diff --git a/crates/switchyard-translation/src/codecs/responses/buffered.rs b/crates/switchyard-translation/src/codecs/responses/buffered.rs
index 998505913..7dded3af2 100644
--- a/crates/switchyard-translation/src/codecs/responses/buffered.rs
+++ b/crates/switchyard-translation/src/codecs/responses/buffered.rs
@@ -33,6 +33,9 @@ use crate::util::{
     push_lossy, stable_id, string_value, validate_request_capabilities,
 };
 
+/// Reasoning controls the OpenAI Responses request body defines.
+const RESPONSES_REASONING_KEYS: [&str; 3] = ["effort", "generate_summary", "summary"];
+
 /// Format codec for OpenAI Responses payloads.
 pub struct OpenAiResponsesCodec;
 
@@ -256,13 +259,31 @@ impl FormatCodec for OpenAiResponsesCodec {
                 json!({"format": encode_responses_text_format(response_format)}),
             );
         }
-        let mut reasoning = request
-            .reasoning
-            .raw
-            .as_ref()
-            .and_then(Value::as_object)
-            .cloned()
-            .unwrap_or_default();
+        let mut reasoning = Map::new();
+        if let Some(raw) = request.reasoning.raw.as_ref().and_then(Value::as_object) {
+            // `raw` holds the source format's reasoning controls verbatim. Only the keys
+            // Responses itself defines can be replayed here. Carrying the rest forward
+            // publishes another provider's spelling under a Responses field name.
+            let mut dropped: Vec<&str> = Vec::new();
+            for (key, value) in raw {
+                if RESPONSES_REASONING_KEYS.contains(&key.as_str()) {
+                    reasoning.insert(key.clone(), value.clone());
+                } else {
+                    dropped.push(key.as_str());
+                }
+            }
+            if !dropped.is_empty() {
+                dropped.sort_unstable();
+                push_lossy(
+                    &mut diagnostics,
+                    _policy,
+                    format!(
+                        "reasoning controls have no OpenAI Responses representation and were dropped: {}",
+                        dropped.join(", ")
+                    ),
+                )?;
+            }
+        }
         if let Some(effort) = &request.reasoning.effort {
             reasoning.insert("effort".to_string(), json!(effort));
         }
diff --git a/crates/switchyard-translation/tests/request_translation.rs b/crates/switchyard-translation/tests/request_translation.rs
index 806e21412..274aabe23 100644
--- a/crates/switchyard-translation/tests/request_translation.rs
+++ b/crates/switchyard-translation/tests/request_translation.rs
@@ -4048,3 +4048,86 @@ fn responses_stored_tool_outputs_stay_tool_results() -> TestResult {
     assert_eq!(output["input"], outputs);
     Ok(())
 }
+
+// Verifies Anthropic thinking controls are not republished under the Responses
+// `reasoning` field, and that dropping them is reported.
+#[test]
+fn anthropic_thinking_controls_are_not_emitted_as_responses_reasoning() -> TestResult {
+    let engine = TranslationEngine::default();
+    let body = json!({
+        "model": "claude-opus-5",
+        "max_tokens": 32000,
+        "messages": [{"role": "user", "content": "hi"}],
+        "thinking": {"type": "enabled", "budget_tokens": 10000}
+    });
+
+    let output = engine.translate_request(
+        WireFormat::AnthropicMessages,
+        WireFormat::OpenAiResponses,
+        &body,
+        &TranslationPolicy::default(),
+    )?;
+
+    assert_eq!(output.body.get("reasoning"), None);
+    assert!(!output.body.to_string().contains("budget_tokens"));
+    assert_eq!(output.diagnostics.len(), 1);
+    assert_eq!(output.diagnostics[0].code, "lossy_conversion");
+    assert!(
+        output.diagnostics[0].message.contains("budget_tokens")
+            && output.diagnostics[0].message.contains("type")
+    );
+    Ok(())
+}
+
+// Verifies a caller asking to be told about information loss is told about
+// reasoning controls the target cannot express.
+#[test]
+fn rejecting_lossy_conversion_rejects_untranslatable_reasoning_controls() -> TestResult {
+    let engine = TranslationEngine::default();
+    let policy = TranslationPolicy {
+        lossy_conversion_policy: LossyConversionPolicy::Reject,
+        ..Default::default()
+    };
+    let body = json!({
+        "model": "claude-opus-5",
+        "max_tokens": 32000,
+        "messages": [{"role": "user", "content": "hi"}],
+        "thinking": {"type": "enabled", "budget_tokens": 10000}
+    });
+
+    let result = engine.translate_request(
+        WireFormat::AnthropicMessages,
+        WireFormat::OpenAiResponses,
+        &body,
+        &policy,
+    );
+
+    assert!(result.is_err());
+    Ok(())
+}
+
+// Verifies Responses reasoning controls that the format does define still carry
+// through a rebuild without a diagnostic.
+#[test]
+fn responses_reasoning_summary_still_carries_through_a_rebuild() -> TestResult {
+    let engine = TranslationEngine::default();
+    let policy = TranslationPolicy::default();
+    let body = json!({
+        "model": "switchyard",
+        "input": "Fix the parser.",
+        "reasoning": {"effort": "high", "summary": "auto"}
+    });
+    let mut request = engine
+        .decode_request(WireFormat::OpenAiResponses, &body, &policy)?
+        .request;
+    request.preservation.requests.clear();
+
+    let output = engine.encode_request(WireFormat::OpenAiResponses, &request, &policy)?;
+
+    assert_eq!(
+        output.body["reasoning"],
+        json!({"effort": "high", "summary": "auto"})
+    );
+    assert!(output.diagnostics.is_empty());
+    Ok(())
+}