diff --git a/common/.eslintrc.js b/common/.eslintrc.js
index 6e7b62cd..e03e5ef5 100644
--- a/common/.eslintrc.js
+++ b/common/.eslintrc.js
@@ -21,6 +21,8 @@ module.exports = {
     },
   ],
   rules: {
+    'no-extra-semi': 'off',
+    'no-unused-vars': 'off',
     'no-constant-condition': ['error', { checkLoops: false }],
     'lodash/import-scope': [2, 'member'],
   },
diff --git a/common/contract.ts b/common/contract.ts
index a8f2bd42..e9b768ea 100644
--- a/common/contract.ts
+++ b/common/contract.ts
@@ -31,8 +31,6 @@ export type FullContract<
 
   closeEmailsSent?: number
 
-  manaLimitPerUser?: number
-
   volume: number
   volume24Hours: number
   volume7Days: number
@@ -97,8 +95,12 @@ export type Numeric = {
 }
 
 export type outcomeType = 'BINARY' | 'MULTI' | 'FREE_RESPONSE' | 'NUMERIC'
-export const OUTCOME_TYPES = ['BINARY', 'MULTI', 'FREE_RESPONSE', 'NUMERIC']
-
+export const OUTCOME_TYPES = [
+  'BINARY',
+  'MULTI',
+  'FREE_RESPONSE',
+  'NUMERIC',
+] as const
 export const MAX_QUESTION_LENGTH = 480
 export const MAX_DESCRIPTION_LENGTH = 10000
 export const MAX_TAG_LENGTH = 60
diff --git a/common/new-bet.ts b/common/new-bet.ts
index b640c0b5..1053ec58 100644
--- a/common/new-bet.ts
+++ b/common/new-bet.ts
@@ -18,18 +18,25 @@ import {
   Multi,
   NumericContract,
 } from './contract'
-import { User } from './user'
 import { noFees } from './fees'
 import { addObjects } from './util/object'
 import { NUMERIC_FIXED_VAR } from './numeric-constants'
 
+export type CandidateBet<T extends Bet> = Omit<T, 'id' | 'userId'>
+export type BetInfo = {
+  newBet: CandidateBet<Bet>
+  newPool?: { [outcome: string]: number }
+  newTotalShares?: { [outcome: string]: number }
+  newTotalBets?: { [outcome: string]: number }
+  newTotalLiquidity?: number
+  newP?: number
+}
+
 export const getNewBinaryCpmmBetInfo = (
-  user: User,
   outcome: 'YES' | 'NO',
   amount: number,
   contract: FullContract<CPMM, Binary>,
-  loanAmount: number,
-  newBetId: string
+  loanAmount: number
 ) => {
   const { shares, newPool, newP, fees } = calculateCpmmPurchase(
     contract,
@@ -37,15 +44,11 @@ export const getNewBinaryCpmmBetInfo = (
     outcome
   )
 
-  const newBalance = user.balance - (amount - loanAmount)
-
   const { pool, p, totalLiquidity } = contract
   const probBefore = getCpmmProbability(pool, p)
   const probAfter = getCpmmProbability(newPool, newP)
 
-  const newBet: Bet = {
-    id: newBetId,
-    userId: user.id,
+  const newBet: CandidateBet<Bet> = {
     contractId: contract.id,
     amount,
     shares,
@@ -60,16 +63,14 @@ export const getNewBinaryCpmmBetInfo = (
   const { liquidityFee } = fees
   const newTotalLiquidity = (totalLiquidity ?? 0) + liquidityFee
 
-  return { newBet, newPool, newP, newBalance, newTotalLiquidity, fees }
+  return { newBet, newPool, newP, newTotalLiquidity }
 }
 
 export const getNewBinaryDpmBetInfo = (
-  user: User,
   outcome: 'YES' | 'NO',
   amount: number,
   contract: FullContract<DPM, Binary>,
-  loanAmount: number,
-  newBetId: string
+  loanAmount: number
 ) => {
   const { YES: yesPool, NO: noPool } = contract.pool
 
@@ -97,9 +98,7 @@ export const getNewBinaryDpmBetInfo = (
   const probBefore = getDpmProbability(contract.totalShares)
   const probAfter = getDpmProbability(newTotalShares)
 
-  const newBet: Bet = {
-    id: newBetId,
-    userId: user.id,
+  const newBet: CandidateBet<Bet> = {
     contractId: contract.id,
     amount,
     loanAmount,
@@ -111,18 +110,14 @@ export const getNewBinaryDpmBetInfo = (
     fees: noFees,
   }
 
-  const newBalance = user.balance - (amount - loanAmount)
-
-  return { newBet, newPool, newTotalShares, newTotalBets, newBalance }
+  return { newBet, newPool, newTotalShares, newTotalBets }
 }
 
 export const getNewMultiBetInfo = (
-  user: User,
   outcome: string,
   amount: number,
   contract: FullContract<DPM, Multi | FreeResponse>,
-  loanAmount: number,
-  newBetId: string
+  loanAmount: number
 ) => {
   const { pool, totalShares, totalBets } = contract
 
@@ -140,9 +135,7 @@ export const getNewMultiBetInfo = (
   const probBefore = getDpmOutcomeProbability(totalShares, outcome)
   const probAfter = getDpmOutcomeProbability(newTotalShares, outcome)
 
-  const newBet: Bet = {
-    id: newBetId,
-    userId: user.id,
+  const newBet: CandidateBet<Bet> = {
     contractId: contract.id,
     amount,
     loanAmount,
@@ -154,18 +147,14 @@ export const getNewMultiBetInfo = (
     fees: noFees,
   }
 
-  const newBalance = user.balance - (amount - loanAmount)
-
-  return { newBet, newPool, newTotalShares, newTotalBets, newBalance }
+  return { newBet, newPool, newTotalShares, newTotalBets }
 }
 
 export const getNumericBetsInfo = (
-  user: User,
   value: number,
   outcome: string,
   amount: number,
-  contract: NumericContract,
-  newBetId: string
+  contract: NumericContract
 ) => {
   const { pool, totalShares, totalBets } = contract
 
@@ -187,9 +176,7 @@ export const getNumericBetsInfo = (
   const probBefore = getDpmOutcomeProbability(totalShares, outcome)
   const probAfter = getDpmOutcomeProbability(newTotalShares, outcome)
 
-  const newBet: NumericBet = {
-    id: newBetId,
-    userId: user.id,
+  const newBet: CandidateBet<NumericBet> = {
     contractId: contract.id,
     value,
     amount,
@@ -203,9 +190,7 @@ export const getNumericBetsInfo = (
     fees: noFees,
   }
 
-  const newBalance = user.balance - amount
-
-  return { newBet, newPool, newTotalShares, newTotalBets, newBalance }
+  return { newBet, newPool, newTotalShares, newTotalBets }
 }
 
 export const getLoanAmount = (yourBets: Bet[], newBetAmount: number) => {
diff --git a/common/new-contract.ts b/common/new-contract.ts
index b70dee37..0b7d294a 100644
--- a/common/new-contract.ts
+++ b/common/new-contract.ts
@@ -27,8 +27,7 @@ export function getNewContract(
   // used for numeric markets
   bucketCount: number,
   min: number,
-  max: number,
-  manaLimitPerUser: number
+  max: number
 ) {
   const tags = parseTags(
     `${question} ${description} ${extraTags.map((tag) => `#${tag}`).join(' ')}`
@@ -70,7 +69,6 @@ export function getNewContract(
       liquidityFee: 0,
       platformFee: 0,
     },
-    manaLimitPerUser,
   })
 
   return contract as Contract
diff --git a/functions/.eslintrc.js b/functions/.eslintrc.js
index c5b8e16f..5d66d2c6 100644
--- a/functions/.eslintrc.js
+++ b/functions/.eslintrc.js
@@ -17,6 +17,7 @@ module.exports = {
     },
   ],
   rules: {
+    'no-extra-semi': 'off',
     'no-unused-vars': 'off',
     'no-constant-condition': ['error', { checkLoops: false }],
     'lodash/import-scope': [2, 'member'],
diff --git a/functions/package.json b/functions/package.json
index d3ee68e2..49352969 100644
--- a/functions/package.json
+++ b/functions/package.json
@@ -28,7 +28,8 @@
     "mailgun-js": "0.22.0",
     "module-alias": "2.2.2",
     "react-query": "3.39.0",
-    "stripe": "8.194.0"
+    "stripe": "8.194.0",
+    "zod": "3.17.2"
   },
   "devDependencies": {
     "@types/mailgun-js": "0.22.12",
diff --git a/functions/src/api.ts b/functions/src/api.ts
index fa3a9aa6..31f14257 100644
--- a/functions/src/api.ts
+++ b/functions/src/api.ts
@@ -1,6 +1,7 @@
 import * as admin from 'firebase-admin'
 import * as functions from 'firebase-functions'
 import * as Cors from 'cors'
+import { z } from 'zod'
 
 import { User, PrivateUser } from '../../common/user'
 import {
@@ -8,10 +9,11 @@ import {
   CORS_ORIGIN_LOCALHOST,
 } from '../../common/envs/constants'
 
+type Output = Record<string, unknown>
 type Request = functions.https.Request
 type Response = functions.Response
-type Handler = (req: Request, res: Response) => Promise<any>
 type AuthedUser = [User, PrivateUser]
+type Handler = (req: Request, user: AuthedUser) => Promise<Output>
 type JwtCredentials = { kind: 'jwt'; data: admin.auth.DecodedIdToken }
 type KeyCredentials = { kind: 'key'; data: string }
 type Credentials = JwtCredentials | KeyCredentials
@@ -19,10 +21,13 @@ type Credentials = JwtCredentials | KeyCredentials
 export class APIError {
   code: number
   msg: string
-  constructor(code: number, msg: string) {
+  details: unknown
+  constructor(code: number, msg: string, details?: unknown) {
     this.code = code
     this.msg = msg
+    this.details = details
   }
+  toJson() {}
 }
 
 export const parseCredentials = async (req: Request): Promise<Credentials> => {
@@ -40,14 +45,11 @@ export const parseCredentials = async (req: Request): Promise<Credentials> => {
     case 'Bearer':
       try {
         const jwt = await admin.auth().verifyIdToken(payload)
-        if (!jwt.user_id) {
-          throw new APIError(403, 'JWT must contain Manifold user ID.')
-        }
         return { kind: 'jwt', data: jwt }
       } catch (err) {
         // This is somewhat suspicious, so get it into the firebase console
         functions.logger.error('Error verifying Firebase JWT: ', err)
-        throw new APIError(403, `Error validating token: ${err}.`)
+        throw new APIError(403, 'Error validating token.')
       }
     case 'Key':
       return { kind: 'key', data: payload }
@@ -63,6 +65,9 @@ export const lookupUser = async (creds: Credentials): Promise<AuthedUser> => {
   switch (creds.kind) {
     case 'jwt': {
       const { user_id } = creds.data
+      if (typeof user_id !== 'string') {
+        throw new APIError(403, 'JWT must contain Manifold user ID.')
+      }
       const [userSnap, privateUserSnap] = await Promise.all([
         users.doc(user_id).get(),
         privateUsers.doc(user_id).get(),
@@ -109,6 +114,27 @@ export const applyCors = (
   })
 }
 
+export const zTimestamp = () => {
+  return z.preprocess((arg) => {
+    return typeof arg == 'number' ? new Date(arg) : undefined
+  }, z.date())
+}
+
+export const validate = <T extends z.ZodTypeAny>(schema: T, val: unknown) => {
+  const result = schema.safeParse(val)
+  if (!result.success) {
+    const issues = result.error.issues.map((i) => {
+      return {
+        field: i.path.join('.') || null,
+        error: i.message,
+      }
+    })
+    throw new APIError(400, 'Error validating request.', issues)
+  } else {
+    return result.data as z.infer<T>
+  }
+}
+
 export const newEndpoint = (methods: [string], fn: Handler) =>
   functions.runWith({ minInstances: 1 }).https.onRequest(async (req, res) => {
     await applyCors(req, res, {
@@ -120,12 +146,17 @@ export const newEndpoint = (methods: [string], fn: Handler) =>
         const allowed = methods.join(', ')
         throw new APIError(405, `This endpoint supports only ${allowed}.`)
       }
-      res.status(200).json(await fn(req, res))
+      const authedUser = await lookupUser(await parseCredentials(req))
+      res.status(200).json(await fn(req, authedUser))
     } catch (e) {
       if (e instanceof APIError) {
-        // Emit a 200 anyway here for now, for backwards compatibility
-        res.status(e.code).json({ message: e.msg })
+        const output: { [k: string]: unknown } = { message: e.msg }
+        if (e.details != null) {
+          output.details = e.details
+        }
+        res.status(e.code).json(output)
       } else {
+        functions.logger.error(e)
         res.status(500).json({ message: 'An unknown error occurred.' })
       }
     }
diff --git a/functions/src/create-answer.ts b/functions/src/create-answer.ts
index bc74dc05..7fbe5416 100644
--- a/functions/src/create-answer.ts
+++ b/functions/src/create-answer.ts
@@ -12,7 +12,6 @@ import { getNewMultiBetInfo } from '../../common/new-bet'
 import { Answer, MAX_ANSWER_LENGTH } from '../../common/answer'
 import { getContract, getValues } from './utils'
 import { sendNewAnswerEmail } from './emails'
-import { Bet } from '../../common/bet'
 
 export const createAnswer = functions.runWith({ minInstances: 1 }).https.onCall(
   async (
@@ -61,11 +60,6 @@ export const createAnswer = functions.runWith({ minInstances: 1 }).https.onCall(
       if (closeTime && Date.now() > closeTime)
         return { status: 'error', message: 'Trading is closed' }
 
-      const yourBetsSnap = await transaction.get(
-        contractDoc.collection('bets').where('userId', '==', userId)
-      )
-      const yourBets = yourBetsSnap.docs.map((doc) => doc.data() as Bet)
-
       const [lastAnswer] = await getValues<Answer>(
         firestore
           .collection(`contracts/${contractId}/answers`)
@@ -99,23 +93,20 @@ export const createAnswer = functions.runWith({ minInstances: 1 }).https.onCall(
       }
       transaction.create(newAnswerDoc, answer)
 
-      const newBetDoc = firestore
-        .collection(`contracts/${contractId}/bets`)
-        .doc()
+      const loanAmount = 0
 
-      const loanAmount = 0 // getLoanAmount(yourBets, amount)
-
-      const { newBet, newPool, newTotalShares, newTotalBets, newBalance } =
+      const { newBet, newPool, newTotalShares, newTotalBets } =
         getNewMultiBetInfo(
-          user,
           answerId,
           amount,
           contract as FullContract<DPM, FreeResponse>,
-          loanAmount,
-          newBetDoc.id
+          loanAmount
         )
 
-      transaction.create(newBetDoc, newBet)
+      const newBalance = user.balance - amount
+      const betDoc = firestore.collection(`contracts/${contractId}/bets`).doc()
+      transaction.create(betDoc, { id: betDoc.id, userId: user.id, ...newBet })
+      transaction.update(userDoc, { balance: newBalance })
       transaction.update(contractDoc, {
         pool: newPool,
         totalShares: newTotalShares,
@@ -124,13 +115,7 @@ export const createAnswer = functions.runWith({ minInstances: 1 }).https.onCall(
         volume: volume + amount,
       })
 
-      if (!isFinite(newBalance)) {
-        throw new Error('Invalid user balance for ' + user.username)
-      }
-
-      transaction.update(userDoc, { balance: newBalance })
-
-      return { status: 'success', answerId, betId: newBetDoc.id, answer }
+      return { status: 'success', answerId, betId: betDoc.id, answer }
     })
 
     const { answer } = result
diff --git a/functions/src/create-contract.ts b/functions/src/create-contract.ts
index 952d396c..42a1c376 100644
--- a/functions/src/create-contract.ts
+++ b/functions/src/create-contract.ts
@@ -1,4 +1,5 @@
 import * as admin from 'firebase-admin'
+import { z } from 'zod'
 
 import {
   Binary,
@@ -17,7 +18,7 @@ import { slugify } from '../../common/util/slugify'
 import { randomString } from '../../common/util/random'
 
 import { chargeUser } from './utils'
-import { APIError, newEndpoint, parseCredentials, lookupUser } from './api'
+import { APIError, newEndpoint, validate, zTimestamp } from './api'
 
 import {
   FIXED_ANTE,
@@ -26,66 +27,45 @@ import {
   getFreeAnswerAnte,
   getNumericAnte,
   HOUSE_LIQUIDITY_PROVIDER_ID,
-  MINIMUM_ANTE,
 } from '../../common/antes'
 import { getNoneAnswer } from '../../common/answer'
 import { getNewContract } from '../../common/new-contract'
 import { NUMERIC_BUCKET_COUNT } from '../../common/numeric-constants'
 
-export const createContract = newEndpoint(['POST'], async (req, _res) => {
-  const [creator, _privateUser] = await lookupUser(await parseCredentials(req))
-  let {
-    question,
-    outcomeType,
-    description,
-    initialProb,
-    closeTime,
-    tags,
-    min,
-    max,
-    manaLimitPerUser,
-  } = req.body || {}
+const bodySchema = z.object({
+  question: z.string().min(1).max(MAX_QUESTION_LENGTH),
+  description: z.string().max(MAX_DESCRIPTION_LENGTH),
+  tags: z.array(z.string().min(1).max(MAX_TAG_LENGTH)).optional(),
+  closeTime: zTimestamp().refine(
+    (date) => date.getTime() > new Date().getTime(),
+    'Close time must be in the future.'
+  ),
+  outcomeType: z.enum(OUTCOME_TYPES),
+})
 
-  if (!question || typeof question != 'string')
-    throw new APIError(400, 'Missing or invalid question field')
+const binarySchema = z.object({
+  initialProb: z.number().min(1).max(99),
+})
 
-  question = question.slice(0, MAX_QUESTION_LENGTH)
+const numericSchema = z.object({
+  min: z.number(),
+  max: z.number(),
+})
 
-  if (typeof description !== 'string')
-    throw new APIError(400, 'Invalid description field')
-
-  description = description.slice(0, MAX_DESCRIPTION_LENGTH)
-
-  if (tags !== undefined && !Array.isArray(tags))
-    throw new APIError(400, 'Invalid tags field')
-
-  tags = (tags || []).map((tag: string) =>
-    tag.toString().slice(0, MAX_TAG_LENGTH)
+export const createContract = newEndpoint(['POST'], async (req, [user, _]) => {
+  const { question, description, tags, closeTime, outcomeType } = validate(
+    bodySchema,
+    req.body
   )
 
-  outcomeType = outcomeType ?? 'BINARY'
-
-  if (!OUTCOME_TYPES.includes(outcomeType))
-    throw new APIError(400, 'Invalid outcomeType')
-
-  if (
-    outcomeType === 'NUMERIC' &&
-    !(
-      min !== undefined &&
-      max !== undefined &&
-      isFinite(min) &&
-      isFinite(max) &&
-      min < max &&
-      max - min > 0.01
-    )
-  )
-    throw new APIError(400, 'Invalid range')
-
-  if (
-    outcomeType === 'BINARY' &&
-    (!initialProb || initialProb < 1 || initialProb > 99)
-  )
-    throw new APIError(400, 'Invalid initial probability')
+  let min, max, initialProb
+  if (outcomeType === 'NUMERIC') {
+    ;({ min, max } = validate(numericSchema, req.body))
+    if (max - min <= 0.01) throw new APIError(400, 'Invalid range.')
+  }
+  if (outcomeType === 'BINARY') {
+    ;({ initialProb } = validate(binarySchema, req.body))
+  }
 
   // Uses utc time on server:
   const today = new Date()
@@ -96,7 +76,7 @@ export const createContract = newEndpoint(['POST'], async (req, _res) => {
 
   const userContractsCreatedTodaySnapshot = await firestore
     .collection(`contracts`)
-    .where('creatorId', '==', creator.id)
+    .where('creatorId', '==', user.id)
     .where('createdTime', '>=', freeMarketResetTime)
     .get()
   console.log('free market reset time: ', freeMarketResetTime)
@@ -104,18 +84,9 @@ export const createContract = newEndpoint(['POST'], async (req, _res) => {
 
   const ante = FIXED_ANTE
 
-  if (
-    ante === undefined ||
-    ante < MINIMUM_ANTE ||
-    (ante > creator.balance && !isFree) ||
-    isNaN(ante) ||
-    !isFinite(ante)
-  )
-    throw new APIError(400, 'Invalid ante')
-
   console.log(
     'creating contract for',
-    creator.username,
+    user.username,
     'on',
     question,
     'ante:',
@@ -123,31 +94,28 @@ export const createContract = newEndpoint(['POST'], async (req, _res) => {
   )
 
   const slug = await getSlug(question)
-
   const contractRef = firestore.collection('contracts').doc()
-
   const contract = getNewContract(
     contractRef.id,
     slug,
-    creator,
+    user,
     question,
     outcomeType,
     description,
-    initialProb,
+    initialProb ?? 0,
     ante,
-    closeTime,
+    closeTime.getTime(),
     tags ?? [],
     NUMERIC_BUCKET_COUNT,
     min ?? 0,
-    max ?? 0,
-    manaLimitPerUser ?? 0
+    max ?? 0
   )
 
-  if (!isFree && ante) await chargeUser(creator.id, ante, true)
+  if (!isFree && ante) await chargeUser(user.id, ante, true)
 
   await contractRef.create(contract)
 
-  const providerId = isFree ? HOUSE_LIQUIDITY_PROVIDER_ID : creator.id
+  const providerId = isFree ? HOUSE_LIQUIDITY_PROVIDER_ID : user.id
 
   if (outcomeType === 'BINARY' && contract.mechanism === 'dpm-2') {
     const yesBetDoc = firestore
@@ -157,7 +125,7 @@ export const createContract = newEndpoint(['POST'], async (req, _res) => {
     const noBetDoc = firestore.collection(`contracts/${contract.id}/bets`).doc()
 
     const { yesBet, noBet } = getAnteBets(
-      creator,
+      user,
       contract as FullContract<DPM, Binary>,
       yesBetDoc.id,
       noBetDoc.id
@@ -183,7 +151,7 @@ export const createContract = newEndpoint(['POST'], async (req, _res) => {
       .collection(`contracts/${contract.id}/answers`)
       .doc('0')
 
-    const noneAnswer = getNoneAnswer(contract.id, creator)
+    const noneAnswer = getNoneAnswer(contract.id, user)
     await noneAnswerDoc.set(noneAnswer)
 
     const anteBetDoc = firestore
@@ -202,7 +170,7 @@ export const createContract = newEndpoint(['POST'], async (req, _res) => {
       .doc()
 
     const anteBet = getNumericAnte(
-      creator,
+      user,
       contract as FullContract<DPM, Numeric>,
       ante,
       anteBetDoc.id
diff --git a/functions/src/place-bet.ts b/functions/src/place-bet.ts
index d97effe4..184ee2df 100644
--- a/functions/src/place-bet.ts
+++ b/functions/src/place-bet.ts
@@ -1,146 +1,112 @@
 import * as admin from 'firebase-admin'
+import { z } from 'zod'
 
-import { APIError, newEndpoint, parseCredentials, lookupUser } from './api'
+import { APIError, newEndpoint, validate } from './api'
 import { Contract } from '../../common/contract'
 import { User } from '../../common/user'
 import {
+  BetInfo,
   getNewBinaryCpmmBetInfo,
   getNewBinaryDpmBetInfo,
   getNewMultiBetInfo,
   getNumericBetsInfo,
 } from '../../common/new-bet'
 import { addObjects, removeUndefinedProps } from '../../common/util/object'
-import { Bet } from '../../common/bet'
 import { redeemShares } from './redeem-shares'
-import { Fees } from '../../common/fees'
 
-export const placeBet = newEndpoint(['POST'], async (req, _res) => {
-  const [bettor, _privateUser] = await lookupUser(await parseCredentials(req))
-  const { amount, outcome, contractId, value } = req.body || {}
+const bodySchema = z.object({
+  contractId: z.string(),
+  amount: z.number().gte(1),
+})
 
-  if (amount < 1 || isNaN(amount) || !isFinite(amount))
-    throw new APIError(400, 'Invalid amount')
+const binarySchema = z.object({
+  outcome: z.enum(['YES', 'NO']),
+})
 
-  if (outcome !== 'YES' && outcome !== 'NO' && isNaN(+outcome))
-    throw new APIError(400, 'Invalid outcome')
+const freeResponseSchema = z.object({
+  outcome: z.string(),
+})
 
-  if (value !== undefined && !isFinite(value))
-    throw new APIError(400, 'Invalid value')
+const numericSchema = z.object({
+  outcome: z.string(),
+  value: z.number(),
+})
 
-  // run as transaction to prevent race conditions
-  return await firestore
-    .runTransaction(async (transaction) => {
-      const userDoc = firestore.doc(`users/${bettor.id}`)
-      const userSnap = await transaction.get(userDoc)
-      if (!userSnap.exists) throw new APIError(400, 'User not found')
-      const user = userSnap.data() as User
+export const placeBet = newEndpoint(['POST'], async (req, [bettor, _]) => {
+  const { amount, contractId } = validate(bodySchema, req.body)
 
-      const contractDoc = firestore.doc(`contracts/${contractId}`)
-      const contractSnap = await transaction.get(contractDoc)
-      if (!contractSnap.exists) throw new APIError(400, 'Invalid contract')
-      const contract = contractSnap.data() as Contract
+  const result = await firestore.runTransaction(async (trans) => {
+    const userDoc = firestore.doc(`users/${bettor.id}`)
+    const userSnap = await trans.get(userDoc)
+    if (!userSnap.exists) throw new APIError(400, 'User not found.')
+    const user = userSnap.data() as User
+    if (user.balance < amount) throw new APIError(400, 'Insufficient balance.')
 
-      const { closeTime, outcomeType, mechanism, collectedFees, volume } =
-        contract
-      if (closeTime && Date.now() > closeTime)
-        throw new APIError(400, 'Trading is closed')
+    const contractDoc = firestore.doc(`contracts/${contractId}`)
+    const contractSnap = await trans.get(contractDoc)
+    if (!contractSnap.exists) throw new APIError(400, 'Contract not found.')
+    const contract = contractSnap.data() as Contract
 
-      const yourBetsSnap = await transaction.get(
-        contractDoc.collection('bets').where('userId', '==', bettor.id)
-      )
-      const yourBets = yourBetsSnap.docs.map((doc) => doc.data() as Bet)
+    const loanAmount = 0
+    const { closeTime, outcomeType, mechanism, collectedFees, volume } =
+      contract
+    if (closeTime && Date.now() > closeTime)
+      throw new APIError(400, 'Trading is closed.')
 
-      const loanAmount = 0 // getLoanAmount(yourBets, amount)
-      if (user.balance < amount) throw new APIError(400, 'Insufficient balance')
-
-      if (outcomeType === 'FREE_RESPONSE') {
-        const answerSnap = await transaction.get(
-          contractDoc.collection('answers').doc(outcome)
-        )
-        if (!answerSnap.exists) throw new APIError(400, 'Invalid contract')
+    const {
+      newBet,
+      newPool,
+      newTotalShares,
+      newTotalBets,
+      newTotalLiquidity,
+      newP,
+    } = await (async (): Promise<BetInfo> => {
+      if (outcomeType == 'BINARY' && mechanism == 'dpm-2') {
+        const { outcome } = validate(binarySchema, req.body)
+        return getNewBinaryDpmBetInfo(outcome, amount, contract, loanAmount)
+      } else if (outcomeType == 'BINARY' && mechanism == 'cpmm-1') {
+        const { outcome } = validate(binarySchema, req.body)
+        return getNewBinaryCpmmBetInfo(outcome, amount, contract, loanAmount)
+      } else if (outcomeType == 'FREE_RESPONSE' && mechanism == 'dpm-2') {
+        const { outcome } = validate(freeResponseSchema, req.body)
+        const answerDoc = contractDoc.collection('answers').doc(outcome)
+        const answerSnap = await trans.get(answerDoc)
+        if (!answerSnap.exists) throw new APIError(400, 'Invalid answer')
+        return getNewMultiBetInfo(outcome, amount, contract, loanAmount)
+      } else if (outcomeType == 'NUMERIC' && mechanism == 'dpm-2') {
+        const { outcome, value } = validate(numericSchema, req.body)
+        return getNumericBetsInfo(value, outcome, amount, contract)
+      } else {
+        throw new APIError(500, 'Contract has invalid type/mechanism.')
       }
+    })()
 
-      const newBetDoc = firestore
-        .collection(`contracts/${contractId}/bets`)
-        .doc()
+    if (newP != null && !isFinite(newP)) {
+      throw new APIError(400, 'Trade rejected due to overflow error.')
+    }
 
-      const {
-        newBet,
-        newPool,
-        newTotalShares,
-        newTotalBets,
-        newBalance,
-        newTotalLiquidity,
-        fees,
-        newP,
-      } =
-        outcomeType === 'BINARY'
-          ? mechanism === 'dpm-2'
-            ? getNewBinaryDpmBetInfo(
-                user,
-                outcome as 'YES' | 'NO',
-                amount,
-                contract,
-                loanAmount,
-                newBetDoc.id
-              )
-            : (getNewBinaryCpmmBetInfo(
-                user,
-                outcome as 'YES' | 'NO',
-                amount,
-                contract,
-                loanAmount,
-                newBetDoc.id
-              ) as any)
-          : outcomeType === 'NUMERIC' && mechanism === 'dpm-2'
-          ? getNumericBetsInfo(
-              user,
-              value,
-              outcome,
-              amount,
-              contract,
-              newBetDoc.id
-            )
-          : getNewMultiBetInfo(
-              user,
-              outcome,
-              amount,
-              contract as any,
-              loanAmount,
-              newBetDoc.id
-            )
+    const newBalance = user.balance - amount - loanAmount
+    const betDoc = contractDoc.collection('bets').doc()
+    trans.create(betDoc, { id: betDoc.id, userId: user.id, ...newBet })
+    trans.update(userDoc, { balance: newBalance })
+    trans.update(
+      contractDoc,
+      removeUndefinedProps({
+        pool: newPool,
+        p: newP,
+        totalShares: newTotalShares,
+        totalBets: newTotalBets,
+        totalLiquidity: newTotalLiquidity,
+        collectedFees: addObjects(newBet.fees, collectedFees),
+        volume: volume + amount,
+      })
+    )
 
-      if (newP !== undefined && !isFinite(newP)) {
-        throw new APIError(400, 'Trade rejected due to overflow error.')
-      }
+    return { betId: betDoc.id }
+  })
 
-      transaction.create(newBetDoc, newBet)
-
-      transaction.update(
-        contractDoc,
-        removeUndefinedProps({
-          pool: newPool,
-          p: newP,
-          totalShares: newTotalShares,
-          totalBets: newTotalBets,
-          totalLiquidity: newTotalLiquidity,
-          collectedFees: addObjects<Fees>(fees ?? {}, collectedFees ?? {}),
-          volume: volume + Math.abs(amount),
-        })
-      )
-
-      if (!isFinite(newBalance)) {
-        throw new APIError(500, 'Invalid user balance for ' + user.username)
-      }
-
-      transaction.update(userDoc, { balance: newBalance })
-
-      return { betId: newBetDoc.id }
-    })
-    .then(async (result) => {
-      await redeemShares(bettor.id, contractId)
-      return result
-    })
+  await redeemShares(bettor.id, contractId)
+  return result
 })
 
 const firestore = admin.firestore()
diff --git a/web/components/numeric-bet-panel.tsx b/web/components/numeric-bet-panel.tsx
index f249e3c3..ebb80dd0 100644
--- a/web/components/numeric-bet-panel.tsx
+++ b/web/components/numeric-bet-panel.tsx
@@ -102,12 +102,10 @@ function NumericBuyPanel(props: {
   const betDisabled = isSubmitting || !betAmount || !bucketChoice || error
 
   const { newBet, newPool, newTotalShares, newTotalBets } = getNumericBetsInfo(
-    { id: 'dummy', balance: 0 } as User, // a little hackish
     value ?? 0,
     bucketChoice ?? 'NaN',
     betAmount ?? 0,
-    contract,
-    'dummy id'
+    contract
   )
 
   const { probAfter: outcomeProb, shares } = newBet
diff --git a/web/components/tags-input.tsx b/web/components/tags-input.tsx
index ff9029c9..6a0cdcda 100644
--- a/web/components/tags-input.tsx
+++ b/web/components/tags-input.tsx
@@ -5,6 +5,7 @@ import { Contract, updateContract } from 'web/lib/firebase/contracts'
 import { Col } from './layout/col'
 import { Row } from './layout/row'
 import { TagsList } from './tags-list'
+import { MAX_TAG_LENGTH } from 'common/contract'
 
 export function TagsInput(props: { contract: Contract; className?: string }) {
   const { contract, className } = props
@@ -36,6 +37,7 @@ export function TagsInput(props: { contract: Contract; className?: string }) {
           className="input input-sm input-bordered resize-none"
           disabled={isSubmitting}
           value={tagText}
+          maxLength={MAX_TAG_LENGTH}
           onChange={(e) => setTagText(e.target.value || '')}
           onKeyDown={(e) => {
             if (e.key === 'Enter' && !e.shiftKey) {
diff --git a/web/pages/create.tsx b/web/pages/create.tsx
index bf035330..342eafeb 100644
--- a/web/pages/create.tsx
+++ b/web/pages/create.tsx
@@ -11,7 +11,11 @@ import { FIXED_ANTE, MINIMUM_ANTE } from 'common/antes'
 import { InfoTooltip } from 'web/components/info-tooltip'
 import { Page } from 'web/components/page'
 import { Row } from 'web/components/layout/row'
-import { MAX_DESCRIPTION_LENGTH, outcomeType } from 'common/contract'
+import {
+  MAX_DESCRIPTION_LENGTH,
+  MAX_QUESTION_LENGTH,
+  outcomeType,
+} from 'common/contract'
 import { formatMoney } from 'common/util/format'
 import { useHasCreatedContractToday } from 'web/hooks/use-has-created-contract-today'
 import { removeUndefinedProps } from 'common/util/object'
@@ -37,6 +41,7 @@ export default function Create() {
                 placeholder="e.g. Will the Democrats win the 2024 US presidential election?"
                 className="input input-bordered resize-none"
                 autoFocus
+                maxLength={MAX_QUESTION_LENGTH}
                 value={question}
                 onChange={(e) => setQuestion(e.target.value || '')}
               />
diff --git a/yarn.lock b/yarn.lock
index 7eecfe4d..982cd49a 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -5410,3 +5410,8 @@ yocto-queue@^0.1.0:
   version "0.1.0"
   resolved "https://registry.yarnpkg.com/yocto-queue/-/yocto-queue-0.1.0.tgz#0294eb3dee05028d31ee1a5fa2c556a6aaf10a1b"
   integrity sha512-rVksvsnNCdJ/ohGc6xgPwyN8eheCxsiLM8mxuE/t/mOVqJewPuO1miLpTHQiRgTKCLexL4MeAFVagts7HmNZ2Q==
+
+zod@3.17.2:
+  version "3.17.2"
+  resolved "https://registry.yarnpkg.com/zod/-/zod-3.17.2.tgz#d20b32146a3b5068f8f71768b4f9a4bfe52cddb0"
+  integrity sha512-L8UPS2J/F3dIA8gsPTvGjd8wSRuwR1Td4AqR2Nw8r8BgcLIbZZ5/tCII7hbTLXTQDhxUnnsFdHwpETGajt5i3A==