diff --git a/src/lake/Lake/Build/Common.lean b/src/lake/Lake/Build/Common.lean
index d1f0205173ba..329470921ed0 100644
--- a/src/lake/Lake/Build/Common.lean
+++ b/src/lake/Lake/Build/Common.lean
@@ -574,7 +574,7 @@ public def resolveArtifact
   match (← getMTime path |>.toBaseIO) with
   | .ok mtime =>
     return {descr, path, mtime}
-  | .error (.noFileOrDirectory ..) =>
+  | .error (.noFileOrDirectory ..) => withLogErrorPos do
     -- we redownload artifacts on any error
     if let some service := service? then
       updateAction .fetch
diff --git a/src/lake/Lake/Config/Cache.lean b/src/lake/Lake/Config/Cache.lean
index a847407eb9a7..28a61b65c456 100644
--- a/src/lake/Lake/Config/Cache.lean
+++ b/src/lake/Lake/Config/Cache.lean
@@ -558,9 +558,10 @@ public def downloadArtifactCore (hash : Hash) (url : String) (path : FilePath) :
   download url path
   let actualHash ← computeFileHash path
   if actualHash != hash then
-    logError s!"{path}: downloaded artifact does not have the expected hash"
+    let errPos ← getLogPos
+    logError s!"{path}: downloaded artifact hash mismatch, got {actualHash}"
     IO.FS.removeFile path
-    failure
+    throw errPos
 
 /-- Uploads a file to an online bucket using the Amazon S3 protocol. -/
 def uploadS3
@@ -700,8 +701,9 @@ public def uploadArtifact
 /-! ## Multi-Artifact Transfer -/
 
 private inductive TransferKind
-| get
-| put
+  | get
+  | put
+  deriving DecidableEq
 
 private structure TransferInfo where
   url : String
@@ -712,13 +714,14 @@ private structure TransferConfig where
   kind : TransferKind
   scope : CacheServiceScope
   infos : Array TransferInfo
+  key : String := ""
 
 private structure TransferState where
   didError : Bool := false
   numSuccesses : Nat := 0
 
-private partial def monitorTransferLoop
-  (cfg : TransferConfig) (h : IO.FS.Handle) (s : TransferState)
+private partial def monitorTransfer
+  (cfg : TransferConfig) (h hOut : IO.FS.Handle) (s : TransferState)
 : LoggerIO TransferState := do
   let line ← h.getLine
   if line.trimAscii.isEmpty then
@@ -726,58 +729,102 @@ private partial def monitorTransferLoop
   else
     let s ← (·.2) <$> StateT.run (s := s) do
       match Json.parse line >>= fromJson? with
-      | .ok (res : JsonObject) =>
-        let some {url, path, descr} := getInfo? res
+      | .ok (out : JsonObject) =>
+        let some info@{url, path, descr} := getInfo? out
           | logError s!"{cfg.scope}: unidentifiable transfer completed: {line.trimAscii}"
             modify ({· with didError := true})
             return
-        match res.get "http_code" with
+        match out.get "http_code" with
         | .ok 200
         | .ok 201 =>
-          let action := match cfg.kind with | .get => "downloaded" | .put => "uploaded"
-          logInfo s!"{cfg.scope}: {action} artifact {descr.hash}\
-            \n  local path: {path}\
-            \n  remote URL: {url}"
-          let actualHash ← computeFileHash path
-          if actualHash != descr.hash then
-            logError s!"downloaded artifact does not have the expected hash"
-            IO.FS.removeFile path
-            modify ({· with didError := true})
-          else
+          match cfg.kind with
+          | .get =>
+            logInfo s!"{cfg.scope}: downloaded artifact {descr.hash}\
+              \n  local path: {path}\
+              \n  remote URL: {url}"
+            let actualHash ← computeFileHash path
+            if actualHash != descr.hash then
+              logError s!"{path}: downloaded artifact hash mismatch, got {actualHash}"
+              IO.FS.removeFile path
+              modify ({· with didError := true})
+            else
+              modify fun s => {s with numSuccesses := s.numSuccesses + 1}
+          | .put =>
+            logInfo s!"{cfg.scope}: uploaded artifact {descr.hash}\
+              \n  local path: {path}\
+              \n  remote URL: {url}"
             modify fun s => {s with numSuccesses := s.numSuccesses + 1}
         | code? =>
-          let msg? := res.getAs String "errormsg"
-          logError (mkFailureMsg descr.hash code? msg?)
-          if cfg.kind matches .get then
-            -- `curl --remove-on-error` can already do this, but only from 7.83 onwards
-            removeFileIfExists path
+          handleFailure info code? out line
           modify ({· with didError := true})
       | .error e =>
         logError s!"curl produced invalid JSON: {e}; received: {line.trimAscii}"
         modify ({· with didError := true})
-    monitorTransferLoop cfg h s
+    monitorTransfer cfg h hOut s
 where
-  getInfo? res :=
-    match res.getAs Nat "urlnum" with
+  getInfo? out :=
+    match out.getAs Nat "urlnum" with
     | .ok i => cfg.infos[i]?
     | _ => none
-  mkFailureMsg hash code?  msg? : String := Id.run do
+  handleFailure info code? out line : LoggerIO Unit := do
     let action := match cfg.kind with | .get => "download" | .put => "upload"
-    let mut msg := s!"{cfg.scope}: failed to {action} artifact {hash}"
+    let mut msg := s!"{cfg.scope}: failed to {action} artifact {info.descr.hash}"
     if let .ok code := code? then
       msg := s!"{msg} (status code: {code})"
-    if let .ok errMsg := msg? then
-      msg := s!"{msg}: {errMsg}"
-    return msg
-
-private def monitorTransfer
-  (cfg : TransferConfig) (args : Array String)
-: LoggerIO Unit := do
+    if let .ok errMsg := out.getAs String "errormsg" then
+      msg := s!"{msg}\n  curl error: {errMsg}"
+    msg := s!"{msg}\
+      \n  local path: {info.path}\
+      \n  remote URL: {info.url}"
+    match cfg.kind with
+    | .get =>
+      if let .ok size := out.getAs Nat "size_download" then
+        if size > 0 then
+          if let .ok contentType := out.getAs String "content_type" then
+            if contentType != artifactContentType then
+              if let .ok resp ← IO.FS.readFile info.path |>.toBaseIO then
+                msg := s!"{msg}\nunexpected response:\n{resp}"
+      removeFileIfExists info.path
+    | .put =>
+      if let .ok size := out.getAs Nat "size_download" then
+        if size > 0 then
+          if let some resp := String.fromUTF8? (← hOut.read size.toUSize) then
+            msg := s!"{msg}\nunexpected response:\n{resp}"
+    logError msg
+    logVerbose s!"curl JSON: {line.trimAsciiEnd}"
+
+private def transferArtifacts
+  (cfg : TransferConfig)
+: LoggerIO Unit := IO.FS.withTempFile fun h path => do
+  let args ← id do
+    match cfg.kind with
+    | .get =>
+      cfg.infos.forM fun info => do
+        h.putStrLn s!"url = {info.url}"
+        h.putStrLn s!"-o {info.path.toString.quote}"
+      h.flush
+      return #[
+        "-Z", "-X", "GET", "-L",
+        "--retry", "3", -- intermittent network errors can occur
+        "-s", "-w", "%{stderr}%{json}\n", "--config", path.toString
+      ]
+    | .put =>
+      cfg.infos.forM fun info => do
+        h.putStrLn s!"-T {info.path.toString.quote}"
+        h.putStrLn s!"url = {info.url}"
+      h.flush
+      return #[
+        "-Z", "-X", "PUT", "-L",
+        "-H", s!"Content-Type: {artifactContentType}",
+        "--retry", "3", -- intermittent network errors can occur
+        "--aws-sigv4", "aws:amz:auto:s3", "--user", cfg.key,
+        "-s", "-w", "%{stderr}%{json}\n", "--config", path.toString
+      ]
   let child ← IO.Process.spawn {
     cmd := "curl", args
     stdout := .piped, stderr := .piped
   }
-  let s ← monitorTransferLoop cfg child.stderr {}
+  let s ← monitorTransfer cfg child.stderr child.stdout {}
   let rc ← child.wait
   let stdout ← child.stdout.readToEnd
   let mut didError := s.didError
@@ -797,27 +844,21 @@ public def downloadArtifacts
    (descrs : Array ArtifactDescr) (cache : Cache)
    (service : CacheService) (scope : CacheServiceScope) (force := false)
 : LoggerIO Unit := do
-  IO.FS.withTempFile fun h path => do
-    IO.FS.createDirAll cache.artifactDir
-    let infos ← descrs.foldlM (init := #[]) fun s descr => do
-      let path := cache.artifactDir / descr.relPath
-      if force then
-        removeFileIfExists path
-      else if (← path.pathExists) then
-        return s
-      let url := service.artifactUrl descr.hash scope
-      h.putStrLn s!"url = {url}"
-      h.putStrLn s!"-o {path.toString.quote}"
-      return s.push {url, path, descr}
-    if infos.isEmpty then
-      return
-    h.flush
-    IO.FS.createDirAll cache.artifactDir
-    monitorTransfer {scope, infos, kind := .get} #[
-      "-Z", "-X", "GET", "-L",
-      "--retry", "3", -- intermittent network errors can occur
-      "-s", "-w", "%{stderr}%{json}\n", "--config", path.toString
-    ]
+  if descrs.isEmpty then
+    logWarning "no artifacts to download"
+    return
+  let infos ← descrs.foldlM (init := #[]) fun s descr => do
+    let path := cache.artifactDir / descr.relPath
+    if force then
+      removeFileIfExists path
+    else if (← path.pathExists) then
+      return s
+    let url := service.artifactUrl descr.hash scope
+    return s.push {url, path, descr}
+  if infos.isEmpty then
+    return
+  IO.FS.createDirAll cache.artifactDir
+  transferArtifacts {scope, infos, kind := .get}
 
 @[deprecated "Deprecated without replacement." (since := "2026-02-27")]
 public def downloadOutputArtifacts
@@ -832,20 +873,13 @@ public def uploadArtifacts
   (descrs : Vector ArtifactDescr n) (paths : Vector FilePath n)
   (service : CacheService) (scope : CacheServiceScope)
 : LoggerIO Unit := do
-  IO.FS.withTempFile fun h path => do
-    let infos ← n.foldM (init := #[]) fun i _ s => do
-      let url := service.artifactUrl descrs[i].hash scope
-      h.putStrLn s!"-T {paths[i].toString.quote}"
-      h.putStrLn s!"url = {url}"
-      return s.push {url, path := paths[i], descr := descrs[i]}
-    h.flush
-    monitorTransfer {scope, infos, kind := .put} #[
-      "-Z", "-X", "PUT", "-L",
-      "-H", s!"Content-Type: {artifactContentType}",
-      "--retry", "3", -- intermittent network errors can occur
-      "--aws-sigv4", "aws:amz:auto:s3", "--user", service.impl.key,
-      "-s", "-w", "%{stderr}%{json}\n", "--config", path.toString
-    ]
+  if n = 0 then
+    logWarning "no artifacts to upload"
+    return
+  let infos ← n.foldM (init := #[]) fun i _ s => do
+    let url := service.artifactUrl descrs[i].hash scope
+    return s.push {url, path := paths[i], descr := descrs[i]}
+  transferArtifacts {scope, infos, kind := .put, key := service.impl.key}
 
 /-! ### Output Transfer -/