diff --git a/packages/tools/src/claude-memory.test.ts b/packages/tools/src/claude-memory.test.ts
index 477b0481..a5a11e1c 100644
--- a/packages/tools/src/claude-memory.test.ts
+++ b/packages/tools/src/claude-memory.test.ts
@@ -292,6 +292,57 @@ describe("ClaudeMemoryTool str_replace replacement literalness", () => {
 	})
 })
 
+describe("ClaudeMemoryTool str_replace uniqueness", () => {
+	let tool: ClaudeMemoryTool
+
+	beforeEach(() => {
+		documentsGetMock.mockReset()
+		updateMock.mockReset().mockResolvedValue({ id: "doc", status: "queued" })
+		mockDocument("status: todo\nnotes\nstatus: todo")
+		tool = new ClaudeMemoryTool("test-api-key")
+	})
+
+	it("refuses to replace an old_str that occurs more than once", async () => {
+		const result = await tool.handleCommand({
+			command: "str_replace",
+			path: FILE_PATH,
+			old_str: "status: todo",
+			new_str: "status: done",
+		})
+
+		expect(result.success).toBe(false)
+		expect(result.error).toContain("Multiple occurrences")
+		expect(result.error).toContain("lines: 1, 3")
+		expect(updateMock).not.toHaveBeenCalled()
+	})
+
+	it("counts overlapping occurrences", async () => {
+		mockDocument("aaa")
+		const result = await tool.handleCommand({
+			command: "str_replace",
+			path: FILE_PATH,
+			old_str: "aa",
+			new_str: "b",
+		})
+
+		expect(result.success).toBe(false)
+		expect(updateMock).not.toHaveBeenCalled()
+	})
+
+	it("still replaces a unique old_str", async () => {
+		const result = await tool.handleCommand({
+			command: "str_replace",
+			path: FILE_PATH,
+			old_str: "notes",
+			new_str: "updated notes",
+		})
+
+		expect(result.success).toBe(true)
+		const stored = updateMock.mock.calls[0]?.[2]?.content as string
+		expect(stored).toBe("status: todo\nupdated notes\nstatus: todo")
+	})
+})
+
 describe("ClaudeMemoryTool path traversal", () => {
 	let tool: ClaudeMemoryTool
 
diff --git a/packages/tools/src/claude-memory.ts b/packages/tools/src/claude-memory.ts
index 6ea73f6e..2f1255c7 100644
--- a/packages/tools/src/claude-memory.ts
+++ b/packages/tools/src/claude-memory.ts
@@ -389,6 +389,23 @@ export class ClaudeMemoryTool {
 				}
 			}
 
+			// old_str must be unique, like Anthropic's memory tool: replacing
+			// only the first of several matches would silently edit the wrong spot.
+			const matchingLines: number[] = []
+			for (
+				let index = originalContent.indexOf(oldStr);
+				index !== -1;
+				index = originalContent.indexOf(oldStr, index + 1)
+			) {
+				matchingLines.push(originalContent.slice(0, index).split("\n").length)
+			}
+			if (matchingLines.length > 1) {
+				return {
+					success: false,
+					error: `No replacement was performed. Multiple occurrences of old_str "${oldStr}" in lines: ${matchingLines.join(", ")}. Please ensure it is unique`,
+				}
+			}
+
 			// Replace the string. The function replacer keeps `$` sequences
 			// in the replacement literal — a bare string here would expand
 			// patterns like $&, $', and $` and silently corrupt the file.