deepseek-harness/docs/rfc/implemented/feature/2026-06-30-hook-protocol-lib.md

RFC: dsh-hook-protocol — the shared Claude Code / Codex hook wire-protocol core

Status: implemented (accepted 2026-06-30)

Context

The hooks subsystem ships two bridge plugins: one that runs a user's existing Claude Code (CC) hooks, one for Codex hooks. Studying the reference implementations (~/repos/refs/claude-code, ~/repos/refs/codex) surfaced a decisive fact: Codex deliberately reimplements a SUBSET of the CC hook protocol. Its engine reads the same hooks.json, uses the same matcher-group shape, the same exit-code/structured-stdout output contract, and the same command-hook execution model — Codex's source even names the engine after Claude's and comments where it "intentionally diverges." So the two bridges would otherwise duplicate the bulk of the protocol.

This RFC introduces @deepseek-ai/dsh-hook-protocol, a library (not a plugin — it registers and injects nothing) holding the genuinely-identical primitives both bridges build on. The split between shared and per-dialect is the design's center of gravity.

Decision

A new packages/hooks/ group with hook-protocol as a pure library. It owns four primitive families and the hook/* session events; each bridge plugin (dsh-hooks-claude, dsh-hooks-codex) owns what genuinely differs.

Shared (here):

• Matcher — matchesMatcher(pattern, query, mode). The ONE axis the dialects differ on is collapsed to the mode parameter: claude treats a pure [A-Za-z0-9_|]+ pattern as a literal (pipe = exact-match alternation) and anything else as a regex; codex is always an unanchored regex. Match-all on absent/''/'*'; an invalid regex matches nothing (never throws into the loop).
• Execution — runHook(bash, hook, options, now). Runs a command hook through the ctx.bash seam rather than a bespoke spawn: the executor already provides the scrubbed-but-overridable env, process-group kills, and timeout the protocol needs, and dsh-bash's stdin/env fields (added in the bash-seam PR for exactly this) are the trusted-plugin surface an in-process bridge is allowed to use. It serializes the bridge-built payload to stdin (trailing newline iff CC), honors the hook's timeoutSec, and never throws (an executor rejection becomes a non-blocking-error HookOutput).
• Decode — parseHookOutput(exit, stdout, stderr), the exit-code + structured-stdout codec, producing a dialect-neutral HookOutput. Exit 0 → lenient JSON parse of stdout; exit 2 → blocking error with stderr as the reason (surfaced as decision: 'block' so no caller needs a separate exit-code branch); other → non-blocking error. Parses the full CC superset (continue/stopReason/suppressOutput/decision/hookSpecificOutput.{permissionDecision,additionalContext,updatedInput}/systemMessage); the bridge honors only the subset meaningful for its dialect.
• Merge — mergeHookOutputs(outputs), folding multiple matched hooks into one most-restrictive MergedHookOutcome: permission precedence deny > ask > allow, halt sticky on the first continue:false, block reasons joined \n\n, context/system-messages accumulated in order.
• hook/* session events — hook/invoked / hook/result, declaration-merged into SessionEventMap (log-only, like compact/* — NOT SurfaceEventTypes), with appendHookInvoked/appendHookResult helpers so the invoked/result pairing and turn-enclosure stay consistent across bridges.

Per-dialect (the bridge plugins): building each event's stdin payload (CC's base+per-event field sets vs Codex's snake_case with turn_id/model extras), the dialect's env + ${CLAUDE_PLUGIN_ROOT} substitution (CC) vs none (Codex), and mapping the neutral HookOutput/MergedHookOutcome onto the harness's seam-specific typed Decisions (PreToolDecision, PromptDecision, ContinuationDecision, PostToolDecision).

Why "shared core + per-dialect adapters", not "one parameterized engine"

A single engine parameterized by a full dialect descriptor was considered and rejected. The payload construction and decision mapping are where the dialects genuinely diverge (different field names, different supported outputs, CC's env/substitution); folding those into a data-driven descriptor would make the bridge logic indirect — a reader of dsh-hooks-claude would have to chase a descriptor to see what payload it sends. Keeping the truly-identical primitives shared (matcher, codec, runner, merge, events) and letting each bridge write its own straightforward payload+mapping keeps each bridge readable standalone, at the cost of a little duplication in the payload shape. The primitives are the part where duplication would actually be dangerous (a divergent matcher or exit-code rule is a correctness bug); the payload is the part where explicitness beats sharing.

Consequences

The two bridge plugins become thin: parse the config file, pick a matcher mode, build the per-event payload+env, call runHook + mergeHookOutputs, map the outcome to a Decision, and append hook/*. The protocol's correctness-critical halves (matcher semantics, exit-code contract, merge precedence) live in one tested place — hook-protocol ships with heavy unit tests (matcher per-mode, codec per exit-code/field, runner plumbing with a stub executor, merge precedence, the hook/* helpers) at per-file 100%. Input rewrite (updatedInput) is parsed but not honored (the deferred pre-tool-input-rewrite RFC); a bridge logs+warns on it. The package is a library, so it has no cordis.yml load path of its own — its real-load-path coverage comes through the bridge plugins that consume it.