From cc3ca9384229f05243affcef86c53364f7e2eb9a Mon Sep 17 00:00:00 2001
From: Matt Perpick <matt@braintrustdata.com>
Date: Fri, 9 Jan 2026 10:44:22 -0500
Subject: [PATCH 1/2] handle prompts, templating, etc.

---
 .claude/settings.json                         |  47 ++
 examples/prompt.rb                            |  95 ++++
 lib/braintrust.rb                             |   1 +
 lib/braintrust/api/functions.rb               |   8 +
 lib/braintrust/prompt.rb                      | 172 +++++++
 test/braintrust/api/functions_test.rb         |  51 ++
 test/braintrust/prompt_test.rb                | 203 ++++++++
 test/fixtures/vcr_cassettes/functions/get.yml | 449 ++++++++++++++++++
 test/fixtures/vcr_cassettes/prompt/load.yml   | 389 +++++++++++++++
 .../vcr_cassettes/prompt/load_not_found.yml   | 143 ++++++
 10 files changed, 1558 insertions(+)
 create mode 100644 .claude/settings.json
 create mode 100644 examples/prompt.rb
 create mode 100644 lib/braintrust/prompt.rb
 create mode 100644 test/braintrust/prompt_test.rb
 create mode 100644 test/fixtures/vcr_cassettes/functions/get.yml
 create mode 100644 test/fixtures/vcr_cassettes/prompt/load.yml
 create mode 100644 test/fixtures/vcr_cassettes/prompt/load_not_found.yml

diff --git a/.claude/settings.json b/.claude/settings.json
new file mode 100644
index 0000000..21984fd
--- /dev/null
+++ b/.claude/settings.json
@@ -0,0 +1,47 @@
+{
+  "permissions": {
+    "allow": [
+      "Bash(git log:*)",
+      "Bash(git branch:*)",
+      "Bash(git remote:*)",
+      "Bash(git fetch:*)",
+      "Bash(git stash:*)",
+      "Bash(git diff:*)",
+      "Bash(git status:*)",
+      "Bash(git show:*)",
+      "Bash(gh repo view:*)",
+      "Bash(gh run list:*)",
+      "Bash(gh run view:*)",
+      "Bash(gh pr view:*)",
+      "Bash(gh pr diff:*)",
+      "Bash(gh pr list:*)",
+      "Bash(gh issue view:*)",
+      "Bash(gh release:*)",
+      "Bash(gh api:*)",
+      "Bash(rake -T:*)",
+      "Bash(bundle show:*)",
+      "Bash(bundle list:*)",
+      "Bash(bundle platform:*)",
+      "Bash(gem search:*)",
+      "Bash(gem list:*)",
+      "Bash(tree:*)",
+      "Bash(find:*)",
+      "Bash(echo:*)",
+      "Bash(sort:*)",
+      "Bash(cat:*)",
+      "Bash(head:*)",
+      "Bash(tail:*)",
+      "Bash(less:*)",
+      "Bash(wc:*)",
+      "Bash(ls:*)",
+      "Bash(pwd:*)",
+      "Bash(which:*)",
+      "Bash(type:*)",
+      "Bash(file:*)",
+      "Bash(VCR_MODE=new_episodes bundle exec ruby:*)",
+      "Bash(VCR_MODE=new_episodes bundle exec rake:*)",
+      "Bash(VCR_MODE=new_episodes rake test:*)"
+    ],
+    "deny": []
+  }
+}
diff --git a/examples/prompt.rb b/examples/prompt.rb
new file mode 100644
index 0000000..32a51e3
--- /dev/null
+++ b/examples/prompt.rb
@@ -0,0 +1,95 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+# Example: Loading and using prompts from Braintrust
+#
+# This example demonstrates how to:
+# 1. Create a prompt (function) on the Braintrust server
+# 2. Load it using Prompt.load
+# 3. Build the prompt with variable substitution
+# 4. Use the built prompt with an LLM client
+#
+# Benefits of loading prompts:
+# - Centralized prompt management in Braintrust UI
+# - Version control and A/B testing for prompts
+# - No code deployment needed for prompt changes
+# - Works with any LLM client (OpenAI, Anthropic, etc.)
+
+require "bundler/setup"
+require "braintrust"
+
+# Initialize Braintrust
+Braintrust.init
+
+project_name = "ruby-sdk-examples"
+prompt_slug = "greeting-prompt-#{Time.now.to_i}"
+
+# First, create a prompt on the server
+# In practice, you would create prompts via the Braintrust UI
+puts "Creating prompt..."
+
+api = Braintrust::API.new
+api.functions.create(
+  project_name: project_name,
+  slug: prompt_slug,
+  function_data: {type: "prompt"},
+  prompt_data: {
+    prompt: {
+      type: "chat",
+      messages: [
+        {
+          role: "system",
+          content: "You are a friendly assistant who speaks {{language}}."
+        },
+        {
+          role: "user",
+          content: "Say hello to {{name}} and wish them a great {{time_of_day}}!"
+        }
+      ]
+    },
+    options: {
+      model: "gpt-4o-mini",
+      params: {temperature: 0.7, max_tokens: 100}
+    }
+  }
+)
+puts "Created prompt: #{prompt_slug}"
+
+# Load the prompt using Prompt.load
+puts "\nLoading prompt..."
+prompt = Braintrust::Prompt.load(project: project_name, slug: prompt_slug)
+
+puts "  ID: #{prompt.id}"
+puts "  Name: #{prompt.name}"
+puts "  Model: #{prompt.model}"
+puts "  Messages: #{prompt.messages.length}"
+
+# Build the prompt with variable substitution
+puts "\nBuilding prompt with variables..."
+params = prompt.build(
+  name: "Alice",
+  language: "Spanish",
+  time_of_day: "morning"
+)
+
+puts "  Model: #{params[:model]}"
+puts "  Temperature: #{params[:temperature]}"
+puts "  Max tokens: #{params[:max_tokens]}"
+puts "  Messages:"
+params[:messages].each do |msg|
+  puts "    [#{msg[:role]}] #{msg[:content]}"
+end
+
+# The params hash is ready to pass to any LLM client:
+#
+# With OpenAI:
+#   client.chat.completions.create(**params)
+#
+# With Anthropic:
+#   client.messages.create(**params)
+
+puts "\nPrompt is ready to use with any LLM client!"
+
+# Clean up - delete the test prompt
+api.functions.delete(id: prompt.id)
+puts "Cleaned up test prompt."
diff --git a/lib/braintrust.rb b/lib/braintrust.rb
index 469cde1..7e6b676 100644
--- a/lib/braintrust.rb
+++ b/lib/braintrust.rb
@@ -5,6 +5,7 @@
 require_relative "braintrust/state"
 require_relative "braintrust/trace"
 require_relative "braintrust/api"
+require_relative "braintrust/prompt"
 require_relative "braintrust/internal/experiments"
 require_relative "braintrust/eval"
 
diff --git a/lib/braintrust/api/functions.rb b/lib/braintrust/api/functions.rb
index ccfea31..66d82b0 100644
--- a/lib/braintrust/api/functions.rb
+++ b/lib/braintrust/api/functions.rb
@@ -85,6 +85,14 @@ def invoke(id:, input:)
         http_post_json("/v1/function/#{id}/invoke", payload)
       end
 
+      # Get a function by ID (includes full prompt_data)
+      # GET /v1/function/{id}
+      # @param id [String] Function UUID
+      # @return [Hash] Full function data including prompt_data
+      def get(id:)
+        http_get("/v1/function/#{id}")
+      end
+
       # Delete a function by ID
       # DELETE /v1/function/{id}
       # @param id [String] Function UUID
diff --git a/lib/braintrust/prompt.rb b/lib/braintrust/prompt.rb
new file mode 100644
index 0000000..d417e95
--- /dev/null
+++ b/lib/braintrust/prompt.rb
@@ -0,0 +1,172 @@
+# frozen_string_literal: true
+
+module Braintrust
+  # Prompt class for loading and building prompts from Braintrust
+  #
+  # @example Load and use a prompt
+  #   prompt = Braintrust::Prompt.load(project: "my-project", slug: "summarizer")
+  #   params = prompt.build(text: "Article to summarize...")
+  #   client.messages.create(**params)
+  class Prompt
+    attr_reader :id, :name, :slug, :project_id
+
+    # Load a prompt from Braintrust
+    #
+    # @param project [String] Project name
+    # @param slug [String] Prompt slug
+    # @param version [String, nil] Specific version (default: latest)
+    # @param defaults [Hash] Default variable values for build()
+    # @param state [State, nil] Braintrust state (default: global)
+    # @return [Prompt]
+    def self.load(project:, slug:, version: nil, defaults: {}, state: nil)
+      state ||= Braintrust.current_state
+      raise Error, "No state available - call Braintrust.init first" unless state
+
+      api = API.new(state: state)
+
+      # Find the function by project + slug
+      result = api.functions.list(project_name: project, slug: slug)
+      function = result.dig("objects")&.first
+      raise Error, "Prompt '#{slug}' not found in project '#{project}'" unless function
+
+      # Fetch full function data including prompt_data
+      full_data = api.functions.get(id: function["id"])
+
+      new(full_data, defaults: defaults)
+    end
+
+    # Initialize a Prompt from function data
+    #
+    # @param data [Hash] Function data from API
+    # @param defaults [Hash] Default variable values for build()
+    def initialize(data, defaults: {})
+      @data = data
+      @defaults = stringify_keys(defaults)
+
+      @id = data["id"]
+      @name = data["name"]
+      @slug = data["slug"]
+      @project_id = data["project_id"]
+    end
+
+    # Get the raw prompt definition
+    # @return [Hash, nil]
+    def prompt
+      @data.dig("prompt_data", "prompt")
+    end
+
+    # Get the prompt messages
+    # @return [Array<Hash>]
+    def messages
+      prompt&.dig("messages") || []
+    end
+
+    # Get the model name
+    # @return [String, nil]
+    def model
+      @data.dig("prompt_data", "options", "model")
+    end
+
+    # Get model options
+    # @return [Hash]
+    def options
+      @data.dig("prompt_data", "options") || {}
+    end
+
+    # Build the prompt with variable substitution
+    #
+    # Returns a hash ready to pass to an LLM client:
+    #   {model: "...", messages: [...], temperature: ..., ...}
+    #
+    # @param variables [Hash] Variables to substitute (e.g., {name: "Alice"})
+    # @param strict [Boolean] Raise error on missing variables (default: false)
+    # @return [Hash] Built prompt ready for LLM client
+    #
+    # @example With keyword arguments
+    #   prompt.build(name: "Alice", task: "coding")
+    #
+    # @example With explicit hash
+    #   prompt.build({name: "Alice"}, strict: true)
+    def build(variables = nil, strict: false, **kwargs)
+      # Support both explicit hash and keyword arguments
+      variables_hash = variables.is_a?(Hash) ? variables : {}
+      vars = @defaults.merge(stringify_keys(variables_hash)).merge(stringify_keys(kwargs))
+
+      # Substitute variables in messages
+      built_messages = messages.map do |msg|
+        {
+          role: msg["role"].to_sym,
+          content: substitute_variables(msg["content"], vars, strict: strict)
+        }
+      end
+
+      # Build result with model and messages
+      result = {
+        model: model,
+        messages: built_messages
+      }
+
+      # Add params (temperature, max_tokens, etc.) to top level
+      params = options.dig("params")
+      if params.is_a?(Hash)
+        params.each do |key, value|
+          result[key.to_sym] = value
+        end
+      end
+
+      result
+    end
+
+    private
+
+    # Substitute {{variable}} placeholders with values
+    def substitute_variables(text, variables, strict:)
+      return text unless text.is_a?(String)
+
+      # Find all {{variable}} patterns
+      missing = []
+
+      result = text.gsub(/\{\{([^}]+)\}\}/) do |match|
+        var_path = ::Regexp.last_match(1).strip
+        value = resolve_variable(var_path, variables)
+
+        if value.nil?
+          missing << var_path
+          match # Keep original placeholder
+        else
+          value.to_s
+        end
+      end
+
+      if strict && missing.any?
+        raise Error, "Missing required variables: #{missing.join(", ")}"
+      end
+
+      result
+    end
+
+    # Resolve a variable path like "user.name" from variables hash
+    def resolve_variable(path, variables)
+      parts = path.split(".")
+      value = variables
+
+      parts.each do |part|
+        return nil unless value.is_a?(Hash)
+        # Try both string and symbol keys
+        value = value[part] || value[part.to_sym]
+        return nil if value.nil?
+      end
+
+      value
+    end
+
+    # Convert hash keys to strings (handles both symbol and string keys)
+    def stringify_keys(hash)
+      return {} unless hash.is_a?(Hash)
+
+      hash.transform_keys(&:to_s).transform_values do |v|
+        v.is_a?(Hash) ? stringify_keys(v) : v
+      end
+    end
+  end
+end
diff --git a/test/braintrust/api/functions_test.rb b/test/braintrust/api/functions_test.rb
index 753c1d2..bdb6e5b 100644
--- a/test/braintrust/api/functions_test.rb
+++ b/test/braintrust/api/functions_test.rb
@@ -378,4 +378,55 @@ def test_helper_validates_prompt_data_has_prompt_key
       end
     end
   end
+
+  def test_functions_get_by_id
+    VCR.use_cassette("functions/get") do
+      api = get_test_api
+      # This test verifies that we can get the full function data by ID,
+      # including prompt_data which is needed for Prompt.load()
+      function_slug = "test-ruby-sdk-get-func"
+
+      # Create a function with prompt_data
+      create_response = api.functions.create(
+        project_name: @project_name,
+        slug: function_slug,
+        function_data: {type: "prompt"},
+        prompt_data: {
+          prompt: {
+            type: "chat",
+            messages: [
+              {role: "system", content: "You are a helpful assistant."},
+              {role: "user", content: "Hello {{name}}"}
+            ]
+          },
+          options: {
+            model: "gpt-4o-mini",
+            params: {temperature: 0.7}
+          }
+        }
+      )
+      function_id = create_response["id"]
+
+      # Get the full function data
+      result = api.functions.get(id: function_id)
+
+      assert_instance_of Hash, result
+      assert_equal function_id, result["id"]
+      assert_equal function_slug, result["slug"]
+
+      # Verify prompt_data is included
+      assert result.key?("prompt_data")
+      prompt_data = result["prompt_data"]
+      assert prompt_data.key?("prompt")
+      assert prompt_data["prompt"].key?("messages")
+      assert_equal 2, prompt_data["prompt"]["messages"].length
+
+      # Verify options are included
+      assert prompt_data.key?("options")
+      assert_equal "gpt-4o-mini", prompt_data["options"]["model"]
+
+      # Clean up
+      api.functions.delete(id: function_id)
+    end
+  end
 end
diff --git a/test/braintrust/prompt_test.rb b/test/braintrust/prompt_test.rb
new file mode 100644
index 0000000..14ed7d1
--- /dev/null
+++ b/test/braintrust/prompt_test.rb
@@ -0,0 +1,203 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+class Braintrust::PromptTest < Minitest::Test
+  def setup
+    # Sample function data as returned by API
+    @function_data = {
+      "id" => "func-123",
+      "name" => "Test Prompt",
+      "slug" => "test-prompt",
+      "project_id" => "proj-456",
+      "prompt_data" => {
+        "prompt" => {
+          "type" => "chat",
+          "messages" => [
+            {"role" => "system", "content" => "You are a helpful assistant."},
+            {"role" => "user", "content" => "Hello {{name}}, please help with {{task}}."}
+          ]
+        },
+        "options" => {
+          "model" => "claude-3-5-sonnet",
+          "params" => {"temperature" => 0.7, "max_tokens" => 1000}
+        }
+      }
+    }
+  end
+
+  def test_prompt_initialization
+    prompt = Braintrust::Prompt.new(@function_data)
+
+    assert_equal "func-123", prompt.id
+    assert_equal "Test Prompt", prompt.name
+    assert_equal "test-prompt", prompt.slug
+    assert_equal "proj-456", prompt.project_id
+  end
+
+  def test_prompt_messages
+    prompt = Braintrust::Prompt.new(@function_data)
+    messages = prompt.messages
+
+    assert_equal 2, messages.length
+    assert_equal "system", messages[0]["role"]
+    assert_equal "You are a helpful assistant.", messages[0]["content"]
+    assert_equal "user", messages[1]["role"]
+  end
+
+  def test_prompt_model
+    prompt = Braintrust::Prompt.new(@function_data)
+
+    assert_equal "claude-3-5-sonnet", prompt.model
+  end
+
+  def test_prompt_options
+    prompt = Braintrust::Prompt.new(@function_data)
+    options = prompt.options
+
+    assert_equal "claude-3-5-sonnet", options["model"]
+    assert_equal 0.7, options["params"]["temperature"]
+    assert_equal 1000, options["params"]["max_tokens"]
+  end
+
+  def test_prompt_raw_returns_prompt_definition
+    prompt = Braintrust::Prompt.new(@function_data)
+    raw = prompt.prompt
+
+    assert_equal "chat", raw["type"]
+    assert raw.key?("messages")
+  end
+
+  def test_build_substitutes_variables
+    prompt = Braintrust::Prompt.new(@function_data)
+    result = prompt.build(name: "Alice", task: "coding")
+
+    assert_equal "claude-3-5-sonnet", result[:model]
+    assert_equal 2, result[:messages].length
+    assert_equal "You are a helpful assistant.", result[:messages][0][:content]
+    assert_equal "Hello Alice, please help with coding.", result[:messages][1][:content]
+  end
+
+  def test_build_with_string_keys
+    prompt = Braintrust::Prompt.new(@function_data)
+    result = prompt.build("name" => "Bob", "task" => "writing")
+
+    assert_equal "Hello Bob, please help with writing.", result[:messages][1][:content]
+  end
+
+  def test_build_includes_params
+    prompt = Braintrust::Prompt.new(@function_data)
+    result = prompt.build(name: "Alice", task: "coding")
+
+    assert_equal 0.7, result[:temperature]
+    assert_equal 1000, result[:max_tokens]
+  end
+
+  def test_build_with_defaults
+    prompt = Braintrust::Prompt.new(@function_data, defaults: {name: "Default User"})
+    result = prompt.build(task: "testing")
+
+    assert_equal "Hello Default User, please help with testing.", result[:messages][1][:content]
+  end
+
+  def test_build_overrides_defaults
+    prompt = Braintrust::Prompt.new(@function_data, defaults: {name: "Default User"})
+    result = prompt.build(name: "Override User", task: "testing")
+
+    assert_equal "Hello Override User, please help with testing.", result[:messages][1][:content]
+  end
+
+  def test_build_strict_raises_on_missing_variable
+    prompt = Braintrust::Prompt.new(@function_data)
+
+    error = assert_raises(Braintrust::Error) do
+      prompt.build({name: "Alice"}, strict: true)
+    end
+
+    assert_match(/missing.*task/i, error.message)
+  end
+
+  def test_build_non_strict_leaves_missing_variables
+    prompt = Braintrust::Prompt.new(@function_data)
+    result = prompt.build(name: "Alice")
+
+    assert_equal "Hello Alice, please help with {{task}}.", result[:messages][1][:content]
+  end
+
+  def test_build_handles_nested_variables
+    data = @function_data.dup
+    data["prompt_data"]["prompt"]["messages"] = [
+      {"role" => "user", "content" => "User: {{user.name}}, Email: {{user.email}}"}
+    ]
+    prompt = Braintrust::Prompt.new(data)
+
+    result = prompt.build(user: {name: "Alice", email: "alice@example.com"})
+
+    assert_equal "User: Alice, Email: alice@example.com", result[:messages][0][:content]
+  end
+end
+
+class Braintrust::PromptLoadTest < Minitest::Test
+  def setup
+    flunk "BRAINTRUST_API_KEY not set" unless ENV["BRAINTRUST_API_KEY"]
+    @project_name = "ruby-sdk-test"
+  end
+
+  def test_prompt_load
+    VCR.use_cassette("prompt/load") do
+      Braintrust.init(blocking_login: true)
+
+      # Create a prompt first
+      api = Braintrust::API.new
+      slug = "test-prompt-load"
+
+      api.functions.create(
+        project_name: @project_name,
+        slug: slug,
+        function_data: {type: "prompt"},
+        prompt_data: {
+          prompt: {
+            type: "chat",
+            messages: [
+              {role: "user", content: "Say hello to {{name}}"}
+            ]
+          },
+          options: {
+            model: "gpt-4o-mini"
+          }
+        }
+      )
+
+      # Load the prompt using Prompt.load
+      prompt = Braintrust::Prompt.load(project: @project_name, slug: slug)
+
+      assert_instance_of Braintrust::Prompt, prompt
+      assert_equal slug, prompt.slug
+      assert_equal "gpt-4o-mini", prompt.model
+      assert_equal 1, prompt.messages.length
+
+      # Test build
+      result = prompt.build(name: "World")
+      assert_equal "Say hello to World", result[:messages][0][:content]
+
+      # Clean up
+      api.functions.delete(id: prompt.id)
+    ensure
+      OpenTelemetry.tracer_provider.shutdown
+    end
+  end
+
+  def test_prompt_load_not_found
+    VCR.use_cassette("prompt/load_not_found") do
+      Braintrust.init(blocking_login: true)
+
+      error = assert_raises(Braintrust::Error) do
+        Braintrust::Prompt.load(project: @project_name, slug: "nonexistent-prompt-xyz")
+      end
+
+      assert_match(/not found/i, error.message)
+    ensure
+      OpenTelemetry.tracer_provider.shutdown
+    end
+  end
+end
diff --git a/test/fixtures/vcr_cassettes/functions/get.yml b/test/fixtures/vcr_cassettes/functions/get.yml
new file mode 100644
index 0000000..908caac
--- /dev/null
+++ b/test/fixtures/vcr_cassettes/functions/get.yml
@@ -0,0 +1,449 @@
+---
+http_interactions:
+- request:
+    method: get
+    uri: https://api.braintrust.dev/v1/function/eaa1d7fa-ddfc-487e-bdf1-8c83c9e5e216
+    body:
+      encoding: US-ASCII
+      string: ''
+    headers:
+      Accept-Encoding:
+      - gzip;q=1.0,deflate;q=0.6,identity;q=0.3
+      Accept:
+      - "*/*"
+      User-Agent:
+      - Ruby
+      Host:
+      - api.braintrust.dev
+      Authorization:
+      - Bearer <BRAINTRUST_API_KEY>
+  response:
+    status:
+      code: 200
+      message: OK
+    headers:
+      Content-Type:
+      - application/json; charset=utf-8
+      Content-Length:
+      - '650'
+      Connection:
+      - keep-alive
+      X-Amz-Cf-Pop:
+      - JFK50-P2
+      - JFK50-P5
+      Date:
+      - Fri, 09 Jan 2026 15:22:45 GMT
+      Access-Control-Allow-Credentials:
+      - 'true'
+      X-Amzn-Requestid:
+      - 999f045e-52bf-431a-ab23-4231e84c599b
+      X-Bt-Internal-Trace-Id:
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From 7e77515f30a16fa0e2da762ac455fec7d8600126 Mon Sep 17 00:00:00 2001
From: Matt Perpick <matt@braintrustdata.com>
Date: Fri, 9 Jan 2026 11:27:22 -0500
Subject: [PATCH 2/2] use mustache templates

---
 .claude/settings.json          |  1 +
 Gemfile.lock                   |  2 ++
 braintrust.gemspec             |  1 +
 examples/prompt.rb             | 41 ++++++++++++++++++---------------
 lib/braintrust/prompt.rb       | 42 +++++++++++++++++-----------------
 test/braintrust/prompt_test.rb |  5 ++--
 6 files changed, 51 insertions(+), 41 deletions(-)

diff --git a/.claude/settings.json b/.claude/settings.json
index 21984fd..bba6f84 100644
--- a/.claude/settings.json
+++ b/.claude/settings.json
@@ -25,6 +25,7 @@
       "Bash(gem search:*)",
       "Bash(gem list:*)",
       "Bash(tree:*)",
+      "Bash(grep:*)",
       "Bash(find:*)",
       "Bash(echo:*)",
       "Bash(sort:*)",
diff --git a/Gemfile.lock b/Gemfile.lock
index cff826e..4ec76b3 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -2,6 +2,7 @@ PATH
   remote: .
   specs:
     braintrust (0.0.12)
+      mustache (~> 1.0)
       openssl (~> 3.3.1)
       opentelemetry-exporter-otlp (~> 0.28)
       opentelemetry-sdk (~> 1.3)
@@ -49,6 +50,7 @@ GEM
       builder
       minitest (>= 5.0)
       ruby-progressbar
+    mustache (1.1.1)
     openssl (3.3.1)
     opentelemetry-api (1.7.0)
     opentelemetry-common (0.23.0)
diff --git a/braintrust.gemspec b/braintrust.gemspec
index 6f1fd68..e7b36fa 100644
--- a/braintrust.gemspec
+++ b/braintrust.gemspec
@@ -31,6 +31,7 @@ Gem::Specification.new do |spec|
   # Runtime dependencies
   spec.add_runtime_dependency "opentelemetry-sdk", "~> 1.3"
   spec.add_runtime_dependency "opentelemetry-exporter-otlp", "~> 0.28"
+  spec.add_runtime_dependency "mustache", "~> 1.0"
 
   # OpenSSL 3.3.1+ fixes macOS CRL (Certificate Revocation List) verification issues
   # that occur with OpenSSL 3.6 + Ruby (certificate verify failed: unable to get certificate CRL).
diff --git a/examples/prompt.rb b/examples/prompt.rb
index 32a51e3..4c0e837 100644
--- a/examples/prompt.rb
+++ b/examples/prompt.rb
@@ -1,26 +1,31 @@
 #!/usr/bin/env ruby
 # frozen_string_literal: true
 
-# Example: Loading and using prompts from Braintrust
+# Example: Loading and executing prompts from Braintrust
 #
 # This example demonstrates how to:
 # 1. Create a prompt (function) on the Braintrust server
 # 2. Load it using Prompt.load
-# 3. Build the prompt with variable substitution
-# 4. Use the built prompt with an LLM client
+# 3. Build the prompt with Mustache variable substitution
+# 4. Execute the prompt with OpenAI and get a response
 #
 # Benefits of loading prompts:
 # - Centralized prompt management in Braintrust UI
 # - Version control and A/B testing for prompts
 # - No code deployment needed for prompt changes
 # - Works with any LLM client (OpenAI, Anthropic, etc.)
+# - Uses standard Mustache templating ({{variable}}, {{object.property}})
 
 require "bundler/setup"
 require "braintrust"
+require "openai"
 
-# Initialize Braintrust
+# Initialize Braintrust with tracing
 Braintrust.init
 
+# Wrap OpenAI client for tracing
+openai = Braintrust::Trace::OpenAI.wrap(OpenAI::Client.new)
+
 project_name = "ruby-sdk-examples"
 prompt_slug = "greeting-prompt-#{Time.now.to_i}"
 
@@ -39,7 +44,7 @@
       messages: [
         {
           role: "system",
-          content: "You are a friendly assistant who speaks {{language}}."
+          content: "You are a friendly assistant. Respond in {{language}}. Keep responses brief (1-2 sentences)."
         },
         {
           role: "user",
@@ -60,11 +65,10 @@
 prompt = Braintrust::Prompt.load(project: project_name, slug: prompt_slug)
 
 puts "  ID: #{prompt.id}"
-puts "  Name: #{prompt.name}"
+puts "  Slug: #{prompt.slug}"
 puts "  Model: #{prompt.model}"
-puts "  Messages: #{prompt.messages.length}"
 
-# Build the prompt with variable substitution
+# Build the prompt with Mustache variable substitution
 puts "\nBuilding prompt with variables..."
 params = prompt.build(
   name: "Alice",
@@ -74,22 +78,23 @@
 
 puts "  Model: #{params[:model]}"
 puts "  Temperature: #{params[:temperature]}"
-puts "  Max tokens: #{params[:max_tokens]}"
 puts "  Messages:"
 params[:messages].each do |msg|
   puts "    [#{msg[:role]}] #{msg[:content]}"
 end
 
-# The params hash is ready to pass to any LLM client:
-#
-# With OpenAI:
-#   client.chat.completions.create(**params)
-#
-# With Anthropic:
-#   client.messages.create(**params)
+# Execute the prompt with OpenAI
+puts "\nExecuting prompt with OpenAI..."
+response = openai.chat.completions.create(**params)
 
-puts "\nPrompt is ready to use with any LLM client!"
+puts "\nResponse:"
+content = response.choices.first.message.content
+puts "  #{content}"
 
 # Clean up - delete the test prompt
+puts "\nCleaning up..."
 api.functions.delete(id: prompt.id)
-puts "Cleaned up test prompt."
+puts "Done!"
+
+# Flush traces
+OpenTelemetry.tracer_provider.shutdown
diff --git a/lib/braintrust/prompt.rb b/lib/braintrust/prompt.rb
index d417e95..dc162ff 100644
--- a/lib/braintrust/prompt.rb
+++ b/lib/braintrust/prompt.rb
@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "mustache"
+
 module Braintrust
   # Prompt class for loading and building prompts from Braintrust
   #
@@ -92,11 +94,11 @@ def build(variables = nil, strict: false, **kwargs)
       variables_hash = variables.is_a?(Hash) ? variables : {}
       vars = @defaults.merge(stringify_keys(variables_hash)).merge(stringify_keys(kwargs))
 
-      # Substitute variables in messages
+      # Render Mustache templates in messages
       built_messages = messages.map do |msg|
         {
           role: msg["role"].to_sym,
-          content: substitute_variables(msg["content"], vars, strict: strict)
+          content: render_template(msg["content"], vars, strict: strict)
         }
       end
 
@@ -119,33 +121,31 @@ def build(variables = nil, strict: false, **kwargs)
 
     private
 
-    # Substitute {{variable}} placeholders with values
-    def substitute_variables(text, variables, strict:)
+    # Render Mustache template with variables
+    def render_template(text, variables, strict:)
       return text unless text.is_a?(String)
 
-      # Find all {{variable}} patterns
-      missing = []
-
-      result = text.gsub(/\{\{([^}]+)\}\}/) do |match|
-        var_path = ::Regexp.last_match(1).strip
-        value = resolve_variable(var_path, variables)
-
-        if value.nil?
-          missing << var_path
-          match # Keep original placeholder
-        else
-          value.to_s
+      if strict
+        # Check for missing variables before rendering
+        missing = find_missing_variables(text, variables)
+        if missing.any?
+          raise Error, "Missing required variables: #{missing.join(", ")}"
         end
       end
 
-      if strict && missing.any?
-        raise Error, "Missing required variables: #{missing.join(", ")}"
-      end
+      Mustache.render(text, variables)
+    end
 
-      result
+    # Find variables in template that are not provided
+    def find_missing_variables(text, variables)
+      # Extract {{variable}} and {{variable.path}} patterns
+      # Mustache uses {{name}} syntax
+      text.scan(/\{\{([^}#^\/!>]+)\}\}/).flatten.map(&:strip).uniq.reject do |var|
+        resolve_variable(var, variables)
+      end
     end
 
-    # Resolve a variable path like "user.name" from variables hash
+    # Check if a variable path exists in the variables hash
     def resolve_variable(path, variables)
       parts = path.split(".")
       value = variables
diff --git a/test/braintrust/prompt_test.rb b/test/braintrust/prompt_test.rb
index 14ed7d1..247de90 100644
--- a/test/braintrust/prompt_test.rb
+++ b/test/braintrust/prompt_test.rb
@@ -117,11 +117,12 @@ def test_build_strict_raises_on_missing_variable
     assert_match(/missing.*task/i, error.message)
   end
 
-  def test_build_non_strict_leaves_missing_variables
+  def test_build_non_strict_replaces_missing_variables_with_empty
+    # Mustache standard behavior: missing variables become empty strings
     prompt = Braintrust::Prompt.new(@function_data)
     result = prompt.build(name: "Alice")
 
-    assert_equal "Hello Alice, please help with {{task}}.", result[:messages][1][:content]
+    assert_equal "Hello Alice, please help with .", result[:messages][1][:content]
   end
 
   def test_build_handles_nested_variables