a8dc6f9776
tool-bash-persistent overwrote the backend's PS1, so terminal-bash prompt readiness never matched and every send degraded to the 3.5s silence tier (idleSilenceMs + handoffGraceMs) under production defaults. The controlled PROMPT_COMMAND now re-asserts PS1 before every prompt, so an in-shell override never survives to the next prompt. The tool initializes with stty -echo alone and detects the no-end-marker fallback through the seam's stdin_read wait reason instead of matching its own prompt text. Tool calls drop from 7180/3560/3566 ms to 355/88/91 ms (spawn+init+echo, echo, pwd; darwin, production defaults). The loader composition suite now pins the fast path by pushing idleSilenceMs beyond the send bound, and a real-PTY case proves PS1 self-healing. Fixes #2585
4.4 KiB
Agent Note: Persistent bash keeps the backend's controlled prompt
Status: implemented
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Problem
dsh-tool-bash-persistent initialized its shell with stty -echo; PS1='__DSH_PERSISTENT_BASH_PROMPT__ ', overwriting the PS1 that dsh-terminal-bash sets in the spawn environment. The backend's prompt readiness requires the printable tail after the OSC 133;D marker to exactly equal the controlled prompt (design), so after initialization no send could ever settle through it. PROMPT_COMMAND survived the override, so the marker kept arriving and every send paid the silence tier plus handoff grace — 3.5 s per tool call under production defaults, 7.2 s for the first call because the initialization send degraded too, and an extra 3.5 s tail after every long command. macOS has no exact stdin-wait tier, and on Linux the exact probe cannot observe a sub-poll-interval command leaving its stdin wait, so the degradation applied to effectively every call. Package tests masked it by configuring idleSilenceMs: 100.
The override existed to give the tool a known prompt for two consumers: a viewport-suffix fallback that detected "shell at a prompt without the end marker", and cosmetic stripping of prompt text from partial output.
Decision
The backend owns its prompt protocol and repairs it itself: the controlled PROMPT_COMMAND re-asserts PS1 after printing the marker, so any in-shell prompt override — this tool's former initialization, a model command, a sourced script — lasts zero prompts. This also protects providers that cannot report foreground state, where the exact prompt text is the only readiness evidence.
The tool stops overwriting PS1 (initialization is stty -echo alone) and replaces its viewport-suffix fallback with the seam's existing signal: a send that settles as stdin_read without the end marker in scrollback returns the captured partial output. The private prompt constant and its stripping are deleted; partial output may now end with the backend's own prompt text, which the tool cannot and should not know.
Alternatives considered
Fix only the tool, leaving PROMPT_COMMAND unchanged. Rejected because the seam would stay silently fragile: any later consumer or model command that touches PS1 reintroduces the 3.5 s degradation with no failing signal, and providers without foreground inspection lose their only readiness factor.
Import the controlled prompt into the tool. Rejected because the prompt is one provider's protocol constant; a Consumer matching it would couple the tool to dsh-terminal-bash specifically, and any other mounted backend would break it again.
Drop the prompt-text factor from backend readiness. Rejected because for providers whose inspectForeground reports nothing, marker-plus-text is the defense against command output that embeds the raw OSC marker sequence; weakening it trades a fast path for a false-settle risk.
Widen handoffGraceMs/idleSilenceMs tuning instead. Rejected because no silence value fixes a dead fast path; it only rebalances how much every call overpays.
Consequences
Measured on darwin with production defaults: raw sends settle in ~86 ms with the controlled prompt intact versus ~3540 ms after an override; tool calls drop from 7180/3560/3566 ms to 355/88/91 ms for spawn+init+echo, echo, and pwd.
The stdin_read fallback is behavior, not only cosmetics: after exec, an interrupt, or an interactive foreground child whose stdin wait the provider proves (the Linux exact tier), the call now returns captured partial output instead of spinning to the command deadline. Where no provider proves the wait (macOS), an interactive child still runs to timeoutMs — recorded as a known limitation in the tool README. Partial output can carry the backend's trailing prompt; complete marker-delimited output is byte-identical to before, which the keyless jsonrpc-agent snapshots confirm.
The loader-composition suite now sets idleSilenceMs above the send bound, so silence can settle nothing and every case fails if prompt readiness regresses; a real-PTY case overrides PS1 in-shell and requires the next send to settle as stdin_read with the healed prompt. The self-repair cannot survive a command that overwrites PROMPT_COMMAND itself; the silence tier remains the bound there, unchanged from the prior design.