diff --git a/CHANGELOG.md b/CHANGELOG.md
index 786f7c0..eeed2c0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,12 @@ Release tags use the form `vX.Y.Z` and match `package.json`. GitHub Releases car
 
 ## [Unreleased]
 
+## [0.5.9] - 2026-07-23
+
+### Fixed
+
+- Long-running streamed responses now send protocol-safe keepalives so clients such as Claude Code do not cancel healthy requests during extended model silence.
+
 ## [0.5.8] - 2026-07-23
 
 ### Fixed
@@ -94,7 +100,8 @@ Release tags use the form `vX.Y.Z` and match `package.json`. GitHub Releases car
 
 See [GitHub Releases](https://github.com/gitcommit90/rerouted/releases) for artifact digests and notes prior to the Keep a Changelog narrative. Notable themes in late 0.4.x included signed/notarized distribution, in-app updates, named routes, OAuth account pools, OpenAI chat completions and Responses routing, and launch hardening.
 
-[Unreleased]: https://github.com/gitcommit90/rerouted/compare/v0.5.8...HEAD
+[Unreleased]: https://github.com/gitcommit90/rerouted/compare/v0.5.9...HEAD
+[0.5.9]: https://github.com/gitcommit90/rerouted/releases/tag/v0.5.9
 [0.5.8]: https://github.com/gitcommit90/rerouted/releases/tag/v0.5.8
 [0.5.7]: https://github.com/gitcommit90/rerouted/releases/tag/v0.5.7
 [0.5.5]: https://github.com/gitcommit90/rerouted/releases/tag/v0.5.5
diff --git a/package-lock.json b/package-lock.json
index 418ea14..a149eaf 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "@gitcommit90/rerouted",
-  "version": "0.5.8",
+  "version": "0.5.9",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "@gitcommit90/rerouted",
-      "version": "0.5.8",
+      "version": "0.5.9",
       "license": "MIT",
       "bin": {
         "rerouted": "src/cli/index.js"
diff --git a/package.json b/package.json
index fe77720..3f0c426 100644
--- a/package.json
+++ b/package.json
@@ -1,7 +1,7 @@
 {
   "name": "@gitcommit90/rerouted",
   "productName": "ReRouted",
-  "version": "0.5.8",
+  "version": "0.5.9",
   "description": "A local AI router for connected accounts, models, named routes, and automatic fallback.",
   "author": "gitcommit90",
   "license": "MIT",
diff --git a/src/lib/anthropic-api.js b/src/lib/anthropic-api.js
index ced7555..381d74f 100644
--- a/src/lib/anthropic-api.js
+++ b/src/lib/anthropic-api.js
@@ -378,6 +378,12 @@ function writeEvent(sink, type, data) {
   sink.write(`event: ${type}\ndata: ${JSON.stringify(data)}\n\n`);
 }
 
+// Claude Code filters SSE comments and ping events before its stream-event
+// watchdog. This zero-impact delta reaches that watchdog without adding content
+// or completing the response; the real final delta remains authoritative.
+const ANTHROPIC_SSE_HEARTBEAT =
+  'event: message_delta\ndata: {"type":"message_delta","delta":{"stop_reason":null,"stop_sequence":null},"usage":{"output_tokens":0}}\n\n';
+
 async function pipeChatCompletionsSseToAnthropic(streamPipe, sink, requestedModel) {
   const parser = createSseParser();
   const id = messageId();
@@ -571,6 +577,7 @@ module.exports = {
   toChatCompletionsBody,
   fromChatCompletion,
   pipeChatCompletionsSseToAnthropic,
+  ANTHROPIC_SSE_HEARTBEAT,
   toAnthropicError,
   estimateInputTokens,
   anthropicUsage,
diff --git a/src/lib/gateway.js b/src/lib/gateway.js
index cd98c32..5222fe7 100644
--- a/src/lib/gateway.js
+++ b/src/lib/gateway.js
@@ -3,6 +3,7 @@
 const http = require("node:http");
 const { DEFAULT_PORT } = require("./constants");
 const logger = require("./logger");
+const { createSseHeartbeat, DEFAULT_SSE_HEARTBEAT_MS } = require("./sse");
 const {
   toChatCompletionsBody,
   fromChatCompletion,
@@ -15,10 +16,15 @@ const {
   pipeChatCompletionsSseToAnthropic,
   toAnthropicError,
   estimateInputTokens,
+  ANTHROPIC_SSE_HEARTBEAT,
 } = require("./anthropic-api");
 
 const MAX_JSON_BODY_BYTES = 32 * 1024 * 1024;
 
+function isClaudeCodeRequest(req) {
+  return /^claude-cli\//i.test(String(req.headers["user-agent"] || ""));
+}
+
 /**
  * OpenAI-compatible HTTP gateway.
  * Auth: Authorization: Bearer <apiKey>
@@ -31,6 +37,7 @@ function createGateway({
   port = DEFAULT_PORT,
   host = "127.0.0.1",
   maxBodyBytes = MAX_JSON_BODY_BYTES,
+  sseHeartbeatMs = DEFAULT_SSE_HEARTBEAT_MS,
   requestActivity,
 } = {}) {
   let server = null;
@@ -329,13 +336,29 @@ function createGateway({
               "Cache-Control": "no-cache",
               Connection: "keep-alive",
             });
+            const heartbeat = anthropicRequest && isClaudeCodeRequest(req)
+              ? createSseHeartbeat(res, {
+                  intervalMs: sseHeartbeatMs,
+                  heartbeat: ANTHROPIC_SSE_HEARTBEAT,
+                })
+              : null;
+            const streamSink = heartbeat?.sink || res;
             try {
               if (responsesRequest) {
-                await pipeChatCompletionsSseToResponses(result.streamPipe, res, body.model, body);
+                await pipeChatCompletionsSseToResponses(
+                  result.streamPipe,
+                  streamSink,
+                  body.model,
+                  body
+                );
               } else if (anthropicRequest) {
-                await pipeChatCompletionsSseToAnthropic(result.streamPipe, res, body.model);
+                await pipeChatCompletionsSseToAnthropic(
+                  result.streamPipe,
+                  streamSink,
+                  body.model
+                );
               } else {
-                await result.streamPipe(res);
+                await result.streamPipe(streamSink);
               }
               activityStatus = 200;
               activityOutcome = "success";
@@ -349,6 +372,8 @@ function createGateway({
                    );
                  }
               }
+            } finally {
+              heartbeat?.stop();
             }
             if (!res.writableEnded) res.end();
             return;
diff --git a/src/lib/sse.js b/src/lib/sse.js
index b650338..c6fea3c 100644
--- a/src/lib/sse.js
+++ b/src/lib/sse.js
@@ -24,6 +24,57 @@ function formatSseData(obj) {
 }
 
 const SSE_DONE = "data: [DONE]\n\n";
+const SSE_KEEPALIVE = ": keepalive\n\n";
+const DEFAULT_SSE_HEARTBEAT_MS = 25_000;
+
+/**
+ * Wrap a client-facing SSE sink and emit the configured liveness frame whenever
+ * the stream has otherwise been silent for the configured interval.
+ */
+function createSseHeartbeat(
+  res,
+  { intervalMs = DEFAULT_SSE_HEARTBEAT_MS, heartbeat = SSE_KEEPALIVE } = {}
+) {
+  let timer = null;
+  let stopped = false;
+
+  function schedule() {
+    if (timer) clearTimeout(timer);
+    timer = null;
+    if (stopped || intervalMs <= 0) return;
+    timer = setTimeout(() => {
+      timer = null;
+      if (stopped || res.writableEnded || res.destroyed) return;
+      res.write(heartbeat);
+      schedule();
+    }, intervalMs);
+    timer.unref?.();
+  }
+
+  const sink = {
+    write(...args) {
+      const written = res.write(...args);
+      schedule();
+      return written;
+    },
+    get writableEnded() {
+      return res.writableEnded;
+    },
+    get destroyed() {
+      return res.destroyed;
+    },
+  };
+
+  schedule();
+  return {
+    sink,
+    stop() {
+      stopped = true;
+      if (timer) clearTimeout(timer);
+      timer = null;
+    },
+  };
+}
 
 /**
  * Parse SSE text stream lines into { event, data } objects.
@@ -126,6 +177,9 @@ module.exports = {
   openaiChunk,
   formatSseData,
   SSE_DONE,
+  SSE_KEEPALIVE,
+  DEFAULT_SSE_HEARTBEAT_MS,
+  createSseHeartbeat,
   createSseParser,
   chunkToString,
   pipeOpenAiSse,
diff --git a/tests/gateway.test.js b/tests/gateway.test.js
index be7e566..9ccec8e 100644
--- a/tests/gateway.test.js
+++ b/tests/gateway.test.js
@@ -1797,6 +1797,157 @@ describe("SSE chunk decoding", () => {
     assert.equal(chunkToString(u8), text);
     assert.notEqual(String(u8), text);
   });
+
+  it("keeps Anthropic Messages clients alive during downstream silence", async () => {
+    const store = createStore(tmpConfig());
+    const apiKey = store.load().apiKey;
+    const gateway = createGateway({
+      store,
+      sseHeartbeatMs: 10,
+      router: {
+        async chatCompletions() {
+          return {
+            ok: true,
+            stream: true,
+            streamPipe: async (sink) => {
+              await new Promise((resolve) => setTimeout(resolve, 35));
+              sink.write(
+                `data: ${JSON.stringify({
+                  choices: [{ delta: { role: "assistant", content: "OK" } }],
+                })}\n\n`
+              );
+              sink.write(
+                `data: ${JSON.stringify({
+                  choices: [{ delta: {}, finish_reason: "stop" }],
+                })}\n\n`
+              );
+              sink.write("data: [DONE]\n\n");
+            },
+          };
+        },
+      },
+    });
+    const server = http.createServer((req, res) => gateway.handle(req, res));
+    await new Promise((resolve) => server.listen(0, "127.0.0.1", resolve));
+
+    try {
+      const response = await fetch(`http://127.0.0.1:${server.address().port}/v1/messages`, {
+        method: "POST",
+        headers: {
+          Authorization: `Bearer ${apiKey}`,
+          "Content-Type": "application/json",
+          "User-Agent": "claude-cli/2.1.218 (external, sdk-cli)",
+        },
+        body: JSON.stringify({
+          model: "route",
+          max_tokens: 8,
+          messages: [],
+          stream: true,
+        }),
+      });
+      const text = await response.text();
+      assert.equal(response.status, 200);
+      assert.match(
+        text,
+        /event: message_delta\ndata: \{"type":"message_delta","delta":\{"stop_reason":null,"stop_sequence":null\},"usage":\{"output_tokens":0\}\}\n\n/
+      );
+      assert.match(text, /event: content_block_delta/);
+      const heartbeatBlock = text.split("\n\n").find(
+        (block) => block.startsWith("event: message_delta") &&
+          block.includes('"stop_reason":null')
+      );
+      const heartbeatData = JSON.parse(
+        heartbeatBlock.split("\n").find((line) => line.startsWith("data: ")).slice(6)
+      );
+      assert.deepEqual(heartbeatData.usage, { output_tokens: 0 });
+
+      const genericResponse = await fetch(
+        `http://127.0.0.1:${server.address().port}/v1/messages`,
+        {
+          method: "POST",
+          headers: { Authorization: `Bearer ${apiKey}`, "Content-Type": "application/json" },
+          body: JSON.stringify({
+            model: "route",
+            max_tokens: 8,
+            messages: [],
+            stream: true,
+          }),
+        }
+      );
+      const genericText = await genericResponse.text();
+      assert.equal(genericResponse.status, 200);
+      assert.equal(
+        genericText.split("\n\n").filter(
+          (block) => block.startsWith("event: message_delta") &&
+            block.includes('"stop_reason":null')
+        ).length,
+        0
+      );
+    } finally {
+      await new Promise((resolve) => server.close(resolve));
+    }
+  });
+
+  it("still cancels an active stream when the client disconnects", async () => {
+    const store = createStore(tmpConfig());
+    const apiKey = store.load().apiKey;
+    let finishActivity;
+    const activityEnded = new Promise((resolve) => {
+      finishActivity = resolve;
+    });
+    const gateway = createGateway({
+      store,
+      sseHeartbeatMs: 10,
+      requestActivity: {
+        begin: () => "activity-1",
+        route() {},
+        end: (_id, result) => finishActivity(result),
+      },
+      router: {
+        async chatCompletions({ signal }) {
+          return {
+            ok: true,
+            stream: true,
+            streamPipe: async () => new Promise((resolve, reject) => {
+              if (signal.aborted) reject(signal.reason);
+              else signal.addEventListener("abort", () => reject(signal.reason), { once: true });
+            }),
+          };
+        },
+      },
+    });
+    const server = http.createServer((req, res) => gateway.handle(req, res));
+    await new Promise((resolve) => server.listen(0, "127.0.0.1", resolve));
+
+    try {
+      await new Promise((resolve, reject) => {
+        const request = http.request({
+          host: "127.0.0.1",
+          port: server.address().port,
+          path: "/v1/messages",
+          method: "POST",
+          headers: {
+            Authorization: `Bearer ${apiKey}`,
+            "Content-Type": "application/json",
+            "User-Agent": "claude-cli/2.1.218 (external, sdk-cli)",
+          },
+        }, (response) => {
+          let received = "";
+          response.on("data", (chunk) => {
+            received += String(chunk);
+            if (!received.includes('"stop_reason":null')) return;
+            response.destroy();
+            resolve();
+          });
+        });
+        request.once("error", reject);
+        request.end(JSON.stringify({ model: "route", max_tokens: 8, messages: [], stream: true }));
+      });
+      assert.deepEqual(await activityEnded, { status: 499, outcome: "canceled" });
+    } finally {
+      await new Promise((resolve) => server.close(resolve));
+    }
+  });
 });
 
 describe("OAuth → OpenAI SSE translation pipes", () => {