diff --git a/src/cli/pm_pkg_command.zig b/src/cli/pm_pkg_command.zig
index a694398c0f..8160b8cbbb 100644
--- a/src/cli/pm_pkg_command.zig
+++ b/src/cli/pm_pkg_command.zig
@@ -221,7 +221,13 @@ pub const PmPkgCommand = struct {
                 Global.exit(1);
             }
 
-            try setValue(ctx.allocator, &root, key, value, parse_json);
+            setValue(ctx.allocator, &root, key, value, parse_json) catch |err| {
+                if (err == error.ArrayAppendToNonArray) {
+                    Output.errGeneric("Property {s} already exists and is not an Array or Object.", .{key});
+                    Global.exit(1);
+                }
+                return err;
+            };
             modified = true;
         }
 
@@ -493,11 +499,131 @@ pub const PmPkgCommand = struct {
         return path_parts;
     }
 
+    fn appendToArray(allocator: std.mem.Allocator, root: *js_ast.Expr, key: []const u8, value: []const u8, parse_json: bool) !void {
+        const new_value = try parseValue(allocator, value, parse_json);
+        
+        // Handle nested keys like "config.tags"
+        if (strings.indexOf(key, ".")) |_| {
+            return try appendToNestedArray(allocator, root, key, new_value);
+        }
+        
+        // Simple case: top-level key
+        const existing = root.get(key);
+        if (existing == null) {
+            // Create new array
+            const array_items = try allocator.alloc(js_ast.Expr, 1);
+            array_items[0] = new_value;
+            const new_array = js_ast.Expr.init(js_ast.E.Array, js_ast.E.Array{
+                .items = js_ast.ExprNodeList.init(array_items),
+                .close_bracket_loc = logger.Loc.Empty,
+                .is_single_line = true,
+            }, logger.Loc.Empty);
+            try root.data.e_object.put(allocator, key, new_array);
+        } else {
+            // Append to existing
+            if (existing.?.data != .e_array) {
+                return error.ArrayAppendToNonArray;
+            }
+            
+            const old_items = existing.?.data.e_array.items.slice();
+            const new_items = try allocator.alloc(js_ast.Expr, old_items.len + 1);
+            @memcpy(new_items[0..old_items.len], old_items);
+            new_items[old_items.len] = new_value;
+            
+            const updated_array = js_ast.Expr.init(js_ast.E.Array, js_ast.E.Array{
+                .items = js_ast.ExprNodeList.init(new_items),
+                .close_bracket_loc = existing.?.data.e_array.close_bracket_loc,
+                .is_single_line = existing.?.data.e_array.is_single_line,
+            }, existing.?.loc);
+            try root.data.e_object.put(allocator, key, updated_array);
+        }
+    }
+    
+    fn appendToNestedArray(allocator: std.mem.Allocator, root: *js_ast.Expr, key: []const u8, new_value: js_ast.Expr) !void {
+        var parts = std.mem.tokenizeScalar(u8, key, '.');
+        var path_parts = std.ArrayList([]const u8).init(allocator);
+        defer path_parts.deinit();
+        
+        while (parts.next()) |part| {
+            try path_parts.append(part);
+        }
+        
+        if (path_parts.items.len == 0) {
+            return error.EmptyKey;
+        }
+        
+        return try appendToNestedArrayRecursive(allocator, root, path_parts.items, new_value);
+    }
+    
+    fn appendToNestedArrayRecursive(allocator: std.mem.Allocator, root: *js_ast.Expr, path: []const []const u8, new_value: js_ast.Expr) !void {
+        if (path.len == 0) return;
+        
+        const current_key = path[0];
+        const remaining_path = path[1..];
+        
+        if (remaining_path.len == 0) {
+            // This is the final key - handle array append
+            const existing = root.get(current_key);
+            
+            if (existing == null) {
+                // Create new array
+                const array_items = try allocator.alloc(js_ast.Expr, 1);
+                array_items[0] = new_value;
+                const new_array = js_ast.Expr.init(js_ast.E.Array, js_ast.E.Array{
+                    .items = js_ast.ExprNodeList.init(array_items),
+                    .close_bracket_loc = logger.Loc.Empty,
+                    .is_single_line = true,
+                }, logger.Loc.Empty);
+                try root.data.e_object.put(allocator, current_key, new_array);
+            } else {
+                // Append to existing
+                if (existing.?.data != .e_array) {
+                    return error.ArrayAppendToNonArray;
+                }
+                
+                const old_items = existing.?.data.e_array.items.slice();
+                const new_items = try allocator.alloc(js_ast.Expr, old_items.len + 1);
+                @memcpy(new_items[0..old_items.len], old_items);
+                new_items[old_items.len] = new_value;
+                
+                const updated_array = js_ast.Expr.init(js_ast.E.Array, js_ast.E.Array{
+                    .items = js_ast.ExprNodeList.init(new_items),
+                    .close_bracket_loc = existing.?.data.e_array.close_bracket_loc,
+                    .is_single_line = existing.?.data.e_array.is_single_line,
+                }, existing.?.loc);
+                try root.data.e_object.put(allocator, current_key, updated_array);
+            }
+            return;
+        }
+        
+        // Navigate deeper
+        var nested_obj = root.get(current_key);
+        if (nested_obj == null or nested_obj.?.data != .e_object) {
+            const new_obj = js_ast.Expr.init(js_ast.E.Object, js_ast.E.Object{}, logger.Loc.Empty);
+            try root.data.e_object.put(allocator, current_key, new_obj);
+            nested_obj = root.get(current_key);
+        }
+        
+        if (nested_obj.?.data != .e_object) {
+            return error.ExpectedObject;
+        }
+        
+        var nested = nested_obj.?;
+        try appendToNestedArrayRecursive(allocator, &nested, remaining_path, new_value);
+        try root.data.e_object.put(allocator, current_key, nested);
+    }
+
     fn setValue(allocator: std.mem.Allocator, root: *js_ast.Expr, key: []const u8, value: []const u8, parse_json: bool) !void {
         if (root.data != .e_object) {
             return error.InvalidRoot;
         }
 
+        // Check for array append syntax (key[]=value)
+        if (strings.endsWith(key, "[]")) {
+            const array_key = key[0..key.len - 2];
+            return try appendToArray(allocator, root, array_key, value, parse_json);
+        }
+
         if (strings.indexOf(key, "[") == null) {
             var parts = std.mem.tokenizeScalar(u8, key, '.');
             var path_parts = std.ArrayList([]const u8).init(allocator);
diff --git a/test/cli/install/bun-pm-pkg.test.ts b/test/cli/install/bun-pm-pkg.test.ts
index 7ffaafa2b9..f2d81aa2a1 100644
--- a/test/cli/install/bun-pm-pkg.test.ts
+++ b/test/cli/install/bun-pm-pkg.test.ts
@@ -1099,6 +1099,127 @@ describe("bun pm pkg", () => {
     });
   });
 
+  describe("array append functionality (npm compatibility)", () => {
+    it("should append to existing array using key[]=value syntax", async () => {
+      const { code } = await runPmPkg(["set", "keywords[]=newkeyword"], testDir!);
+      expect(code).toBe(0);
+
+      const { output: getOutput } = await runPmPkg(["get", "keywords"], testDir!);
+      const parsed = JSON.parse(getOutput);
+      expect(Array.isArray(parsed)).toBe(true);
+      expect(parsed).toContain("test");
+      expect(parsed).toContain("package");
+      expect(parsed).toContain("newkeyword");
+      expect(parsed).toHaveLength(3);
+    });
+
+    it("should create new array when property doesn't exist using key[]=value syntax", async () => {
+      const { code } = await runPmPkg(["set", "newarrayfield[]=firstitem"], testDir!);
+      expect(code).toBe(0);
+
+      const { output: getOutput } = await runPmPkg(["get", "newarrayfield"], testDir!);
+      const parsed = JSON.parse(getOutput);
+      expect(Array.isArray(parsed)).toBe(true);
+      expect(parsed).toEqual(["firstitem"]);
+    });
+
+    it("should append multiple items to array in single command", async () => {
+      const { code } = await runPmPkg(["set", "keywords[]=first", "keywords[]=second"], testDir!);
+      expect(code).toBe(0);
+
+      const { output: getOutput } = await runPmPkg(["get", "keywords"], testDir!);
+      const parsed = JSON.parse(getOutput);
+      expect(Array.isArray(parsed)).toBe(true);
+      expect(parsed).toContain("test");
+      expect(parsed).toContain("package");
+      expect(parsed).toContain("first");
+      expect(parsed).toContain("second");
+      expect(parsed).toHaveLength(4);
+    });
+
+    it("should handle nested array append syntax", async () => {
+      const { code } = await runPmPkg(["set", "config.tags[]=newtag"], testDir!);
+      expect(code).toBe(0);
+
+      const { output: getOutput } = await runPmPkg(["get", "config.tags"], testDir!);
+      const parsed = JSON.parse(getOutput);
+      expect(Array.isArray(parsed)).toBe(true);
+      expect(parsed).toEqual(["newtag"]);
+    });
+
+    it("should append to nested existing array", async () => {
+      // First create a nested array
+      const { code: setupCode } = await runPmPkg(["set", 'nested.items=["existing"]', "--json"], testDir!);
+      expect(setupCode).toBe(0);
+
+      // Then append to it
+      const { code } = await runPmPkg(["set", "nested.items[]=appended"], testDir!);
+      expect(code).toBe(0);
+
+      const { output: getOutput } = await runPmPkg(["get", "nested.items"], testDir!);
+      const parsed = JSON.parse(getOutput);
+      expect(Array.isArray(parsed)).toBe(true);
+      expect(parsed).toEqual(["existing", "appended"]);
+    });
+
+    it("should error when trying to append to non-array property", async () => {
+      const { error, code } = await runPmPkg(["set", "name[]=invalid"], testDir!, false);
+      expect(code).toBe(1);
+      expect(error).toContain("Property name[] already exists and is not an Array or Object");
+    });
+
+    it("should handle array append with JSON values", async () => {
+      const { code } = await runPmPkg(["set", 'numbers[]=42', "--json"], testDir!);
+      expect(code).toBe(0);
+
+      const { output: getOutput } = await runPmPkg(["get", "numbers"], testDir!);
+      const parsed = JSON.parse(getOutput);
+      expect(Array.isArray(parsed)).toBe(true);
+      expect(parsed).toEqual([42]);
+      expect(typeof parsed[0]).toBe("number");
+    });
+
+    it("should handle array append with complex JSON objects", async () => {
+      const { code } = await runPmPkg(["set", 'contributors[]={"name":"New Contributor","email":"new@example.com"}', "--json"], testDir!);
+      expect(code).toBe(0);
+
+      const { output: getOutput } = await runPmPkg(["get", "contributors"], testDir!);
+      const parsed = JSON.parse(getOutput);
+      expect(Array.isArray(parsed)).toBe(true);
+      expect(parsed).toHaveLength(3); // Original 2 plus 1 new
+      expect(parsed[2]).toEqual({
+        name: "New Contributor",
+        email: "new@example.com"
+      });
+    });
+
+    it("should maintain array order when appending", async () => {
+      const { code } = await runPmPkg(["set", "order[]=first", "order[]=second", "order[]=third"], testDir!);
+      expect(code).toBe(0);
+
+      const { output: getOutput } = await runPmPkg(["get", "order"], testDir!);
+      const parsed = JSON.parse(getOutput);
+      expect(parsed).toEqual(["first", "second", "third"]);
+    });
+
+    it("should preserve existing array structure when appending", async () => {
+      // Get original keywords array
+      const { output: originalOutput } = await runPmPkg(["get", "keywords"], testDir!);
+      const originalKeywords = JSON.parse(originalOutput);
+
+      // Append new item
+      const { code } = await runPmPkg(["set", "keywords[]=appended"], testDir!);
+      expect(code).toBe(0);
+
+      // Verify structure is preserved with new item added
+      const { output: newOutput } = await runPmPkg(["get", "keywords"], testDir!);
+      const newKeywords = JSON.parse(newOutput);
+      
+      expect(newKeywords.slice(0, originalKeywords.length)).toEqual(originalKeywords);
+      expect(newKeywords[newKeywords.length - 1]).toBe("appended");
+    });
+  });
+
   // npm does the actual "" key, but bun right now doesn't support it
   describe.todo("empty string key compatibility", () => {
     let emptyKeyDir: string;