diff --git a/frontend/src/pages/AddBucketForm.test.tsx b/frontend/src/pages/AddBucketForm.test.tsx
new file mode 100644
index 0000000..1abbb04
--- /dev/null
+++ b/frontend/src/pages/AddBucketForm.test.tsx
@@ -0,0 +1,306 @@
+import { render, screen, waitFor } from '@testing-library/react'
+import userEvent from '@testing-library/user-event'
+import { delay, http, HttpResponse } from 'msw'
+import { describe, expect, it } from 'vitest'
+import { server } from '../test/server'
+import AddBucketForm from './AddBucketForm'
+
+const SCENARIO_ID = '11111111-1111-1111-1111-111111111111'
+const SCENARIO_IRI = `/api/scenarios/${SCENARIO_ID}`
+
+/**
+ * AddBucketForm's contract (locked for #54 story 2): it takes the owning
+ * scenario's IRI, the scenario's EXISTING buckets (so it can compute the next
+ * priority itself — no priority field is shown to the user), and an
+ * `onCreated` callback fired after a successful 201 so the host page can
+ * refetch the scenario. No auth harness is needed here — the form talks to
+ * the API directly via `api.post`, which is exercised through MSW, and does
+ * not itself probe `/api/me` (that's AuthProvider's job at the page level).
+ */
+function renderForm({
+  existingBuckets = [],
+  onCreated = () => {},
+}: {
+  existingBuckets?: { priority: number }[]
+  onCreated?: () => void
+} = {}) {
+  render(
+    <AddBucketForm
+      scenario={SCENARIO_IRI}
+      existingBuckets={existingBuckets}
+      onCreated={onCreated}
+    />,
+  )
+}
+
+describe('AddBucketForm', () => {
+  it('keeps the form fields hidden until the "Add bucket" toggle is clicked', async () => {
+    const user = userEvent.setup()
+    renderForm()
+
+    expect(screen.queryByLabelText(/name/i)).not.toBeInTheDocument()
+
+    await user.click(screen.getByRole('button', { name: /add bucket/i }))
+
+    expect(screen.getByLabelText(/name/i)).toBeInTheDocument()
+  })
+
+  it('submits a fixed_limit bucket to /api/buckets as JSON-LD referencing the scenario', async () => {
+    let requestBody: unknown
+    let contentType: string | null = null
+    server.use(
+      http.post('http://localhost/api/buckets', async ({ request }) => {
+        contentType = request.headers.get('Content-Type')
+        requestBody = await request.json()
+        return HttpResponse.json(
+          { '@id': '/api/buckets/22222222-2222-2222-2222-222222222222' },
+          { status: 201 },
+        )
+      }),
+    )
+
+    const user = userEvent.setup()
+    renderForm()
+
+    await user.click(screen.getByRole('button', { name: /add bucket/i }))
+    await user.type(screen.getByLabelText(/name/i), 'Rent')
+    await user.type(screen.getByLabelText(/allocation/i), '100')
+    await user.click(
+      screen.getByRole('button', { name: /save|create|submit/i }),
+    )
+
+    await waitFor(() => expect(requestBody).not.toBeUndefined())
+    expect(contentType).toBe('application/ld+json')
+    expect(requestBody).toMatchObject({
+      scenario: SCENARIO_IRI,
+      name: 'Rent',
+      allocationValue: 10000,
+      type: 'need',
+      allocationType: 'fixed_limit',
+      priority: 1,
+    })
+  })
+
+  it('computes the next priority as one past the highest existing priority', async () => {
+    let requestBody: { priority?: number } | undefined
+    server.use(
+      http.post('http://localhost/api/buckets', async ({ request }) => {
+        requestBody = (await request.json()) as { priority?: number }
+        return HttpResponse.json(
+          { '@id': '/api/buckets/22222222-2222-2222-2222-222222222222' },
+          { status: 201 },
+        )
+      }),
+    )
+
+    const user = userEvent.setup()
+    renderForm({ existingBuckets: [{ priority: 1 }, { priority: 2 }] })
+
+    await user.click(screen.getByRole('button', { name: /add bucket/i }))
+    await user.type(screen.getByLabelText(/name/i), 'Dining Out')
+    await user.type(screen.getByLabelText(/allocation/i), '50')
+    await user.click(
+      screen.getByRole('button', { name: /save|create|submit/i }),
+    )
+
+    await waitFor(() => expect(requestBody).not.toBeUndefined())
+    expect(requestBody?.priority).toBe(3)
+  })
+
+  it('assigns priority 1 for the first bucket in an empty scenario', async () => {
+    let requestBody: { priority?: number } | undefined
+    server.use(
+      http.post('http://localhost/api/buckets', async ({ request }) => {
+        requestBody = (await request.json()) as { priority?: number }
+        return HttpResponse.json(
+          { '@id': '/api/buckets/22222222-2222-2222-2222-222222222222' },
+          { status: 201 },
+        )
+      }),
+    )
+
+    const user = userEvent.setup()
+    renderForm({ existingBuckets: [] })
+
+    await user.click(screen.getByRole('button', { name: /add bucket/i }))
+    await user.type(screen.getByLabelText(/name/i), 'Rent')
+    await user.type(screen.getByLabelText(/allocation/i), '100')
+    await user.click(
+      screen.getByRole('button', { name: /save|create|submit/i }),
+    )
+
+    await waitFor(() => expect(requestBody).not.toBeUndefined())
+    expect(requestBody?.priority).toBe(1)
+  })
+
+  it('disables the submit button while the request is in flight', async () => {
+    server.use(
+      http.post('http://localhost/api/buckets', async () => {
+        await delay(50)
+        return HttpResponse.json(
+          { '@id': '/api/buckets/22222222-2222-2222-2222-222222222222' },
+          { status: 201 },
+        )
+      }),
+    )
+
+    const user = userEvent.setup()
+    renderForm()
+
+    await user.click(screen.getByRole('button', { name: /add bucket/i }))
+    await user.type(screen.getByLabelText(/name/i), 'Rent')
+    await user.type(screen.getByLabelText(/allocation/i), '100')
+
+    const submitButton = screen.getByRole('button', {
+      name: /save|create|submit/i,
+    })
+    await user.click(submitButton)
+
+    expect(submitButton).toBeDisabled()
+  })
+
+  it('fires onCreated with the response after a successful submit', async () => {
+    server.use(
+      http.post('http://localhost/api/buckets', () =>
+        HttpResponse.json(
+          { '@id': '/api/buckets/22222222-2222-2222-2222-222222222222' },
+          { status: 201 },
+        ),
+      ),
+    )
+
+    let created = false
+    const user = userEvent.setup()
+    renderForm({ onCreated: () => (created = true) })
+
+    await user.click(screen.getByRole('button', { name: /add bucket/i }))
+    await user.type(screen.getByLabelText(/name/i), 'Rent')
+    await user.type(screen.getByLabelText(/allocation/i), '100')
+    await user.click(
+      screen.getByRole('button', { name: /save|create|submit/i }),
+    )
+
+    await waitFor(() => expect(created).toBe(true))
+  })
+
+  it('blocks submit with a field error when allocation value is left blank for a fixed_limit bucket', async () => {
+    let requestMade = false
+    server.use(
+      http.post('http://localhost/api/buckets', () => {
+        requestMade = true
+        return HttpResponse.json(
+          { '@id': '/api/buckets/22222222-2222-2222-2222-222222222222' },
+          { status: 201 },
+        )
+      }),
+    )
+
+    const user = userEvent.setup()
+    renderForm()
+
+    await user.click(screen.getByRole('button', { name: /add bucket/i }))
+    await user.type(screen.getByLabelText(/name/i), 'Rent')
+    await user.click(
+      screen.getByRole('button', { name: /save|create|submit/i }),
+    )
+
+    expect(await screen.findByRole('alert')).toBeInTheDocument()
+    expect(requestMade).toBe(false)
+  })
+
+  it('resets and collapses the form after a successful add', async () => {
+    server.use(
+      http.post('http://localhost/api/buckets', () =>
+        HttpResponse.json(
+          { '@id': '/api/buckets/22222222-2222-2222-2222-222222222222' },
+          { status: 201 },
+        ),
+      ),
+    )
+
+    const user = userEvent.setup()
+    renderForm()
+
+    await user.click(screen.getByRole('button', { name: /add bucket/i }))
+    await user.type(screen.getByLabelText(/name/i), 'Rent')
+    await user.type(screen.getByLabelText(/allocation/i), '100')
+    await user.click(
+      screen.getByRole('button', { name: /save|create|submit/i }),
+    )
+
+    expect(
+      await screen.findByRole('button', { name: /add bucket/i }),
+    ).toBeInTheDocument()
+    expect(screen.queryByLabelText(/name/i)).not.toBeInTheDocument()
+
+    await user.click(screen.getByRole('button', { name: /add bucket/i }))
+    expect(screen.getByLabelText(/name/i)).toHaveValue('')
+  })
+
+  it('falls back to the generic form error for a violation with no inline field', async () => {
+    server.use(
+      http.post('http://localhost/api/buckets', () =>
+        HttpResponse.json(
+          {
+            violations: [
+              {
+                propertyPath: 'priority',
+                message: 'This priority is already used.',
+              },
+            ],
+          },
+          { status: 422 },
+        ),
+      ),
+    )
+
+    const user = userEvent.setup()
+    renderForm()
+
+    await user.click(screen.getByRole('button', { name: /add bucket/i }))
+    await user.type(screen.getByLabelText(/name/i), 'Rent')
+    await user.type(screen.getByLabelText(/allocation/i), '100')
+    await user.click(
+      screen.getByRole('button', { name: /save|create|submit/i }),
+    )
+
+    expect(
+      await screen.findByText('Could not add the bucket. Please try again.'),
+    ).toBeInTheDocument()
+  })
+
+  it('renders a name violation inline without the generic form error', async () => {
+    server.use(
+      http.post('http://localhost/api/buckets', () =>
+        HttpResponse.json(
+          {
+            violations: [
+              {
+                propertyPath: 'name',
+                message: 'This value is already used.',
+              },
+            ],
+          },
+          { status: 422 },
+        ),
+      ),
+    )
+
+    const user = userEvent.setup()
+    renderForm()
+
+    await user.click(screen.getByRole('button', { name: /add bucket/i }))
+    await user.type(screen.getByLabelText(/name/i), 'Rent')
+    await user.type(screen.getByLabelText(/allocation/i), '100')
+    await user.click(
+      screen.getByRole('button', { name: /save|create|submit/i }),
+    )
+
+    expect(
+      await screen.findByText('This value is already used.'),
+    ).toBeInTheDocument()
+    expect(
+      screen.queryByText('Could not add the bucket. Please try again.'),
+    ).not.toBeInTheDocument()
+  })
+})
diff --git a/frontend/src/pages/AddBucketForm.tsx b/frontend/src/pages/AddBucketForm.tsx
new file mode 100644
index 0000000..a4b1b54
--- /dev/null
+++ b/frontend/src/pages/AddBucketForm.tsx
@@ -0,0 +1,210 @@
+import { useState, type FormEvent } from 'react'
+import Button from '../components/ui/Button'
+import FieldError from '../components/ui/FieldError'
+import Input from '../components/ui/Input'
+import { api } from '../lib/api'
+import { dollarsToCents } from '../lib/bucketInput'
+import { parseViolations, type FieldErrors } from '../lib/fieldErrors'
+import type { BucketAllocationType, BucketType } from '../types/api'
+
+interface AddBucketFormProps {
+  /** IRI of the owning scenario — sent as the POST body's `scenario`. */
+  scenario: string
+  /** Existing buckets, used only to compute the next (hidden) priority. */
+  existingBuckets: { priority: number }[]
+  /** Fired after a successful create so the host page can refetch. */
+  onCreated: () => void
+}
+
+/** Priority is unique per scenario and auto-assigned — one past the highest. */
+function nextPriority(existingBuckets: { priority: number }[]): number {
+  if (existingBuckets.length === 0) {
+    return 1
+  }
+  return Math.max(...existingBuckets.map((bucket) => bucket.priority)) + 1
+}
+
+/** Violation propertyPaths this form renders inline; anything else falls back
+ * to the generic form error so it can't fail silently. */
+const INLINE_ERROR_FIELDS = new Set(['name', 'allocationValue'])
+
+/**
+ * The add-bucket form, hosted on the scenario show page behind an "Add bucket"
+ * toggle. Priority is not shown — it's auto-assigned (unique per scenario). On
+ * submit it POSTs to /api/buckets as JSON-LD (via `api.post`) referencing the
+ * scenario IRI, then calls `onCreated` so the page refetches the scenario.
+ */
+export default function AddBucketForm({
+  scenario,
+  existingBuckets,
+  onCreated,
+}: AddBucketFormProps) {
+  const [open, setOpen] = useState(false)
+  const [name, setName] = useState('')
+  const [type, setType] = useState<BucketType>('need')
+  const [allocationType, setAllocationType] =
+    useState<BucketAllocationType>('fixed_limit')
+  const [allocationValue, setAllocationValue] = useState('')
+  const [bufferMultiplier, setBufferMultiplier] = useState('0')
+  const [fieldErrors, setFieldErrors] = useState<FieldErrors>({})
+  const [formError, setFormError] = useState<string | null>(null)
+  const [pending, setPending] = useState(false)
+
+  function resetFields() {
+    setName('')
+    setType('need')
+    setAllocationType('fixed_limit')
+    setAllocationValue('')
+    setBufferMultiplier('0')
+    setFieldErrors({})
+    setFormError(null)
+  }
+
+  async function handleSubmit(event: FormEvent<HTMLFormElement>) {
+    event.preventDefault()
+    setFieldErrors({})
+    setFormError(null)
+
+    // A fixed_limit bucket needs a numeric cap — blank would submit a bogus
+    // value, so reject it inline before hitting the API. `percentage` needs one
+    // too, but that guard rides with the per-allocationType field logic in
+    // TODO(#54 story 4) — `unlimited` legitimately carries no value.
+    if (allocationType === 'fixed_limit' && allocationValue.trim() === '') {
+      setFieldErrors({ allocationValue: ['Please enter an allocation value.'] })
+      return
+    }
+
+    setPending(true)
+    try {
+      await api.post('/buckets', {
+        scenario,
+        name,
+        type,
+        allocationType,
+        allocationValue: dollarsToCents(allocationValue),
+        bufferMultiplier,
+        priority: nextPriority(existingBuckets),
+      })
+      // Reset and collapse so the next add starts clean — priority is
+      // auto-assigned off the (soon-refetched) buckets, so nothing to carry.
+      resetFields()
+      setOpen(false)
+      onCreated()
+    } catch (err: unknown) {
+      const errors = parseViolations(err)
+      if (errors !== null) {
+        setFieldErrors(errors)
+        // Priority is auto-assigned and has no input, so a priority collision
+        // (UniquePriorityPerScenario, 422) would render nowhere — surface any
+        // violation the form can't show inline via the generic form error
+        // rather than failing silently.
+        if (
+          Object.keys(errors).some((path) => !INLINE_ERROR_FIELDS.has(path))
+        ) {
+          setFormError('Could not add the bucket. Please try again.')
+        }
+      } else {
+        setFormError('Could not add the bucket. Please try again.')
+      }
+    } finally {
+      setPending(false)
+    }
+  }
+
+  if (!open) {
+    return (
+      <Button glow onClick={() => setOpen(true)}>
+        Add bucket
+      </Button>
+    )
+  }
+
+  const nameErrors = fieldErrors.name
+  const allocationValueErrors = fieldErrors.allocationValue
+
+  return (
+    <form className="space-y-4" onSubmit={handleSubmit}>
+      <div className="space-y-1">
+        <Input
+          label="Name"
+          required
+          value={name}
+          onChange={(e) => setName(e.target.value)}
+          aria-invalid={nameErrors !== undefined}
+          aria-describedby={nameErrors ? 'bucket-name-errors' : undefined}
+        />
+        <FieldError id="bucket-name-errors" messages={nameErrors} />
+      </div>
+
+      <label className="block space-y-1">
+        <span className="text-primary block font-mono text-sm uppercase tracking-wide">
+          Type
+        </span>
+        <select
+          className="border-primary text-primary w-full border-2 bg-background px-3 py-2 font-mono"
+          value={type}
+          onChange={(e) => setType(e.target.value as BucketType)}
+        >
+          <option value="need">Need</option>
+          <option value="want">Want</option>
+          <option value="overflow">Overflow</option>
+        </select>
+      </label>
+
+      <label className="block space-y-1">
+        <span className="text-primary block font-mono text-sm uppercase tracking-wide">
+          Cap type
+        </span>
+        <select
+          className="border-primary text-primary w-full border-2 bg-background px-3 py-2 font-mono"
+          value={allocationType}
+          onChange={(e) =>
+            setAllocationType(e.target.value as BucketAllocationType)
+          }
+        >
+          <option value="fixed_limit">Fixed limit</option>
+          <option value="percentage">Percentage</option>
+          <option value="unlimited">Unlimited</option>
+        </select>
+      </label>
+
+      <div className="space-y-1">
+        <Input
+          label="Allocation value"
+          type="number"
+          min={0}
+          step="0.01"
+          value={allocationValue}
+          onChange={(e) => setAllocationValue(e.target.value)}
+          aria-invalid={allocationValueErrors !== undefined}
+          aria-describedby={
+            allocationValueErrors ? 'bucket-allocation-errors' : undefined
+          }
+        />
+        <FieldError
+          id="bucket-allocation-errors"
+          messages={allocationValueErrors}
+        />
+      </div>
+
+      <Input
+        label="Buffer multiplier"
+        type="number"
+        min={0}
+        step="0.01"
+        value={bufferMultiplier}
+        onChange={(e) => setBufferMultiplier(e.target.value)}
+      />
+
+      {formError !== null && (
+        <p role="alert" className="text-destructive font-mono text-sm">
+          {formError}
+        </p>
+      )}
+
+      <Button type="submit" glow disabled={pending} className="w-full">
+        {pending ? 'Saving…' : 'Save bucket'}
+      </Button>
+    </form>
+  )
+}
diff --git a/frontend/src/pages/ScenarioShowPage.test.tsx b/frontend/src/pages/ScenarioShowPage.test.tsx
index a73d6cd..b7bcd74 100644
--- a/frontend/src/pages/ScenarioShowPage.test.tsx
+++ b/frontend/src/pages/ScenarioShowPage.test.tsx
@@ -1,4 +1,5 @@
 import { render, screen } from '@testing-library/react'
+import userEvent from '@testing-library/user-event'
 import { http, HttpResponse } from 'msw'
 import { MemoryRouter, Route, Routes } from 'react-router'
 import { describe, expect, it } from 'vitest'
@@ -89,11 +90,11 @@ describe('ScenarioShowPage', () => {
     // Priority: assert each bucket's own priority is shown somewhere within
     // its own rendered content, rather than a page-wide loose number match
     // (which could coincidentally match allocation/other digits instead).
-    const rentPriorityMarker = await screen.findByText((_, element) =>
-      element?.textContent === 'Priority 1',
+    const rentPriorityMarker = await screen.findByText(
+      (_, element) => element?.textContent === 'Priority 1',
     )
-    const diningPriorityMarker = await screen.findByText((_, element) =>
-      element?.textContent === 'Priority 2',
+    const diningPriorityMarker = await screen.findByText(
+      (_, element) => element?.textContent === 'Priority 2',
     )
     expect(rentPriorityMarker).toBeInTheDocument()
     expect(diningPriorityMarker).toBeInTheDocument()
@@ -121,7 +122,9 @@ describe('ScenarioShowPage', () => {
     expect(
       await screen.findByText(/this scenario has no buckets yet/i),
     ).toBeInTheDocument()
-    expect(screen.queryByText('Priority', { exact: false })).not.toBeInTheDocument()
+    expect(
+      screen.queryByText('Priority', { exact: false }),
+    ).not.toBeInTheDocument()
   })
 
   it('renders the scenario name without a description paragraph when description is null', async () => {
@@ -181,7 +184,10 @@ describe('ScenarioShowPage', () => {
         HttpResponse.json({ id: 'u1', email: 'a@b.com' }),
       ),
       http.get(`http://localhost/api/scenarios/${SCENARIO_ID}`, () =>
-        HttpResponse.json({ message: 'Internal Server Error' }, { status: 500 }),
+        HttpResponse.json(
+          { message: 'Internal Server Error' },
+          { status: 500 },
+        ),
       ),
     )
 
@@ -191,4 +197,65 @@ describe('ScenarioShowPage', () => {
       /could not load this scenario/i,
     )
   })
+
+  it('shows a newly added bucket after a successful add-bucket submit, via a refetch', async () => {
+    // Stateful GET: the first call (initial mount) returns an empty scenario,
+    // every call after the POST returns the scenario WITH the new bucket. This
+    // proves the add flow re-fetches the scenario rather than relying on an
+    // optimistic insert — the new bucket can only appear via the second GET.
+    let scenarioCreated = false
+    server.use(
+      http.get('http://localhost/api/me', () =>
+        HttpResponse.json({ id: 'u1', email: 'a@b.com' }),
+      ),
+      http.get(`http://localhost/api/scenarios/${SCENARIO_ID}`, () =>
+        HttpResponse.json({
+          '@context': '/api/contexts/Scenario',
+          '@id': `/api/scenarios/${SCENARIO_ID}`,
+          '@type': 'Scenario',
+          name: 'Emergency Fund',
+          description: 'Rainy day savings',
+          buckets: scenarioCreated
+            ? [
+                {
+                  '@id': '/api/buckets/22222222-2222-2222-2222-222222222222',
+                  '@type': 'Bucket',
+                  name: 'Rent',
+                  type: 'need',
+                  priority: 1,
+                  sortOrder: 0,
+                  allocationType: 'fixed_limit',
+                  allocationValue: 10000,
+                  startingAmount: 0,
+                  bufferMultiplier: '0.00',
+                },
+              ]
+            : [],
+        }),
+      ),
+      http.post('http://localhost/api/buckets', () => {
+        scenarioCreated = true
+        return HttpResponse.json(
+          { '@id': '/api/buckets/22222222-2222-2222-2222-222222222222' },
+          { status: 201 },
+        )
+      }),
+    )
+
+    const user = userEvent.setup()
+    renderPage()
+
+    expect(
+      await screen.findByText(/this scenario has no buckets yet/i),
+    ).toBeInTheDocument()
+
+    await user.click(screen.getByRole('button', { name: /add bucket/i }))
+    await user.type(screen.getByLabelText(/name/i), 'Rent')
+    await user.type(screen.getByLabelText(/allocation/i), '100')
+    await user.click(
+      screen.getByRole('button', { name: /save|create|submit/i }),
+    )
+
+    expect(await screen.findByText('Rent')).toBeInTheDocument()
+  })
 })
diff --git a/frontend/src/pages/ScenarioShowPage.tsx b/frontend/src/pages/ScenarioShowPage.tsx
index c5c5882..fc4d5f4 100644
--- a/frontend/src/pages/ScenarioShowPage.tsx
+++ b/frontend/src/pages/ScenarioShowPage.tsx
@@ -7,6 +7,7 @@ import { ApiError, api } from '../lib/api'
 import { bucketCapacity } from '../lib/bucketCapacity'
 import { formatAllocation } from '../lib/bucketDisplay'
 import type { Bucket, Scenario } from '../types/api'
+import AddBucketForm from './AddBucketForm'
 
 type LoadState =
   | { status: 'loading' }
@@ -44,6 +45,9 @@ function BucketCard({ bucket }: { bucket: Bucket }) {
 export default function ScenarioShowPage() {
   const { id } = useParams<{ id: string }>()
   const [state, setState] = useState<LoadState>({ status: 'loading' })
+  // Bumped after a successful add so the effect re-fetches the scenario, pulling
+  // in the new bucket from the embedded-buckets GET (the single source of truth).
+  const [reloadKey, setReloadKey] = useState(0)
 
   useEffect(() => {
     let active = true
@@ -71,7 +75,7 @@ export default function ScenarioShowPage() {
     return () => {
       active = false
     }
-  }, [id])
+  }, [id, reloadKey])
 
   return (
     <AuthedLayout>
@@ -120,6 +124,12 @@ export default function ScenarioShowPage() {
                 ))}
               </ul>
             )}
+
+            <AddBucketForm
+              scenario={state.scenario['@id']}
+              existingBuckets={state.scenario.buckets}
+              onCreated={() => setReloadKey((key) => key + 1)}
+            />
           </>
         )}
       </div>