fix: re-add lightweight MCP servers section to system prompt for OpenAI compatibility

OpenAI models (e.g., GPT-5.2) fail to call MCP tools because they need
explicit system prompt context about available MCP servers and the
mcp--serverName--toolName naming convention. Claude and Gemini models
handle this from native tool definitions alone, but OpenAI models do not.

This commit adds a lightweight MCP SERVERS section to the system prompt
that includes:
- The mcp--serverName--toolName naming convention explanation
- A list of connected servers and their tool names
- Server-specific instructions (from the MCP protocol instructions field)

This does NOT duplicate tool schemas or descriptions (already provided
via native tool definitions), keeping the prompt compact.

Closes #11317

Author: roomote

src/core/prompts/sections/__tests__/mcp-servers.spec.ts

@@ -0,0 +1,156 @@
+import type { McpServer, McpTool } from "@roo-code/types"
+
+import type { McpHub } from "../../../../services/mcp/McpHub"
+
+import { getMcpServersSection } from "../mcp-servers"
+
+describe("getMcpServersSection", () => {
+	const createMockTool = (name: string, description = "Test tool", enabledForPrompt?: boolean): McpTool => ({
+		name,
+		description,
+		inputSchema: {
+			type: "object",
+			properties: {},
+		},
+		...(enabledForPrompt !== undefined ? { enabledForPrompt } : {}),
+	})
+
+	const createMockServer = (
+		name: string,
+		tools: McpTool[],
+		options: { instructions?: string; source?: "global" | "project" } = {},
+	): McpServer => ({
+		name,
+		config: JSON.stringify({ type: "stdio", command: "test" }),
+		status: "connected",
+		source: options.source ?? "global",
+		tools,
+		instructions: options.instructions,
+	})
+
+	const createMockMcpHub = (servers: McpServer[]): Partial<McpHub> => ({
+		getServers: vi.fn().mockReturnValue(servers),
+	})
+
+	it("should return empty string when mcpHub is undefined", () => {
+		const result = getMcpServersSection(undefined)
+		expect(result).toBe("")
+	})
+
+	it("should return empty string when no servers are available", () => {
+		const mockHub = createMockMcpHub([])
+		const result = getMcpServersSection(mockHub as McpHub)
+		expect(result).toBe("")
+	})
+
+	it("should return empty string when servers have no enabled tools", () => {
+		const server = createMockServer("testServer", [])
+		const mockHub = createMockMcpHub([server])
+		const result = getMcpServersSection(mockHub as McpHub)
+		expect(result).toBe("")
+	})
+
+	it("should return empty string when all tools are disabled", () => {
+		const server = createMockServer("testServer", [
+			createMockTool("tool1", "Tool 1", false),
+			createMockTool("tool2", "Tool 2", false),
+		])
+		const mockHub = createMockMcpHub([server])
+		const result = getMcpServersSection(mockHub as McpHub)
+		expect(result).toBe("")
+	})
+
+	it("should include MCP SERVERS header", () => {
+		const server = createMockServer("testServer", [createMockTool("testTool")])
+		const mockHub = createMockMcpHub([server])
+		const result = getMcpServersSection(mockHub as McpHub)
+		expect(result).toContain("MCP SERVERS")
+	})
+
+	it("should explain the naming convention", () => {
+		const server = createMockServer("testServer", [createMockTool("testTool")])
+		const mockHub = createMockMcpHub([server])
+		const result = getMcpServersSection(mockHub as McpHub)
+		expect(result).toContain("mcp--serverName--toolName")
+	})
+
+	it("should list server name as heading", () => {
+		const server = createMockServer("context7", [createMockTool("resolve-library-id")])
+		const mockHub = createMockMcpHub([server])
+		const result = getMcpServersSection(mockHub as McpHub)
+		expect(result).toContain("## context7")
+	})
+
+	it("should list tool names using the mcp-- naming convention", () => {
+		const server = createMockServer("context7", [
+			createMockTool("resolve-library-id"),
+			createMockTool("get-library-docs"),
+		])
+		const mockHub = createMockMcpHub([server])
+		const result = getMcpServersSection(mockHub as McpHub)
+		expect(result).toContain("mcp--context7--resolve-library-id")
+		expect(result).toContain("mcp--context7--get-library-docs")
+	})
+
+	it("should include server instructions when provided", () => {
+		const server = createMockServer("conport", [createMockTool("init-db")], {
+			instructions: "Always initialize the database before performing queries.",
+		})
+		const mockHub = createMockMcpHub([server])
+		const result = getMcpServersSection(mockHub as McpHub)
+		expect(result).toContain("Server Instructions:")
+		expect(result).toContain("Always initialize the database before performing queries.")
+	})
+
+	it("should not include server instructions section when not provided", () => {
+		const server = createMockServer("testServer", [createMockTool("testTool")])
+		const mockHub = createMockMcpHub([server])
+		const result = getMcpServersSection(mockHub as McpHub)
+		expect(result).not.toContain("Server Instructions:")
+	})
+
+	it("should list multiple servers", () => {
+		const server1 = createMockServer("context7", [createMockTool("resolve-library-id")])
+		const server2 = createMockServer("git", [createMockTool("git-status")])
+		const mockHub = createMockMcpHub([server1, server2])
+		const result = getMcpServersSection(mockHub as McpHub)
+		expect(result).toContain("## context7")
+		expect(result).toContain("## git")
+		expect(result).toContain("mcp--context7--resolve-library-id")
+		expect(result).toContain("mcp--git--git-status")
+	})
+
+	it("should filter out disabled tools", () => {
+		const server = createMockServer("testServer", [
+			createMockTool("enabledTool", "Enabled tool"),
+			createMockTool("disabledTool", "Disabled tool", false),
+		])
+		const mockHub = createMockMcpHub([server])
+		const result = getMcpServersSection(mockHub as McpHub)
+		expect(result).toContain("mcp--testServer--enabledTool")
+		expect(result).not.toContain("mcp--testServer--disabledTool")
+	})
+
+	it("should skip servers with only disabled tools", () => {
+		const serverWithDisabledTools = createMockServer("disabledServer", [createMockTool("tool1", "Tool 1", false)])
+		const serverWithEnabledTools = createMockServer("enabledServer", [createMockTool("tool1", "Tool 1")])
+		const mockHub = createMockMcpHub([serverWithDisabledTools, serverWithEnabledTools])
+		const result = getMcpServersSection(mockHub as McpHub)
+		expect(result).not.toContain("## disabledServer")
+		expect(result).toContain("## enabledServer")
+	})
+
+	it("should skip servers with undefined tools", () => {
+		const serverWithUndefinedTools: McpServer = {
+			name: "noTools",
+			config: JSON.stringify({ type: "stdio", command: "test" }),
+			status: "connected",
+			tools: undefined,
+		}
+		const serverWithTools = createMockServer("withTools", [createMockTool("tool1")])
+		const mockHub = createMockMcpHub([serverWithUndefinedTools, serverWithTools])
+		const result = getMcpServersSection(mockHub as McpHub)
+		expect(result).not.toContain("## noTools")
+		expect(result).toContain("## withTools")
+	})
+})

src/core/prompts/sections/index.ts

@@ -8,3 +8,4 @@ export { getCapabilitiesSection } from "./capabilities"
 export { getModesSection } from "./modes"
 export { markdownFormattingSection } from "./markdown-formatting"
 export { getSkillsSection } from "./skills"
+export { getMcpServersSection } from "./mcp-servers"

src/core/prompts/sections/mcp-servers.ts

@@ -0,0 +1,64 @@
+import type { McpTool } from "@roo-code/types"
+
+import { McpHub } from "../../../services/mcp/McpHub"
+import { buildMcpToolName } from "../../../utils/mcp-name"
+
+/**
+ * Generates a lightweight MCP servers section for the system prompt.
+ *
+ * This provides the model with context about connected MCP servers, including:
+ * - The naming convention for MCP tool calls (mcp--serverName--toolName)
+ * - A list of connected servers and their available tools
+ * - Server-specific instructions (from the MCP protocol's `instructions` field)
+ *
+ * This does NOT duplicate tool schemas or descriptions, since those are already
+ * provided via native tool definitions. The purpose is to give the model enough
+ * context to understand how to use these tools, which is especially important
+ * for models like OpenAI's GPT series that benefit from explicit system prompt
+ * guidance about available MCP tools.
+ */
+export function getMcpServersSection(mcpHub?: McpHub): string {
+	if (!mcpHub) {
+		return ""
+	}
+
+	const servers = mcpHub.getServers()
+
+	if (servers.length === 0) {
+		return ""
+	}
+
+	// Build per-server entries with tool names and optional instructions
+	const serverEntries: string[] = []
+
+	for (const server of servers) {
+		const enabledTools = (server.tools || []).filter((tool: McpTool) => tool.enabledForPrompt !== false)
+
+		if (enabledTools.length === 0) {
+			continue
+		}
+
+		const toolNames = enabledTools.map((tool: McpTool) => `  - ${buildMcpToolName(server.name, tool.name)}`)
+
+		let entry = `## ${server.name}\n`
+		entry += `Tools:\n${toolNames.join("\n")}`
+
+		if (server.instructions) {
+			entry += `\n\nServer Instructions:\n${server.instructions}`
+		}
+
+		serverEntries.push(entry)
+	}
+
+	if (serverEntries.length === 0) {
+		return ""
+	}
+
+	return `====
+
+MCP SERVERS
+
+The following MCP (Model Context Protocol) servers are connected and provide additional tools you can use. MCP tools are called as native tool calls using the naming convention \`mcp--serverName--toolName\`. When a task could benefit from an MCP tool, prefer using it.
+
+${serverEntries.join("\n\n")}`
+}

src/core/prompts/system.ts

@@ -26,6 +26,7 @@ import {
 	addCustomInstructions,
 	markdownFormattingSection,
 	getSkillsSection,
+	getMcpServersSection,
 } from "./sections"
 
 // Helper function to get prompt component, filtering out empty objects
@@ -86,6 +87,8 @@ async function generatePrompt(
 	// Tools catalog is not included in the system prompt.
 	const toolsCatalog = ""
 
+	const mcpServersSection = shouldIncludeMcp ? getMcpServersSection(mcpHub) : ""
+
 	const basePrompt = `${roleDefinition}
 
 ${markdownFormattingSection()}
@@ -95,7 +98,7 @@ ${getSharedToolUseSection()}${toolsCatalog}
 	${getToolUseGuidelinesSection()}
 
 ${getCapabilitiesSection(cwd, shouldIncludeMcp ? mcpHub : undefined)}
-
+${mcpServersSection ? `\n${mcpServersSection}` : ""}
 ${modesSection}
 ${skillsSection ? `\n${skillsSection}` : ""}
 ${getRulesSection(cwd, settings)}