feat: add CLI backend fallback

src/config/types.ts

@@ -1355,6 +1355,45 @@ export type AgentContextPruningConfig = {
   };
 };
 
+export type CliBackendConfig = {
+  /** CLI command to execute (absolute path or on PATH). */
+  command: string;
+  /** Base args applied to every invocation. */
+  args?: string[];
+  /** Output parsing mode (default: json). */
+  output?: "json" | "text";
+  /** Prompt input mode (default: arg). */
+  input?: "arg" | "stdin";
+  /** Max prompt length for arg mode (if exceeded, stdin is used). */
+  maxPromptArgChars?: number;
+  /** Extra env vars injected for this CLI. */
+  env?: Record<string, string>;
+  /** Env vars to remove before launching this CLI. */
+  clearEnv?: string[];
+  /** Flag used to pass model id (e.g. --model). */
+  modelArg?: string;
+  /** Model aliases mapping (config model id → CLI model id). */
+  modelAliases?: Record<string, string>;
+  /** Flag used to pass session id (e.g. --session-id). */
+  sessionArg?: string;
+  /** When to pass session ids. */
+  sessionMode?: "always" | "existing" | "none";
+  /** JSON fields to read session id from (in order). */
+  sessionIdFields?: string[];
+  /** Flag used to pass system prompt. */
+  systemPromptArg?: string;
+  /** System prompt behavior (append vs replace). */
+  systemPromptMode?: "append" | "replace";
+  /** When to send system prompt. */
+  systemPromptWhen?: "first" | "always" | "never";
+  /** Flag used to pass image paths. */
+  imageArg?: string;
+  /** How to pass multiple images. */
+  imageMode?: "repeat" | "list";
+  /** Serialize runs for this CLI. */
+  serialize?: boolean;
+};
+
 export type AgentDefaultsConfig = {
   /** Primary model and fallbacks (provider/model). */
   model?: AgentModelListConfig;
@@ -1370,6 +1409,8 @@ export type AgentDefaultsConfig = {
   userTimezone?: string;
   /** Optional display-only context window override (used for % in status UIs). */
   contextTokens?: number;
+  /** Optional CLI backends for text-only fallback (claude-cli, etc.). */
+  cliBackends?: Record<string, CliBackendConfig>;
   /** Opt-in: prune old tool results from the LLM context to reduce token usage. */
   contextPruning?: AgentContextPruningConfig;
   /** Default thinking level when no /think directive is present. */