diff --git a/services/platform-service/src/lib/cosmos-init.ts b/services/platform-service/src/lib/cosmos-init.ts
index 2e185f45..18af8750 100644
--- a/services/platform-service/src/lib/cosmos-init.ts
+++ b/services/platform-service/src/lib/cosmos-init.ts
@@ -79,6 +79,10 @@ const CONTAINER_DEFS: Record<string, ContainerConfig> = {
   agent_evaluation_cases: { partitionKeyPath: '/suiteId' },
   agent_evaluation_runs: { partitionKeyPath: '/productId', defaultTtl: 30 * 86400 },
   agent_evaluation_results: { partitionKeyPath: '/runId', defaultTtl: 30 * 86400 },
+  // AI budget and cost governance
+  ai_budget_policies: { partitionKeyPath: '/productId' },
+  ai_budget_spend_entries: { partitionKeyPath: '/productId', defaultTtl: 30 * 86400 },
+  ai_budget_alerts: { partitionKeyPath: '/productId', defaultTtl: 30 * 86400 },
   // Telemetry (client diagnostics — see docs/WINDSURF/CLIENT_TELEMETRY_DESIGN.md)
   telemetry_events: { partitionKeyPath: '/pk', defaultTtl: 30 * 86400 },
   telemetry_error_clusters: { partitionKeyPath: '/pk', defaultTtl: 90 * 86400 },
diff --git a/services/platform-service/src/modules/ai-budgets/repository.test.ts b/services/platform-service/src/modules/ai-budgets/repository.test.ts
new file mode 100644
index 00000000..d8486efd
--- /dev/null
+++ b/services/platform-service/src/modules/ai-budgets/repository.test.ts
@@ -0,0 +1,73 @@
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
+import { MemoryDatastoreProvider } from '@bytelyst/datastore';
+import { _resetDatastoreProvider, setProvider } from '../../lib/datastore.js';
+import * as repo from './repository.js';
+
+describe('ai budget repository', () => {
+  beforeEach(() => {
+    setProvider(new MemoryDatastoreProvider());
+  });
+
+  afterEach(() => {
+    _resetDatastoreProvider();
+  });
+
+  it('stores policies, spend entries, and alerts', async () => {
+    await repo.createPolicy({
+      id: 'aibudget_1',
+      productId: 'lysnrai',
+      scopeType: 'agent',
+      scopeId: 'agt_1',
+      name: 'Incident Agent Budget',
+      status: 'active',
+      period: 'monthly',
+      budgetUsd: 25,
+      softThreshold: 0.8,
+      hardThreshold: 1,
+      modelAllowlist: ['gpt-4o-mini'],
+      createdBy: 'admin_1',
+      createdAt: '2026-03-15T00:00:00.000Z',
+      updatedAt: '2026-03-15T00:00:00.000Z',
+    });
+
+    await repo.createSpendEntry({
+      id: 'aispend_1',
+      productId: 'lysnrai',
+      policyId: 'aibudget_1',
+      scopeType: 'agent',
+      scopeId: 'agt_1',
+      agentId: 'agt_1',
+      agentVersionId: 'agt_1:v2',
+      model: 'gpt-4o-mini',
+      tokensUsed: 500,
+      costUsd: 4.2,
+      source: 'run',
+      verdict: 'allow',
+      recordedAt: '2026-03-15T00:10:00.000Z',
+    });
+
+    await repo.createAlerts([
+      {
+        id: 'aialert_1',
+        productId: 'lysnrai',
+        policyId: 'aibudget_1',
+        scopeType: 'agent',
+        scopeId: 'agt_1',
+        severity: 'warn',
+        message: 'Approaching budget',
+        spendUsd: 20,
+        budgetUsd: 25,
+        percentUsed: 0.8,
+        createdAt: '2026-03-15T00:11:00.000Z',
+      },
+    ]);
+
+    const policies = await repo.listPolicies('lysnrai', { limit: 20 });
+    const spend = await repo.listSpendEntries('lysnrai', { policyId: 'aibudget_1' });
+    const alerts = await repo.listAlerts('lysnrai', { limit: 20 });
+
+    expect(policies).toHaveLength(1);
+    expect(spend[0].costUsd).toBe(4.2);
+    expect(alerts[0].severity).toBe('warn');
+  });
+});
diff --git a/services/platform-service/src/modules/ai-budgets/repository.ts b/services/platform-service/src/modules/ai-budgets/repository.ts
new file mode 100644
index 00000000..5e44f955
--- /dev/null
+++ b/services/platform-service/src/modules/ai-budgets/repository.ts
@@ -0,0 +1,100 @@
+import { NotFoundError } from '../../lib/errors.js';
+import { getCollection } from '../../lib/datastore.js';
+import type {
+  BudgetAlertDoc,
+  BudgetPolicyDoc,
+  BudgetSpendEntryDoc,
+  ListBudgetAlertsQuery,
+  ListBudgetPoliciesQuery,
+} from './types.js';
+
+function policyCollection() {
+  return getCollection<BudgetPolicyDoc>('ai_budget_policies', '/productId');
+}
+
+function spendCollection() {
+  return getCollection<BudgetSpendEntryDoc>('ai_budget_spend_entries', '/productId');
+}
+
+function alertCollection() {
+  return getCollection<BudgetAlertDoc>('ai_budget_alerts', '/productId');
+}
+
+export async function createPolicy(doc: BudgetPolicyDoc): Promise<BudgetPolicyDoc> {
+  return policyCollection().create(doc);
+}
+
+export async function listPolicies(
+  productId: string,
+  query: ListBudgetPoliciesQuery
+): Promise<BudgetPolicyDoc[]> {
+  return policyCollection().findMany({
+    filter: {
+      productId,
+      ...(query.scopeType ? { scopeType: query.scopeType } : {}),
+      ...(query.scopeId ? { scopeId: query.scopeId } : {}),
+      ...(query.status ? { status: query.status } : {}),
+    },
+    sort: { createdAt: -1 },
+    limit: query.limit,
+  });
+}
+
+export async function getPolicy(id: string, productId: string): Promise<BudgetPolicyDoc> {
+  const policy = await policyCollection().findById(id, productId);
+  if (!policy) throw new NotFoundError(`Budget policy '${id}' not found`);
+  return policy;
+}
+
+export async function updatePolicy(
+  id: string,
+  productId: string,
+  updates: Partial<BudgetPolicyDoc>
+): Promise<BudgetPolicyDoc> {
+  const updated = await policyCollection().update(id, productId, {
+    ...updates,
+    updatedAt: new Date().toISOString(),
+  });
+  if (!updated) throw new NotFoundError(`Budget policy '${id}' not found`);
+  return updated;
+}
+
+export async function createSpendEntry(doc: BudgetSpendEntryDoc): Promise<BudgetSpendEntryDoc> {
+  return spendCollection().create(doc);
+}
+
+export async function listSpendEntries(
+  productId: string,
+  filter: { policyId?: string; scopeType?: string; scopeId?: string; since?: string } = {}
+): Promise<BudgetSpendEntryDoc[]> {
+  return spendCollection().findMany({
+    filter: {
+      productId,
+      ...(filter.policyId ? { policyId: filter.policyId } : {}),
+      ...(filter.scopeType ? { scopeType: filter.scopeType } : {}),
+      ...(filter.scopeId ? { scopeId: filter.scopeId } : {}),
+      ...(filter.since ? { recordedAt: { $gte: filter.since } } : {}),
+    } as import('@bytelyst/datastore').FilterMap,
+    sort: { recordedAt: -1 },
+    limit: 1000,
+  });
+}
+
+export async function createAlerts(docs: BudgetAlertDoc[]): Promise<BudgetAlertDoc[]> {
+  const writes = docs.map(doc => alertCollection().create(doc));
+  return Promise.all(writes);
+}
+
+export async function listAlerts(
+  productId: string,
+  query: ListBudgetAlertsQuery
+): Promise<BudgetAlertDoc[]> {
+  return alertCollection().findMany({
+    filter: {
+      productId,
+      ...(query.severity ? { severity: query.severity } : {}),
+    },
+    sort: { createdAt: -1 },
+    limit: query.limit,
+  });
+}
diff --git a/services/platform-service/src/modules/ai-budgets/routes.test.ts b/services/platform-service/src/modules/ai-budgets/routes.test.ts
new file mode 100644
index 00000000..8b1c0720
--- /dev/null
+++ b/services/platform-service/src/modules/ai-budgets/routes.test.ts
@@ -0,0 +1,133 @@
+import Fastify from 'fastify';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+const repoMock = {
+  listPolicies: vi.fn(),
+  createPolicy: vi.fn(),
+  getPolicy: vi.fn(),
+  updatePolicy: vi.fn(),
+  createSpendEntry: vi.fn(),
+  listSpendEntries: vi.fn(),
+  createAlerts: vi.fn(),
+  listAlerts: vi.fn(),
+};
+
+vi.mock('./repository.js', () => repoMock);
+
+async function buildApp(payload?: { sub: string; productId: string; role?: string }) {
+  const { aiBudgetRoutes } = await import('./routes.js');
+  const app = Fastify({ logger: false });
+  if (payload) {
+    app.addHook('onRequest', async req => {
+      req.jwtPayload = payload;
+    });
+  }
+  await app.register(aiBudgetRoutes, { prefix: '/api' });
+  return app;
+}
+
+describe('aiBudgetRoutes', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it('POST /ai-budgets/policies creates a policy', async () => {
+    repoMock.createPolicy.mockResolvedValue({ id: 'aibudget_1' });
+
+    const app = await buildApp({ sub: 'admin_1', productId: 'lysnrai', role: 'admin' });
+    const res = await app.inject({
+      method: 'POST',
+      url: '/api/ai-budgets/policies',
+      payload: {
+        scopeType: 'agent',
+        scopeId: 'agt_1',
+        name: 'Incident Budget',
+        budgetUsd: 20,
+      },
+    });
+
+    expect(res.statusCode).toBe(200);
+    expect(repoMock.createPolicy).toHaveBeenCalledWith(
+      expect.objectContaining({
+        scopeType: 'agent',
+        scopeId: 'agt_1',
+        budgetUsd: 20,
+      })
+    );
+  });
+
+  it('GET /ai-budgets/policies/:id/status summarizes current spend', async () => {
+    repoMock.getPolicy.mockResolvedValue({
+      id: 'aibudget_1',
+      productId: 'lysnrai',
+      scopeType: 'agent',
+      scopeId: 'agt_1',
+      name: 'Incident Budget',
+      period: 'monthly',
+      budgetUsd: 10,
+      softThreshold: 0.8,
+      hardThreshold: 1,
+    });
+    repoMock.listSpendEntries.mockResolvedValue([
+      { costUsd: 3, tokensUsed: 100, recordedAt: '2026-03-15T00:00:00.000Z' },
+      { costUsd: 6, tokensUsed: 200, recordedAt: '2026-03-15T01:00:00.000Z' },
+    ]);
+
+    const app = await buildApp({ sub: 'admin_1', productId: 'lysnrai', role: 'admin' });
+    const res = await app.inject({
+      method: 'GET',
+      url: '/api/ai-budgets/policies/aibudget_1/status',
+    });
+
+    expect(res.statusCode).toBe(200);
+    expect(res.json()).toMatchObject({
+      spendUsd: 9,
+      tokensUsed: 300,
+      verdict: 'warn',
+    });
+  });
+
+  it('POST /ai-budgets/spend blocks disallowed models and persists alerts', async () => {
+    repoMock.getPolicy.mockResolvedValue({
+      id: 'aibudget_1',
+      productId: 'lysnrai',
+      scopeType: 'agent',
+      scopeId: 'agt_1',
+      name: 'Incident Budget',
+      status: 'active',
+      period: 'monthly',
+      budgetUsd: 50,
+      softThreshold: 0.8,
+      hardThreshold: 1,
+      modelAllowlist: ['gpt-4o-mini'],
+    });
+    repoMock.createSpendEntry.mockResolvedValue({ id: 'aispend_1', verdict: 'block' });
+    repoMock.createAlerts.mockResolvedValue([]);
+
+    const app = await buildApp({ sub: 'admin_1', productId: 'lysnrai', role: 'admin' });
+    const res = await app.inject({
+      method: 'POST',
+      url: '/api/ai-budgets/spend',
+      payload: {
+        policyId: 'aibudget_1',
+        scopeType: 'agent',
+        scopeId: 'agt_1',
+        model: 'gpt-4.1',
+        costUsd: 2,
+      },
+    });
+
+    expect(res.statusCode).toBe(200);
+    expect(repoMock.createAlerts).toHaveBeenCalled();
+    expect(repoMock.createSpendEntry).toHaveBeenCalledWith(
+      expect.objectContaining({
+        verdict: 'block',
+        model: 'gpt-4.1',
+      })
+    );
+  });
+});
diff --git a/services/platform-service/src/modules/ai-budgets/routes.ts b/services/platform-service/src/modules/ai-budgets/routes.ts
new file mode 100644
index 00000000..439e9c09
--- /dev/null
+++ b/services/platform-service/src/modules/ai-budgets/routes.ts
@@ -0,0 +1,251 @@
+import { randomUUID } from 'node:crypto';
+import type { FastifyInstance } from 'fastify';
+import { BadRequestError, ForbiddenError } from '../../lib/errors.js';
+import {
+  BudgetAlertDoc,
+  BudgetPolicyDoc,
+  BudgetSpendEntryDoc,
+  CreateBudgetPolicySchema,
+  ListBudgetAlertsQuerySchema,
+  ListBudgetPoliciesQuerySchema,
+  RecordBudgetSpendSchema,
+  UpdateBudgetPolicySchema,
+} from './types.js';
+import * as repo from './repository.js';
+
+function requireAdmin(req: { jwtPayload?: { sub?: string; role?: string; productId?: string } }): {
+  userId: string;
+  productId: string;
+} {
+  const payload = req.jwtPayload;
+  if (!payload?.sub) throw new ForbiddenError('Authentication required');
+  if (!payload.role || !['super_admin', 'admin'].includes(payload.role)) {
+    throw new ForbiddenError('Admin access required');
+  }
+  return {
+    userId: payload.sub,
+    productId: payload.productId ?? process.env.DEFAULT_PRODUCT_ID ?? 'lysnrai',
+  };
+}
+
+function periodStart(now: Date, period: 'daily' | 'monthly'): string {
+  if (period === 'daily') {
+    return now.toISOString().slice(0, 10);
+  }
+  return `${now.getUTCFullYear()}-${String(now.getUTCMonth() + 1).padStart(2, '0')}-01`;
+}
+
+function toIsoDayBoundary(day: string): string {
+  return `${day}T00:00:00.000Z`;
+}
+
+function validationError(message: string): never {
+  throw new BadRequestError(message);
+}
+
+export async function aiBudgetRoutes(app: FastifyInstance) {
+  app.get('/ai-budgets/policies', async req => {
+    const access = requireAdmin(req);
+    const parsed = ListBudgetPoliciesQuerySchema.safeParse(req.query);
+    if (!parsed.success) {
+      validationError(parsed.error.issues.map(issue => issue.message).join('; '));
+    }
+    return repo.listPolicies(access.productId, parsed.data);
+  });
+
+  app.post('/ai-budgets/policies', async req => {
+    const access = requireAdmin(req);
+    const parsed = CreateBudgetPolicySchema.safeParse(req.body);
+    if (!parsed.success) {
+      validationError(parsed.error.issues.map(issue => issue.message).join('; '));
+    }
+
+    const now = new Date().toISOString();
+    const doc: BudgetPolicyDoc = {
+      id: `aibudget_${randomUUID()}`,
+      productId: access.productId,
+      scopeType: parsed.data.scopeType,
+      scopeId: parsed.data.scopeId,
+      name: parsed.data.name,
+      status: 'active',
+      period: parsed.data.period,
+      budgetUsd: parsed.data.budgetUsd,
+      softThreshold: parsed.data.softThreshold,
+      hardThreshold: parsed.data.hardThreshold,
+      modelAllowlist: parsed.data.modelAllowlist,
+      metadata: parsed.data.metadata,
+      createdBy: access.userId,
+      createdAt: now,
+      updatedAt: now,
+    };
+    return repo.createPolicy(doc);
+  });
+
+  app.get('/ai-budgets/policies/:id', async req => {
+    const access = requireAdmin(req);
+    const { id } = req.params as { id: string };
+    return repo.getPolicy(id, access.productId);
+  });
+
+  app.patch('/ai-budgets/policies/:id', async req => {
+    const access = requireAdmin(req);
+    const { id } = req.params as { id: string };
+    const parsed = UpdateBudgetPolicySchema.safeParse(req.body);
+    if (!parsed.success) {
+      validationError(parsed.error.issues.map(issue => issue.message).join('; '));
+    }
+    return repo.updatePolicy(id, access.productId, parsed.data);
+  });
+
+  app.get('/ai-budgets/policies/:id/status', async req => {
+    const access = requireAdmin(req);
+    const { id } = req.params as { id: string };
+    const policy = await repo.getPolicy(id, access.productId);
+    const since = toIsoDayBoundary(periodStart(new Date(), policy.period));
+    const entries = await repo.listSpendEntries(access.productId, {
+      policyId: id,
+      since,
+    });
+    const spendUsd = entries.reduce((sum, entry) => sum + entry.costUsd, 0);
+    const tokensUsed = entries.reduce((sum, entry) => sum + entry.tokensUsed, 0);
+    const percentUsed = policy.budgetUsd > 0 ? spendUsd / policy.budgetUsd : 0;
+
+    return {
+      policy,
+      spendUsd,
+      tokensUsed,
+      percentUsed,
+      verdict:
+        percentUsed >= policy.hardThreshold
+          ? 'block'
+          : percentUsed >= policy.softThreshold
+            ? 'warn'
+            : 'allow',
+      entries: entries.length,
+    };
+  });
+
+  app.get('/ai-budgets/alerts', async req => {
+    const access = requireAdmin(req);
+    const parsed = ListBudgetAlertsQuerySchema.safeParse(req.query);
+    if (!parsed.success) {
+      validationError(parsed.error.issues.map(issue => issue.message).join('; '));
+    }
+    return repo.listAlerts(access.productId, parsed.data);
+  });
+
+  app.post('/ai-budgets/spend', async req => {
+    const access = requireAdmin(req);
+    const parsed = RecordBudgetSpendSchema.safeParse(req.body);
+    if (!parsed.success) {
+      validationError(parsed.error.issues.map(issue => issue.message).join('; '));
+    }
+
+    const candidatePolicies = parsed.data.policyId
+      ? [await repo.getPolicy(parsed.data.policyId, access.productId)]
+      : await repo.listPolicies(access.productId, {
+          scopeType: parsed.data.scopeType,
+          scopeId: parsed.data.scopeId,
+          status: 'active',
+          limit: 100,
+        });
+
+    const now = new Date();
+    const alerts: BudgetAlertDoc[] = [];
+    let overallVerdict: 'allow' | 'warn' | 'block' = 'allow';
+
+    for (const policy of candidatePolicies) {
+      if (policy.status !== 'active') continue;
+
+      if (parsed.data.model && policy.modelAllowlist.length > 0) {
+        const allowed = policy.modelAllowlist.includes(parsed.data.model);
+        if (!allowed) {
+          overallVerdict = 'block';
+          alerts.push({
+            id: `aialert_${randomUUID()}`,
+            productId: access.productId,
+            policyId: policy.id,
+            scopeType: policy.scopeType,
+            scopeId: policy.scopeId,
+            severity: 'block',
+            message: `Model '${parsed.data.model}' is not allowed by policy '${policy.name}'`,
+            spendUsd: parsed.data.costUsd,
+            budgetUsd: policy.budgetUsd,
+            percentUsed: 0,
+            createdAt: now.toISOString(),
+          });
+          continue;
+        }
+      }
+
+      const since = toIsoDayBoundary(periodStart(now, policy.period));
+      const entries = await repo.listSpendEntries(access.productId, {
+        policyId: policy.id,
+        since,
+      });
+      const spendUsd = entries.reduce((sum, entry) => sum + entry.costUsd, 0) + parsed.data.costUsd;
+      const percentUsed = policy.budgetUsd > 0 ? spendUsd / policy.budgetUsd : 0;
+
+      if (percentUsed >= policy.hardThreshold) {
+        overallVerdict = 'block';
+        alerts.push({
+          id: `aialert_${randomUUID()}`,
+          productId: access.productId,
+          policyId: policy.id,
+          scopeType: policy.scopeType,
+          scopeId: policy.scopeId,
+          severity: 'block',
+          message: `AI budget hard limit exceeded for policy '${policy.name}'`,
+          spendUsd,
+          budgetUsd: policy.budgetUsd,
+          percentUsed,
+          createdAt: now.toISOString(),
+        });
+      } else if (percentUsed >= policy.softThreshold && overallVerdict !== 'block') {
+        overallVerdict = 'warn';
+        alerts.push({
+          id: `aialert_${randomUUID()}`,
+          productId: access.productId,
+          policyId: policy.id,
+          scopeType: policy.scopeType,
+          scopeId: policy.scopeId,
+          severity: 'warn',
+          message: `AI budget nearing threshold for policy '${policy.name}'`,
+          spendUsd,
+          budgetUsd: policy.budgetUsd,
+          percentUsed,
+          createdAt: now.toISOString(),
+        });
+      }
+    }
+
+    const entry: BudgetSpendEntryDoc = {
+      id: `aispend_${randomUUID()}`,
+      productId: access.productId,
+      policyId: candidatePolicies.length === 1 ? candidatePolicies[0].id : parsed.data.policyId,
+      scopeType: parsed.data.scopeType,
+      scopeId: parsed.data.scopeId,
+      agentId: parsed.data.agentId,
+      agentVersionId: parsed.data.agentVersionId,
+      runId: parsed.data.runId,
+      evaluationRunId: parsed.data.evaluationRunId,
+      model: parsed.data.model,
+      tokensUsed: parsed.data.tokensUsed,
+      costUsd: parsed.data.costUsd,
+      source: parsed.data.source,
+      verdict: overallVerdict,
+      recordedAt: now.toISOString(),
+    };
+
+    const saved = await repo.createSpendEntry(entry);
+    if (alerts.length > 0) {
+      await repo.createAlerts(alerts);
+    }
+
+    return {
+      verdict: overallVerdict,
+      entry: saved,
+      alerts,
+    };
+  });
+}
diff --git a/services/platform-service/src/modules/ai-budgets/types.ts b/services/platform-service/src/modules/ai-budgets/types.ts
new file mode 100644
index 00000000..6fa77478
--- /dev/null
+++ b/services/platform-service/src/modules/ai-budgets/types.ts
@@ -0,0 +1,124 @@
+import { z } from 'zod';
+
+export const BudgetScopeTypeSchema = z.enum(['product', 'agent']);
+export const BudgetPolicyStatusSchema = z.enum(['active', 'paused', 'archived']);
+export const BudgetPeriodSchema = z.enum(['daily', 'monthly']);
+export const BudgetAlertSeveritySchema = z.enum(['warn', 'block']);
+export const BudgetVerdictSchema = z.enum(['allow', 'warn', 'block']);
+
+export const BudgetPolicySchema = z.object({
+  id: z.string().min(1),
+  productId: z.string().min(1),
+  scopeType: BudgetScopeTypeSchema,
+  scopeId: z.string().min(1),
+  name: z.string().min(1),
+  status: BudgetPolicyStatusSchema,
+  period: BudgetPeriodSchema,
+  budgetUsd: z.number().min(0),
+  softThreshold: z.number().min(0).max(1),
+  hardThreshold: z.number().min(0).max(1),
+  modelAllowlist: z.array(z.string()).default([]),
+  metadata: z.record(z.unknown()).optional(),
+  createdBy: z.string().min(1),
+  createdAt: z.string(),
+  updatedAt: z.string(),
+});
+
+export type BudgetPolicyDoc = z.infer<typeof BudgetPolicySchema> & {
+  _ts?: number;
+  _etag?: string;
+};
+
+export const BudgetSpendEntrySchema = z.object({
+  id: z.string().min(1),
+  productId: z.string().min(1),
+  policyId: z.string().optional(),
+  scopeType: BudgetScopeTypeSchema,
+  scopeId: z.string().min(1),
+  agentId: z.string().optional(),
+  agentVersionId: z.string().optional(),
+  runId: z.string().optional(),
+  evaluationRunId: z.string().optional(),
+  model: z.string().optional(),
+  tokensUsed: z.number().int().min(0).default(0),
+  costUsd: z.number().min(0),
+  source: z.string().optional(),
+  verdict: BudgetVerdictSchema,
+  recordedAt: z.string(),
+});
+
+export type BudgetSpendEntryDoc = z.infer<typeof BudgetSpendEntrySchema> & {
+  _ts?: number;
+  _etag?: string;
+};
+
+export const BudgetAlertSchema = z.object({
+  id: z.string().min(1),
+  productId: z.string().min(1),
+  policyId: z.string().min(1),
+  scopeType: BudgetScopeTypeSchema,
+  scopeId: z.string().min(1),
+  severity: BudgetAlertSeveritySchema,
+  message: z.string().min(1),
+  spendUsd: z.number().min(0),
+  budgetUsd: z.number().min(0),
+  percentUsed: z.number().min(0),
+  createdAt: z.string(),
+});
+
+export type BudgetAlertDoc = z.infer<typeof BudgetAlertSchema> & {
+  _ts?: number;
+  _etag?: string;
+};
+
+export const CreateBudgetPolicySchema = z.object({
+  scopeType: BudgetScopeTypeSchema,
+  scopeId: z.string().min(1),
+  name: z.string().min(1),
+  period: BudgetPeriodSchema.default('monthly'),
+  budgetUsd: z.number().min(0.01),
+  softThreshold: z.number().min(0).max(1).default(0.8),
+  hardThreshold: z.number().min(0).max(1).default(1),
+  modelAllowlist: z.array(z.string()).default([]),
+  metadata: z.record(z.unknown()).optional(),
+});
+
+export const UpdateBudgetPolicySchema = z.object({
+  name: z.string().min(1).optional(),
+  status: BudgetPolicyStatusSchema.optional(),
+  period: BudgetPeriodSchema.optional(),
+  budgetUsd: z.number().min(0.01).optional(),
+  softThreshold: z.number().min(0).max(1).optional(),
+  hardThreshold: z.number().min(0).max(1).optional(),
+  modelAllowlist: z.array(z.string()).optional(),
+  metadata: z.record(z.unknown()).optional(),
+});
+
+export const RecordBudgetSpendSchema = z.object({
+  scopeType: BudgetScopeTypeSchema,
+  scopeId: z.string().min(1),
+  policyId: z.string().min(1).optional(),
+  agentId: z.string().optional(),
+  agentVersionId: z.string().optional(),
+  runId: z.string().optional(),
+  evaluationRunId: z.string().optional(),
+  model: z.string().optional(),
+  tokensUsed: z.number().int().min(0).default(0),
+  costUsd: z.number().min(0),
+  source: z.string().optional(),
+});
+
+export const ListBudgetPoliciesQuerySchema = z.object({
+  scopeType: BudgetScopeTypeSchema.optional(),
+  scopeId: z.string().min(1).optional(),
+  status: BudgetPolicyStatusSchema.optional(),
+  limit: z.coerce.number().min(1).max(100).default(20),
+});
+
+export const ListBudgetAlertsQuerySchema = z.object({
+  severity: BudgetAlertSeveritySchema.optional(),
+  limit: z.coerce.number().min(1).max(100).default(50),
+});
+
+export type ListBudgetPoliciesQuery = z.infer<typeof ListBudgetPoliciesQuerySchema>;
+export type ListBudgetAlertsQuery = z.infer<typeof ListBudgetAlertsQuerySchema>;
diff --git a/services/platform-service/src/server.ts b/services/platform-service/src/server.ts
index 0d5fbb72..5d516bf7 100644
--- a/services/platform-service/src/server.ts
+++ b/services/platform-service/src/server.ts
@@ -37,6 +37,7 @@ import { magicLinkRoutes } from './modules/auth/magic-link/routes.js';
 import { auditRoutes } from './modules/audit/routes.js';
 import { agentRoutes } from './modules/agents/routes.js';
 import { agentEvalRoutes } from './modules/agent-evals/routes.js';
+import { aiBudgetRoutes } from './modules/ai-budgets/routes.js';
 import { notificationRoutes } from './modules/notifications/routes.js';
 import { flagRoutes } from './modules/flags/routes.js';
 import { rateLimitRoutes } from './modules/ratelimit/routes.js';
@@ -143,6 +144,7 @@ await app.register(magicLinkRoutes, { prefix: '/api' });
 await app.register(auditRoutes, { prefix: '/api' });
 await app.register(agentRoutes, { prefix: '/api' });
 await app.register(agentEvalRoutes, { prefix: '/api' });
+await app.register(aiBudgetRoutes, { prefix: '/api' });
 await app.register(notificationRoutes, { prefix: '/api' });
 await app.register(flagRoutes, { prefix: '/api' });
 await app.register(rateLimitRoutes, { prefix: '/api' });