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---
title: "Extending the Compression Pipeline"
version: 3.8.44
lastUpdated: 2026-07-02
---
# Extending the Compression Pipeline
> **TL;DR**: OmniRoute's compression engine is **pluggable** — you can register custom engines, ship language packs for new languages, and compose stacked pipelines. This guide shows how.
**Related guides:**
- [COMPRESSION_GUIDE.md](./COMPRESSION_GUIDE.md) — Full pipeline overview
- [COMPRESSION_ENGINES.md](./COMPRESSION_ENGINES.md) — Engine registry and built-in engines
- [RTK_COMPRESSION.md](./RTK_COMPRESSION.md) — RTK engine and custom filters
- [COMPRESSION_RULES_FORMAT.md](./COMPRESSION_RULES_FORMAT.md) — Rule pack format reference
---
## Overview
The compression system has **3 extension points**:
| Extension point | Use case | Difficulty |
| -------------------- | ------------------------------------------------------------------------ | ---------- |
| **Custom engine** | Add a brand-new compression algorithm (e.g., domain-specific summarizer) | Advanced |
| **Language pack** | Add support for a new natural language (e.g., Hindi, Arabic) | Medium |
| **Stacked pipeline** | Compose existing engines in a custom order | Beginner |
```
┌─────────────────────────────────────────────────────────────┐
│ Compression Strategy │
│ │
│ Input messages ──▶ getEffectiveMode() ──▶ mode │
│ │ │
│ ┌───────────────────────┼──────────┐ │
│ │ │ │ │ │ │
│ ▼ ▼ ▼ ▼ │ │
│ "rtk" "lite" "standard" "stacked" │
│ │ │ │ │ │ │
│ ▼ ▼ ▼ ▼ │ │
│ RTK Lite Caveman engines[] │
│ engine engine engine chained │
│ │ │ │ │ │ │
│ └─────────┴─────────┴─────────┘ │ │
│ │ │
│ ▼ │
│ Compressed output │
└─────────────────────────────────────────────────────────────┘
The strategy selector is MODE-BASED: each request selects ONE mode
(rtk / lite / standard / aggressive / ultra / stacked / off).
Only mode "stacked" chains multiple engines in sequence.
Default auto-trigger mode is "lite" (not a 3-tier priority chain).
```
---
## Writing a Custom Compression Engine
The engine interface (`open-sse/services/compression/engines/types.ts`) is the contract every engine must satisfy. It has 5 required methods.
### The `CompressionEngine` Interface
```ts
interface CompressionEngine {
id: string; // Unique engine ID
name: string; // Display name
description: string; // Short description
icon: string; // Icon (emoji or URL)
targets: CompressionEngineTarget[]; // ["messages", "tool_results", "code_blocks"]
stackable: boolean; // Can be used in a stacked pipeline
stackPriority: number; // Order in stacked pipelines (lower = earlier)
metadata: CompressionEngineMetadata;
apply(body, options?): CompressionResult;
compress(body, config?): CompressionResult;
getConfigSchema(): EngineConfigField[];
validateConfig(config): EngineValidationResult;
}
```
### Minimal Example: Whitespace Engine
The simplest possible engine — strip extra whitespace from messages.
````ts
import type { CompressionEngine } from "omniroute/compression/engines/types";
import { registerCompressionEngine } from "omniroute/compression/engines/registry";
function preserveCodeBlocks(text: string): string {
// Split by code block markers and preserve whitespace inside them
const parts = text.split(/(```[\s\S]*?```)/);
return parts
.map((part) => {
if (part.startsWith("```")) {
return part; // Don't modify code blocks
}
return part.replace(/\n{3,}/g, "\n\n"); // Only apply to prose
})
.join("");
}
const whitespaceEngine: CompressionEngine = {
id: "whitespace",
name: "Whitespace Stripper",
description: "Removes extra whitespace and blank lines",
icon: "📝",
targets: ["messages", "tool_results"],
stackable: true,
stackPriority: 100, // Run AFTER caveman/rtk
metadata: {
id: "whitespace",
name: "Whitespace Stripper",
description: "Removes extra whitespace and blank lines",
inputScope: "messages",
targetLatencyMs: 5,
supportsPreview: true,
stable: true,
},
apply(body, options) {
return this.compress(body, options?.config);
},
compress(body, config = {}) {
let originalLength = 0;
let compressedLength = 0;
// Traverse message array — handle both string and multipart content
const compressedBody = (body.messages || []).map((msg) => {
if (typeof msg.content === "string") {
originalLength += msg.content.length;
let compressed = msg.content
.replace(/[ \t]+/g, " ")
.replace(/\n{3,}/g, "\n\n")
.replace(/^\s+|\s+$/gm, "");
compressedLength += compressed.length;
return { ...msg, content: compressed };
}
// Multipart content: traverse parts, compress text parts only
if (Array.isArray(msg.content)) {
const newParts = msg.content.map((part) => {
if (part.type === "text" && typeof part.text === "string") {
originalLength += part.text.length;
let compressed = part.text
.replace(/[ \t]+/g, " ")
.replace(/\n{3,}/g, "\n\n")
.replace(/^\s+|\s+$/gm, "");
compressedLength += compressed.length;
return { ...part, text: compressed };
}
return part; // preserve image_url, tool_use, etc.
});
return { ...msg, content: newParts };
}
return msg;
});
return {
body: { ...body, messages: compressedBody },
stats: {
originalTokens: Math.ceil(originalLength / 4),
compressedTokens: Math.ceil(compressedLength / 4),
savingsPercent: originalLength > 0 ? 100 * (1 - compressedLength / originalLength) : 0,
techniques: ["whitespace-collapse"],
engineId: "whitespace",
},
};
},
getConfigSchema() {
return [
{
key: "preserveCodeBlocks",
type: "boolean",
label: "Preserve code blocks",
defaultValue: true,
description: "Don't touch whitespace inside ```code``` blocks",
},
];
},
validateConfig(config) {
if (config.preserveCodeBlocks !== undefined && typeof config.preserveCodeBlocks !== "boolean") {
return { valid: false, errors: ["preserveCodeBlocks must be a boolean"] };
}
return { valid: true, errors: [] };
},
};
// Register globally
registerCompressionEngine(whitespaceEngine);
````
### Where to Place Custom Engines
```
~/.omniroute/compression/engines/my-engine.ts # User-level
<project>/compression-engines/my-engine.ts # Project-level (loaded on startup)
```
Or load programmatically from a plugin:
```ts
// In your plugin
import {
registerCompressionEngine,
unregisterCompressionEngine,
} from "@omniroute/open-sse/services/compression/engines/registry";
import { myEngine } from "./engines/my-engine";
export default definePlugin({
name: "my-compression-plugin",
// The plugin SDK exposes onRequest / onResponse / onError hooks. Register the
// engine when the plugin module loads (or on first onRequest); unregister it
// from your own teardown path.
onRequest: async (ctx) => {
registerCompressionEngine(myEngine);
},
});
// On teardown:
// unregisterCompressionEngine("my-engine");
```
### Testing Your Engine
Register your engine in a plugin or startup function. Once registered, the engine will be available
in the strategy selector via its `id`. Test integration by composing it in a stacked pipeline:
---
## Creating Language Packs
Caveman-style compression uses **language-specific rule packs** to handle fillers, hedging, and verbose patterns in each natural language. OmniRoute ships with **6 language packs**: `en`, `es`, `fr`, `de`, `ja`, `pt-BR`.
### Pack Structure
A language pack is a directory of **JSON files** under `open-sse/services/compression/rules/<language>/`:
```
open-sse/services/compression/rules/
├── en/
│ ├── filler.json # Pleasantries, hedging, politeness
│ ├── context.json # Context-reducing rules
│ ├── dedup.json # Deduplication rules
│ ├── structural.json # Punctuation, formatting
│ └── ultra.json # Aggressive compression rules
├── es/ (same structure)
├── fr/ (same structure)
├── de/ (same structure)
├── ja/ (same structure)
└── pt-BR/ (same structure)
```
### Rule Anatomy
Each rule has this shape (from `open-sse/services/compression/ruleLoader.ts`):
```ts
interface FileRule {
name: string; // Human-readable name (kebab-case)
pattern: string; // JavaScript regex pattern
replacement?: string; // What to replace the match with
replacementMap?: Record<string, string>; // OR a key→replacement map
flags?: string; // Regex flags ("gi" typically)
context?: "all" | "user" | "system" | "assistant";
category?: "filler" | "context" | "structural" | "dedup" | "terse" | "ultra";
minIntensity?: "lite" | "full" | "ultra"; // Skip below this intensity
description?: string; // Documentation
}
```
### Example: Adding Hindi Filler Rules
```json
{
"language": "hi",
"category": "filler",
"rules": [
{
"name": "polite_opener",
"pattern": "\\b(?:नमस्ते|नमस्कार|आदरणीय)\\b[,!\\s]*",
"replacement": "",
"context": "all",
"category": "filler",
"minIntensity": "lite",
"description": "Strip polite openers like 'नमस्ते'"
},
{
"name": "filler_actually",
"pattern": "\\b(?:असल में|वास्तव में|दरअसल)\\b\\s*",
"replacement": "",
"context": "all",
"category": "filler",
"minIntensity": "lite",
"description": "Strip 'actually' fillers"
},
{
"name": "verbose_plea",
"pattern": "\\b(?:कृपया|कृपया आप|अनुरोध है कि आप)\\b\\s*",
"replacement": "",
"context": "all",
"category": "filler",
"minIntensity": "full",
"description": "Strip 'please' in Hindi"
}
]
}
```
### Validation
Rule packs are validated against `_schema.json` on load. A pack with bad structure will fail to load and log an error:
```
RULE_LOADER: pack "hi/filler.json" failed validation:
- rules.0.pattern: Invalid regex
- rules.1.context: must be one of [all, user, system, assistant]
```
Validation runs automatically when a pack is loaded (against `_schema.json`); an
invalid pack is rejected and the error above is logged. There is no separate
`npm run` script for pack validation — load the pack (e.g. start the server or
exercise the compression path) and watch the logs.
### Loading a Custom Language Pack
```ts
import { loadRulePack } from "omniroute/compression/ruleLoader";
await loadRulePack("./my-custom-rules/hi/filler.json");
```
Or place in a recognized location:
```
~/.omniroute/compression/rules/hi/filler.json # User-level
<project>/.compression/rules/hi/filler.json # Project-level
```
### Best Practices for Language Packs
1. **Start with `filler`** — these are the highest-impact rules
2. **Use `minIntensity`** to gate aggressive rules — protects against over-compression
3. **Include test cases** — add `tests[]` array in the JSON to verify behavior
4. **Order matters** — earlier rules apply first; place high-impact rules first
5. **Be conservative with `replacement`** — empty string is usually correct; never introduce new content
### Translation Strategy
When localizing rule packs to a new language:
1. **Translate the rule names** — they appear in debug output
2. **Adapt the regex patterns** — direct translation often fails (word boundaries differ)
3. **Test against real conversations** — the pack should be safe on actual input
4. **Match cultural conventions** — Japanese packs, for instance, have more honorific fillers than English
---
## Stacked Pipelines
A **stacked pipeline** runs multiple engines in sequence, with each engine's output feeding the next. This is how `mode: stacked` works internally.
### How Stacking Works
```
Input (10,000 tokens)
┌──────────┐
│ Engine │ priority 10
│ A │ ──▶ output: 6,000 tokens (-40%)
└────┬─────┘
┌──────────┐
│ Engine │ priority 50
│ B │ ──▶ output: 2,400 tokens (-60%)
└────┬─────┘
┌──────────┐
│ Engine │ priority 100
│ C │ ──▶ output: 1,200 tokens (-80%)
└────┬─────┘
Final output (1,200 tokens, ~88% savings combined)
```
When `mode: "stacked"` is selected, engines execute sequentially in the order specified in the `pipeline` array.
The output of engine N becomes the input of engine N+1.
### Compression Modes
OmniRoute selects **ONE mode per request** based on configuration, auto-trigger thresholds, and combo overrides.
The available modes are defined in `open-sse/services/compression/types.ts` (type `CompressionMode`):
| Mode | Engines | Use case |
| ------------ | -------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `off` | None | Disable all compression |
| `rtk` | RTK only | Command-output heavy sessions (80%+ savings) |
| `lite` | Lite only | Conservative compression (fast, safe) |
| `standard` | Caveman | Prose compression with language packs |
| `aggressive` | Caveman + Aggressive | Aggressive prose + aggressive final pass |
| `ultra` | Ultra | Maximum compression (lossy, last resort). Optionally routed through the **LLMLingua-2** SLM engine when `ultra.modelPath` is set (fail-opens to the rule-based path when the model is unavailable). |
| `stacked` | Custom pipeline | Compose engines in any order (see below) |
> Beyond the mode engines above, the registry also ships specialized stackable engines —
> **CCR**, **headroom**, **ionizer**, and **session-dedup** — documented in
> [COMPRESSION_ENGINES.md](./COMPRESSION_ENGINES.md#additional-built-in-engines).
Mode selection is determined by `getEffectiveMode()` in `open-sse/services/compression/strategySelector.ts`:
1. If compression is disabled: `"off"`
2. If a combo override exists: use the override
3. If auto-trigger threshold is exceeded: use `autoTriggerMode` (default: `"lite"`)
4. Otherwise: use `defaultMode`
### The Default Stacked Pipeline
When `mode: "stacked"` is explicitly configured, the default pipeline composes:
1. **RTK** — strip command output noise (~80% savings on terminal output)
2. **Caveman** — remove fillers, terse-ify prose (~46% on remaining text)
3. **Lite** — final whitespace + dedup pass
This composition achieves **78-95% savings** on tool-heavy sessions.
### Configuring Stacked Pipelines
In combo config:
```json
{
"compression": {
"mode": "stacked",
"pipeline": [
{ "engine": "rtk", "config": { "intensity": "aggressive" } },
{ "engine": "caveman", "config": { "intensity": "full" } },
{ "engine": "lite", "config": {} }
]
}
}
```
You can omit engines, add custom ones, or reorder them.
### State Passing
Engines can read metadata from the request context (in `options`):
```ts
compress(body, config) {
// Read metadata from previous engines
const original = options?.compressionComboId; // "my-coding-combo"
// ...
}
```
The metadata is **read-only** — engines cannot mutate the request context, only their own body output.
### Execution Order Gotchas
| Engine order | Effect |
| ----------------------------------- | ------------------------------------------------------------------------------ |
| RTK → Caveman → Lite | **Recommended** (strips noise first, then language, then whitespace) |
| Lite → RTK → Caveman | Bad — Lite strips whitespace from raw output, making RTK pattern matching fail |
| Caveman → RTK | Bad — Caveman may rewrite text in ways that RTK doesn't recognize |
| Any order with `tool_results` first | Better — tool output is the noisiest content |
### When NOT to Stack
Stacking isn't always better:
- **Simple messages** (no tool output) — single Caveman or Lite is enough
- **Cost-sensitive** — each engine adds ~5-50ms latency
- **Specific tools** — RTK alone is usually sufficient for shell output
### Building a Custom Pipeline
There is no named-pipeline registry. A stacked pipeline is just an **inline array
of steps** passed to `applyStackedCompression()` (exported from
`@omniroute/open-sse/services/compression/strategySelector`):
```ts
import { applyStackedCompression } from "@omniroute/open-sse/services/compression/strategySelector";
const result = applyStackedCompression(body, [
{ engine: "rtk", intensity: "aggressive" },
{ engine: "caveman", intensity: "full" },
]);
```
When you don't pass a pipeline, it defaults to `rtk(standard) → caveman(full)`.
To drive it from config, set `mode: "stacked"` and provide the step array under
`stackedPipeline` (read from `config.stackedPipeline`):
```json
{
"compression": {
"mode": "stacked",
"stackedPipeline": [
{ "engine": "rtk", "intensity": "aggressive" },
{ "engine": "caveman", "intensity": "full" }
]
}
}
```
---
## Upstream Sync Policy
OmniRoute's compression engines credit several upstream projects in the README
("inspired by RTK, Caveman, LLMLingua-2, Troglodita"). A common contributor
question is: **when upstream RTK adds a new tool filter or Caveman adds a rule
pack, how does that reach OmniRoute?** This section is the authoritative answer.
### Vendored copies vs. independent implementations
| Engine | Relationship to upstream | Location |
| ---------------------------- | ------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------- |
| **RTK** | **Independent reimplementation** (inspired-by, not a copy) | `open-sse/services/compression/engines/rtk/` |
| **Caveman** | **Independent reimplementation** (inspired-by) | `open-sse/services/compression/engines/cavemanAdapter.ts` |
| **Headroom** | Mostly internal; only the `gcf/` codec is **genuinely vendored** from `gcf-typescript` (MIT, SPDX-marked, generic profile only) | `open-sse/services/compression/engines/headroom/gcf/` |
| **LLMLingua-2 / Troglodita** | Inspired-by (drive the `llmlingua` + `session-dedup` engines) | `open-sse/services/compression/engines/llmlingua/`, `session-dedup` |
Key point: **RTK and Caveman are clean-room TypeScript implementations of the
_ideas_ (filter rules, rule packs), not vendored source trees.** There is no
upstream copy to `git pull` from — which is exactly why the README says
"inspired by" rather than "bundled".
### How upstream improvements are merged
There is **no automated upstream-release tracking and no `compression-sync`
label** — by design. Because the engines are reimplementations, an upstream RTK
filter or Caveman rule pack is not merged as code; it is **re-expressed as a new
rule/filter in OmniRoute's own format** (see
[COMPRESSION_RULES_FORMAT.md](./COMPRESSION_RULES_FORMAT.md)) and lands ad-hoc via
a normal PR. The extension points above (custom engine, language pack, RTK filter)
are the sanctioned way to contribute one.
Recent examples of exactly this flow:
- RTK filters for Gradle & `dotnet` build output (v3.8.42)
- RTK filters for kubectl / docker-build / composer / gh (#2824)
- Caveman Indonesian language pack (#3975), plus German / French / Japanese / Chinese packs
### Headroom (input-compression proxy)
Headroom is **fully internal** — a pinned vendored `gcf` codec snapshot plus
OmniRoute's own `smartcrusher` / `toon` / `tabular` layers. There is no live
upstream to track beyond the vendored copy; updates to `gcf` are refreshed
manually when the codec changes and re-validated against the compression budget
gate (`check:compression-budget`).
### Proposing an upstream-inspired improvement
1. **Don't vendor** — re-express the upstream rule/filter in OmniRoute's format.
2. Add it via the matching extension point below (language pack, RTK filter, or
custom engine).
3. Reference the upstream project in the PR description (attribution), not by
copying its license-bearing source.
4. Include tests and confirm the `check:compression-budget` gate still passes.
---
## Best Practices
### Engine Development
1. **Always implement `validateConfig`** — engines without validation cause silent failures
2. **Set realistic `targetLatencyMs`** — used by the strategy selector to choose engines
3. **Use `getConfigSchema` for the dashboard** — never hide config from users
4. **Support `stackable: true` if your engine is pure** — engines with side effects shouldn't stack
5. **Write inline tests** — engines should be verifiable in <1s
### Language Pack Development
1. **Start with `lite` intensity** — your rules should be safe at the lowest setting
2. **Use `context` to scope rules** — `user` only rules can't accidentally affect system prompts
3. **Avoid capturing JSON keys** — `\\bword\\b` can match inside JSON, breaking structured data
4. **Test with edge cases** — empty input, unicode, RTL text, emojis
5. **Use existing packs as templates** — `en/filler.json` is the most-developed example
### Pipeline Design
1. **Profile before optimizing** — measure with `compression_stats` first
2. **Prefer composition over reimplementation** — extend Caveman rules before writing a new engine
3. **Document the order rationale** — comment why engine A before engine B
4. **Test at all 3 intensity levels** — `lite` is fast but lossy, `ultra` is slow but precise
---
## Reference: Built-in Engines
| Engine ID | Stackable | Default stackPriority | Targets |
| -------------------- | --------- | --------------------- | ----------------------------------- |
| `lite` | Yes | 5 | messages, tool_results |
| `rtk` | Yes | 10 | tool_results |
| `standard` (caveman) | Yes | 20 | messages, tool_results, code_blocks |
| `aggressive` | Yes | 30 | messages |
| `ultra` | Yes | 40 | messages, code_blocks |
### See Also
- [COMPRESSION_GUIDE.md](./COMPRESSION_GUIDE.md) — Pipeline overview
- [COMPRESSION_ENGINES.md](./COMPRESSION_ENGINES.md) — Engine registry reference
- [COMPRESSION_RULES_FORMAT.md](./COMPRESSION_RULES_FORMAT.md) — Rule format spec
- [COMPRESSION_LANGUAGE_PACKS.md](./COMPRESSION_LANGUAGE_PACKS.md) — Language pack details
- [RTK_COMPRESSION.md](./RTK_COMPRESSION.md) — RTK engine and custom filters
- Source: `open-sse/services/compression/` (117 files, ~250KB)