diff --git a/backend/go/cloud-proxy/provider_anthropic.go b/backend/go/cloud-proxy/provider_anthropic.go
index d8382d454..d86f2ab8e 100644
--- a/backend/go/cloud-proxy/provider_anthropic.go
+++ b/backend/go/cloud-proxy/provider_anthropic.go
@@ -142,19 +142,12 @@ func buildAnthropicRequest(opts *pb.PredictOptions, cfg *proxyConfig, stream boo
 	if req.MaxTokens <= 0 {
 		req.MaxTokens = anthropicDefaultMaxTokens
 	}
-	// Newer Anthropic models 400 when both temperature and top_p are
-	// set ("`temperature` and `top_p` cannot both be specified for
-	// this model. Please use only one.") even though their docs only
-	// "recommend" picking one. The OpenAI-compatible chat UI almost
-	// always sends both with default values, so prefer temperature
-	// and drop top_p when both are present.
-	if t := opts.GetTemperature(); t != 0 {
-		v := float64(t)
-		req.Temperature = &v
-	} else if t := opts.GetTopP(); t != 0 {
-		v := float64(t)
-		req.TopP = &v
-	}
+	// Do not forward temperature/top_p. Newer Anthropic reasoning models reject
+	// requests that carry temperature ("`temperature` is deprecated for this
+	// model"), and the OpenAI-compatible clients typically send only the
+	// server-side DEFAULT sampling values rather than user intent — dropping
+	// them loses nothing and lets the upstream apply its own defaults.
+	_ = opts
 
 	req.Tools = convertOpenAITools(opts.GetTools())
 	req.ToolChoice = convertOpenAIToolChoice(opts.GetToolChoice())
diff --git a/backend/go/cloud-proxy/provider_anthropic_test.go b/backend/go/cloud-proxy/provider_anthropic_test.go
index d46db7b12..6119c97cf 100644
--- a/backend/go/cloud-proxy/provider_anthropic_test.go
+++ b/backend/go/cloud-proxy/provider_anthropic_test.go
@@ -3,7 +3,6 @@ package main
 import (
 	"encoding/json"
 	"io"
-	"math"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -75,15 +74,16 @@ func TestPredict_Anthropic_BasicMessages(t *testing.T) {
 	g.Expect(captured.Messages).To(HaveLen(1))
 	g.Expect(captured.Messages[0].Role).To(Equal("user"))
 	g.Expect(captured.MaxTokens).To(Equal(int32(32)))
-	g.Expect(captured.Temperature).NotTo(BeNil())
-	g.Expect(*captured.Temperature).To(Equal(0.5))
-	// Anthropic 400s when both temperature and top_p are set; the
-	// translator must prefer temperature and drop top_p.
+	// Newer Anthropic reasoning models reject requests carrying temperature
+	// ("`temperature` is deprecated for this model"); clients typically send
+	// only default sampling values, so the translator forwards neither.
+	g.Expect(captured.Temperature).To(BeNil())
 	g.Expect(captured.TopP).To(BeNil())
 	g.Expect(captured.Stream).To(BeFalse())
 }
 
-// When only top_p is set, it should be forwarded.
+// Sampling parameters are not forwarded at all — the upstream applies its
+// own defaults (newest models reject explicit temperature/top_p).
 func TestPredict_Anthropic_TopPOnly(t *testing.T) {
 	g := NewWithT(t)
 	srv, captured := fakeAnthropicUpstream(t, func(_ anthropicRequest) (int, string, string) {
@@ -99,11 +99,7 @@ func TestPredict_Anthropic_TopPOnly(t *testing.T) {
 	})
 	g.Expect(err).NotTo(HaveOccurred())
 	g.Expect(captured.Temperature).To(BeNil())
-	// PredictOptions.TopP is float32 on the wire; the translator widens
-	// to float64 so 0.9 round-trips as 0.8999999761581421… — compare
-	// with a small tolerance rather than exact equality.
-	g.Expect(captured.TopP).NotTo(BeNil())
-	g.Expect(math.Abs(*captured.TopP - 0.9)).To(BeNumerically("<=", 1e-6))
+	g.Expect(captured.TopP).To(BeNil())
 }
 
 func TestPredict_Anthropic_DefaultsMaxTokens(t *testing.T) {
diff --git a/backend/go/cloud-proxy/provider_openai.go b/backend/go/cloud-proxy/provider_openai.go
index d4911b6b7..2fab6c06f 100644
--- a/backend/go/cloud-proxy/provider_openai.go
+++ b/backend/go/cloud-proxy/provider_openai.go
@@ -30,7 +30,7 @@ type openAIRequest struct {
 	Stream           bool            `json:"stream,omitempty"`
 	Temperature      *float64        `json:"temperature,omitempty"`
 	TopP             *float64        `json:"top_p,omitempty"`
-	MaxTokens        *int32          `json:"max_tokens,omitempty"`
+	MaxTokens        *int32          `json:"max_completion_tokens,omitempty"` // newer OpenAI models reject max_tokens ("use max_completion_tokens instead")
 	Stop             []string        `json:"stop,omitempty"`
 	FrequencyPenalty *float64        `json:"frequency_penalty,omitempty"`
 	PresencePenalty  *float64        `json:"presence_penalty,omitempty"`
@@ -107,14 +107,10 @@ func buildOpenAIRequest(opts *pb.PredictOptions, cfg *proxyConfig, stream bool)
 		Tools:      parseRawJSON(opts.GetTools()),
 		ToolChoice: parseRawJSON(opts.GetToolChoice()),
 	}
-	if t := opts.GetTemperature(); t != 0 {
-		v := float64(t)
-		req.Temperature = &v
-	}
-	if t := opts.GetTopP(); t != 0 {
-		v := float64(t)
-		req.TopP = &v
-	}
+	// Do not forward temperature/top_p. Newer OpenAI reasoning models reject
+	// temperature as deprecated, and clients typically send only default
+	// sampling values rather than user intent — let the upstream apply its
+	// own defaults.
 	if n := opts.GetTokens(); n > 0 {
 		req.MaxTokens = &n
 	}
diff --git a/backend/go/cloud-proxy/provider_openai_test.go b/backend/go/cloud-proxy/provider_openai_test.go
index 9ce4334db..6db03a664 100644
--- a/backend/go/cloud-proxy/provider_openai_test.go
+++ b/backend/go/cloud-proxy/provider_openai_test.go
@@ -74,8 +74,9 @@ func TestPredict_OpenAI_BasicChat(t *testing.T) {
 	g.Expect(captured.Messages).To(HaveLen(2))
 	g.Expect(captured.Messages[0].Role).To(Equal("system"))
 	g.Expect(captured.Messages[1].Role).To(Equal("user"))
-	g.Expect(captured.Temperature).NotTo(BeNil())
-	g.Expect(*captured.Temperature).To(Equal(0.5))
+	// Sampling parameters are not forwarded (newest models reject explicit
+	// temperature); token limit is serialized as max_completion_tokens.
+	g.Expect(captured.Temperature).To(BeNil())
 	g.Expect(captured.MaxTokens).NotTo(BeNil())
 	g.Expect(*captured.MaxTokens).To(Equal(int32(32)))
 	g.Expect(captured.Stream).To(BeFalse())