Description

--cache-strategy content does not enable content hashing. A property name casing mismatch between Prettier and file-entry-cache v11 causes the option to be silently ignored, leaving file size as the only change-detection check. Any file modification that preserves the byte count is invisible to the cache.

This also affects the default behaviour (no --cache-strategy flag), since the default is content.

Root cause

In src/cli/format-results-cache.js (line 46–48), Prettier passes:

const useChecksum = cacheStrategy === "content";   // lowercase 's'
const fileEntryCacheOptions = {
  useChecksum,          // ← lowercase 's'
  useModifiedTime: !useChecksum,
};

But file-entry-cache v11.1.2 reads options?.useCheckSum (capital S):

// file-entry-cache v11 — constructor
if (options?.useCheckSum) {        // ← capital 'S'
  this._useCheckSum = options.useCheckSum;
}

JavaScript property access is case-sensitive, so options.useCheckSum is undefined and _useCheckSum stays false.

Net effect

Only file size is compared. Content hashing never runs.

History

This is a regression from the file-entry-cache v10 → v11 upgrade. In v10, the property was named useChecksum (lowercase), matching what Prettier passes. In v11, it was renamed to useCheckSum (capital S). Related: #17278 (the opposite problem in v10 — mtime checked when it shouldn't be).

Reproduction

# 1. Create a file and format it (populates cache)
echo '| A   | B   |' > test.md
echo '|---|---|' >> test.md
echo '| 1   | 2   |' >> test.md
prettier --write --cache --cache-strategy content test.md

# 2. Replace with different content of the SAME byte count
# (Prettier reformats |---| to | --- | — same length due to padding)
printf '| A   | B   |\n|---|---|\n| 1   | 2   |\n' > test.md

# 3. Cache falsely reports the file as clean
prettier --check --cache --cache-strategy content test.md
# ✅ "All matched files use Prettier code style!"  ← WRONG

# 4. Without cache, correctly detects the issue
prettier --check test.md
# ❌ "Code style issues found"                      ← CORRECT

A simpler reproduction using any two files with identical byte counts:

# Create two files with different content but same size (11 bytes each)
echo '0123456789' > a.txt    # 11 bytes
echo 'abcdefghij' > b.txt    # 11 bytes

# Cache a.txt as "formatted"
cp a.txt test.txt
prettier --write --cache --cache-strategy content test.txt

# Replace with b.txt (same size, different content)
cp b.txt test.txt

# Cache reports clean — only size was checked
prettier --check --cache --cache-strategy content test.txt
# "All matched files use Prettier code style!"

Suggested fix

One-character change — match the casing expected by file-entry-cache v11:

-    const useChecksum = cacheStrategy === "content";
+    const useCheckSum = cacheStrategy === "content";
     const fileEntryCacheOptions = {
-      useChecksum,
-      useModifiedTime: !useChecksum,
+      useCheckSum,
+      useModifiedTime: !useCheckSum,
       restrictAccessToCwd: false,
     };

Versions

• Prettier: 3.8.1
• file-entry-cache: 11.1.2 (bundled)
• Node.js: 22.x
• OS: macOS / Linux (reproducible on both)