diff --git a/scripts/remote.sh b/scripts/remote.sh
index e17de5846..6f2d0b9b9 100644
--- a/scripts/remote.sh
+++ b/scripts/remote.sh
@@ -285,6 +285,17 @@ cmd_doctor() {
 
 # --- shared HTTP helpers (B1: never put secrets in curl's own argv/ps) ---
 
+# _remote_curl_path <path> — render <path> for embedding INSIDE a curl -K config
+# file. On Windows/Git Bash, MSYS translates POSIX paths to Windows form only for
+# a native binary's argv, NOT for paths read from a config file's contents, so an
+# embedded /tmp or /c/.. path is unopenable by native curl (→ curl fails → the
+# caller's HTTP 000). cygpath -m yields a Windows drive path with FORWARD slashes;
+# -w is wrong here because curl's config parser treats backslashes as escapes.
+# Capability-gated on cygpath so macOS/Linux (no cygpath) are unchanged.
+_remote_curl_path() {
+  if command -v cygpath >/dev/null 2>&1; then cygpath -m "$1"; else printf '%s' "$1"; fi
+}
+
 # _remote_http_post_json <url> <body_file> <out_body_file> <out_header_file> -> prints http_code
 # Posts <body_file> as the request body via a curl -K config file, so the
 # body (which holds the token) never appears in curl's own argv/ps. The
@@ -311,11 +322,11 @@ _remote_http_post_json() {
     printf 'request = "POST"\n'
     printf 'header = "Content-Type: application/json"\n'
     printf 'header = "Agmsg-Protocol-Version: 1"\n'
-    printf 'dump-header = "%s"\n' "$header_fifo"
+    printf 'dump-header = "%s"\n' "$(_remote_curl_path "$header_fifo")"
     printf 'connect-timeout = "10"\n'
     printf 'max-time = "15"\n'
     printf 'max-filesize = "2097152"\n'
-    printf 'data = "@%s"\n' "$body_file"
+    printf 'data = "@%s"\n' "$(_remote_curl_path "$body_file")"
   } > "$cfg"
   if curl_output=$(curl -sS -o "$out_file" -w '%{http_code}' -K "$cfg" 2>/dev/null); then
     :
diff --git a/tests/test_remote_curl_config_paths.bats b/tests/test_remote_curl_config_paths.bats
new file mode 100644
index 000000000..ef5b208f1
--- /dev/null
+++ b/tests/test_remote_curl_config_paths.bats
@@ -0,0 +1,306 @@
+#!/usr/bin/env bats
+# PATHS EMBEDDED IN A CURL CONFIG FILE ARE NOT TRANSLATED FOR YOU (#850).
+#
+# `_remote_http_post_json` hands curl its arguments in a `-K` config file so the
+# request body -- which carries the token -- never appears in curl's argv. On
+# Windows that has a consequence the POSIX side never sees: MSYS rewrites POSIX
+# paths into Windows form for a native binary's ARGV, and does not touch the
+# CONTENTS of a file that binary reads. So `data = "@/tmp/x"` reaches native
+# curl as the literal string `/tmp/x`, which it cannot open. curl fails, and the
+# caller reports HTTP 000 -- a network-shaped symptom for a path-shaped fault.
+#
+# THE SAME BYTES, TWO CONSUMERS. That is what makes this hard to test honestly:
+# `/tmp/x` is openable by a POSIX curl and not by a native Windows one, so a
+# path string is only right or wrong RELATIVE TO WHO READS IT. The stubs here
+# are therefore capability-paired, and each test states which pairing it runs:
+#
+#   PATH        an allowlist sandbox -- cygpath is ABSENT because this file did
+#               not link it, not because the host happens to lack one. Each arm
+#               asserts the capability it relies on before relying on it.
+#   curl        told which consumer it is playing. As `native-windows` it
+#               refuses a POSIX path the way real curl.exe would; as `posix` it
+#               refuses a Windows path. Neither accepts both.
+#   cygpath     -m yields forward slashes, -w yields backslashes -- really both,
+#               because telling them apart is the whole reason the fix names -m.
+#
+# An earlier version of this file got the second point wrong: its stub accepted
+# any absolute POSIX path whatever the platform, so it could not have failed on
+# the untranslated path that broke the user, and the file's own header claimed
+# otherwise. Review caught it. What follows is written so that the claim and the
+# stub say the same thing.
+
+load test_helper
+
+# Externals `remote.sh` needs to source, plus those `_remote_http_post_json`
+# and the curl stub call. Derived by sourcing under an empty PATH and adding
+# what it asked for, then reading the function for the rest: mktemp, mkfifo,
+# chmod, python3 (the bounded-copy reader), rm, rmdir.
+#
+# An allowlist rather than a subtraction, matching `path_without_python3` in
+# test_helper. A subtraction cannot express "cygpath is absent" on a machine
+# where cygpath lives in the same directory as mktemp, which is every Git Bash.
+SANDBOX_TOOLS=(bash dirname mktemp mkfifo chmod rm rmdir sed cp cat grep python3 uname)
+
+setup() {
+  setup_test_env
+
+  CFG_CAPTURE="$BATS_TEST_TMPDIR/captured-config"
+  export CFG_CAPTURE
+
+  # The Windows drive prefix the fake cygpath maps onto. Any string works; what
+  # matters is that the stubs agree, so the curl stub can undo it exactly the
+  # way a native binary resolves a Windows path back to the same file.
+  FAKE_ROOT="C:/msys64"
+  export FAKE_ROOT
+
+  STUB_SRC="$BATS_TEST_TMPDIR/stubs"
+  mkdir -p "$STUB_SRC"
+
+  cat > "$STUB_SRC/curl" <<'STUB'
+#!/usr/bin/env bash
+# Stands in for whichever curl the config is destined for. STUB_CURL_CONSUMER
+# decides which one, and the two do NOT accept the same strings -- that
+# asymmetry is the defect being tested, so a stub without it proves nothing.
+set -u
+cfg=""; out=""; prev=""
+for arg in "$@"; do
+  case "$prev" in
+    -K) cfg="$arg" ;;
+    -o) out="$arg" ;;
+  esac
+  prev="$arg"
+done
+[ -n "$cfg" ] || { echo "STUB_CURL: no -K config" >&2; exit 2; }
+cp "$cfg" "$CFG_CAPTURE"
+
+# Resolve an embedded path to something this consumer can open, or fail.
+#
+#   native-windows  only a Windows path opens. A POSIX path is a literal
+#                   filename with no such directory -- the reported bug.
+#   posix           only a POSIX path opens. A Windows path is a filename
+#                   containing a colon, which is not a path here.
+resolve() {
+  case "$STUB_CURL_CONSUMER:$1" in
+    "native-windows:$FAKE_ROOT"/*) printf '%s' "/${1#"$FAKE_ROOT"/}" ;;
+    native-windows:*) return 1 ;;
+    posix:/*) printf '%s' "$1" ;;
+    posix:*) return 1 ;;
+    *) return 1 ;;
+  esac
+}
+
+data_field="$(sed -n 's/^data = "@\(.*\)"$/\1/p' "$cfg")"
+if [ -n "$data_field" ]; then
+  if ! body="$(resolve "$data_field")" || [ ! -r "$body" ]; then
+    echo "STUB_CURL($STUB_CURL_CONSUMER): cannot open data path: $data_field" >&2
+    exit 26
+  fi
+fi
+
+hdr_field="$(sed -n 's/^dump-header = "\(.*\)"$/\1/p' "$cfg")"
+if [ -n "$hdr_field" ]; then
+  if ! hdr="$(resolve "$hdr_field")"; then
+    echo "STUB_CURL($STUB_CURL_CONSUMER): cannot open dump-header path: $hdr_field" >&2
+    exit 23
+  fi
+  printf 'HTTP/1.1 200 OK\r\n\r\n' > "$hdr" || {
+    echo "STUB_CURL($STUB_CURL_CONSUMER): dump-header not writable: $hdr_field" >&2
+    exit 23
+  }
+fi
+
+[ -z "$out" ] || printf '{"ok":true}' > "$out"
+printf '200'
+STUB
+  chmod +x "$STUB_SRC/curl"
+
+  cat > "$STUB_SRC/cygpath" <<'STUB'
+#!/usr/bin/env bash
+set -u
+mode="$1"; path="$2"
+case "$mode" in
+  -m) printf '%s%s' "$FAKE_ROOT" "$path" ;;
+  -w) printf '%s%s' "${FAKE_ROOT//\//\\}" "${path//\//\\}" ;;
+  *) echo "stub cygpath: unexpected mode $mode" >&2; exit 64 ;;
+esac
+STUB
+  chmod +x "$STUB_SRC/cygpath"
+}
+
+teardown() { teardown_test_env; }
+
+# A PATH holding the allowlist, the curl stub, and cygpath only when asked.
+# Fails the test if the host is missing a tool: a silently short sandbox would
+# make the helper fail for a reason that has nothing to do with the fix.
+sandbox_path() {
+  local want_cygpath="$1" dir tool src
+  dir="$(mktemp -d "$BATS_TEST_TMPDIR/sandbox.XXXXXX")"
+  for tool in "${SANDBOX_TOOLS[@]}"; do
+    src="$(command -v "$tool")" || { echo "host lacks $tool" >&2; return 1; }
+    ln -s "$src" "$dir/$tool"
+  done
+  ln -s "$STUB_SRC/curl" "$dir/curl"
+  [ "$want_cygpath" = "yes" ] && ln -s "$STUB_SRC/cygpath" "$dir/cygpath"
+  printf '%s' "$dir"
+}
+
+# Runs the real helper under a sandbox PATH, and prints the http code. The
+# consumer the curl stub plays is named by the caller, so a test cannot
+# accidentally get a curl that accepts whatever the config happens to say.
+# STDERR GOES TO A FILE, NOT INTO $output. bats merges the two streams, and the
+# helper's stderr is not always empty: under load on CI, bash reported
+# "Interrupted system call" opening the header fifo, which turned an $output of
+# "000" into a diagnostic line followed by "000". Two cases failed on macOS CI
+# and passed here, on the same code -- the difference was timing, not platform.
+#
+# The http code is the only thing on stdout, so separating the streams is what
+# makes an exact comparison meaningful. It also matches the harness in the
+# stderr tests (#853), where the separation is the subject rather than an
+# accident.
+post_under() {
+  local bin="$1" consumer="$2" body_file="$3" extra="${4:-}"
+  ERR_FILE="$BATS_TEST_TMPDIR/helper-stderr"
+  run env PATH="$bin" STUB_CURL_CONSUMER="$consumer" CFG_CAPTURE="$CFG_CAPTURE" \
+    FAKE_ROOT="$FAKE_ROOT" bash -c '
+    set -uo pipefail
+    . '"$SCRIPTS"'/remote.sh 2>/dev/null
+    '"$extra"'
+    _remote_http_post_json "https://example.invalid/v1/x" "'"$body_file"'" \
+      "'"$BATS_TEST_TMPDIR"'/out-body" "'"$BATS_TEST_TMPDIR"'/out-header" 2>"'"$ERR_FILE"'"
+  '
+}
+
+@test "the sandbox decides whether cygpath exists — both directions (#850)" {
+  # The control on the instrument. Every arm below rests on the capability
+  # being what this file says it is, and on the host that is true by accident:
+  # macOS has no cygpath at all. If the sandbox were not really removing it,
+  # the negative arm would pass on any machine and fail on a Windows runner --
+  # which is the one machine this fix is for.
+  local without with
+  without="$(sandbox_path no)"
+  with="$(sandbox_path yes)"
+
+  run env PATH="$without" bash -c 'command -v cygpath >/dev/null 2>&1 && echo PRESENT || echo ABSENT'
+  [ "$output" = "ABSENT" ]
+
+  run env PATH="$with" bash -c 'command -v cygpath >/dev/null 2>&1 && echo PRESENT || echo ABSENT'
+  [ "$output" = "PRESENT" ]
+}
+
+@test "without cygpath the embedded paths are passed through byte for byte (#850)" {
+  # macOS and Linux, and the assertion that says the fix costs them nothing:
+  # the config must hold the exact strings the caller built.
+  local bin; bin="$(sandbox_path no)"
+  body="$BATS_TEST_TMPDIR/body.json"
+  printf '{"t":"secret"}' > "$body"
+
+  post_under "$bin" posix "$body"
+  [ "$status" -eq 0 ]
+  [ "$output" = "200" ]
+
+  # The data path is one this test chose, so it can be compared exactly.
+  grep -q -F -- "data = \"@$body\"" "$CFG_CAPTURE"
+
+  # The header path is generated inside the helper, so what is asserted is its
+  # shape: still POSIX, and untouched by any translation.
+  hdr="$(sed -n 's/^dump-header = "\(.*\)"$/\1/p' "$CFG_CAPTURE")"
+  [ -n "$hdr" ]
+  case "$hdr" in /*) : ;; *) echo "not a POSIX path: $hdr"; return 1 ;; esac
+  case "$hdr" in *"$FAKE_ROOT"*) echo "translated with no cygpath present: $hdr"; return 1 ;; esac
+}
+
+@test "with cygpath the DATA path is rendered in mixed Windows form (#850)" {
+  # Two fields, two tests, because they are two effects of one line and can
+  # regress apart. Asserting both in one test says "something is untranslated"
+  # -- true of either, pointing at neither.
+  local bin; bin="$(sandbox_path yes)"
+  body="$BATS_TEST_TMPDIR/body.json"
+  printf '{"t":"secret"}' > "$body"
+
+  post_under "$bin" native-windows "$body"
+  [ "$status" -eq 0 ]
+
+  # Deliberately NOT asserting the 200 here. The stub refuses to open either
+  # field's path when it is untranslated, so a request fails whichever half is
+  # wrong -- and asserting the outcome in both field tests made them both go
+  # red for either mutation, which is the attribution this split exists to get.
+  # The outcome has its own test below.
+  grep -q -F -- "data = \"@$FAKE_ROOT$body\"" "$CFG_CAPTURE"
+}
+
+@test "with cygpath the DUMP-HEADER path is rendered in mixed Windows form (#850)" {
+  # The half that is easy to leave behind: the body path is the one a reader
+  # thinks of as "the file", and the header sink is generated inside the helper.
+  # Translate one and not the other and curl opens the body, fails on the
+  # header, and the caller reports 000 -- which reads as a header problem.
+  local bin; bin="$(sandbox_path yes)"
+  body="$BATS_TEST_TMPDIR/body.json"
+  printf '{"t":"secret"}' > "$body"
+
+  post_under "$bin" native-windows "$body"
+  [ "$status" -eq 0 ]
+
+  # Config bytes only, for the reason given in the DATA case above.
+  hdr="$(sed -n 's/^dump-header = "\(.*\)"$/\1/p' "$CFG_CAPTURE")"
+  case "$hdr" in "$FAKE_ROOT"/*) : ;; *) echo "header path not translated: $hdr"; return 1 ;; esac
+}
+
+@test "with both fields rendered, the request actually completes (#850)" {
+  # The outcome the two field tests deliberately leave alone. A native curl has
+  # to be able to open BOTH paths for the call to return a code at all, so this
+  # is the one assertion that fails for either field -- and that is its job:
+  # whatever regresses, the user's request stops working.
+  local bin; bin="$(sandbox_path yes)"
+  body="$BATS_TEST_TMPDIR/body.json"
+  printf '{"t":"secret"}' > "$body"
+
+  post_under "$bin" native-windows "$body"
+  [ "$status" -eq 0 ]
+  [ "$output" = "200" ]
+}
+
+@test "the rendered paths carry forward slashes, never backslashes (#850)" {
+  # `cygpath -w` also produces a valid Windows path, and it is the wrong one:
+  # curl's config parser reads a backslash as an escape, so the path arrives
+  # corrupted. Written against the config text rather than against the flag,
+  # because what breaks a user is the bytes curl parses.
+  local bin; bin="$(sandbox_path yes)"
+  body="$BATS_TEST_TMPDIR/body.json"
+  printf '{"t":"secret"}' > "$body"
+
+  post_under "$bin" native-windows "$body"
+  [ "$status" -eq 0 ]
+
+  refute grep -q '\\' "$CFG_CAPTURE"
+}
+
+@test "an untranslated POSIX path reaching native curl is the reported 000 (#850)" {
+  # The defect itself, and the control on the stub. `_remote_curl_path` is
+  # replaced AFTER sourcing, so the production helper is still the one under
+  # test -- only its renderer is reduced to the passthrough it was before the
+  # fix. The consumer is the native Windows one, which cannot open /tmp/...
+  #
+  # Without this case the assertions above could all be passing on a config no
+  # curl would accept, which is exactly how the previous version of this file
+  # was wrong.
+  local bin; bin="$(sandbox_path yes)"
+  body="$BATS_TEST_TMPDIR/body.json"
+  printf '{"t":"secret"}' > "$body"
+
+  post_under "$bin" native-windows "$body" '_remote_curl_path() { printf "%s" "$1"; }'
+  [ "$status" -eq 0 ]
+  [ "$output" = "000" ]
+}
+
+@test "a Windows path reaching a POSIX curl is equally a 000 (#850)" {
+  # The mirror, so "native-windows refuses POSIX paths" is not read as "this
+  # stub refuses whatever the test needs it to". Each consumer refuses the
+  # other's form, and the fix is what puts the right form in front of each.
+  local bin; bin="$(sandbox_path no)"
+  body="$BATS_TEST_TMPDIR/body.json"
+  printf '{"t":"secret"}' > "$body"
+
+  post_under "$bin" posix "$body" '_remote_curl_path() { printf "%s%s" "$FAKE_ROOT" "$1"; }'
+  [ "$status" -eq 0 ]
+  [ "$output" = "000" ]
+}