Session options

function buildSessionOptions<TConfig extends BaseSessionConfig>(
  config: TConfig,
  runDir: string,
  spec: AgentSpec<TConfig>,
  options?: { autoCompactEnabled?: boolean; run?: RunFolder },
): Promise<{
  options: Options; // for the SDK's query()
  transcriptLogger: TranscriptLogger;
  transcriptPath: string; // <runDir>/transcript.jsonl
  modeControl: ModeControl;
  language: Language; // the resolved language
}>;

• config: your session config.
• runDir: this run's folder. The transcript, the response file and the kit's MCP servers use it. With a runs folder, pass the folder the chat gives your opener (run.dir).
• spec: your AgentSpec.
• options.autoCompactEnabled: the SDK's automatic context compaction; true by default.
• options.run: keeps the SDK's transcript in the run folder and resumes its session if it has one. See Runs and resuming.

What goes into Options

System prompt

spec.buildSystemPrompt(config), then the knowledge base section if config.knowledgeDir is set and spec.knowledgeBase !== false, then the reply language line unless spec.replyInLanguage === false.

Tools

tools (what exists) and allowedTools (what's pre-approved) are the same list, built from the configuration:

Condition	Tools added
always	WebFetch, WebSearch, TodoWrite
knowledgeDir or sourcesDir	Read, Write, Edit, Glob, Grep; with the built-in knowledge base reached through its tools (the default), only Read, Glob, Grep for sourcesDir and extraWritableDirs, plus Write, Edit for the latter
file tools or at least one plugin	Skill
spec.buildSubagents() returns something	Agent, Bash

TodoWrite is the SDK's task list for long jobs (the session's env is process.env plus CLAUDE_CODE_ENABLE_TASKS=0, without which the CLI replaces it with its own Task tools): the chats show the list instead of the calls (see Ink chat). Nothing else of the CLI's built-in tools is available. Bash is there only because the SDK won't spawn a subagent whose own tools aren't in the session's; a hook denies it to the main agent.

MCP servers and permissions

mcpServers holds spec.buildMcpServers(config, runDir) plus the kit's own, when they apply:

Server	Tool	When
approvals	request_human_approval, ask_human, present_plan	mode is not autonomous
manualLogin	request_manual_login	mode is not autonomous and spec.manualInterventionTexts is set
sourceFiles	list_sources, extract_text, save_to_sources, download_to_sources, and request_file, retire_source outside autonomous mode	config.sourcesDir is set
time	current_time, date_math	always (in config.timeZone, or the system's)
knowledge	the knowledge_* tools (knowledge_retire outside autonomous mode)	the built-in knowledge base, unless spec.knowledgeTools is "files"

canUseTool is allowAnyMcpTool: every mcp__* call is approved, anything else is denied (the built-in tools above are already pre-approved, so they never reach it). disallowedTools is spec.disallowedTools ?? [] and wins over everything.

Hooks

PreToolUse, in this order:

1. the transcript logger;
2. the file scope gate, with file tools;
3. the step gate, outside autonomous mode, active only while the mode is interactive;
4. the plan gate, likewise outside autonomous mode, active only while the mode is plan (see modes);
5. with subagents, the three subagent gates: type, Bash, foreground.

PostToolUse: the transcript logger.

The result also has knowledgeStore, the knowledge base's store, when the agent reaches it through its tools (see Knowledge store).

Skills, plugins and the working directory

When the session has file tools or at least one plugin:

• cwd is config.projectDir;
• additionalDirectories are the searchable folders (knowledgeDir, sourcesDir, extraWritableDirs);
• plugins are spec.pluginRoots(config), plus the knowledge plugin when the knowledge base is on, each loaded as a local plugin with its MCP discovery skipped;
• skills is spec.skills ?? "all"; a list gets the knowledge base's skills added when it's on.

Otherwise the SDK's defaults apply (the process's working directory, no plugins).

Settings

Option	Value	Why
settingSources	spec.settingSources ?? ["project"]	the runner's own Claude Code settings never reach the agent unless asked for
settings.autoCompactEnabled	options.autoCompactEnabled ?? true	long chats keep working
settings.autoMemoryEnabled	false	the runner's Claude Code memory is not the agent's
settings.includeGitInstructions	false	no kit agent commits code, and the git context skewed the reply language
includePartialMessages	true	replies stream token by token
maxTurns	400	a generous cap per query

See Permissions and isolation for the reasons.

Subagents

agents is what spec.buildSubagents(config) returned, with the reply language line appended to each subagent's prompt.

Runs

With options.run: sessionStore is a run store for run.dir, and resume is run.sessionId when it has one.

Adding your own options

The result is a plain object: spread it to add or override anything the kit doesn't set for you.

const { options } = await buildSessionOptions(config, runDir, spec);

const session: Options = {
  ...options,
  model: "claude-sonnet-5",
  maxTurns: 50,
  hooks: {
    ...options.hooks,
    PostToolUse: [...(options.hooks?.PostToolUse ?? []), { hooks: [myAuditHook] }],
  },
};

Keep the kit's hooks when you add yours: replacing hooks wholesale would drop every guardrail. See Your own hooks.

Calling it more than once

Every call builds a fresh set of servers and hooks bound to its runDir. Build the options once per session, and again for another run folder (that's what a session opener does on /resume). The resolved language, the mode control and the transcript logger belong to that call.