diff --git a/cmd/harnesscli/go_code_script_test.go b/cmd/harnesscli/go_code_script_test.go
index 30846ab8..4a5a8583 100644
--- a/cmd/harnesscli/go_code_script_test.go
+++ b/cmd/harnesscli/go_code_script_test.go
@@ -528,3 +528,75 @@ func describeStuckProcesses(wrapperPID int) string {
 	}
 	return b.String()
 }
+
+// TestGoCodeScriptRejectsExtraPromptArguments pins issue #1435: the wrapper
+// must not silently discard part of the user's prompt.
+//
+// `prompt="$1"` took only the first positional, so `go-code explain this repo`
+// — the natural unquoted form — ran against `-prompt explain` and threw the
+// rest away with no warning and exit 0. The run looked successful and a short
+// prompt often still produces plausible output, so the truncation could go
+// unnoticed indefinitely.
+//
+// The assertion is on the user-visible consequence: either the whole prompt
+// reaches harnesscli, or the wrapper refuses and says why.
+func TestGoCodeScriptRejectsExtraPromptArguments(t *testing.T) {
+	scriptPath, err := filepath.Abs(filepath.Join("..", "..", "scripts", "go-code.sh"))
+	if err != nil {
+		t.Fatalf("resolve go-code script path: %v", err)
+	}
+
+	newStubs := func(t *testing.T, recordFile string) string {
+		t.Helper()
+		binDir := t.TempDir()
+		writeExecutable(t, filepath.Join(binDir, "curl"), "#!/usr/bin/env bash\nexit 0\n")
+		writeExecutable(t, filepath.Join(binDir, "harnessd"), "#!/usr/bin/env bash\nexit 0\n")
+		writeExecutable(t, filepath.Join(binDir, "harnesscli"), "#!/usr/bin/env bash\nprintf '%s\\n' \"$*\" >> \"$RECORD_FILE\"\n")
+		return binDir
+	}
+
+	t.Run("extra positional arguments are refused", func(t *testing.T) {
+		tmp := t.TempDir()
+		recordFile := filepath.Join(tmp, "harnesscli.args")
+		binDir := newStubs(t, recordFile)
+
+		cmd := exec.Command("bash", scriptPath, "explain this repo", "and also this")
+		cmd.Env = append(os.Environ(),
+			"PATH="+binDir+string(os.PathListSeparator)+os.Getenv("PATH"),
+			"HARNESS_ADDR=:19910", "RECORD_FILE="+recordFile)
+		out, err := cmd.CombinedOutput()
+
+		if err == nil {
+			t.Fatalf("expected a non-zero exit when extra prompt arguments are given, got success\n%s", out)
+		}
+		if raw, statErr := os.ReadFile(recordFile); statErr == nil && len(raw) > 0 {
+			t.Fatalf("harnesscli was invoked with a truncated prompt instead of the wrapper refusing: %q", raw)
+		}
+		if !bytes.Contains(out, []byte("and also this")) {
+			t.Errorf("refusal should name the offending argument so the user can see what was dropped, got:\n%s", out)
+		}
+	})
+
+	// Control: the normal path must keep working, or a fix that refuses
+	// everything would satisfy the assertion above.
+	t.Run("a single quoted prompt still passes through whole", func(t *testing.T) {
+		tmp := t.TempDir()
+		recordFile := filepath.Join(tmp, "harnesscli.args")
+		binDir := newStubs(t, recordFile)
+
+		cmd := exec.Command("bash", scriptPath, "explain this repo")
+		cmd.Env = append(os.Environ(),
+			"PATH="+binDir+string(os.PathListSeparator)+os.Getenv("PATH"),
+			"HARNESS_ADDR=:19911", "RECORD_FILE="+recordFile)
+		if out, err := cmd.CombinedOutput(); err != nil {
+			t.Fatalf("single quoted prompt should succeed: %v\n%s", err, out)
+		}
+		raw, err := os.ReadFile(recordFile)
+		if err != nil {
+			t.Fatalf("read record file: %v", err)
+		}
+		if !bytes.Contains(raw, []byte("-prompt explain this repo")) {
+			t.Errorf("whole prompt should reach harnesscli, got %q", raw)
+		}
+	})
+}
diff --git a/docs/logs/engineering-log.md b/docs/logs/engineering-log.md
index 2aa892d2..44cf43d9 100644
--- a/docs/logs/engineering-log.md
+++ b/docs/logs/engineering-log.md
@@ -1,5 +1,35 @@
 # Engineering Log
 
+## 2026-09-08 — Issue #1435 go-code silently dropped prompt arguments
+
+- Symptom: `go-code explain this repo` — the natural unquoted form — ran against
+  `-prompt explain` and discarded the rest, with no warning and exit 0. Proven
+  with stubs: `go-code "explain this repo" "and also this"` reached harnesscli
+  as `-prompt explain this repo`.
+- Cause: the prompt branch of the argument `case` assigned `prompt="$1"` and
+  never read `$2` onward. The CLI branch beside it captures the rest with
+  `cli_args=("$@")`, and the flag branches `die` on anything unknown — prompt
+  mode was the only path that accepted input and threw part of it away.
+- Why it stayed hidden: the run looks successful, and a truncated prompt often
+  still produces plausible output, so nothing signals the loss. No test covered
+  multi-argument prompt mode.
+- Fix: refuse, and name the fix. `go-code explain this repo` now exits non-zero
+  with `unexpected extra argument: this. Quote the whole prompt as one argument,
+  e.g. go-code "explain this repo"` — the message reconstructs the correct
+  command rather than only complaining.
+- Joining the arguments was considered and rejected: it guesses at intent and
+  silently reinterprets a shell-splitting mistake, where refusing teaches the
+  rule once.
+- Test: `TestGoCodeScriptRejectsExtraPromptArguments`, with a control asserting
+  a single quoted prompt still passes through whole — otherwise a fix that
+  refused everything would satisfy the first assertion.
+- Provenance: found by the untrusted external reviewer (`gpt-6-astra` via the
+  Surplus proxy) reading only the script, then confirmed by running the stub
+  reproduction rather than by reading. Six findings on this file, of which two
+  were confirmed, three were legitimate low-severity hardening (PID reuse in
+  `stop_server`, predictable tmp paths, unvalidated `HARNESS_ADDR`), and one was
+  rejected as speculative.
+
 ## 2026-09-08 — Issue #1432 wide labels destroyed the cancel hint
 
 - Symptom: `shortenLabel` guarantees the `(esc to interrupt)` hint survives at
diff --git a/scripts/go-code.sh b/scripts/go-code.sh
index 3c196f3f..5e4274a8 100755
--- a/scripts/go-code.sh
+++ b/scripts/go-code.sh
@@ -405,6 +405,16 @@ main() {
     *)
       mode="prompt"
       prompt="$1"
+      shift
+      # Refuse rather than silently truncate. `prompt="$1"` alone discarded
+      # every later positional, so the natural unquoted form —
+      # `go-code explain this repo` — ran against `-prompt explain` and threw
+      # the rest away with exit 0. A short prompt still produces plausible
+      # output, so the loss was invisible. Joining the words instead would guess
+      # at intent; refusing teaches the rule once. Issue #1435.
+      if [[ $# -gt 0 ]]; then
+        die "unexpected extra argument: $1. Quote the whole prompt as one argument, e.g. go-code \"${prompt} $*\""
+      fi
       ;;
   esac