deepseek-harness/packages/experimental/hook-protocol

description, kind

description	kind
The shared hook rules behind the Claude Code and Codex bridges — what a hook can do and what happens when it runs — for users and maintainers of the hooks subsystem.	package-library

@deepseek-ai/dsh-hook-protocol

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Summary

dsh-hook-protocol makes both bridges handle your hooks identically: it defines what a hook can do and what happens when it runs. You never install or configure it yourself — choose dsh-hooks-claude-code or dsh-hooks-codex, point it at your existing hooks.json, and these rules apply to your hooks. Through either bridge, a hook can block a prompt or tool call with a message the model sees, attach extra context to the conversation, or ask the run to stop. Only command hooks run; http, mcp_tool, prompt, and agent handlers are skipped with a warning.

Table of Contents

• Use this package
• Understand the implementation
• Further Exploration
• Model Experience
• Known Limitations and Deferred Work
• Dev Note

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Use this package

You don't install or configure this package directly — mounting dsh-hooks-claude-code or dsh-hooks-codex applies these rules to your existing hooks.json hooks. Use this page to learn what a hook can do and what happens when it runs; the two bridge pages list which events each dialect supports.

When to choose it

Choose dsh-hooks-claude-code or dsh-hooks-codex when you have existing Claude Code or Codex hooks and want them to keep working during agent runs. You never choose this package directly. Avoid the whole group for bespoke behavior with no reference-tool equivalent: a native Cordis plugin has the full harness API with no hook protocol in between.

What a hook can do

• Block an action with a message — a hook that exits with code 2 stops the prompt or tool call, and its error output is shown as the reason.
• Ask before a tool runs — a Claude Code hook can request confirmation instead of blocking outright; the Codex bridge does not surface this option.
• Attach context — a hook can return extra text that the model sees in the next request.
• Run on chosen moments — a hook config selects which events it fires on by name or pattern; an absent, empty, or '*' pattern means every event of that kind.
• Fail without stopping the run — any exit code other than 2 is a non-blocking failure: the action proceeds and the failure is logged, and a hook that cannot be started at all is treated the same way.
• Ask the run to stop — a hook can request that the run halt ({"continue": false}); the request is recorded but has no run-level effect (see Known Limitations).

What you see when hooks run

• When a hook blocks, the action does not happen and the hook's message is shown.
• When a hook attaches context, the model sees that text in its next request.
• A hook that fails — a bad command, a crash, or any exit other than 2 — is logged and does not stop the agent.
• If the hook config cannot be read or parsed, the bridge logs a warning and no hooks run; the agent still starts.
• Configs that mix hook types still work: http, mcp_tool, prompt, and agent handlers are skipped with a warning, and their command hooks run.

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Understand the implementation

Implementation internals — click to expand

This section explains the design decisions behind the library and points at the code that realizes them; the observable behavior is fully covered in Use this package.

Processing pipeline

selectHookGroups selects nonempty matching groups in config order. Each bridge resolves the current working directory only when this selection contains commands to run. appendHookInvoked omits absent or undefined matchers from the recorded event.

The library is a chain of single-purpose steps, one function each: validate the matcher pattern, run the command through the dsh-shell executor, decode the outcome, merge every matched hook's outcome into one most-restrictive result, and record the durable hook/* event pair. The matcher's mode parameter is the single axis the dialects differ on — claude-code interprets a pattern as literal alternatives or a regex, codex always as an unanchored regex. Every step degrades to a contained outcome instead of throwing, so a hook can never crash the calling turn: an invalid regex is a non-match, an executor rejection becomes a HookOutput with no exit code, exit 2 blocks with stderr as the reason, and every other failure stays non-blocking. Merging applies deny > ask > allow precedence, keeps the first continue: false stop sticky, and accumulates context in hook order. Detached runs are tracked so fiber.dispose() reaches quiescence. The steps live in src/matcher.ts, src/runner.ts, src/codec.ts, src/merge.ts, src/events.ts, and src/detached.ts.

hook/* session events

The hook/invoked and hook/result events are declaration-merged into SessionEventMap as log-only records: like compaction/*, they are not surface events and carry no surfaceOp. A hook/result pairs with its hook/invoked by handlerId, and appendHookResult owns the decision rule. Payloads and per-event JSDoc live in the generated persistence log event catalog.

Invocation and result records must sit inside an open turn: UserPromptSubmit, PreToolUse, PostToolUse, and Stop satisfy that relation by construction, while SessionStart runs before turn 1 and gets no hook/* record — its injected context is delivered instead.

Design philosophy

• One axis of difference collapsed into mode. The dialects differ only in how a matcher pattern is interpreted, so the matcher takes the mode as a parameter instead of duplicating the engine.
• The executor owns process control. Commands run through the dsh-shell executor rather than a bespoke spawn: the executor already provides the scrubbed-but-overridable environment, process-group cancellation, and timeout the protocol needs.
• Never throw into the loop. Every failure mode — malformed JSON, an invalid regex, an executor rejection — degrades to a contained outcome or a non-match, so a hook can never crash the calling turn.
• Log-only, turn-enclosed events. The hook/* records are durable evidence of what ran and what it decided; they are not surface events.

The hook-protocol-lib Agent Note records the shared-versus-per-dialect split and the alternatives considered.

Source map

File	Role
src/index.ts	Public exports of every primitive and event helper
src/matcher.ts	Matcher validation, dialect matching, and nonempty command-group selection
src/runner.ts	runHook execution through ctx.shell and DEFAULT_HOOK_TIMEOUT_MS
src/codec.ts	Exit-code and structured-stdout decoding into HookOutput
src/merge.ts	Most-restrictive merge and the MergedHookOutcome type
src/events.ts	hook/* event declaration, append helpers, stderr summary
src/detached.ts	Detached-run quiescence tracking
src/types.ts	HookOutput, MatcherGroup, CommandHook, and the hook/* payload types

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Further Exploration

Read these pages when the package-level contract is not enough. They move from the shared rules to the bridges that apply them and the extension points they program against.

• Hooks group map — the sibling group page and its package table.
• Hook protocol library Agent Note — why the protocol core is shared and what each bridge owns.
• Hook bridges Agent Note — how the two bridges use these primitives.
• Interception extension-points reference — the typed-Decision surface the bridges map onto.
• Generated persistence log event catalog — the hook/* event payloads and per-event JSDoc.

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Model Experience

Indirectly, through dsh-hooks-claude-code and dsh-hooks-codex, which are the only consumers that render decoded hook output into model context.

KV Cache effect

No direct invalidation; the named consumers own any request-prefix changes.

Known Limitations and Deferred Work

These limits describe what hooks cannot do through the shared engine yet. They are current package constraints, not a task backlog.

• HookOutput.updatedInput is parsed but not honored — input rewrite is a deferred consistency-design problem (the pre-tool-input-rewrite Agent Note); a bridge logs and warns when a hook sets it.
• A folded halt has no run-level effect — mergeHookOutputs folds continue: false into a sticky stop, but the interception points have no hard-halt primitive, so a bridge records the halt and keeps the hook's per-point effect.
• Only the command-hook shape runs — the protocol executes { type: 'command', command, timeout? }; a bridge parses-and-skips the other shapes its dialect defines (http, mcp_tool, prompt, agent).

Dev Note

Working context for maintainers — click to expand

This Dev Note is working context for maintainers: open questions and directions that are not decided. It is explicitly non-authoritative — shipped behavior, limits, and accepted rationale live in the sections above, the package code, and the linked Agent Notes.

Future: run-level halt

A hook that asks to halt the whole run (continue: false) is folded into MergedHookOutcome.stop but not applied anywhere: the interception points lack a hard-halt primitive, and mid-turn requests record the halt in hook/result instead. A run-level halt mechanism would let the bridges honor it; no design exists yet.