%s: cannot combine --formatVersion 1 with --writePoolBundle

Error message

%s: cannot combine --formatVersion 1 with --writePoolBundle or --usePoolBundle\n

What it means

genrb forbids combining formatVersion 1 with pool bundles. Pool bundles (written via --writePoolBundle or consumed via --usePoolBundle) rely on the strings-v2 layout and a keys pool that only exists in formatVersion 2 and above; formatVersion 1 predates that infrastructure. The check at genrb.cpp:169-173 catches s[0] == '1' together with either pool flag being set and sets illegalArg=true.

Source

Thrown at deps/icu-small/source/tools/genrb/genrb.cpp:172

        illegalArg = true;
    }
    if(options[WRITE_POOL_BUNDLE].doesOccur && options[USE_POOL_BUNDLE].doesOccur) {
        fprintf(stderr, "%s: cannot combine --writePoolBundle and --usePoolBundle\n", argv[0]);
        illegalArg = true;
    }
    if (options[ICU4X_MODE].doesOccur && !options[UCADATA].doesOccur) {
        fprintf(stderr, "%s: --icu4xMode requires --ucadata\n", argv[0]);
        illegalArg = true;
    }
    if(options[FORMAT_VERSION].doesOccur) {
        const char *s = options[FORMAT_VERSION].value;
        if(uprv_strlen(s) != 1 || (s[0] < '1' || '3' < s[0])) {
            fprintf(stderr, "%s: unsupported --formatVersion %s\n", argv[0], s);
            illegalArg = true;
        } else if(s[0] == '1' &&
                  (options[WRITE_POOL_BUNDLE].doesOccur || options[USE_POOL_BUNDLE].doesOccur)
        ) {
            fprintf(stderr, "%s: cannot combine --formatVersion 1 with --writePoolBundle or --usePoolBundle\n", argv[0]);
            illegalArg = true;
        } else {
            setFormatVersion(s[0] - '0');
        }
    }

    if((options[JAVA_PACKAGE].doesOccur || options[BUNDLE_NAME].doesOccur) &&
            !options[WRITE_JAVA].doesOccur) {
        fprintf(stderr,
                "%s error: command line argument --java-package or --bundle-name "
                "without --write-java\n",
                argv[0]);
        illegalArg = true;
    }

    if(options[VERSION].doesOccur) {
        fprintf(stderr,
                "%s version %s (ICU version %s).\n"

View on GitHub (pinned to 1b2de5e052)

Solutions

Use --formatVersion 2 or 3 (or omit it) when enabling pool bundles: `genrb --formatVersion 3 --usePoolBundle pool/ root.txt`.
Drop --writePoolBundle / --usePoolBundle if you truly must emit formatVersion 1 files.
Regenerate the pool bundle with the same or higher formatVersion as the consuming bundles.

Example fix

// before
genrb --formatVersion 1 --usePoolBundle ./pool root.txt
// after
genrb --formatVersion 3 --usePoolBundle ./pool root.txt

Defensive patterns

Strategy: validation

Validate before calling

# Reject formatVersion 1 combined with pool-bundle flags
if [ "$FORMAT_VERSION" = "1" ] && { [ -n "$WRITE_POOL_BUNDLE" ] || [ -n "$USE_POOL_BUNDLE" ]; }; then
  echo "ERROR: formatVersion 1 is incompatible with pool bundles" >&2
  exit 2
fi

Prevention

When introducing pool bundles, bump formatVersion to 3 in the same commit.
Document the formatVersion floor for pool-bundle builds in your data build README.

When it happens

Trigger: Running `genrb --formatVersion 1 --writePoolBundle out/ root.txt` or `genrb --formatVersion 1 --usePoolBundle pool/ root.txt`. Either pool flag combined with formatVersion 1 triggers the branch.

Common situations: Trying to produce smaller backwards-compatible .res files while also adopting the pool-bundle optimization; copy-pasting a formatVersion 1 flag from an older build script into a modern data build that enables pool bundles.

Related errors

AI-assisted analysis of nodejs/node@1b2de5e052 (2026-08-13). Data as JSON: /api/errors/14e5f05d8c1aba18. Report an issue: GitHub.