diff --git a/CHANGELOG.md b/CHANGELOG.md
index 846df76..5f7e0dd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,12 @@
 # Changelog
 
+## Unreleased
+
+- `base_url` is now validated when the client is built: it must be an absolute
+  `https` URL with no userinfo, query, or fragment. Plain `http` is allowed only
+  for loopback hosts. Anything else raises `ConfigurationError` instead of
+  sending the API key in cleartext or to a host the URL does not name.
+
 ## 0.1.0 - 2026-09-18
 
 Provider-neutral release. One `Client`, two providers behind it.
diff --git a/README.md b/README.md
index 79638d6..0526e65 100644
--- a/README.md
+++ b/README.md
@@ -87,6 +87,13 @@ client.model      # => "jev-latest" (resolved after aliasing)
 
 Both providers send `User-Agent: ruby_decision_model/<version>`.
 
+`base_url` is validated when the client is built. It must be an absolute
+`https` URL with no userinfo, query, or fragment; plain `http` is accepted only
+for `localhost`, `127.0.0.1`, and `[::1]`, where nothing leaves the machine.
+Anything else raises `ConfigurationError`, because every request carries the API
+key in an `Authorization` header and a plaintext or redirected base URL hands
+that key to whoever is listening.
+
 ### Model aliases
 
 Each provider resolves a few friendly names to its own canonical model name.
@@ -171,7 +178,7 @@ return is still accepted and treated as having no headers, which means no
 
 | Error | Meaning |
 | --- | --- |
-| `ConfigurationError` | No provider could be resolved, missing api_key, unknown provider, or bad `retry:` value |
+| `ConfigurationError` | No provider could be resolved, missing api_key, unknown provider, bad `base_url`, or bad `retry:` value |
 | `RequestError` | Questions hash was empty |
 | `TransportError` (`TimeoutError`) | Network or timeout failure after retries, carries `#cause_error` |
 | `ApiError` | Non-2xx response, carries `#status`, `#body`, and `#headers` |
diff --git a/lib/ruby_decision_model/client.rb b/lib/ruby_decision_model/client.rb
index 3457726..eaa0351 100644
--- a/lib/ruby_decision_model/client.rb
+++ b/lib/ruby_decision_model/client.rb
@@ -40,6 +40,8 @@ def initialize(provider: nil, api_key: nil, model: nil, base_url: nil, timeout:
               "api_key is required for #{@provider.name}: pass api_key: or set #{@provider.env_var}"
       end
 
+      @provider.validate_base_url!
+
       @model = @provider.resolve_model(model)
       @timeout = timeout
       @transport = transport || default_transport
diff --git a/lib/ruby_decision_model/providers/base.rb b/lib/ruby_decision_model/providers/base.rb
index 85f7089..62b6c6d 100644
--- a/lib/ruby_decision_model/providers/base.rb
+++ b/lib/ruby_decision_model/providers/base.rb
@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "json"
+require "uri"
 
 module RubyDecisionModel
   module Providers
@@ -59,6 +60,45 @@ def api_key?
         !(api_key.nil? || api_key.to_s.strip.empty?)
       end
 
+      # Hosts that never leave the machine, where plain http carries no risk.
+      LOOPBACK_HOSTS = %w[localhost 127.0.0.1 ::1 [::1]].freeze
+
+      # Every request carries the API key in an Authorization header, so the
+      # base URL has to be one that keeps it secret. Raises unless it is an
+      # absolute http(s) URL, and unless it is https or a loopback host.
+      # Called once when a Client is built, so a bad URL fails before the
+      # first request rather than after the key has already gone out.
+      def validate_base_url!
+        uri = begin
+          URI.parse(base_url)
+        rescue URI::InvalidURIError => e
+          raise ConfigurationError, "base_url is not a valid URL: #{base_url.inspect} (#{e.message})"
+        end
+
+        unless uri.is_a?(URI::HTTP) && !uri.host.to_s.empty?
+          raise ConfigurationError, "base_url must be an absolute http(s) URL, got #{base_url.inspect}"
+        end
+
+        unless uri.userinfo.nil?
+          raise ConfigurationError, "base_url must not carry userinfo, got #{base_url.sub(uri.userinfo, '...').inspect}"
+        end
+
+        # endpoint_path is appended to base_url as a string. A query or a
+        # fragment would swallow it -- "https://host/p?x=1" + "/v1/systemone"
+        # posts to /p with the path buried in the query -- so reject both
+        # rather than build a URL that points somewhere else than it reads.
+        unless uri.query.nil? && uri.fragment.nil?
+          raise ConfigurationError,
+                "base_url must not carry a query or fragment, got #{base_url.inspect}"
+        end
+
+        return if uri.is_a?(URI::HTTPS) || LOOPBACK_HOSTS.include?(uri.host.downcase)
+
+        raise ConfigurationError,
+              "base_url must use https: over plain http the #{env_var} bearer token is sent in " \
+              "cleartext. Got #{base_url.inspect}. http is allowed only for #{LOOPBACK_HOSTS.join(', ')}."
+      end
+
       # Nil or blank means the provider default. Known aliases resolve to the
       # provider's canonical name. Anything else passes through untouched.
       def resolve_model(model)
diff --git a/test/base_url_test.rb b/test/base_url_test.rb
new file mode 100644
index 0000000..168e06f
--- /dev/null
+++ b/test/base_url_test.rb
@@ -0,0 +1,92 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+# The API key travels in an Authorization header on every request, so the base
+# URL is the thing that decides whether it stays secret.
+class BaseUrlTest < Minitest::Test
+  def build(base_url)
+    RubyDecisionModel::Client.new(
+      provider: :typesafe, api_key: "ts-secret", base_url: base_url, transport: FakeTransport.new([])
+    )
+  end
+
+  def test_https_base_url_is_accepted
+    client = build("https://api.typesafe.ai")
+    assert_equal "https://api.typesafe.ai/v1/systemone", client.provider.url
+  end
+
+  def test_plain_http_base_url_is_rejected
+    error = assert_raises(RubyDecisionModel::ConfigurationError) { build("http://api.typesafe.ai") }
+    assert_match(/https/, error.message)
+    assert_match(/cleartext/, error.message)
+  end
+
+  def test_plain_http_is_allowed_for_loopback_hosts
+    %w[http://localhost:4000 http://127.0.0.1:4000 http://[::1]:4000].each do |url|
+      client = build(url)
+      assert_equal "#{url}/v1/systemone", client.provider.url
+    end
+  end
+
+  def test_loopback_host_matching_ignores_case
+    client = build("http://LOCALHOST:4000")
+    assert_equal "http://LOCALHOST:4000/v1/systemone", client.provider.url
+  end
+
+  def test_non_http_scheme_is_rejected
+    error = assert_raises(RubyDecisionModel::ConfigurationError) { build("ftp://api.typesafe.ai") }
+    assert_match(/absolute http\(s\) URL/, error.message)
+  end
+
+  def test_base_url_without_a_host_is_rejected
+    error = assert_raises(RubyDecisionModel::ConfigurationError) { build("api.typesafe.ai") }
+    assert_match(/absolute http\(s\) URL/, error.message)
+  end
+
+  def test_unparseable_base_url_is_rejected
+    error = assert_raises(RubyDecisionModel::ConfigurationError) { build("https://exa mple.com") }
+    assert_match(/not a valid URL/, error.message)
+  end
+
+  def test_userinfo_in_base_url_is_rejected_and_not_echoed
+    error = assert_raises(RubyDecisionModel::ConfigurationError) { build("https://user:pw@api.typesafe.ai") }
+    assert_match(/userinfo/, error.message)
+    refute_includes error.message, "pw"
+  end
+
+  def test_query_or_fragment_in_base_url_is_rejected
+    # "https://api.typesafe.ai/p?x=1" + "/v1/systemone" would POST to /p.
+    ["https://api.typesafe.ai/p?x=1", "https://api.typesafe.ai/p#frag"].each do |url|
+      error = assert_raises(RubyDecisionModel::ConfigurationError) { build(url) }
+      assert_match(/query or fragment/, error.message)
+    end
+  end
+
+  def test_userinfo_host_confusion_is_rejected
+    # Reads as trusted.example, resolves to evil.example.
+    error = assert_raises(RubyDecisionModel::ConfigurationError) do
+      build("https://trusted.example@evil.example/p")
+    end
+    assert_match(/userinfo/, error.message)
+  end
+
+  def test_env_supplied_http_base_url_is_rejected_too
+    error = assert_raises(RubyDecisionModel::ConfigurationError) do
+      RubyDecisionModel::Client.new(
+        provider: RubyDecisionModel::Providers::Typesafe.new(api_key: "k", base_url: "http://evil.example.com"),
+        transport: FakeTransport.new([])
+      )
+    end
+    assert_match(/cleartext/, error.message)
+  end
+
+  def test_the_default_base_urls_pass_validation
+    with_env("TYPESAFE_API_KEY" => "ts-key", "OPENROUTER_API_KEY" => "or-key") do
+      RubyDecisionModel::Providers.names.each do |name|
+        client = RubyDecisionModel::Client.new(provider: name, transport: FakeTransport.new([]))
+        assert_match(%r{\Ahttps://}, client.base_url)
+      end
+    end
+  end
+end