# Agent Note: Interception seams — the typed-Decision surface a hook programs against

Status: implemented

English | [中文](2026-06-30-interception-seams.zh.md)

## Problem

The harness needs a hooks subsystem: users extend or gate the agent at lifecycle points the way Claude Code (CC) and Codex do. The key reframe driving this design is that **"native hooks" are not a package** — a native hook is just an ordinary Cordis plugin subscribing to the canonical lifecycle events. So the real product is a *powerful, well-typed canonical event surface*; the CC/Codex bridges (the `dsh-hooks-claude` / `dsh-hooks-codex` packages) are merely translators that map an external shell-hook protocol onto that same surface. Anything a bridge can do, a plain plugin can do directly — more powerfully (no serialization boundary, full `ctx`, typed returns).

The surface needs distinct contracts for per-prompt policy (CC's `UserPromptSubmit`), session-start observation (CC's `SessionStart`), pre-tool policy, around-dispatch control, post-tool transformation, final-result observation, and continuation with a model-facing reason. Conflating those phases gives plugins mutation channels they do not need and makes finality depend on listener ordering. The [event-domain-semantics Agent Note](../architecture/2026-06-30-event-domain-semantics.md) supplies the three-domain rule and the typed-Decision idiom; this Agent Note applies them to the lifecycle seams.

## Decision

The canonical surface separates transformable policy, around-dispatch control, and observe-only notification. Policy waterfalls return small seam-specific **typed Decision unions**; wrappers return normalized results; notifications receive immutable snapshots and cannot affect the outcome. The set covers the hook points in scope (`session-start`, `prompt-submit`, `pre-tool`, `post-tool`, `stop`-via-continuation) while leaving non-hook execution policy independently composable.

**Agent events** (`dsh-agent`):
- `agent/session-start(agent, source)` — emit, once before turn 1, carrying a `SessionStartSource` (`startup` for a fresh/forked create, `resume` for a reloaded persisted session; `clear`/`compact` reserved). A pure notification — it CANNOT block startup (a deliberate gap: a bridge logs/injects, it does not gate startup). A listener seeds context via `agent.inject()`.
- `agent/prompt-submit(agent, content, source, signal, next) → PromptDecision` — waterfall, fired for one claimed queued message before the loop opens a turn or appends `user/message`. The explicit admission signal is placed before the final `next`; `allow` optionally rewrites the prompt `content` or attaches separately sourced `additionalContexts[]`, while `block` discards the candidate without creating session history.

**`agent/turn-stopping`** is an awaited notification at the natural stop boundary. A listener that needs another step calls `agent.steer()` with explicitly sourced model-facing content; the loop then re-reads the outbox and either continues or closes the turn.

### The tool pipeline gives each phase one kind of authority

Every call follows `tools/pre-execute` → guards → `tools/execute` → dispatch → `tools/post-execute` → definition-owned `finalizeContent` → `tools/result`. The registry snapshots caller input, materializes and freezes arguments, assigns an opaque token, and snapshots the visible definition's final-content callback before policy begins. Nested calls carry only the parent token. Identity remains immutable; only `signal` may change around dispatch. The log, UI, and tool body therefore agree on what ran.

- **`tools/pre-execute`** is the extensible waterfall gate. Its `PreToolDecision` allows, denies, or asks. Deny skips `tools/execute` and core dispatch. Ask resolves through the optional approval seam: only `allowed-once` continues through guards and dispatch; rejection, cancellation, an unavailable channel, a missing approval service, or an agent-less call becomes a normalized denial. Every resolved decision still reaches post-policy; a throwing listener becomes a final normalized failure.
- **`ctx.tools.guard()`** installs synchronous scope-aware policy after the whole pre-execute waterfall. A guard may deny or abstain, never force-allow, so listener ordering cannot resurrect an operation that a final invariant forbids.
- **`tools/execute`** is the around-dispatch waterfall for timeout, retry, and metrics plugins. A wrapper delegates to core dispatch with `next()`, may replace and restore the required `exec.signal` before doing so but cannot remove it, and receives the already-normalized canonical success/failure result of a thrown or unknown tool; a wrapper-authored success short-circuits dispatch and is re-normalized through the resolved output declaration.
- **`tools/post-execute`** is the inspect/transform waterfall. Its `PostToolDecision` accepts, blocks with feedback, replaces either presentation content or canonical value, or attaches `additionalContexts`. Value replacement revalidates and recomputes presentation; content replacement preserves programmatic value and is not a confidentiality boundary. The returned decision is the supported transform channel.
- **`ToolDefinition.finalizeContent`** is an optional synchronous, total, content-only boundary snapshotted with the visible definition at call creation. It runs exactly once after the registry has normalized and losslessly snapshotted the candidate outcome, including pre-, around-, or post-listener failures that bypass later waterfalls and errors discovered while snapshotting another result field. It may replace `content` or preserve it with `undefined`, but cannot rewrite `isError`, structured error identity, contexts, or presentation metadata. This is where a tool enforces its own last-mile content invariant without converting policy failures into weaker block decisions.
- **`tools/result`** is the synchronous contained notification after every transform, lossless-JSON materialization, and the outer error boundary. It receives the same frozen execution identity and an immutable snapshot of the authoritative result; observer failures are contained per listener and cannot change or reject `ToolRegistry.execute()`'s returned outcome.

Core dispatch and the tool body sit inside normalization boundaries, so tool, listener, invalid canonical value, renderer/projector, non-JSON presentation, and identity-shape failures resolve as JSON-safe `isError` results rather than escaping the turn. A post-execute listener can therefore inspect a thrown tool; definition-owned final content invariants also cover outer pipeline and candidate-materialization failures; and a final observer sees the execution-local canonical value beside exactly the presentation fields the session log can persist. The [canonical tool-output contract](../architecture/2026-07-20-canonical-tool-output-contract.md) owns the value/projection and durability rules.

### Three load-bearing loop decisions

1. **Run prompt policy before opening the turn.** A blocked prompt creates no turn or durable event. On allow, the loop stages the rewritten prompt followed by every returned `additionalContexts` entry, opens the turn, and drains that outbox before the first step. Each claimed ordinary-send item is the sole direct prompt in its turn under the [one-send-one-turn simplification](../simplification/2026-07-17-one-send-one-turn.md).

2. **Post-tool `additionalContexts` and asynchronous injections enter the active-batch FIFO and append when that batch settles.** `content`/`feedback` shape the result `execute()` returns, but each context is a separate sourced `user/message`, and a single step or composite tool can produce many. Appending context immediately would interleave `result(c1) → context → result(c2)` or place nested context before its outer result, breaking tool-call/result adjacency. `ToolRunContext.deferContext()` therefore collects nested-dispatch context through failures, `execute()` surfaces the ordered array on `ToolExecutionResult`, and the loop accepts it into the same FIFO as `agent.inject()` calls made during execution. The FIFO appends after every recorded result when the batch settles, including before an interrupted turn closes. An accepted outer call preserves deferred contexts before decision contexts; an outer block discards deferred contexts and exposes only contexts explicitly supplied by the blocking decision.

3. **A stopping listener requests continuation through the steering channel**, so the next step's top-of-loop drain records it as steering for the continued turn — next-*step* steering within the SAME turn, not a next-*turn* prompt.

### Pre-tool input rewrite is a separate consistency decision

`PreToolDecision` cannot rewrite arguments. History and the audit call are logged before execution, and UI presentation reads the same input, so the registry seals arguments before policy. A valid rewrite must update history, audit, presentation, and execution before identity is created; that contract belongs to the [input-rewrite proposal](../../proposed/feature/2026-06-30-pre-tool-input-rewrite.md).

### Boundaries

The seam package does **not** declare `hook/*` session events (the durable hook-invocation log); those belong to `dsh-hook-protocol`, because a native plugin uses typed decisions without an external hook log. The native-plugin integration test (`packages/core/agent-loop/tests/interception.spec.ts`) composes the seams through the real loop with no `hook/*` protocol. Compaction (`PreCompact`/`PostCompact`), Notification, and Codex `PermissionRequest` remain outside this decision. The [approval seam](2026-07-06-approval-seam.md) resolves `ask` decisions through `ctx.approval`; terminal monotonic stopping is expressed by tool-result data, while `agent/turn-stopping` is the last chance to steer another step.

## Alternatives considered

- **Shipping pre-tool INPUT rewrite as part of this seam set** — deferred as the over-reach signal; the section above carries the consistency problem (audit, history, and presentation all read `tool/call.arguments` logged before execution), and [the pre-tool input-rewrite proposal](../../proposed/feature/2026-06-30-pre-tool-input-rewrite.md) owns the design.
- **Declaring the durable `hook/*` SessionEvents alongside the seams** — rejected: a native plugin uses the typed Decisions with no hook log at all (the worked example proves it), so the durable log belongs to [the hook-protocol library](2026-06-30-hook-protocol-lib.md), not the seam surface.

## Consequences

The canonical interception surface is uniformly typed without giving every extension the same power: hooks return decisions, execution wrappers wrap, terminal guards only deny, and final observers only observe. The loop owns session-start, pre-turn prompt admission, post-tool context buffering, and stopping; `dsh-tools` owns identity sealing and the five-phase execution pipeline. Their contracts are documented in [architecture.md](../../../../docs/architecture.md), package READMEs, [core interception decisions](../../../../docs/core-data-structures/core.md#interception-decisions), and [tool structures](../../../../docs/core-data-structures/tools.md). The ACP bridge settles an admission rejection as `cancelled` after the agent becomes idle with no owned turn, while hook-driven snapshots verify the observable bridge behavior end to end.
