fix: replay DeepSeek compaction as user handoff context

AGENTS.md

@@ -141,7 +141,9 @@ The non-negotiable details:
     for a compact handoff summary, and return exactly one synthetic `compaction` output item before
     `response.completed`. Encrypt the summary with AES-256-GCM using a key derived from the stable
     router token; on later DeepSeek requests, decrypt only DSCodex-prefixed compaction items and
-    restore them as assistant summary context. A compaction item that cannot be decrypted
+    restore them as user handoff context. Automatic compaction resumes the same turn without a
+    new user message; restoring the final summary as `assistant` while tools are enabled makes
+    DeepSeek demand a missing `reasoning_text` and reject the continuation. A compaction item that cannot be decrypted
     (GPT-sealed after a provider switch, or a rotated token) must be dropped, never forwarded raw
     to DeepSeek. Never route compaction through GPT or store the summary as plaintext in the
     rollout file.

src/proxy.mjs

@@ -178,9 +178,14 @@ function convertInputItem(item, compactionSecret) {
   if (item.type === "compaction") {
     const summary = openCompaction(item.encrypted_content, compactionSecret);
     if (summary) {
+      // Automatic compaction resumes the same DeepSeek turn without a new user
+      // message. If the restored summary is the final `assistant` item and tools
+      // are enabled, DeepSeek treats it as its own prior thinking turn and rejects
+      // the continuation because no matching `reasoning_text` was replayed.
+      // Replay the compact summary as user handoff context instead.
       return {
         type: "message",
-        role: "assistant",
+        role: "user",
         content: [{ type: "input_text", text: `[Compacted prior context]\n${summary}` }],
       };
     }

test/proxy.test.mjs

@@ -277,21 +277,22 @@ test("adapts Codex remote compaction v2 to a DeepSeek summary and restores it on
   assert.equal(observed[0].model, "deepseek-flash");
   assert.match(observed[0].input.at(-1).content[0].text, /compact handoff summary/i);
 
+  // Automatic compaction resumes the same turn. There is no new user message
+  // after the compaction item, and the continuation still has tools enabled.
   const replayResponse = await fetch(route(proxyUrl), {
     method: "POST",
     headers: { "content-type": "application/json" },
     body: JSON.stringify({
       model: "deepseek/deepseek-v4-pro",
-      input: [
-        compactItem,
-        { type: "message", role: "user", content: [{ type: "input_text", text: "Continue" }] },
-      ],
+      tools: [{ type: "function", name: "shell" }],
+      parallel_tool_calls: false,
+      input: [compactItem],
     }),
   });
   assert.equal(replayResponse.status, 200);
   await replayResponse.text();
   assert.equal(observed[1].input.some((item) => item.type === "compaction"), false);
-  const restored = observed[1].input.find((item) => item.role === "assistant");
+  const restored = observed[1].input.find((item) => item.role === "user");
   assert.match(restored.content[0].text, /Compacted prior context/);
   assert.match(restored.content[0].text, /tests and a restart are still pending/);
   const gptReplay = await fetch(route(proxyUrl), {