Make `pi -p` show what it’s doing (live output in print mode)

[prathamdby]

Right now pi -p feels like a black box: it runs the prompt, sits silently, then prints the final assistant text and exits. Other CLIs (e.g. OpenCode) stream realtime progress even in their “print” / non-interactive mode: partial assistant output, tool calls, tool output, errors, etc.

I want the same experience in pi, without breaking the current “final-only” behavior that people rely on in scripts.

Current behavior

• pi -p "prompt" (text mode): waits for completion, prints only the final assistant text, exits
• pi --mode json "prompt": prints the full JSON event stream in realtime

What I want

Add a streaming option to print mode that emits human-readable realtime output.

CLI behavior

• New flag: --stream (-s)
• Defaulting:
  • If stdout is a TTY and --stream is not specified: streaming should be auto-enabled
  • If stdout is non-TTY (piped/redirected) and --stream is not specified: keep the existing final-only output (backwards compatible)
• --mode json stays exactly as-is (raw JSON events)

Output format (human-readable)

• Plain text by default (minimal/no ANSI), with stable prefixes for non-text events:
  • [thinking] ...
  • [tool <name>] calling...
  • [tool <name>] output:
  • [error] ...
• Partial assistant text should stream as it arrives (append to stdout)
• Tool output should use sane truncation defaults:
  • truncate very large outputs
  • clearly mark truncation ([truncated])
  • if there’s a known “full output path” available, print it

Important: no duplicate final output

If partial assistant text is streamed, the final assistant response should not be printed again in full at the end. (No double-printing.)

Where this likely lives

• packages/coding-agent/src/cli/args.ts — add --stream / -s
• packages/coding-agent/src/main.ts — wire flag into print mode
• packages/coding-agent/src/modes/print-mode.ts — subscribe to session events and render them to stdout
• packages/coding-agent/test/* — add tests for streaming + deduplication
• packages/coding-agent/README.md — document --stream
• packages/coding-agent/CHANGELOG.md — Unreleased entry

Implementation sketch

1. Parse --stream / -s and decide:
   • streamingEnabled = parsed.stream || (process.stdout.isTTY && parsed.stream !== false)
2. In runPrintMode():
   • If streamingEnabled: subscribe to session events and print as they arrive
     • incremental text chunks → write to stdout
     • tool call start/end/results → prefixed lines/blocks
     • thinking/progress → brief lines
     • errors/aborts → stderr, non-zero exit on error
   • If not enabled: keep current final-only behavior
3. Ensure stdout is drained before exit (avoid losing the tail of streamed output)

Acceptance criteria

• pi -p --stream "prompt" streams:
  • partial assistant text live
  • tool calls + tool output in chronological order with clear prefixes
  • thinking/progress lines
  • errors/aborts to stderr; non-zero exit on error
  • no duplicate final assistant output
• pi -p "prompt" > out.txt stays final-only unless --stream is explicitly passed
• pi --mode json "prompt" unchanged
• Tests cover streaming and deduplication

Examples

• Human (live): pi -p --stream "summarize my repo"
• Script (unchanged): pi -p "summarize my repo" > result.txt
• JSON (unchanged): pi --mode json "summarize my repo"

[badlogic]

This can be an extension that calls shutdown on agent_end. I do not want to add yet another mode.

[patrick-kidger]

@prathamdby - If you take a look at #796 you'll find I was also after the same thing: the observable/human-readable/streaming behavior of interactive mode, coupled with the scriping usability of print mode.

This problem can be solved with what I have there: by launching in interactive mode, with an extension that calls ctx.shutdown once the agent has completed its response.

In my own personal wrapper for pi this enables me to separate out each piece separately — any combination of (streaming UI output/no streaming UI output) x (final result output/no final result output) x (waiting on completion/halting on completion) — rather than the way particular choices here are coupled together in default pi. I think it should work for you too!

[virtuald]

What about instead of a separate mode, just a separate command the renders the json mode output? pi -p --mode-json @whatever | pi render-json. Should be easy enough to keep it clean and isolated from the rest of the code?

... but probably could just be an npm package I suppose.

[yogibear54]

For anyone like me that landed here wanting to have a stream flag ... I created an extension to do that after seeing Mario's comment - https://github.com/yogibear54/my-own-pi-stuff/blob/main/extensions/stream-output/index.ts