diff --git a/backend/internal/service/openai_codex_transform.go b/backend/internal/service/openai_codex_transform.go
index 4ec038e0..a266d6a0 100644
--- a/backend/internal/service/openai_codex_transform.go
+++ b/backend/internal/service/openai_codex_transform.go
@@ -124,6 +124,14 @@ func applyCodexOAuthTransform(reqBody map[string]any, isCodexCLI bool, isCompact
 		"top_p",
 		"frequency_penalty",
 		"presence_penalty",
+		// prompt_cache_retention is a newer Responses API parameter (cache TTL).
+		// The ChatGPT internal Codex endpoint rejects it with
+		// "Unsupported parameter: prompt_cache_retention". Defense-in-depth
+		// for any OAuth path that reaches this transform — the Cursor
+		// Responses-shape short-circuit in ForwardAsChatCompletions strips
+		// it earlier too, but we keep this line so other OAuth callers are
+		// equally protected.
+		"prompt_cache_retention",
 	} {
 		if _, ok := reqBody[key]; ok {
 			delete(reqBody, key)
diff --git a/backend/internal/service/openai_codex_transform_test.go b/backend/internal/service/openai_codex_transform_test.go
index 889ac615..993ade07 100644
--- a/backend/internal/service/openai_codex_transform_test.go
+++ b/backend/internal/service/openai_codex_transform_test.go
@@ -481,6 +481,26 @@ func TestExtractSystemMessagesFromInput(t *testing.T) {
 	})
 }
 
+// TestApplyCodexOAuthTransform_StripsPromptCacheRetention is a regression
+// test: some clients (e.g. Cursor cloud via the Responses-shape compat path)
+// send prompt_cache_retention, but the ChatGPT internal Codex endpoint
+// rejects it with "Unsupported parameter: prompt_cache_retention".
+func TestApplyCodexOAuthTransform_StripsPromptCacheRetention(t *testing.T) {
+	reqBody := map[string]any{
+		"model":                  "gpt-5.1",
+		"prompt_cache_retention": "24h",
+		"input": []any{
+			map[string]any{"role": "user", "content": "hi"},
+		},
+	}
+
+	applyCodexOAuthTransform(reqBody, false, false)
+
+	_, stillThere := reqBody["prompt_cache_retention"]
+	require.False(t, stillThere,
+		"prompt_cache_retention must be stripped before forwarding to Codex upstream")
+}
+
 func TestApplyCodexOAuthTransform_ExtractsSystemMessages(t *testing.T) {
 	reqBody := map[string]any{
 		"model": "gpt-5.1",
diff --git a/backend/internal/service/openai_cursor_warmup_pipeline_test.go b/backend/internal/service/openai_cursor_warmup_pipeline_test.go
index 8ade9dbb..19bb13d6 100644
--- a/backend/internal/service/openai_cursor_warmup_pipeline_test.go
+++ b/backend/internal/service/openai_cursor_warmup_pipeline_test.go
@@ -153,3 +153,47 @@ func TestCursorMixedShape_JSONRoundtrip(t *testing.T) {
 	require.True(t, ok, "input must decode to a Go []any after round-trip")
 	require.Len(t, inputArr, 1)
 }
+
+// TestCursorMixedShape_StripsUnsupportedFields mirrors the strip loop in
+// ForwardAsChatCompletions (isResponsesShape branch). Cursor cloud sends
+// prompt_cache_retention, safety_identifier, metadata and stream_options
+// as top-level Responses API parameters, which Codex upstreams reject with
+// "Unsupported parameter: ...". The fix must remove them from the raw body
+// before it is forwarded, for BOTH OAuth and API Key account types.
+func TestCursorMixedShape_StripsUnsupportedFields(t *testing.T) {
+	cursorBody := []byte(`{
+		"model": "gpt-5.4",
+		"stream": true,
+		"prompt_cache_retention": "24h",
+		"safety_identifier": "cursor-user-xyz",
+		"metadata": {"trace_id":"abc","caller":"cursor"},
+		"stream_options": {"include_usage": true},
+		"input": [{"role":"user","content":"hi"}]
+	}`)
+
+	// Sanity: the test fixture contains every field the production code strips.
+	for _, field := range cursorResponsesUnsupportedFields {
+		require.True(t, gjson.GetBytes(cursorBody, field).Exists(),
+			"test fixture must contain %s", field)
+	}
+
+	// Run the exact same loop as the production code.
+	result := cursorBody
+	for _, field := range cursorResponsesUnsupportedFields {
+		if stripped, err := sjson.DeleteBytes(result, field); err == nil {
+			result = stripped
+		}
+	}
+
+	// All unsupported fields must be gone.
+	for _, field := range cursorResponsesUnsupportedFields {
+		assert.False(t, gjson.GetBytes(result, field).Exists(),
+			"%s must be stripped", field)
+	}
+
+	// Everything else must survive intact.
+	assert.Equal(t, "gpt-5.4", gjson.GetBytes(result, "model").String())
+	assert.Equal(t, true, gjson.GetBytes(result, "stream").Bool())
+	assert.True(t, gjson.GetBytes(result, "input").IsArray())
+	assert.Equal(t, "user", gjson.GetBytes(result, "input.0.role").String())
+}
diff --git a/backend/internal/service/openai_gateway_chat_completions.go b/backend/internal/service/openai_gateway_chat_completions.go
index c827fd7b..ac7d28a7 100644
--- a/backend/internal/service/openai_gateway_chat_completions.go
+++ b/backend/internal/service/openai_gateway_chat_completions.go
@@ -21,6 +21,22 @@ import (
 	"go.uber.org/zap"
 )
 
+// cursorResponsesUnsupportedFields are top-level Responses API parameters that
+// Codex upstreams reject with "Unsupported parameter: ...". They must be
+// stripped when forwarding a raw client body through the Responses-shape
+// short-circuit in ForwardAsChatCompletions (see isResponsesShape branch).
+// The normal Chat Completions → Responses conversion path is unaffected
+// because ChatCompletionsRequest has no fields for these parameters — unknown
+// fields are dropped naturally by json.Unmarshal. Kept semantically in sync
+// with the list in openai_gateway_service.go:2034 used by the /v1/responses
+// passthrough path.
+var cursorResponsesUnsupportedFields = []string{
+	"prompt_cache_retention",
+	"safety_identifier",
+	"metadata",
+	"stream_options",
+}
+
 // ForwardAsChatCompletions accepts a Chat Completions request body, converts it
 // to OpenAI Responses API format, forwards to the OpenAI upstream, and converts
 // the response back to Chat Completions format. All account types (OAuth and API
@@ -81,6 +97,16 @@ func (s *OpenAIGatewayService) ForwardAsChatCompletions(
 		if err != nil {
 			return nil, fmt.Errorf("rewrite model in responses-shape body: %w", err)
 		}
+		// Strip Responses API parameters that no Codex upstream accepts.
+		// Because this branch forwards the raw body (the normal path rebuilds
+		// it from ChatCompletionsRequest and drops unknown fields naturally),
+		// we must filter these fields explicitly here — otherwise the upstream
+		// rejects the request with "Unsupported parameter: ...".
+		for _, field := range cursorResponsesUnsupportedFields {
+			if stripped, derr := sjson.DeleteBytes(responsesBody, field); derr == nil {
+				responsesBody = stripped
+			}
+		}
 		// Minimal stub populated from the raw body so downstream billing
 		// propagation (ServiceTier, ReasoningEffort) keeps working.
 		responsesReq = &apicompat.ResponsesRequest{