OmniRoute/docs/compression/EXTENDING_COMPRESSION.md

---
title: "Extending the Compression Pipeline"
version: 3.8.40
lastUpdated: 2026-06-28
---

# 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" }
    ]
  }
}
```

---

## 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)