Hardik Gupta

Posted on Apr 17

Bloom Filters for AI Agents: The Small Cache Trick That Saved My Retrieval Pipeline

#bloomfilter #agents #typescript #performance

Bloom filters felt like a purely academic data structure - until an agent pipeline started repeating work. At that point, they became immediately practical.

Problem

The system needed a fast, low-cost way to check whether something had probably been seen before.

Not certainty. A strong enough signal to avoid redundant work.

Failure Mode

The agent repeatedly:

revisited identical document IDs
re-triggered the same tool calls
reprocessed items already handled minutes earlier

This created:

unnecessary latency
increased compute cost
degraded pipeline efficiency

A lightweight pre-check layer was required.

Approach

Introduce a Bloom filter as a front-line gate:

If definitely new → process
If possibly seen → verify via authoritative store

Properties leveraged:

No false negatives
Acceptable false positives

Mental Model

A Bloom filter consists of:

a fixed-size bit array
multiple hash functions
a probabilistic membership check

Insert

hash value multiple times
set corresponding bits to 1

Query

if any bit is 0 → definitely not present
if all bits are 1 → possibly present

Implementation

class BloomFilter {
  private bits = new Uint8Array(2048);
  private readonly seeds = [17, 31, 53, 73];

  private hash(value: string, seed: number) {
    let hash = seed;
    for (let i = 0; i < value.length; i++) {
      hash = (hash * 33 + value.charCodeAt(i)) % this.bits.length;
    }
    return hash;
  }

  add(value: string) {
    for (const seed of this.seeds) {
      this.bits[this.hash(value, seed)] = 1;
    }
  }

  has(value: string) {
    return this.seeds.every(
      (seed) => this.bits[this.hash(value, seed)] === 1
    );
  }
}

Where It Fit in My Agent Stack

I ended up using Bloom filters in three key places:

1. Event Deduplication

Before the agent processes anything, I filter out repeated inputs. This alone removed a lot of noise.

2. Retrieval Optimization

While scanning candidate documents, I skip anything that has likely been seen before. This reduced unnecessary lookups.

3. Tool Call Short-Circuiting

This was the biggest win.

Agents tend to repeat tool calls when context becomes messy. A Bloom filter doesn’t fix reasoning, but it stops the system from wasting cycles on the same targets again and again.

The Tradeoff I Respect

I don’t use Bloom filters when I need certainty.

I use them when I need:

speed
low memory usage
a fast first-pass filter

They are not a source of truth.

They are a guardrail.

Final Take

Bloom filters work best as a front-line defense against wasted effort.

They don’t fix reasoning.

They don’t improve intelligence.

What they do is enforce discipline in the system - quietly, efficiently, and at scale.

In agent pipelines, that’s often exactly what is missing.

Discussion

How do you handle deduplication in your AI workflows?

Redis / Postgres with exact checks?
Probabilistic structures like Bloom or Cuckoo filters?
Something hybrid?

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