diff --git a/src/automerge/AutomergeToTLStore.ts b/src/automerge/AutomergeToTLStore.ts
index 9db1da2..d0dd894 100644
--- a/src/automerge/AutomergeToTLStore.ts
+++ b/src/automerge/AutomergeToTLStore.ts
@@ -11,39 +11,23 @@ function isValidIndexKey(index: string): boolean {
     return false
   }
 
-  // The first character indicates the integer part length:
-  // 'a' = 1 digit, 'b' = 2 digits, etc. for positive integers
-  // 'Z' = 1 digit, 'Y' = 2 digits, etc. for negative integers
-  // But for normal shapes, 'a' followed by a digit is the most common pattern
+  // tldraw uses fractional indexing where:
+  // - First character is a lowercase letter indicating integer part length (a=1, b=2, c=3, etc.)
+  // - Followed by alphanumeric characters for the value and optional jitter
+  // Examples: "a0", "a1", "b10", "b99", "c100", "a1V4rr", "b10Lz"
+  //
+  // Also uppercase letters for negative indices (Z=1, Y=2, etc.)
 
-  // Simple invalid patterns that are definitely wrong:
-  // - Just a number like "1", "2"
-  // - Old format like "b1", "c1" (letter + single digit that's not a valid fractional index)
-  // - Empty or whitespace
-
-  // Valid fractional indices from tldraw start with 'a' for small positive numbers
-  // and follow with digits + optional alphanumeric jitter
-  // Pattern: starts with 'a', followed by at least one digit, then optional alphanumeric chars
-
-  // Simple patterns that are DEFINITELY invalid for tldraw:
-  // "b1", "c1", "d1" etc - these are old non-fractional indices
-  if (/^[b-z]\d$/i.test(index)) {
-    return false
-  }
-
-  // Valid tldraw indices should start with lowercase 'a' followed by digits
-  // and optionally more alphanumeric characters for the fractional part
-  // Examples from actual tldraw: "a0", "a1", "a24sT", "a1V4rr"
-  if (/^a\d/.test(index)) {
+  // Valid fractional index: lowercase letter followed by alphanumeric characters
+  if (/^[a-z][a-zA-Z0-9]+$/.test(index)) {
     return true
   }
 
-  // Also allow 'Z' prefix for very high indices (though rare)
-  if (/^Z[a-z]/i.test(index)) {
+  // Also allow uppercase prefix for negative/very high indices
+  if (/^[A-Z][a-zA-Z0-9]+$/.test(index)) {
     return true
   }
 
-  // If none of the above, it's likely invalid
   return false
 }
 
diff --git a/src/automerge/MinimalSanitization.ts b/src/automerge/MinimalSanitization.ts
index ece4131..1613e85 100644
--- a/src/automerge/MinimalSanitization.ts
+++ b/src/automerge/MinimalSanitization.ts
@@ -23,13 +23,15 @@ function minimalSanitizeRecord(record: any): any {
     // NOTE: Index assignment is handled by assignSequentialIndices() during format conversion
     // Here we only ensure index exists with a valid format, not strictly validate
     // This preserves layer order that was established during conversion
-    // Valid formats: a1, a2, a10, a1V, a1Lz, etc. (fractional indexing)
+    // tldraw uses fractional indexing: a0, a1, b10, c100, a1V4rr, etc.
+    // - First letter (a-z) indicates integer part length (a=1 digit, b=2 digits, etc.)
+    // - Uppercase (A-Z) for negative/special indices
     if (!sanitized.index || typeof sanitized.index !== 'string' || sanitized.index.length === 0) {
       // Only assign default if truly missing
       sanitized.index = 'a1'
-    } else if (!/^a\d/.test(sanitized.index) && !/^Z[a-z]/i.test(sanitized.index)) {
-      // Accept any index starting with 'a' + digit, or 'Z' prefix
-      // Only reset clearly invalid formats
+    } else if (!/^[a-zA-Z][a-zA-Z0-9]+$/.test(sanitized.index)) {
+      // Accept any letter followed by alphanumeric characters
+      // Only reset clearly invalid formats (e.g., numbers, empty, single char)
       console.warn(`⚠️ MinimalSanitization: Invalid index format "${sanitized.index}" for shape ${sanitized.id}`)
       sanitized.index = 'a1'
     }
diff --git a/src/automerge/useAutomergeSyncRepo.ts b/src/automerge/useAutomergeSyncRepo.ts
index 3b393ef..23438bd 100644
--- a/src/automerge/useAutomergeSyncRepo.ts
+++ b/src/automerge/useAutomergeSyncRepo.ts
@@ -20,31 +20,23 @@ import { getDocumentId, saveDocumentId } from "./documentIdMapping"
 function isValidTldrawIndex(index: string): boolean {
   if (!index || typeof index !== 'string' || index.length === 0) return false
 
-  // The first character indicates the integer part length:
-  // 'a' = 1 digit, 'b' = 2 digits, etc. for positive integers
-  // 'Z' = 1 digit, 'Y' = 2 digits, etc. for negative integers
-  // But for normal shapes, 'a' followed by a digit is the most common pattern
+  // tldraw uses fractional indexing where:
+  // - First character is a lowercase letter indicating integer part length (a=1, b=2, c=3, etc.)
+  // - Followed by alphanumeric characters for the value and optional jitter
+  // Examples: "a0", "a1", "b10", "b99", "c100", "a1V4rr", "b10Lz"
+  //
+  // Also uppercase letters for negative indices (Z=1, Y=2, etc.)
 
-  // Simple patterns that are DEFINITELY invalid for tldraw:
-  // "b1", "c1", "d1" etc - these are old non-fractional indices (single letter + single digit)
-  // These were used before tldraw switched to fractional indexing
-  if (/^[b-z]\d$/i.test(index)) {
-    return false
-  }
-
-  // Valid tldraw indices should start with lowercase 'a' followed by digits
-  // and optionally more alphanumeric characters for the fractional/jitter part
-  // Examples from actual tldraw: "a0", "a1", "a24sT", "a1V4rr"
-  if (/^a\d/.test(index)) {
+  // Valid fractional index: lowercase letter followed by alphanumeric characters
+  if (/^[a-z][a-zA-Z0-9]+$/.test(index)) {
     return true
   }
 
-  // Also allow 'Z' prefix for very high indices (though rare)
-  if (/^Z[a-z]/i.test(index)) {
+  // Also allow uppercase prefix for negative/very high indices
+  if (/^[A-Z][a-zA-Z0-9]+$/.test(index)) {
     return true
   }
 
-  // If none of the above, it's likely invalid
   return false
 }
 
diff --git a/src/routes/Board.tsx b/src/routes/Board.tsx
index 54fec9d..4fc7ddc 100644
--- a/src/routes/Board.tsx
+++ b/src/routes/Board.tsx
@@ -68,20 +68,18 @@ import "@/css/style.css"
 import "@/css/obsidian-browser.css"
 
 // Helper to validate and fix tldraw IndexKey format
-// Valid: "a0", "a1", "a24sT", "a1V4rr" - Invalid: "b1", "c1" (old format)
+// tldraw uses fractional indexing: a0, a1, b10, c100, a1V4rr, etc.
+// - First letter (a-z) indicates integer part length (a=1 digit, b=2 digits, etc.)
+// - Uppercase (A-Z) for negative/special indices
 function sanitizeIndex(index: any): IndexKey {
   if (!index || typeof index !== 'string' || index.length === 0) {
     return 'a1' as IndexKey
   }
-  // Old format "b1", "c1" etc are invalid (single letter + single digit)
-  if (/^[b-z]\d$/i.test(index)) {
-    return 'a1' as IndexKey
-  }
-  // Valid: starts with 'a' followed by at least one digit
-  if (/^a\d/.test(index)) {
+  // Valid: letter followed by alphanumeric characters
+  if (/^[a-zA-Z][a-zA-Z0-9]+$/.test(index)) {
     return index as IndexKey
   }
-  // Fallback
+  // Fallback for invalid formats
   return 'a1' as IndexKey
 }
 
diff --git a/worker/AutomergeDurableObject.ts b/worker/AutomergeDurableObject.ts
index ba85b70..260dc2c 100644
--- a/worker/AutomergeDurableObject.ts
+++ b/worker/AutomergeDurableObject.ts
@@ -747,11 +747,12 @@ export class AutomergeDurableObject {
     shapesNeedingIndex.sort((a, b) => a.originalIndex - b.originalIndex)
 
     // Check if shapes already have valid indices we should preserve
-    // Valid index: starts with 'a' followed by digits, optionally followed by alphanumeric jitter
+    // Valid tldraw fractional index: starts with a lowercase letter followed by alphanumeric characters
+    // Examples: a1, a2, b1, c10, a1V, a1Lz, etc. (the letter increments as indices grow)
     const isValidIndex = (idx: any): boolean => {
       if (!idx || typeof idx !== 'string' || idx.length === 0) return false
-      // Valid fractional index format: a1, a2, a1V, a10, a1Lz, etc.
-      if (/^a\d/.test(idx)) return true
+      // Valid fractional index format: lowercase letter followed by alphanumeric (a1, b1, c10, a1V, etc.)
+      if (/^[a-z][a-zA-Z0-9]+$/.test(idx)) return true
       // Also allow 'Z' prefix for very high indices
       if (/^Z[a-z]/i.test(idx)) return true
       return false