diff --git a/.claude/settings.local.json b/.claude/settings.local.json
new file mode 100644
index 00000000000..4a1edf13a3d
--- /dev/null
+++ b/.claude/settings.local.json
@@ -0,0 +1,7 @@
+{
+  "permissions": {
+    "allow": [
+      "Bash(grep:*)"
+    ]
+  }
+}
diff --git a/src/libstore/http-binary-cache-store.cc b/src/libstore/http-binary-cache-store.cc
index d4361264edf..e93147be69b 100644
--- a/src/libstore/http-binary-cache-store.cc
+++ b/src/libstore/http-binary-cache-store.cc
@@ -203,9 +203,16 @@ void HttpBinaryCacheStore::getFile(const std::string & path, Sink & sink)
     try {
         getFileTransfer()->download(std::move(request), sink);
     } catch (FileTransferError & e) {
-        if (e.error == FileTransfer::NotFound || e.error == FileTransfer::Forbidden)
+        if (e.error == FileTransfer::NotFound)
             throw NoSuchBinaryCacheFile(
                 "file '%s' does not exist in binary cache '%s'", path, config->getHumanReadableURI());
+        if (e.error == FileTransfer::Forbidden)
+            throw NoSuchBinaryCacheFile(
+                "%s\nUnable to access file '%s' in binary cache '%s'. "
+                "If the binary cache requires authentication, check that your access token is valid and not expired.",
+                e.what(),
+                path,
+                config->getHumanReadableURI());
         maybeDisable();
         throw;
     }
diff --git a/tests/functional/binary-cache-auth-errors.sh b/tests/functional/binary-cache-auth-errors.sh
new file mode 100755
index 00000000000..8b725f223e2
--- /dev/null
+++ b/tests/functional/binary-cache-auth-errors.sh
@@ -0,0 +1,115 @@
+#!/usr/bin/env bash
+
+# Test that HTTP binary cache errors are properly distinguished:
+# - 404 errors should say "does not exist in binary cache"
+# - 401/403 errors should mention authentication and token expiration
+
+source common.sh
+
+TODO_NixOS
+
+needLocalStore "'--no-require-sigs' can't be used with the daemon"
+
+# Create a binary cache with some content
+clearStore
+clearCache
+outPath=$(nix-build dependencies.nix --no-out-link)
+nix copy --to "file://$cacheDir" "$outPath"
+
+# Function to start the test HTTP server with a specific return code
+TEST_SERVER="$_NIX_TEST_SOURCE_DIR/binary-cache-auth-errors_test-http-server.py"
+startHttpServer() {
+    local return_code=$1
+    CACHE_DIR="$cacheDir" RETURN_CODE="$return_code" python3 "$TEST_SERVER" > "$TEST_ROOT/port" 2>&1 &
+    SERVER_PID=$!
+    sleep 1
+    HTTP_PORT=$(cat "$TEST_ROOT/port")
+}
+
+# Function to verify a pattern does NOT appear in a log file
+expectNoMatch() {
+    local pattern=$1
+    local logfile=$2
+    local error_code=$3
+
+    if grep -q "$pattern" "$logfile"; then
+        echo "ERROR: $error_code error incorrectly says '$pattern'"
+        echo "Log contents:"
+        cat "$logfile"
+        exit 1
+    fi
+}
+
+cleanup() {
+    kill $SERVER_PID 2>/dev/null || true
+}
+trap cleanup EXIT
+
+# Start the HTTP server on a random port
+startHttpServer 200
+
+# Test 1: Normal operation (200 OK) should work
+clearStore
+nix-store --substituters "http://127.0.0.1:$HTTP_PORT" --no-require-sigs -r "$outPath" 2>&1 | tee "$TEST_ROOT/log-200"
+[ -x "$outPath/program" ]
+expectNoMatch "does not exist in binary cache" "$TEST_ROOT/log-200" "200"
+expectNoMatch "HTTP error" "$TEST_ROOT/log-200" "200"
+
+# Stop the server and restart with 404 errors
+kill $SERVER_PID
+sleep 1
+
+# Test 2: 404 errors should say "does not exist in binary cache"
+clearStore
+startHttpServer 404
+
+if nix-store --substituters "http://127.0.0.1:$HTTP_PORT" --no-require-sigs -r "$outPath" 2>&1 | tee "$TEST_ROOT/log-404"; then
+    echo "Expected substitution to fail with 404"
+    exit 1
+fi
+
+echo "=== 404 Log contents ==="
+cat "$TEST_ROOT/log-404"
+echo "=== End 404 Log ==="
+
+grepQuiet "does not exist in binary cache" "$TEST_ROOT/log-404"
+
+# Stop the server and restart with 401 errors
+kill $SERVER_PID
+sleep 1
+
+# Test 3: 401 errors should mention authentication and NOT say "does not exist"
+clearStore
+startHttpServer 401
+
+if nix-store --substituters "http://127.0.0.1:$HTTP_PORT" --no-require-sigs -r "$outPath" 2>&1 | tee "$TEST_ROOT/log-401"; then
+    echo "Expected substitution to fail with 401"
+    exit 1
+fi
+
+grepQuiet "HTTP error 401" "$TEST_ROOT/log-401"
+grepQuiet "access token" "$TEST_ROOT/log-401"
+
+# Verify it does NOT say "does not exist" for 401 errors
+expectNoMatch "does not exist in binary cache" "$TEST_ROOT/log-401" "401"
+
+# Stop the server and restart with 403 errors
+kill $SERVER_PID
+sleep 1
+
+# Test 4: 403 errors should also mention authentication and NOT say "does not exist"
+clearStore
+startHttpServer 403
+
+if nix-store --substituters "http://127.0.0.1:$HTTP_PORT" --no-require-sigs -r "$outPath" 2>&1 | tee "$TEST_ROOT/log-403"; then
+    echo "Expected substitution to fail with 403"
+    exit 1
+fi
+
+grepQuiet "HTTP error 403" "$TEST_ROOT/log-403"
+grepQuiet "access token" "$TEST_ROOT/log-403"
+
+# Verify it does NOT say "does not exist" for 403 errors
+expectNoMatch "does not exist in binary cache" "$TEST_ROOT/log-403" "403"
+
+echo "All HTTP error tests passed!"
diff --git a/tests/functional/binary-cache-auth-errors_test-http-server.py b/tests/functional/binary-cache-auth-errors_test-http-server.py
new file mode 100755
index 00000000000..278d6e55245
--- /dev/null
+++ b/tests/functional/binary-cache-auth-errors_test-http-server.py
@@ -0,0 +1,63 @@
+#!/usr/bin/env python3
+"""
+Test HTTP server for binary cache authentication error testing.
+
+This server serves files from a directory but can be configured to return
+specific HTTP status codes (401, 403, 404) for NAR files while still serving
+metadata files (.narinfo, nix-cache-info) normally.
+
+Environment variables:
+- CACHE_DIR: Directory to serve files from
+- RETURN_CODE: HTTP status code to return for NAR files (default: 200)
+- TEST_SERVER_PORT: Port to bind to (0 = random port, default)
+"""
+
+import http.server
+import socketserver
+import os
+import sys
+from urllib.parse import urlparse, parse_qs
+
+PORT = int(os.environ.get('TEST_SERVER_PORT', 0))
+CACHE_DIR = os.environ.get('CACHE_DIR', '')
+RETURN_CODE = int(os.environ.get('RETURN_CODE', 200))
+
+class TestHandler(http.server.SimpleHTTPRequestHandler):
+    def do_GET(self):
+        # Remove leading slash
+        path = self.path.lstrip('/')
+        file_path = os.path.join(CACHE_DIR, path)
+
+        # Check if we should return an error for NAR files
+        # Always serve nix-cache-info and .narinfo files normally
+        if RETURN_CODE != 200 and (path.startswith('nar/') or path.endswith('.nar') or path.endswith('.nar.xz')):
+            self.send_response(RETURN_CODE)
+            if RETURN_CODE == 401:
+                self.send_header('WWW-Authenticate', 'Bearer realm="test"')
+            self.send_header('Content-Length', '0')
+            self.end_headers()
+            return
+
+        # Otherwise, serve files from the cache directory
+        if os.path.isfile(file_path):
+            self.send_response(200)
+            self.send_header('Content-Type', 'application/octet-stream')
+            with open(file_path, 'rb') as f:
+                content = f.read()
+                self.send_header('Content-Length', str(len(content)))
+                self.end_headers()
+                self.wfile.write(content)
+        else:
+            self.send_response(404)
+            self.send_header('Content-Length', '0')
+            self.end_headers()
+
+    def log_message(self, format, *args):
+        # Suppress logging
+        pass
+
+# Bind to a random port if PORT is 0
+with socketserver.TCPServer(("127.0.0.1", PORT), TestHandler) as httpd:
+    # Print the actual port we're listening on
+    print(httpd.server_address[1], flush=True)
+    httpd.serve_forever()
diff --git a/tests/functional/meson.build b/tests/functional/meson.build
index d917d91c3f3..aaa3530c016 100644
--- a/tests/functional/meson.build
+++ b/tests/functional/meson.build
@@ -65,6 +65,7 @@ suites = [
       'user-envs.sh',
       'user-envs-migration.sh',
       'binary-cache.sh',
+      'binary-cache-auth-errors.sh',
       'multiple-outputs.sh',
       'nix-build.sh',
       'gc-concurrent.sh',