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QVeris · Stock Quote APIProvider Comparison

Stock Quote API for AI Agents: 6 Best Providers Compared on Fields, Latency & Free Tiers (2026)

A technical comparison of 6 stock quote API providers — Polygon.io, Alpaca, Finnhub, Twelve Data, Alpha Vantage, and Marketstack — benchmarked on quote fields, latency, free tier limits, and AI agent integration patterns.

6
Providers Compared
8
Quote Fields Benchmarked
10,000+
Unified Capabilities
97M+
MCP SDK Downloads
✓ Verified Data
TL;DR
Problem: Stock quote APIs differ significantly on which fields they return — bid/ask spread, VWAP, extended hours data, and pre/post-market prices are not universally available. AI agents that depend on missing fields silently degrade.
Solution: Compare 6 stock quote APIs on the 8 fields that matter most for AI agent workflows — then use a unified capability routing layer to abstract provider differences behind a consistent interface.
Result: Your AI agent gets complete quote data without writing provider-specific parsing logic or handling missing fields from different providers.

What is a Stock Quote API for AI Agents?

A stock quote API returns structured price data for publicly traded securities — including last price, bid/ask spread, volume, day high/low, and intraday range. For AI agents, the critical differentiators are which quote fields are available, whether data is real-time or delayed, and whether extended-hours data (pre-market and after-hours) is included.

Unlike a real-time stock price API that focuses on streaming price updates, a stock quote API provides a complete snapshot of the current market state for a given symbol — the full picture of supply (ask), demand (bid), and recent activity (last price, volume, VWAP). This snapshot data is what AI agents need for execution quality assessment, spread analysis, and pre/post-market monitoring.

The challenge for AI agent developers: not all quote APIs return the same fields. Some provide bid/ask spread but not VWAP. Others include pre-market prices but not after-hours. An AI agent that assumes a field is always present will silently degrade when a provider omits it. This guide benchmarks 6 providers on exactly which fields they return — so you know before you integrate.

The 8 Quote Fields That Matter for AI Agents

These eight fields define what a stock quote API can tell your agent. Missing fields are not just gaps — they are capabilities your agent loses.

FieldWhat It IsWhy AI Agents Need It
Last PriceMost recent trade priceCore price signal — the baseline for every agent workflow
Bid PriceHighest buy offer on the order bookBest available exit price; essential for execution quality assessment
Ask PriceLowest sell offer on the order bookBest available entry price; combined with bid = spread signal
VWAPVolume-weighted average price (intraday)Institutional benchmark — tells your agent if a trade was above or below the market average
VolumeTotal shares traded todayMomentum and liquidity confirmation — price moves without volume may be noise
Day High/LowIntraday price rangeRange-based condition triggers — breakout detection, support/resistance proximity
Pre-Market PricePrice before regular market openEarly signal for overnight news, earnings reactions, and gap-up/gap-down detection
After-Hours PricePrice after regular market closeEarnings announcements typically come after close — agents need this to monitor post-release reactions

6 Stock Quote APIs Compared

Quote field availability is the primary differentiator. Latency, free tier generosity, and AI agent SDK support follow.

Stock Quote API Comparison — Quote Fields, Latency, Free Tier, and AI Agent Compatibility. Updated June 2026.
ProviderBid/AskVWAPExtended HoursPre/Post MarketFree TierLatencyStarting Price
Polygon.io✓✓✓✓Delayed only<10ms$29/mo
Alpaca✓✓✓✓Real-time free<100msFree
Finnhub✓✗✗✗300 calls/daySub-secondFree
Twelve Data✓✓✗✗800 req/daySub-secondFree
Alpha Vantage✓✗✗✗25 req/day1-5sFree
Marketstack✗✗✗✗100 req/day EOD15min delayed$9.99/mo
Legend: Genuinely Free — production-usable without payment. Limited Free — usable for development; may require paid for production. Paid Only — no meaningful free tier. Quote field availability verified against official docs as of June 2026.
Stock Quote Field Coverage by Provider — 8 Critical Fields

Provider Deep Dives

1. Polygon.io — Most Complete Quote Fields

Best for: Production AI agents that need every quote field with institutional-grade latency
  • Strengths: All 8 quote fields available, WebSocket real-time streaming, broad multi-asset coverage (stocks, options, forex, crypto), <10ms latency on paid plans
  • Limitations: Free tier is delayed data only — real-time requires $29/mo minimum; no genuine free tier for development
  • AI Agent Fit: Production agents that need complete quote snapshots with sub-10ms latency

2. Alpaca — Best Free Tier for Complete Quotes

Best for: US equity AI agents that need all 8 fields at zero cost
  • Strengths: All 8 quote fields available on the free tier, real-time data with no daily call cap, WebSocket streaming included, Python and JS SDKs
  • Limitations: Primarily US equity coverage — international and multi-asset coverage is limited compared to Polygon.io or Twelve Data
  • AI Agent Fit: The go-to choice for US stock monitoring agents on a budget

3. Finnhub — Simple Quote, Real-Time Free

Best for: Lightweight monitoring agents that only need basic bid/ask/last price
  • Strengths: Real-time bid/ask and last price on the free tier, WebSocket support included, 300 calls/day with 60 calls/min rate limit
  • Limitations: No VWAP — a significant gap for execution-quality agents; no extended hours or pre/post-market data
  • AI Agent Fit: Good for basic price monitoring; insufficient for agents that need execution benchmarks

4. Twelve Data — Best Balance of Fields and Free Tier

Best for: Agents that need VWAP alongside bid/ask on a generous free tier
  • Strengths: Bid/ask and VWAP included on the free tier, 800 requests/day — the highest daily REST limit among free quote APIs, 50+ exchanges covered
  • Limitations: Free tier data is delayed; no extended hours pricing on any tier below paid plans; WebSocket requires paid
  • AI Agent Fit: Best option for agents needing VWAP with a meaningful free daily allowance

5. Alpha Vantage — Accessible but Severely Limited

Best for: Prototyping and learning — not for production AI agents
  • Strengths: Simple REST API with clear documentation, bid/ask included, covers stocks, forex, and crypto in one API key
  • Limitations: Only 25 requests/day — the most restrictive free tier; no VWAP; 15-minute delayed data; no WebSocket support
  • AI Agent Fit: Useful for initial prototyping only. Any automated agent workflow will exhaust the daily quota in minutes

6. Marketstack — EOD Only, Not for Real-Time Agents

Best for: Historical research and end-of-day analysis — not real-time quotes
  • Strengths: Broad global coverage with 80,000+ tickers, 100 requests/day on free tier for EOD data, affordable paid tier at $9.99/month
  • Limitations: No bid/ask on any tier; EOD data only on free tier (15-minute delayed); no VWAP, extended hours, or real-time streaming
  • AI Agent Fit: Suitable for historical analysis agents. Not suitable for any real-time monitoring workflow

Free Tier Comparison for AI Agent Development

ProviderFree Tier TypeDaily LimitReal-Time?Credit Card?Commercial Use?Best For
AlpacaGenuinely FreeNo hard cap (fair use)YesNoDev use freeProduction US equity agents
FinnhubLimited Free300 calls/dayYes (limited)NoRestrictedDevelopment + low-volume prod
Twelve DataLimited Free800 req/dayDelayed (free)NoNon-commercialMulti-asset research agents
Alpha VantageLimited Free25 req/dayDelayed 15minNoNon-commercialPrototyping only
Polygon.ioDelayed OnlyNone (paid)Paid onlyRequiredYes (paid)Production (paid plans)
MarketstackEOD Only100 req/day (EOD)15min delayedRequiredYes (paid)Historical/EOD analysis

Key takeaway: Alpaca is the only provider offering genuinely free real-time quotes with all 8 fields — no daily cap, no credit card, and commercial dev use permitted. For agents that need VWAP on a free tier, Twelve Data provides the best balance (800 req/day, VWAP included, but delayed). For production agents requiring institutional latency and every quote field, Polygon.io is the standard — but it starts at $29/month with no free real-time tier.

Pre-Market and After-Hours Quotes for AI Agents

Extended hours data is not a luxury — it is operational necessity for several AI agent patterns. Earnings announcements are released after market close. Overnight news breaks before the open. An agent that only monitors regular-hours quotes is blind to the most important price moves of the day.

🌅

Pre-Market Quotes (4:00 AM – 9:30 AM ET)

Capture overnight reactions to earnings, geopolitical events, and sector news. Gap-up and gap-down detection requires pre-market prices — without them, your agent sees the move only after the open, when the opportunity has passed. Available from: Polygon.io, Alpaca.

🌙

After-Hours Quotes (4:00 PM – 8:00 PM ET)

Monitor post-earnings price reactions, guidance updates, and late-breaking corporate news. Most earnings releases hit at 4:00–4:30 PM ET. An agent without after-hours data won't see the market's verdict until the next morning. Available from: Polygon.io, Alpaca.

Only Polygon.io and Alpaca provide pre-market and after-hours quotes — and Alpaca includes them on the free tier. Finnhub, Twelve Data, and Alpha Vantage do not support extended hours data on any standard tier. If your agent monitors earnings reactions, overnight news impact, or pre-market gap signals, provider choice is effectively reduced to two options.

Unified Stock Quote Access via Capability Routing

Different providers name the same quote fields differently. Polygon.io returns VWAP as vw. Twelve Data returns it as vwap. Alpha Vantage does not return VWAP at all. An AI agent that hardcodes field names per provider accumulates parsing debt with every new integration — and silently drops data when a provider omits a field the agent expects.

QVeris solves this with a Discover → Inspect → Call protocol. Your agent describes what quote data it needs. QVeris discovers which providers support those fields, lets the agent inspect schemas and field availability before calling, and returns normalized field names regardless of which provider answers. The same vwap field is always called vwap — no matter who provided the data.

qveris_quote.py — Terminal
# Without unified access — different field names per provider: # Polygon: data["vw"] # Twelve Data: data["vwap"] # Alpha Vantage: not available at all # Every new provider = new field-mapping code. # With QVeris — consistent field names, normalized across providers: import requests response = requests.post( "https://api.qveris.ai/v1/call", headers={"Authorization": f"Bearer {QVERIS_API_KEY}"}, json={ "capability": "stock_quote", "parameters": { "symbol": "AAPL", "fields": ["price", "bid", "ask", "vwap", "pre_market", "after_hours"] } } ) # Same field names. Every provider. Every time. # Discover and Inspect are free forever. # Docs: https://qveris.ai/docs | Pricing: https://qveris.ai/pricing

The routing layer handles provider discovery, field-name normalization, and response formatting. Your agent code stays consistent — it requests vwap and receives vwap, regardless of whether Polygon.io, Twelve Data, or Alpaca ultimately answered the call. Read the docs → or view pricing →.

Getting Started Checklist

☐Identify which quote fields your AI agent requires (bid/ask, VWAP, extended hours, or basic price only)
☐Check if your agent needs pre-market or after-hours data
☐Evaluate free tier daily limits against your agent's query volume
☐Test WebSocket stability if you need streaming quotes
☐Verify commercial use terms before production deployment
☐Consider unified access if you need consistent field names across providers
☐Start with QVeris: 1,000 credits on signup + 100 daily
Start Building with QVeris →

QVeris provides a capability routing layer. Quote data comes from third-party providers. Field availability and limits are subject to change — verify official docs.

Get Complete Quote Data for Your AI Agent

QVeris normalizes quote fields across providers so your agent gets consistent bid/ask, VWAP, and extended hours data — without per-provider parsing logic. Discover and Inspect are free forever.

Explore QVeris →View Pricing

Real-Time Stock Price API for AI Agents →

Compare 6 real-time stock price APIs with WebSocket streaming and AI agent integration.

Stock API Free Comparison →

Every free stock API compared with real tier limits and upgrade paths.

Free Financial API Comparison →

Compare 12 free financial APIs across stock, crypto, forex, news, and macro categories.

Market Data API for AI Agents →

Complete comparison of 7 market data providers across equities, crypto, forex, and options.

Stock Quote API FAQ

What is a stock quote API?
A stock quote API returns structured price data for publicly traded securities — including last price, bid/ask spread, volume, and intraday range. For AI agents, the key differentiator is which fields are available and whether data is real-time or delayed. Unlike a streaming price feed, a quote API provides a complete snapshot of current market state for a given symbol.
Which stock quote API has the best free tier?
Alpaca offers the most generous free tier — real-time US equity quotes with no daily call cap, including bid/ask spread, VWAP, and pre/post-market prices. No credit card is required. For agents needing VWAP on a free tier with higher daily limits, Twelve Data (800 req/day) is the best alternative — though data is delayed on the free tier.
Do free stock quote APIs include bid/ask spread?
Alpaca, Finnhub, Twelve Data, and Alpha Vantage all include bid/ask on their free tiers. Marketstack does not include bid/ask on any tier — it provides last price and EOD data only. If your agent needs spread analysis (bid-ask width as a liquidity signal), avoid Marketstack for quote workflows.
Which stock quote API supports pre-market prices?
Polygon.io and Alpaca both support pre-market and after-hours quotes — and Alpaca includes extended hours data on the free tier. Finnhub, Twelve Data, and Alpha Vantage do not provide extended hours data. If your agent monitors earnings reactions or overnight news impact, your provider choices are effectively limited to Polygon.io and Alpaca.
What is VWAP and which APIs provide it for free?
VWAP (Volume-Weighted Average Price) is the average execution price weighted by trading volume throughout the day — a key benchmark for institutional trading and AI agent execution-quality assessment. Alpaca provides VWAP on its free tier. Twelve Data includes VWAP on its free tier (delayed data). Polygon.io provides VWAP on paid plans. Finnhub and Alpha Vantage do not provide VWAP on any tier.
How do AI agents handle different quote field names across providers?
Different providers use different field names for the same data — Polygon calls VWAP vw, Twelve Data calls it vwap, and Alpha Vantage does not provide it at all. Without normalization, your agent code must maintain per-provider field maps. A unified routing layer like QVeris normalizes field names across providers — your agent requests vwap and receives vwap, regardless of which provider answered. See the Unified Quote Access section for a code example.

References & Sources

  1. Polygon.io Documentation — polygon.io/docs
  2. Alpaca Markets Documentation — docs.alpaca.markets
  3. Finnhub Documentation — finnhub.io/docs/api
  4. Twelve Data Documentation — twelvedata.com/docs
  5. Alpha Vantage Documentation — alphavantage.co/documentation
  6. Marketstack Documentation — marketstack.com/documentation