diff --git a/src/domain/analysisView/EnvironmentStateWriteSimulation.test.ts b/src/domain/analysisView/EnvironmentStateWriteSimulation.test.ts
index 1f7bbe2375..d388b52a35 100644
--- a/src/domain/analysisView/EnvironmentStateWriteSimulation.test.ts
+++ b/src/domain/analysisView/EnvironmentStateWriteSimulation.test.ts
@@ -277,6 +277,97 @@ describe("slug rules learned from a later call", () => {
   });
 });
 
+describe("clock rules learned across calls", () => {
+  it("serves a clock only when every recorded call changed that field", () => {
+    const oldFirst = "2026-10-01T09:00:00Z";
+    const oldSecond = "2026-10-01T10:00:00Z";
+    const described = (
+      entry: Book,
+      description: string,
+      publishedAt: string,
+    ): Book => ({ ...entry, description, published_at: publishedAt });
+    const first = described(FIRST_BOOK, "original", oldFirst);
+    const second = described(SECOND_BOOK, "other", oldSecond);
+    const published = described(
+      renamedBook(FIRST_BOOK, "Renamed Book", 3),
+      "original",
+      bookClock(3),
+    );
+    /** The first call stamps `published_at`; `later` is the second call's answer. */
+    const rows = (later: Book, sent: Book): TimedCaptureRow[] => [
+      ["GET", "/api/books", [first, second]],
+      ["GET", "/api/books/1", first],
+      ["GET", "/api/books/2", second],
+      ["PATCH", "/api/books/1", published, 200, { name: "Renamed Book" }],
+      ["GET", "/api/books/1", published],
+      ["PATCH", "/api/books/2", later, 200, sent],
+      ["GET", "/api/books/2", later],
+    ];
+    // The description edit stamps `updated_at` and leaves `published_at` as it was.
+    const edited = described(
+      { ...SECOND_BOOK, updated_at: bookClock(5) },
+      "revised",
+      oldSecond,
+    );
+    expect(
+      gated(rows(edited, { description: "revised" }), "patch_books_by_id"),
+    ).toMatchObject({ served: false, reason: "unexplained-change" });
+    const stamped = described(
+      renamedBook(SECOND_BOOK, "Second Edition", 5),
+      "other",
+      bookClock(5),
+    );
+    const plan = gated(
+      rows(stamped, { name: "Second Edition" }),
+      "patch_books_by_id",
+    );
+    expect(plan).toMatchObject({
+      served: true,
+      checked_record_ordinals: [4, 6],
+    });
+    expect(plan.rules).toContainEqual({
+      field: "published_at",
+      rule: "clock",
+      format: { kind: "iso", separator: "T", fraction: 0, zone: "Z" },
+    });
+  });
+
+  it("leaves a clock unserved when an earlier call kept the field a later call stamped", () => {
+    const oldFirst = "2026-10-01T09:00:00Z";
+    const oldSecond = "2026-10-01T10:00:00Z";
+    const described = (
+      entry: Book,
+      description: string,
+      publishedAt: string,
+    ): Book => ({ ...entry, description, published_at: publishedAt });
+    const first = described(FIRST_BOOK, "original", oldFirst);
+    const second = described(SECOND_BOOK, "other", oldSecond);
+    const edited = described(
+      { ...SECOND_BOOK, updated_at: bookClock(3) },
+      "revised",
+      oldSecond,
+    );
+    const published = described(
+      renamedBook(FIRST_BOOK, "Renamed Book", 5),
+      "original",
+      bookClock(5),
+    );
+    const rows: TimedCaptureRow[] = [
+      ["GET", "/api/books", [first, second]],
+      ["GET", "/api/books/1", first],
+      ["GET", "/api/books/2", second],
+      ["PATCH", "/api/books/2", edited, 200, { description: "revised" }],
+      ["GET", "/api/books/2", edited],
+      ["PATCH", "/api/books/1", published, 200, { name: "Renamed Book" }],
+      ["GET", "/api/books/1", published],
+    ];
+    expect(gated(rows, "patch_books_by_id")).toMatchObject({
+      served: false,
+      reason: "unexplained-change",
+    });
+  });
+});
+
 describe("checked lines", () => {
   /** A list page that echoes its own request path, as some APIs do. */
   const page = (path: string, items: JsonValue[]): JsonValue => ({
@@ -1057,6 +1148,30 @@ describe("what a check leaves pending", () => {
     });
   });
 
+  it("learns an update from a read begun after the write, not from one that overlapped it", () => {
+    const named = (name: string) => ({ id: 1, name });
+    const other = { id: 2, name: "B" };
+    const rows: TimedCaptureRow[] = [
+      ["GET", "/api/books", [named("A"), other]],
+      ["GET", "/api/books/1", named("A")],
+      ["GET", "/api/books/2", other],
+      // The patch runs from second 3 to 5.5 and returns no entity. A read begun while it still runs shows the old name.
+      ["PATCH", "/api/books/1", "", 204, { name: "Z" }, 2500],
+      ["GET", "/api/books/1", named("A")],
+      ["GET", "/api/books/1", named("A")],
+      ["GET", "/api/books/1", named("Z")],
+      ["GET", "/api/books", [named("Z"), other]],
+    ];
+    expect(gated(rows, "patch_books_by_id")).toMatchObject({
+      served: true,
+      observation: { ordinal: 6, pointer: "" },
+      body: { accepted: ["name"], fixed: [] },
+      rules: [{ field: "name", rule: "echo" }],
+      checked_record_ordinals: [7],
+      contradicted_by: null,
+    });
+  });
+
   it("checks nothing by a range read the collection has outgrown", () => {
     const rows: TimedCaptureRow[] = [
       ["GET", "/api/books?_start=0&_end=2", [FIRST_BOOK, SECOND_BOOK]],
diff --git a/src/domain/analysisView/environmentStateWriteAttribution.ts b/src/domain/analysisView/environmentStateWriteAttribution.ts
index 71c3970a1d..d46a2462d0 100644
--- a/src/domain/analysisView/environmentStateWriteAttribution.ts
+++ b/src/domain/analysisView/environmentStateWriteAttribution.ts
@@ -19,12 +19,16 @@ export type PendingTargets = Map<
   { until: number; readonly at: Map<string, number> }
 >;
 
+/** Whether a write that ended at `until` was still running when a read began at `start`, so the read may show either state. */
+export const overlapsWrite = (until: number, start: number): boolean =>
+  until > start;
+
 /** Whether a modelled write to the target was still running when a read began at `start`, so the read may show either state. */
 export const runningAt = (
   pending: PendingTargets,
   key: string,
   start: number,
-): boolean => (pending.get(key)?.until ?? -Infinity) > start;
+): boolean => overlapsWrite(pending.get(key)?.until ?? -Infinity, start);
 
 /** The tools whose writes a read exercises: those that changed its targets since they were last checked, less those that borrowed the read to learn their effect. */
 export const exercisedBy = (
diff --git a/src/domain/analysisView/environmentStateWriteEffects.ts b/src/domain/analysisView/environmentStateWriteEffects.ts
index ca3696f492..1c424d75f4 100644
--- a/src/domain/analysisView/environmentStateWriteEffects.ts
+++ b/src/domain/analysisView/environmentStateWriteEffects.ts
@@ -313,13 +313,15 @@ const contradictsEarlierCalls = (
   const ruled = new Set(draft.plan.rules.map((rule) => rule.field));
   return rules.some(
     (rule) =>
-      unsentChange(rule) &&
+      (unsentChange(rule) || rule.rule === "clock") &&
       !ruled.has(rule.field) &&
       draft.updates.some(({ shown, sent }) => {
         // An unruled field an earlier call showed is one it left as it was.
         const value = shown.get(rule.field);
         if (value === undefined) return false;
         if (rule.rule === "increment") return integerOf(value) !== undefined;
+        // Serving stamps a clock on every call, so a field an earlier call kept contradicts it.
+        if (rule.rule === "clock") return true;
         const from = rule.rule === "slug" ? sent.get(rule.from) : undefined;
         return typeof from === "string" && slugOf(from) !== value;
       }),
diff --git a/src/domain/analysisView/environmentStateWriteObserve.ts b/src/domain/analysisView/environmentStateWriteObserve.ts
index 9994d2192d..f9c4c4febe 100644
--- a/src/domain/analysisView/environmentStateWriteObserve.ts
+++ b/src/domain/analysisView/environmentStateWriteObserve.ts
@@ -1,4 +1,5 @@
 import type { JsonValue } from "../jsonValue.js";
+import { overlapsWrite } from "./environmentStateWriteAttribution.js";
 import {
   collectionPathOf,
   isObject,
@@ -62,7 +63,7 @@ export const newEntityIn = (
 export interface ObservationSearch {
   readonly records: readonly StateRecordFacts[];
   readonly from: number;
-  /** A timed write's completion; reads begun before it cannot observe its effect. */
+  /** A timed write's completion. A read begun while it still ran may show the prior state. */
   readonly after?: number;
   readonly viewOf: (ordinal: number) => StateView | undefined;
   readonly withBody: <T>(
@@ -77,6 +78,7 @@ const succeeded = (record: StateRecordFacts): boolean =>
 /**
  * The first later read, before the next write, that shows a written entity: an item read of
  * `id` when given, else an item read of an unheld id or a collection read listing exactly one.
+ * A read begun while the write still ran may show the earlier state, so it is not chosen.
  */
 export const laterObservation = (
   search: ObservationSearch,
@@ -93,7 +95,9 @@ export const laterObservation = (
 ): Observed | undefined => {
   for (const record of search.records.slice(search.from + 1)) {
     if (target.isWrite(record)) return undefined;
-    if (search.after !== undefined && record.start < search.after) continue;
+    // The same overlap the check path ignores: the write was still running when this read began.
+    if (search.after !== undefined && overlapsWrite(search.after, record.start))
+      continue;
     const view = search.viewOf(record.ordinal);
     if (
       !succeeded(record) ||