diff --git a/src/provider/codex-responses-adapter.ts b/src/provider/codex-responses-adapter.ts
index 06d36d9b3..2d3b2ffb3 100644
--- a/src/provider/codex-responses-adapter.ts
+++ b/src/provider/codex-responses-adapter.ts
@@ -56,14 +56,27 @@ type ResponsesInputItem =
   | { type: "function_call_output"; call_id: string; output: string }
   | { type: "reasoning"; summary: never[]; encrypted_content: string };
 
+// A thinking block's `signature` is opaque ciphertext a specific backend
+// issued for a specific model; only that backend can decrypt it. `turn.model`
+// records which model produced the turn, so comparing it against the model
+// this request is being built for is enough provenance to tell whether a
+// signature is safe to replay — no separate provenance field is needed.
+// Switching models means turns from the old model simply stop qualifying, so
+// a poisoned history self-heals on the very next request instead of being
+// replayed forever.
+export function signatureForModel(turn: ConversationTurn, requestModel: string, signature: string): string | undefined {
+  return turn.model === requestModel ? signature : undefined;
+}
+
 // Map one internal turn to zero or more Responses items. Assistant text uses
 // `output_text` parts; user/system text uses `input_text`. Tool calls become
 // `function_call` items (arguments serialized to a JSON string) and tool
 // results become `function_call_output` items. Reasoning blocks are echoed
 // back only when they carry the opaque `encrypted_content` the backend issued
-// (held in a thinking block's signature), which is required for multi-turn
-// reasoning continuity.
-function toResponsesItems(turn: ConversationTurn): ResponsesInputItem[] {
+// (held in a thinking block's signature) AND that backend is the one this
+// request is going to — replaying it to a different provider gets a 400 it
+// cannot recover from.
+function toResponsesItems(turn: ConversationTurn, requestModel: string): ResponsesInputItem[] {
   const items: ResponsesInputItem[] = [];
   const textKind: "input_text" | "output_text" = turn.role === "assistant" ? "output_text" : "input_text";
   const textParts: ResponsesContentPart[] = [];
@@ -99,7 +112,10 @@ function toResponsesItems(turn: ConversationTurn): ResponsesInputItem[] {
       items.push({ type: "function_call_output", call_id: block.callId, output: toolResultText(block) });
     } else if (block.type === "thinking" && typeof block.signature === "string" && block.signature.length > 0) {
       flushText();
-      items.push({ type: "reasoning", summary: [], encrypted_content: block.signature });
+      const encryptedContent = signatureForModel(turn, requestModel, block.signature);
+      if (encryptedContent !== undefined) {
+        items.push({ type: "reasoning", summary: [], encrypted_content: encryptedContent });
+      }
     }
   }
   flushText();
@@ -154,7 +170,7 @@ function buildRequest(
   model: string,
   options: InferenceOptions,
 ): BuiltRequest {
-  const conversation = messages.flatMap(toResponsesItems);
+  const conversation = messages.flatMap((turn) => toResponsesItems(turn, model));
   // Corbits Code's prompt cannot live in `instructions` (the backend pins that to
   // the official Codex prompt), so it leads the input as a developer message.
   const input =
diff --git a/src/provider/grok-responses-adapter.ts b/src/provider/grok-responses-adapter.ts
index 5dcc5ba94..c0787925f 100644
--- a/src/provider/grok-responses-adapter.ts
+++ b/src/provider/grok-responses-adapter.ts
@@ -15,7 +15,7 @@ import {
   XAI_CLIENT_VERSION,
   XAI_USER_AGENT,
 } from "../auth/xai/constants.js";
-import { createResponsesBlockIndexer, parseResponse } from "./codex-responses-adapter.js";
+import { createResponsesBlockIndexer, parseResponse, signatureForModel } from "./codex-responses-adapter.js";
 
 // Adapter for the grok-cli OAuth proxy (cli-chat-proxy.grok.com), which serves
 // the OpenAI Responses API at /v1/responses. The request shape mirrors the grok
@@ -52,7 +52,7 @@ function toolResultText(block: Extract<ContentBlock, { type: "tool_result" }>):
 // Map one internal turn to Responses items. Text-only messages keep the string
 // shape grok sends; messages with image blocks switch to Responses content parts
 // so the model receives the actual pixels instead of only a text placeholder.
-function toResponsesItems(turn: ConversationTurn): ResponsesInputItem[] {
+function toResponsesItems(turn: ConversationTurn, requestModel: string): ResponsesInputItem[] {
   const items: ResponsesInputItem[] = [];
   const role = turn.role;
   const parts: ResponsesInputContentPart[] = [];
@@ -95,7 +95,10 @@ function toResponsesItems(turn: ConversationTurn): ResponsesInputItem[] {
       items.push({ type: "function_call_output", call_id: block.callId, output: toolResultText(block) });
     } else if (block.type === "thinking" && typeof block.signature === "string" && block.signature.length > 0) {
       flushMessage();
-      items.push({ type: "reasoning", summary: [], encrypted_content: block.signature });
+      const encryptedContent = signatureForModel(turn, requestModel, block.signature);
+      if (encryptedContent !== undefined) {
+        items.push({ type: "reasoning", summary: [], encrypted_content: encryptedContent });
+      }
     }
   }
   flushMessage();
@@ -135,7 +138,7 @@ function buildRequest(
   model: string,
   options: InferenceOptions,
 ): BuiltRequest {
-  const conversation = dedupeToolOutputs(messages.flatMap(toResponsesItems));
+  const conversation = dedupeToolOutputs(messages.flatMap((turn) => toResponsesItems(turn, model)));
   const systemMessage: ResponsesInputItem | undefined =
     options.systemPrompt !== undefined
       ? { type: "message", role: "system", content: options.systemPrompt }
diff --git a/src/provider/openai-responses-adapter.ts b/src/provider/openai-responses-adapter.ts
index e68ce9494..27661cb92 100644
--- a/src/provider/openai-responses-adapter.ts
+++ b/src/provider/openai-responses-adapter.ts
@@ -13,6 +13,7 @@ import {
   createResponsesBlockIndexer,
   isResponsesStreamTerminal,
   parseResponse,
+  signatureForModel,
 } from "./codex-responses-adapter.js";
 
 // Generic OpenAI Responses API adapter (POST /responses). Used by OpenCode Go
@@ -41,7 +42,7 @@ function toolResultText(block: Extract<ContentBlock, { type: "tool_result" }>):
   return parts.join("");
 }
 
-function toResponsesItems(turn: ConversationTurn): ResponsesInputItem[] {
+function toResponsesItems(turn: ConversationTurn, requestModel: string): ResponsesInputItem[] {
   const items: ResponsesInputItem[] = [];
   const role = turn.role;
   const parts: ResponsesInputContentPart[] = [];
@@ -90,7 +91,10 @@ function toResponsesItems(turn: ConversationTurn): ResponsesInputItem[] {
       items.push({ type: "function_call_output", call_id: block.callId, output: toolResultText(block) });
     } else if (block.type === "thinking" && typeof block.signature === "string" && block.signature.length > 0) {
       flushMessage();
-      items.push({ type: "reasoning", summary: [], encrypted_content: block.signature });
+      const encryptedContent = signatureForModel(turn, requestModel, block.signature);
+      if (encryptedContent !== undefined) {
+        items.push({ type: "reasoning", summary: [], encrypted_content: encryptedContent });
+      }
     }
   }
   flushMessage();
@@ -125,7 +129,7 @@ function buildRequest(
   model: string,
   options: InferenceOptions,
 ): BuiltRequest {
-  const conversation = dedupeToolOutputs(messages.flatMap(toResponsesItems));
+  const conversation = dedupeToolOutputs(messages.flatMap((turn) => toResponsesItems(turn, model)));
   const systemMessage: ResponsesInputItem | undefined =
     options.systemPrompt !== undefined
       ? { type: "message", role: "system", content: options.systemPrompt }
diff --git a/tests/unit/codex-responses-adapter.test.ts b/tests/unit/codex-responses-adapter.test.ts
index ee933a7b7..4ca6ea417 100644
--- a/tests/unit/codex-responses-adapter.test.ts
+++ b/tests/unit/codex-responses-adapter.test.ts
@@ -110,6 +110,7 @@ describe("codex-responses buildRequest", () => {
       userTurn("solve the hard problem"),
       {
         role: "assistant",
+        model: "gpt-5-codex",
         timestamp: 0,
         content: [
           { type: "thinking", thinking: "internal steps...", signature: "ENC_BLOB_123" },
@@ -125,6 +126,48 @@ describe("codex-responses buildRequest", () => {
     ]);
   });
 
+  test("drops a reasoning signature issued for a different model after a provider switch", () => {
+    // The signature was minted by grok-4.5; the request now targets a Codex
+    // model. Replaying it would 400 with an undecryptable-content error, so
+    // the reasoning item must be omitted while the surrounding turn survives.
+    const turns: ConversationTurn[] = [
+      userTurn("solve the hard problem"),
+      {
+        role: "assistant",
+        model: "grok-4.5",
+        timestamp: 0,
+        content: [
+          { type: "thinking", thinking: "internal steps...", signature: "FOREIGN_BLOB" },
+          { type: "text", text: "The answer is 42." },
+        ],
+      },
+    ];
+    const body = JSON.parse(adapter().buildRequest(turns, "gpt-5-codex", baseOptions).body) as Record<string, unknown>;
+    expect(body["input"]).toEqual([
+      { type: "message", role: "user", content: [{ type: "input_text", text: "solve the hard problem" }] },
+      { type: "message", role: "assistant", content: [{ type: "output_text", text: "The answer is 42." }] },
+    ]);
+  });
+
+  test("recovers an already-poisoned session: a foreign signature is dropped on every subsequent request", () => {
+    const poisonedHistory: ConversationTurn[] = [
+      userTurn("turn 1"),
+      {
+        role: "assistant",
+        model: "grok-4.5",
+        timestamp: 0,
+        content: [{ type: "thinking", thinking: "...", signature: "FOREIGN_BLOB" }, { type: "text", text: "ok" }],
+      },
+      userTurn("turn 2"),
+    ];
+    const firstRetry = JSON.parse(adapter().buildRequest(poisonedHistory, "gpt-5-codex", baseOptions).body) as Record<string, unknown>;
+    const secondRetry = JSON.parse(adapter().buildRequest(poisonedHistory, "gpt-5-codex", baseOptions).body) as Record<string, unknown>;
+    for (const body of [firstRetry, secondRetry]) {
+      const input = body["input"] as Array<Record<string, unknown>>;
+      expect(input.some((item) => item["type"] === "reasoning")).toBe(false);
+    }
+  });
+
   test("omits the account-id header when no account id is supplied", () => {
     const req = adapter().buildRequest([userTurn("x")], "gpt-5-codex", { providerOptions: { [CODEX_SESSION_ID_OPTION]: "s" } });
     expect(req.headers["chatgpt-account-id"]).toBeUndefined();