diff --git a/packages/tools/src/claude-memory.test.ts b/packages/tools/src/claude-memory.test.ts
index 7337441a..622470e9 100644
--- a/packages/tools/src/claude-memory.test.ts
+++ b/packages/tools/src/claude-memory.test.ts
@@ -180,6 +180,80 @@ describe("ClaudeMemoryTool exact-file matching", () => {
 	})
 })
 
+describe("ClaudeMemoryTool insert line semantics", () => {
+	let tool: ClaudeMemoryTool
+
+	beforeEach(() => {
+		searchExecute.mockReset()
+		addMock.mockReset()
+		mockDocument(FILE_CONTENT)
+		tool = new ClaudeMemoryTool("test-api-key")
+	})
+
+	// The memory_20250818 spec: insert_text is inserted AFTER line insert_line,
+	// 0 inserts at the beginning of the file, and the valid range is [0, n_lines].
+
+	it("insert_line: 0 inserts at the beginning of the file", async () => {
+		const result = await tool.handleCommand({
+			command: "insert",
+			path: FILE_PATH,
+			insert_line: 0,
+			insert_text: "header",
+		})
+
+		expect(result.success).toBe(true)
+		const stored = addMock.mock.calls[0]?.[0]?.content as string
+		expect(stored).toBe("header\nline1\nline2\nline3\nline4\nline5")
+	})
+
+	it("inserts AFTER the given line, not before it", async () => {
+		const result = await tool.handleCommand({
+			command: "insert",
+			path: FILE_PATH,
+			insert_line: 2,
+			insert_text: "after2",
+		})
+
+		expect(result.success).toBe(true)
+		const stored = addMock.mock.calls[0]?.[0]?.content as string
+		// Regression guard: the old 1-based insert-BEFORE landed this one line early.
+		expect(stored).toBe("line1\nline2\nafter2\nline3\nline4\nline5")
+	})
+
+	it("insert_line: n_lines appends at the end of the file", async () => {
+		const result = await tool.handleCommand({
+			command: "insert",
+			path: FILE_PATH,
+			insert_line: 5,
+			insert_text: "tail",
+		})
+
+		expect(result.success).toBe(true)
+		const stored = addMock.mock.calls[0]?.[0]?.content as string
+		expect(stored).toBe("line1\nline2\nline3\nline4\nline5\ntail")
+	})
+
+	it("rejects insert_line outside [0, n_lines] without writing", async () => {
+		const below = await tool.handleCommand({
+			command: "insert",
+			path: FILE_PATH,
+			insert_line: -1,
+			insert_text: "x",
+		})
+		expect(below.success).toBe(false)
+		expect(below.error).toContain("[0, 5]")
+
+		const above = await tool.handleCommand({
+			command: "insert",
+			path: FILE_PATH,
+			insert_line: 6,
+			insert_text: "x",
+		})
+		expect(above.success).toBe(false)
+		expect(addMock).not.toHaveBeenCalled()
+	})
+})
+
 describe("ClaudeMemoryTool str_replace replacement literalness", () => {
 	let tool: ClaudeMemoryTool
 
diff --git a/packages/tools/src/claude-memory.ts b/packages/tools/src/claude-memory.ts
index 867c4d82..1184bf4b 100644
--- a/packages/tools/src/claude-memory.ts
+++ b/packages/tools/src/claude-memory.ts
@@ -490,16 +490,18 @@ export class ClaudeMemoryTool {
 			const originalContent = readResult.document.content
 			const lines = originalContent.split("\n")
 
-			// Validate line number
-			if (insertLine < 1 || insertLine > lines.length + 1) {
+			// Validate line number. Per the memory_20250818 spec the valid range
+			// is [0, n_lines]: text is inserted AFTER `insert_line`, and 0 means
+			// the beginning of the file.
+			if (insertLine < 0 || insertLine > lines.length) {
 				return {
 					success: false,
-					error: `Invalid line number: ${insertLine}. File has ${lines.length} lines.`,
+					error: `Invalid insert_line parameter: ${insertLine}. It should be within the range of lines of the file: [0, ${lines.length}]`,
 				}
 			}
 
-			// Insert the text (insertLine is 1-based)
-			lines.splice(insertLine - 1, 0, insertText)
+			// Insert the text after line `insertLine` (0-based insert-after)
+			lines.splice(insertLine, 0, insertText)
 			const newContent = lines.join("\n")
 
 			// Update the document
@@ -517,7 +519,7 @@ export class ClaudeMemoryTool {
 
 			return {
 				success: true,
-				content: `Text inserted at line ${insertLine} in file: ${filePath}`,
+				content: `Text inserted after line ${insertLine} in file: ${filePath}`,
 			}
 		} catch (error) {
 			return {
diff --git a/packages/tools/src/tool-operations.test.ts b/packages/tools/src/tool-operations.test.ts
index 8efef91d..63ff8e24 100644
--- a/packages/tools/src/tool-operations.test.ts
+++ b/packages/tools/src/tool-operations.test.ts
@@ -309,7 +309,7 @@ describe("ClaudeMemoryTool", () => {
 		const result = await tool.handleCommand({
 			command: "insert",
 			path: FILE_PATH,
-			insert_line: 2,
+			insert_line: 1,
 			insert_text: "",
 		})