diff --git a/CHANGELOG.md b/CHANGELOG.md
index c0cc2989c..795d7f5fb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- **OpenAI Responses API usage parsing robustness**: Hardened `build_anthropic_usage_from_responses()` and the Responses → Anthropic SSE translator so a missing or malformed upstream `usage` no longer produces `"usage": null` in `message_delta`. This unblocks strict Anthropic clients (notably the VSCode Claude Code extension) that crashed with "Cannot read properties of null (reading 'output_tokens')" against providers such as Codex OAuth and DashScope's `compatible-mode/v1/responses` endpoint. Added OpenAI field-name fallbacks (`prompt_tokens` / `completion_tokens`), null/empty/partial object handling, and preserved cache token fields even when input/output tokens are missing (#2422).
+
 ## [3.14.1] - 2026-04-23
 
 Development since v3.14.0 focuses on Codex OAuth stability, tray usage visibility, Skills import/install reliability, Gemini session restore paths, and simplifying Hermes configuration health handling.
diff --git a/src-tauri/src/proxy/providers/streaming_responses.rs b/src-tauri/src/proxy/providers/streaming_responses.rs
index 1eba5033e..c7f63a14e 100644
--- a/src-tauri/src/proxy/providers/streaming_responses.rs
+++ b/src-tauri/src/proxy/providers/streaming_responses.rs
@@ -170,9 +170,12 @@ pub fn create_anthropic_sse_stream_from_responses<E: std::error::Error + Send +
                                 }
 
                                 has_sent_message_start = true;
-                                // Build usage with cache tokens if available
+                                // Build usage with defensive null handling
+                                // Some() wrapper ensures build function always receives valid input
+                                // Fallback to empty object {} if usage field missing, ensuring message_start
+                                // event always has valid usage structure for VSCode Extension compatibility
                                 let start_usage = build_anthropic_usage_from_responses(
-                                    response_obj.get("usage"),
+                                    Some(response_obj.get("usage").unwrap_or(&json!({}))),
                                 );
 
                                 let event = json!({
@@ -670,9 +673,12 @@ pub fn create_anthropic_sse_stream_from_responses<E: std::error::Error + Send +
                                 }
                                 fallback_open_index = None;
 
-                                let usage_json = response_obj.get("usage").map(|u| {
-                                    build_anthropic_usage_from_responses(Some(u))
-                                });
+                                // Defensive: Always build usage_json, even if usage field missing
+                                // Some() wrapper with fallback to {} ensures build_anthropic_usage_from_responses
+                                // always receives valid input, preventing null pointer errors in VSCode Extension
+                                let usage_json = build_anthropic_usage_from_responses(
+                                    Some(response_obj.get("usage").unwrap_or(&json!({})))
+                                );
 
                                 // Emit message_delta (with usage + stop_reason)
                                 let delta_event = json!({
diff --git a/src-tauri/src/proxy/providers/transform_responses.rs b/src-tauri/src/proxy/providers/transform_responses.rs
index ddff5136e..5a5d6dc2b 100644
--- a/src-tauri/src/proxy/providers/transform_responses.rs
+++ b/src-tauri/src/proxy/providers/transform_responses.rs
@@ -218,12 +218,28 @@ pub(crate) fn map_responses_stop_reason(
 
 /// Build Anthropic-style usage JSON from Responses API usage, including cache tokens.
 ///
-/// Priority order:
+/// **Robustness Features**:
+/// - Handles null, missing, empty objects, and partial objects gracefully
+/// - Supports OpenAI field name variants (prompt_tokens/completion_tokens) as fallbacks
+/// - Always returns valid structure: {"input_tokens": N, "output_tokens": N}
+/// - Preserves cache token fields even when input/output tokens are missing
+///
+/// **Field Name Resolution Priority**:
+/// 1. input_tokens: Anthropic `input_tokens` → OpenAI `prompt_tokens` → default 0
+/// 2. output_tokens: Anthropic `output_tokens` → OpenAI `completion_tokens` → default 0
+/// 3. cache_read_input_tokens: Direct field → nested input_tokens_details.cached_tokens → prompt_tokens_details.cached_tokens
+/// 4. cache_creation_input_tokens: Direct field only
+///
+/// **Cache Token Priority Order**:
 /// 1. OpenAI nested details (`input_tokens_details.cached_tokens`, `prompt_tokens_details.cached_tokens`) as initial value
 /// 2. Direct Anthropic-style fields (`cache_read_input_tokens`, `cache_creation_input_tokens`) override if present
+///
+/// **Logging**:
+/// - Warns on empty objects {} or partial objects (only one field present)
+/// - Debug logs when using OpenAI field name fallbacks
 pub(crate) fn build_anthropic_usage_from_responses(usage: Option<&Value>) -> Value {
     let u = match usage {
-        Some(v) if !v.is_null() => v,
+        Some(v) if !v.is_null() && v.is_object() => v,
         _ => {
             return json!({
                 "input_tokens": 0,
@@ -232,15 +248,56 @@ pub(crate) fn build_anthropic_usage_from_responses(usage: Option<&Value>) -> Val
         }
     };
 
-    let input = u.get("input_tokens").and_then(|v| v.as_u64()).unwrap_or(0);
-    let output = u.get("output_tokens").and_then(|v| v.as_u64()).unwrap_or(0);
+    // Detect empty object {} and log warning
+    if u.as_object().map(|obj| obj.is_empty()).unwrap_or(false) {
+        log::warn!("[Responses] Empty usage object received, using defaults");
+        return json!({
+            "input_tokens": 0,
+            "output_tokens": 0
+        });
+    }
+
+    // Extract input_tokens with OpenAI field name fallback
+    // Priority: input_tokens (Anthropic) → prompt_tokens (OpenAI) → 0
+    let input = u
+        .get("input_tokens")
+        .and_then(|v| v.as_u64())
+        .or_else(|| {
+            let prompt_tokens = u.get("prompt_tokens").and_then(|v| v.as_u64());
+            if prompt_tokens.is_some() {
+                log::debug!(
+                    "[Responses] Using OpenAI field name fallback 'prompt_tokens' for input_tokens"
+                );
+            }
+            prompt_tokens
+        })
+        .unwrap_or(0);
+
+    // Extract output_tokens with OpenAI field name fallback
+    // Priority: output_tokens (Anthropic) → completion_tokens (OpenAI) → 0
+    let output = u.get("output_tokens")
+        .and_then(|v| v.as_u64())
+        .or_else(|| {
+            let completion_tokens = u.get("completion_tokens").and_then(|v| v.as_u64());
+            if completion_tokens.is_some() {
+                log::debug!("[Responses] Using OpenAI field name fallback 'completion_tokens' for output_tokens");
+            }
+            completion_tokens
+        })
+        .unwrap_or(0);
+
+    // Log if only one field present (partial object). Streaming chunks legitimately
+    // arrive with partial usage, so this stays at debug level to avoid noise.
+    if (input == 0 && output > 0) || (input > 0 && output == 0) {
+        log::debug!("[Responses] Partial usage object: {:?}", u);
+    }
 
     let mut result = json!({
         "input_tokens": input,
         "output_tokens": output
     });
 
-    // Step 1: OpenAI nested details as fallback
+    // Step 1: OpenAI nested details as fallback for cache tokens
     // OpenAI Responses API: input_tokens_details.cached_tokens
     if let Some(cached) = u
         .pointer("/input_tokens_details/cached_tokens")
@@ -259,6 +316,7 @@ pub(crate) fn build_anthropic_usage_from_responses(usage: Option<&Value>) -> Val
     }
 
     // Step 2: Direct Anthropic-style fields override (authoritative if present)
+    // These preserve cache tokens even if input/output_tokens are missing
     if let Some(v) = u.get("cache_read_input_tokens") {
         result["cache_read_input_tokens"] = v.clone();
     }
@@ -1352,4 +1410,106 @@ mod tests {
             "非 Codex OAuth 路径下 tools 在客户端未送时不应被注入"
         );
     }
+
+    // ==================== Usage Field Robustness Tests ====================
+
+    #[test]
+    fn test_build_usage_from_null_parameter() {
+        let result = build_anthropic_usage_from_responses(None);
+        assert_eq!(result["input_tokens"], json!(0));
+        assert_eq!(result["output_tokens"], json!(0));
+    }
+
+    #[test]
+    fn test_build_usage_from_null_json_value() {
+        let result = build_anthropic_usage_from_responses(Some(&json!(null)));
+        assert_eq!(result["input_tokens"], json!(0));
+        assert_eq!(result["output_tokens"], json!(0));
+    }
+
+    #[test]
+    fn test_build_usage_from_empty_object() {
+        let result = build_anthropic_usage_from_responses(Some(&json!({})));
+        assert_eq!(result["input_tokens"], json!(0));
+        assert_eq!(result["output_tokens"], json!(0));
+    }
+
+    #[test]
+    fn test_build_usage_from_partial_input_only() {
+        let result = build_anthropic_usage_from_responses(Some(&json!({
+            "input_tokens": 100
+        })));
+        assert_eq!(result["input_tokens"], json!(100));
+        assert_eq!(result["output_tokens"], json!(0));
+    }
+
+    #[test]
+    fn test_build_usage_from_partial_output_only() {
+        let result = build_anthropic_usage_from_responses(Some(&json!({
+            "output_tokens": 50
+        })));
+        assert_eq!(result["input_tokens"], json!(0));
+        assert_eq!(result["output_tokens"], json!(50));
+    }
+
+    #[test]
+    fn test_build_usage_with_openai_field_names() {
+        let result = build_anthropic_usage_from_responses(Some(&json!({
+            "prompt_tokens": 120,
+            "completion_tokens": 45
+        })));
+        assert_eq!(result["input_tokens"], json!(120));
+        assert_eq!(result["output_tokens"], json!(45));
+    }
+
+    #[test]
+    fn test_build_usage_anthropic_names_precedence() {
+        let result = build_anthropic_usage_from_responses(Some(&json!({
+            "input_tokens": 100,
+            "prompt_tokens": 120,
+            "output_tokens": 50,
+            "completion_tokens": 45
+        })));
+        assert_eq!(result["input_tokens"], json!(100)); // Anthropic name takes precedence
+        assert_eq!(result["output_tokens"], json!(50)); // Anthropic name takes precedence
+    }
+
+    #[test]
+    fn test_build_usage_cache_tokens_from_nested_details() {
+        let result = build_anthropic_usage_from_responses(Some(&json!({
+            "input_tokens": 100,
+            "output_tokens": 50,
+            "input_tokens_details": {
+                "cached_tokens": 80
+            }
+        })));
+        assert_eq!(result["input_tokens"], json!(100));
+        assert_eq!(result["output_tokens"], json!(50));
+        assert_eq!(result["cache_read_input_tokens"], json!(80));
+    }
+
+    #[test]
+    fn test_build_usage_cache_tokens_direct_override() {
+        let result = build_anthropic_usage_from_responses(Some(&json!({
+            "input_tokens": 100,
+            "output_tokens": 50,
+            "input_tokens_details": {
+                "cached_tokens": 80
+            },
+            "cache_read_input_tokens": 100
+        })));
+        assert_eq!(result["cache_read_input_tokens"], json!(100)); // Direct field overrides nested
+    }
+
+    #[test]
+    fn test_build_usage_cache_tokens_without_input_output() {
+        let result = build_anthropic_usage_from_responses(Some(&json!({
+            "cache_read_input_tokens": 60,
+            "cache_creation_input_tokens": 20
+        })));
+        assert_eq!(result["input_tokens"], json!(0));
+        assert_eq!(result["output_tokens"], json!(0));
+        assert_eq!(result["cache_read_input_tokens"], json!(60));
+        assert_eq!(result["cache_creation_input_tokens"], json!(20));
+    }
 }