diff --git a/docs/migration/model-list-migration.md b/docs/migration/model-list-migration.md
index 0d4af719c..41b196b29 100644
--- a/docs/migration/model-list-migration.md
+++ b/docs/migration/model-list-migration.md
@@ -113,6 +113,7 @@ The `model` field uses a protocol prefix format: `[protocol/]model-identifier`
 | `api_base` | No | API endpoint URL |
 | `api_key` | No* | API authentication key |
 | `proxy` | No | HTTP proxy URL |
+| `headers` | No | Custom HTTP headers (`Authorization` and `Content-Type` are reserved and will be ignored) |
 | `auth_method` | No | Authentication method: `oauth`, `token` |
 | `connect_mode` | No | Connection mode for CLI providers: `stdio`, `grpc` |
 | `rpm` | No | Requests per minute limit |
diff --git a/pkg/config/config.go b/pkg/config/config.go
index 440fb31e4..7c5b9f706 100644
--- a/pkg/config/config.go
+++ b/pkg/config/config.go
@@ -464,7 +464,7 @@ type ModelConfig struct {
 	APIBase string `json:"api_base,omitempty"` // API endpoint URL
 	APIKey  string `json:"api_key"`            // API authentication key
 	Proxy   string `json:"proxy,omitempty"`    // HTTP proxy URL
-	Headers map[string]string `json:"headers,omitempty"`  // Custom HTTP headers
+	Headers map[string]string `json:"headers,omitempty"`  // Custom HTTP headers (Authorization and Content-Type are reserved and will be ignored)
 
 	// Special providers (CLI-based, OAuth, etc.)
 	AuthMethod  string `json:"auth_method,omitempty"`  // Authentication method: oauth, token
diff --git a/pkg/providers/openai_compat/provider.go b/pkg/providers/openai_compat/provider.go
index 09f8eeea6..38408c2ea 100644
--- a/pkg/providers/openai_compat/provider.go
+++ b/pkg/providers/openai_compat/provider.go
@@ -184,6 +184,9 @@ func (p *Provider) Chat(
 		req.Header.Set("Authorization", "Bearer "+p.apiKey)
 	}
 	for key, value := range p.customHeaders {
+		if k := http.CanonicalHeaderKey(key); k == "Authorization" || k == "Content-Type" {
+			continue
+		}
 		req.Header.Set(key, value)
 	}
 
@@ -301,7 +304,7 @@ func parseResponse(body []byte) (*LLMResponse, error) {
 type openaiMessage struct {
 	Role       string     `json:"role"`
 	Content    string     `json:"content"`
-	Reasoning  string     `json:"reasoning_content,omitempty"`
+	ReasoningContent string `json:"reasoning_content,omitempty"`
 	ToolCalls  []ToolCall `json:"tool_calls,omitempty"`
 	ToolCallID string     `json:"tool_call_id,omitempty"`
 }
@@ -315,7 +318,7 @@ func stripSystemParts(messages []Message) []openaiMessage {
 		out[i] = openaiMessage{
 			Role:       m.Role,
 			Content:    m.Content,
-			Reasoning:  m.ReasoningContent,
+			ReasoningContent: m.ReasoningContent,
 			ToolCalls:  m.ToolCalls,
 			ToolCallID: m.ToolCallID,
 		}
diff --git a/pkg/providers/openai_compat/provider_test.go b/pkg/providers/openai_compat/provider_test.go
index 7247fea3e..8a4aafb1b 100644
--- a/pkg/providers/openai_compat/provider_test.go
+++ b/pkg/providers/openai_compat/provider_test.go
@@ -361,3 +361,119 @@ func TestProvider_FunctionalOptionRequestTimeoutNonPositive(t *testing.T) {
 		t.Fatalf("http timeout = %v, want %v", p.httpClient.Timeout, defaultRequestTimeout)
 	}
 }
+
+func TestProviderChat_SendsCustomHeaders(t *testing.T) {
+	var gotHeader string
+
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotHeader = r.Header.Get("X-Custom-Key")
+		resp := map[string]any{
+			"choices": []map[string]any{
+				{
+					"message":       map[string]any{"content": "ok"},
+					"finish_reason": "stop",
+				},
+			},
+		}
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(resp)
+	}))
+	defer server.Close()
+
+	p := NewProvider("key", server.URL, "", WithCustomHeaders(map[string]string{
+		"X-Custom-Key": "myvalue",
+	}))
+	_, err := p.Chat(t.Context(), []Message{{Role: "user", Content: "hi"}}, nil, "gpt-4o", nil)
+	if err != nil {
+		t.Fatalf("Chat() error = %v", err)
+	}
+	if gotHeader != "myvalue" {
+		t.Fatalf("X-Custom-Key = %q, want %q", gotHeader, "myvalue")
+	}
+}
+
+func TestProviderChat_SkipsReservedHeaders(t *testing.T) {
+	var gotAuth, gotContentType string
+
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotAuth = r.Header.Get("Authorization")
+		gotContentType = r.Header.Get("Content-Type")
+		resp := map[string]any{
+			"choices": []map[string]any{
+				{
+					"message":       map[string]any{"content": "ok"},
+					"finish_reason": "stop",
+				},
+			},
+		}
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(resp)
+	}))
+	defer server.Close()
+
+	p := NewProvider("mykey", server.URL, "", WithCustomHeaders(map[string]string{
+		"Authorization": "Bearer OVERRIDE",
+		"Content-Type":  "text/plain",
+		"X-Extra":       "allowed",
+	}))
+	_, err := p.Chat(t.Context(), []Message{{Role: "user", Content: "hi"}}, nil, "gpt-4o", nil)
+	if err != nil {
+		t.Fatalf("Chat() error = %v", err)
+	}
+	if gotAuth != "Bearer mykey" {
+		t.Fatalf("Authorization = %q, want %q", gotAuth, "Bearer mykey")
+	}
+	if gotContentType != "application/json" {
+		t.Fatalf("Content-Type = %q, want %q", gotContentType, "application/json")
+	}
+}
+
+func TestProviderChat_RoundTripsReasoningContent(t *testing.T) {
+	var reqBody map[string]any
+
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if err := json.NewDecoder(r.Body).Decode(&reqBody); err != nil {
+			http.Error(w, err.Error(), http.StatusBadRequest)
+			return
+		}
+		resp := map[string]any{
+			"choices": []map[string]any{
+				{
+					"message":       map[string]any{"content": "ok"},
+					"finish_reason": "stop",
+				},
+			},
+		}
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(resp)
+	}))
+	defer server.Close()
+
+	p := NewProvider("key", server.URL, "")
+	_, err := p.Chat(
+		t.Context(),
+		[]Message{
+			{Role: "user", Content: "1+1=?"},
+			{Role: "assistant", Content: "2", ReasoningContent: "let me think..."},
+			{Role: "user", Content: "thanks"},
+		},
+		nil,
+		"gpt-4o",
+		nil,
+	)
+	if err != nil {
+		t.Fatalf("Chat() error = %v", err)
+	}
+
+	msgs, ok := reqBody["messages"].([]any)
+	if !ok {
+		t.Fatalf("messages is not []any")
+	}
+	assistantMsg, ok := msgs[1].(map[string]any)
+	if !ok {
+		t.Fatalf("messages[1] is not map[string]any")
+	}
+	if got := assistantMsg["reasoning_content"]; got != "let me think..." {
+		t.Fatalf("reasoning_content = %v, want %q", got, "let me think...")
+	}
+}