diff --git a/src/mitm/modify.rs b/src/mitm/modify.rs
index bebf2cc..94e25bc 100644
--- a/src/mitm/modify.rs
+++ b/src/mitm/modify.rs
@@ -206,8 +206,13 @@ pub fn modify_request(body: &[u8], tool_ctx: Option<&ToolContext>) -> Option<Vec
                 changes.push("remove toolConfig (no tools)".to_string());
             }
         }
+    }
 
-        // Also strip functionCall/functionResponse from conversation history
+    // ── 3b. ALWAYS strip old functionCall/functionResponse from history ───
+    // Even when custom tools are injected, the LS history contains function
+    // call parts for LS-internal tools we stripped. Google rejects these as
+    // MALFORMED_FUNCTION_CALL because the referenced tools don't exist.
+    if STRIP_ALL_TOOLS {
         if let Some(contents) = json
             .pointer_mut("/request/contents")
             .and_then(|v| v.as_array_mut())
diff --git a/src/mitm/proxy.rs b/src/mitm/proxy.rs
index 7e31924..b1aec73 100644
--- a/src/mitm/proxy.rs
+++ b/src/mitm/proxy.rs
@@ -726,6 +726,12 @@ async fn handle_http_over_tls(
                     }
                     headers_parsed = true;
 
+                    // Log error response bodies for debugging
+                    if resp.code.unwrap_or(0) >= 400 {
+                        let body_preview = String::from_utf8_lossy(&header_buf[hdr_end..]);
+                        warn!(domain, status = resp.code.unwrap_or(0), body = %body_preview, "MITM: upstream error response");
+                    }
+
                     // Save body for usage parsing
                     response_body_buf.extend_from_slice(&header_buf[hdr_end..]);