shell: don't let blank "cmd" shadow "command" alias

cmp.Or treats any non-empty string as non-zero, so a whitespace-only
"cmd" would win over a valid "command" alias and trip the empty-arg
guard with a misleading error. Select the alias when "cmd" is blank.

Addresses PR #2481 review comments.

Assisted-By: docker-agent

Author: trungutt

pkg/tools/builtin/shell.go

@@ -110,7 +110,9 @@ type RunShellArgs struct {
 // models (particularly ones biased by Anthropic's built-in bash tool and other
 // ecosystems that use "command") occasionally emit "command" instead. Accepting
 // both prevents a wasted turn on an empty-command error while keeping the
-// canonical contract unchanged. When both keys are present, "cmd" wins.
+// canonical contract unchanged. When "cmd" is present with a non-blank value
+// it wins; a blank (empty or whitespace-only) "cmd" falls back to "command"
+// so a valid alias is not silently shadowed.
 func (a *RunShellArgs) UnmarshalJSON(data []byte) error {
 	var raw struct {
 		Cmd     string `json:"cmd"`
@@ -121,7 +123,7 @@ func (a *RunShellArgs) UnmarshalJSON(data []byte) error {
 	if err := json.Unmarshal(data, &raw); err != nil {
 		return err
 	}
-	a.Cmd = cmp.Or(raw.Cmd, raw.Command)
+	a.Cmd = preferNonBlank(raw.Cmd, raw.Command)
 	a.Cwd = raw.Cwd
 	a.Timeout = raw.Timeout
 	return nil
@@ -143,11 +145,22 @@ func (a *RunShellBackgroundArgs) UnmarshalJSON(data []byte) error {
 	if err := json.Unmarshal(data, &raw); err != nil {
 		return err
 	}
-	a.Cmd = cmp.Or(raw.Cmd, raw.Command)
+	a.Cmd = preferNonBlank(raw.Cmd, raw.Command)
 	a.Cwd = raw.Cwd
 	return nil
 }
 
+// preferNonBlank returns primary when it has a non-whitespace character;
+// otherwise it returns fallback. The chosen value is returned unmodified so
+// that whitespace inside a legitimate command (e.g. trailing newlines in a
+// heredoc) is preserved.
+func preferNonBlank(primary, fallback string) string {
+	if strings.TrimSpace(primary) != "" {
+		return primary
+	}
+	return fallback
+}
+
 type ViewBackgroundJobArgs struct {
 	JobID string `json:"job_id" jsonschema:"The ID of the background job to view"`
 }

pkg/tools/builtin/shell_test.go

@@ -85,6 +85,16 @@ func TestRunShellArgs_UnmarshalJSON_AcceptsCmdAndCommand(t *testing.T) {
 			input:   `{"cmd":"from-cmd","command":"from-command"}`,
 			wantCmd: "from-cmd",
 		},
+		{
+			name:    "blank cmd falls back to command alias",
+			input:   `{"cmd":"   ","command":"from-command"}`,
+			wantCmd: "from-command",
+		},
+		{
+			name:    "empty cmd falls back to command alias",
+			input:   `{"cmd":"","command":"from-command"}`,
+			wantCmd: "from-command",
+		},
 		{
 			name:    "empty object leaves cmd empty",
 			input:   `{}`,
@@ -115,6 +125,11 @@ func TestRunShellBackgroundArgs_UnmarshalJSON_AcceptsCmdAndCommand(t *testing.T)
 	var viaCommand RunShellBackgroundArgs
 	require.NoError(t, json.Unmarshal([]byte(`{"command":"sleep 1"}`), &viaCommand))
 	assert.Equal(t, "sleep 1", viaCommand.Cmd)
+
+	// A blank "cmd" must not shadow a valid "command" alias.
+	var blankCmd RunShellBackgroundArgs
+	require.NoError(t, json.Unmarshal([]byte(`{"cmd":"   ","command":"sleep 1"}`), &blankCmd))
+	assert.Equal(t, "sleep 1", blankCmd.Cmd)
 }
 
 // Exercises the end-to-end dispatch path: a tool-call whose raw arguments