diff --git a/packages/support/invariants/README.md b/packages/support/invariants/README.md
index b26170d48e..94c2682f9d 100644
--- a/packages/support/invariants/README.md
+++ b/packages/support/invariants/README.md
@@ -40,6 +40,10 @@ Agent status (per agent):
 
 - **legal transitions only** — `idle↔running` and `(idle|running)→disposed`. A no-op transition (`setStatus` dedups, so it never fires) and leaving the terminal `disposed` state are violations.
 
+Model requests (on `llm/stream`):
+
+- **a loop-built request is exactly what the log reconstructs** — a frozen request with a live `sessionId` (the loop-built marker; hand-built one-shots like compaction's summarize are unfrozen and skipped) must carry frozen `messages` deep-equal to the derivation over the log prefix strictly before the in-flight step's `step/start` (rebuilt through a FRESH `Session`, so the live cache cannot vouch for itself — and boundary-correct: content logged after `step/start` legitimately belongs to the next request), and every non-content field must equal the fold of the log's `request/header*` events (see [the reconstructability RFC](../../../docs/rfc/implemented/architecture/2026-07-05-reconstructable-requests.md)). Registered with `prepend: true` so a short-circuiting `llm/stream` listener (the replay adapter) cannot silence it; prepend orders it against append-registered listeners only — correctness rests on the seq-bounded rebuild, never listener timing.
+
 On any violation it throws `InvariantError` (`code: 'INVARIANT'`).
 
 ## Why runtime, not deep-readonly types
diff --git a/packages/support/invariants/src/index.ts b/packages/support/invariants/src/index.ts
index 08d4365f4b..f35fbafa1d 100644
--- a/packages/support/invariants/src/index.ts
+++ b/packages/support/invariants/src/index.ts
@@ -21,9 +21,10 @@
 
 import type { Context } from 'cordis'
 import { HarnessError } from '@deepseek-ai/dsh-llm'
-import type { CallId } from '@deepseek-ai/dsh-llm'
+import type { CallId, GenerateOptions } from '@deepseek-ai/dsh-llm'
 import type { Agent, AgentStatus } from '@deepseek-ai/dsh-agent'
-import type { Session, SessionEvent, SurfaceEventType } from '@deepseek-ai/dsh-session'
+import { Session, SessionId, foldRequestHeader } from '@deepseek-ai/dsh-session'
+import type { SessionEvent, SurfaceEventType } from '@deepseek-ai/dsh-session'
 
 export const name = 'invariants'
 export const inject = ['sessions']
@@ -360,4 +361,74 @@ export function apply(ctx: Context, config: Config = {}): void {
     checkTransition(lastStatus.get(agent), status)
     lastStatus.set(agent, status)
   })
+
+  // Request-reconstruction cross-check (the reconstructability RFC): a
+  // loop-built request — frozen envelope + live sessionId is the marker; a
+  // hand-built one-shot (compaction summarize) is unfrozen and skipped — must
+  // be EXACTLY what the session log reconstructs:
+  //
+  // - messages: the derivation over the log prefix strictly before the
+  //   in-flight step's `step/start` (the reconstruction boundary). Compared
+  //   against a FRESH Session built over that prefix — the same projection
+  //   code with zero shared state, so the live cache under test cannot vouch
+  //   for itself. Boundary-correct by construction: content appended after
+  //   the boundary (an `agent/request`-window inject) is legitimately absent
+  //   from this request, and a current-surface comparison would false-fire.
+  // - header: every non-content field must equal the fold of the log's
+  //   `request/header*` events — the loop logs the header event BEFORE
+  //   dispatch, so the fold already covers this request.
+  //
+  // Registered with `prepend: true` so a short-circuiting llm/stream listener
+  // (the replay adapter returns its chunks without calling next()) cannot
+  // silence the check by registering first. Prepend beats APPEND-registered
+  // listeners only — two prepended listeners have no defined mutual order
+  // (cordis unshift) — which is fine: correctness rests on the seq-bounded
+  // fold below, never on listener timing.
+  ctx.on('llm/stream', (options: GenerateOptions, next) => {
+    if (options.sessionId === undefined || !Object.isFrozen(options)) return next()
+    // GenerateOptions types sessionId as Branded<'SessionId'>, which IS
+    // SessionId (dsh-llm cannot import it without a cycle) — no cast needed.
+    const session = ctx.sessions.get(options.sessionId)
+    if (!session) return next()
+    if (!Object.isFrozen(options.messages)) {
+      throw new InvariantError('a loop-built request must carry a frozen messages array')
+    }
+
+    const events = session.events
+    // seq === index (checked above), so the last step/start's seq bounds the
+    // prefix directly. The in-flight step's step/start is necessarily the
+    // last one: the loop cannot open another step while this call streams.
+    let boundary = -1
+    for (let i = events.length - 1; i >= 0; i -= 1) {
+      if (events[i]?.type === 'step/start') {
+        boundary = i
+        break
+      }
+    }
+    if (boundary === -1) {
+      throw new InvariantError('a loop-built request with no step/start in its session log')
+    }
+    const rebuilt = new Session(SessionId(`${String(session.id)}-invariant-rebuild`), structuredClone(events.slice(0, boundary)))
+    // JSON equality is sound here: both sides are structuredClones produced by
+    // the same projection code path, so key insertion order matches when the
+    // values do.
+    if (JSON.stringify(options.messages) !== JSON.stringify(rebuilt.deriveMessages())) {
+      throw new InvariantError(`llm request for session "${String(session.id)}" diverges from the boundary derivation (log-reconstruction desync)`)
+    }
+
+    const header = foldRequestHeader(events)
+    if (header === undefined) {
+      throw new InvariantError('a loop-built request with no request/header event in its session log')
+    }
+    const headerMatches = options.model === header.config.model
+      && options.system === header.system
+      && options.temperature === header.config.temperature
+      && options.maxTokens === header.config.maxTokens
+      && JSON.stringify(options.stop) === JSON.stringify(header.config.stop)
+      && JSON.stringify(options.tools ?? []) === JSON.stringify(header.tools ?? [])
+    if (!headerMatches) {
+      throw new InvariantError(`llm request for session "${String(session.id)}" diverges from the folded request header`)
+    }
+    return next()
+  }, { prepend: true })
 }
diff --git a/packages/support/invariants/tests/invariants.spec.ts b/packages/support/invariants/tests/invariants.spec.ts
index d7e95c1103..489cfb9817 100644
--- a/packages/support/invariants/tests/invariants.spec.ts
+++ b/packages/support/invariants/tests/invariants.spec.ts
@@ -670,3 +670,113 @@ describe('surface invariants', () => {
       .toThrow(/cannot carry surfaceOp/)
   })
 })
+
+describe('request-reconstruction cross-check (llm/stream)', () => {
+  /** Session with a boundary: one derivable user message, an open step, and the header event the loop would have logged. */
+  async function requestSetup() {
+    const { ctx } = await setup({ freeze: false })
+    const session = ctx.sessions.create(SessionId('req-check'))
+    session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
+    session.append('user/message', { content: [{ type: 'text', text: 'hi' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
+    const boundary = session.deriveMessages()
+    session.append('step/start', { turn: 1, step: 1 })
+    session.append('request/header', { header: { config: { model: 'm' } }, reason: 'initial' })
+    return { ctx, session, boundary }
+  }
+
+  /** Dispatch the llm/stream waterfall with a stub core, collecting the check's verdict. */
+  function dispatch(ctx: Context, options: unknown): void {
+    // The invariants listener runs synchronously at dispatch time (its checks
+    // precede next()); the stub core just yields nothing.
+    void ctx.waterfall('llm/stream', options as never, () => (async function* () {})() as never)
+  }
+
+  it('passes a frozen request that equals the boundary derivation + the folded header', async () => {
+    const { ctx, session, boundary } = await requestSetup()
+    const options = Object.freeze({ model: 'm', messages: Object.freeze(boundary), sessionId: session.id })
+    expect(() => { dispatch(ctx, options) }).not.toThrow()
+  })
+
+  it('is boundary-correct: content logged after step/start is legitimately absent from this request', async () => {
+    const { ctx, session, boundary } = await requestSetup()
+    // An agent/request-window inject: lands in the log after the boundary,
+    // belongs to the NEXT request. A current-surface comparison would
+    // false-fire here; the seq-bounded rebuild must not.
+    session.append('context/message', { content: [{ type: 'text', text: '[late]' }], source: { kind: 'plugin', plugin: 'x' } }, { surfaceOp: 'append' })
+    const options = Object.freeze({ model: 'm', messages: Object.freeze(boundary), sessionId: session.id })
+    expect(() => { dispatch(ctx, options) }).not.toThrow()
+  })
+
+  it('rejects a frozen request whose messages diverge from the boundary derivation', async () => {
+    const { ctx, session, boundary } = await requestSetup()
+    const messages = [...boundary, { role: 'user', content: [{ type: 'text', text: 'phantom' }] }]
+    const options = Object.freeze({ model: 'm', messages: Object.freeze(messages), sessionId: session.id })
+    expect(() => { dispatch(ctx, options) }).toThrow(/diverges from the boundary derivation/)
+  })
+
+  it('rejects a frozen request whose fields diverge from the folded header', async () => {
+    const { ctx, session, boundary } = await requestSetup()
+    const options = Object.freeze({ model: 'other', messages: Object.freeze(boundary), sessionId: session.id })
+    expect(() => { dispatch(ctx, options) }).toThrow(/diverges from the folded request header/)
+  })
+
+  it('rejects a loop-built request with no header event or no step/start in its log', async () => {
+    const { ctx } = await setup({ freeze: false })
+    const session = ctx.sessions.create(SessionId('req-bare'))
+    session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
+    const bare = Object.freeze({ model: 'm', messages: Object.freeze([]), sessionId: session.id })
+    expect(() => { dispatch(ctx, bare) }).toThrow(/no step\/start/)
+
+    session.append('step/start', { turn: 1, step: 1 })
+    expect(() => { dispatch(ctx, bare) }).toThrow(/no request\/header event/)
+  })
+
+  it('rejects a frozen request carrying an unfrozen messages array', async () => {
+    const { ctx, session, boundary } = await requestSetup()
+    const options = Object.freeze({ model: 'm', messages: [...boundary], sessionId: session.id })
+    expect(() => { dispatch(ctx, options) }).toThrow(/frozen messages array/)
+  })
+
+  it('skips hand-built (unfrozen) requests — compaction summarize is out of scope', async () => {
+    const { ctx, session } = await requestSetup()
+    // Unfrozen envelope + arbitrary messages: a direct one-shot call.
+    const options = { model: 'summarizer', messages: [{ role: 'user', content: [{ type: 'text', text: 'summarize!' }] }], sessionId: session.id }
+    expect(() => { dispatch(ctx, options) }).not.toThrow()
+  })
+
+  it('skips requests without a sessionId or with an unknown session', async () => {
+    const { ctx } = await requestSetup()
+    expect(() => { dispatch(ctx, Object.freeze({ model: 'm', messages: Object.freeze([]) })) }).not.toThrow()
+    expect(() => { dispatch(ctx, Object.freeze({ model: 'm', messages: Object.freeze([]), sessionId: SessionId('ghost') })) }).not.toThrow()
+  })
+})
+
+describe('request cross-check ordering (prepend)', () => {
+  it('runs ahead of a short-circuiting llm/stream listener registered before it', async () => {
+    // The replay adapter returns its chunks WITHOUT calling next(), which
+    // would silence a later-registered check — snapshot compositions load
+    // replay before the app bundle that loads invariants. The check prepends,
+    // so it fires ahead of append-registered listeners regardless of load
+    // order. (Prepend orders it against APPENDED listeners only; correctness
+    // rests on the seq-bounded rebuild, not on listener timing.)
+    const ctx = new Context()
+    await ctx.plugin(SessionStore)
+    ctx.on('llm/stream', () => (async function* () {})() as never) // short-circuits, no next()
+    await ctx.plugin(Invariants, { freeze: false })
+
+    const session = ctx.sessions.create(SessionId('prepend-check'))
+    session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
+    session.append('user/message', { content: [{ type: 'text', text: 'hi' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
+    session.append('step/start', { turn: 1, step: 1 })
+    session.append('request/header', { header: { config: { model: 'm' } }, reason: 'initial' })
+
+    const divergent = Object.freeze({
+      model: 'm',
+      messages: Object.freeze([{ role: 'user', content: [{ type: 'text', text: 'phantom' }] }]),
+      sessionId: session.id,
+    })
+    expect(() => {
+      void ctx.waterfall('llm/stream', divergent as never, () => (async function* () {})() as never)
+    }).toThrow(/diverges from the boundary derivation/)
+  })
+})