deepseek-harness/packages/pty/tool-pty

@deepseek-ai/dsh-tool-pty

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Six model-facing tools over ctx.pty: terminal_open, terminal_send, terminal_read, terminal_signal, terminal_close, and terminal_list. Every operation requires the exact initiating Agent, so a model cannot address another agent's terminal even if it learns the id.

terminal_send(run_in_background: true) reuses ctx.tasks; task preflight and the PTY service's exclusive per-session send reservation occur before the task id is returned, completion is collected with task_output, and task_kill delivers SIGINT to the foreground process group. Foreground sends use terminal call/result cards. Background sends use a generic execute card; open, read, signal, close, and list use generic execute, read, execute, delete, and read cards respectively. None declares source locations.

Config

key	default	meaning
enableRunInBackground	true	expose and accept run_in_background; false omits the schema field and rejects a forced undeclared argument
maxResultBytes	262144	UTF-8 cap (minimum 64) for each complete terminal result or PTY task output after wait, session, pagination, truncation, and task-status metadata

Both values are validated at load. The minimum result cap keeps every registry-issued session or task id visible in its creation acknowledgement. When a result exceeds maxResultBytes, rendering reserves space for control metadata and a truncation marker when they fit; cuts preserve UTF-8 boundaries. Each terminal definition's final-content callback applies the same cap after normalized pre-, around-, and post-execute policy failures, denials, short-circuits, replacements, or blocks; a structured multi-block policy result retains its shape.

Model Experience

System prompt

What the model sees

The plugin contributes this fixed guidance section:

Terminal guidance

Use a terminal session only when work needs persistent terminal state or interactive stdin; prefer bash/read/write/edit for bounded one-shot operations. Track every terminal session id and close sessions that no longer matter. An inferred_idle or timeout result does not prove the foreground command exited.

Token effect

Small fixed input cost on every request while the plugin is active.

KV Cache effect

Prefix-stable while the registration scope and guidance text are unchanged.

Tool schemas

What the model sees

The six generated schemas are listed in the dsh-tool-pty catalog section. Their fixed schema tokens are present whenever this plugin is active; agent-scoped tool filtering may hide them.

Token effect

Fixed schema cost on requests where the tools are visible.

KV Cache effect

Prefix-stable while tool visibility and definitions are unchanged.

Tool results and task context

What the model sees

Spawn returns the id and bounded MOTD. Send/read return bounded terminal text plus readiness/history markers. Background mode returns a generic task id. Every terminal-owned or policy-produced single-text result is capped by maxResultBytes after normalized tool or pipeline errors, denials, short-circuits, replacements, blocks, and generic task status text. Structured multi-block policy results retain their shape. Results remain in session history until compaction; incremental task reads do not repeat consumed output. Programmatic callers receive typed session snapshots, bounded provider read/send DTOs, signal and close outcomes, or { kind: "background", taskId }; Native rendering applies the presentation cap above.

Token effect

Terminal-owned and policy-produced single-text results are data-dependent and bounded by maxResultBytes; a policy that deliberately substitutes structured multi-block content owns that content's bound. Each returned result remains in history until compaction.

KV Cache effect

Append-only; new results follow the reusable request prefix.

Known Limitations and Deferred Work

• No named key sequence, TUI, BEL, resize, auto-start, or cross-agent sharing schema is exposed.
• Background mode requires both @deepseek-ai/dsh-tasks and its model-facing control surface.