Architecture Patterns
PromptPack has grown from a simple prompt-packaging format into a full-stack specification for conversational AI systems. This page explains how the major building blocks relate to each other and when to use each one.
How the Pieces Fit Together
A PromptPack is organized in layers. Lower layers are simpler and more universal; higher layers are optional and compose on top of the lower ones.
┌─────────────────────────────────────────────┐
│ Agents Inter-system discovery (A2A) │ v1.3
├─────────────────────────────────────────────┤
│ Workflow Intra-pack state machine │ v1.3
├─────────────────────────────────────────────┤
│ Evals Async quality measurement │ v1.2
├─────────────────────────────────────────────┤
│ Validators Inline guardrails │ v1.0
├─────────────────────────────────────────────┤
│ Tools & Fragments Shared resources │ v1.0
├─────────────────────────────────────────────┤
│ Prompts Core behavior definitions │ v1.0
└─────────────────────────────────────────────┘
Prompts define what the LLM does. Tools & Fragments provide shared resources that prompts reference. Validators add inline guardrails that block bad output. Evals add async quality measurement that scores and reports. Workflow orchestrates transitions between prompts via a state machine. Agents expose prompts as discoverable services via the A2A protocol.
Each layer is optional — a valid PromptPack only requires id, name, version, template_engine, and at least one prompt. You adopt higher layers only when you need them.
Workflows vs Agents
These two v1.3 features solve different problems and can be used independently or together.
| Aspect | Workflow | Agents |
|---|---|---|
| Purpose | Orchestrate transitions within a pack | Make prompts discoverable across systems |
| Mechanism | State machine with event-driven transitions | A2A Agent Cards with metadata and tags |
| Scope | Intra-pack (states reference prompt keys) | Inter-system (external services discover agents) |
| Requires | workflow top-level field | agents top-level field |
| Entry point | workflow.entry — first state in the machine | agents.entry — default agent for incoming requests |
| Key benefit | Automated routing without caller logic | Multi-agent discovery and interoperability |
Decision Matrix
| Scenario | Use Workflow? | Use Agents? |
|---|---|---|
| Single prompt, no routing needed | — | — |
| Multiple prompts, caller chooses which to invoke | — | — |
| Automated routing between prompts based on events | Yes | — |
| Prompts need to be discoverable by external systems | — | Yes |
| Automated routing and external discovery | Yes | Yes |
| Standalone agents communicating via A2A (no internal state machine) | — | Yes |
Workflow and agents are orthogonal. A prompt can participate in a workflow state and be an agent member simultaneously. The workflow manages intra-pack state; agents manage inter-system discovery.
Validators vs Evals
Both assess LLM output quality, but they operate at different points in the pipeline and serve different purposes.
| Aspect | Validators | Evals |
|---|---|---|
| Runs when? | Inline, every response | Async, on schedule or trigger |
| Blocks output? | Yes (when fail_on_violation: true) | No — scores and reports |
| Scope | Prompt-level only | Prompt-level or pack-level |
| Type system | Enum with custom escape hatch | Free string (runtimes define types) |
| Metrics | None | Prometheus-style metric declarations |
| Version introduced | v1.0 | v1.2 |
Decision Matrix
| Scenario | Use Validators? | Use Evals? |
|---|---|---|
| Block toxic or unsafe content before it reaches users | Yes | — |
| Monitor tone quality over time with dashboards | — | Yes |
| Enforce character limits on social media responses | Yes | — |
| Measure brand voice consistency across all prompts | — | Yes |
| PII detection that must never leak | Yes | — |
| Sample 10% of responses for LLM-judge quality scoring | — | Yes |
| Both block bad output and track quality trends | Yes | Yes |
Validators and evals are complementary, not competitive. Use validators for hard safety guardrails and evals for continuous quality monitoring. The same prompt engineer typically authors both.
Orchestration Patterns
The workflow state machine supports several common multi-prompt patterns. Choose based on your use case.
Router + Specialists
A triage prompt classifies requests and routes to specialized prompts.
┌─ billing ─────┐
triage ──┤ ├── closing
└─ technical ───┘
When to use: Customer support, help desks, any system where incoming requests need classification before handling.
Pipeline
Prompts execute in sequence, each processing the output of the previous one.
intake ── analyze ── draft ── review
When to use: Document processing, content generation pipelines, multi-step analysis.
Agent Mesh
Multiple agents communicate via A2A without a central workflow. Each agent discovers and invokes others through tool references.
researcher ←→ fact_checker ←→ writer
When to use: Loosely coupled agents that need to collaborate without rigid sequencing. Use the agents section without workflow.
Hybrid
Combine workflow orchestration internally with agent discovery externally.
[Workflow: triage → specialist → closing]
↕ A2A
[External systems discover and invoke agents]
When to use: Systems that need both internal routing logic and external interoperability.
Multimodal Integration
The media configuration (v1.1+) composes with all other features:
- With Prompts: Each prompt can declare its own
mediaconfig — supported types, format restrictions, size limits, and multimodal examples withpartsarrays - With Validators: Validators run on the text output regardless of whether the input was multimodal
- With Evals: Evals can assess multimodal interactions (e.g., an LLM judge evaluating whether the response correctly interpreted an image)
- With Workflow: A workflow state can route to a prompt that accepts images, while another state routes to a text-only prompt
- With Agents: Agent input/output modes (
input_modes,output_modes) declare which MIME types the agent supports, complementing the prompt-levelmediaconfig
Multimodal and text-only prompts can coexist in the same pack. A pack might have an image-aware product_lookup prompt and a text-only catalog_writer prompt — the media config is per-prompt, not per-pack.
Feature Compatibility Matrix
| Feature | Version | Combines With |
|---|---|---|
| Prompts | v1.0 | Everything |
| Tools & Fragments | v1.0 | Prompts, Workflow, Agents |
| Validators | v1.0 | Prompts |
| Parameters | v1.0 | Prompts, Model Overrides |
| Tested Models | v1.0 | Prompts |
| Model Overrides | v1.0 | Prompts |
| Media (multimodal) | v1.1 | Prompts, Agents (MIME types) |
| Evals | v1.2 | Prompts (prompt-level), Pack (pack-level) |
| Workflow | v1.3 | Prompts (via prompt_task), Agents |
| Agents | v1.3 | Prompts (via members), Workflow |
Next Steps
- Build your first workflow: How to Add a Workflow
- Set up agents: How to Set Up Agents
- Add quality monitoring: How to Add Evals
- See full examples: Real-World Examples
- Design rationale: RFC 0005: Workflow Extension · RFC 0007: Agents Extension · RFC 0006: Evals Extension