Add request header rewriting to RewritePlugin

DevProxy.Integration.Tests/ManipulationAndMockingIntegrationTests.cs

@@ -53,6 +53,102 @@ public async Task Rewrite_RewritesRequestUrl_OriginServesRewrittenPath()
         Assert.Equal(HttpStatusCode.ServiceUnavailable, response.StatusCode);
     }
 
+    [Theory]
+    [InlineData("original")]
+    [InlineData(null)]
+    public async Task Rewrite_SetsRequestHeader_WhenUrlMatches(string? originalValue)
+    {
+        await using var origin = await FakeOrigin.StartAsync();
+        var urls = KestrelProxyHarness.BuildUrlsToWatch(origin.Host);
+        var config = PluginConfig.FromJson("""
+            {
+              "rewrites": [
+                { "in": { "url": "/headers$" }, "out": { "headers": [ { "name": "x-probe", "value": "rewritten" } ] } }
+              ]
+            }
+            """);
+        var plugin = new RewritePlugin(
+            SharedHttpClient,
+            NullLogger<RewritePlugin>.Instance,
+            urls,
+            ProxyConfig,
+            config);
+
+        await using var proxy = await KestrelProxyHarness.StartAsync(origin.Host, [plugin]);
+        using var client = proxy.CreateHttpClient();
+        using var request = new HttpRequestMessage(HttpMethod.Get, new Uri($"http://{origin.Host}/headers"));
+        if (originalValue is not null)
+        {
+            request.Headers.Add("X-Probe", originalValue);
+        }
+
+        using var response = await client.SendAsync(request);
+        var body = await response.Content.ReadAsStringAsync();
+
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+        Assert.Equal("probe=rewritten", body);
+        Assert.Contains(origin.ReceivedRequests, r => r.PathAndQuery == "/headers");
+    }
+
+    [Fact]
+    public async Task Rewrite_RewritesUrlAndHeader_MatchingHeaderAgainstOriginalUrl()
+    {
+        await using var origin = await FakeOrigin.StartAsync();
+        var urls = KestrelProxyHarness.BuildUrlsToWatch(origin.Host);
+        var config = PluginConfig.FromJson("""
+            {
+              "rewrites": [
+                { "in": { "url": "/get$" }, "out": { "url": "/headers", "headers": [ { "name": "X-Probe", "value": "rewritten" } ] } }
+              ]
+            }
+            """);
+        var plugin = new RewritePlugin(
+            SharedHttpClient,
+            NullLogger<RewritePlugin>.Instance,
+            urls,
+            ProxyConfig,
+            config);
+
+        await using var proxy = await KestrelProxyHarness.StartAsync(origin.Host, [plugin]);
+        using var client = proxy.CreateHttpClient();
+
+        using var response = await client.GetAsync(new Uri($"http://{origin.Host}/get"));
+        var body = await response.Content.ReadAsStringAsync();
+
+        Assert.Contains(origin.ReceivedRequests, r => r.PathAndQuery == "/headers");
+        Assert.Equal("probe=rewritten", body);
+    }
+
+    [Fact]
+    public async Task Rewrite_LeavesRequestHeader_WhenUrlDoesNotMatch()
+    {
+        await using var origin = await FakeOrigin.StartAsync();
+        var urls = KestrelProxyHarness.BuildUrlsToWatch(origin.Host);
+        var config = PluginConfig.FromJson("""
+            {
+              "rewrites": [
+                { "in": { "url": "/other$" }, "out": { "headers": [ { "name": "X-Probe", "value": "rewritten" } ] } }
+              ]
+            }
+            """);
+        var plugin = new RewritePlugin(
+            SharedHttpClient,
+            NullLogger<RewritePlugin>.Instance,
+            urls,
+            ProxyConfig,
+            config);
+
+        await using var proxy = await KestrelProxyHarness.StartAsync(origin.Host, [plugin]);
+        using var client = proxy.CreateHttpClient();
+        using var request = new HttpRequestMessage(HttpMethod.Get, new Uri($"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("probe=original", body);
+    }
+
     [Fact]
     public async Task Rewrite_RewritesWebSocketUrl_OriginReceivesRewrittenPath()
     {

DevProxy.Plugins/Manipulation/RewritePlugin.cs

@@ -12,9 +12,16 @@
 
 namespace DevProxy.Plugins.Manipulation;
 
+public sealed class RewriteHeader
+{
+    public string? Name { get; set; }
+    public string? Value { get; set; }
+}
+
 public sealed class RewriteRule
 {
     public string? Url { get; set; }
+    public IEnumerable<RewriteHeader>? Headers { get; set; }
 }
 
 public sealed class RequestRewrite
@@ -82,22 +89,53 @@ public override Task BeforeRequestAsync(ProxyRequestArgs e, CancellationToken ca
 
         foreach (var rewrite in Configuration.Rewrites)
         {
-            if (string.IsNullOrEmpty(rewrite.In?.Url) ||
-                string.IsNullOrEmpty(rewrite.Out?.Url))
+            if (string.IsNullOrEmpty(rewrite.In?.Url) || rewrite.Out is null)
+            {
+                continue;
+            }
+
+            var headers = rewrite.Out.Headers?
+                .Where(h => h is not null && !string.IsNullOrEmpty(h.Name) && h.Value is not null)
+                .ToArray() ?? [];
+            if (string.IsNullOrEmpty(rewrite.Out.Url) && headers.Length == 0)
             {
                 continue;
             }
 
-            var newUrl = Regex.Replace(request.Url, rewrite.In.Url, rewrite.Out.Url, RegexOptions.IgnoreCase);
+            // Match against the URL before this rule rewrites it, so that
+            // 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 newUrl = Regex.Replace(request.Url, rewrite.In.Url, rewrite.Out.Url, RegexOptions.IgnoreCase);
+
+                if (request.Url.Equals(newUrl, StringComparison.OrdinalIgnoreCase))
+                {
+                    Logger.LogRequest($"{rewrite.In.Url}", MessageType.Skipped, new LoggingContext(e.ProxySession));
+                }
+                else
+                {
+                    Logger.LogRequest($"{rewrite.In.Url} > {newUrl}", MessageType.Processed, new LoggingContext(e.ProxySession));
+                    request.Url = newUrl;
+                }
+            }
+
+            if (headers.Length == 0)
+            {
+                continue;
+            }
 
-            if (request.Url.Equals(newUrl, StringComparison.OrdinalIgnoreCase))
+            if (!urlMatches)
             {
-                Logger.LogRequest($"{rewrite.In?.Url}", MessageType.Skipped, new LoggingContext(e.ProxySession));
+                Logger.LogRequest($"{rewrite.In.Url} (headers)", MessageType.Skipped, new LoggingContext(e.ProxySession));
+                continue;
             }
-            else
+
+            foreach (var header in headers)
             {
-                Logger.LogRequest($"{rewrite.In?.Url} > {newUrl}", MessageType.Processed, new LoggingContext(e.ProxySession));
-                request.Url = newUrl;
+                request.Headers.Replace(header.Name!, header.Value!);
+                Logger.LogRequest($"{rewrite.In.Url} > header {header.Name}", MessageType.Processed, new LoggingContext(e.ProxySession));
             }
         }
 

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

@@ -33,10 +33,32 @@
               "url": {
                 "type": "string",
                 "pattern": "^.*$",
-                "description": "URL to rewrite the request to. Can use capture groups from the 'in' pattern."
+                "description": "URL to rewrite the request to. Can use capture groups from the 'in' pattern. If omitted, the request URL isn't changed."
+              },
+              "headers": {
+                "type": "array",
+                "description": "Request headers to set on requests matching the 'in' URL. Replaces the value of an existing header (matched case-insensitively) or adds the header if it's missing.",
+                "items": {
+                  "type": "object",
+                  "properties": {
+                    "name": {
+                      "type": "string",
+                      "pattern": "^.+$",
+                      "description": "Name of the request header."
+                    },
+                    "value": {
+                      "type": "string",
+                      "description": "Literal value to set for the request header."
+                    }
+                  },
+                  "required": ["name", "value"]
+                }
               }
             },
-            "required": ["url"]
+            "anyOf": [
+              { "required": ["url"] },
+              { "required": ["headers"] }
+            ]
           }
         },
         "required": ["in", "out"]

skills/dev-proxy/references/configuration.md

@@ -302,7 +302,7 @@ None of these have configuration properties.
 
 ### RewritePlugin
 
-Rewrites request URLs using regex capture groups.
+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.
 
 | 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.
+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.
 
 | Property | Default | Description |
 |----------|---------|-------------|