RFD 010: PTY Infrastructure and Interactive Tool SDK

• Status: Discussion
• Category: Design
• Authors: Jean Mertz git@jeanmertz.com
• Date: 2026-02-23

Summary

This RFD introduces a jp_pty crate for managing pseudo-terminal sessions and a jp_tool::interactive SDK module for building stateful tools that drive interactive CLI programs. Together they provide the infrastructure that tool authors need to wrap programs like git add --patch, vim, or a persistent shell session — exposing them through the stateful tool protocol defined in RFD 009.

Motivation

RFD 009 defines the protocol for stateful tools: the ToolState state machine, ToolCommand dispatch, handle registry, and per-tool action sets. But it deliberately leaves open how a tool manages an interactive subprocess. A tool author who wants to build a git tool with interactive staging support needs to:

1. Spawn git add --patch in a pseudo-terminal (PTY) so it behaves as if a human is at the keyboard.
2. Read the terminal screen as text (not raw escape sequences) to return as ToolState::Running { content }.
3. Write keystrokes to the PTY when the assistant sends Apply.
4. Detect when the process exits and return ToolState::Stopped.
5. Implement the AnyToolHandle trait so JP's handle registry can manage the session.

Each of these steps involves low-level systems programming: PTY creation, process forking, terminal emulation, non-blocking I/O. Without shared infrastructure, every tool author would reimplement this from scratch.

This RFD provides two layers:

• jp_pty: A standalone crate that handles PTY lifecycle and terminal emulation. No dependency on JP's tool system — it can be used independently (e.g., for PTY-based testing).
• jp_tool::interactive: An SDK module that bridges jp_pty to the stateful tool protocol. It provides the AnyToolHandle implementation, screen-to-content conversion, and helpers for defining tool commands.

Design

User experience (tool author)

A tool author building an interactive git tool writes something like:

use jp_tool::interactive::{InteractiveTool, Command, PtyHandle};

pub struct GitTool;

impl InteractiveTool for GitTool {
    fn commands(&self) -> Vec<Command> {
        vec![
            Command::new("stage")
                .description("Interactive staging with --patch")
                .accepts_input(true),
            Command::new("rebase")
                .description("Interactive rebase")
                .accepts_input(true),
            Command::new("log")
                .description("View git log")
                .accepts_input(false),
        ]
    }

    fn spawn(&self, command: &str, args: &[String], root: &Path) -> Result<PtyHandle> {
        let mut cmd = vec!["git"];
        match command {
            "stage" => cmd.extend(["add", "--patch"]),
            "rebase" => cmd.extend(["rebase", "--interactive"]),
            "log" => cmd.extend(["log", "--oneline", "-20"]),
            _ => return Err(/* unknown command */),
        };
        cmd.extend(args.iter().map(|s| s.as_str()));

        PtyHandle::spawn(cmd, root, PtyConfig::default())
    }
}

The SDK handles:

• Generating the oneOf JSON schema with action variants from commands()
• Creating the AnyToolHandle implementation that maps Fetch → screen read, Apply → PTY write, Abort → SIGTERM/SIGKILL
• Screen text extraction from the terminal emulator
• Activity detection (waiting for output to settle before capturing)

The tool author focuses on what commands their tool supports and how to translate them into shell commands. The PTY mechanics are invisible.

jp_pty crate

A standalone crate with no dependency on JP's tool system. It provides three things: PTY session management, terminal emulation, and screen text extraction.

Session

A Session represents a running program in a pseudo-terminal:

pub struct Session {
    master_fd: OwnedFd,
    child_pid: Pid,
    emulator: Emulator,
    config: PtyConfig,
}

pub struct PtyConfig {
    pub rows: u16,       // default: 24
    pub cols: u16,       // default: 80
    pub term: String,    // default: "xterm-256color"
}

impl Session {
    /// Spawn a command in a new PTY session.
    pub fn spawn(cmd: &[&str], cwd: &Path, config: PtyConfig) -> Result<Self>;

    /// Write bytes to the PTY (keystrokes, input data).
    pub fn write(&self, data: &[u8]) -> Result<()>;

    /// Read the current screen as plain text.
    ///
    /// Processes any pending PTY output through the terminal emulator
    /// before capturing the screen.
    pub fn screen_text(&mut self) -> String;

    /// Check if the child process is still running.
    /// Returns the exit code if it has exited.
    pub fn try_wait(&mut self) -> Option<i32>;

    /// Wait for new output activity, up to a timeout.
    ///
    /// Returns `true` if new output arrived, `false` on timeout.
    pub fn wait_for_activity(&mut self, timeout: Duration) -> bool;

    /// Send SIGTERM, wait for grace period, then SIGKILL if needed.
    pub fn kill(&mut self, grace: Duration);

    /// Resize the terminal.
    pub fn resize(&mut self, rows: u16, cols: u16) -> Result<()>;
}

PTY creation and process spawning

The session creation flow:

1. openpty() with the configured Winsize creates a master/slave pair.
2. Fork the process (see Fork safety below).
3. In the child: setsid(), redirect stdin/stdout/stderr to the slave fd, set the slave as controlling terminal via TIOCSCTTY, set $TERM, exec() the command.
4. In the parent: close the slave fd, set master fd to non-blocking, spawn a background reader task.

The background reader continuously reads from the master fd and feeds bytes to the terminal emulator. This happens on a dedicated thread (not a tokio task) because PTY reads are blocking I/O that shouldn't occupy the async runtime.

// Background reader (runs on a dedicated thread)
fn reader_loop(master_fd: RawFd, emulator: Arc<Mutex<Emulator>>) {
    let mut buf = [0u8; 4096];
    loop {
        match nix::unistd::read(master_fd, &mut buf) {
            Ok(0) => break,        // PTY closed
            Ok(n) => {
                emulator.lock().process(&buf[..n]);
            }
            Err(Errno::EAGAIN) => {
                // Non-blocking read, no data — poll/sleep
                poll_for_input(master_fd, Duration::from_millis(50));
            }
            Err(_) => break,
        }
    }
}

Terminal emulation

The emulator processes raw PTY output (ANSI escape sequences, cursor movement, scrolling, alternate screen buffer, etc.) and maintains a character grid. The screen_text() method extracts the grid as plain text with trailing whitespace trimmed per line.

Recommended crate: alacritty_terminal. It provides battle-tested VT100/ xterm-256color parsing and a complete screen buffer. It handles the full range of escape sequences that modern programs emit.

Fallback: vt100. Lighter weight, smaller dependency footprint. Handles basic VT100 and common xterm extensions. May struggle with programs that use advanced features (24-bit color, bracket paste mode, etc.), but those features don't affect screen text extraction.

Not recommended: custom emulator. Terminal emulation has decades of edge cases. Writing a new one is a deep rabbit hole that wouldn't serve JP's goals.

The choice between alacritty_terminal and vt100 should be made during implementation based on binary size impact and actual compatibility with the programs we need to support. The Session API is the same regardless — the emulator is an internal detail.

Screen text extraction

The screen is returned as one line per terminal row, trailing whitespace trimmed, empty trailing lines removed:

pick abc1234 first commit
pick def5678 fixup! first commit
pick 789abcd second commit

# Rebase abc1234..789abcd onto 000fedc (3 commands)
#
# Commands:
# p, pick <commit> = use commit
# s, squash <commit> = use commit, but meld into previous commit

No ANSI codes, no cursor markers. The assistant sees clean text. Cursor position is available via a separate cursor_position() method if the tool author wants to include it in the content.

Fork safety

fork() in a multi-threaded process (which tokio creates) is a known hazard. Only async-signal-safe functions should be called between fork() and exec(). The pattern used here — setsid(), dup2(), ioctl(), exec() — is safe in practice, but we should consider alternatives:

• posix_spawn: Avoids fork entirely. More limited (can't call setsid() directly), but some platforms support POSIX_SPAWN_SETSID.
• command-fds crate: Wraps std::process::Command with fd passing, avoiding manual fork/exec.
• Pre-fork: Create the PTY and fork before the tokio runtime starts. Impractical for on-demand session creation.

The recommended approach for the initial implementation: use fork() with the minimal safe sequence. Document the constraint. Revisit if we encounter issues.

Platform support

PTY APIs (openpty, fork, setsid, TIOCSCTTY) are Unix-only. This crate targets Linux and macOS. Windows support (via ConPTY) is out of scope for this RFD.

jp_tool::interactive SDK module

This module bridges jp_pty::Session to the stateful tool protocol from RFD 009. It lives in the jp_tool crate (or a new jp_tool_interactive crate if the dependency on jp_pty should be optional).

InteractiveTool trait

The primary interface for tool authors:

pub trait InteractiveTool: Send + Sync {
    /// The commands this tool supports.
    fn commands(&self) -> Vec<Command>;

    /// Spawn a session for the given command.
    fn spawn(
        &self,
        command: &str,
        args: &[String],
        root: &Path,
    ) -> Result<PtyHandle, ToolError>;
}

A Command describes one subcommand the tool exposes:

pub struct Command {
    pub name: String,
    pub description: Option<String>,
    pub accepts_input: bool,
    pub args_schema: Option<Value>,  // JSON schema for command-specific args
}

accepts_input controls whether the apply action appears in the generated schema for this tool.

PtyHandle

The AnyToolHandle implementation for PTY-backed tools:

pub struct PtyHandle {
    session: jp_pty::Session,
    settle_timeout: Duration,
}

impl PtyHandle {
    /// Spawn a command in a PTY.
    pub fn spawn(
        cmd: &[&str],
        cwd: &Path,
        config: PtyConfig,
    ) -> Result<Self, ToolError>;
}

impl AnyToolHandle for PtyHandle {
    fn fetch(&mut self) -> ToolState {
        self.session.wait_for_activity(self.settle_timeout);
        let content = self.session.screen_text();

        match self.session.try_wait() {
            Some(0) => ToolState::Stopped {
                result: Ok(content),
            },
            Some(code) => ToolState::Stopped {
                result: Err(ToolError {
                    message: format!("Process exited with code {code}"),
                    trace: vec![],
                    transient: false,
                }),
            },
            None => ToolState::Running {
                content: Some(content),
            },
        }
    }

    fn apply(&mut self, input: Value) -> ToolState {
        if let Some(text) = input.as_str() {
            let _ = self.session.write(text.as_bytes());
        }
        // Wait for the program to process the input
        self.session.wait_for_activity(self.settle_timeout);
        self.fetch()
    }

    fn abort(&mut self) -> ToolState {
        self.session.kill(Duration::from_secs(2));
        let content = self.session.screen_text();
        ToolState::Stopped {
            result: Ok(content),
        }
    }
}

The settle_timeout controls how long to wait for output to settle after sending input. Default: 100ms. This is a trade-off between responsiveness (return quickly) and completeness (wait for the program to finish rendering). The tool author can override it per-command if needed.

Schema generation

The SDK generates the oneOf JSON schema from the tool's commands() list:

impl InteractiveTool for T {
    fn to_schema(&self) -> Value {
        let commands: Vec<&str> = self.commands()
            .iter()
            .map(|c| c.name.as_str())
            .collect();

        let accepts_input = self.commands()
            .iter()
            .any(|c| c.accepts_input);

        let mut variants = vec![
            // spawn variant
            json!({
                "properties": {
                    "action": { "const": "spawn" },
                    "command": { "enum": commands },
                    "args": { "type": "array", "items": { "type": "string" } }
                },
                "required": ["action", "command"]
            }),
            // fetch variant
            json!({
                "properties": {
                    "action": { "const": "fetch" },
                    "id": { "type": "string" }
                },
                "required": ["action", "id"]
            }),
        ];

        if accepts_input {
            variants.push(json!({
                "properties": {
                    "action": { "const": "apply" },
                    "id": { "type": "string" },
                    "input": { "type": "string" }
                },
                "required": ["action", "id", "input"]
            }));
        }

        // abort variant
        variants.push(json!({
            "properties": {
                "action": { "const": "abort" },
                "id": { "type": "string" }
            },
            "required": ["action", "id"]
        }));

        json!({ "type": "object", "oneOf": variants })
    }
}

Non-PTY stateful tools

Not every stateful tool needs a PTY. A tool that runs cargo check in the background only needs a child process with captured stdout/stderr — no terminal emulation. The SDK should support this with a ProcessHandle alongside PtyHandle:

pub struct ProcessHandle {
    child: tokio::process::Child,
    stdout_buf: String,
    stderr_buf: String,
}

impl AnyToolHandle for ProcessHandle {
    fn fetch(&mut self) -> ToolState {
        // Read any new stdout/stderr
        self.drain_output();

        match self.child.try_wait() {
            Ok(Some(status)) => ToolState::Stopped {
                result: if status.success() {
                    Ok(self.stdout_buf.clone())
                } else {
                    Err(ToolError {
                        message: format!(
                            "Process exited with code {}",
                            status.code().unwrap_or(-1)
                        ),
                        trace: vec![self.stderr_buf.clone()],
                        transient: false,
                    })
                },
            },
            Ok(None) => ToolState::Running {
                content: if self.stdout_buf.is_empty() {
                    None
                } else {
                    Some(self.stdout_buf.clone())
                },
            },
            Err(e) => ToolState::Stopped {
                result: Err(ToolError {
                    message: e.to_string(),
                    trace: vec![],
                    transient: false,
                }),
            },
        }
    }

    fn apply(&mut self, _input: Value) -> ToolState {
        // ProcessHandle doesn't accept input — this should
        // never be called because the tool's schema omits `apply`.
        self.fetch()
    }

    fn abort(&mut self) -> ToolState {
        let _ = self.child.start_kill();
        self.fetch()
    }
}

This lets a tool like cargo_check become stateful (async with polling) using the same SDK, without bringing in PTY dependencies.

The git tool — first consumer

The git tool is the primary motivating example and the first consumer of the SDK. It exposes three interactive subcommands:

Command	Shell command	Needs PTY	Accepts input
stage	git add --patch [args]	Yes	Yes
rebase	git rebase --interactive [args]	Yes	Yes
log	git log --oneline -20 [args]	No	No

stage and rebase need a PTY because they render full-screen interactive UIs. log is a simple command that produces output and exits — it uses ProcessHandle rather than PtyHandle.

The tool implementation is approximately 50 lines of Rust (the InteractiveTool impl shown in the User Experience section). The SDK and jp_pty handle everything else.

Crate structure

crates/
├── jp_pty/                     # Standalone PTY management (new)
│   ├── src/
│   │   ├── lib.rs              # Public API: Session, PtyConfig
│   │   ├── emulator.rs         # Terminal emulator wrapper
│   │   ├── process.rs          # Fork/exec, child management
│   │   └── screen.rs           # Screen text extraction
│   └── Cargo.toml              # deps: nix, alacritty_terminal or vt100
│
├── jp_tool/                    # Existing tool types
│   ├── src/
│   │   ├── lib.rs              # ToolState, ToolCommand, etc. (from RFD 009)
│   │   └── interactive.rs      # InteractiveTool, PtyHandle, ProcessHandle (new)
│   └── Cargo.toml              # optional dep on jp_pty
│
└── jp_tool_git/                # The git tool (new)
    ├── src/
    │   └── lib.rs              # InteractiveTool impl for git
    └── Cargo.toml              # deps: jp_tool

jp_pty has no dependency on jp_tool. jp_tool::interactive depends on jp_pty behind an optional feature flag so tools that don't need PTY support don't pay the dependency cost.

Drawbacks

Binary size. alacritty_terminal is a significant dependency. It pulls in a parser, screen buffer, and color handling that adds to the binary. Feature gating (interactive feature on jp_tool) mitigates this — users who don't need interactive tools don't pay the cost.

Platform restriction. PTY APIs are Unix-only. The jp_pty crate does not compile on Windows. Any tool that uses PtyHandle is Unix-only. Tools using ProcessHandle work everywhere.

Terminal emulation fidelity. Even with alacritty_terminal, some programs may render in ways that produce confusing plain-text output (e.g., progress bars that overwrite lines, split-pane layouts). The assistant sees the final screen state, not the animation. This is inherent to the approach.

Fork safety. fork() in a multi-threaded process is technically problematic. The minimal post-fork sequence we use is safe in practice but not guaranteed by POSIX. This is a known risk shared with many other Rust projects that spawn subprocesses.

Screen settling heuristic. The settle_timeout approach (wait N ms for output to stop) is imperfect. A program might produce a burst of output, pause briefly, then produce more. The assistant might see an intermediate state. More sophisticated approaches (tracking cursor position, detecting known prompts) add complexity for marginal benefit.

Alternatives

portable-pty instead of raw PTY APIs

The portable-pty crate provides a higher-level, cross-platform PTY abstraction (including Windows ConPTY).

Rejected for now because: We only target Unix initially, and the raw API gives us more control over the fork/exec sequence. portable-pty is worth revisiting if Windows support becomes a priority.

vt100 instead of alacritty_terminal

The vt100 crate is smaller and simpler.

Not yet decided. This should be evaluated during implementation based on: (a) binary size difference, (b) whether programs we need to support use escape sequences that vt100 doesn't handle. Both crates expose similar screen-buffer APIs, so switching later is feasible.

Expose raw PTY tools to the assistant

Instead of domain tools (git) that use PTY internally, expose generic terminal_start/terminal_input/terminal_output tools.

Rejected in RFD 009 because it exposes the wrong abstraction. The assistant should call domain tools, not manage terminals. This RFD provides the infrastructure that makes domain tools easy to build.

Run interactive tools via external daemon (interminai)

Use the interminai daemon to manage PTY sessions, communicating over Unix sockets.

Rejected in RFD 009 because it introduces operational complexity (external binary, daemon lifecycle, socket management) that conflicts with JP's single-binary philosophy. The in-process approach in this RFD absorbs the concept (PTY + terminal emulator + screen-as-text) without the daemon architecture. interminai's protocol design (start/input/output/wait/stop) directly informed the action set in RFD 009.

Non-Goals

• Windows support. ConPTY is a different API with different semantics. A future RFD can address it.
• Real-time screen streaming to the user. The user sees tool call results, not live terminal output. Real-time mirroring is a potential enhancement.
• GUI application support. Strictly terminal/CLI programs.
• Custom terminal emulator. We use an existing crate, not roll our own.

Risks and Open Questions

alacritty_terminal API stability

alacritty_terminal is the internals of the Alacritty terminal emulator, published as a crate. It is not designed as a stable library API — breaking changes between versions are expected. We should pin to a specific version and wrap it behind our own Emulator abstraction so a version bump doesn't ripple through the codebase.

Activity detection vs. fixed timeout

The settle_timeout approach assumes output stops arriving within N ms. This fails for programs that:

• Produce output in bursts with pauses between them
• Show a spinner or progress indicator that updates continuously
• Wait for network I/O before rendering the final state

A more robust approach might combine the timeout with heuristics: detect when the cursor returns to a known prompt position, or when the screen content hasn't changed for N ms. This is worth prototyping but shouldn't block the initial implementation.

Screen text for alternate-screen programs

Programs like vim or htop use the terminal's alternate screen buffer. When they exit, the terminal restores the primary buffer — the alternate screen content disappears. For tools that need to capture the alternate screen (e.g., an editor tool), the screen capture must happen while the program is running, not after it exits. The PtyHandle.fetch() implementation handles this correctly (it reads the current screen state), but tool authors should be aware of this behavior.

Large screen output and token cost

A 24×80 terminal screen is ~1920 characters per capture. A 50-step interaction sends ~96KB of screen text to the assistant. At $3/M input tokens, that's roughly $0.10 per interaction. For frequent use, this adds up. Potential optimizations (sending diffs, only changed lines, or compressed representations) are worth exploring but are out of scope for this RFD.

Shared jp_pty with testing infrastructure

Issue #392 proposes PTY-based end-to-end CLI testing. The jp_pty crate could serve both purposes: tool sessions and test infrastructure. The API surface is the same (spawn, write, read screen, wait). This is a potential synergy but shouldn't constrain the design of either use case.

Implementation Plan

Phase 1: jp_pty — PTY session management

Create the crates/jp_pty/ crate:

1. PTY creation (openpty, Winsize configuration)
2. Child process spawning (fork + exec, setsid, fd setup)
3. Non-blocking master fd, background reader thread
4. Terminal emulator integration (start with alacritty_terminal, evaluate vt100 as alternative)
5. screen_text() — extract character grid as trimmed plain text
6. try_wait() — non-blocking child status check
7. wait_for_activity() — block until output arrives or timeout
8. write() — send bytes to PTY master
9. kill() — graceful shutdown (SIGTERM → wait → SIGKILL)

No dependency on JP's tool system. Can be reviewed and merged independently.

Tests: Spawn echo hello, verify screen contains "hello". Spawn cat, write input, read it back. Spawn a program that exits, verify exit code. Test non-blocking reads. Test kill/cleanup.

Phase 2: jp_tool::interactive — SDK module

Add interactive.rs to jp_tool (or create jp_tool_interactive crate):

1. InteractiveTool trait definition
2. Command struct
3. PtyHandle — AnyToolHandle impl wrapping jp_pty::Session
4. ProcessHandle — AnyToolHandle impl for non-PTY stateful tools
5. Schema generation from commands() list
6. settle_timeout configuration

Depends on Phase 1 and RFD 009 Phase 2 (AnyToolHandle trait).

Tests: Unit tests for schema generation. Integration tests with PtyHandle spawning real programs. Integration tests with ProcessHandle.

Phase 3: jp_tool_git — first interactive tool

Create the crates/jp_tool_git/ crate:

1. InteractiveTool impl for git
2. stage command (PTY, accepts input)
3. rebase command (PTY, accepts input)
4. log command (process, no input)
5. Register as a built-in tool

Depends on Phase 2 and RFD 009 Phase 4 (stateful tool dispatch).

Tests: Integration tests: spawn git add --patch on a test repo, send input, verify staging result. End-to-end test with mock assistant driving the full spawn/fetch/apply/fetch cycle.

Phase 4: Polish and documentation

1. Feature gate jp_pty dependency behind interactive feature
2. Evaluate alacritty_terminal vs vt100 binary size impact
3. Document how to build a custom interactive tool
4. Add examples to the jp_tool::interactive module docs

References

• RFD 009: Stateful Tool Protocol — defines ToolState, ToolCommand, AnyToolHandle, handle registry, and per-tool action sets that this RFD builds on.
• interminai — prior art for PTY-based AI tool interaction. Validates the approach and informs the screen-as-text design.
• alacritty_terminal — recommended terminal emulator crate.
• vt100 — lightweight terminal emulator alternative.
• portable-pty — cross-platform PTY abstraction (future consideration for Windows).
• #392 — PTY-based end-to-end CLI testing infrastructure (potential shared use of jp_pty).
• #92 — stream output to text editor (related interactive workflow).