refactor: replace text-based edit_diagram with ID-based operations (#267)

* refactor: replace text-based edit_diagram with ID-based operations

- Add applyDiagramOperations() function using DOMParser for ID lookup
- New schema: operations array with type (update/add/delete), cell_id, new_xml
- Update chat-panel.tsx handler for new operations format
- Update OperationsDisplay component to show operation type and cell_id
- Simplify system prompts with new ID-based examples
- Add ID validation for add operations
- Add warning for edges referencing deleted cells

* fix: add ID validation to update operation and remove dead code

- Add ID mismatch validation to update operation (consistency with add)
- Remove orphaned replaceXMLParts function (~300 lines of dead code)
- Update cell_id schema description for clarity
- Add unit tests for applyDiagramOperations (11 tests)

app/api/chat/route.ts

@@ -408,33 +408,37 @@ Notes:
                 }),
             },
             edit_diagram: {
-                description: `Edit specific parts of the current diagram by replacing exact line matches. Use this tool to make targeted fixes without regenerating the entire XML.
-CRITICAL: Copy-paste the EXACT search pattern from the "Current diagram XML" in system context. Do NOT reorder attributes or reformat - the attribute order in draw.io XML varies and you MUST match it exactly.
-IMPORTANT: Keep edits concise:
-- COPY the exact mxCell line from the current XML (attribute order matters!)
-- Only include the lines that are changing, plus 1-2 surrounding lines for context if needed
-- Break large changes into multiple smaller edits
-- Each search must contain complete lines (never truncate mid-line)
-- First match only - be specific enough to target the right element
+                description: `Edit the current diagram by ID-based operations (update/add/delete cells).
 
-⚠️ JSON ESCAPING: Every " inside string values MUST be escaped as \\". Example: x=\\"100\\" y=\\"200\\" - BOTH quotes need backslashes!`,
+Operations:
+- update: Replace an existing cell by its id. Provide cell_id and complete new_xml.
+- add: Add a new cell. Provide cell_id (new unique id) and new_xml.
+- delete: Remove a cell by its id. Only cell_id is needed.
+
+For update/add, new_xml must be a complete mxCell element including mxGeometry.
+
+⚠️ JSON ESCAPING: Every " inside new_xml MUST be escaped as \\". Example: id=\\"5\\" value=\\"Label\\"`,
                 inputSchema: z.object({
-                    edits: z
+                    operations: z
                         .array(
                             z.object({
-                                search: z
+                                type: z
+                                    .enum(["update", "add", "delete"])
+                                    .describe("Operation type"),
+                                cell_id: z
                                     .string()
                                     .describe(
-                                        "EXACT lines copied from current XML (preserve attribute order!)",
+                                        "The id of the mxCell. Must match the id attribute in new_xml.",
                                     ),
-                                replace: z
+                                new_xml: z
                                     .string()
-                                    .describe("Replacement lines"),
+                                    .optional()
+                                    .describe(
+                                        "Complete mxCell XML element (required for update/add)",
+                                    ),
                             }),
                         )
-                        .describe(
-                            "Array of search/replace pairs to apply sequentially",
-                        ),
+                        .describe("Array of operations to apply"),
                 }),
             },
             append_diagram: {

components/chat-message-display.tsx

@@ -10,9 +10,7 @@ import {
     Cpu,
     FileCode,
     FileText,
-    Minus,
     Pencil,
-    Plus,
     RotateCcw,
     ThumbsDown,
     ThumbsUp,
@@ -38,9 +36,10 @@ import {
 import ExamplePanel from "./chat-example-panel"
 import { CodeBlock } from "./code-block"
 
-interface EditPair {
-    search: string
-    replace: string
+interface DiagramOperation {
+    type: "update" | "add" | "delete"
+    cell_id: string
+    new_xml?: string
 }
 
 // Tool part interface for type safety
@@ -48,49 +47,44 @@ interface ToolPartLike {
     type: string
     toolCallId: string
     state?: string
-    input?: { xml?: string; edits?: EditPair[] } & Record<string, unknown>
+    input?: {
+        xml?: string
+        operations?: DiagramOperation[]
+    } & Record<string, unknown>
     output?: string
 }
 
-function EditDiffDisplay({ edits }: { edits: EditPair[] }) {
+function OperationsDisplay({ operations }: { operations: DiagramOperation[] }) {
     return (
         <div className="space-y-3">
-            {edits.map((edit, index) => (
+            {operations.map((op, index) => (
                 <div
-                    key={`${(edit.search || "").slice(0, 50)}-${(edit.replace || "").slice(0, 50)}-${index}`}
+                    key={`${op.type}-${op.cell_id}-${index}`}
                     className="rounded-lg border border-border/50 overflow-hidden bg-background/50"
                 >
                     <div className="px-3 py-1.5 bg-muted/40 border-b border-border/30 flex items-center gap-2">
-                        <span className="text-xs font-medium text-muted-foreground">
-                            Change {index + 1}
+                        <span
+                            className={`text-[10px] font-medium uppercase tracking-wide ${
+                                op.type === "delete"
+                                    ? "text-red-600"
+                                    : op.type === "add"
+                                      ? "text-green-600"
+                                      : "text-blue-600"
+                            }`}
+                        >
+                            {op.type}
+                        </span>
+                        <span className="text-xs text-muted-foreground">
+                            cell_id: {op.cell_id}
                         </span>
                     </div>
-                    <div className="divide-y divide-border/30">
-                        {/* Search (old) */}
+                    {op.new_xml && (
                         <div className="px-3 py-2">
-                            <div className="flex items-center gap-1.5 mb-1.5">
-                                <Minus className="w-3 h-3 text-red-500" />
-                                <span className="text-[10px] font-medium text-red-600 uppercase tracking-wide">
-                                    Remove
-                                </span>
-                            </div>
-                            <pre className="text-[11px] font-mono text-red-700 bg-red-50 rounded px-2 py-1.5 overflow-x-auto whitespace-pre-wrap break-all">
-                                {edit.search}
+                            <pre className="text-[11px] font-mono text-foreground/80 bg-muted/30 rounded px-2 py-1.5 overflow-x-auto whitespace-pre-wrap break-all">
+                                {op.new_xml}
                             </pre>
                         </div>
-                        {/* Replace (new) */}
-                        <div className="px-3 py-2">
-                            <div className="flex items-center gap-1.5 mb-1.5">
-                                <Plus className="w-3 h-3 text-green-500" />
-                                <span className="text-[10px] font-medium text-green-600 uppercase tracking-wide">
-                                    Add
-                                </span>
-                            </div>
-                            <pre className="text-[11px] font-mono text-green-700 bg-green-50 rounded px-2 py-1.5 overflow-x-auto whitespace-pre-wrap break-all">
-                                {edit.replace}
-                            </pre>
-                        </div>
-                    </div>
+                    )}
                 </div>
             ))}
         </div>
@@ -582,9 +576,9 @@ export function ChatMessageDisplay({
                         {typeof input === "object" && input.xml ? (
                             <CodeBlock code={input.xml} language="xml" />
                         ) : typeof input === "object" &&
-                          input.edits &&
-                          Array.isArray(input.edits) ? (
-                            <EditDiffDisplay edits={input.edits} />
+                          input.operations &&
+                          Array.isArray(input.operations) ? (
+                            <OperationsDisplay operations={input.operations} />
                         ) : typeof input === "object" &&
                           Object.keys(input).length > 0 ? (
                             <CodeBlock

components/chat-panel.tsx

@@ -323,8 +323,12 @@ ${finalXml}
                     }
                 }
             } else if (toolCall.toolName === "edit_diagram") {
-                const { edits } = toolCall.input as {
-                    edits: Array<{ search: string; replace: string }>
+                const { operations } = toolCall.input as {
+                    operations: Array<{
+                        type: "update" | "add" | "delete"
+                        cell_id: string
+                        new_xml?: string
+                    }>
                 }
 
                 let currentXml = ""
@@ -338,8 +342,36 @@ ${finalXml}
                         currentXml = await onFetchChart(false)
                     }
 
-                    const { replaceXMLParts } = await import("@/lib/utils")
-                    const editedXml = replaceXMLParts(currentXml, edits)
+                    const { applyDiagramOperations } = await import(
+                        "@/lib/utils"
+                    )
+                    const { result: editedXml, errors } =
+                        applyDiagramOperations(currentXml, operations)
+
+                    // Check for operation errors
+                    if (errors.length > 0) {
+                        const errorMessages = errors
+                            .map(
+                                (e) =>
+                                    `- ${e.type} on cell_id="${e.cellId}": ${e.message}`,
+                            )
+                            .join("\n")
+
+                        addToolOutput({
+                            tool: "edit_diagram",
+                            toolCallId: toolCall.toolCallId,
+                            state: "output-error",
+                            errorText: `Some operations failed:\n${errorMessages}
+
+Current diagram XML:
+\`\`\`xml
+${currentXml}
+\`\`\`
+
+Please check the cell IDs and retry.`,
+                        })
+                        return
+                    }
 
                     // loadDiagram validates and returns error if invalid
                     const validationError = onDisplayChart(editedXml)
@@ -359,7 +391,7 @@ Current diagram XML:
 ${currentXml}
 \`\`\`
 
-Please fix the edit to avoid structural issues (e.g., duplicate IDs, invalid references).`,
+Please fix the operations to avoid structural issues.`,
                         })
                         return
                     }
@@ -367,7 +399,7 @@ Please fix the edit to avoid structural issues (e.g., duplicate IDs, invalid ref
                     addToolOutput({
                         tool: "edit_diagram",
                         toolCallId: toolCall.toolCallId,
-                        output: `Successfully applied ${edits.length} edit(s) to the diagram.`,
+                        output: `Successfully applied ${operations.length} operation(s) to the diagram.`,
                     })
                 } catch (error) {
                     console.error("[edit_diagram] Failed:", error)
@@ -375,7 +407,6 @@ Please fix the edit to avoid structural issues (e.g., duplicate IDs, invalid ref
                     const errorMessage =
                         error instanceof Error ? error.message : String(error)
 
-                    // Use addToolOutput with state: 'output-error' for proper error signaling
                     addToolOutput({
                         tool: "edit_diagram",
                         toolCallId: toolCall.toolCallId,
@@ -387,7 +418,7 @@ Current diagram XML:
 ${currentXml || "No XML available"}
 \`\`\`
 
-Please retry with an adjusted search pattern or use display_diagram if retries are exhausted.`,
+Please check cell IDs and retry, or use display_diagram to regenerate.`,
                     })
                 }
             } else if (toolCall.toolName === "append_diagram") {

lib/system-prompts.ts

@@ -88,19 +88,15 @@ Note that:
 - NEVER include XML comments (<!-- ... -->) in your generated XML. Draw.io strips comments, which breaks edit_diagram patterns.
 
 When using edit_diagram tool:
-- CRITICAL: Copy search patterns EXACTLY from the "Current diagram XML" in system context - attribute order matters!
-- Always include the element's id attribute for unique targeting: {"search": "<mxCell id=\\"5\\"", ...}
-- Include complete elements (mxCell + mxGeometry) for reliable matching
-- Preserve exact whitespace, indentation, and line breaks
-- BAD: {"search": "value=\\"Label\\"", ...} - too vague, matches multiple elements
-- GOOD: {"search": "<mxCell id=\\"3\\" value=\\"Old\\" style=\\"...\\">", "replace": "<mxCell id=\\"3\\" value=\\"New\\" style=\\"...\\">"}
-- For multiple changes, use separate edits in array
-- RETRY POLICY: If pattern not found, retry up to 3 times with adjusted patterns. After 3 failures, use display_diagram instead.
+- Use operations: update (modify cell by id), add (new cell), delete (remove cell by id)
+- For update/add: provide cell_id and complete new_xml (full mxCell element including mxGeometry)
+- For delete: only cell_id is needed
+- Find the cell_id from "Current diagram XML" in system context
+- Example update: {"operations": [{"type": "update", "cell_id": "3", "new_xml": "<mxCell id=\\"3\\" value=\\"New Label\\" style=\\"rounded=1;\\" vertex=\\"1\\" parent=\\"1\\">\\n  <mxGeometry x=\\"100\\" y=\\"100\\" width=\\"120\\" height=\\"60\\" as=\\"geometry\\"/>\\n</mxCell>"}]}
+- Example delete: {"operations": [{"type": "delete", "cell_id": "5"}]}
+- Example add: {"operations": [{"type": "add", "cell_id": "new1", "new_xml": "<mxCell id=\\"new1\\" value=\\"New Box\\" style=\\"rounded=1;\\" vertex=\\"1\\" parent=\\"1\\">\\n  <mxGeometry x=\\"400\\" y=\\"200\\" width=\\"120\\" height=\\"60\\" as=\\"geometry\\"/>\\n</mxCell>"}]}
 
-⚠️ CRITICAL JSON ESCAPING: When outputting edit_diagram tool calls, you MUST escape ALL double quotes inside string values:
-- CORRECT: "y=\\"119\\""  (both quotes escaped)
-- WRONG: "y="119\\""  (missing backslash before first quote - causes JSON parse error!)
-- Every " inside a JSON string value needs \\" - no exceptions!
+⚠️ JSON ESCAPING: Every " inside new_xml MUST be escaped as \\". Example: id=\\"5\\" value=\\"Label\\"
 
 ## Draw.io XML Structure Reference
 
@@ -268,69 +264,43 @@ const EXTENDED_ADDITIONS = `
 
 ### edit_diagram Details
 
-**CRITICAL RULES:**
-- Copy-paste the EXACT search pattern from the "Current diagram XML" in system context
-- Do NOT reorder attributes or reformat - the attribute order in draw.io XML varies and you MUST match it exactly
-- Only include the lines that are changing, plus 1-2 surrounding lines for context if needed
-- Break large changes into multiple smaller edits
-- Each search must contain complete lines (never truncate mid-line)
-- First match only - be specific enough to target the right element
+edit_diagram uses ID-based operations to modify cells directly by their id attribute.
+
+**Operations:**
+- **update**: Replace an existing cell. Provide cell_id and new_xml.
+- **add**: Add a new cell. Provide cell_id (new unique id) and new_xml.
+- **delete**: Remove a cell. Only cell_id is needed.
 
 **Input Format:**
 \`\`\`json
 {
-  "edits": [
-    {
-      "search": "EXACT lines copied from current XML (preserve attribute order!)",
-      "replace": "Replacement lines"
-    }
+  "operations": [
+    {"type": "update", "cell_id": "3", "new_xml": "<mxCell ...complete element...>"},
+    {"type": "add", "cell_id": "new1", "new_xml": "<mxCell ...new element...>"},
+    {"type": "delete", "cell_id": "5"}
   ]
 }
 \`\`\`
 
-## edit_diagram Best Practices
+**Examples:**
 
-### Core Principle: Unique & Precise Patterns
-Your search pattern MUST uniquely identify exactly ONE location in the XML. Before writing a search pattern:
-1. Review the "Current diagram XML" in the system context
-2. Identify the exact element(s) to modify by their unique id attribute
-3. Include enough context to ensure uniqueness
-
-### Pattern Construction Rules
-
-**Rule 1: Always include the element's id attribute**
+Change label:
 \`\`\`json
-{"search": "<mxCell id=\\"node5\\"", "replace": "<mxCell id=\\"node5\\" value=\\"New Label\\""}
+{"operations": [{"type": "update", "cell_id": "3", "new_xml": "<mxCell id=\\"3\\" value=\\"New Label\\" style=\\"rounded=1;\\" vertex=\\"1\\" parent=\\"1\\">\\n  <mxGeometry x=\\"100\\" y=\\"100\\" width=\\"120\\" height=\\"60\\" as=\\"geometry\\"/>\\n</mxCell>"}]}
 \`\`\`
 
-**Rule 2: Include complete XML elements when possible**
+Add new shape:
 \`\`\`json
-{
-  "search": "<mxCell id=\\"3\\" value=\\"Old\\" style=\\"rounded=1;\\" vertex=\\"1\\" parent=\\"1\\">\\n  <mxGeometry x=\\"100\\" y=\\"100\\" width=\\"120\\" height=\\"60\\" as=\\"geometry\\"/>\\n</mxCell>",
-  "replace": "<mxCell id=\\"3\\" value=\\"New\\" style=\\"rounded=1;\\" vertex=\\"1\\" parent=\\"1\\">\\n  <mxGeometry x=\\"100\\" y=\\"100\\" width=\\"120\\" height=\\"60\\" as=\\"geometry\\"/>\\n</mxCell>"
-}
+{"operations": [{"type": "add", "cell_id": "new1", "new_xml": "<mxCell id=\\"new1\\" value=\\"New Box\\" style=\\"rounded=1;fillColor=#dae8fc;\\" vertex=\\"1\\" parent=\\"1\\">\\n  <mxGeometry x=\\"400\\" y=\\"200\\" width=\\"120\\" height=\\"60\\" as=\\"geometry\\"/>\\n</mxCell>"}]}
 \`\`\`
 
-**Rule 3: Preserve exact whitespace and formatting**
-Copy the search pattern EXACTLY from the current XML, including leading spaces, line breaks (\\n), and attribute order.
+Delete cell:
+\`\`\`json
+{"operations": [{"type": "delete", "cell_id": "5"}]}
+\`\`\`
 
-### Good vs Bad Patterns
-
-**BAD:** \`{"search": "value=\\"Label\\""}\` - Too vague, matches multiple elements
-**BAD:** \`{"search": "<mxCell value=\\"X\\" id=\\"5\\""}\` - Reordered attributes won't match
-**GOOD:** \`{"search": "<mxCell id=\\"5\\" parent=\\"1\\" style=\\"...\\" value=\\"Old\\" vertex=\\"1\\">"}\` - Uses unique id with full context
-
-### ⚠️ JSON Escaping (CRITICAL)
-Every double quote inside JSON string values MUST be escaped with backslash:
-- **CORRECT:** \`"x=\\"100\\" y=\\"200\\""\` - both quotes escaped
-- **WRONG:** \`"x=\\"100\\" y="200\\""\` - missing backslash causes JSON parse error!
-
-### Error Recovery
-If edit_diagram fails with "pattern not found":
-1. **First retry**: Check attribute order - copy EXACTLY from current XML
-2. **Second retry**: Expand context - include more surrounding lines
-3. **Third retry**: Try matching on just \`<mxCell id="X"\` prefix + full replacement
-4. **After 3 failures**: Fall back to display_diagram to regenerate entire diagram
+**Error Recovery:**
+If cell_id not found, check "Current diagram XML" for correct IDs. Use display_diagram if major restructuring is needed
 
 
 

lib/utils.ts

@@ -377,303 +377,223 @@ export function replaceNodes(currentXML: string, nodes: string): string {
     }
 }
 
-/**
- * Create a character count dictionary from a string
- * Used for attribute-order agnostic comparison
- */
-function charCountDict(str: string): Map<string, number> {
-    const dict = new Map<string, number>()
-    for (const char of str) {
-        dict.set(char, (dict.get(char) || 0) + 1)
+// ============================================================================
+// ID-based Diagram Operations
+// ============================================================================
+
+export interface DiagramOperation {
+    type: "update" | "add" | "delete"
+    cell_id: string
+    new_xml?: string
 }
-    return dict
+
+export interface OperationError {
+    type: "update" | "add" | "delete"
+    cellId: string
+    message: string
+}
+
+export interface ApplyOperationsResult {
+    result: string
+    errors: OperationError[]
 }
 
 /**
- * Compare two strings by character frequency (order-agnostic)
+ * Apply diagram operations (update/add/delete) using ID-based lookup.
+ * This replaces the text-matching approach with direct DOM manipulation.
+ *
+ * @param xmlContent - The full mxfile XML content
+ * @param operations - Array of operations to apply
+ * @returns Object with result XML and any errors
  */
-function sameCharFrequency(a: string, b: string): boolean {
-    const trimmedA = a.trim()
-    const trimmedB = b.trim()
-    if (trimmedA.length !== trimmedB.length) return false
-
-    const dictA = charCountDict(trimmedA)
-    const dictB = charCountDict(trimmedB)
-
-    if (dictA.size !== dictB.size) return false
-
-    for (const [char, count] of dictA) {
-        if (dictB.get(char) !== count) return false
-    }
-    return true
-}
-
-/**
- * Replace specific parts of XML content using search and replace pairs
- * @param xmlContent - The original XML string
- * @param searchReplacePairs - Array of {search: string, replace: string} objects
- * @returns The updated XML string with replacements applied
- */
-export function replaceXMLParts(
+export function applyDiagramOperations(
     xmlContent: string,
-    searchReplacePairs: Array<{ search: string; replace: string }>,
-): string {
-    // Format the XML first to ensure consistent line breaks
-    let result = formatXML(xmlContent)
+    operations: DiagramOperation[],
+): ApplyOperationsResult {
+    const errors: OperationError[] = []
 
-    for (const { search, replace } of searchReplacePairs) {
-        // Also format the search content for consistency
-        const formattedSearch = formatXML(search)
-        const searchLines = formattedSearch.split("\n")
+    // Parse the XML
+    const parser = new DOMParser()
+    const doc = parser.parseFromString(xmlContent, "text/xml")
 
-        // Split into lines for exact line matching
-        const resultLines = result.split("\n")
-
-        // Remove trailing empty line if exists (from the trailing \n in search content)
-        if (searchLines[searchLines.length - 1] === "") {
-            searchLines.pop()
-        }
-
-        // Always search from the beginning - pairs may not be in document order
-        const startLineNum = 0
-
-        // Try to find match using multiple strategies
-        let matchFound = false
-        let matchStartLine = -1
-        let matchEndLine = -1
-
-        // First try: exact match
-        for (
-            let i = startLineNum;
-            i <= resultLines.length - searchLines.length;
-            i++
-        ) {
-            let matches = true
-
-            for (let j = 0; j < searchLines.length; j++) {
-                if (resultLines[i + j] !== searchLines[j]) {
-                    matches = false
-                    break
+    // Check for parse errors
+    const parseError = doc.querySelector("parsererror")
+    if (parseError) {
+        return {
+            result: xmlContent,
+            errors: [
+                {
+                    type: "update",
+                    cellId: "",
+                    message: `XML parse error: ${parseError.textContent}`,
+                },
+            ],
         }
     }
 
-            if (matches) {
-                matchStartLine = i
-                matchEndLine = i + searchLines.length
-                matchFound = true
-                break
+    // Find the root element (inside mxGraphModel)
+    const root = doc.querySelector("root")
+    if (!root) {
+        return {
+            result: xmlContent,
+            errors: [
+                {
+                    type: "update",
+                    cellId: "",
+                    message: "Could not find <root> element in XML",
+                },
+            ],
         }
     }
 
-        // Second try: line-trimmed match (fallback)
-        if (!matchFound) {
-            for (
-                let i = startLineNum;
-                i <= resultLines.length - searchLines.length;
-                i++
-            ) {
-                let matches = true
+    // Build a map of cell IDs to elements
+    const cellMap = new Map<string, Element>()
+    root.querySelectorAll("mxCell").forEach((cell) => {
+        const id = cell.getAttribute("id")
+        if (id) cellMap.set(id, cell)
+    })
 
-                for (let j = 0; j < searchLines.length; j++) {
-                    const originalTrimmed = resultLines[i + j].trim()
-                    const searchTrimmed = searchLines[j].trim()
-
-                    if (originalTrimmed !== searchTrimmed) {
-                        matches = false
-                        break
-                    }
+    // Process each operation
+    for (const op of operations) {
+        if (op.type === "update") {
+            const existingCell = cellMap.get(op.cell_id)
+            if (!existingCell) {
+                errors.push({
+                    type: "update",
+                    cellId: op.cell_id,
+                    message: `Cell with id="${op.cell_id}" not found`,
+                })
+                continue
             }
 
-                if (matches) {
-                    matchStartLine = i
-                    matchEndLine = i + searchLines.length
-                    matchFound = true
-                    break
-                }
-            }
+            if (!op.new_xml) {
+                errors.push({
+                    type: "update",
+                    cellId: op.cell_id,
+                    message: "new_xml is required for update operation",
+                })
+                continue
             }
 
-        // Third try: substring match as last resort (for single-line XML)
-        if (!matchFound) {
-            // Try to find as a substring in the entire content
-            const searchStr = search.trim()
-            const resultStr = result
-            const index = resultStr.indexOf(searchStr)
-
-            if (index !== -1) {
-                // Found as substring - replace it
-                result =
-                    resultStr.substring(0, index) +
-                    replace.trim() +
-                    resultStr.substring(index + searchStr.length)
-                // Re-format after substring replacement
-                result = formatXML(result)
-                continue // Skip the line-based replacement below
-            }
-        }
-
-        // Fourth try: character frequency match (attribute-order agnostic)
-        // This handles cases where the model generates XML with different attribute order
-        if (!matchFound) {
-            for (
-                let i = startLineNum;
-                i <= resultLines.length - searchLines.length;
-                i++
-            ) {
-                let matches = true
-
-                for (let j = 0; j < searchLines.length; j++) {
-                    if (
-                        !sameCharFrequency(resultLines[i + j], searchLines[j])
-                    ) {
-                        matches = false
-                        break
-                    }
-                }
-
-                if (matches) {
-                    matchStartLine = i
-                    matchEndLine = i + searchLines.length
-                    matchFound = true
-                    break
-                }
-            }
-        }
-
-        // Fifth try: Match by mxCell id attribute
-        // Extract id from search pattern and find the element with that id
-        if (!matchFound) {
-            const idMatch = search.match(/id="([^"]+)"/)
-            if (idMatch) {
-                const searchId = idMatch[1]
-                // Find lines that contain this id
-                for (let i = startLineNum; i < resultLines.length; i++) {
-                    if (resultLines[i].includes(`id="${searchId}"`)) {
-                        // Found the element with matching id
-                        // Now find the extent of this element (it might span multiple lines)
-                        let endLine = i + 1
-                        const line = resultLines[i].trim()
-
-                        // Check if it's a self-closing tag or has children
-                        if (!line.endsWith("/>")) {
-                            // Find the closing tag or the end of the mxCell block
-                            let depth = 1
-                            while (endLine < resultLines.length && depth > 0) {
-                                const currentLine = resultLines[endLine].trim()
-                                if (
-                                    currentLine.startsWith("<") &&
-                                    !currentLine.startsWith("</") &&
-                                    !currentLine.endsWith("/>")
-                                ) {
-                                    depth++
-                                } else if (currentLine.startsWith("</")) {
-                                    depth--
-                                }
-                                endLine++
-                            }
-                        }
-
-                        matchStartLine = i
-                        matchEndLine = endLine
-                        matchFound = true
-                        break
-                    }
-                }
-            }
-        }
-
-        // Sixth try: Match by value attribute (label text)
-        // Extract value from search pattern and find elements with that value
-        if (!matchFound) {
-            const valueMatch = search.match(/value="([^"]*)"/)
-            if (valueMatch) {
-                const searchValue = valueMatch[0] // Use full match like value="text"
-                for (let i = startLineNum; i < resultLines.length; i++) {
-                    if (resultLines[i].includes(searchValue)) {
-                        // Found element with matching value
-                        let endLine = i + 1
-                        const line = resultLines[i].trim()
-
-                        if (!line.endsWith("/>")) {
-                            let depth = 1
-                            while (endLine < resultLines.length && depth > 0) {
-                                const currentLine = resultLines[endLine].trim()
-                                if (
-                                    currentLine.startsWith("<") &&
-                                    !currentLine.startsWith("</") &&
-                                    !currentLine.endsWith("/>")
-                                ) {
-                                    depth++
-                                } else if (currentLine.startsWith("</")) {
-                                    depth--
-                                }
-                                endLine++
-                            }
-                        }
-
-                        matchStartLine = i
-                        matchEndLine = endLine
-                        matchFound = true
-                        break
-                    }
-                }
-            }
-        }
-
-        // Seventh try: Normalized whitespace match
-        // Collapse all whitespace and compare
-        if (!matchFound) {
-            const normalizeWs = (s: string) => s.replace(/\s+/g, " ").trim()
-            const normalizedSearch = normalizeWs(search)
-
-            for (
-                let i = startLineNum;
-                i <= resultLines.length - searchLines.length;
-                i++
-            ) {
-                // Build a normalized version of the candidate lines
-                const candidateLines = resultLines.slice(
-                    i,
-                    i + searchLines.length,
+            // Parse the new XML
+            const newDoc = parser.parseFromString(
+                `<wrapper>${op.new_xml}</wrapper>`,
+                "text/xml",
             )
-                const normalizedCandidate = normalizeWs(
-                    candidateLines.join(" "),
+            const newCell = newDoc.querySelector("mxCell")
+            if (!newCell) {
+                errors.push({
+                    type: "update",
+                    cellId: op.cell_id,
+                    message: "new_xml must contain an mxCell element",
+                })
+                continue
+            }
+
+            // Validate ID matches
+            const newCellId = newCell.getAttribute("id")
+            if (newCellId !== op.cell_id) {
+                errors.push({
+                    type: "update",
+                    cellId: op.cell_id,
+                    message: `ID mismatch: cell_id is "${op.cell_id}" but new_xml has id="${newCellId}"`,
+                })
+                continue
+            }
+
+            // Import and replace the node
+            const importedNode = doc.importNode(newCell, true)
+            existingCell.parentNode?.replaceChild(importedNode, existingCell)
+
+            // Update the map with the new element
+            cellMap.set(op.cell_id, importedNode)
+        } else if (op.type === "add") {
+            // Check if ID already exists
+            if (cellMap.has(op.cell_id)) {
+                errors.push({
+                    type: "add",
+                    cellId: op.cell_id,
+                    message: `Cell with id="${op.cell_id}" already exists`,
+                })
+                continue
+            }
+
+            if (!op.new_xml) {
+                errors.push({
+                    type: "add",
+                    cellId: op.cell_id,
+                    message: "new_xml is required for add operation",
+                })
+                continue
+            }
+
+            // Parse the new XML
+            const newDoc = parser.parseFromString(
+                `<wrapper>${op.new_xml}</wrapper>`,
+                "text/xml",
             )
-
-                if (normalizedCandidate === normalizedSearch) {
-                    matchStartLine = i
-                    matchEndLine = i + searchLines.length
-                    matchFound = true
-                    break
-                }
-            }
+            const newCell = newDoc.querySelector("mxCell")
+            if (!newCell) {
+                errors.push({
+                    type: "add",
+                    cellId: op.cell_id,
+                    message: "new_xml must contain an mxCell element",
+                })
+                continue
             }
 
-        if (!matchFound) {
-            throw new Error(
-                `Search pattern not found in the diagram. The pattern may not exist in the current structure.`,
+            // Validate ID matches
+            const newCellId = newCell.getAttribute("id")
+            if (newCellId !== op.cell_id) {
+                errors.push({
+                    type: "add",
+                    cellId: op.cell_id,
+                    message: `ID mismatch: cell_id is "${op.cell_id}" but new_xml has id="${newCellId}"`,
+                })
+                continue
+            }
+
+            // Import and append the node
+            const importedNode = doc.importNode(newCell, true)
+            root.appendChild(importedNode)
+
+            // Add to map
+            cellMap.set(op.cell_id, importedNode)
+        } else if (op.type === "delete") {
+            const existingCell = cellMap.get(op.cell_id)
+            if (!existingCell) {
+                errors.push({
+                    type: "delete",
+                    cellId: op.cell_id,
+                    message: `Cell with id="${op.cell_id}" not found`,
+                })
+                continue
+            }
+
+            // Check for edges referencing this cell (warning only, still delete)
+            const referencingEdges = root.querySelectorAll(
+                `mxCell[source="${op.cell_id}"], mxCell[target="${op.cell_id}"]`,
+            )
+            if (referencingEdges.length > 0) {
+                const edgeIds = Array.from(referencingEdges)
+                    .map((e) => e.getAttribute("id"))
+                    .join(", ")
+                console.warn(
+                    `[applyDiagramOperations] Deleting cell "${op.cell_id}" which is referenced by edges: ${edgeIds}`,
                 )
             }
 
-        // Replace the matched lines
-        const replaceLines = replace.split("\n")
-
-        // Remove trailing empty line if exists
-        if (replaceLines[replaceLines.length - 1] === "") {
-            replaceLines.pop()
+            // Remove the node
+            existingCell.parentNode?.removeChild(existingCell)
+            cellMap.delete(op.cell_id)
+        }
     }
 
-        // Perform the replacement
-        const newResultLines = [
-            ...resultLines.slice(0, matchStartLine),
-            ...replaceLines,
-            ...resultLines.slice(matchEndLine),
-        ]
+    // Serialize back to string
+    const serializer = new XMLSerializer()
+    const result = serializer.serializeToString(doc)
 
-        result = newResultLines.join("\n")
-    }
-
-    return result
+    return { result, errors }
 }
 
 // ============================================================================

scripts/test-diagram-operations.mjs

@@ -0,0 +1,251 @@
+/**
+ * Simple test script for applyDiagramOperations function
+ * Run with: node scripts/test-diagram-operations.mjs
+ */
+
+import { JSDOM } from "jsdom"
+
+// Set up DOMParser for Node.js environment
+const dom = new JSDOM()
+globalThis.DOMParser = dom.window.DOMParser
+globalThis.XMLSerializer = dom.window.XMLSerializer
+
+// Import the function (we'll inline it since it's not ESM exported)
+function applyDiagramOperations(xmlContent, operations) {
+    const errors = []
+    const parser = new DOMParser()
+    const doc = parser.parseFromString(xmlContent, "text/xml")
+
+    const parseError = doc.querySelector("parsererror")
+    if (parseError) {
+        return {
+            result: xmlContent,
+            errors: [{ type: "update", cellId: "", message: `XML parse error: ${parseError.textContent}` }],
+        }
+    }
+
+    const root = doc.querySelector("root")
+    if (!root) {
+        return {
+            result: xmlContent,
+            errors: [{ type: "update", cellId: "", message: "Could not find <root> element in XML" }],
+        }
+    }
+
+    const cellMap = new Map()
+    root.querySelectorAll("mxCell").forEach((cell) => {
+        const id = cell.getAttribute("id")
+        if (id) cellMap.set(id, cell)
+    })
+
+    for (const op of operations) {
+        if (op.type === "update") {
+            const existingCell = cellMap.get(op.cell_id)
+            if (!existingCell) {
+                errors.push({ type: "update", cellId: op.cell_id, message: `Cell with id="${op.cell_id}" not found` })
+                continue
+            }
+            if (!op.new_xml) {
+                errors.push({ type: "update", cellId: op.cell_id, message: "new_xml is required for update operation" })
+                continue
+            }
+            const newDoc = parser.parseFromString(`<wrapper>${op.new_xml}</wrapper>`, "text/xml")
+            const newCell = newDoc.querySelector("mxCell")
+            if (!newCell) {
+                errors.push({ type: "update", cellId: op.cell_id, message: "new_xml must contain an mxCell element" })
+                continue
+            }
+            const newCellId = newCell.getAttribute("id")
+            if (newCellId !== op.cell_id) {
+                errors.push({ type: "update", cellId: op.cell_id, message: `ID mismatch: cell_id is "${op.cell_id}" but new_xml has id="${newCellId}"` })
+                continue
+            }
+            const importedNode = doc.importNode(newCell, true)
+            existingCell.parentNode?.replaceChild(importedNode, existingCell)
+            cellMap.set(op.cell_id, importedNode)
+        } else if (op.type === "add") {
+            if (cellMap.has(op.cell_id)) {
+                errors.push({ type: "add", cellId: op.cell_id, message: `Cell with id="${op.cell_id}" already exists` })
+                continue
+            }
+            if (!op.new_xml) {
+                errors.push({ type: "add", cellId: op.cell_id, message: "new_xml is required for add operation" })
+                continue
+            }
+            const newDoc = parser.parseFromString(`<wrapper>${op.new_xml}</wrapper>`, "text/xml")
+            const newCell = newDoc.querySelector("mxCell")
+            if (!newCell) {
+                errors.push({ type: "add", cellId: op.cell_id, message: "new_xml must contain an mxCell element" })
+                continue
+            }
+            const newCellId = newCell.getAttribute("id")
+            if (newCellId !== op.cell_id) {
+                errors.push({ type: "add", cellId: op.cell_id, message: `ID mismatch: cell_id is "${op.cell_id}" but new_xml has id="${newCellId}"` })
+                continue
+            }
+            const importedNode = doc.importNode(newCell, true)
+            root.appendChild(importedNode)
+            cellMap.set(op.cell_id, importedNode)
+        } else if (op.type === "delete") {
+            const existingCell = cellMap.get(op.cell_id)
+            if (!existingCell) {
+                errors.push({ type: "delete", cellId: op.cell_id, message: `Cell with id="${op.cell_id}" not found` })
+                continue
+            }
+            existingCell.parentNode?.removeChild(existingCell)
+            cellMap.delete(op.cell_id)
+        }
+    }
+
+    const serializer = new XMLSerializer()
+    const result = serializer.serializeToString(doc)
+    return { result, errors }
+}
+
+// Test data
+const sampleXml = `<?xml version="1.0" encoding="UTF-8"?>
+<mxfile>
+  <diagram>
+    <mxGraphModel>
+      <root>
+        <mxCell id="0"/>
+        <mxCell id="1" parent="0"/>
+        <mxCell id="2" value="Box A" style="rounded=1;" vertex="1" parent="1">
+          <mxGeometry x="100" y="100" width="120" height="60" as="geometry"/>
+        </mxCell>
+        <mxCell id="3" value="Box B" style="rounded=1;" vertex="1" parent="1">
+          <mxGeometry x="300" y="100" width="120" height="60" as="geometry"/>
+        </mxCell>
+        <mxCell id="4" value="" style="edgeStyle=orthogonalEdgeStyle;" edge="1" parent="1" source="2" target="3">
+          <mxGeometry relative="1" as="geometry"/>
+        </mxCell>
+      </root>
+    </mxGraphModel>
+  </diagram>
+</mxfile>`
+
+let passed = 0
+let failed = 0
+
+function test(name, fn) {
+    try {
+        fn()
+        console.log(`✓ ${name}`)
+        passed++
+    } catch (e) {
+        console.log(`✗ ${name}`)
+        console.log(`  Error: ${e.message}`)
+        failed++
+    }
+}
+
+function assert(condition, message) {
+    if (!condition) throw new Error(message || "Assertion failed")
+}
+
+// Tests
+test("Update operation changes cell value", () => {
+    const { result, errors } = applyDiagramOperations(sampleXml, [
+        {
+            type: "update",
+            cell_id: "2",
+            new_xml: '<mxCell id="2" value="Updated Box A" style="rounded=1;" vertex="1" parent="1"><mxGeometry x="100" y="100" width="120" height="60" as="geometry"/></mxCell>',
+        },
+    ])
+    assert(errors.length === 0, `Expected no errors, got: ${JSON.stringify(errors)}`)
+    assert(result.includes('value="Updated Box A"'), "Updated value should be in result")
+    assert(!result.includes('value="Box A"'), "Old value should not be in result")
+})
+
+test("Update operation fails for non-existent cell", () => {
+    const { errors } = applyDiagramOperations(sampleXml, [
+        { type: "update", cell_id: "999", new_xml: '<mxCell id="999" value="Test"/>' },
+    ])
+    assert(errors.length === 1, "Should have one error")
+    assert(errors[0].message.includes("not found"), "Error should mention not found")
+})
+
+test("Update operation fails on ID mismatch", () => {
+    const { errors } = applyDiagramOperations(sampleXml, [
+        { type: "update", cell_id: "2", new_xml: '<mxCell id="WRONG" value="Test"/>' },
+    ])
+    assert(errors.length === 1, "Should have one error")
+    assert(errors[0].message.includes("ID mismatch"), "Error should mention ID mismatch")
+})
+
+test("Add operation creates new cell", () => {
+    const { result, errors } = applyDiagramOperations(sampleXml, [
+        {
+            type: "add",
+            cell_id: "new1",
+            new_xml: '<mxCell id="new1" value="New Box" style="rounded=1;" vertex="1" parent="1"><mxGeometry x="500" y="100" width="120" height="60" as="geometry"/></mxCell>',
+        },
+    ])
+    assert(errors.length === 0, `Expected no errors, got: ${JSON.stringify(errors)}`)
+    assert(result.includes('id="new1"'), "New cell should be in result")
+    assert(result.includes('value="New Box"'), "New cell value should be in result")
+})
+
+test("Add operation fails for duplicate ID", () => {
+    const { errors } = applyDiagramOperations(sampleXml, [
+        { type: "add", cell_id: "2", new_xml: '<mxCell id="2" value="Duplicate"/>' },
+    ])
+    assert(errors.length === 1, "Should have one error")
+    assert(errors[0].message.includes("already exists"), "Error should mention already exists")
+})
+
+test("Add operation fails on ID mismatch", () => {
+    const { errors } = applyDiagramOperations(sampleXml, [
+        { type: "add", cell_id: "new1", new_xml: '<mxCell id="WRONG" value="Test"/>' },
+    ])
+    assert(errors.length === 1, "Should have one error")
+    assert(errors[0].message.includes("ID mismatch"), "Error should mention ID mismatch")
+})
+
+test("Delete operation removes cell", () => {
+    const { result, errors } = applyDiagramOperations(sampleXml, [{ type: "delete", cell_id: "3" }])
+    assert(errors.length === 0, `Expected no errors, got: ${JSON.stringify(errors)}`)
+    assert(!result.includes('id="3"'), "Deleted cell should not be in result")
+    assert(result.includes('id="2"'), "Other cells should remain")
+})
+
+test("Delete operation fails for non-existent cell", () => {
+    const { errors } = applyDiagramOperations(sampleXml, [{ type: "delete", cell_id: "999" }])
+    assert(errors.length === 1, "Should have one error")
+    assert(errors[0].message.includes("not found"), "Error should mention not found")
+})
+
+test("Multiple operations in sequence", () => {
+    const { result, errors } = applyDiagramOperations(sampleXml, [
+        {
+            type: "update",
+            cell_id: "2",
+            new_xml: '<mxCell id="2" value="Updated" style="rounded=1;" vertex="1" parent="1"><mxGeometry x="100" y="100" width="120" height="60" as="geometry"/></mxCell>',
+        },
+        {
+            type: "add",
+            cell_id: "new1",
+            new_xml: '<mxCell id="new1" value="Added" style="rounded=1;" vertex="1" parent="1"><mxGeometry x="500" y="100" width="120" height="60" as="geometry"/></mxCell>',
+        },
+        { type: "delete", cell_id: "3" },
+    ])
+    assert(errors.length === 0, `Expected no errors, got: ${JSON.stringify(errors)}`)
+    assert(result.includes('value="Updated"'), "Updated value should be present")
+    assert(result.includes('id="new1"'), "Added cell should be present")
+    assert(!result.includes('id="3"'), "Deleted cell should not be present")
+})
+
+test("Invalid XML returns parse error", () => {
+    const { errors } = applyDiagramOperations("<not valid xml", [{ type: "delete", cell_id: "1" }])
+    assert(errors.length === 1, "Should have one error")
+})
+
+test("Missing root element returns error", () => {
+    const { errors } = applyDiagramOperations("<mxfile></mxfile>", [{ type: "delete", cell_id: "1" }])
+    assert(errors.length === 1, "Should have one error")
+    assert(errors[0].message.includes("root"), "Error should mention root element")
+})
+
+// Summary
+console.log(`\n${passed} passed, ${failed} failed`)
+process.exit(failed > 0 ? 1 : 0)