diff --git a/src/adapters/devin.ts b/src/adapters/devin.ts
index 9260976058..ab7abfa0a9 100644
--- a/src/adapters/devin.ts
+++ b/src/adapters/devin.ts
@@ -160,15 +160,44 @@ function assistantToolCalls(message: OcxAssistantMessage): Array<{ id: string; n
 
 function assistantText(message: OcxAssistantMessage): string {
   return message.content
-    // Thinking stays out of the replayed content. Cognition has no reasoning
-    // replay field, and folding chain-of-thought into assistant text sends it
-    // back as visible prior output - which the model then treats as something
-    // it said to the user.
+    // Thinking stays out of the replayed TEXT: folding chain-of-thought into
+    // assistant text sends it back as visible prior output, which the model
+    // then treats as something it said to the user. It is replayed in its own
+    // field instead — see assistantThinking below.
     .map((part) => (part.type === "text" ? part.text : ""))
     .filter(Boolean)
     .join("\n");
 }
 
+/**
+ * The assistant turn's own reasoning, for replay in ChatMessagePrompt #11.
+ *
+ * This adapter previously asserted that Cognition has no reasoning-replay
+ * field and dropped the thinking outright, so a reasoning model restarted its
+ * chain on every turn of a tool loop. The field exists: two independent
+ * clients of the same service write #11 thinking with #12 signature and #18
+ * signature_type on the assistant prompt.
+ *
+ * The signature attests the thinking it was produced with, so a block without
+ * one contributes its text and nothing else rather than borrowing a neighbour's.
+ */
+function assistantThinking(
+  message: OcxAssistantMessage,
+): { thinking?: string; signature?: string } {
+  const blocks = message.content.filter(
+    (part): part is Extract<typeof part, { type: "thinking" }> => part.type === "thinking",
+  );
+  if (blocks.length === 0) return {};
+  const thinking = blocks.map(b => b.thinking).filter(Boolean).join("\n");
+  // Only one signature can ride the prompt, so take the last block that has
+  // one: that is the block the turn actually ended on.
+  const signature = blocks.filter(b => b.signature).at(-1)?.signature;
+  return {
+    ...(thinking ? { thinking } : {}),
+    ...(signature ? { signature } : {}),
+  };
+}
+
 export function mapOcxMessagesToDevin(parsed: OcxParsedRequest): ChatHistoryItem[] {
   const items: ChatHistoryItem[] = [];
   // Cognition is not an OpenAI host, and this adapter does advertise a real
@@ -203,11 +232,15 @@ function mapOneMessage(message: OcxMessage): ChatHistoryItem | undefined {
   if (message.role === "assistant") {
     const toolCalls = assistantToolCalls(message);
     const text = assistantText(message);
-    if (!text && toolCalls.length === 0) return undefined;
+    const reasoning = assistantThinking(message);
+    // A turn that produced only reasoning is still worth replaying: dropping it
+    // is what makes the next turn re-derive the same chain.
+    if (!text && toolCalls.length === 0 && !reasoning.thinking) return undefined;
     return {
       role: "assistant",
       content: text || "",
       ...(toolCalls.length > 0 ? { tool_calls: toolCalls } : {}),
+      ...reasoning,
     };
   }
   if (message.role === "toolResult") {
@@ -335,6 +368,12 @@ export function createDevinAdapter(
             if (event.text) emit({ type: "thinking_delta", thinking: event.text });
             continue;
           }
+          if (event.kind === "reasoning_signature") {
+            // Carried back out so the next turn can replay it in the prompt's
+            // signature field; an unsigned replay is what the service ignores.
+            emit({ type: "thinking_signature", signature: event.signature });
+            continue;
+          }
           if (event.kind === "tool_call_start") {
             closeOpenTool();
             openToolId = event.id;
diff --git a/src/adapters/devin/cloud-direct/chat.ts b/src/adapters/devin/cloud-direct/chat.ts
index d67a7696e1..3e31e97c5c 100644
--- a/src/adapters/devin/cloud-direct/chat.ts
+++ b/src/adapters/devin/cloud-direct/chat.ts
@@ -120,6 +120,9 @@ export function allocateCascadeId(): string {
  *   #4 num_tokens: int                          (rough estimate)
  *   #5 safe_for_code_telemetry: bool            (1 = ok to log)
  *   #10 images: repeated ImageData              (multimodal)
+ *   #11 thinking: string                        (assistant reasoning, replayed)
+ *   #12 signature: string                       (opaque attestation for #11)
+ *   #18 signature_type: string
  * }
  *
  * ImageData (exa.codeium_common_pb.ImageData) {
@@ -153,7 +156,13 @@ function encodeChatToolCall(tc: { id: string; name: string; arguments: string })
 function encodeChatMessagePrompt(
   content: ContentPart[],
   source: number,
-  opts?: { toolCallId?: string; toolCalls?: Array<{ id: string; name: string; arguments: string }> },
+  opts?: {
+    toolCallId?: string;
+    toolCalls?: Array<{ id: string; name: string; arguments: string }>;
+    thinking?: string;
+    signature?: string;
+    signatureType?: string;
+  },
 ): Buffer {
   const textParts = content.filter((p): p is { type: 'text'; text: string } => p.type === 'text');
   const imageParts = content.filter((p): p is { type: 'image'; mimeType: string; base64Data: string; caption?: string } => p.type === 'image');
@@ -178,6 +187,14 @@ function encodeChatMessagePrompt(
   for (const img of imageParts) {
     parts.push(encodeMessage(10, encodeImageData(img)));
   }
+  // Reasoning replay. This adapter used to assert that Cognition has no
+  // reasoning-replay field and drop the assistant's own thinking, so a
+  // reasoning model restarted its chain on every turn of a tool loop. Two
+  // independent clients of the same service write it here: #11 thinking,
+  // #12 signature, #18 signature_type on the assistant prompt.
+  if (opts?.thinking) parts.push(encodeString(11, opts.thinking));
+  if (opts?.signature) parts.push(encodeString(12, opts.signature));
+  if (opts?.signatureType) parts.push(encodeString(18, opts.signatureType));
   return Buffer.concat(parts);
 }
 
@@ -342,6 +359,15 @@ export interface ChatHistoryItem {
    * each ChatToolCall has #1 id, #2 name, #3 arguments_json).
    */
   tool_calls?: Array<{ id: string; name: string; arguments: string }>;
+  /**
+   * For `role: 'assistant'` only — the model's own reasoning from that turn,
+   * replayed so a reasoning model does not restart its chain on the next one.
+   * Encoded as ChatMessagePrompt #11 with its #12 signature and #18
+   * signature_type.
+   */
+  thinking?: string;
+  signature?: string;
+  signature_type?: string;
 }
 
 /**
@@ -399,6 +425,12 @@ export interface ToolDef {
 export type CloudChatEvent =
   | { kind: 'text'; text: string }
   | { kind: 'reasoning'; text: string }
+  /**
+   * `delta_signature` (#10) — the opaque attestation for the reasoning this
+   * turn produced. Without decoding it there is nothing to put in the prompt's
+   * #12 on the next turn, so the replay would always be unsigned.
+   */
+  | { kind: 'reasoning_signature'; signature: string }
   | { kind: 'tool_call_start'; id: string; name: string }
   | {
       kind: 'tool_call_args';
@@ -592,6 +624,9 @@ function buildGetChatMessageRequest(args: BuildArgs): Buffer {
         {
           toolCallId: m.role === 'tool' ? m.tool_call_id : undefined,
           toolCalls: m.role === 'assistant' ? m.tool_calls : undefined,
+          thinking: m.role === 'assistant' ? m.thinking : undefined,
+          signature: m.role === 'assistant' ? m.signature : undefined,
+          signatureType: m.role === 'assistant' ? m.signature_type : undefined,
         },
       ),
     ),
@@ -716,6 +751,9 @@ export function* decodeChatFrame(proto: Buffer): Generator<CloudChatEvent> {
       // block instead of inline with the answer.
       const s = (f.value as Buffer).toString('utf8');
       if (s) yield { kind: 'reasoning', text: s };
+    } else if (f.num === 10 && f.wire === 2 && Buffer.isBuffer(f.value)) {
+      const s = (f.value as Buffer).toString('utf8');
+      if (s) yield { kind: 'reasoning_signature', signature: s };
     } else if (f.num === 6 && f.wire === 2 && Buffer.isBuffer(f.value)) {
       let id: string | undefined;
       let name: string | undefined;
diff --git a/tests/providers/devin-hardening.test.ts b/tests/providers/devin-hardening.test.ts
index 6d85a61007..319b133061 100644
--- a/tests/providers/devin-hardening.test.ts
+++ b/tests/providers/devin-hardening.test.ts
@@ -1,5 +1,6 @@
 import { describe, expect, test } from "bun:test";
 import { normalizeDevinModelId } from "../../src/adapters/devin";
+import { mapOcxMessagesToDevin } from "../../src/adapters/devin";
 import { parseDevinAuthPaste, refreshDevinToken } from "../../src/oauth/devin";
 import { DEVIN_DEFAULT_API_SERVER, resolveDevinApiBaseUrl, validateDevinApiBaseUrl } from "../../src/oauth/devin/api-base";
 import { registerUser } from "../../src/oauth/devin/register-user";
@@ -444,3 +445,76 @@ describe("devin status classification across the newly reachable trailer codes",
     expect(cls(504)).toEqual({ status: 504, retryable: true });
   });
 });
+
+describe("devin reasoning replay", () => {
+  const parsedWith = (messages: unknown[]) => ({
+    context: { messages, tools: undefined, systemPrompt: undefined },
+    options: { toolChoice: undefined },
+  }) as never;
+  function uvarint(value: number): number[] {
+    const out: number[] = [];
+    let v = value;
+    do { const b = v & 0x7f; v = Math.floor(v / 128); out.push(v > 0 ? b | 0x80 : b); } while (v > 0);
+    return out;
+  }
+  function lenDelim(num: number, payload: Buffer): Buffer {
+    return Buffer.concat([Buffer.from([...uvarint((num << 3) | 2), ...uvarint(payload.length)]), payload]);
+  }
+  function fieldsOf(buf: Buffer): Record<number, Buffer[]> {
+    const out: Record<number, Buffer[]> = {};
+    for (const f of iterFields(buf)) {
+      if (Buffer.isBuffer(f.value)) (out[f.num] ??= []).push(f.value);
+    }
+    return out;
+  }
+
+  test("an assistant turn's thinking and signature ride the prompt instead of being dropped", () => {
+    // The adapter used to assert this field did not exist and drop the chain,
+    // so a reasoning model re-derived it on every turn of a tool loop.
+    const history = mapOcxMessagesToDevin(parsedWith([
+      { role: "user", content: [{ type: "text", text: "hi" }] },
+      {
+        role: "assistant",
+        content: [
+          { type: "thinking", thinking: "step one", signature: "sig-abc" },
+          { type: "text", text: "answer" },
+        ],
+      },
+    ]));
+    const assistant = history.find(m => m.role === "assistant");
+    expect(assistant?.thinking).toBe("step one");
+    expect(assistant?.signature).toBe("sig-abc");
+    // Reasoning must not leak into the visible text.
+    expect(assistant?.content).toBe("answer");
+  });
+
+  test("a turn that produced only reasoning is still replayed", () => {
+    const history = mapOcxMessagesToDevin(parsedWith([
+      { role: "user", content: [{ type: "text", text: "hi" }] },
+      { role: "assistant", content: [{ type: "thinking", thinking: "only thought" }] },
+    ]));
+    expect(history.find(m => m.role === "assistant")?.thinking).toBe("only thought");
+  });
+
+  test("the encoded prompt carries thinking at #11 and its signature at #12", () => {
+    const req = buildGetChatMessageRequestForTests({
+      apiKey: "devin-session-token$x",
+      modelUid: "swe-2",
+      messages: [
+        { role: "user", content: "hi" },
+        { role: "assistant", content: "answer", thinking: "step one", signature: "sig-abc" },
+      ],
+      cascadeId: "c",
+    } as never);
+    const prompts = fieldsOf(req)[3] ?? [];
+    const assistantPrompt = prompts.map(fieldsOf).find(p => p[11]);
+    expect(assistantPrompt?.[11]?.[0]?.toString("utf8")).toBe("step one");
+    expect(assistantPrompt?.[12]?.[0]?.toString("utf8")).toBe("sig-abc");
+  });
+
+  test("the response signature is decoded so there is something to replay", () => {
+    const frame = lenDelim(10, Buffer.from("sig-from-cloud", "utf8"));
+    const events = [...decodeChatFrame(frame)];
+    expect(events).toEqual([{ kind: "reasoning_signature", signature: "sig-from-cloud" }]);
+  });
+});