-    await user.click(other);
-    expect(other).toHaveAttribute("aria-checked", "false");
-    expect(screen.getByRole("button", { name: "Submit" })).toBeEnabled();
-    // Critically: submit and assert "other" is NOT in the payload.
+    expect(friends).toHaveAttribute("aria-checked", "true");
+    expect(search).toHaveAttribute("aria-checked", "false");
+
+    // Pick a second option — the first must clear and Submit stays
+    // enabled with exactly one pick in the payload.
+    await user.click(search);
+    expect(friends).toHaveAttribute("aria-checked", "false");
+    expect(search).toHaveAttribute("aria-checked", "true");
+
     await user.click(screen.getByRole("button", { name: "Submit" }));
     await waitFor(() => {
       expect(mockSaveQuestionnaire).toHaveBeenCalledTimes(1);
     });
     const sent = mockSaveQuestionnaire.mock.calls[0]![0];
-    expect(sent.source).toEqual(["friends_colleagues"]);
-    expect(sent.source).not.toContain("other");
+    // Server schema is still `source: string[]` for back-compat with
+    // v2 rows; the client always sends a single-element array.
+    expect(sent.source).toEqual(["search"]);
+    expect(sent.source).not.toContain("friends_colleagues");
   });
 
   it("defers the entrance by ~700ms when the user has not opted into reduced motion", async () => {
diff --git a/packages/views/onboarding/source-backfill-modal.tsx b/packages/views/onboarding/source-backfill-modal.tsx
index 492dc2327..39532aecc 100644
--- a/packages/views/onboarding/source-backfill-modal.tsx
+++ b/packages/views/onboarding/source-backfill-modal.tsx
@@ -196,56 +196,45 @@ function SourceBackfillDialogBody({
     [t],
   );
 
-  const selected = useMemo<readonly string[]>(
-    () => [
-      ...answers.source,
-      ...(!answers.source.includes("other") && answers.source_other
-        ? ["other"]
-        : []),
-    ],
-    [answers.source, answers.source_other],
-  );
-
+  // Single-select: at most one slug in `source` at any time. The server
+  // schema keeps the array (back-compat with the v2 multi-select shape
+  // and with existing rows), but the modal UI commits exactly one pick
+  // — primary-source attribution is the documented industry default for
+  // HDYHAU prompts (Fairing, Recast, HockeyStack) and gives the team
+  // clean channel weights without splitting users across N buckets.
+  const pickedSlug: string | null = answers.source[0] ?? null;
   const otherOption = options.find((o) => o.isOther) ?? null;
-  // Single source of truth for "is Other ticked": derive from `source`
-  // directly, NOT a parallel useState flag. The previous version kept a
-  // `pendingOther` state set to true on every Other click — which meant
-  // a second click toggled "other" off in `source` but left
-  // `pendingOther` stuck at true, so `otherActive` (its OR with
-  // selected) never flipped back and the card visually stayed selected
-  // (caught in UAT).
-  const otherSelected = otherOption
-    ? selected.includes(otherOption.slug)
-    : false;
+  const otherSelected = pickedSlug === otherOption?.slug;
   const otherFilled = (answers.source_other ?? "").trim().length > 0;
-  const hasNonOtherSelection = selected.some(
-    (slug) => slug !== otherOption?.slug,
-  );
   const canSubmit =
     !busy &&
-    selected.length > 0 &&
-    (hasNonOtherSelection || !otherSelected || otherFilled);
+    pickedSlug !== null &&
+    // If the user picked Other, gate Submit on having typed something —
+    // an empty Other selection isn't useful attribution data.
+    (!otherSelected || otherFilled);
 
   const handleSelect = useCallback(
     (option: QuestionOption) => {
       if (option.isOther) {
-        setAnswers((a) => {
-          const has = a.source.includes("other");
-          return has
-            ? { ...a, source: a.source.filter((s) => s !== "other"), source_other: null }
-            : { ...a, source: [...a.source, "other"], source_skipped: false };
-        });
+        const slug = option.slug as Source;
+        setAnswers((a) => ({
+          ...a,
+          source: [slug],
+          // Picking Other doesn't carry text from a prior Other pick
+          // forward: the text input auto-focuses fresh so the user can
+          // type immediately. A previous text value would be misleading.
+          source_other: a.source[0] === "other" ? a.source_other : null,
+          source_skipped: false,
+        }));
         return;
       }
       const slug = option.slug as Source;
-      setAnswers((a) => {
-        const has = a.source.includes(slug);
-        return {
-          ...a,
-          source: has ? a.source.filter((s) => s !== slug) : [...a.source, slug],
-          source_skipped: false,
-        };
-      });
+      setAnswers((a) => ({
+        ...a,
+        source: [slug],
+        source_other: null,
+        source_skipped: false,
+      }));
     },
     [],
   );
@@ -318,7 +307,7 @@ function SourceBackfillDialogBody({
       </div>
 
       <fieldset
-        role="group"
+        role="radiogroup"
         aria-label={t(($) => $.questions.source.question)}
         className="m-0 grid grid-cols-1 gap-2 p-0 px-6 pt-4 sm:grid-cols-2"
       >
@@ -334,17 +323,16 @@ function SourceBackfillDialogBody({
               onOtherChange={handleOtherChange}
               onConfirm={submit}
               placeholder={t(($) => $.questions.source.other_placeholder)}
-              mode="checkbox"
-              allowToggleOff
+              mode="radio"
             />
           ) : (
             <IconOptionCard
               key={option.slug}
               icon={option.icon}
               label={option.label}
-              selected={selected.includes(option.slug)}
+              selected={pickedSlug === option.slug}
               onSelect={() => handleSelect(option)}
-              mode="checkbox"
+              mode="radio"
             />
           ),
         )}
diff --git a/packages/views/onboarding/steps/step-source.test.tsx b/packages/views/onboarding/steps/step-source.test.tsx
index 37a39b0db..447834127 100644
--- a/packages/views/onboarding/steps/step-source.test.tsx
+++ b/packages/views/onboarding/steps/step-source.test.tsx
@@ -41,54 +41,41 @@ function renderStep(answers: QuestionnaireAnswers = EMPTY) {
   return { onChange, onAdvance, onSkip, onBack };
 }
 
-describe("StepSource (multi-select)", () => {
+describe("StepSource (single-select primary source)", () => {
   beforeEach(() => vi.restoreAllMocks());
 
-  it("clicking a non-Other option appends the slug to the array", async () => {
+  it("clicking a non-Other option writes a one-element source array", async () => {
     const user = userEvent.setup();
     const { onChange, onAdvance } = renderStep();
 
-    await user.click(screen.getByRole("checkbox", { name: /linkedin/i }));
+    await user.click(screen.getByRole("radio", { name: /linkedin/i }));
 
     expect(onChange).toHaveBeenCalledWith({
       source: ["social_linkedin"],
+      source_other: null,
       source_skipped: false,
     });
     // A click only records — it must NOT auto-advance.
     expect(onAdvance).not.toHaveBeenCalled();
   });
 
-  it("clicking an already-selected option removes it (toggle)", async () => {
+  it("picking a second option replaces the first (no stacking)", async () => {
     const user = userEvent.setup();
     const { onChange } = renderStep({
       ...EMPTY,
       source: ["social_linkedin"],
     });
 
-    await user.click(screen.getByRole("checkbox", { name: /linkedin/i }));
+    await user.click(screen.getByRole("radio", { name: /twitter/i }));
 
     expect(onChange).toHaveBeenCalledWith({
-      source: [],
+      source: ["social_x"],
+      source_other: null,
       source_skipped: false,
     });
   });
 
-  it("multi-select stacks several picks", async () => {
-    const user = userEvent.setup();
-    const { onChange } = renderStep({
-      ...EMPTY,
-      source: ["social_linkedin"],
-    });
-
-    await user.click(screen.getByRole("checkbox", { name: /twitter/i }));
-
-    expect(onChange).toHaveBeenCalledWith({
-      source: ["social_linkedin", "social_x"],
-      source_skipped: false,
-    });
-  });
-
-  it("Skip clears the array + other and marks the step skipped, then calls onSkip", async () => {
+  it("Skip clears source + source_other and marks the step skipped, then calls onSkip", async () => {
     const user = userEvent.setup();
     const { onChange, onSkip } = renderStep();
 
@@ -102,14 +89,15 @@ describe("StepSource (multi-select)", () => {
     expect(onSkip).toHaveBeenCalledTimes(1);
   });
 
-  it("Other: clicking adds 'other' to the array; free-text writes to source_other", async () => {
+  it("Other: clicking writes `source: ['other']` and lets the user type into source_other", async () => {
     const user = userEvent.setup();
     const { onChange } = renderStep();
 
-    await user.click(screen.getByRole("checkbox", { name: /^other$/i }));
+    await user.click(screen.getByRole("radio", { name: /^other$/i }));
 
     expect(onChange).toHaveBeenCalledWith({
       source: ["other"],
+      source_other: null,
       source_skipped: false,
     });
 
@@ -117,4 +105,21 @@ describe("StepSource (multi-select)", () => {
     await user.type(input, "x");
     expect(onChange).toHaveBeenLastCalledWith({ source_other: "x" });
   });
+
+  it("switching away from Other clears source_other so a stale value can't leak", async () => {
+    const user = userEvent.setup();
+    const { onChange } = renderStep({
+      ...EMPTY,
+      source: ["other"],
+      source_other: "a podcast",
+    });
+
+    await user.click(screen.getByRole("radio", { name: /linkedin/i }));
+
+    expect(onChange).toHaveBeenCalledWith({
+      source: ["social_linkedin"],
+      source_other: null,
+      source_skipped: false,
+    });
+  });
 });
diff --git a/packages/views/onboarding/steps/step-source.tsx b/packages/views/onboarding/steps/step-source.tsx
index a9f9e732b..fa9476fff 100644
--- a/packages/views/onboarding/steps/step-source.tsx
+++ b/packages/views/onboarding/steps/step-source.tsx
@@ -56,41 +56,25 @@ export function StepSource({
     { slug: "other", icon: <MoreHorizontal className="h-4 w-4" />, label: t(($) => $.questions.source.other), isOther: true },
   ];
 
-  // Multi-select: source already had several "yes I heard via X" intents
-  // collapsed into a single radio pick. Letting users tick multiple
-  // channels keeps attribution honest without making them rank them.
-  // `other` is a Source enum value so it stacks into the array alongside
-  // regular picks; the free-text input lives in `source_other`.
-  const selected: readonly string[] = [
-    ...(answers.source ?? []),
-    ...(!answers.source?.includes("other") && answers.source_other
-      ? ["other"]
-      : []),
-  ];
+  // Single-select on the primary acquisition source. The server schema
+  // keeps `source` as a string array for back-compat with v2 multi-
+  // select rows, but the UI only ever commits a one-element array —
+  // primary-source attribution is the documented industry default for
+  // HDYHAU prompts (Fairing, Recast, HockeyStack) and keeps channel
+  // weights clean for analytics.
+  const selected: readonly string[] = answers.source?.[0] ? [answers.source[0]] : [];
 
-  const toggle = (slug: string) => {
-    const current = answers.source ?? [];
-    if (slug === "other") {
-      // Toggling Other: add/remove from the array; clear the text when
-      // removing so a re-tick starts from an empty input.
-      if (current.includes("other")) {
-        onChange({
-          source: current.filter((s) => s !== "other"),
-          source_other: null,
-        });
-      } else {
-        onChange({
-          source: [...current, "other"],
-          source_skipped: false,
-        });
-      }
-      return;
-    }
+  const pick = (slug: string) => {
     const typed = slug as Source;
-    const next = current.includes(typed)
-      ? current.filter((s) => s !== typed)
-      : [...current, typed];
-    onChange({ source: next, source_skipped: false });
+    onChange({
+      source: [typed],
+      // Switching off Other clears the free-text input so a stale
+      // value can't leak into the next pick. Picking Other keeps the
+      // existing text so the user can finish typing without losing
+      // their input.
+      source_other: typed === "other" ? answers.source_other : null,
+      source_skipped: false,
+    });
   };
 
   return (
@@ -104,14 +88,13 @@ export function StepSource({
       otherValue={answers.source_other ?? ""}
       onOtherChange={(v) => onChange({ source_other: v })}
       otherPlaceholder={t(($) => $.questions.source.other_placeholder)}
-      onAnswer={toggle}
+      onAnswer={pick}
       onAdvance={onAdvance}
       onSkip={() => {
         onChange({ source: [], source_other: null, source_skipped: true });
         onSkip();
       }}
       onBack={onBack}
-      multiSelect
     />
   );
 }