Free Real-Time Stock API: Data, Feeds & Limits

TL;DR

How to verify real-time stock data

A “real-time” label is not enough. Measure freshness from the exchange timestamp, identify the underlying feed, and confirm what the license permits.

Real-time stock price is one field, not the whole feed

A real-time stock price API may return only the most recent eligible trade. A broader stock-data feed can also deliver bid and ask quotes, trade size and conditions, OHLCV aggregates, market status, halts, symbol metadata, and corrections. These records answer different questions. A watchlist may need a last price and session state; a spread monitor needs bid and ask; an execution model needs event-level trades and quotes; a chart usually needs interval bars. Define the record type before comparing providers or you may pay for speed while receiving the wrong data.

Feed coverage and the clocks behind “real time”

Low latency does not guarantee complete market coverage. For U.S. equities, a provider may distribute consolidated information derived from securities information processors, data from selected exchanges, FINRA facilities, OTC sources, or a broker-specific view. A single-venue feed can arrive quickly and still differ from the national market because trading occurs across venues. Record the feed or venue and do not describe one source as “the market” unless its documented coverage supports that statement.

Normalize timestamps to a common internal representation, but retain original values and precision. Timezone conversion alone cannot repair a provider timestamp that represents bar close rather than trade time. During onboarding, measure event age for several liquid and thinly traded symbols in regular, pre-market, after-hours, halted, and closed states. Unchanged prices are not automatically stale when no eligible event has occurred.

What “free” means for real-time stock data

Compare verified provider boundaries, not marketing labels

Alpha Vantage currently says most free datasets allow up to 25 requests per day and that U.S. real-time and 15-minute delayed market data are premium-only. Financial Modeling Prep requires an API key for documented requests and lists quote, historical, company, calendar, and market-data endpoints; actual endpoint access and delay depend on the current plan. Massive (formerly Polygon.io) documents U.S. stock trades, quotes, aggregates, snapshots, and reference records sourced through exchanges, securities information processors, FINRA facilities, and OTC markets, with feed access controlled by plan and entitlement. Recheck these boundaries when implementing because plans can change.

Build WebSocket streaming that survives production

A WebSocket connection is only the transport. A reliable consumer must authenticate, subscribe, confirm subscription state, process heartbeats, retain sequence or event identifiers, detect gaps, reconnect with bounded backoff, and resubscribe without creating duplicate handlers. It must also protect itself when messages arrive faster than downstream storage or user interfaces can process them.

Do not turn aggressive REST polling into fake streaming

Polling a quote endpoint every second can exhaust a free quota, synchronize traffic spikes across users, and still miss every intermediate trade. Use request coalescing so many users share one upstream refresh, add jitter to scheduled polls, and choose cadence from the provider update frequency and product promise. If continuous event coverage is a true requirement, select an entitled streaming feed rather than trying to reconstruct one from snapshots.

A resilient real-time stock data integration

Measure data age before accepting a normalized event

This provider-neutral Python function makes freshness measurable. It accepts an already parsed event timestamp, compares it with client receipt time in UTC, rejects future events beyond a small clock-skew allowance, and labels values that exceed the product’s maximum permitted age. The threshold must come from the product requirement and market state—not from the word “real-time” in a plan name.

from datetime import datetime, timezone

def validate_freshness(event_time, max_age_seconds, clock_skew_seconds=2):
    received_at = datetime.now(timezone.utc)
    event_time = event_time.astimezone(timezone.utc)
    age = (received_at - event_time).total_seconds()

    if age < -clock_skew_seconds:
        raise ValueError("Event timestamp is unexpectedly in the future")

    return {
        "event_time": event_time.isoformat(),
        "received_at": received_at.isoformat(),
        "age_seconds": max(age, 0),
        "is_stale": age > max_age_seconds,
    }

Run this after schema and identity validation, then retain the result with the raw payload. The function cannot prove that a provider timestamp is semantically correct or that the feed is complete; those require documentation review and cross-source tests. During closed or halted markets, use market state to choose an appropriate threshold and message instead of generating continuous false stale-data alerts.

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