Incident Report: vLLM Embeddings Broken by encoding_format Parameter

LiteLLM incident report. Published 2026-02-18.

Date: Feb 16, 2026 Duration: ~3 hours Severity: High (for vLLM embedding users) Status: Resolved

Summary

A commit (dbcae4a) intended to fix OpenAI SDK behavior broke vLLM embeddings by explicitly passing encoding_format=None in API requests. vLLM rejects this with error: "unknown variant \``, expected float or base64".

• vLLM embedding calls: Complete failure - all requests rejected
• Other providers: No impact - OpenAI and other providers functioned normally
• Other vLLM functionality: No impact - only embeddings were affected

Background

The encoding_format parameter for embeddings specifies whether vectors should be returned as float arrays or base64 encoded strings. Different providers have different expectations:

• OpenAI SDK: If encoding_format is omitted, the SDK adds a default value of "float"
• vLLM: Strictly validates encoding_format - only accepts "float", "base64", or complete omission. Rejects None or empty string values.

Root cause

A well-intentioned fix for OpenAI SDK behavior inadvertently broke vLLM embeddings:

The Breaking Change (dbcae4a):

In litellm/main.py, the code was changed to explicitly set encoding_format=None instead of omitting it:

# Added in dbcae4a
if encoding_format is not None:
    optional_params["encoding_format"] = encoding_format
else:
    # Omitting causes openai sdk to add default value of "float"
    optional_params["encoding_format"] = None

This fix worked correctly for OpenAI - explicitly passing None prevented the SDK from adding its default value. However, vLLM’s strict parameter validation rejected None values, causing all embedding requests to fail.

The Fix

Fix deployed (55348dd). The solution filters out None and empty string values from optional_params before sending requests to OpenAI-like providers (including vLLM).

In litellm/llms/openai_like/embedding/handler.py:

# Before (broken)
data = {"model": model, "input": input, **optional_params}

# After (fixed)
filtered_optional_params = {k: v for k, v in optional_params.items() if v not in (None, '')}
data = {"model": model, "input": input, **filtered_optional_params}

This ensures:

• Valid values ("float", "base64") are preserved and sent
• None and empty string values are filtered out (parameter omitted entirely)
• OpenAI SDK no longer adds defaults because liteLLM handles the parameter upstream

Remediation