diff --git a/.junie/guidelines.md b/.junie/guidelines.md
new file mode 100644
index 0000000..2909bb6
--- /dev/null
+++ b/.junie/guidelines.md
@@ -0,0 +1,158 @@
+# ChainForge — Development Guidelines
+
+This document captures project-specific practices for building, testing, and extending ChainForge. It assumes an advanced Ruby developer familiar with Bundler, RSpec, Docker, and MongoDB.
+
+## Build and Configuration
+
+- Ruby: 3.2.2 (see `Gemfile` and `Dockerfile`).
+- App stack: Sinatra 4.x, Mongoid 7.0.x, MongoDB.
+- Environment configuration is consumed via `dotenv` and `Mongoid`:
+  - `Mongoid.load!('./config/mongoid.yml', ENV['ENVIRONMENT'] || :development)` in `main.rb`
+  - `Mongoid.load!('./config/mongoid.yml', ENV['ENVIRONMENT'] || :test)` in `spec/spec_helper.rb`
+- Required ENV (both development and test environments):
+  - `ENVIRONMENT` — one of `development` or `test` (symbol allowed by Mongoid). Defaults to `development` in app, `test` in specs when unset.
+  - `MONGO_DB_NAME` — database name (e.g., `chain_forge_dev` or `chain_forge_test`).
+  - `MONGO_DB_HOST` — MongoDB host (e.g., `127.0.0.1`).
+  - `MONGO_DB_PORT` — MongoDB port (e.g., `27017`).
+- Mongoid config (`config/mongoid.yml`) uses only these ENV variables (no auth block). Ensure the vars are set or use Docker.
+
+### Installing dependencies
+
+```bash
+# Ruby 3.2.2 assumed
+gem install bundler -v 2.4.13
+bundle install
+```
+
+### Running via Docker (recommended for a clean Mongo)
+
+```bash
+docker-compose up -d
+# Exposes app on :1910 and mongo on :27017 by default
+```
+
+### Running app locally
+
+```bash
+# Ensure MongoDB is reachable at MONGO_DB_HOST:MONGO_DB_PORT and DB exists/auto-creates
+ENVIRONMENT=development \
+MONGO_DB_NAME=chain_forge_dev \
+MONGO_DB_HOST=127.0.0.1 \
+MONGO_DB_PORT=27017 \
+bundle exec ruby main.rb -p 1910
+```
+
+## Testing
+
+RSpec is configured in `spec/spec_helper.rb` and loads Mongoid in `:test` unless `ENVIRONMENT` overrides.
+
+### Baseline run (verified)
+
+The following has been verified to pass for unit-level specs on a fresh run with MongoDB available (e.g., via `docker-compose up -d`):
+
+```bash
+ENVIRONMENT=test \
+MONGO_DB_NAME=chain_forge_test \
+MONGO_DB_HOST=127.0.0.1 \
+MONGO_DB_PORT=27017 \
+bundle exec rspec spec/block_spec.rb spec/blockchain_spec.rb
+```
+
+Notes:
+- The full suite currently includes API specs that may fail depending on content-type behavior and rack/test expectations. If you want green runs while iterating on models, scope to the unit specs above or filter via `-e/--example` or `--pattern`.
+- Full-suite run example (may include failing API specs today):
+
+```bash
+ENVIRONMENT=test MONGO_DB_NAME=chain_forge_test MONGO_DB_HOST=127.0.0.1 MONGO_DB_PORT=27017 \
+bundle exec rspec
+```
+
+### Focused runs and filters
+
+- Single file: `bundle exec rspec spec/block_spec.rb`
+- Single example: `bundle exec rspec spec/block_spec.rb:37`
+- By example name: `bundle exec rspec -e 'validates the blockchain'`
+
+### Coverage
+
+SimpleCov is available (loaded on demand). To enable coverage, set an env var before running:
+
+```bash
+COVERAGE=true ENVIRONMENT=test MONGO_DB_NAME=chain_forge_test MONGO_DB_HOST=127.0.0.1 MONGO_DB_PORT=27017 \
+bundle exec rspec
+# Coverage report will be in coverage/index.html
+```
+
+### Adding a new spec (demonstrated workflow)
+
+The following process was executed and verified end-to-end; replicate as needed:
+
+1. Create a new spec file under `spec/`, e.g. `spec/demo_math_spec.rb`:
+   ```ruby
+   # frozen_string_literal: true
+   require 'rspec'
+
+   RSpec.describe 'Math demo' do
+     it 'adds numbers' do
+       expect(1 + 1).to eq(2)
+     end
+   end
+   ```
+2. Run just that spec:
+   ```bash
+   ENVIRONMENT=test MONGO_DB_NAME=chain_forge_test MONGO_DB_HOST=127.0.0.1 MONGO_DB_PORT=27017 \
+   bundle exec rspec spec/demo_math_spec.rb
+   ```
+   Expected: 1 example, 0 failures.
+3. Remove the file once done (to keep repo clean):
+   ```bash
+   rm spec/demo_math_spec.rb
+   ```
+
+### Test data and MongoDB
+
+- Specs create and persist documents via Mongoid (e.g., `Block`, `Blockchain`). Use a dedicated `MONGO_DB_NAME` for tests to avoid polluting dev data.
+- No cleaning strategy is configured. If isolation becomes necessary, consider adding `database_cleaner-mongoid` or dropping the test DB between runs.
+
+## Additional Development Notes
+
+### API behavior
+
+- `main.rb` sets `content_type :json` for POST requests in a `before` block. If API specs expect more strict headers (e.g., `application/json` vs `text/html`), ensure routes are executed in the same Rack env as tests (Sinatra + rack-test). If content-type mismatches persist, review middleware or how tests mount the app.
+
+### Code style and linting
+
+- RuboCop with `rubocop-rspec` is configured in `.rubocop.yml`.
+  - Target Ruby: 3.2
+  - Selected Metrics/Style cops are tuned; specs and config directories are largely excluded from method/ block length checks.
+- Run lint:
+  ```bash
+  bundle exec rubocop
+  ```
+
+### Project structure
+
+- Core domain:
+  - `src/block.rb` — `Block` model with SHA256 hashing and `valid_data?` verification.
+  - `src/blockchain.rb` — `Blockchain` aggregate with `add_genesis_block`, `add_block`, `integrity_valid?`.
+- App entrypoint: `main.rb` (Sinatra routes for creating chains, adding blocks, validating block data).
+- Tests: `spec/` directory with unit specs for `Block` and `Blockchain`, and API specs.
+
+### Debugging tips
+
+- Use focused RSpec runs (`-e`, `:line_number`).
+- Inspect Mongo documents in a console by starting `irb` with the same environment:
+  ```bash
+  ENVIRONMENT=development MONGO_DB_NAME=chain_forge_dev MONGO_DB_HOST=127.0.0.1 MONGO_DB_PORT=27017 \
+  bundle exec irb -r ./src/blockchain -r ./src/block
+  ```
+- When debugging hashing/time-sensitive assertions, compare against `created_at.to_i` (used in hash calculation).
+
+### Docker notes
+
+- The provided `docker-compose.yml` spins up `app` and `db`. The `app` container runs `ruby main.rb -p 1910` per `Dockerfile` CMD. For test runs, it’s simpler to run RSpec on the host and use the `db` service for Mongo.
+- If you want to run tests inside the container, `docker-compose exec app bash` then run the same `ENVIRONMENT=test` commands with `MONGO_DB_HOST=db`.
+
+---
+
+This document reflects commands verified on 2025-11-08 23:58 local time. Keep it updated as the test suite and APIs evolve.
\ No newline at end of file
diff --git a/.rubocop.yml b/.rubocop.yml
index 09bccff..9486493 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -34,6 +34,8 @@ Metrics/BlockLength:
   Exclude:
     - 'spec/**/*'
     - 'config/**/*'
+  AllowedMethods:
+    - namespace
 
 # Style
 Style/Documentation:
diff --git a/Gemfile b/Gemfile
index 03c3a2c..b66ec0e 100644
--- a/Gemfile
+++ b/Gemfile
@@ -4,7 +4,9 @@ source 'https://rubygems.org'
 ruby '3.2.2'
 
 gem 'dotenv', '~> 2.7'
+gem 'dry-validation', '~> 1.10'
 gem 'mongoid', '~> 7.0.5'
+gem 'rack-attack', '~> 6.7'
 gem 'rackup', '~> 2.1.0'
 gem 'rspec', '~> 3.10'
 gem 'sinatra', '~> 4.0.0'
diff --git a/Gemfile.lock b/Gemfile.lock
index ca3a3ab..e3b2d55 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -11,16 +11,53 @@ GEM
       zeitwerk (~> 2.3)
     ast (2.4.3)
     base64 (0.2.0)
+    bigdecimal (3.3.1)
     bson (4.15.0)
     concurrent-ruby (1.2.3)
     diff-lcs (1.5.1)
     docile (1.4.1)
     dotenv (2.8.1)
+    dry-configurable (1.3.0)
+      dry-core (~> 1.1)
+      zeitwerk (~> 2.6)
+    dry-core (1.1.0)
+      concurrent-ruby (~> 1.0)
+      logger
+      zeitwerk (~> 2.6)
+    dry-inflector (1.2.0)
+    dry-initializer (3.2.0)
+    dry-logic (1.6.0)
+      bigdecimal
+      concurrent-ruby (~> 1.0)
+      dry-core (~> 1.1)
+      zeitwerk (~> 2.6)
+    dry-schema (1.14.1)
+      concurrent-ruby (~> 1.0)
+      dry-configurable (~> 1.0, >= 1.0.1)
+      dry-core (~> 1.1)
+      dry-initializer (~> 3.2)
+      dry-logic (~> 1.5)
+      dry-types (~> 1.8)
+      zeitwerk (~> 2.6)
+    dry-types (1.8.3)
+      bigdecimal (~> 3.0)
+      concurrent-ruby (~> 1.0)
+      dry-core (~> 1.0)
+      dry-inflector (~> 1.0)
+      dry-logic (~> 1.4)
+      zeitwerk (~> 2.6)
+    dry-validation (1.11.1)
+      concurrent-ruby (~> 1.0)
+      dry-core (~> 1.1)
+      dry-initializer (~> 3.2)
+      dry-schema (~> 1.14)
+      zeitwerk (~> 2.6)
     i18n (1.14.1)
       concurrent-ruby (~> 1.0)
     json (2.16.0)
     language_server-protocol (3.17.0.5)
     lint_roller (1.1.0)
+    logger (1.7.0)
     minitest (5.22.0)
     mongo (2.19.3)
       bson (>= 4.14.1, < 5.0.0)
@@ -37,6 +74,8 @@ GEM
     prism (1.6.0)
     racc (1.8.1)
     rack (3.0.8)
+    rack-attack (6.8.0)
+      rack (>= 1.0, < 4)
     rack-protection (4.0.0)
       base64 (>= 0.1.0)
       rack (>= 3.0.0, < 4)
@@ -124,7 +163,9 @@ PLATFORMS
 
 DEPENDENCIES
   dotenv (~> 2.7)
+  dry-validation (~> 1.10)
   mongoid (~> 7.0.5)
+  rack-attack (~> 6.7)
   rack-test (~> 2.1)
   rackup (~> 2.1.0)
   rspec (~> 3.10)
diff --git a/config/rack_attack.rb b/config/rack_attack.rb
new file mode 100644
index 0000000..99cb2c0
--- /dev/null
+++ b/config/rack_attack.rb
@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+# Configure Rack::Attack for rate limiting
+module Rack
+  class Attack
+    # Throttle all requests by IP (60 requests per minute)
+    throttle('req/ip', limit: 60, period: 60, &:ip)
+
+    # Throttle POST requests to /chain by IP (10 per minute)
+    throttle('chain/ip', limit: 10, period: 60) do |req|
+      req.ip if req.path == '/chain' && req.post?
+    end
+
+    # Throttle POST requests to block creation (30 per minute)
+    throttle('block/ip', limit: 30, period: 60) do |req|
+      req.ip if req.path.match?(%r{^/chain/.+/block$}) && req.post?
+    end
+
+    # Custom response for throttled requests
+    self.throttled_responder = lambda do |_env|
+      [
+        429,
+        { 'Content-Type' => 'application/json' },
+        [{ error: 'Rate limit exceeded. Please try again later.' }.to_json]
+      ]
+    end
+  end
+end
diff --git a/main.rb b/main.rb
index 7ff7dbb..fb8cc7c 100644
--- a/main.rb
+++ b/main.rb
@@ -5,10 +5,16 @@
 require 'json'
 require 'mongoid'
 require 'dotenv/load'
+require 'rack/attack'
 require_relative 'src/blockchain'
+require_relative 'src/validators'
+require_relative 'config/rack_attack'
 
 Mongoid.load!('./config/mongoid.yml', ENV['ENVIRONMENT'] || :development)
 
+# Enable Rack::Attack middleware (disabled in test environment)
+use Rack::Attack unless ENV['ENVIRONMENT'] == 'test'
+
 # Set default content type for JSON responses
 before do
   content_type :json if request.post?
@@ -33,10 +39,14 @@
 
   post '/chain/:id/block' do
     block_data = parse_json_body
+    validation = BlockDataContract.new.call(block_data)
+
+    halt 400, { errors: validation.errors.to_h }.to_json if validation.failure?
+
     chain_id = params[:id]
     blockchain = find_block_chain(chain_id)
-    difficulty = validate_difficulty(block_data['difficulty'])
-    block = blockchain.add_block(block_data['data'], difficulty: difficulty)
+    difficulty = validation[:difficulty] || 2
+    block = blockchain.add_block(validation[:data], difficulty: difficulty)
 
     {
       chain_id: chain_id,
@@ -49,13 +59,17 @@
 
   post '/chain/:id/block/:block_id/valid' do
     block_data = parse_json_body
+    validation = BlockDataContract.new.call(block_data)
+
+    halt 400, { errors: validation.errors.to_h }.to_json if validation.failure?
+
     chain_id = params[:id]
     block_id = params[:block_id]
     blockchain = find_block_chain(chain_id)
     block = blockchain.blocks.find(block_id)
     raise 'Block not found' unless block
 
-    valid = block.valid_data?(block_data['data'])
+    valid = block.valid_data?(validation[:data])
 
     {
       chain_id: chain_id,
@@ -99,11 +113,4 @@ def find_block_chain(chain_id)
 
     blockchain
   end
-
-  def validate_difficulty(difficulty)
-    difficulty = difficulty.nil? ? 2 : difficulty.to_i
-    halt 422, { error: 'Difficulty must be a positive integer' }.to_json if difficulty <= 0
-    halt 422, { error: 'Difficulty must be between 1 and 10' }.to_json if difficulty > 10
-    difficulty
-  end
 end
diff --git a/src/validators.rb b/src/validators.rb
new file mode 100644
index 0000000..451d84b
--- /dev/null
+++ b/src/validators.rb
@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+require 'dry-validation'
+
+# Validator for block data
+class BlockDataContract < Dry::Validation::Contract
+  params do
+    required(:data).filled(:string)
+    optional(:difficulty).maybe(:integer)
+  end
+
+  rule(:difficulty) do
+    if value
+      key.failure('must be a positive integer') if value <= 0
+      key.failure('must be between 1 and 10') if value > 10
+    end
+  end
+end