diff --git a/internal/provider/anthropic/anthropic.go b/internal/provider/anthropic/anthropic.go
index dc6fa52a93..39ebbf7aeb 100644
--- a/internal/provider/anthropic/anthropic.go
+++ b/internal/provider/anthropic/anthropic.go
@@ -166,6 +166,7 @@ func (c *client) Stream(ctx context.Context, req provider.Request) (<-chan provi
 		httpReq.Header.Set("Accept", "text/event-stream")
 		httpReq.Header.Set("x-api-key", c.apiKey)
 		httpReq.Header.Set("anthropic-version", anthropicVersion)
+		httpReq.Header.Set("User-Agent", "Reasonix")
 		return httpReq, nil
 	}
 	resp, err := provider.SendWithRetry(ctx, c.http, c.sendOpts(), newReq)
diff --git a/internal/provider/anthropic/anthropic_test.go b/internal/provider/anthropic/anthropic_test.go
index f19ddb663c..6250ef298a 100644
--- a/internal/provider/anthropic/anthropic_test.go
+++ b/internal/provider/anthropic/anthropic_test.go
@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"io"
 	"net/http"
+	"net/http/httptest"
 	"strings"
 	"testing"
 
@@ -113,6 +114,34 @@ func TestMapStopReason(t *testing.T) {
 	}
 }
 
+// TestStreamSendsReasonixUserAgent verifies outbound Messages API requests carry
+// an identifying User-Agent so an upstream gateway can attribute the traffic to
+// Reasonix instead of Go's default UA (#5226).
+func TestStreamSendsReasonixUserAgent(t *testing.T) {
+	var gotUA string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotUA = r.Header.Get("User-Agent")
+		w.Header().Set("Content-Type", "text/event-stream")
+		_, _ = io.WriteString(w, "event: message_stop\ndata: {\"type\":\"message_stop\"}\n\n")
+	}))
+	defer srv.Close()
+
+	p, err := New(provider.Config{Name: "anthropic", BaseURL: srv.URL, Model: "claude-opus-4-8", APIKey: "k"})
+	if err != nil {
+		t.Fatalf("New: %v", err)
+	}
+	ch, err := p.Stream(context.Background(), provider.Request{Messages: []provider.Message{{Role: provider.RoleUser, Content: "hi"}}})
+	if err != nil {
+		t.Fatalf("Stream: %v", err)
+	}
+	for range ch { // drain
+	}
+
+	if gotUA != "Reasonix" {
+		t.Errorf("User-Agent = %q, want %q", gotUA, "Reasonix")
+	}
+}
+
 const sseFixture = `event: message_start
 data: {"type":"message_start","message":{"usage":{"input_tokens":100,"cache_creation_input_tokens":0,"cache_read_input_tokens":50,"output_tokens":1}}}
 
diff --git a/internal/provider/openai/openai.go b/internal/provider/openai/openai.go
index f2d0f0fc49..d94bd00fa7 100644
--- a/internal/provider/openai/openai.go
+++ b/internal/provider/openai/openai.go
@@ -204,6 +204,7 @@ func (c *client) Stream(ctx context.Context, req provider.Request) (<-chan provi
 			httpReq.Header.Set("Authorization", "Bearer "+c.apiKey)
 		}
 		httpReq.Header.Set("Accept", "text/event-stream")
+		httpReq.Header.Set("User-Agent", "Reasonix")
 		return httpReq, nil
 	}
 	resp, err := provider.SendWithRetry(ctx, c.http, c.sendOpts(), newReq)
diff --git a/internal/provider/openai/openai_test.go b/internal/provider/openai/openai_test.go
index 44f140d0dc..8570442e8f 100644
--- a/internal/provider/openai/openai_test.go
+++ b/internal/provider/openai/openai_test.go
@@ -125,6 +125,34 @@ func TestStreamAuthError(t *testing.T) {
 	}
 }
 
+// TestStreamSendsReasonixUserAgent verifies outbound requests carry an
+// identifying User-Agent so an upstream gateway can attribute the traffic to
+// Reasonix instead of seeing Go's default UA (#5226).
+func TestStreamSendsReasonixUserAgent(t *testing.T) {
+	var gotUA string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotUA = r.Header.Get("User-Agent")
+		w.Header().Set("Content-Type", "text/event-stream")
+		_, _ = io.WriteString(w, "data: [DONE]\n\n")
+	}))
+	defer srv.Close()
+
+	p, err := New(provider.Config{Name: "deepseek", BaseURL: srv.URL, Model: "deepseek-v4", APIKey: "k"})
+	if err != nil {
+		t.Fatalf("New: %v", err)
+	}
+	ch, err := p.Stream(context.Background(), provider.Request{Messages: []provider.Message{{Role: provider.RoleUser, Content: "hi"}}})
+	if err != nil {
+		t.Fatalf("Stream: %v", err)
+	}
+	for range ch { // drain
+	}
+
+	if gotUA != "Reasonix" {
+		t.Errorf("User-Agent = %q, want %q", gotUA, "Reasonix")
+	}
+}
+
 // TestBuildRequestAlwaysSerializesContent guards the DeepSeek 400 regression:
 // DeepSeek rejects a message missing the `content` field, so every message must
 // serialize one. A pure tool_calls assistant turn carries null (OpenAI-spec,