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Replace localhost with 127.0.0.1 for correct Codex handshake #1057

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Replace localhost with 127.0.0.1 for correct Codex handshake
hvesuk Apr 11, 2026
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hvesuk Apr 21, 2026
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15 changes: 13 additions & 2 deletions MCPForUnity/Editor/Helpers/CodexConfigHelper.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -36,7 +36,7 @@ public static string BuildCodexServerBlock(string uvPath)
if (useHttpTransport)
{
// HTTP mode: Use url field
string httpUrl = HttpEndpointUtility.GetMcpRpcUrl();
string httpUrl = GetCodexHttpUrl();
unityMCP["url"] = new TomlString { Value = httpUrl };

// Enable Codex's Rust MCP client for HTTP/SSE transport
Expand Down Expand Up @@ -191,7 +191,7 @@ private static TomlTable CreateUnityMcpTable(string uvPath)
if (useHttpTransport)
{
// HTTP mode: Use url field
string httpUrl = HttpEndpointUtility.GetMcpRpcUrl();
string httpUrl = GetCodexHttpUrl();
unityMCP["url"] = new TomlString { Value = httpUrl };
}
else
Expand Down Expand Up @@ -229,6 +229,17 @@ private static TomlTable CreateUnityMcpTable(string uvPath)
return unityMCP;
}

private static string GetCodexHttpUrl()
{
// Codex has intermittently failed the MCP handshake when configured with hostname-based
// loopback URLs such as localhost/::1, even though Unity MCP itself is reachable.
// Keep normalizing loopback hosts to 127.0.0.1 for Codex until the client routes them
// reliably again. If Codex fixes that behavior in the future, this is the workaround
// to revisit or remove before changing shared endpoint generation.
return HttpEndpointUtility.GetMcpRpcUrl()
.Replace("http://localhost:", "http://127.0.0.1:");
Comment on lines +232 to +240
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@sourcery-ai sourcery-ai Bot Apr 11, 2026

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suggestion (bug_risk): Make the localhost-to-127.0.0.1 conversion more robust (https, non-prefix matches, future changes).

This relies on HttpEndpointUtility.GetMcpRpcUrl() always returning http://localhost:<port>. If it ever returns https://localhost:... or localhost appears elsewhere in the URL, this string replace may fail or corrupt the URL. Instead, parse the URL with Uri, check that Host == "localhost", and rebuild it with Host = "127.0.0.1" while preserving the original scheme, port, path, and query to avoid these assumptions.

Suggested implementation:

        private static string GetCodexHttpUrl()
        {
            var rawUrl = HttpEndpointUtility.GetMcpRpcUrl();

            if (string.IsNullOrWhiteSpace(rawUrl))
            {
                return rawUrl;
            }

            if (!Uri.TryCreate(rawUrl, UriKind.Absolute, out var uri))
            {
                // If parsing fails, don't try to be clever – just return the original value
                return rawUrl;
            }

            if (!string.Equals(uri.Host, "localhost", StringComparison.OrdinalIgnoreCase))
            {
                // Only rewrite when the host is actually localhost
                return rawUrl;
            }

            var builder = new UriBuilder(uri)
            {
                Host = "127.0.0.1"
            };

            return builder.Uri.ToString();
        }

If System is not already imported at the top of CodexConfigHelper.cs, add using System; so that Uri, UriBuilder, and StringComparison resolve correctly.

}
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@coderabbitai coderabbitai Bot Apr 11, 2026

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⚠️ Potential issue | 🟠 Major

🧩 Analysis chain

🏁 Script executed:

#!/bin/bash
set -euo pipefail

echo "== Locate HttpEndpointUtility and inspect URL construction =="
fd -a "HttpEndpointUtility.cs" | while read -r f; do
  echo "--- $f ---"
  rg -n -C3 'GetBaseUrl|GetMcpRpcUrl|localhost|https|Uri|port' "$f"
done

echo
echo "== Find localhost URL patterns and transport-related usage across C# files =="
rg -n -C2 --type=cs 'http://localhost|https://localhost|GetMcpRpcUrl\(|UseHttpTransport'

Repository: CoplayDev/unity-mcp

Length of output: 50375

🏁 Script executed:

#!/bin/bash
set -euo pipefail

echo "=== Inspect CodexConfigHelper.cs GetCodexHttpUrl method ==="
fd -a "CodexConfigHelper.cs" | while read -r f; do
  echo "--- $f (lines 230-250) ---"
  sed -n '230,250p' "$f"
done

echo
echo "=== Inspect HttpEndpointUtility GetLocalBaseUrl and GetRemoteBaseUrl ==="
fd -a "HttpEndpointUtility.cs" | while read -r f; do
  echo "--- GetLocalBaseUrl method ---"
  sed -n '33,90p' "$f"
  echo
  echo "--- GetRemoteBaseUrl method ---"
  sed -n '103,130p' "$f" 2>/dev/null || echo "(not in expected range)"
done

echo
echo "=== Check for HTTPS usage with localhost ==="
rg -n 'https.*localhost|localhost.*https' --type=cs -i

echo
echo "=== Check what AppendPathSegment does ==="
rg -n -A10 'AppendPathSegment' --type=cs | head -30

Repository: CoplayDev/unity-mcp

Length of output: 4312

🏁 Script executed:

#!/bin/bash
set -euo pipefail

echo "=== Find DefaultLocalBaseUrl and DefaultRemoteBaseUrl constants ==="
rg -n 'DefaultLocalBaseUrl|DefaultRemoteBaseUrl|LocalPrefKey|RemotePrefKey' --type=cs | head -20

echo
echo "=== Find NormalizeBaseUrl implementation ==="
rg -n -A30 'private static string NormalizeBaseUrl' --type=cs

echo
echo "=== Check what schemes are assigned in the normalization process ==="
rg -n 'http:|https:' --type=cs -C2 | grep -A2 -B2 'scheme\|http://'  | head -40

Repository: CoplayDev/unity-mcp

Length of output: 10211

🏁 Script executed:

#!/bin/bash
set -euo pipefail

echo "=== Search for GetCodexHttpUrl usage ==="
rg -n 'GetCodexHttpUrl' --type=cs

echo
echo "=== Context around CodexConfigHelper GetCodexHttpUrl and its callers ==="
fd -a "CodexConfigHelper.cs" | while read -r f; do
  echo "--- Full context (lines 220-250) ---"
  sed -n '220,250p' "$f"
done

echo
echo "=== Check if localhost ever appears in remote scope settings ==="
rg -n 'HttpRemoteBaseUrl|RemoteBaseUrl.*localhost' --type=cs

echo
echo "=== Find any tests for CodexConfigHelper ==="
fd -a "*CodexConfigHelper*Test*" --type f

Repository: CoplayDev/unity-mcp

Length of output: 4263

🏁 Script executed:

#!/bin/bash
set -euo pipefail

echo "=== Context of GetCodexHttpUrl calls at lines 39 and 194 ==="
fd -a "CodexConfigHelper.cs" | while read -r f; do
  echo "--- Lines 30-50 (first call at 39) ---"
  sed -n '30,50p' "$f"
  echo
  echo "--- Lines 185-205 (second call at 194) ---"
  sed -n '185,205p' "$f"
done

echo
echo "=== Check if GetCodexHttpUrl result is only used for local scope ==="
fd -a "CodexConfigHelper.cs" | while read -r f; do
  echo "--- Full method context around GetCodexHttpUrl calls ---"
  sed -n '25,75p' "$f"
  echo
  sed -n '175,220p' "$f"
done

echo
echo "=== Look for tests of CodexConfigHelper with HTTP URLs ==="
fd -a "CodexConfigHelperTests.cs" | head -1 | xargs -I {} sed -n '460,540p' {}

Repository: CoplayDev/unity-mcp

Length of output: 10227

🏁 Script executed:

#!/bin/bash
set -euo pipefail

echo "=== Check if IsRemoteScope can be true when useHttpTransport=true ==="
rg -n -B5 -A5 'GetCodexHttpUrl\|GetMcpRpcUrl\|IsRemoteScope' MCPForUnity/Editor/Helpers/CodexConfigHelper.cs | head -30

echo
echo "=== Verify remote scope can use localhost ==="
rg -n 'HttpRemoteBaseUrl|RemoteBaseUrl|IsRemoteScope' MCPForUnity/Editor/Helpers/HttpEndpointUtility.cs | head -20

echo
echo "=== Check actual URL construction for remote scope ==="
fd -a "HttpEndpointUtility.cs" | while read -r f; do
  echo "--- NormalizeBaseUrl with remoteScope examples ---"
  sed -n '312,340p' "$f"
done

Repository: CoplayDev/unity-mcp

Length of output: 132

🏁 Script executed:

#!/bin/bash
set -euo pipefail

echo "=== Check if IsRemoteScope can be true and what URLs it produces ==="
fd -a "HttpEndpointUtility.cs" | while read -r f; do
  echo "--- GetBaseUrl and IsRemoteScope logic ---"
  sed -n '29,32p' "$f"
  echo
  echo "--- NormalizeBaseUrl remoteScope=true branch (lines 312-340) ---"
  sed -n '312,340p' "$f"
done

echo
echo "=== Check if GetRemoteBaseUrl can return a localhost URL ==="
rg -n 'GetRemoteBaseUrl|DefaultRemoteBaseUrl|RemotePrefKey' MCPForUnity/Editor/Helpers/HttpEndpointUtility.cs | head -15

echo
echo "=== Verify: Can a user configure RemoteBaseUrl with localhost? ==="
rg -n -C3 'SaveRemoteBaseUrl|SetHttpRemoteBaseUrl' --type=cs | head -40

echo
echo "=== Check validation of remote URLs ==="
fd -a "HttpEndpointUtility.cs" | while read -r f; do
  echo "--- ValidateRemoteBaseUrl method ---"
  sed -n '255,290p' "$f"
done

Repository: CoplayDev/unity-mcp

Length of output: 7806

Rewrite localhost URL handling to use URI-based host replacement instead of string replacement.

The current implementation only matches http://localhost: exactly. When the HTTP Remote scope is active and configured with a localhost URL (e.g., localhost:9000), it normalizes to https://localhost:..., which the .Replace() call silently misses.

Suggested fix 
 private static string GetCodexHttpUrl()
 {
-    return HttpEndpointUtility.GetMcpRpcUrl()
-        .Replace("http://localhost:", "http://127.0.0.1:");
+    string rawUrl = HttpEndpointUtility.GetMcpRpcUrl();
+    if (!Uri.TryCreate(rawUrl, UriKind.Absolute, out Uri uri))
+    {
+        return rawUrl;
+    }
+
+    if (!string.Equals(uri.Host, "localhost", StringComparison.OrdinalIgnoreCase))
+    {
+        return rawUrl;
+    }
+
+    var builder = new UriBuilder(uri)
+    {
+        Host = "127.0.0.1"
+    };
+
+    return builder.Uri.ToString();
 }
🤖 Prompt for AI Agents 
Verify each finding against the current code and only fix it if needed.

In `@MCPForUnity/Editor/Helpers/CodexConfigHelper.cs` around lines 232 - 236,
GetCodexHttpUrl currently uses string.Replace on the raw URL from
HttpEndpointUtility.GetMcpRpcUrl(), which fails when the scheme changes (e.g.,
https) or the host appears in different forms; instead parse the value returned
by HttpEndpointUtility.GetMcpRpcUrl() into a System.Uri (or UriBuilder), detect
if the host is "localhost" or IPLoopback, and if so set the host to "127.0.0.1"
while preserving scheme, port, path and query, then return the rebuilt URL
string; update GetCodexHttpUrl to use Uri/UriBuilder logic and handle
exceptions/fallbacks to the original string from
HttpEndpointUtility.GetMcpRpcUrl().


/// <summary>
/// Ensures the features table contains the rmcp_client flag for HTTP/SSE transport.
/// </summary>
Expand Down