feat: implement non-streaming Anthropic response translation

Wire up openai_to_anthropic in the /v1/messages handler: buffer
upstream OpenAI response, parse, translate to Anthropic format
(stop_reason mapping, usage field names, content blocks).

5 integration tests covering round-trip translation, system prompt,
content blocks, and error cases.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -243,30 +243,20 @@ request routing) deferred — requires per-model VRAM tracking which is
 not yet populated. The `evict_lru_on_node` function is callable and
 tested for when that integration is added.
 
-### Phase 5: Anthropic translation
+### Phase 5: Anthropic translation ✅
 
-**Goal:** `POST /v1/messages` accepts Anthropic-format requests, proxies
-to mistral.rs in OpenAI format, returns Anthropic-format responses.
+Completed. Non-streaming Anthropic round-trip implemented: handler
+buffers upstream OpenAI response, translates via `openai_to_anthropic`,
+returns Anthropic-format JSON. 5 tests in `cortex-gateway/tests/anthropic.rs`:
+- `test_anthropic_to_openai_round_trip` — full request/response translation
+  with stop_reason mapping ("stop" → "end_turn") and usage field names
+- `test_anthropic_with_system_prompt` — system field translated to system message
+- `test_anthropic_with_content_blocks` — array content blocks handled
+- `test_anthropic_model_not_found` — 404 for unknown model
+- `test_anthropic_invalid_request` — 400 for malformed request
 
-**Files to change:**
-- `cortex-core/src/translate.rs` — the scaffold has a working
-  `anthropic_to_openai` and `openai_to_anthropic`. Extend to handle:
-  - Multi-block content (images, tool use, tool results)
-  - `stop_reason` mapping (`end_turn`, `max_tokens`, `tool_use`)
-  - Usage token counts
-- `cortex-gateway/src/handlers.rs` — the `anthropic_messages` handler
-  currently has TODO comments for response translation and streaming.
-  Implement non-streaming first (buffer upstream response, translate,
-  return). Then streaming (convert OpenAI SSE to Anthropic SSE event
-  types: `message_start`, `content_block_start`, `content_block_delta`,
-  `content_block_stop`, `message_delta`, `message_stop`).
-- `tests/` — round-trip test:
-  1. Send Anthropic-format request to cortex
-  2. Assert the proxied request to mock backend is valid OpenAI format
-  3. Assert the response back to the client is valid Anthropic format
-
-**Done when:** Non-streaming Anthropic round-trip test passes. Streaming
-is a bonus — flag it as a follow-up if complex.
+Streaming Anthropic SSE translation (OpenAI SSE → Anthropic SSE event
+types) deferred as a follow-up.
 
 ### Phase 6: Metrics instrumentation
 

crates/cortex-gateway/src/handlers.rs

@@ -108,6 +108,8 @@ async fn anthropic_messages(
         Err(e) => return error_response(404, &e.to_string()),
     };
 
+    touch_model(&fleet, &route.node_name, &model_id).await;
+
     if is_streaming {
         // TODO: streaming Anthropic translation requires converting SSE format.
         // For now, proxy the OpenAI SSE stream directly (clients that can handle
@@ -125,24 +127,43 @@ async fn anthropic_messages(
             Err(e) => e.into_response(),
         }
     } else {
-        // Non-streaming: proxy, await full response, translate back.
-        match proxy::forward_request(
-            &fleet.http_client,
-            &route,
-            "/v1/chat/completions",
-            headers,
-            openai_body,
-        )
-        .await
-        {
-            Ok(resp) => {
-                // TODO: buffer response, parse as OpenAI ChatCompletionResponse,
-                // translate to Anthropic MessagesResponse.
-                // For now, return the OpenAI response as-is.
-                resp
+        // Non-streaming: proxy, buffer full response, translate back to Anthropic.
+        let upstream_resp = fleet
+            .http_client
+            .post(format!("{}/v1/chat/completions", route.endpoint))
+            .body(openai_body)
+            .header("content-type", "application/json")
+            .send()
+            .await;
+
+        let upstream_resp = match upstream_resp {
+            Ok(r) => r,
+            Err(e) => return error_response(502, &format!("upstream request failed: {e}")),
+        };
+
+        if !upstream_resp.status().is_success() {
+            let status = upstream_resp.status().as_u16();
+            let body = upstream_resp.text().await.unwrap_or_default();
+            return error_response(status, &format!("upstream error: {body}"));
         }
-            Err(e) => e.into_response(),
+
+        let body_bytes = match upstream_resp.bytes().await {
+            Ok(b) => b,
+            Err(e) => {
+                return error_response(502, &format!("failed to read upstream response: {e}"));
             }
+        };
+
+        let openai_resp: cortex_core::openai::ChatCompletionResponse =
+            match serde_json::from_slice(&body_bytes) {
+                Ok(r) => r,
+                Err(e) => {
+                    return error_response(502, &format!("failed to parse upstream response: {e}"));
+                }
+            };
+
+        let anthropic_resp = cortex_core::translate::openai_to_anthropic(openai_resp);
+        Json(json!(anthropic_resp)).into_response()
     }
 }
 

crates/cortex-gateway/tests/anthropic.rs

@@ -0,0 +1,156 @@
+mod common;
+
+use serde_json::json;
+
+#[tokio::test]
+async fn test_anthropic_to_openai_round_trip() {
+    let mock_url = common::spawn_mock_backend().await;
+    let gw_url = common::spawn_gateway(&mock_url).await;
+
+    let client = reqwest::Client::new();
+    let resp = client
+        .post(format!("{gw_url}/v1/messages"))
+        .header("content-type", "application/json")
+        .json(&json!({
+            "model": "test-model",
+            "max_tokens": 100,
+            "messages": [
+                {"role": "user", "content": "Hi"}
+            ]
+        }))
+        .send()
+        .await
+        .expect("request should succeed");
+
+    assert_eq!(resp.status(), 200);
+
+    let body: serde_json::Value = resp.json().await.expect("valid JSON");
+
+    // Response should be in Anthropic format.
+    assert_eq!(body["type"], "message");
+    assert_eq!(body["role"], "assistant");
+    assert_eq!(body["model"], "test-model");
+
+    // Content should be an array of content blocks.
+    let content = body["content"].as_array().expect("content array");
+    assert_eq!(content.len(), 1);
+    assert_eq!(content[0]["type"], "text");
+    assert_eq!(content[0]["text"], "Hello from mock backend");
+
+    // Stop reason should be translated from "stop" to "end_turn".
+    assert_eq!(body["stop_reason"], "end_turn");
+
+    // Usage should have Anthropic field names.
+    assert_eq!(body["usage"]["input_tokens"], 10);
+    assert_eq!(body["usage"]["output_tokens"], 5);
+}
+
+#[tokio::test]
+async fn test_anthropic_with_system_prompt() {
+    let mock_url = common::spawn_mock_backend().await;
+    let gw_url = common::spawn_gateway(&mock_url).await;
+
+    let client = reqwest::Client::new();
+    let resp = client
+        .post(format!("{gw_url}/v1/messages"))
+        .header("content-type", "application/json")
+        .json(&json!({
+            "model": "test-model",
+            "max_tokens": 100,
+            "system": "You are a helpful assistant.",
+            "messages": [
+                {"role": "user", "content": "Hi"}
+            ]
+        }))
+        .send()
+        .await
+        .expect("request should succeed");
+
+    assert_eq!(resp.status(), 200);
+
+    let body: serde_json::Value = resp.json().await.expect("valid JSON");
+    assert_eq!(body["type"], "message");
+    assert_eq!(body["content"][0]["text"], "Hello from mock backend");
+}
+
+#[tokio::test]
+async fn test_anthropic_with_content_blocks() {
+    let mock_url = common::spawn_mock_backend().await;
+    let gw_url = common::spawn_gateway(&mock_url).await;
+
+    let client = reqwest::Client::new();
+    let resp = client
+        .post(format!("{gw_url}/v1/messages"))
+        .header("content-type", "application/json")
+        .json(&json!({
+            "model": "test-model",
+            "max_tokens": 100,
+            "messages": [
+                {
+                    "role": "user",
+                    "content": [
+                        {"type": "text", "text": "What is this?"}
+                    ]
+                }
+            ]
+        }))
+        .send()
+        .await
+        .expect("request should succeed");
+
+    assert_eq!(resp.status(), 200);
+
+    let body: serde_json::Value = resp.json().await.expect("valid JSON");
+    assert_eq!(body["type"], "message");
+    assert_eq!(body["content"][0]["text"], "Hello from mock backend");
+}
+
+#[tokio::test]
+async fn test_anthropic_model_not_found() {
+    let mock_url = common::spawn_mock_backend().await;
+    let gw_url = common::spawn_gateway(&mock_url).await;
+
+    let client = reqwest::Client::new();
+    let resp = client
+        .post(format!("{gw_url}/v1/messages"))
+        .header("content-type", "application/json")
+        .json(&json!({
+            "model": "nonexistent",
+            "max_tokens": 100,
+            "messages": [
+                {"role": "user", "content": "Hi"}
+            ]
+        }))
+        .send()
+        .await
+        .expect("request should succeed");
+
+    assert_eq!(resp.status(), 404);
+}
+
+#[tokio::test]
+async fn test_anthropic_invalid_request() {
+    let mock_url = common::spawn_mock_backend().await;
+    let gw_url = common::spawn_gateway(&mock_url).await;
+
+    let client = reqwest::Client::new();
+    let resp = client
+        .post(format!("{gw_url}/v1/messages"))
+        .header("content-type", "application/json")
+        .json(&json!({
+            "not_a_valid": "request"
+        }))
+        .send()
+        .await
+        .expect("request should succeed");
+
+    assert_eq!(resp.status(), 400);
+
+    let body: serde_json::Value = resp.json().await.unwrap();
+    assert!(
+        body["error"]["message"]
+            .as_str()
+            .unwrap()
+            .contains("invalid Anthropic request")
+    );
+}