Support environment variables in RewritePlugin replacement values

DevProxy.Integration.Tests/ManipulationAndMockingIntegrationTests.cs

@@ -149,6 +149,81 @@ public async Task Rewrite_LeavesRequestHeader_WhenUrlDoesNotMatch()
         Assert.Equal("probe=original", body);
     }
 
+    [Fact]
+    public async Task Rewrite_ExpandsEnvironmentVariablesInUrlAndHeader()
+    {
+        var suffix = Guid.NewGuid().ToString("N");
+        var pathVar = $"DEVPROXY_TEST_PATH_{suffix}";
+        var headerVar = $"DEVPROXY_TEST_HEADER_{suffix}";
+        Environment.SetEnvironmentVariable(pathVar, "/head$1");
+        Environment.SetEnvironmentVariable(headerVar, "from-env");
+        try
+        {
+            await using var origin = await FakeOrigin.StartAsync();
+            var urls = KestrelProxyHarness.BuildUrlsToWatch(origin.Host);
+            var config = PluginConfig.FromJson($$"""
+                {
+                  "rewrites": [
+                    { "in": { "url": "/get(ers)$" }, "out": { "url": "${env:{{pathVar}}}", "headers": [ { "name": "X-Probe", "value": "v-${env:{{headerVar}}}" } ] } }
+                  ]
+                }
+                """);
+            var plugin = new RewritePlugin(
+                SharedHttpClient,
+                NullLogger<RewritePlugin>.Instance,
+                urls,
+                ProxyConfig,
+                config);
+
+            await using var proxy = await KestrelProxyHarness.StartAsync(origin.Host, [plugin]);
+            using var client = proxy.CreateHttpClient();
+
+            using var response = await client.GetAsync(new Uri($"http://{origin.Host}/geters"));
+            var body = await response.Content.ReadAsStringAsync();
+
+            // /head + capture group "ers" referenced from the env var value.
+            Assert.Contains(origin.ReceivedRequests, r => r.PathAndQuery == "/headers");
+            Assert.Equal("probe=v-from-env", body);
+        }
+        finally
+        {
+            Environment.SetEnvironmentVariable(pathVar, null);
+            Environment.SetEnvironmentVariable(headerVar, null);
+        }
+    }
+
+    [Fact]
+    public async Task Rewrite_SkipsRule_WhenEnvironmentVariableMissing()
+    {
+        var missingVar = $"DEVPROXY_TEST_MISSING_{Guid.NewGuid():N}";
+        await using var origin = await FakeOrigin.StartAsync();
+        var urls = KestrelProxyHarness.BuildUrlsToWatch(origin.Host);
+        var config = PluginConfig.FromJson($$"""
+            {
+              "rewrites": [
+                { "in": { "url": "/headers$" }, "out": { "url": "/status/503", "headers": [ { "name": "X-Probe", "value": "${env:{{missingVar}}}" } ] } }
+              ]
+            }
+            """);
+        var plugin = new RewritePlugin(
+            SharedHttpClient,
+            NullLogger<RewritePlugin>.Instance,
+            urls,
+            ProxyConfig,
+            config);
+
+        await using var proxy = await KestrelProxyHarness.StartAsync(origin.Host, [plugin]);
+        using var client = proxy.CreateHttpClient();
+        using var request = new HttpRequestMessage(HttpMethod.Get, new Uri($"http://{origin.Host}/headers"));
+        request.Headers.Add("X-Probe", "original");
+
+        using var response = await client.SendAsync(request);
+        var body = await response.Content.ReadAsStringAsync();
+
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+        Assert.Equal("probe=original", body);
+    }
+
     [Fact]
     public async Task Rewrite_RewritesWebSocketUrl_OriginReceivesRewrittenPath()
     {

DevProxy.Plugins/Manipulation/RewritePlugin.cs

@@ -36,7 +36,7 @@ public sealed class RewritePluginConfiguration
     public string RewritesFile { get; set; } = "rewrites.json";
 }
 
-public sealed class RewritePlugin(
+public sealed partial class RewritePlugin(
     HttpClient httpClient,
     ILogger<RewritePlugin> logger,
     ISet<UrlToWatch> urlsToWatch,
@@ -106,9 +106,31 @@ public override Task BeforeRequestAsync(ProxyRequestArgs e, CancellationToken ca
             // header rewrites apply to the same requests as the URL rewrite.
             var urlMatches = Regex.IsMatch(request.Url, rewrite.In.Url, RegexOptions.IgnoreCase);
 
-            if (!string.IsNullOrEmpty(rewrite.Out.Url))
+            var outUrl = rewrite.Out.Url;
+            if (urlMatches)
             {
-                var newUrl = Regex.Replace(request.Url, rewrite.In.Url, rewrite.Out.Url, RegexOptions.IgnoreCase);
+                var missingVariables = new HashSet<string>(StringComparer.Ordinal);
+                outUrl = ExpandEnvironmentVariables(outUrl, missingVariables);
+                headers = [.. headers.Select(h => new RewriteHeader
+                {
+                    Name = h.Name,
+                    Value = ExpandEnvironmentVariables(h.Value, missingVariables)
+                })];
+
+                if (missingVariables.Count > 0)
+                {
+                    Logger.LogError(
+                        "Skipping rewrite {InUrl} for {RequestUrl}. The following environment variables aren't set: {MissingVariables}",
+                        rewrite.In.Url,
+                        request.Url,
+                        string.Join(", ", missingVariables));
+                    continue;
+                }
+            }
+
+            if (!string.IsNullOrEmpty(outUrl))
+            {
+                var newUrl = Regex.Replace(request.Url, rewrite.In.Url, outUrl, RegexOptions.IgnoreCase);
 
                 if (request.Url.Equals(newUrl, StringComparison.OrdinalIgnoreCase))
                 {
@@ -143,6 +165,29 @@ public override Task BeforeRequestAsync(ProxyRequestArgs e, CancellationToken ca
         return Task.CompletedTask;
     }
 
+    private static string? ExpandEnvironmentVariables(string? value, HashSet<string> missingVariables)
+    {
+        if (string.IsNullOrEmpty(value))
+        {
+            return value;
+        }
+
+        return EnvironmentVariableTokenRegex().Replace(value, match =>
+        {
+            var name = match.Groups["name"].Value;
+            var envValue = Environment.GetEnvironmentVariable(name);
+            if (envValue is null)
+            {
+                _ = missingVariables.Add(name);
+                return match.Value;
+            }
+            return envValue;
+        });
+    }
+
+    [GeneratedRegex(@"\$\{env:(?<name>[^}]+)\}")]
+    private static partial Regex EnvironmentVariableTokenRegex();
+
     protected override void Dispose(bool disposing)
     {
         if (disposing)

schemas/v4.0.0/rewriteplugin.rewritesfile.schema.json

@@ -33,7 +33,7 @@
               "url": {
                 "type": "string",
                 "pattern": "^.*$",
-                "description": "URL to rewrite the request to. Can use capture groups from the 'in' pattern. If omitted, the request URL isn't changed."
+                "description": "URL to rewrite the request to. Can use capture groups from the 'in' pattern and reference environment variables using ${env:VAR_NAME}. Environment variable values are inserted as-is, so they can also reference capture groups. If a referenced environment variable isn't set, the rule is skipped. If omitted, the request URL isn't changed."
               },
               "headers": {
                 "type": "array",
@@ -48,7 +48,7 @@
                     },
                     "value": {
                       "type": "string",
-                      "description": "Literal value to set for the request header."
+                      "description": "Value to set for the request header. Can reference environment variables using ${env:VAR_NAME}. If a referenced environment variable isn't set, the rule is skipped."
                     }
                   },
                   "required": ["name", "value"]

skills/dev-proxy/references/configuration.md

@@ -302,7 +302,7 @@ None of these have configuration properties.
 
 ### RewritePlugin
 
-Rewrites request URLs using regex capture groups and sets request header values. Each rule matches on `in.url`; `out.url` (optional) rewrites the URL, and `out.headers` (optional, `[{ "name", "value" }]`) replaces the header value or adds the header if it's missing.
+Rewrites request URLs using regex capture groups and sets request header values. Each rule matches on `in.url`; `out.url` (optional) rewrites the URL, and `out.headers` (optional, `[{ "name", "value" }]`) replaces the header value or adds the header if it's missing. `out.url` and header values can reference environment variables with `${env:VAR_NAME}` (values are inserted as-is, so in `out.url` they can reference capture groups); if a referenced variable isn't set, Dev Proxy logs an error and skips the rule.
 
 | Property | Default | Description |
 |----------|---------|-------------|

skills/dev-proxy/references/plugin-catalog.md

@@ -117,7 +117,7 @@ Simulates token-based rate limiting for LLM APIs.
 
 ### RewritePlugin
 
-Rewrites request URLs using regex capture groups and sets request header values. Each rule matches on `in.url`; `out.url` (optional) rewrites the URL, and `out.headers` (optional, `[{ "name", "value" }]`) replaces the header value or adds the header if it's missing.
+Rewrites request URLs using regex capture groups and sets request header values. Each rule matches on `in.url`; `out.url` (optional) rewrites the URL, and `out.headers` (optional, `[{ "name", "value" }]`) replaces the header value or adds the header if it's missing. `out.url` and header values can reference environment variables with `${env:VAR_NAME}` (values are inserted as-is, so in `out.url` they can reference capture groups); if a referenced variable isn't set, Dev Proxy logs an error and skips the rule.
 
 | Property | Default | Description |
 |----------|---------|-------------|