Are there any historical price data APIs that include deep historical coverage and corporate action adjustments?

Yes — several APIs do, though the exact combination of deep history + corporate-action-adjusted prices varies a lot by vendor and asset class.

Good candidates

1) Polygon.io

• Coverage: strong for US equities; intraday and daily history depending on plan
• Adjustments: supports split/dividend-adjusted data for aggregates
• Best for: modern developer-friendly API, equities focused

2) Tiingo

• Coverage: long daily history for many US equities/ETFs
• Adjustments: adjusted prices included; split/dividend data available
• Best for: clean end-of-day equity data with adjustments

3) Nasdaq Data Link / Quandl datasets

• Coverage: depends on dataset, some go very deep historically
• Adjustments: many premium datasets are adjusted; corporate actions often included separately or baked in
• Best for: flexibility, research workflows, alternative vendors

4) Alpha Vantage

• Coverage: decent, but not as deep or robust as institutional sources
• Adjustments: adjusted daily series available
• Best for: lightweight use, prototyping

5) EOD Historical Data

• Coverage: broad global equities with long history
• Adjustments: adjusted OHLC data and corporate actions
• Best for: international equities and reasonably deep history

6) Intrinio

• Coverage: broad market data products, depth varies by package
• Adjustments: corporate action-aware pricing options
• Best for: professional use, enterprise integrations

7) Bloomberg / Refinitiv / FactSet / S&P Capital IQ

• Coverage: very deep, institutional-grade
• Adjustments: extensive corporate action handling
• Best for: institutional research/compliance-heavy environments
• Tradeoff: expensive and licensing-heavy

What to check when comparing

To make sure an API really fits your use case, verify:

• Adjusted close vs adjusted OHLC
• Whether adjustments are split-only or split + dividend
• Whether the vendor provides raw prices and corporate actions separately
• Survivorship-bias-free coverage for delisted securities
• History depth: daily back to IPO, or only a limited window
• Support for global markets if needed

Quick recommendation

• If you want a developer-friendly API: Polygon or Tiingo
• If you want global/long history: EOD Historical Data
• If you want institutional-grade depth: Bloomberg/Refinitiv/FactSet

If you want, I can also give you a comparison table of the best APIs by:

• history depth
• adjustment quality
• pricing
• ease of use
• asset class coverage