core: extract session entry stepping logic into dedicated module

Move the step function from session-entry.ts to session-entry-stepper.ts and remove immer dependency. Add static fromEvent factory methods to Synthetic, Assistant, and Compaction classes for cleaner event-to-entry conversion.

3 files changed, 446 insertions, 173 deletions

diff --git a/packages/opencode/src/v2/session-entry-stepper.ts b/packages/opencode/src/v2/session-entry-stepper.ts
new file mode 100644
index 000000000..3d642579d
--- /dev/null
+++ b/packages/opencode/src/v2/session-entry-stepper.ts
@@ -0,0 +1,253 @@
+import { castDraft, produce, type WritableDraft } from "immer"
+import { SessionEvent } from "./session-event"
+import { SessionEntry } from "./session-entry"
+
+export type MemoryState = {
+  entries: SessionEntry.Entry[]
+  pending: SessionEntry.Entry[]
+}
+
+export interface Adapter<Result> {
+  readonly getCurrentAssistant: () => SessionEntry.Assistant | undefined
+  readonly updateAssistant: (assistant: SessionEntry.Assistant) => void
+  readonly appendEntry: (entry: SessionEntry.Entry) => void
+  readonly appendPending: (entry: SessionEntry.Entry) => void
+  readonly finish: () => Result
+}
+
+export function memory(state: MemoryState): Adapter<MemoryState> {
+  const activeAssistantIndex = () =>
+    state.entries.findLastIndex((entry) => entry.type === "assistant" && !entry.time.completed)
+
+  return {
+    getCurrentAssistant() {
+      const index = activeAssistantIndex()
+      if (index < 0) return
+      const assistant = state.entries[index]
+      return assistant?.type === "assistant" ? assistant : undefined
+    },
+    updateAssistant(assistant) {
+      const index = activeAssistantIndex()
+      if (index < 0) return
+      const current = state.entries[index]
+      if (current?.type !== "assistant") return
+      state.entries[index] = assistant
+    },
+    appendEntry(entry) {
+      state.entries.push(entry)
+    },
+    appendPending(entry) {
+      state.pending.push(entry)
+    },
+    finish() {
+      return state
+    },
+  }
+}
+
+export function stepWith<Result>(adapter: Adapter<Result>, event: SessionEvent.Event): Result {
+  const currentAssistant = adapter.getCurrentAssistant()
+  type DraftAssistant = WritableDraft<SessionEntry.Assistant>
+  type DraftTool = WritableDraft<SessionEntry.AssistantTool>
+  type DraftText = WritableDraft<SessionEntry.AssistantText>
+  type DraftReasoning = WritableDraft<SessionEntry.AssistantReasoning>
+
+  const latestTool = (assistant: DraftAssistant | undefined, callID?: string) =>
+    assistant?.content.findLast(
+      (item): item is DraftTool => item.type === "tool" && (callID === undefined || item.callID === callID),
+    )
+
+  const latestText = (assistant: DraftAssistant | undefined) =>
+    assistant?.content.findLast((item): item is DraftText => item.type === "text")
+
+  const latestReasoning = (assistant: DraftAssistant | undefined) =>
+    assistant?.content.findLast((item): item is DraftReasoning => item.type === "reasoning")
+
+  SessionEvent.Event.match(event, {
+    prompt: (event) => {
+      const entry = SessionEntry.User.fromEvent(event)
+      if (currentAssistant) {
+        adapter.appendPending(entry)
+        return
+      }
+      adapter.appendEntry(entry)
+    },
+    synthetic: (event) => {
+      adapter.appendEntry(SessionEntry.Synthetic.fromEvent(event))
+    },
+    "step.started": (event) => {
+      if (currentAssistant) {
+        adapter.updateAssistant(
+          produce(currentAssistant, (draft) => {
+            draft.time.completed = event.timestamp
+          }),
+        )
+      }
+      adapter.appendEntry(SessionEntry.Assistant.fromEvent(event))
+    },
+    "step.ended": (event) => {
+      if (currentAssistant) {
+        adapter.updateAssistant(
+          produce(currentAssistant, (draft) => {
+            draft.time.completed = event.timestamp
+            draft.cost = event.cost
+            draft.tokens = event.tokens
+          }),
+        )
+      }
+    },
+    "text.started": () => {
+      if (currentAssistant) {
+        adapter.updateAssistant(
+          produce(currentAssistant, (draft) => {
+            draft.content.push({
+              type: "text",
+              text: "",
+            })
+          }),
+        )
+      }
+    },
+    "text.delta": (event) => {
+      if (currentAssistant) {
+        adapter.updateAssistant(
+          produce(currentAssistant, (draft) => {
+            const match = latestText(draft)
+            if (match) match.text += event.delta
+          }),
+        )
+      }
+    },
+    "text.ended": () => {},
+    "tool.input.started": (event) => {
+      if (currentAssistant) {
+        adapter.updateAssistant(
+          produce(currentAssistant, (draft) => {
+            draft.content.push({
+              type: "tool",
+              callID: event.callID,
+              name: event.name,
+              time: {
+                created: event.timestamp,
+              },
+              state: {
+                status: "pending",
+                input: "",
+              },
+            })
+          }),
+        )
+      }
+    },
+    "tool.input.delta": (event) => {
+      if (currentAssistant) {
+        adapter.updateAssistant(
+          produce(currentAssistant, (draft) => {
+            const match = latestTool(draft, event.callID)
+            // oxlint-disable-next-line no-base-to-string -- event.delta is a Schema.String (runtime string)
+            if (match && match.state.status === "pending") match.state.input += event.delta
+          }),
+        )
+      }
+    },
+    "tool.input.ended": () => {},
+    "tool.called": (event) => {
+      if (currentAssistant) {
+        adapter.updateAssistant(
+          produce(currentAssistant, (draft) => {
+            const match = latestTool(draft, event.callID)
+            if (match) {
+              match.time.ran = event.timestamp
+              match.state = {
+                status: "running",
+                input: event.input,
+              }
+            }
+          }),
+        )
+      }
+    },
+    "tool.success": (event) => {
+      if (currentAssistant) {
+        adapter.updateAssistant(
+          produce(currentAssistant, (draft) => {
+            const match = latestTool(draft, event.callID)
+            if (match && match.state.status === "running") {
+              match.state = {
+                status: "completed",
+                input: match.state.input,
+                output: event.output ?? "",
+                title: event.title,
+                metadata: event.metadata ?? {},
+                attachments: [...(event.attachments ?? [])],
+              }
+            }
+          }),
+        )
+      }
+    },
+    "tool.error": (event) => {
+      if (currentAssistant) {
+        adapter.updateAssistant(
+          produce(currentAssistant, (draft) => {
+            const match = latestTool(draft, event.callID)
+            if (match && match.state.status === "running") {
+              match.state = {
+                status: "error",
+                error: event.error,
+                input: match.state.input,
+                metadata: event.metadata ?? {},
+              }
+            }
+          }),
+        )
+      }
+    },
+    "reasoning.started": () => {
+      if (currentAssistant) {
+        adapter.updateAssistant(
+          produce(currentAssistant, (draft) => {
+            draft.content.push({
+              type: "reasoning",
+              text: "",
+            })
+          }),
+        )
+      }
+    },
+    "reasoning.delta": (event) => {
+      if (currentAssistant) {
+        adapter.updateAssistant(
+          produce(currentAssistant, (draft) => {
+            const match = latestReasoning(draft)
+            if (match) match.text += event.delta
+          }),
+        )
+      }
+    },
+    "reasoning.ended": (event) => {
+      if (currentAssistant) {
+        adapter.updateAssistant(
+          produce(currentAssistant, (draft) => {
+            const match = latestReasoning(draft)
+            if (match) match.text = event.text
+          }),
+        )
+      }
+    },
+    retried: () => {},
+    compacted: (event) => {
+      adapter.appendEntry(SessionEntry.Compaction.fromEvent(event))
+    },
+  })
+
+  return adapter.finish()
+}
+
+export function step(old: MemoryState, event: SessionEvent.Event): MemoryState {
+  return produce(old, (draft) => {
+    stepWith(memory(draft as MemoryState), event)
+  })
+}
+
+export * as SessionEntryStepper from "./session-entry-stepper"
diff --git a/packages/opencode/src/v2/session-entry.ts b/packages/opencode/src/v2/session-entry.ts
index 08122428a..97c5fc7ce 100644
--- a/packages/opencode/src/v2/session-entry.ts
+++ b/packages/opencode/src/v2/session-entry.ts
@@ -1,6 +1,5 @@
 import { Schema } from "effect"
 import { SessionEvent } from "./session-event"
-import { castDraft, produce } from "immer"
 
 export const ID = SessionEvent.ID
 export type ID = Schema.Schema.Type<typeof ID>
@@ -40,7 +39,14 @@ export class Synthetic extends Schema.Class<Synthetic>("Session.Entry.Synthetic"
   ...SessionEvent.Synthetic.fields,
   ...Base,
   type: Schema.Literal("synthetic"),
-}) {}
+}) {
+  static fromEvent(event: SessionEvent.Synthetic) {
+    return new Synthetic({
+      ...event,
+      time: { created: event.timestamp },
+    })
+  }
+}
 
 export class ToolStatePending extends Schema.Class<ToolStatePending>("Session.Entry.ToolState.Pending")({
   status: Schema.Literal("pending"),
@@ -122,13 +128,32 @@ export class Assistant extends Schema.Class<Assistant>("Session.Entry.Assistant"
     created: Schema.DateTimeUtc,
     completed: Schema.DateTimeUtc.pipe(Schema.optional),
   }),
-}) {}
+}) {
+  static fromEvent(event: SessionEvent.Step.Started) {
+    return new Assistant({
+      id: event.id,
+      type: "assistant",
+      time: {
+        created: event.timestamp,
+      },
+      content: [],
+    })
+  }
+}
 
 export class Compaction extends Schema.Class<Compaction>("Session.Entry.Compaction")({
   ...SessionEvent.Compacted.fields,
   type: Schema.Literal("compaction"),
   ...Base,
-}) {}
+}) {
+  static fromEvent(event: SessionEvent.Compacted) {
+    return new Compaction({
+      ...event,
+      type: "compaction",
+      time: { created: event.timestamp },
+    })
+  }
+}
 
 export const Entry = Schema.Union([User, Synthetic, Assistant, Compaction]).pipe(Schema.toTaggedUnion("type"))
 
@@ -136,151 +161,6 @@ export type Entry = Schema.Schema.Type<typeof Entry>
 
 export type Type = Entry["type"]
 
-export type History = {
-  entries: Entry[]
-  pending: Entry[]
-}
-
-export function step(old: History, event: SessionEvent.Event): History {
-  return produce(old, (draft) => {
-    const lastAssistant = draft.entries.findLast((x) => x.type === "assistant")
-    const pendingAssistant = lastAssistant && !lastAssistant.time.completed ? lastAssistant : undefined
-    type DraftContent = NonNullable<typeof pendingAssistant>["content"][number]
-    type DraftTool = Extract<DraftContent, { type: "tool" }>
-
-    const latestTool = (callID?: string) =>
-      pendingAssistant?.content.findLast(
-        (item): item is DraftTool => item.type === "tool" && (callID === undefined || item.callID === callID),
-      )
-    const latestText = () => pendingAssistant?.content.findLast((item) => item.type === "text")
-    const latestReasoning = () => pendingAssistant?.content.findLast((item) => item.type === "reasoning")
-
-    SessionEvent.Event.match(event, {
-      prompt: (event) => {
-        const entry = User.fromEvent(event)
-        if (pendingAssistant) {
-          draft.pending.push(castDraft(entry))
-          return
-        }
-        draft.entries.push(castDraft(entry))
-      },
-      synthetic: (event) => {
-        draft.entries.push(new Synthetic({ ...event, time: { created: event.timestamp } }))
-      },
-      "step.started": (event) => {
-        if (pendingAssistant) pendingAssistant.time.completed = event.timestamp
-        draft.entries.push({
-          id: event.id,
-          type: "assistant",
-          time: {
-            created: event.timestamp,
-          },
-          content: [],
-        })
-      },
-      "step.ended": (event) => {
-        if (!pendingAssistant) return
-        pendingAssistant.time.completed = event.timestamp
-        pendingAssistant.cost = event.cost
-        pendingAssistant.tokens = event.tokens
-      },
-      "text.started": () => {
-        if (!pendingAssistant) return
-        pendingAssistant.content.push({
-          type: "text",
-          text: "",
-        })
-      },
-      "text.delta": (event) => {
-        if (!pendingAssistant) return
-        const match = latestText()
-        if (match) match.text += event.delta
-      },
-      "text.ended": () => {},
-      "tool.input.started": (event) => {
-        if (!pendingAssistant) return
-        pendingAssistant.content.push({
-          type: "tool",
-          callID: event.callID,
-          name: event.name,
-          time: {
-            created: event.timestamp,
-          },
-          state: {
-            status: "pending",
-            input: "",
-          },
-        })
-      },
-      "tool.input.delta": (event) => {
-        if (!pendingAssistant) return
-        const match = latestTool(event.callID)
-        // oxlint-disable-next-line no-base-to-string -- event.delta is a Schema.String (runtime string)
-        if (match) match.state.input += event.delta
-      },
-      "tool.input.ended": () => {},
-      "tool.called": (event) => {
-        if (!pendingAssistant) return
-        const match = latestTool(event.callID)
-        if (match) {
-          match.time.ran = event.timestamp
-          match.state = {
-            status: "running",
-            input: event.input,
-          }
-        }
-      },
-      "tool.success": (event) => {
-        if (!pendingAssistant) return
-        const match = latestTool(event.callID)
-        if (match && match.state.status === "running") {
-          match.state = {
-            status: "completed",
-            input: match.state.input,
-            output: event.output ?? "",
-            title: event.title,
-            metadata: event.metadata ?? {},
-            attachments: [...(event.attachments ?? [])],
-          }
-        }
-      },
-      "tool.error": (event) => {
-        if (!pendingAssistant) return
-        const match = latestTool(event.callID)
-        if (match && match.state.status === "running") {
-          match.state = {
-            status: "error",
-            error: event.error,
-            input: match.state.input,
-            metadata: event.metadata ?? {},
-          }
-        }
-      },
-      "reasoning.started": () => {
-        if (!pendingAssistant) return
-        pendingAssistant.content.push({
-          type: "reasoning",
-          text: "",
-        })
-      },
-      "reasoning.delta": (event) => {
-        if (!pendingAssistant) return
-        const match = latestReasoning()
-        if (match) match.text += event.delta
-      },
-      "reasoning.ended": (event) => {
-        if (!pendingAssistant) return
-        const match = latestReasoning()
-        if (match) match.text = event.text
-      },
-      retried: () => {},
-      compacted: (event) => {
-        draft.entries.push(new Compaction({ ...event, type: "compaction", time: { created: event.timestamp } }))
-      },
-    })
-  })
-}
-
 /*
 export interface Interface {
   readonly decode: (row: typeof SessionEntryTable.$inferSelect) => Entry
diff --git a/packages/opencode/test/session/session-entry.test.ts b/packages/opencode/test/session/session-entry-stepper.test.ts
index dea8da20a..a81b4c2be 100644
--- a/packages/opencode/test/session/session-entry.test.ts
+++ b/packages/opencode/test/session/session-entry-stepper.test.ts
@@ -2,6 +2,7 @@ import { describe, expect, test } from "bun:test"
 import * as DateTime from "effect/DateTime"
 import * as FastCheck from "effect/testing/FastCheck"
 import { SessionEntry } from "../../src/v2/session-entry"
+import { SessionEntryStepper } from "../../src/v2/session-entry-stepper"
 import { SessionEvent } from "../../src/v2/session-event"
 
 const time = (n: number) => DateTime.makeUnsafe(n)
@@ -29,8 +30,8 @@ function assistant() {
   })
 }
 
-function history() {
-  const state: SessionEntry.History = {
+function memoryState() {
+  const state: SessionEntryStepper.MemoryState = {
     entries: [],
     pending: [],
   }
@@ -38,51 +39,189 @@ function history() {
 }
 
 function active() {
-  const state: SessionEntry.History = {
+  const state: SessionEntryStepper.MemoryState = {
     entries: [assistant()],
     pending: [],
   }
   return state
 }
 
-function run(events: SessionEvent.Event[], state = history()) {
-  return events.reduce<SessionEntry.History>((state, event) => SessionEntry.step(state, event), state)
+function run(events: SessionEvent.Event[], state = memoryState()) {
+  return events.reduce<SessionEntryStepper.MemoryState>((state, event) => SessionEntryStepper.step(state, event), state)
 }
 
-function last(state: SessionEntry.History) {
+function last(state: SessionEntryStepper.MemoryState) {
   const entry = [...state.pending, ...state.entries].reverse().find((x) => x.type === "assistant")
   expect(entry?.type).toBe("assistant")
   return entry?.type === "assistant" ? entry : undefined
 }
 
-function texts_of(state: SessionEntry.History) {
+function texts_of(state: SessionEntryStepper.MemoryState) {
   const entry = last(state)
   if (!entry) return []
   return entry.content.filter((x): x is SessionEntry.AssistantText => x.type === "text")
 }
 
-function reasons(state: SessionEntry.History) {
+function reasons(state: SessionEntryStepper.MemoryState) {
   const entry = last(state)
   if (!entry) return []
   return entry.content.filter((x): x is SessionEntry.AssistantReasoning => x.type === "reasoning")
 }
 
-function tools(state: SessionEntry.History) {
+function tools(state: SessionEntryStepper.MemoryState) {
   const entry = last(state)
   if (!entry) return []
   return entry.content.filter((x): x is SessionEntry.AssistantTool => x.type === "tool")
 }
 
-function tool(state: SessionEntry.History, callID: string) {
+function tool(state: SessionEntryStepper.MemoryState, callID: string) {
   return tools(state).find((x) => x.callID === callID)
 }
 
-describe("session-entry step", () => {
-  describe("seeded pending assistant", () => {
+function adapterStore() {
+  return {
+    committed: [] as SessionEntry.Entry[],
+    deferred: [] as SessionEntry.Entry[],
+  }
+}
+
+function adapterFor(store: ReturnType<typeof adapterStore>): SessionEntryStepper.Adapter<typeof store> {
+  const activeAssistantIndex = () =>
+    store.committed.findLastIndex((entry) => entry.type === "assistant" && !entry.time.completed)
+
+  const getCurrentAssistant = () => {
+    const index = activeAssistantIndex()
+    if (index < 0) return
+    const assistant = store.committed[index]
+    return assistant?.type === "assistant" ? assistant : undefined
+  }
+
+  return {
+    getCurrentAssistant,
+    updateAssistant(assistant) {
+      const index = activeAssistantIndex()
+      if (index < 0) return
+      const current = store.committed[index]
+      if (current?.type !== "assistant") return
+      store.committed[index] = assistant
+    },
+    appendEntry(entry) {
+      store.committed.push(entry)
+    },
+    appendPending(entry) {
+      store.deferred.push(entry)
+    },
+    finish() {
+      return store
+    },
+  }
+}
+
+describe("session-entry-stepper", () => {
+  describe("stepWith", () => {
+    test("reduces through a custom adapter", () => {
+      const store = adapterStore()
+      store.committed.push(assistant())
+
+      SessionEntryStepper.stepWith(adapterFor(store), SessionEvent.Prompt.create({ text: "hello", timestamp: time(1) }))
+      SessionEntryStepper.stepWith(adapterFor(store), SessionEvent.Reasoning.Started.create({ timestamp: time(2) }))
+      SessionEntryStepper.stepWith(adapterFor(store), SessionEvent.Reasoning.Delta.create({ delta: "thinking", timestamp: time(3) }))
+      SessionEntryStepper.stepWith(adapterFor(store), SessionEvent.Reasoning.Ended.create({ text: "thought", timestamp: time(4) }))
+      SessionEntryStepper.stepWith(adapterFor(store), SessionEvent.Text.Started.create({ timestamp: time(5) }))
+      SessionEntryStepper.stepWith(adapterFor(store), SessionEvent.Text.Delta.create({ delta: "world", timestamp: time(6) }))
+      SessionEntryStepper.stepWith(
+        adapterFor(store),
+        SessionEvent.Step.Ended.create({
+          reason: "stop",
+          cost: 1,
+          tokens: {
+            input: 1,
+            output: 2,
+            reasoning: 3,
+            cache: {
+              read: 4,
+              write: 5,
+            },
+          },
+          timestamp: time(7),
+        }),
+      )
+
+      expect(store.deferred).toHaveLength(1)
+      expect(store.deferred[0]?.type).toBe("user")
+      expect(store.committed).toHaveLength(1)
+      expect(store.committed[0]?.type).toBe("assistant")
+      if (store.committed[0]?.type !== "assistant") return
+
+      expect(store.committed[0].content).toEqual([
+        { type: "reasoning", text: "thought" },
+        { type: "text", text: "world" },
+      ])
+      expect(store.committed[0].time.completed).toEqual(time(7))
+    })
+  })
+
+  describe("memory", () => {
+    test("tracks and replaces the current assistant", () => {
+      const state = active()
+      const adapter = SessionEntryStepper.memory(state)
+      const current = adapter.getCurrentAssistant()
+
+      expect(current?.type).toBe("assistant")
+      if (!current) return
+
+      adapter.updateAssistant(
+        new SessionEntry.Assistant({
+          ...current,
+          content: [new SessionEntry.AssistantText({ type: "text", text: "done" })],
+          time: {
+            ...current.time,
+            completed: time(1),
+          },
+        }),
+      )
+
+      expect(adapter.getCurrentAssistant()).toBeUndefined()
+      expect(state.entries[0]?.type).toBe("assistant")
+      if (state.entries[0]?.type !== "assistant") return
+
+      expect(state.entries[0].content).toEqual([{ type: "text", text: "done" }])
+      expect(state.entries[0].time.completed).toEqual(time(1))
+    })
+
+    test("appends committed and pending entries", () => {
+      const state = memoryState()
+      const adapter = SessionEntryStepper.memory(state)
+      const committed = SessionEntry.User.fromEvent(SessionEvent.Prompt.create({ text: "committed", timestamp: time(1) }))
+      const pending = SessionEntry.User.fromEvent(SessionEvent.Prompt.create({ text: "pending", timestamp: time(2) }))
+
+      adapter.appendEntry(committed)
+      adapter.appendPending(pending)
+
+      expect(state.entries).toEqual([committed])
+      expect(state.pending).toEqual([pending])
+    })
+
+    test("stepWith through memory records reasoning", () => {
+      const state = active()
+
+      SessionEntryStepper.stepWith(SessionEntryStepper.memory(state), SessionEvent.Reasoning.Started.create({ timestamp: time(1) }))
+      SessionEntryStepper.stepWith(SessionEntryStepper.memory(state), SessionEvent.Reasoning.Delta.create({ delta: "draft", timestamp: time(2) }))
+      SessionEntryStepper.stepWith(
+        SessionEntryStepper.memory(state),
+        SessionEvent.Reasoning.Ended.create({ text: "final", timestamp: time(3) }),
+      )
+
+      expect(reasons(state)).toEqual([{ type: "reasoning", text: "final" }])
+    })
+  })
+
+  describe("step", () => {
+    describe("seeded pending assistant", () => {
     test("stores prompts in entries when no assistant is pending", () => {
       FastCheck.assert(
         FastCheck.property(word, (body) => {
-          const next = SessionEntry.step(history(), SessionEvent.Prompt.create({ text: body, timestamp: time(1) }))
+          const next = SessionEntryStepper.step(memoryState(), SessionEvent.Prompt.create({ text: body, timestamp: time(1) }))
           expect(next.entries).toHaveLength(1)
           expect(next.entries[0]?.type).toBe("user")
           if (next.entries[0]?.type !== "user") return
@@ -95,7 +234,7 @@ describe("session-entry step", () => {
     test("stores prompts in pending when an assistant is pending", () => {
       FastCheck.assert(
         FastCheck.property(word, (body) => {
-          const next = SessionEntry.step(active(), SessionEvent.Prompt.create({ text: body, timestamp: time(1) }))
+          const next = SessionEntryStepper.step(active(), SessionEvent.Prompt.create({ text: body, timestamp: time(1) }))
           expect(next.pending).toHaveLength(1)
           expect(next.pending[0]?.type).toBe("user")
           if (next.pending[0]?.type !== "user") return
@@ -110,8 +249,8 @@ describe("session-entry step", () => {
         FastCheck.property(texts, (parts) => {
           const next = parts.reduce(
             (state, part, i) =>
-              SessionEntry.step(state, SessionEvent.Text.Delta.create({ delta: part, timestamp: time(i + 2) })),
-            SessionEntry.step(active(), SessionEvent.Text.Started.create({ timestamp: time(1) })),
+              SessionEntryStepper.step(state, SessionEvent.Text.Delta.create({ delta: part, timestamp: time(i + 2) })),
+            SessionEntryStepper.step(active(), SessionEvent.Text.Started.create({ timestamp: time(1) })),
           )
 
           expect(texts_of(next)).toEqual([
@@ -302,7 +441,7 @@ describe("session-entry step", () => {
             },
             timestamp: time(n),
           })
-          const next = SessionEntry.step(active(), event)
+          const next = SessionEntryStepper.step(active(), event)
           const entry = last(next)
           expect(entry).toBeDefined()
           if (!entry) return
@@ -316,12 +455,12 @@ describe("session-entry step", () => {
     })
   })
 
-  describe("known reducer gaps", () => {
+    describe("known reducer gaps", () => {
     test("prompt appends immutably when no assistant is pending", () => {
       FastCheck.assert(
         FastCheck.property(word, (body) => {
-          const old = history()
-          const next = SessionEntry.step(old, SessionEvent.Prompt.create({ text: body, timestamp: time(1) }))
+          const old = memoryState()
+          const next = SessionEntryStepper.step(old, SessionEvent.Prompt.create({ text: body, timestamp: time(1) }))
           expect(old).not.toBe(next)
           expect(old.entries).toHaveLength(0)
           expect(next.entries).toHaveLength(1)
@@ -334,7 +473,7 @@ describe("session-entry step", () => {
       FastCheck.assert(
         FastCheck.property(word, (body) => {
           const old = active()
-          const next = SessionEntry.step(old, SessionEvent.Prompt.create({ text: body, timestamp: time(1) }))
+          const next = SessionEntryStepper.step(old, SessionEvent.Prompt.create({ text: body, timestamp: time(1) }))
           expect(old).not.toBe(next)
           expect(old.pending).toHaveLength(0)
           expect(next.pending).toHaveLength(1)
@@ -651,7 +790,7 @@ describe("session-entry step", () => {
     test("records synthetic events", () => {
       FastCheck.assert(
         FastCheck.property(word, (body) => {
-          const next = SessionEntry.step(history(), SessionEvent.Synthetic.create({ text: body, timestamp: time(1) }))
+          const next = SessionEntryStepper.step(memoryState(), SessionEvent.Synthetic.create({ text: body, timestamp: time(1) }))
           expect(next.entries).toHaveLength(1)
           expect(next.entries[0]?.type).toBe("synthetic")
           if (next.entries[0]?.type !== "synthetic") return
@@ -664,8 +803,8 @@ describe("session-entry step", () => {
     test("records compaction events", () => {
       FastCheck.assert(
         FastCheck.property(FastCheck.boolean(), maybe(FastCheck.boolean()), (auto, overflow) => {
-          const next = SessionEntry.step(
-            history(),
+          const next = SessionEntryStepper.step(
+            memoryState(),
             SessionEvent.Compacted.create({ auto, overflow, timestamp: time(1) }),
           )
           expect(next.entries).toHaveLength(1)
@@ -677,5 +816,6 @@ describe("session-entry step", () => {
         { numRuns: 50 },
       )
     })
+    })
   })
 })