diff --git a/types/channels-session/reducer.ts b/types/channels-session/reducer.ts
index 7f657a05..88a277b7 100644
--- a/types/channels-session/reducer.ts
+++ b/types/channels-session/reducer.ts
@@ -32,6 +32,27 @@ import {
 import type { SessionAction } from '../action-origin.generated.js';
 import { softAssertNever } from '../common/reducer-helpers.js';
 
+// ─── Injectable clock ──────────────────────────────────────────────────────────
+
+/**
+ * Provider for the `modifiedAt` timestamps the session reducer stamps. Defaults to `Date.now`.
+ *
+ * The reducer is documented as pure, and "the same reducer code runs on both server and client,
+ * which is what makes write-ahead possible" — but reading the ambient wall clock makes an identical
+ * `(state, action)` pair produce different `summary.modifiedAt` depending on when it runs. The Go,
+ * Kotlin, and Rust clients already expose an injectable now-seam for exactly this (Go `SetNowProvider`,
+ * Kotlin `currentTimestampProvider`, Rust `MOCK_NOW_MS`); the TypeScript client was the only one
+ * forcing consumers to monkeypatch the process-global `Date.now`. This brings TS to parity.
+ *
+ * Tests/hosts set this for deterministic output; production leaves the default. See issue #186.
+ */
+export let currentTimestampProvider: () => number = () => Date.now();
+
+/** Override {@link currentTimestampProvider} (e.g. tests/hosts needing deterministic `modifiedAt`). */
+export function setCurrentTimestampProvider(provider: () => number): void {
+  currentTimestampProvider = provider;
+}
+
 // ─── Helpers ─────────────────────────────────────────────────────────────────
 
 /** Extracts the common base fields shared by all tool call lifecycle states. */
@@ -154,7 +175,7 @@ function endTurn(
     ...state,
     turns: [...state.turns, turn],
     activeTurn: undefined,
-    summary: { ...state.summary, modifiedAt: Date.now() },
+    summary: { ...state.summary, modifiedAt: currentTimestampProvider() },
   };
   delete next.inputRequests;
   return {
@@ -174,7 +195,7 @@ function upsertInputRequest(state: SessionState, request: SessionInputRequest):
     inputRequests.push(request);
   }
   const next = { ...state, inputRequests };
-  return { ...next, summary: { ...next.summary, status: withStatusFlag(summaryStatus(next), SessionStatus.IsRead, false), modifiedAt: Date.now() } };
+  return { ...next, summary: { ...next.summary, status: withStatusFlag(summaryStatus(next), SessionStatus.IsRead, false), modifiedAt: currentTimestampProvider() } };
 }
 
 /**
@@ -297,7 +318,7 @@ export function sessionReducer(state: SessionState, action: SessionAction, log?:
       };
       next = {
         ...next,
-        summary: { ...next.summary, status: withStatusFlag(summaryStatus(next), SessionStatus.IsRead, false), modifiedAt: Date.now() },
+        summary: { ...next.summary, status: withStatusFlag(summaryStatus(next), SessionStatus.IsRead, false), modifiedAt: currentTimestampProvider() },
       };
 
       // If this turn was auto-started from a pending message, remove it
@@ -519,7 +540,7 @@ export function sessionReducer(state: SessionState, action: SessionAction, log?:
     case ActionType.SessionTitleChanged:
       return {
         ...state,
-        summary: { ...state.summary, title: action.title, modifiedAt: Date.now() },
+        summary: { ...state.summary, title: action.title, modifiedAt: currentTimestampProvider() },
       };
 
     case ActionType.SessionUsage:
@@ -542,13 +563,13 @@ export function sessionReducer(state: SessionState, action: SessionAction, log?:
     case ActionType.SessionModelChanged:
       return {
         ...state,
-        summary: { ...state.summary, model: action.model, modifiedAt: Date.now() },
+        summary: { ...state.summary, model: action.model, modifiedAt: currentTimestampProvider() },
       };
 
     case ActionType.SessionAgentChanged:
       return {
         ...state,
-        summary: { ...state.summary, agent: action.agent, modifiedAt: Date.now() },
+        summary: { ...state.summary, agent: action.agent, modifiedAt: currentTimestampProvider() },
       };
 
     case ActionType.SessionIsReadChanged:
@@ -588,7 +609,7 @@ export function sessionReducer(state: SessionState, action: SessionAction, log?:
         },
         summary: {
           ...state.summary,
-          modifiedAt: Date.now(),
+          modifiedAt: currentTimestampProvider(),
         },
       };
 
@@ -748,7 +769,7 @@ export function sessionReducer(state: SessionState, action: SessionAction, log?:
         ...state,
         turns,
         activeTurn: undefined,
-        summary: { ...state.summary, modifiedAt: Date.now() },
+        summary: { ...state.summary, modifiedAt: currentTimestampProvider() },
       };
       delete next.inputRequests;
       return {
@@ -783,7 +804,7 @@ export function sessionReducer(state: SessionState, action: SessionAction, log?:
       return {
         ...state,
         inputRequests: updated,
-        summary: { ...state.summary, modifiedAt: Date.now() },
+        summary: { ...state.summary, modifiedAt: currentTimestampProvider() },
       };
     }
 
@@ -803,7 +824,7 @@ export function sessionReducer(state: SessionState, action: SessionAction, log?:
       }
       return {
         ...next,
-        summary: { ...next.summary, status: summaryStatus(next), modifiedAt: Date.now() },
+        summary: { ...next.summary, status: summaryStatus(next), modifiedAt: currentTimestampProvider() },
       };
     }
 
diff --git a/types/index.ts b/types/index.ts
index 5a250428..d1ec31d2 100644
--- a/types/index.ts
+++ b/types/index.ts
@@ -229,6 +229,8 @@ export {
   annotationsReducer,
   resourceWatchReducer,
   isClientDispatchable,
+  currentTimestampProvider,
+  setCurrentTimestampProvider,
 } from './reducers.js';
 
 // Command types
diff --git a/types/reducers.test.ts b/types/reducers.test.ts
index 4a99a3dd..a1572b1b 100644
--- a/types/reducers.test.ts
+++ b/types/reducers.test.ts
@@ -25,6 +25,7 @@ import {
   annotationsReducer,
   resourceWatchReducer,
   isClientDispatchable,
+  setCurrentTimestampProvider,
 } from './reducers.js';
 import { IS_CLIENT_DISPATCHABLE } from './action-origin.generated.js';
 import { ActionType } from './actions.js';
@@ -106,21 +107,20 @@ const fixtures: Fixture[] = fixtureFiles.map(f => {
 // ─── Fixture-Driven Reducer Tests ────────────────────────────────────────────
 
 /**
- * The reducers call Date.now() for modifiedAt timestamps.
- * We mock it to a fixed value (9999) matching what was used during
- * fixture generation, so expected values match exactly.
+ * The reducers stamp modifiedAt via the injectable `currentTimestampProvider` (issue #186).
+ * We override it to a fixed value (9999) matching what was used during fixture generation, so
+ * expected values match exactly — no longer monkeypatching the process-global `Date.now` (which was
+ * racy under concurrency). This mirrors the Go/Kotlin/Rust fixture runners, which inject the same now.
  */
 const MOCK_NOW = 9999;
-let originalDateNow: typeof Date.now;
 
 describe('reducer fixtures', () => {
   beforeEach(() => {
-    originalDateNow = Date.now;
-    Date.now = () => MOCK_NOW;
+    setCurrentTimestampProvider(() => MOCK_NOW);
   });
 
   afterEach(() => {
-    Date.now = originalDateNow;
+    setCurrentTimestampProvider(() => Date.now());
   });
 
   for (const fixture of fixtures) {
diff --git a/types/reducers.ts b/types/reducers.ts
index eb2d615a..e02c6332 100644
--- a/types/reducers.ts
+++ b/types/reducers.ts
@@ -6,7 +6,7 @@
  */
 
 export { rootReducer } from './channels-root/reducer.js';
-export { sessionReducer } from './channels-session/reducer.js';
+export { sessionReducer, currentTimestampProvider, setCurrentTimestampProvider } from './channels-session/reducer.js';
 export { terminalReducer } from './channels-terminal/reducer.js';
 export { changesetReducer } from './channels-changeset/reducer.js';
 export { annotationsReducer } from './channels-annotations/reducer.js';