diff --git a/lib/services/remediation-service.test.ts b/lib/services/remediation-service.test.ts
new file mode 100644
index 0000000..15ae5c5
--- /dev/null
+++ b/lib/services/remediation-service.test.ts
@@ -0,0 +1,258 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+
+// Mock postgresClient and phishing-audit BEFORE importing the module under test.
+const queryMock = vi.fn();
+const transactionMock = vi.fn();
+vi.mock('./postgres-client', () => ({
+  postgresClient: {
+    query: (...args: unknown[]) => queryMock(...args),
+    transaction: (...args: unknown[]) => transactionMock(...args),
+  },
+}));
+
+const writeAuditEventMock = vi.fn();
+vi.mock('./phishing-audit', () => ({
+  writeAuditEvent: (...args: unknown[]) => writeAuditEventMock(...args),
+}));
+
+// eslint-disable-next-line import/first -- imported after vi.mock hoisting
+import {
+  approveRemediationActions,
+  remediateApprovedActions,
+  markCampaignFalsePositive,
+  RemediationValidationError,
+  RemediationConflictError,
+} from './remediation-service';
+
+/** Records every SQL call made through the fake transaction client. */
+let clientCalls: Array<{ sql: string; params: unknown[] }> = [];
+
+interface MockRows {
+  classification?: unknown[];
+  insertRemediation?: unknown[];
+  remediationRows?: unknown[];
+  campaign?: unknown[];
+  guardRows?: unknown[];
+}
+
+function makeClient(rows: MockRows) {
+  return {
+    query: vi.fn(async (sql: string, params?: unknown[]) => {
+      clientCalls.push({ sql, params: params ?? [] });
+
+      if (sql.includes('FROM classifications')) {
+        return { rows: rows.classification ?? [], rowCount: rows.classification?.length ?? 0 };
+      }
+      if (sql.includes('INSERT INTO remediation_actions')) {
+        const insertRows = rows.insertRemediation ?? [];
+        return { rows: [insertRows.shift() ?? { id: 'unstaged-action-id' }], rowCount: 1 };
+      }
+      if (sql.includes('SELECT id::text, action_type, status') && sql.includes('FOR UPDATE')) {
+        return { rows: rows.remediationRows ?? [], rowCount: rows.remediationRows?.length ?? 0 };
+      }
+      if (sql.includes('UPDATE remediation_actions SET status')) {
+        return { rows: [], rowCount: 1 };
+      }
+      if (sql.includes('FROM remediation_actions WHERE campaign_id') && sql.includes("status IN")) {
+        return { rows: rows.guardRows ?? [], rowCount: rows.guardRows?.length ?? 0 };
+      }
+      if (sql.includes('SELECT status FROM campaigns')) {
+        return { rows: rows.campaign ?? [], rowCount: rows.campaign?.length ?? 0 };
+      }
+      if (sql.includes('UPDATE campaigns SET status')) {
+        return { rows: [], rowCount: 1 };
+      }
+      throw new Error(`Unstaged query in test mock: ${sql}`);
+    }),
+  };
+}
+
+function callsContaining(needle: string) {
+  return clientCalls.filter((c) => c.sql.includes(needle));
+}
+
+function stage(rows: MockRows) {
+  transactionMock.mockImplementation(async (callback: (client: unknown) => Promise<unknown>) =>
+    callback(makeClient(rows))
+  );
+}
+
+beforeEach(() => {
+  queryMock.mockReset();
+  transactionMock.mockReset();
+  writeAuditEventMock.mockReset();
+  writeAuditEventMock.mockResolvedValue('audit-id');
+  clientCalls = [];
+});
+
+// =============================================================================
+// approveRemediationActions
+// =============================================================================
+
+describe('approveRemediationActions', () => {
+  it('rejects an action type that is NOT in the latest classification recommended_actions', async () => {
+    stage({
+      classification: [{ recommended_actions: ['warn_user'] }],
+    });
+
+    await expect(
+      approveRemediationActions('campaign-1', [{ actionType: 'block_sender' }], 'operator@example.com')
+    ).rejects.toThrow(RemediationValidationError);
+  });
+
+  it('throws RemediationValidationError when no classification row exists for the campaign', async () => {
+    stage({ classification: [] });
+
+    await expect(
+      approveRemediationActions('campaign-1', [{ actionType: 'warn_user' }], 'operator@example.com')
+    ).rejects.toThrow(RemediationValidationError);
+  });
+
+  it('inserts one remediation_actions row per requested action plus one audit row, inside one transaction', async () => {
+    stage({
+      classification: [{ recommended_actions: ['block_sender', 'purge_message'] }],
+      insertRemediation: [{ id: 'action-1' }, { id: 'action-2' }],
+    });
+
+    const result = await approveRemediationActions(
+      'campaign-1',
+      [
+        { actionType: 'block_sender', params: { sender: 'evil@example.com' } },
+        { actionType: 'purge_message', params: {} },
+      ],
+      'operator@example.com'
+    );
+
+    expect(transactionMock).toHaveBeenCalledTimes(1);
+    const insertCalls = callsContaining('INSERT INTO remediation_actions');
+    expect(insertCalls).toHaveLength(2);
+    expect(insertCalls[0].sql).toContain("'approved'");
+    expect(insertCalls[0].params).toEqual([
+      'campaign-1',
+      'block_sender',
+      JSON.stringify({ sender: 'evil@example.com' }),
+      'operator@example.com',
+    ]);
+
+    expect(writeAuditEventMock).toHaveBeenCalledTimes(1);
+    const [auditArgs, auditClient] = writeAuditEventMock.mock.calls[0];
+    expect(auditArgs).toMatchObject({
+      campaignId: 'campaign-1',
+      actor: 'operator@example.com',
+      eventType: 'remediation_approved',
+    });
+    expect(auditClient).toBeDefined();
+
+    expect(result).toHaveLength(2);
+    expect(result[0]).toMatchObject({ id: 'action-1', actionType: 'block_sender', status: 'approved' });
+  });
+});
+
+// =============================================================================
+// remediateApprovedActions
+// =============================================================================
+
+describe('remediateApprovedActions', () => {
+  it('transitions every status=approved row to completed and writes one audit row per transitioned action', async () => {
+    stage({
+      remediationRows: [
+        { id: 'action-1', action_type: 'block_sender', status: 'approved' },
+        { id: 'action-2', action_type: 'purge_message', status: 'approved' },
+      ],
+    });
+
+    const result = await remediateApprovedActions('campaign-1', 'operator@example.com');
+
+    const updateCalls = callsContaining('UPDATE remediation_actions SET status');
+    expect(updateCalls).toHaveLength(2);
+    expect(writeAuditEventMock).toHaveBeenCalledTimes(2);
+    expect(writeAuditEventMock.mock.calls[0][0]).toMatchObject({
+      campaignId: 'campaign-1',
+      eventType: 'remediation_completed',
+    });
+    expect(result.actions.every((a) => a.status === 'completed')).toBe(true);
+  });
+
+  it('is idempotent: a second call transitions nothing and writes no second audit row (REMED-04)', async () => {
+    // First call: two approved rows.
+    stage({
+      remediationRows: [
+        { id: 'action-1', action_type: 'block_sender', status: 'approved' },
+        { id: 'action-2', action_type: 'purge_message', status: 'approved' },
+      ],
+    });
+    await remediateApprovedActions('campaign-1', 'operator@example.com');
+    expect(writeAuditEventMock).toHaveBeenCalledTimes(2);
+
+    // Second call: same rows are now already completed — status filter finds nothing to transition.
+    clientCalls = [];
+    stage({
+      remediationRows: [
+        { id: 'action-1', action_type: 'block_sender', status: 'completed' },
+        { id: 'action-2', action_type: 'purge_message', status: 'completed' },
+      ],
+    });
+    const result2 = await remediateApprovedActions('campaign-1', 'operator@example.com');
+
+    expect(callsContaining('UPDATE remediation_actions SET status')).toHaveLength(0);
+    // Still exactly 2 total audit calls across both calls (no additional on the second).
+    expect(writeAuditEventMock).toHaveBeenCalledTimes(2);
+    expect(result2.actions.every((a) => a.alreadyCompleted)).toBe(true);
+  });
+
+  it('raises an explicit failure (RemediationValidationError) when the campaign has zero remediation_actions rows', async () => {
+    stage({ remediationRows: [] });
+
+    await expect(remediateApprovedActions('campaign-1', 'operator@example.com')).rejects.toThrow(
+      RemediationValidationError
+    );
+    expect(writeAuditEventMock).not.toHaveBeenCalled();
+  });
+});
+
+// =============================================================================
+// markCampaignFalsePositive (D-04 guard)
+// =============================================================================
+
+describe('markCampaignFalsePositive', () => {
+  it('throws RemediationConflictError when any remediation_actions row has status approved/completed', async () => {
+    stage({
+      guardRows: [{ id: 'action-1' }],
+    });
+
+    await expect(
+      markCampaignFalsePositive('campaign-1', 'operator@example.com')
+    ).rejects.toThrow(RemediationConflictError);
+    expect(callsContaining('UPDATE campaigns SET status')).toHaveLength(0);
+  });
+
+  it('sets campaigns.status to false_positive and writes one audit row when there are no approved/completed rows', async () => {
+    stage({
+      guardRows: [],
+      campaign: [{ status: 'open' }],
+    });
+
+    const result = await markCampaignFalsePositive('campaign-1', 'operator@example.com', 'confirmed benign');
+
+    expect(callsContaining('UPDATE campaigns SET status')).toHaveLength(1);
+    expect(writeAuditEventMock).toHaveBeenCalledTimes(1);
+    const [auditArgs] = writeAuditEventMock.mock.calls[0];
+    expect(auditArgs).toMatchObject({
+      campaignId: 'campaign-1',
+      eventType: 'campaign_marked_false_positive',
+      payload: { previousStatus: 'open', reason: 'confirmed benign' },
+    });
+    expect(result).toMatchObject({ campaignId: 'campaign-1', status: 'false_positive' });
+  });
+
+  it('throws RemediationValidationError when the campaign does not exist', async () => {
+    stage({
+      guardRows: [],
+      campaign: [],
+    });
+
+    await expect(
+      markCampaignFalsePositive('campaign-missing', 'operator@example.com')
+    ).rejects.toThrow(RemediationValidationError);
+  });
+});