Input Messages (stdin)

Undocumented Protocol

The --input-format stream-json stdin protocol is officially undocumented. The following is reverse-engineered from working implementations. It may change without notice.

These NDJSON messages are written to Claude Code's stdin when using --input-format stream-json. Each message is a single JSON object followed by a newline (\n).

User Message

Send a chat message to Claude.

{"type":"user","message":{"role":"user","content":"What does this project do?"}}

Field	Type	Description
type	"user"	Message type
message.role	"user"	Always "user"
message.content	string	The message text

With createMessage:

claude.stdin.write(createMessage.user('What does this project do?'))

Tool Approval

Approve a pending tool execution. Send this after receiving a tool_use event.

{"type":"approve","tool_use_id":"toolu_abc123"}

Field	Type	Description
type	"approve"	Approval type
tool_use_id	string	The id from the tool_use content block

With createMessage:

claude.stdin.write(createMessage.approve('toolu_abc123'))

Tool Denial

Deny a pending tool execution.

{"type":"deny","tool_use_id":"toolu_abc123"}

With createMessage:

claude.stdin.write(createMessage.deny('toolu_abc123'))

Tool Result with Content

Send a tool result with user-provided content. Used for interactive tools like AskUserQuestion.

{"type":"tool_result","tool_use_id":"toolu_abc123","content":"The user selected option B"}

Field	Type	Description
type	"tool_result"	Result type
tool_use_id	string	The id from the tool_use content block
content	string	The result content

With createMessage:

claude.stdin.write(createMessage.toolResult('toolu_abc123', 'The user selected option B'))

Abort (Mid-Turn)

Mid-turn abort is not done via stdin — it requires sending SIGINT to the Claude Code process.

import { kill } from 'process'

// Send SIGINT to abort the current turn
kill(claude.pid!, 'SIGINT')

WARNING

SIGINT aborts the current turn but does not kill the process. Claude Code handles it gracefully and waits for the next stdin message.

Message Flow Example

YOUR CODE                           CLAUDE CODE
   │                                     │
   │  user message ──────────────────>   │
   │                                     │  ← processes, starts thinking
   │   <──────────────── system/init     │
   │   <──────────────── assistant       │  (thinking)
   │   <──────────────── assistant       │  (thinking + text)
   │   <──────────────── assistant       │  (thinking + text + tool_use)
   │                                     │
   │  approve ───────────────────────>   │
   │                                     │  ← executes tool
   │   <──────────────── user            │  (tool_result)
   │   <──────────────── assistant       │  (new text)
   │   <──────────────── result          │  (turn complete)
   │                                     │