diff --git a/src/core/prompts/sections/__tests__/custom-instructions.spec.ts b/src/core/prompts/sections/__tests__/custom-instructions.spec.ts
index d015748315..be9c25bcaa 100644
--- a/src/core/prompts/sections/__tests__/custom-instructions.spec.ts
+++ b/src/core/prompts/sections/__tests__/custom-instructions.spec.ts
@@ -775,6 +775,86 @@ describe("addCustomInstructions", () => {
 		expect(readFileMock).toHaveBeenCalledWith(expect.stringContaining("AGENTS.md"), "utf-8")
 	})
 
+	it("should load AGENT.md (singular) when AGENTS.md is not found", async () => {
+		// Simulate no .roo/rules-test-mode directory
+		statMock.mockRejectedValueOnce({ code: "ENOENT" })
+
+		// Mock lstat to indicate AGENTS.md doesn't exist but AGENT.md does
+		lstatMock.mockImplementation((filePath: PathLike) => {
+			const pathStr = filePath.toString()
+			if (pathStr.endsWith("AGENTS.md")) {
+				return Promise.reject({ code: "ENOENT" })
+			}
+			if (pathStr.endsWith("AGENT.md")) {
+				return Promise.resolve({
+					isSymbolicLink: vi.fn().mockReturnValue(false),
+				})
+			}
+			return Promise.reject({ code: "ENOENT" })
+		})
+
+		readFileMock.mockImplementation((filePath: PathLike) => {
+			const pathStr = filePath.toString()
+			if (pathStr.endsWith("AGENT.md")) {
+				return Promise.resolve("Agent rules from AGENT.md file (singular)")
+			}
+			return Promise.reject({ code: "ENOENT" })
+		})
+
+		const result = await addCustomInstructions(
+			"mode instructions",
+			"global instructions",
+			"/fake/path",
+			"test-mode",
+			{ settings: { maxConcurrentFileReads: 5, todoListEnabled: true, useAgentRules: true } },
+		)
+
+		expect(result).toContain("# Agent Rules Standard (AGENT.md):")
+		expect(result).toContain("Agent rules from AGENT.md file (singular)")
+		expect(readFileMock).toHaveBeenCalledWith(expect.stringContaining("AGENT.md"), "utf-8")
+	})
+
+	it("should prefer AGENTS.md over AGENT.md when both exist", async () => {
+		// Simulate no .roo/rules-test-mode directory
+		statMock.mockRejectedValueOnce({ code: "ENOENT" })
+
+		// Mock lstat to indicate both files exist
+		lstatMock.mockImplementation((filePath: PathLike) => {
+			const pathStr = filePath.toString()
+			if (pathStr.endsWith("AGENTS.md") || pathStr.endsWith("AGENT.md")) {
+				return Promise.resolve({
+					isSymbolicLink: vi.fn().mockReturnValue(false),
+				})
+			}
+			return Promise.reject({ code: "ENOENT" })
+		})
+
+		readFileMock.mockImplementation((filePath: PathLike) => {
+			const pathStr = filePath.toString()
+			if (pathStr.endsWith("AGENTS.md")) {
+				return Promise.resolve("Agent rules from AGENTS.md file (plural)")
+			}
+			if (pathStr.endsWith("AGENT.md")) {
+				return Promise.resolve("Agent rules from AGENT.md file (singular)")
+			}
+			return Promise.reject({ code: "ENOENT" })
+		})
+
+		const result = await addCustomInstructions(
+			"mode instructions",
+			"global instructions",
+			"/fake/path",
+			"test-mode",
+			{ settings: { maxConcurrentFileReads: 5, todoListEnabled: true, useAgentRules: true } },
+		)
+
+		// Should contain AGENTS.md content (preferred) and not AGENT.md
+		expect(result).toContain("# Agent Rules Standard (AGENTS.md):")
+		expect(result).toContain("Agent rules from AGENTS.md file (plural)")
+		expect(result).not.toContain("Agent rules from AGENT.md file (singular)")
+		expect(readFileMock).toHaveBeenCalledWith(expect.stringContaining("AGENTS.md"), "utf-8")
+	})
+
 	it("should return empty string when no instructions provided", async () => {
 		// Simulate no .roo/rules directory
 		statMock.mockRejectedValueOnce({ code: "ENOENT" })
diff --git a/src/core/prompts/sections/custom-instructions.ts b/src/core/prompts/sections/custom-instructions.ts
index 22fb6122ec..2d70e45419 100644
--- a/src/core/prompts/sections/custom-instructions.ts
+++ b/src/core/prompts/sections/custom-instructions.ts
@@ -217,40 +217,46 @@ export async function loadRuleFiles(cwd: string): Promise<string> {
 }
 
 /**
- * Load AGENTS.md file from the project root if it exists
+ * Load AGENTS.md or AGENT.md file from the project root if it exists
+ * Checks for both AGENTS.md (standard) and AGENT.md (alternative) for compatibility
  */
 async function loadAgentRulesFile(cwd: string): Promise<string> {
-	try {
-		const agentsPath = path.join(cwd, "AGENTS.md")
-		let resolvedPath = agentsPath
+	// Try both filenames - AGENTS.md (standard) first, then AGENT.md (alternative)
+	const filenames = ["AGENTS.md", "AGENT.md"]
 
-		// Check if AGENTS.md exists and handle symlinks
+	for (const filename of filenames) {
 		try {
-			const stats = await fs.lstat(agentsPath)
-			if (stats.isSymbolicLink()) {
-				// Create a temporary fileInfo array to use with resolveSymLink
-				const fileInfo: Array<{ originalPath: string; resolvedPath: string }> = []
-
-				// Use the existing resolveSymLink function to handle symlink resolution
-				await resolveSymLink(agentsPath, fileInfo, 0)
-
-				// Extract the resolved path from fileInfo
-				if (fileInfo.length > 0) {
-					resolvedPath = fileInfo[0].resolvedPath
+			const agentPath = path.join(cwd, filename)
+			let resolvedPath = agentPath
+
+			// Check if file exists and handle symlinks
+			try {
+				const stats = await fs.lstat(agentPath)
+				if (stats.isSymbolicLink()) {
+					// Create a temporary fileInfo array to use with resolveSymLink
+					const fileInfo: Array<{ originalPath: string; resolvedPath: string }> = []
+
+					// Use the existing resolveSymLink function to handle symlink resolution
+					await resolveSymLink(agentPath, fileInfo, 0)
+
+					// Extract the resolved path from fileInfo
+					if (fileInfo.length > 0) {
+						resolvedPath = fileInfo[0].resolvedPath
+					}
 				}
+			} catch (err) {
+				// If lstat fails (file doesn't exist), try next filename
+				continue
 			}
-		} catch (err) {
-			// If lstat fails (file doesn't exist), return empty
-			return ""
-		}
 
-		// Read the content from the resolved path
-		const content = await safeReadFile(resolvedPath)
-		if (content) {
-			return `# Agent Rules Standard (AGENTS.md):\n${content}`
+			// Read the content from the resolved path
+			const content = await safeReadFile(resolvedPath)
+			if (content) {
+				return `# Agent Rules Standard (${filename}):\n${content}`
+			}
+		} catch (err) {
+			// Silently ignore errors - agent rules files are optional
 		}
-	} catch (err) {
-		// Silently ignore errors - AGENTS.md is optional
 	}
 	return ""
 }
diff --git a/src/core/protect/RooProtectedController.ts b/src/core/protect/RooProtectedController.ts
index e6d57b22b5..f4d80c4d64 100644
--- a/src/core/protect/RooProtectedController.ts
+++ b/src/core/protect/RooProtectedController.ts
@@ -21,6 +21,7 @@ export class RooProtectedController {
 		".vscode/**",
 		".rooprotected", // For future use
 		"AGENTS.md",
+		"AGENT.md", // Alternative singular form for compatibility
 	]
 
 	constructor(cwd: string) {
diff --git a/src/core/protect/__tests__/RooProtectedController.spec.ts b/src/core/protect/__tests__/RooProtectedController.spec.ts
index 0ee6135e81..f19cf17548 100644
--- a/src/core/protect/__tests__/RooProtectedController.spec.ts
+++ b/src/core/protect/__tests__/RooProtectedController.spec.ts
@@ -48,6 +48,10 @@ describe("RooProtectedController", () => {
 			expect(controller.isWriteProtected("AGENTS.md")).toBe(true)
 		})
 
+		it("should protect AGENT.md file", () => {
+			expect(controller.isWriteProtected("AGENT.md")).toBe(true)
+		})
+
 		it("should not protect other files starting with .roo", () => {
 			expect(controller.isWriteProtected(".roosettings")).toBe(false)
 			expect(controller.isWriteProtected(".rooconfig")).toBe(false)
@@ -147,6 +151,7 @@ describe("RooProtectedController", () => {
 				".vscode/**",
 				".rooprotected",
 				"AGENTS.md",
+				"AGENT.md",
 			])
 		})
 	})