Claude Code stream-json protocol

How internal/claude drives the claude CLI. Worked out from the Agent SDKs (@anthropic-ai/claude-agent-sdk on npm, claude-agent-sdk on PyPI); the Python SDK's _internal/query.py and _internal/transport/subprocess_cli.py are the most readable references. The CLI version the tests last passed against is in the system/init event of any transcript under internal/claude/testdata/transcripts/ (claude_code_version).

Launch

claude --output-format stream-json --input-format stream-json --verbose \
  --permission-mode bypassPermissions --allow-dangerously-skip-permissions \
  --settings '{"attribution":{"commit":"","pr":""}}' \
  --mcp-config '{"mcpServers":{"byre":{"type":"sdk","name":"byre"}}}' \
  [--model M] [--resume=SESSION_ID] [--fork-session]

No --print; stream-json input implies non-interactive.
--resume= uses the equals form so a dash-leading value can't be parsed as a flag.
Environment: CLAUDECODE is removed so the child doesn't think it is nested; GIT_AUTHOR_*/GIT_COMMITTER_* set the commit identity; attribution set to empty strings removes the co-author trailer.

Framing

NDJSON both ways: one JSON object per line on stdin and stdout. Non-JSON stdout lines are ignored. Stdin EOF ends the process.

Messages

User turn, written to stdin: {"type":"user","message":{"role":"user","content":"..."},"parent_tool_use_id":null,"session_id":"..."}.
Stdout events: system (subtype init carries session_id, model, tools, MCP server status), assistant and user (Anthropic message content blocks: text, tool_use, tool_result), result (ends a turn; subtype is success or error_during_execution after an interrupt, with cost, duration, turn count).
The init event arrives with the first turn, not at launch, so the session ID is unknown until the first message is sent.

Control channel

Either side sends {"type":"control_request","request_id":ID,"request":{"subtype":...}} and the other answers {"type":"control_response","response":{"subtype":"success"|"error","request_id":ID,"response":{...},"error":"..."}}.

Host → CLI:

initialize must be first. It carries hook registrations: {"hooks":{"Stop":[{"matcher":null,"hookCallbackIds":["hook_0"]}],...}}.
interrupt ends the current turn; the turn's result follows almost immediately and a running Bash tool is killed. The session stays usable.
set_model, set_permission_mode, mcp_status, get_context_usage, rewind_files, stop_task exist and are not used yet.

CLI → host:

hook_callback with callback_id and input (the hook's input JSON). The response body is the hook output. A Stop hook returning {"decision":"block","reason":"..."} keeps the agent going with the reason fed back to it.
mcp_message with server_name and a JSON-RPC message for an sdk-type MCP server; answer with {"mcp_response": <JSON-RPC response>}. Notifications still need a response.
can_use_tool only arrives when a permission prompt would show; under bypassPermissions it should not.
control_cancel_request abandons an in-flight request.

MCP tools are deferred

The CLI hides MCP tool schemas behind its ToolSearch tool: an agent calls ToolSearch with select:mcp__byre__report before it can call report, costing an extra model turn every session.

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