diff --git a/crates/engines/src/base.rs b/crates/engines/src/base.rs
index df68750b..6385ffa6 100644
--- a/crates/engines/src/base.rs
+++ b/crates/engines/src/base.rs
@@ -332,7 +332,7 @@ pub(crate) fn parse_json_reply(reply: UpstreamResponse) -> GResult<(u16, Value)>
         ));
     };
     let v: Value = serde_json::from_slice(bytes)
-        .map_err(|e| GatewayError::internal("parse upstream response").with_source(e))?;
+        .map_err(|e| crate::engine::unparsed_reply(reply.status, "parse upstream response", e))?;
     if let Some(err) = crate::engine::vendor_error(reply.status, &v) {
         return Err(err);
     }
diff --git a/crates/engines/src/claude_engine.rs b/crates/engines/src/claude_engine.rs
index 2ab0abeb..33622e4f 100644
--- a/crates/engines/src/claude_engine.rs
+++ b/crates/engines/src/claude_engine.rs
@@ -2,13 +2,13 @@
 //! blocks, and streaming (the standard anthropic SSE event sequence). Marks
 //! `is_messages_protocol` so the usage extractor applies the Anthropic map.
 
-use gw_models::{GResult, GatewayError, GatewayResponse};
+use gw_models::{GResult, GatewayResponse};
 use gw_protocol::object;
 use gw_protocol::reasoning::{ThinkingDialect, is_thinking_block};
 use serde_json::{Map, Value, json};
 
 use crate::base::base_engine;
-use crate::engine::{EngineOutcome, ModelEngine, StreamChunk};
+use crate::engine::{EngineOutcome, ModelEngine, StreamChunk, unparsed_reply};
 use crate::transport::{UpstreamBody, UpstreamRequest};
 
 const DEFAULT_MAX_TOKENS: i64 = 1024;
@@ -238,7 +238,7 @@ impl ClaudeEngine {
 
     fn parse_json(&self, status: u16, bytes: &[u8]) -> GResult<EngineOutcome> {
         let v: Value = serde_json::from_slice(bytes)
-            .map_err(|e| GatewayError::internal("parse anthropic response").with_source(e))?;
+            .map_err(|e| unparsed_reply(status, "parse anthropic response", e))?;
         self.parse_value(status, v)
     }
 
@@ -367,7 +367,7 @@ impl ModelEngine for ClaudeEngine {
                 match reply.body {
                     UpstreamBody::Json(b) => {
                         let v: Value = serde_json::from_slice(&b).map_err(|e| {
-                            GatewayError::internal("parse converse response").with_source(e)
+                            unparsed_reply(reply.status, "parse converse response", e)
                         })?;
                         if let Some(err) = crate::engine::vendor_error(reply.status, &v) {
                             return Err(err);
@@ -490,8 +490,7 @@ impl<'a> SseState<'a> {
                 }
             }
             "content_block_delta" => {
-                // the native event is forwarded whole, so its text is read; the chat
-                // surface renders from the chunk instead, so its text is moved
+                // the native event keeps its delta; the chat surface moves it out
                 if self.preserve_native {
                     if let Some(t) = v["delta"]["text"].as_str() {
                         self.full.push_str(t);
@@ -510,7 +509,6 @@ impl<'a> SseState<'a> {
                     }
                     native_chunk.delta = t;
                 }
-                // the native event keeps its delta; the chat surface moves it out
                 if self.preserve_native {
                     if let Some(t) = v["delta"]["thinking"].as_str() {
                         self.append_thinking(t);
diff --git a/crates/engines/src/engine.rs b/crates/engines/src/engine.rs
index 1296db66..2a489e04 100644
--- a/crates/engines/src/engine.rs
+++ b/crates/engines/src/engine.rs
@@ -131,6 +131,15 @@ pub fn vendor_error(http_status: u16, v: &Value) -> Option<GatewayError> {
     })
 }
 
+pub(crate) fn unparsed_reply(
+    status: u16,
+    what: impl Into<String>,
+    e: serde_json::Error,
+) -> GatewayError {
+    vendor_error(status, &Value::Null)
+        .unwrap_or_else(|| GatewayError::internal(what).with_source(e))
+}
+
 /// MiniMax reports business errors as `base_resp.status_code != 0` on an HTTP 200.
 pub(crate) fn reject_minimax_error(v: &Value) -> GResult<()> {
     let code = v["base_resp"]["status_code"].as_i64().unwrap_or(0);
diff --git a/crates/engines/src/factory.rs b/crates/engines/src/factory.rs
index aca70e9c..8a18572c 100644
--- a/crates/engines/src/factory.rs
+++ b/crates/engines/src/factory.rs
@@ -53,7 +53,7 @@ pub fn get_engine(
                 400,
                 format!(
                     "realtime model `{}` is served on the /v1/realtime websocket surface, not the chat surface",
-                    p.as_str()
+                    request.model_param_v2.unwrap_or_default().model_name
                 ),
             ));
         }
@@ -92,6 +92,17 @@ mod tests {
         assert_eq!(dispatched, Protocol::ALL.len() - 1);
     }
 
+    #[test]
+    fn a_realtime_refusal_names_the_model() {
+        let t: SharedTransport = Arc::new(MockTransport);
+        let request = GatewayRequest {
+            model_param_v2: Some(ModelParamV2::with_name(Protocol::Realtime, "gpt-rt")),
+            ..Default::default()
+        };
+        let err = get_engine(request, t).err().unwrap();
+        assert!(err.message.contains("`gpt-rt`"), "{}", err.message);
+    }
+
     #[test]
     fn rejects_missing_param() {
         let t: SharedTransport = Arc::new(MockTransport);
diff --git a/crates/engines/src/families.rs b/crates/engines/src/families.rs
index 6953e153..6f3cf73c 100644
--- a/crates/engines/src/families.rs
+++ b/crates/engines/src/families.rs
@@ -1397,7 +1397,7 @@ impl ResponsesEngine {
     /// Non-streaming Responses reply: full `output` array + `usage`.
     fn parse_json(&self, status: u16, bytes: &[u8]) -> GResult<EngineOutcome> {
         let mut v: Value = serde_json::from_slice(bytes)
-            .map_err(|e| GatewayError::internal("parse responses reply").with_source(e))?;
+            .map_err(|e| crate::engine::unparsed_reply(status, "parse responses reply", e))?;
         if let Some(err) = crate::engine::vendor_error(status, &v) {
             return Err(err);
         }
diff --git a/crates/engines/src/mock_transport.rs b/crates/engines/src/mock_transport.rs
index e0e43c4f..4186be4c 100644
--- a/crates/engines/src/mock_transport.rs
+++ b/crates/engines/src/mock_transport.rs
@@ -908,8 +908,6 @@ impl Transport for MockTransport {
             self.ernie_reply(&req)
         } else if u.contains("minimax") {
             self.minimax_reply(&req)
-        } else if u.contains("meta.llama") {
-            self.llama_reply(&req)
         } else if u.contains("/messages") {
             self.anthropic_reply(&req)
         } else if u.contains(":generateContent") || u.contains(":streamGenerateContent") {
diff --git a/crates/engines/src/openai_engine.rs b/crates/engines/src/openai_engine.rs
index 0f99bfff..71031b52 100644
--- a/crates/engines/src/openai_engine.rs
+++ b/crates/engines/src/openai_engine.rs
@@ -3,7 +3,7 @@
 
 use std::borrow::Cow;
 
-use gw_models::{GResult, GatewayError, GatewayResponse};
+use gw_models::{GResult, GatewayResponse};
 use gw_protocol::object;
 use gw_protocol::reasoning::is_thinking_block;
 use serde_json::{Map, Value, json};
@@ -143,7 +143,7 @@ impl OpenAiEngine {
 
     fn parse_json(&self, status: u16, body: &[u8]) -> GResult<EngineOutcome> {
         let mut v: Value = serde_json::from_slice(body)
-            .map_err(|e| GatewayError::internal("parse openai response").with_source(e))?;
+            .map_err(|e| crate::engine::unparsed_reply(status, "parse openai response", e))?;
         // surface vendor error envelopes instead of silently returning empty
         if let Some(err) = crate::engine::vendor_error(status, &v) {
             return Err(err);
diff --git a/crates/engines/src/pump.rs b/crates/engines/src/pump.rs
index 3066d390..84581909 100644
--- a/crates/engines/src/pump.rs
+++ b/crates/engines/src/pump.rs
@@ -30,7 +30,7 @@ pub struct PumpResult {
 pub(crate) fn reject_json_error(what: &str, status: u16, body: &UpstreamBody) -> GResult<()> {
     if let UpstreamBody::Json(b) = body {
         let v: Value = serde_json::from_slice(b)
-            .map_err(|e| GatewayError::internal(format!("parse {what} reply")).with_source(e))?;
+            .map_err(|e| crate::engine::unparsed_reply(status, format!("parse {what} reply"), e))?;
         if let Some(err) = crate::engine::vendor_error(status, &v) {
             return Err(err);
         }
diff --git a/crates/engines/src/transport.rs b/crates/engines/src/transport.rs
index c63f7472..a2fded5e 100644
--- a/crates/engines/src/transport.rs
+++ b/crates/engines/src/transport.rs
@@ -14,8 +14,8 @@ pub use crate::mock_transport::MockTransport;
 
 /// Fixed "created" timestamp for deterministic mock payloads.
 pub const MOCK_CREATED: i64 = 1_720_000_000;
-/// 1x1 PNG-ish placeholder bytes, base64. Deterministic image/audio payload.
-pub const MOCK_B64: &str = "TU9DS0JZVEVT"; // "MOCKBYTES"
+/// Deterministic image/audio payload: base64 of "MOCKBYTES".
+pub const MOCK_B64: &str = "TU9DS0JZVEVT";
 pub(crate) const DEFAULT_CONNECT_RETRIES: u32 = 1;
 
 /// Wire headers an engine attaches; names are always literals.
diff --git a/crates/handler/tests/fallback.rs b/crates/handler/tests/fallback.rs
index b070db14..17e4dc40 100644
--- a/crates/handler/tests/fallback.rs
+++ b/crates/handler/tests/fallback.rs
@@ -28,6 +28,8 @@ impl Transport for Vendor {
         let (status, body): (u16, &'static [u8]) = match body["model"].as_str() {
             Some("broken") => (503, br#"{"error":{"message":"vendor down"}}"#),
             Some("throttled") => (429, br#"{"error":{"message":"rate limited"}}"#),
+            Some("proxied") => (502, b"<html><body>502 Bad Gateway</body></html>"),
+            Some("oversized") => (413, b"Request Entity Too Large"),
             Some("healthy") => (
                 200,
                 br#"{"model":"healthy","choices":[{"message":{"content":"ok"}}],"usage":{"prompt_tokens":1,"completion_tokens":1,"total_tokens":2}}"#,
@@ -55,6 +57,8 @@ models:
   - {{name: unavailable, protocol: openai-chat, provider: absent, fallback_models: [broken, throttled, healthy]}}
   - {{name: broken, protocol: openai-chat, fallback_models: [throttled, healthy]}}
   - {{name: throttled, protocol: openai-chat}}
+  - {{name: proxied, protocol: openai-chat, fallback_models: [healthy]}}
+  - {{name: oversized, protocol: openai-chat, fallback_models: [healthy]}}
   - {{name: healthy, protocol: openai-chat, qpm: {model_qpm}}}
 accounts: [{{name: a, provider: p, protocols: [openai-chat]}}]"
     );
@@ -179,6 +183,29 @@ async fn an_exhausted_chain_reports_the_last_upstream_error() {
     assert!(err.message.contains("rate limited"), "{}", err.message);
 }
 
+#[tokio::test]
+async fn a_non_json_error_body_keeps_the_vendor_status() {
+    let (h, vendor) = handler(100.0, "tenants: [{name: t}]", 100).expect("fallback config");
+    let ak = h.state().auth.authenticate("k").await.expect("access key");
+    let ctx = h
+        .run(request("proxied", true), ak.clone())
+        .await
+        .expect("a proxy's 502 page falls back");
+    let trail = ctx.decisions_line();
+    assert!(trail.contains("fallback: proxied -> healthy"), "{trail}");
+    let err = h
+        .run(request("oversized", true), ak)
+        .await
+        .err()
+        .expect("a 413 page is the vendor's refusal");
+    assert_eq!((err.http_status, err.original_status()), (413, Some(413)));
+    assert_eq!(
+        vendor.calls(),
+        3,
+        "a 4xx page neither fails over nor falls back"
+    );
+}
+
 #[tokio::test]
 async fn fallback_consumes_each_request_limit_once() {
     for (qps, limits, denial) in [
diff --git a/crates/models/src/request.rs b/crates/models/src/request.rs
index e0c6ba3c..efe50388 100644
--- a/crates/models/src/request.rs
+++ b/crates/models/src/request.rs
@@ -6,8 +6,7 @@ pub use domain::*;
 /// Everything an engine needs to serve one request.
 #[derive(Debug, Default, Clone)]
 pub struct GatewayRequest {
-    /// Arc'd: selected from the shared pool per request and cloned along the
-    /// engine seam — sharing beats copying six strings per hop.
+    /// Arc'd: selected from the shared pool per request and cloned along the engine seam.
     pub account: Option<std::sync::Arc<Account>>,
     pub message: Vec<ChatMsg>,
     pub stream: bool,
diff --git a/crates/state/src/store.rs b/crates/state/src/store.rs
index 3973bd7f..7ef1ebe4 100644
--- a/crates/state/src/store.rs
+++ b/crates/state/src/store.rs
@@ -67,13 +67,10 @@ pub struct BillingRecord {
     pub tenant: String,
     /// Effective end user: the key's `owner` if set, else request metadata; empty
     /// when neither is present. The precise per-user billing dimension.
-    #[serde(default)]
     pub user_id: String,
     /// Ingress correlation id, joins this row to the access log and audit events.
-    #[serde(default)]
     pub request_id: String,
     /// Unix seconds the call was billed — the billing-period axis.
-    #[serde(default)]
     pub created_at_epoch_secs: i64,
     /// Public model the caller requested.
     pub model: String,
@@ -88,18 +85,14 @@ pub struct BillingRecord {
     pub total_tokens: i64,
     pub cost_micros: i64,
     /// What the serving account's vendor charged us (zero = untracked).
-    #[serde(default)]
     pub vendor_cost_micros: i64,
     /// Non-token units billed (TTS characters, transcription seconds, rerank
     /// search units); their price is folded into `cost_micros`.
-    #[serde(default)]
     pub billed_units: i64,
     /// PTU spilled over to a paygo account (a failover occurred).
-    #[serde(default)]
     pub ptu_spillover: bool,
     /// Token counts were estimated (an aborted stream billed from delivered
     /// text), not read from a vendor usage payload.
-    #[serde(default)]
     pub estimated: bool,
 }