diff --git a/crates/anthropic/src/anthropic.rs b/crates/anthropic/src/anthropic.rs
index ba79eb2315f96f52d7f695ae2e94c616763f94a0..db6f88d8b2ff5794ddbaaf8bf62c3b9dfababf7c 100644
--- a/crates/anthropic/src/anthropic.rs
+++ b/crates/anthropic/src/anthropic.rs
@@ -288,33 +288,43 @@ impl Model {
     }
 
     pub fn mode(&self) -> AnthropicModelMode {
-        if self.supports_adaptive_thinking() {
-            AnthropicModelMode::AdaptiveThinking
-        } else if self.supports_thinking() {
-            AnthropicModelMode::Thinking {
+        match self {
+            Self::Custom { mode, .. } => mode.clone(),
+            _ if self.supports_adaptive_thinking() => AnthropicModelMode::AdaptiveThinking,
+            _ if self.supports_thinking() => AnthropicModelMode::Thinking {
                 budget_tokens: Some(4_096),
-            }
-        } else {
-            AnthropicModelMode::Default
+            },
+            _ => AnthropicModelMode::Default,
         }
     }
 
     pub fn supports_thinking(&self) -> bool {
-        matches!(
-            self,
-            Self::ClaudeOpus4
-                | Self::ClaudeOpus4_1
-                | Self::ClaudeOpus4_5
-                | Self::ClaudeOpus4_6
-                | Self::ClaudeSonnet4
-                | Self::ClaudeSonnet4_5
-                | Self::ClaudeSonnet4_6
-                | Self::ClaudeHaiku4_5
-        )
+        match self {
+            Self::Custom { mode, .. } => {
+                matches!(
+                    mode,
+                    AnthropicModelMode::Thinking { .. } | AnthropicModelMode::AdaptiveThinking
+                )
+            }
+            _ => matches!(
+                self,
+                Self::ClaudeOpus4
+                    | Self::ClaudeOpus4_1
+                    | Self::ClaudeOpus4_5
+                    | Self::ClaudeOpus4_6
+                    | Self::ClaudeSonnet4
+                    | Self::ClaudeSonnet4_5
+                    | Self::ClaudeSonnet4_6
+                    | Self::ClaudeHaiku4_5
+            ),
+        }
     }
 
     pub fn supports_adaptive_thinking(&self) -> bool {
-        matches!(self, Self::ClaudeOpus4_6 | Self::ClaudeSonnet4_6)
+        match self {
+            Self::Custom { mode, .. } => matches!(mode, AnthropicModelMode::AdaptiveThinking),
+            _ => matches!(self, Self::ClaudeOpus4_6 | Self::ClaudeSonnet4_6),
+        }
     }
 
     pub fn beta_headers(&self) -> Option<String> {
@@ -1110,6 +1120,65 @@ impl From<ApiError> for language_model_core::LanguageModelCompletionError {
     }
 }
 
+#[test]
+fn custom_mode_thinking_is_preserved() {
+    let model = Model::Custom {
+        name: "my-custom-model".to_string(),
+        max_tokens: 8192,
+        display_name: None,
+        tool_override: None,
+        cache_configuration: None,
+        max_output_tokens: None,
+        default_temperature: None,
+        extra_beta_headers: vec![],
+        mode: AnthropicModelMode::Thinking {
+            budget_tokens: Some(2048),
+        },
+    };
+    assert_eq!(
+        model.mode(),
+        AnthropicModelMode::Thinking {
+            budget_tokens: Some(2048)
+        }
+    );
+    assert!(model.supports_thinking());
+}
+
+#[test]
+fn custom_mode_adaptive_is_preserved() {
+    let model = Model::Custom {
+        name: "my-custom-model".to_string(),
+        max_tokens: 8192,
+        display_name: None,
+        tool_override: None,
+        cache_configuration: None,
+        max_output_tokens: None,
+        default_temperature: None,
+        extra_beta_headers: vec![],
+        mode: AnthropicModelMode::AdaptiveThinking,
+    };
+    assert_eq!(model.mode(), AnthropicModelMode::AdaptiveThinking);
+    assert!(model.supports_adaptive_thinking());
+    assert!(model.supports_thinking());
+}
+
+#[test]
+fn custom_mode_default_disables_thinking() {
+    let model = Model::Custom {
+        name: "my-custom-model".to_string(),
+        max_tokens: 8192,
+        display_name: None,
+        tool_override: None,
+        cache_configuration: None,
+        max_output_tokens: None,
+        default_temperature: None,
+        extra_beta_headers: vec![],
+        mode: AnthropicModelMode::Default,
+    };
+    assert!(!model.supports_thinking());
+    assert!(!model.supports_adaptive_thinking());
+}
+
 #[test]
 fn test_match_window_exceeded() {
     let error = ApiError {