diff --git a/crates/aether-ai/formats/src/formats/openai/request_contract.rs b/crates/aether-ai/formats/src/formats/openai/request_contract.rs
index 7eb7107bd..b145d7c69 100644
--- a/crates/aether-ai/formats/src/formats/openai/request_contract.rs
+++ b/crates/aether-ai/formats/src/formats/openai/request_contract.rs
@@ -381,10 +381,21 @@ mod tests {
 
     #[test]
     fn finalization_strips_non_replayable_responses_reasoning_history() {
+        let gemini_carrier =
+            crate::formats::openai::responses::encode_gemini_tool_signature_carrier(
+                "opaque-gemini-thought-signature",
+            )
+            .expect("Gemini signature carrier");
         let mut body = json!({
             "model": "gpt-5.4",
             "input": [
                 {"type": "reasoning", "id": "rs_provider_123", "summary": []},
+                {
+                    "type": "reasoning",
+                    "id": "rs_aether_55070860f6d45c6b8f6fa11efd9dff8a",
+                    "summary": [],
+                    "encrypted_content": gemini_carrier
+                },
                 {
                     "type": "reasoning",
                     "id": "item_72d3bd8d367d01977ace23f1",
diff --git a/crates/aether-ai/formats/src/formats/openai/responses/mod.rs b/crates/aether-ai/formats/src/formats/openai/responses/mod.rs
index 9a9328ff8..36053f67e 100644
--- a/crates/aether-ai/formats/src/formats/openai/responses/mod.rs
+++ b/crates/aether-ai/formats/src/formats/openai/responses/mod.rs
@@ -167,9 +167,10 @@ pub fn normalize_openai_responses_message_item_ids(body: &mut Value) -> usize {
 /// Removes reasoning history items that cannot be replayed against an OpenAI Responses backend.
 ///
 /// Reasoning IDs are opaque provider references and must never be repaired by changing their
-/// prefix. Foreign IDs (for example `item_...`) are therefore removed. Aether-synthesized
-/// reasoning summaries are also removed unless they carry encrypted reasoning state that can be
-/// replayed statelessly.
+/// prefix. Foreign IDs (for example `item_...`) are therefore removed. Aether's Gemini signature
+/// carriers are also removed: they are intentionally transported through the Responses
+/// `encrypted_content` field so they can be restored on a later Gemini tool turn, but they are not
+/// OpenAI ciphertext and must never be replayed to an OpenAI/Codex backend.
 pub fn strip_incompatible_openai_responses_reasoning_items(
     body: &mut Value,
     provider_api_format: &str,
@@ -221,6 +222,13 @@ fn openai_responses_reasoning_item_is_replayable(
     if object.get("type").and_then(Value::as_str) != Some("reasoning") {
         return true;
     }
+    if object
+        .get("encrypted_content")
+        .and_then(Value::as_str)
+        .is_some_and(|value| value.starts_with(GEMINI_TOOL_SIGNATURE_CARRIER_PREFIX))
+    {
+        return false;
+    }
     if policy == OpenAiResponsesReasoningReplayPolicy::DeepSeekOpaque
         && deepseek_opaque_reasoning_item_is_replayable(object)
     {
@@ -475,6 +483,43 @@ mod tests {
         assert_eq!(input[2]["id"], "item_message_123");
     }
 
+    #[test]
+    fn strips_gemini_signature_carriers_before_openai_replay() {
+        let gemini_item_id = openai_responses_synthetic_reasoning_item_id("resp_gemini", 0);
+        let openai_item_id = openai_responses_synthetic_reasoning_item_id("resp_openai", 0);
+        let carrier = encode_gemini_tool_signature_carrier_with_direction(
+            "opaque-gemini-thought-signature",
+            GeminiToolSignatureCarrierDirection::Next,
+        )
+        .expect("Gemini signature carrier");
+        let mut body = json!({
+            "input": [
+                {
+                    "type": "reasoning",
+                    "id": gemini_item_id,
+                    "summary": [],
+                    "encrypted_content": carrier
+                },
+                {
+                    "type": "reasoning",
+                    "id": openai_item_id,
+                    "summary": [],
+                    "encrypted_content": "provider-encrypted-state"
+                },
+                {"type": "reasoning", "id": "rs_provider_123", "summary": []}
+            ]
+        });
+
+        assert_eq!(
+            strip_incompatible_openai_responses_reasoning_items(&mut body, "openai:responses"),
+            1
+        );
+        let input = body["input"].as_array().expect("input array");
+        assert_eq!(input.len(), 2);
+        assert_eq!(input[0]["encrypted_content"], "provider-encrypted-state");
+        assert_eq!(input[1]["id"], "rs_provider_123");
+    }
+
     #[test]
     fn reasoning_item_sanitizer_is_scoped_to_responses_targets() {
         let mut body = json!({