fast2flow

Overview

fast2flow is a high-performance routing extension that routes incoming messages to the appropriate flow using deterministic token-based scoring (BM25) with optional LLM fallback.

Core concept: A user sends a message like “refund please” → fast2flow checks tenant-specific indexes → returns a routing directive (Dispatch, Respond, Deny, or Continue).

Key principles:

• Deterministic first — Token-based BM25 scoring for predictable, explainable routing
• Fail-open — Errors, timeouts, or missing indexes produce a Continue directive
• Time-bounded — Hard timeout enforcement via time_budget_ms
• Policy-driven — Runtime behavior changes without code changes

Architecture

Incoming Message
    │
    ▼
┌──────────────────────────────────────────────────┐
│                fast2flow Pipeline                 │
│                                                  │
│  ┌────────────────────────────────────────────┐  │
│  │  1. Hook Filter                            │  │
│  │  Allow/deny lists, respond rules, policy   │  │
│  └────────────────────────────────────────────┘  │
│                      │                           │
│  ┌────────────────────────────────────────────┐  │
│  │  2. Index Lookup                           │  │
│  │  Load TF-IDF index for tenant scope        │  │
│  └────────────────────────────────────────────┘  │
│                      │                           │
│  ┌────────────────────────────────────────────┐  │
│  │  3. Deterministic Strategy (BM25)          │  │
│  │  Token scoring with title boosting (2x)    │  │
│  └────────────────────────────────────────────┘  │
│                      │                           │
│  ┌────────────────────────────────────────────┐  │
│  │  4. Confidence Gate                        │  │
│  │  min_confidence threshold check            │  │
│  └────────────────────────────────────────────┘  │
│                      │                           │
│  ┌────────────────────────────────────────────┐  │
│  │  5. LLM Fallback (optional)                │  │
│  │  OpenAI or Ollama for ambiguous cases      │  │
│  └────────────────────────────────────────────┘  │
└──────────────────────────────────────────────────┘
    │
    ▼
Routing Directive (Dispatch / Respond / Deny / Continue)

Routing Directives

Every routing decision produces one of four directives:

Directive	Purpose	Fields
dispatch	Route to a specific flow	target, confidence, reason
respond	Return an immediate response	message
deny	Block the request	reason
continue	No decision — let the caller handle it	—

// Dispatch to a flow
{"type": "dispatch", "target": "support-pack:refund_request", "confidence": 0.92, "reason": "BM25 match"}

// Auto-respond without routing
{"type": "respond", "message": "Use the self-service refund form at /refund."}

// Block the request
{"type": "deny", "reason": "Denied by scope policy"}

// Pass through (fail-open default)
{"type": "continue"}

Installation

fast2flow is distributed as a .gtpack artifact via GHCR:

# Pull from GHCR
oras pull ghcr.io/greentic-biz/providers/routing-hook/fast2flow.gtpack:latest

# Or reference a specific version
oras pull ghcr.io/greentic-biz/providers/routing-hook/fast2flow.gtpack:v0.4.6

The pack registers a post_ingress hook that intercepts messages before they reach any flow.

WASM Components

fast2flow ships three WASM components (targeting wasm32-wasip2):

Component	Purpose	Operation
Indexer	Builds a searchable TF-IDF index from flow metadata	build, update
Matcher	Fast BM25-based intent matching against the index	match
Router	Orchestrates the full routing pipeline	route

These components are coordinated by three flows defined in the pack:

# flows/index.ygtc  — Runs at deploy time to build indexes
# flows/match.ygtc  — Runtime BM25 intent matching
# flows/route.ygtc  — Full routing pipeline with LLM fallback

Bundle Workflow

fast2flow indexes flows from your bundle’s .ygtc files. The indexer scans your bundle directory, extracts metadata (title, description, tags), and builds a TF-IDF index with BM25 scoring.

Bundle Structure

my-bundle/
├── packs/
│   ├── support-pack/
│   │   └── flows/
│   │       ├── refund.ygtc
│   │       ├── shipping.ygtc
│   │       └── faq.ygtc
│   └── hr-pack/
│       └── flows/
│           ├── leave.ygtc
│           └── booking.ygtc

Flow Definition (.ygtc)

Each flow file provides the metadata used for intent matching:

refund.ygtc

id: refund_request
title: Process Refund Request
description: Handle customer refund requests for orders and payments
type: messaging
tags:
  - refund
  - payment
  - billing
  - return
start: collect_info

nodes:
  collect_info:
    templating.handlebars:
      text: "Please provide your order number for the refund."
    routing:
      - out: true

Tip

Title words receive a 2x TF-IDF boost, so choose descriptive titles for better routing accuracy.

Building the Index

Use the CLI to build an index from your bundle:

greentic-fast2flow bundle index \
  --bundle ./my-bundle \
  --output ./state/indexes \
  --tenant demo \
  --team default \
  --verbose

This produces:

• index.json — TF-IDF index with term frequencies and document frequencies
• intents.md — Human-readable intent documentation

Validating a Bundle

greentic-fast2flow bundle validate --bundle ./my-bundle

Policy Configuration

Policies control routing behavior at runtime without code changes. They are JSON files loaded from /mnt/registry/fast2flow-policy.json or a custom path.

Policy Structure

fast2flow-policy.json

{
  "stage_order": ["scope", "channel", "provider"],
  "default": {
    "min_confidence": 0.5,
    "llm_min_confidence": 0.5,
    "candidate_limit": 20
  },
  "scope_overrides": [],
  "channel_overrides": [],
  "provider_overrides": []
}

Policy Rules

All rule fields are optional — only specified fields are applied:

Field	Type	Description
min_confidence	f32	Minimum BM25 score to dispatch (0.0–1.0)
llm_min_confidence	f32	Minimum LLM confidence to dispatch (0.0–1.0)
candidate_limit	usize	Maximum candidates to evaluate
allow_channels	string[]	Whitelist channels (null = allow all)
deny_channels	string[]	Blacklist channels
allow_providers	string[]	Whitelist providers (null = allow all)
deny_providers	string[]	Blacklist providers
allow_scopes	string[]	Whitelist scopes (null = allow all)
deny_scopes	string[]	Blacklist scopes
respond_rules	object[]	Auto-respond rules (keyword matching)

Override Examples

Overrides are applied in stage order (scope → channel → provider) with priority sorting within each stage.

Scope override — stricter confidence for a VIP tenant:

{
  "id": "vip-tenant",
  "priority": 10,
  "scope": "tenant-vip",
  "rules": {
    "min_confidence": 0.8,
    "candidate_limit": 10
  }
}

Channel override — auto-respond on email channel:

{
  "id": "email-autorespond",
  "priority": 20,
  "channel": "email",
  "rules": {
    "respond_rules": [
      {
        "needle": "refund",
        "message": "Refund requests via email take 3–5 business days. Use chat for instant support.",
        "mode": "contains"
      }
    ]
  }
}

Provider override — restrict to specific provider:

{
  "id": "slack-only",
  "priority": 30,
  "provider": "slack",
  "rules": {
    "deny_providers": ["telegram"]
  }
}

Respond Rules

Auto-respond rules match text before the routing pipeline runs:

{
  "needle": "business hours",
  "message": "Our business hours are Mon–Fri 9AM–5PM UTC.",
  "mode": "contains"
}

Supported modes: exact, contains (default), regex.

Policy Management CLI

# Print default policy
greentic-fast2flow policy print-default

# Validate a policy file
greentic-fast2flow policy validate --file ./my-policy.json

LLM Fallback

When the deterministic BM25 strategy produces low confidence scores, fast2flow can fall back to an LLM for classification.

Provider	Environment Variables
OpenAI	FAST2FLOW_OPENAI_API_KEY_PATH, FAST2FLOW_OPENAI_MODEL_PATH
Ollama	FAST2FLOW_OLLAMA_ENDPOINT_PATH, FAST2FLOW_OLLAMA_MODEL_PATH
Disabled	FAST2FLOW_LLM_PROVIDER=disabled (default)

# Enable OpenAI fallback
FAST2FLOW_LLM_PROVIDER=openai \
FAST2FLOW_OPENAI_API_KEY_PATH=/run/secrets/openai-key \
greentic-fast2flow-routing-host < request.json

Caution

LLM providers are only available in the native host binary. WASM component runtime requires FAST2FLOW_LLM_PROVIDER=disabled.

CLI Reference

Bundle Commands

# Build TF-IDF index from bundle
greentic-fast2flow bundle index \
  --bundle ./my-bundle \
  --output ./indexes \
  --tenant demo \
  --team default \
  --generate-docs \
  --verbose

# Validate bundle has indexable flows
greentic-fast2flow bundle validate --bundle ./my-bundle

Index Commands

# Build index from flow definitions JSON
greentic-fast2flow index build \
  --scope tenant-a \
  --flows flows.json \
  --output /tmp/indexes

# Inspect a built index
greentic-fast2flow index inspect \
  --scope tenant-a \
  --input /tmp/indexes

Route Commands

# Simulate a routing decision
greentic-fast2flow route simulate \
  --scope tenant-a \
  --text "I need a refund" \
  --indexes-path /tmp/indexes

Policy Commands

# Print default policy template
greentic-fast2flow policy print-default

# Validate policy file
greentic-fast2flow policy validate --file policy.json

Environment Variables

Variable	Default	Description
FAST2FLOW_LLM_PROVIDER	disabled	LLM provider: disabled, openai, ollama
FAST2FLOW_POLICY_PATH	/mnt/registry/fast2flow-policy.json	Policy file path
FAST2FLOW_TRACE_POLICY	—	Set to 1 to emit policy trace to stderr
FAST2FLOW_MIN_CONFIDENCE	0.5	Default minimum confidence threshold
FAST2FLOW_LLM_MIN_CONFIDENCE	0.5	Default LLM minimum confidence
FAST2FLOW_CANDIDATE_LIMIT	20	Default max candidates

Performance

fast2flow is optimized for low-latency routing:

Stage	Typical Latency
Hook filter (allow/deny)	< 0.1ms
BM25 index lookup	< 1ms
Policy resolution	< 0.1ms
LLM fallback (if enabled)	200–500ms

Tip

Start with deterministic BM25 routing. Only enable LLM fallback if your flows have overlapping intents that BM25 cannot distinguish.

Best Practices

1. Write descriptive titles — Title words get 2x TF-IDF boost for better scoring
2. Use specific tags — Tags are the primary signal for BM25 matching
3. Set appropriate thresholds — Start with min_confidence: 0.5 and tune up
4. Use policies for overrides — Change behavior per scope/channel/provider without redeploying
5. Monitor Continue rate — High Continue output indicates gaps in your flow coverage
6. Keep LLM as fallback — Deterministic routing is faster and more predictable

Next Steps

• Flows Guide — Learn about .ygtc flow definitions
• Packs Guide — Understand .gtpack distribution
• Control Chains Reference — Hook registration and policies