Add amber mcp serve: stateless Streamable HTTP MCP server

shard.lock

@@ -24,6 +24,10 @@ shards:
     git: https://github.com/crystal-loot/exception_page.git
     version: 0.2.2
 
+  mcprotocol:
+    git: https://github.com/crimson-knight/mcprotocol.git
+    version: 1.0.0+git.commit.f4a52b03c768471f823e813a82281d6bc6f959b7
+
   micrate:
     git: https://github.com/amberframework/micrate.git
     version: 0.15.1+git.commit.647fac0490a522956fef305c720e8ebf1422a87d

shard.yml

@@ -47,6 +47,12 @@ dependencies:
     github: elorest/compiled_license
     version: ~> 1.2.2
 
+  # MCP schema bindings for `amber mcp serve`. Branch pin until the
+  # 2026-07-28 / 2025-11-25 revision support lands in a tagged release.
+  mcprotocol:
+    github: crimson-knight/mcprotocol
+    branch: feature/mcp-2026-07-28
+
 development_dependencies:
   ameba:
     github: crystal-ameba/ameba

spec/commands/mcp_command_spec.cr

@@ -0,0 +1,73 @@
+require "spec"
+require "../../src/amber_cli"
+
+private def parsed_command(args : Array(String)) : AmberCLI::Commands::McpCommand
+  command = AmberCLI::Commands::McpCommand.new("mcp")
+  command.option_parser.unknown_args do |unknown_args, _|
+    command.remaining_arguments.concat(unknown_args)
+  end
+  command.option_parser.parse(args)
+  command
+end
+
+describe AmberCLI::Commands::McpCommand do
+  describe "registration" do
+    it "is reachable as `amber mcp`" do
+      AmberCLI::Core::CommandRegistry.find_command("mcp").should eq(AmberCLI::Commands::McpCommand)
+    end
+  end
+
+  describe "#setup_command_options" do
+    it "defaults to the loopback endpoint on port 5757" do
+      command = parsed_command(["serve"])
+
+      command.host.should eq("127.0.0.1")
+      command.port.should eq(5757)
+      command.allow_remote?.should be_false
+      command.remaining_arguments.should eq(["serve"])
+    end
+
+    it "accepts --host and --port" do
+      command = parsed_command(["serve", "--host=127.0.0.1", "--port=9999"])
+
+      command.host.should eq("127.0.0.1")
+      command.port.should eq(9999)
+    end
+
+    it "accepts --allow-remote" do
+      parsed_command(["serve", "--allow-remote"]).allow_remote?.should be_true
+    end
+  end
+
+  describe "#validate_bind!" do
+    it "refuses --host 0.0.0.0 without --allow-remote" do
+      command = parsed_command(["serve", "--host=0.0.0.0"])
+
+      error = expect_raises(AmberCLI::Commands::McpCommand::RemoteBindRefusedError) do
+        command.validate_bind!
+      end
+
+      error.message.to_s.should contain("Refusing to bind 0.0.0.0")
+      error.message.to_s.should contain("no authentication")
+      error.message.to_s.should contain("--allow-remote")
+    end
+
+    it "refuses a wildcard IPv6 bind without --allow-remote" do
+      command = parsed_command(["serve", "--host=::"])
+
+      expect_raises(AmberCLI::Commands::McpCommand::RemoteBindRefusedError) do
+        command.validate_bind!
+      end
+    end
+
+    it "permits 0.0.0.0 once --allow-remote is given" do
+      command = parsed_command(["serve", "--host=0.0.0.0", "--allow-remote"])
+
+      command.validate_bind!
+    end
+
+    it "permits the loopback default" do
+      parsed_command(["serve"]).validate_bind!
+    end
+  end
+end