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feat(agent): add the agent/request-messages request-only message seam

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A new waterfall near request construction lets plugins contribute
request-ONLY messages framing the derived history: RequestMessages
{ before, after } with a frozen empty seed, fired inside the open step
after the agent/request config waterfall, so the step/start boundary
snapshot and its same-sync-frame invariant are untouched. The request
becomes messagePrefix + boundary snapshot + messageSuffix.

Contributions never enter session history — deriveMessages() is
unchanged — so the request header is their durable record:
EpochHeader gains messagePrefix/messageSuffix (canonical absence for
empty arrays), request/header-delta replaces either array whole with
an empty array encoding the transition back to absence, and the
dev-mode reconstruction cross-check now expects the folded header's
framing around the boundary derivation.

This is the seam for per-request advisory context that must be
model-visible now without becoming durable history (a skills catalog,
an environment reminder), keeping the base system prompt
workspace-independent and provider prefix caches stable. The docs
carry the channel cost model: session-frozen content belongs in
before, low-frequency change notices belong in durable history via
inject() (paid once, prefix-cached thereafter), and after is reserved
for small frequently-refreshed state snapshots re-paid on every
request they ride. No shipped producer yet, so ACP snapshot fixtures
are byte-identical.
This commit is contained in:
Yichen Jiang
2026-07-07 19:42:30 +08:00
parent e477d76199
commit 17bd71e530
20 changed files with 553 additions and 119 deletions
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42
docs/core-data-structures/core.md
View File

@@ -127,6 +127,12 @@ Source: [`packages/llm/llm/src/types.ts`](../../packages/llm/llm/src/types.ts)
```ts type-equiv
interface GenerateOptions {
model: string
/**
* Ordered conversation messages, exactly as the provider sees them (after
* the `system` slot). A loop-built request assembles them as
* `EpochHeader.messagePrefix` + the derived history + `messageSuffix`
* (dsh-agent-loop); a hand-built one-shot passes any list.
*/
messages: Message[]
/** System prompt text (adapters map to the provider's system slot). */
system?: string
@@ -187,7 +193,9 @@ The model-facing `ToolSchema` is the wire shape; the registered `ToolDefinition`
### The request envelope: `LlmCallConfig` and the logged header
Requests are built by the loop, not shaped per call: the non-content half of a request — the `EpochHeader`: this call configuration plus the rendered system prompt and assembled tool schemas — is logged session state (`request/header` snapshot and delta events, [session.md](session.md#the-request-header-events-requestheader-and-requestheader-delta)), so every conversation request is a pure function of the session log ([reconstructability RFC](../rfc/implemented/architecture/2026-07-05-reconstructable-requests.md)). The `agent/request` waterfall receives a frozen `LlmCallConfig` seed and a listener returns a replacement to switch model or sampling — the loop logs whatever the request actually uses. Loop-built requests arrive at `llm/stream` deep-frozen; mutation throws.
Requests are built by the loop, not shaped per call: the non-history half of a request — the `EpochHeader`: this call configuration plus the rendered system prompt, assembled tool schemas, and any request-only messages — is logged session state (`request/header` snapshot and delta events, [session.md](session.md#the-request-header-events-requestheader-and-requestheader-delta)), so every conversation request is a pure function of the session log ([reconstructability RFC](../rfc/implemented/architecture/2026-07-05-reconstructable-requests.md)). The `agent/request` waterfall receives a frozen `LlmCallConfig` seed and a listener returns a replacement to switch model or sampling; the `agent/request-messages` waterfall contributes request-only messages framing the derived history (recorded as the header's `messagePrefix`/`messageSuffix`) — the loop logs whatever the request actually uses. Loop-built requests arrive at `llm/stream` deep-frozen; mutation throws.
On the wire, a loop-built request reads in this order: the `system` slot (the rendered prompt assembly) → `messagePrefix` (request-only `before` contributions) → the derived history — the boundary snapshot, whose tail is the newest `user/message` on a turn's first step and the previous step's tool results on later steps — → `messageSuffix` (request-only `after` contributions, the last thing the model reads). The framing arrays never enter the derived history; their durable record is the header events, and the dev invariant recomputes exactly this equation against every loop-built request.
FIXME(call-config-shape): revisit the exact definition of this type — which fields are genuinely epoch-level for cache purposes (`model` certainly; the sampling scalars sit here out of caution), and where provider-specific extras (reasoning options, extra body params) belong when an adapter needs them.
@@ -320,7 +328,7 @@ interface Agent {
}
```
`AgentStatus` is `'idle' | 'running' | 'disposed'`. `AgentId` is a branded string. `AgentOptions` (`model?`) is merge-extensible — plugins add creation options by declaration merging; the persona is NOT an agent option but the `dsh-system-prompt` plugin's `persona` config, shared context-wide. The `agent/*` event taxonomy (lifecycle emits incl. `agent/session-start`, the serial `agent/pre-step` surface-mutation seam, and the `agent/prompt-submit`/`agent/request`/`agent/step-result`/`agent/turn-continuation` waterfalls) is in [architecture.md § Event taxonomy](../architecture.md#event-taxonomy); turn/step boundaries are durable `session/event` records, not `agent/*` emits.
`AgentStatus` is `'idle' | 'running' | 'disposed'`. `AgentId` is a branded string. `AgentOptions` (`model?`) is merge-extensible — plugins add creation options by declaration merging; the persona is NOT an agent option but the `dsh-system-prompt` plugin's `persona` config, shared context-wide. The `agent/*` event taxonomy (lifecycle emits incl. `agent/session-start`, the serial `agent/pre-step` surface-mutation seam, and the `agent/prompt-submit`/`agent/request`/`agent/request-messages`/`agent/step-result`/`agent/turn-continuation` waterfalls) is in [architecture.md § Event taxonomy](../architecture.md#event-taxonomy); turn/step boundaries are durable `session/event` records, not `agent/*` emits.
## Interception decisions
@@ -357,6 +365,36 @@ type ContinuationDecision =
type SessionStartSource = 'startup' | 'resume' | 'clear' | 'compact'
```
`agent/request-messages` returns a `RequestMessages` — request-only `before`/`after` messages framing the derived history for ONE request. Not a Decision union: the seam contributes content instead of vetoing, so the shape is the contribution itself; the loop records the non-empty arrays as the header's `messagePrefix`/`messageSuffix` ([the request envelope](#the-request-envelope-llmcallconfig-and-the-logged-header)), and `deriveMessages()` never returns them:
```ts type-equiv
interface RequestMessages {
/** Messages placed before the derived history in the request. */
before: Message[]
/** Messages placed after the derived history in the request. */
after: Message[]
}
```
Listeners read the already-fixed request facts from a `RequestMessagesContext` (decide what to contribute from these; never mutate them):
```ts type-equiv
interface RequestMessagesContext {
/** The rendered system prompt this request will carry. */
system: string
/** The prompt assembly the system prompt was rendered from (sections + tools). */
assembly: PromptAssembly
/**
* The boundary snapshot: the derived history this request will carry between
* `before` and `after`. A frozen snapshot — treat it as read-only; content
* for the NEXT request flows through the log channels.
*/
boundaryMessages: readonly Message[]
/** Aborts in-flight listener work when the step is torn down. */
signal: AbortSignal
}
```
## `ToolDefinition`
The one pipeline-authoring type that is core: what every registered tool *is* — a model-facing `ToolSchema` plus an `execute` function and optional UI presenters. A tool author rarely constructs it by hand (the `defineTool` DSL builds it with typed args), but it is the contract the registry holds and the loop dispatches through.

30
docs/core-data-structures/session.md
View File

@@ -74,16 +74,15 @@ interface SessionEventMap {
*/
'request/header': { header: EpochHeader; reason: RequestHeaderReason }
/**
* Amendment to the folded {@link EpochHeader}: at least one of a
* {@link SystemDelta}, a {@link ToolsDelta}, or a whole replacement
* {@link LlmCallConfig} (four scalars — not worth diffing). Appended by the
* loop inside the step, before dispatch, when the header for this request
* differs from the fold of the log so far; the writer verifies
* `applyHeaderDelta(previous, delta)` reproduces the new header exactly and
* falls back to a `'fallback'` `request/header` snapshot when it cannot, so
* a logged delta ALWAYS round-trips. NOT a {@link SurfaceEventType}.
* Amendment to the folded {@link EpochHeader}: system line-trim, name-keyed
* tools delta, whole replacement config, or whole replacement request-only
* message arrays (an EMPTY array encodes the transition to "none"). The
* writer verifies `applyHeaderDelta(previous, delta)` reproduces the new
* header exactly and falls back to a `'fallback'` `request/header` snapshot
* when it cannot, so a logged delta ALWAYS round-trips. NOT a
* {@link SurfaceEventType}.
*/
'request/header-delta': { system?: SystemDelta; tools?: ToolsDelta; config?: LlmCallConfig }
'request/header-delta': { system?: SystemDelta; tools?: ToolsDelta; config?: LlmCallConfig; messagePrefix?: Message[]; messageSuffix?: Message[] }
}
```
@@ -100,7 +99,7 @@ export interface TodoItem {
### The request header events: `request/header` and `request/header-delta`
The request envelope — the `EpochHeader` (call config + rendered system prompt + assembled tool schemas) — is logged session state, so every conversation request is a pure function of the log (the reconstructability RFC). A `request/header` snapshot (reason `'initial' | 'resume' | 'fallback'`) anchors the fold at conversation birth, process boundaries, and delta-encoding fallbacks; `request/header-delta` events amend it mid-run. `foldRequestHeader(events)` reconstructs the header any request was built under; the writer round-trip-verifies every delta before logging it, so a well-formed log always folds. Neither is a `SurfaceEventType` — they produce no LLM message.
The request envelope — the `EpochHeader` (call config + rendered system prompt + assembled tool schemas + request-only messages) — is logged session state, so every conversation request is a pure function of the log (the reconstructability RFC). A `request/header` snapshot (reason `'initial' | 'resume' | 'fallback'`) anchors the fold at conversation birth, process boundaries, and delta-encoding fallbacks; `request/header-delta` events amend it mid-run. `foldRequestHeader(events)` reconstructs the header any request was built under; the writer round-trip-verifies every delta before logging it, so a well-formed log always folds. Neither is a `SurfaceEventType` — they produce no LLM message.
```ts type-equiv
export interface EpochHeader {
@@ -110,10 +109,19 @@ export interface EpochHeader {
system?: string
/** Assembled tool schemas; absent for a tool-less request. */
tools?: ToolSchema[]
/**
* Request-only messages sent BEFORE the derived history (the
* `agent/request-messages` waterfall's `before` contributions). Not session
* history — `deriveMessages()` never returns them — so the header is their
* only durable record; absent when the request carried none.
*/
messagePrefix?: Message[]
/** Request-only messages sent AFTER the derived history; absent when none. */
messageSuffix?: Message[]
}
```
Canonical form: an empty system prompt and an empty tool list are ABSENT fields, matching how requests are built. The delta payloads (`SystemDelta` — a common-prefix/suffix line trim; `ToolsDelta` — name-keyed added/removed/changed) live beside the events in [`packages/core/session/src/types.ts`](../../packages/core/session/src/types.ts).
Canonical form: an empty system prompt, an empty tool list, and empty request-only message arrays are ABSENT fields, matching how requests are built. `messagePrefix`/`messageSuffix` are the durable record of the `agent/request-messages` waterfall's contributions (the request is `messagePrefix + derived history + messageSuffix`); their deltas replace the array whole, an empty array encoding the transition back to absence. The other delta payloads (`SystemDelta` — a common-prefix/suffix line trim; `ToolsDelta` — name-keyed added/removed/changed) live beside the events in [`packages/core/session/src/types.ts`](../../packages/core/session/src/types.ts).
## `SessionEvent<T>` — one log entry