diff --git a/tools-sync/scripts/sync_mcp_registrations.ps1 b/tools-sync/scripts/sync_mcp_registrations.ps1
index b87d912..4ae2055 100644
--- a/tools-sync/scripts/sync_mcp_registrations.ps1
+++ b/tools-sync/scripts/sync_mcp_registrations.ps1
@@ -116,6 +116,16 @@ function Get-PortableMcps($Map, [string]$Side) {
     return $result
 }
 
+function ConvertTo-PortablePath([string]$Value) {
+    # `claude mcp add-json` rejects Windows backslash paths ("Invalid configuration:
+    # : Invalid input"); forward slashes work on Windows and are what the CLI itself
+    # writes. Only rewrite strings that actually carry a drive-letter path, so flags
+    # and non-path args are left alone (`--extension-file=C:\..` is rewritten too).
+    if ($null -eq $Value) { return $Value }
+    if ($Value -match '[A-Za-z]:\\') { return ($Value -replace '\\', '/') }
+    return $Value
+}
+
 function Get-ComparableJson($Mcp) {
     $envMap = [ordered]@{}
     foreach ($key in @($Mcp.env.Keys) | Sort-Object) {
@@ -126,10 +136,13 @@ function Get-ComparableJson($Mcp) {
     foreach ($key in @($Mcp.headers.Keys) | Sort-Object) {
         $headers[$key] = [string]$Mcp.headers[$key]
     }
+    # `C:\x\y` and `C:/x/y` are the same target on Windows -- the two CLIs write
+    # different separators, so compare on a single normalized form or every path
+    # arg reads as drift.
     $value = [ordered]@{
         transport = $Mcp.transport
-        command = $Mcp.command
-        args = @($Mcp.args)
+        command = (ConvertTo-PortablePath $Mcp.command)
+        args = @(@($Mcp.args) | ForEach-Object { ConvertTo-PortablePath $_ })
         env = $envMap
         url = $Mcp.url
         headers = $headers
@@ -175,21 +188,32 @@ function Add-ToCodex($Mcp) {
 }
 
 function Add-ToClaude($Mcp) {
+    # Deliberately NOT `claude mcp add-json`: Windows PowerShell 5.1 strips the
+    # double quotes out of a JSON argument on its way to a native exe, so claude.exe
+    # sees malformed JSON and dies with "Invalid configuration: : Invalid input".
+    # The flag form takes plain tokens and is immune. (Separately, add-json rejects
+    # backslash paths even when quoting is clean -- hence ConvertTo-PortablePath.)
     if ($Mcp.transport -eq "stdio") {
-        $envMap = [ordered]@{}
+        $arguments = @("mcp", "add", $Mcp.name, "--scope", "user")
         foreach ($key in $Mcp.env.Keys) {
-            if ($CodexOnlyEnv -notcontains $key) { $envMap[$key] = $Mcp.env[$key] }
+            if ($CodexOnlyEnv -contains $key) { continue }
+            $arguments += @("--env", "$key=$($Mcp.env[$key])")
         }
-        $cfg = [ordered]@{ type = "stdio"; command = $Mcp.command; args = @($Mcp.args); env = $envMap }
-    } else {
-        if ($Mcp.bearer_token_env_var) {
-            throw "Codex bearer_token_env_var has no lossless Claude JSON translation: $($Mcp.name)"
-        }
-        $cfg = [ordered]@{ type = $Mcp.transport; url = $Mcp.url }
-        if ($Mcp.headers.Count -gt 0) { $cfg.headers = $Mcp.headers }
+        $arguments += "--"
+        $arguments += (ConvertTo-PortablePath $Mcp.command)
+        foreach ($arg in @($Mcp.args)) { $arguments += (ConvertTo-PortablePath $arg) }
+        Invoke-External "claude add $($Mcp.name)" "claude" $arguments
+        return
     }
-    $json = $cfg | ConvertTo-Json -Depth 8 -Compress
-    Invoke-External "claude add $($Mcp.name)" "claude" @("mcp", "add-json", "--scope", "user", $Mcp.name, $json)
+    if ($Mcp.bearer_token_env_var) {
+        throw "Codex bearer_token_env_var has no lossless Claude translation: $($Mcp.name)"
+    }
+    $arguments = @("mcp", "add", $Mcp.name, "--scope", "user",
+                   "--transport", $Mcp.transport, $Mcp.url)
+    foreach ($key in $Mcp.headers.Keys) {
+        $arguments += @("--header", "$key`: $($Mcp.headers[$key])")
+    }
+    Invoke-External "claude add $($Mcp.name)" "claude" $arguments
 }
 
 function Remove-FromClaude([string]$Name) {