redis/redis-py · error · DataError

The supported algorithms are

Error message

The supported algorithms are: {supported_algos_str}

What it means

Raised by stralgo() when the algo argument is not 'LCS'. LCS (longest common substring) is currently the only algorithm the STRALGO command implements, so the client rejects any other value immediately rather than sending an invalid command to the server. It is a DataError.

Solutions

Pass algo='LCS' (uppercase) to stralgo().
If you need a different string algorithm, use the dedicated LCS key command (client.lcs()) instead of stralgo().

Example fix

# before
client.stralgo(algo='LCS_STR', value1='ohmytext', value2='mynewtext')

# after
client.stralgo(algo='LCS', value1='ohmytext', value2='mynewtext')

Defensive patterns

Strategy: validation

Validate before calling

if algo != 'LCS':
    raise ValueError("stralgo algo must be 'LCS'")
client.stralgo(algo=algo, value1=v1, value2=v2)

Type guard

from typing import Literal

def is_supported_algo(a: str) -> bool:
    return a == 'LCS'

# usage
algo: Literal['LCS'] = 'LCS'

Try / catch

from redis.exceptions import DataError
try:
    client.stralgo(algo=algo, value1=v1, value2=v2)
except DataError as e:
    if 'supported algorithms' in str(e):
        algo = 'LCS'
        client.stralgo(algo=algo, value1=v1, value2=v2)

Prevention

Always pass algo='LCS' literally; there is currently no alternative.
Prefer client.lcs(key1, key2) for the key-based LCS use case, which has no algo parameter.

When it happens

Trigger: Calling client.stralgo(algo='EDIT', value1=..., value2=...), client.stralgo('lcs', ...) (wrong case), or passing any string other than 'LCS' as the algo parameter.

Common situations: Misspelling the algorithm name, using wrong case, or assuming a newer algorithm is available when targeting an older Redis server. Programmatic algo selection where the source value is not constrained.

Related errors

AI-assisted analysis of redis/redis-py@6a6b581b48 (2026-08-10). Data as JSON: /api/errors/ff77e39cbfb18a19. Report an issue: GitHub.

Appendix: source

Thrown at redis/commands/core.py:4620

        ``algo`` Right now must be LCS
        ``value1`` and ``value2`` Can be two strings or two keys
        ``specific_argument`` Specifying if the arguments to the algorithm
        will be keys or strings. strings is the default.
        ``len`` Returns just the len of the match.
        ``idx`` Returns the match positions in each string.
        ``minmatchlen`` Restrict the list of matches to the ones of a given
        minimal length. Can be provided only when ``idx`` set to True.
        ``withmatchlen`` Returns the matches with the len of the match.
        Can be provided only when ``idx`` set to True.

        For more information, see https://redis.io/commands/stralgo
        """
        # check validity
        supported_algo = ["LCS"]
        if algo not in supported_algo:
            supported_algos_str = ", ".join(supported_algo)
            raise DataError(f"The supported algorithms are: {supported_algos_str}")
        if specific_argument not in ["keys", "strings"]:
            raise DataError("specific_argument can be only keys or strings")
        if len and idx:
            raise DataError("len and idx cannot be provided together.")

        pieces: list[EncodableT] = [algo, specific_argument.upper(), value1, value2]
        if len:
            pieces.append(b"LEN")
        if idx:
            pieces.append(b"IDX")
        try:
            int(minmatchlen)
            pieces.extend([b"MINMATCHLEN", minmatchlen])
        except TypeError:
            pass
        if withmatchlen:
            pieces.append(b"WITHMATCHLEN")

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