dspy-ruby

name	dspy-ruby
description	Build type-safe LLM applications with DSPy.rb -- Ruby's programmatic prompt framework with signatures, modules, agents, and optimization. Use when implementing predictable AI features, creating LLM signatures and modules, configuring language model providers, building agent systems with tools, optimizing prompts, or testing LLM-powered functionality in Ruby applications.

DSPy.rb

Build LLM apps like you build software. Type-safe, modular, testable.

DSPy.rb brings software engineering best practices to LLM development. Instead of tweaking prompts, define what you want with Ruby types and let DSPy handle the rest.

Overview

DSPy.rb is a Ruby framework for building language model applications with programmatic prompts. It provides:

• Type-safe signatures — Define inputs/outputs with Sorbet types
• Modular components — Compose and reuse LLM logic
• Automatic optimization — Use data to improve prompts, not guesswork
• Production-ready — Built-in observability, testing, and error handling

Core Concepts

1. Signatures

Define interfaces between your app and LLMs using Ruby types:

class EmailClassifier < DSPy::Signature
  description "Classify customer support emails by category and priority"

  class Priority < T::Enum
    enums do
      Low = new('low')
      Medium = new('medium')
      High = new('high')
      Urgent = new('urgent')
    end
  end

  input do
    const :email_content, String
    const :sender, String
  end

  output do
    const :category, String
    const :priority, Priority  # Type-safe enum with defined values
    const :confidence, Float
  end
end

2. Modules

Build complex workflows from simple building blocks:

• Predict — Basic LLM calls with signatures
• ChainOfThought — Step-by-step reasoning
• ReAct — Tool-using agents
• CodeAct — Dynamic code generation agents (install the dspy-code_act gem)

3. Tools & Toolsets

Create type-safe tools for agents with comprehensive Sorbet support:

# Enum-based tool with automatic type conversion
class CalculatorTool < DSPy::Tools::Base
  tool_name 'calculator'
  tool_description 'Performs arithmetic operations with type-safe enum inputs'

  class Operation < T::Enum
    enums do
      Add = new('add')
      Subtract = new('subtract')
      Multiply = new('multiply')
      Divide = new('divide')
    end
  end

  sig { params(operation: Operation, num1: Float, num2: Float).returns(T.any(Float, String)) }
  def call(operation:, num1:, num2:)
    case operation
    when Operation::Add then num1 + num2
    when Operation::Subtract then num1 - num2
    when Operation::Multiply then num1 * num2
    when Operation::Divide
      return "Error: Division by zero" if num2 == 0
      num1 / num2
    end
  end
end

# Multi-tool toolset with rich types
class DataToolset < DSPy::Tools::Toolset
  toolset_name "data_processing"

  class Format < T::Enum
    enums do
      JSON = new('json')
      CSV = new('csv')
      XML = new('xml')
    end
  end

  tool :convert, description: "Convert data between formats"
  tool :validate, description: "Validate data structure"

  sig { params(data: String, from: Format, to: Format).returns(String) }
  def convert(data:, from:, to:)
    "Converted from #{from.serialize} to #{to.serialize}"
  end

  sig { params(data: String, format: Format).returns(T::Hash[String, T.any(String, Integer, T::Boolean)]) }
  def validate(data:, format:)
    { valid: true, format: format.serialize, row_count: 42, message: "Data validation passed" }
  end
end

4. Type System & Discriminators

DSPy.rb uses sophisticated type discrimination for complex data structures:

• Automatic _type field injection — DSPy adds discriminator fields to structs for type safety
• Union type support — T.any() types automatically disambiguated by _type
• Reserved field name — Avoid defining your own _type fields in structs
• Recursive filtering — _type fields filtered during deserialization at all nesting levels

5. Optimization

Improve accuracy with real data:

• MIPROv2 — Advanced multi-prompt optimization with bootstrap sampling and Bayesian optimization
• GEPA — Genetic-Pareto Reflective Prompt Evolution with feedback maps, experiment tracking, and telemetry
• Evaluation — Comprehensive framework with built-in and custom metrics, error handling, and batch processing

Quick Start

# Install
gem 'dspy'

# Configure
DSPy.configure do |c|
  c.lm = DSPy::LM.new('openai/gpt-4o-mini', api_key: ENV['OPENAI_API_KEY'])
end

# Define a task
class SentimentAnalysis < DSPy::Signature
  description "Analyze sentiment of text"

  input do
    const :text, String
  end

  output do
    const :sentiment, String  # positive, negative, neutral
    const :score, Float       # 0.0 to 1.0
  end
end

# Use it
analyzer = DSPy::Predict.new(SentimentAnalysis)
result = analyzer.call(text: "This product is amazing!")
puts result.sentiment  # => "positive"
puts result.score      # => 0.92

Provider Adapter Gems

Two strategies for connecting to LLM providers:

Per-provider adapters (direct SDK access)

# Gemfile
gem 'dspy'
gem 'dspy-openai'    # OpenAI, OpenRouter, Ollama
gem 'dspy-anthropic' # Claude
gem 'dspy-gemini'    # Gemini

Each adapter gem pulls in the official SDK (openai, anthropic, gemini-ai).

Unified adapter via RubyLLM (recommended for multi-provider)

# Gemfile
gem 'dspy'
gem 'dspy-ruby_llm'  # Routes to any provider via ruby_llm
gem 'ruby_llm'

RubyLLM handles provider routing based on the model name. Use the ruby_llm/ prefix:

DSPy.configure do |c|
  c.lm = DSPy::LM.new('ruby_llm/gemini-2.5-flash', structured_outputs: true)
  # c.lm = DSPy::LM.new('ruby_llm/claude-sonnet-4-20250514', structured_outputs: true)
  # c.lm = DSPy::LM.new('ruby_llm/gpt-4o-mini', structured_outputs: true)
end

Events System

DSPy.rb ships with a structured event bus for observing runtime behavior.

Module-Scoped Subscriptions (preferred for agents)

class MyAgent < DSPy::Module
  subscribe 'lm.tokens', :track_tokens, scope: :descendants

  def track_tokens(_event, attrs)
    @total_tokens += attrs.fetch(:total_tokens, 0)
  end
end

Global Subscriptions (for observability/integrations)

subscription_id = DSPy.events.subscribe('score.create') do |event, attrs|
  Langfuse.export_score(attrs)
end

# Wildcards supported
DSPy.events.subscribe('llm.*') { |name, attrs| puts "[#{name}] tokens=#{attrs[:total_tokens]}" }

Event names use dot-separated namespaces (llm.generate, react.iteration_complete). Every event includes module metadata (module_path, module_leaf, module_scope.ancestry_token) for filtering.

Lifecycle Callbacks

Rails-style lifecycle hooks ship with every DSPy::Module:

• before — Runs ahead of forward for setup (metrics, context loading)
• around — Wraps forward, calls yield, and lets you pair setup/teardown logic
• after — Fires after forward returns for cleanup or persistence

class InstrumentedModule < DSPy::Module
  before :setup_metrics
  around :manage_context
  after :log_metrics

  def forward(question:)
    @predictor.call(question: question)
  end

  private

  def setup_metrics
    @start_time = Time.now
  end

  def manage_context
    load_context
    result = yield
    save_context
    result
  end

  def log_metrics
    duration = Time.now - @start_time
    Rails.logger.info "Prediction completed in #{duration}s"
  end
end

Execution order: before → around (before yield) → forward → around (after yield) → after. Callbacks are inherited from parent classes and execute in registration order.

Fiber-Local LM Context

Override the language model temporarily using fiber-local storage:

fast_model = DSPy::LM.new("openai/gpt-4o-mini", api_key: ENV['OPENAI_API_KEY'])

DSPy.with_lm(fast_model) do
  result = classifier.call(text: "test")  # Uses fast_model inside this block
end
# Back to global LM outside the block

LM resolution hierarchy: Instance-level LM → Fiber-local LM (DSPy.with_lm) → Global LM (DSPy.configure).

Use configure_predictor for fine-grained control over agent internals:

agent = DSPy::ReAct.new(MySignature, tools: tools)
agent.configure { |c| c.lm = default_model }
agent.configure_predictor('thought_generator') { |c| c.lm = powerful_model }

Evaluation Framework

Systematically test LLM application performance with DSPy::Evals:

metric = DSPy::Metrics.exact_match(field: :answer, case_sensitive: false)
evaluator = DSPy::Evals.new(predictor, metric: metric)
result = evaluator.evaluate(test_examples, display_table: true)
puts "Pass Rate: #{(result.pass_rate * 100).round(1)}%"

Built-in metrics: exact_match, contains, numeric_difference, composite_and. Custom metrics return true/false or a DSPy::Prediction with score: and feedback: fields.

Use DSPy::Example for typed test data and export_scores: true to push results to Langfuse.

GEPA Optimization

GEPA (Genetic-Pareto Reflective Prompt Evolution) uses reflection-driven instruction rewrites:

gem 'dspy-gepa'

teleprompter = DSPy::Teleprompt::GEPA.new(
  metric: metric,
  reflection_lm: DSPy::ReflectionLM.new('openai/gpt-4o-mini', api_key: ENV['OPENAI_API_KEY']),
  feedback_map: feedback_map,
  config: { max_metric_calls: 600, minibatch_size: 6 }
)

result = teleprompter.compile(program, trainset: train, valset: val)
optimized_program = result.optimized_program

The metric must return DSPy::Prediction.new(score:, feedback:) so the reflection model can reason about failures. Use feedback_map to target individual predictors in composite modules.

Typed Context Pattern

Replace opaque string context blobs with T::Struct inputs. Each field gets its own description: annotation in the JSON schema the LLM sees:

class NavigationContext < T::Struct
  const :workflow_hint, T.nilable(String),
        description: "Current workflow phase guidance for the agent"
  const :action_log, T::Array[String], default: [],
        description: "Compact one-line-per-action history of research steps taken"
  const :iterations_remaining, Integer,
        description: "Budget remaining. Each tool call costs 1 iteration."
end

class ToolSelectionSignature < DSPy::Signature
  input do
    const :query, String
    const :context, NavigationContext  # Structured, not an opaque string
  end

  output do
    const :tool_name, String
    const :tool_args, String, description: "JSON-encoded arguments"
  end
end

Benefits: type safety at compile time, per-field descriptions in the LLM schema, easy to test as value objects, extensible by adding const declarations.

Schema Formats (BAML / TOON)

Control how DSPy describes signature structure to the LLM:

• JSON Schema (default) — Standard format, works with structured_outputs: true
• BAML (schema_format: :baml) — 84% token reduction for Enhanced Prompting mode. Requires sorbet-baml gem.
• TOON (schema_format: :toon, data_format: :toon) — Table-oriented format for both schemas and data. Enhanced Prompting mode only.

BAML and TOON apply only when structured_outputs: false. With structured_outputs: true, the provider receives JSON Schema directly.

Storage System

Persist and reload optimized programs with DSPy::Storage::ProgramStorage:

storage = DSPy::Storage::ProgramStorage.new(storage_path: "./dspy_storage")
storage.save_program(result.optimized_program, result, metadata: { optimizer: 'MIPROv2' })

Supports checkpoint management, optimization history tracking, and import/export between environments.

Rails Integration

Directory Structure

Organize DSPy components using Rails conventions:

app/
  entities/          # T::Struct types shared across signatures
  signatures/        # DSPy::Signature definitions
  tools/             # DSPy::Tools::Base implementations
    concerns/        # Shared tool behaviors (error handling, etc.)
  modules/           # DSPy::Module orchestrators
  services/          # Plain Ruby services that compose DSPy modules
config/
  initializers/
    dspy.rb          # DSPy + provider configuration
    feature_flags.rb # Model selection per role
spec/
  signatures/        # Schema validation tests
  tools/             # Tool unit tests
  modules/           # Integration tests with VCR
  vcr_cassettes/     # Recorded HTTP interactions

Initializer

# config/initializers/dspy.rb
Rails.application.config.after_initialize do
  next if Rails.env.test? && ENV["DSPY_ENABLE_IN_TEST"].blank?

  RubyLLM.configure do |config|
    config.gemini_api_key = ENV["GEMINI_API_KEY"] if ENV["GEMINI_API_KEY"].present?
    config.anthropic_api_key = ENV["ANTHROPIC_API_KEY"] if ENV["ANTHROPIC_API_KEY"].present?
    config.openai_api_key = ENV["OPENAI_API_KEY"] if ENV["OPENAI_API_KEY"].present?
  end

  model = ENV.fetch("DSPY_MODEL", "ruby_llm/gemini-2.5-flash")
  DSPy.configure do |config|
    config.lm = DSPy::LM.new(model, structured_outputs: true)
    config.logger = Rails.logger
  end

  # Langfuse observability (optional)
  if ENV["LANGFUSE_PUBLIC_KEY"].present? && ENV["LANGFUSE_SECRET_KEY"].present?
    DSPy::Observability.configure!
  end
end

Feature-Flagged Model Selection

Use different models for different roles (fast/cheap for classification, powerful for synthesis):

# config/initializers/feature_flags.rb
module FeatureFlags
  SELECTOR_MODEL = ENV.fetch("DSPY_SELECTOR_MODEL", "ruby_llm/gemini-2.5-flash-lite")
  SYNTHESIZER_MODEL = ENV.fetch("DSPY_SYNTHESIZER_MODEL", "ruby_llm/gemini-2.5-flash")
end

Then override per-tool or per-predictor:

class ClassifyTool < DSPy::Tools::Base
  def call(query:)
    predictor = DSPy::Predict.new(ClassifyQuery)
    predictor.configure { |c| c.lm = DSPy::LM.new(FeatureFlags::SELECTOR_MODEL, structured_outputs: true) }
    predictor.call(query: query)
  end
end

Schema-Driven Signatures

Prefer typed schemas over string descriptions. Let the type system communicate structure to the LLM rather than prose in the signature description.

Entities as Shared Types

Define reusable T::Struct and T::Enum types in app/entities/ and reference them across signatures:

# app/entities/search_strategy.rb
class SearchStrategy < T::Enum
  enums do
    SingleSearch = new("single_search")
    DateDecomposition = new("date_decomposition")
  end
end

# app/entities/scored_item.rb
class ScoredItem < T::Struct
  const :id, String
  const :score, Float, description: "Relevance score 0.0-1.0"
  const :verdict, String, description: "relevant, maybe, or irrelevant"
  const :reason, String, default: ""
end

Schema vs Description: When to Use Each

Use schemas (T::Struct/T::Enum) for:

• Multi-field outputs with specific types
• Enums with defined values the LLM must pick from
• Nested structures, arrays of typed objects
• Outputs consumed by code (not displayed to users)

Use string descriptions for:

• Simple single-field outputs where the type is String
• Natural language generation (summaries, answers)
• Fields where constraint guidance helps (e.g., description: "YYYY-MM-DD format")

Rule of thumb: If you'd write a case statement on the output, it should be a T::Enum. If you'd call .each on it, it should be T::Array[SomeStruct].

Tool Patterns

Tools That Wrap Predictions

A common pattern: tools encapsulate a DSPy prediction, adding error handling, model selection, and serialization:

class RerankTool < DSPy::Tools::Base
  tool_name "rerank"
  tool_description "Score and rank search results by relevance"

  MAX_ITEMS = 200
  MIN_ITEMS_FOR_LLM = 5

  sig { params(query: String, items: T::Array[T::Hash[Symbol, T.untyped]]).returns(T::Hash[Symbol, T.untyped]) }
  def call(query:, items: [])
    return { scored_items: items, reranked: false } if items.size < MIN_ITEMS_FOR_LLM

    capped_items = items.first(MAX_ITEMS)
    predictor = DSPy::Predict.new(RerankSignature)
    predictor.configure { |c| c.lm = DSPy::LM.new(FeatureFlags::SYNTHESIZER_MODEL, structured_outputs: true) }

    result = predictor.call(query: query, items: capped_items)
    { scored_items: result.scored_items, reranked: true }
  rescue => e
    Rails.logger.warn "[RerankTool] LLM rerank failed: #{e.message}"
    { error: "Rerank failed: #{e.message}", scored_items: items, reranked: false }
  end
end

Key patterns:

• Short-circuit LLM calls when unnecessary (small data, trivial cases)
• Cap input size to prevent token overflow
• Per-tool model selection via configure
• Graceful error handling with fallback data

Error Handling Concern

module ErrorHandling
  extend ActiveSupport::Concern

  private

  def safe_predict(signature_class, **inputs)
    predictor = DSPy::Predict.new(signature_class)
    yield predictor if block_given?
    predictor.call(**inputs)
  rescue Faraday::Error, Net::HTTPError => e
    Rails.logger.error "[#{self.class.name}] API error: #{e.message}"
    nil
  rescue JSON::ParserError => e
    Rails.logger.error "[#{self.class.name}] Invalid LLM output: #{e.message}"
    nil
  end
end

Observability

Tracing with DSPy::Context

Wrap operations in spans for Langfuse/OpenTelemetry visibility:

result = DSPy::Context.with_span(
  operation: "tool_selector.select",
  "dspy.module" => "ToolSelector",
  "tool_selector.tools" => tool_names.join(",")
) do
  @predictor.call(query: query, context: context, available_tools: schemas)
end

Setup for Langfuse

# Gemfile
gem 'dspy-o11y'
gem 'dspy-o11y-langfuse'

# .env
LANGFUSE_PUBLIC_KEY=pk-...
LANGFUSE_SECRET_KEY=sk-...
DSPY_TELEMETRY_BATCH_SIZE=5

Every DSPy::Predict, DSPy::ReAct, and tool call is automatically traced when observability is configured.

Score Reporting

Report evaluation scores to Langfuse:

DSPy.score(name: "relevance", value: 0.85, trace_id: current_trace_id)

Testing

VCR Setup for Rails

VCR.configure do |config|
  config.cassette_library_dir = "spec/vcr_cassettes"
  config.hook_into :webmock
  config.configure_rspec_metadata!
  config.filter_sensitive_data('<GEMINI_API_KEY>') { ENV['GEMINI_API_KEY'] }
  config.filter_sensitive_data('<OPENAI_API_KEY>') { ENV['OPENAI_API_KEY'] }
end

Signature Schema Tests

Test that signatures produce valid schemas without calling any LLM:

RSpec.describe ClassifyResearchQuery do
  it "has required input fields" do
    schema = described_class.input_json_schema
    expect(schema[:required]).to include("query")
  end

  it "has typed output fields" do
    schema = described_class.output_json_schema
    expect(schema[:properties]).to have_key(:search_strategy)
  end
end

Tool Tests with Mocked Predictions

RSpec.describe RerankTool do
  let(:tool) { described_class.new }

  it "skips LLM for small result sets" do
    expect(DSPy::Predict).not_to receive(:new)
    result = tool.call(query: "test", items: [{ id: "1" }])
    expect(result[:reranked]).to be false
  end

  it "calls LLM for large result sets", :vcr do
    items = 10.times.map { |i| { id: i.to_s, title: "Item #{i}" } }
    result = tool.call(query: "relevant items", items: items)
    expect(result[:reranked]).to be true
  end
end

Resources

• references/core-concepts.md — Signatures, modules, predictors, type system deep-dive
• references/toolsets.md — Tools::Base, Tools::Toolset DSL, type safety, testing
• references/providers.md — Provider adapters, RubyLLM, fiber-local LM context, compatibility matrix
• references/optimization.md — MIPROv2, GEPA, evaluation framework, storage system
• references/observability.md — Event system, dspy-o11y gems, Langfuse, score reporting
• assets/signature-template.rb — Signature scaffold with T::Enum, Date/Time, defaults, union types
• assets/module-template.rb — Module scaffold with .call(), lifecycle callbacks, fiber-local LM
• assets/config-template.rb — Rails initializer with RubyLLM, observability, feature flags

Key URLs

• Homepage: https://oss.vicente.services/dspy.rb/
• GitHub: https://github.com/vicentereig/dspy.rb
• Documentation: https://oss.vicente.services/dspy.rb/getting-started/

Guidelines for Copilot

When helping users with DSPy.rb:

1. Schema over prose — Define output structure with T::Struct and T::Enum types, not string descriptions
2. Entities in app/entities/ — Extract shared types so signatures stay thin
3. Per-tool model selection — Use predictor.configure { |c| c.lm = ... } to pick the right model per task
4. Short-circuit LLM calls — Skip the LLM for trivial cases (small data, cached results)
5. Cap input sizes — Prevent token overflow by limiting array sizes before sending to LLM
6. Test schemas without LLM — Validate input_json_schema and output_json_schema in unit tests
7. VCR for integration tests — Record real HTTP interactions, never mock LLM responses by hand
8. Trace with spans — Wrap tool calls in DSPy::Context.with_span for observability
9. Graceful degradation — Always rescue LLM errors and return fallback data

Signature Best Practices

Keep description concise — The signature description should state the goal, not the field details:

# Good — concise goal
class ParseOutline < DSPy::Signature
  description 'Extract block-level structure from HTML as a flat list of skeleton sections.'

  input do
    const :html, String, description: 'Raw HTML to parse'
  end

  output do
    const :sections, T::Array[Section], description: 'Block elements: headings, paragraphs, code blocks, lists'
  end
end

Use defaults over nilable arrays — For OpenAI structured outputs compatibility:

# Good — works with OpenAI structured outputs
class ASTNode < T::Struct
  const :children, T::Array[ASTNode], default: []
end

Recursive Types with $defs

DSPy.rb supports recursive types in structured outputs using JSON Schema $defs:

class TreeNode < T::Struct
  const :value, String
  const :children, T::Array[TreeNode], default: []  # Self-reference
end

The schema generator automatically creates #/$defs/TreeNode references for recursive types, compatible with OpenAI and Gemini structured outputs.

Field Descriptions for T::Struct

DSPy.rb extends T::Struct to support field-level description: kwargs that flow to JSON Schema:

class ASTNode < T::Struct
  const :node_type, NodeType, description: 'The type of node (heading, paragraph, etc.)'
  const :text, String, default: "", description: 'Text content of the node'
  const :level, Integer, default: 0  # No description — field is self-explanatory
  const :children, T::Array[ASTNode], default: []
end

When to use field descriptions: complex field semantics, enum-like strings, constrained values, nested structs with ambiguous names. When to skip: self-explanatory fields like name, id, url, or boolean flags.

Version

Current: 0.34.3