diff --git a/crates/forge_app/src/dto/anthropic/response.rs b/crates/forge_app/src/dto/anthropic/response.rs
index ba32540fe7..08fe6db801 100644
--- a/crates/forge_app/src/dto/anthropic/response.rs
+++ b/crates/forge_app/src/dto/anthropic/response.rs
@@ -24,11 +24,16 @@ pub struct ListModelResponse {
 pub struct Model {
     pub id: String,
     pub display_name: Option<String>,
+    /// Context window size reported by the API. When present, takes precedence
+    /// over the hardcoded fallback in `get_context_length`.
+    pub context_length: Option<u64>,
 }
 
 impl From<Model> for forge_domain::Model {
     fn from(value: Model) -> Self {
-        let context_length = get_context_length(&value.id);
+        let context_length = value
+            .context_length
+            .or_else(|| get_context_length(&value.id));
         let input_modalities = if value.id.contains("claude-3")
             || value.id.contains("claude-4")
             || value.id.contains("claude-sonnet")
@@ -768,6 +773,7 @@ mod tests {
         let fixture = Model {
             id: "claude-sonnet-4-5-20250929".to_string(),
             display_name: Some("Claude 3.5 Sonnet (New)".to_string()),
+            context_length: None,
         };
 
         let actual: forge_domain::Model = fixture.into();
@@ -782,6 +788,7 @@ mod tests {
         let fixture = Model {
             id: "unknown-claude-model".to_string(),
             display_name: Some("Unknown Model".to_string()),
+            context_length: None,
         };
 
         let actual: forge_domain::Model = fixture.into();
@@ -790,6 +797,35 @@ mod tests {
         assert_eq!(actual.id.as_str(), "unknown-claude-model");
     }
 
+    #[test]
+    fn test_model_conversion_api_context_length_takes_precedence() {
+        // When the API reports context_length (e.g., 1M with beta header),
+        // it should take precedence over the hardcoded fallback (200K).
+        let fixture = Model {
+            id: "claude-sonnet-4-5-20250929".to_string(),
+            display_name: Some("Claude Sonnet 4.5".to_string()),
+            context_length: Some(1_048_576),
+        };
+
+        let actual: forge_domain::Model = fixture.into();
+
+        assert_eq!(actual.context_length, Some(1_048_576));
+    }
+
+    #[test]
+    fn test_model_conversion_unknown_model_with_api_context_length() {
+        // Even for unknown models, API-provided context_length should be used.
+        let fixture = Model {
+            id: "claude-future-6-0".to_string(),
+            display_name: Some("Claude Future".to_string()),
+            context_length: Some(500_000),
+        };
+
+        let actual: forge_domain::Model = fixture.into();
+
+        assert_eq!(actual.context_length, Some(500_000));
+    }
+
     #[test]
     fn test_ping_event_with_string_cost() {
         // Fixture: OpenCode Zen sends cost as a string in a ping event
diff --git a/crates/forge_repo/src/provider/anthropic.rs b/crates/forge_repo/src/provider/anthropic.rs
index 3292f5ab9f..e19ea99a51 100644
--- a/crates/forge_repo/src/provider/anthropic.rs
+++ b/crates/forge_repo/src/provider/anthropic.rs
@@ -81,6 +81,14 @@ impl<H: HttpInfra> Anthropic<H> {
             }
         }
 
+        // Append provider-level custom headers (e.g., for proxies that require
+        // different header names like `api-key` instead of `x-api-key`)
+        if let Some(custom_headers) = &self.provider.custom_headers {
+            for (k, v) in custom_headers {
+                headers.push((k.clone(), v.clone()));
+            }
+        }
+
         headers
     }
 }
@@ -855,4 +863,63 @@ mod tests {
             "Vertex AI requests should include anthropic_version"
         );
     }
+
+    #[test]
+    fn test_get_headers_includes_custom_headers() {
+        let chat_url = Url::parse("https://proxy.example.com/v1/messages").unwrap();
+        let model_url = Url::parse("https://proxy.example.com/v1/models").unwrap();
+
+        let mut custom = std::collections::HashMap::new();
+        custom.insert("api-key".to_string(), "my-proxy-key".to_string());
+        custom.insert("x-custom-tag".to_string(), "forge".to_string());
+
+        let provider = Provider {
+            id: forge_app::domain::ProviderId::ANTHROPIC,
+            provider_type: forge_domain::ProviderType::Llm,
+            response: Some(forge_app::domain::ProviderResponse::Anthropic),
+            url: chat_url,
+            credential: Some(forge_domain::AuthCredential {
+                id: forge_app::domain::ProviderId::ANTHROPIC,
+                auth_details: forge_domain::AuthDetails::ApiKey(forge_domain::ApiKey::from(
+                    "sk-test-key".to_string(),
+                )),
+                url_params: std::collections::HashMap::new(),
+            }),
+            auth_methods: vec![forge_domain::AuthMethod::ApiKey],
+            url_params: vec![],
+            models: Some(forge_domain::ModelSource::Url(model_url)),
+            custom_headers: Some(custom),
+        };
+
+        let fixture = Anthropic::new(
+            Arc::new(MockHttpClient::new()),
+            provider,
+            "2023-06-01".to_string(),
+            false,
+        );
+
+        let actual = fixture.get_headers();
+
+        // Custom headers should be present
+        assert!(
+            actual
+                .iter()
+                .any(|(k, v)| k == "api-key" && v == "my-proxy-key"),
+            "custom_headers should be appended to request headers"
+        );
+        assert!(
+            actual
+                .iter()
+                .any(|(k, v)| k == "x-custom-tag" && v == "forge"),
+            "all custom_headers entries should be included"
+        );
+
+        // Standard headers should still be present
+        assert!(
+            actual
+                .iter()
+                .any(|(k, v)| k == "x-api-key" && v == "sk-test-key"),
+            "standard x-api-key should still be present"
+        );
+    }
 }