diff --git a/apps/server/integration/OrchestrationEngineHarness.integration.ts b/apps/server/integration/OrchestrationEngineHarness.integration.ts
index 152ed1d608..87c81f08c8 100644
--- a/apps/server/integration/OrchestrationEngineHarness.integration.ts
+++ b/apps/server/integration/OrchestrationEngineHarness.integration.ts
@@ -48,7 +48,7 @@ import { CheckpointReactorLive } from "../src/orchestration/Layers/CheckpointRea
 import { OrchestrationEngineLive } from "../src/orchestration/Layers/OrchestrationEngine.ts";
 import { OrchestrationProjectionPipelineLive } from "../src/orchestration/Layers/ProjectionPipeline.ts";
 import { OrchestrationProjectionSnapshotQueryLive } from "../src/orchestration/Layers/ProjectionSnapshotQuery.ts";
-import { RuntimeReceiptBusLive } from "../src/orchestration/Layers/RuntimeReceiptBus.ts";
+import { RuntimeReceiptBusTest } from "../src/orchestration/Layers/RuntimeReceiptBus.ts";
 import { OrchestrationReactorLive } from "../src/orchestration/Layers/OrchestrationReactor.ts";
 import { ProviderCommandReactorLive } from "../src/orchestration/Layers/ProviderCommandReactor.ts";
 import { ProviderRuntimeIngestionLive } from "../src/orchestration/Layers/ProviderRuntimeIngestion.ts";
@@ -297,7 +297,7 @@ export const makeOrchestrationIntegrationHarness = (
       ProjectionPendingApprovalRepositoryLive,
       checkpointStoreLayer,
       providerLayer,
-      RuntimeReceiptBusLive,
+      RuntimeReceiptBusTest,
     );
     const serverSettingsLayer = ServerSettingsService.layerTest();
     const runtimeIngestionLayer = ProviderRuntimeIngestionLive.pipe(
@@ -376,7 +376,7 @@ export const makeOrchestrationIntegrationHarness = (
       runtime.runPromise(reactor.start().pipe(Scope.provide(scope))),
     ).pipe(Effect.orDie);
     const receiptHistory = yield* Ref.make<ReadonlyArray<OrchestrationRuntimeReceipt>>([]);
-    yield* Stream.runForEach(runtimeReceiptBus.stream, (receipt) =>
+    yield* Stream.runForEach(runtimeReceiptBus.streamEventsForTest, (receipt) =>
       Ref.update(receiptHistory, (history) => [...history, receipt]).pipe(Effect.asVoid),
     ).pipe(Effect.forkIn(scope));
     yield* Effect.sleep(10);
diff --git a/apps/server/integration/providerService.integration.test.ts b/apps/server/integration/providerService.integration.test.ts
index ef03a1ab5c..1ca6fa1b83 100644
--- a/apps/server/integration/providerService.integration.test.ts
+++ b/apps/server/integration/providerService.integration.test.ts
@@ -86,6 +86,7 @@ const collectEventsDuring = <A, E, R>(
       Effect.forkScoped,
     );
 
+    yield* Effect.sleep("50 millis");
     yield* action;
 
     return yield* Effect.forEach(
@@ -104,7 +105,6 @@ const runTurn = (input: {
 }) =>
   Effect.gen(function* () {
     yield* input.harness.queueTurnResponse(input.threadId, input.response);
-
     return yield* collectEventsDuring(
       input.provider.streamEvents,
       input.response.events.length,
@@ -116,7 +116,7 @@ const runTurn = (input: {
     );
   });
 
-it.effect("replays typed runtime fixture events", () =>
+it.live("replays typed runtime fixture events", () =>
   Effect.gen(function* () {
     const fixture = yield* makeIntegrationFixture;
 
@@ -149,7 +149,7 @@ it.effect("replays typed runtime fixture events", () =>
   }).pipe(Effect.provide(NodeServices.layer)),
 );
 
-it.effect("replays file-changing fixture turn events", () =>
+it.live("replays file-changing fixture turn events", () =>
   Effect.gen(function* () {
     const fixture = yield* makeIntegrationFixture;
     const { join } = yield* Path.Path;
@@ -188,7 +188,7 @@ it.effect("replays file-changing fixture turn events", () =>
   }).pipe(Effect.provide(NodeServices.layer)),
 );
 
-it.effect("runs multi-turn tool/approval flow", () =>
+it.live("runs multi-turn tool/approval flow", () =>
   Effect.gen(function* () {
     const fixture = yield* makeIntegrationFixture;
     const { join } = yield* Path.Path;
@@ -242,7 +242,7 @@ it.effect("runs multi-turn tool/approval flow", () =>
   }).pipe(Effect.provide(NodeServices.layer)),
 );
 
-it.effect("rolls back provider conversation state only", () =>
+it.live("rolls back provider conversation state only", () =>
   Effect.gen(function* () {
     const fixture = yield* makeIntegrationFixture;
     const { join } = yield* Path.Path;
diff --git a/apps/server/src/orchestration/Layers/CheckpointReactor.test.ts b/apps/server/src/orchestration/Layers/CheckpointReactor.test.ts
index ab9f633e02..72adb175f9 100644
--- a/apps/server/src/orchestration/Layers/CheckpointReactor.test.ts
+++ b/apps/server/src/orchestration/Layers/CheckpointReactor.test.ts
@@ -96,7 +96,9 @@ function createProviderServiceHarness(
     listSessions,
     getCapabilities: () => Effect.succeed({ sessionModelSwitch: "in-session" }),
     rollbackConversation,
-    streamEvents: Stream.fromPubSub(runtimeEventPubSub),
+    get streamEvents() {
+      return Stream.fromPubSub(runtimeEventPubSub);
+    },
   };
 
   const emit = (event: LegacyProviderRuntimeEvent): void => {
diff --git a/apps/server/src/orchestration/Layers/ProviderCommandReactor.test.ts b/apps/server/src/orchestration/Layers/ProviderCommandReactor.test.ts
index ca3dc04517..fe6cb9caf5 100644
--- a/apps/server/src/orchestration/Layers/ProviderCommandReactor.test.ts
+++ b/apps/server/src/orchestration/Layers/ProviderCommandReactor.test.ts
@@ -208,7 +208,9 @@ describe("ProviderCommandReactor", () => {
           sessionModelSwitch: input?.sessionModelSwitch ?? "in-session",
         }),
       rollbackConversation: () => unsupported(),
-      streamEvents: Stream.fromPubSub(runtimeEventPubSub),
+      get streamEvents() {
+        return Stream.fromPubSub(runtimeEventPubSub);
+      },
     };
 
     const orchestrationLayer = OrchestrationEngineLive.pipe(
diff --git a/apps/server/src/orchestration/Layers/ProviderRuntimeIngestion.test.ts b/apps/server/src/orchestration/Layers/ProviderRuntimeIngestion.test.ts
index 6c27e1010c..85f4d966e3 100644
--- a/apps/server/src/orchestration/Layers/ProviderRuntimeIngestion.test.ts
+++ b/apps/server/src/orchestration/Layers/ProviderRuntimeIngestion.test.ts
@@ -98,7 +98,9 @@ function createProviderServiceHarness() {
     listSessions: () => Effect.succeed([...runtimeSessions]),
     getCapabilities: () => Effect.succeed({ sessionModelSwitch: "in-session" }),
     rollbackConversation: () => unsupported(),
-    streamEvents: Stream.fromPubSub(runtimeEventPubSub),
+    get streamEvents() {
+      return Stream.fromPubSub(runtimeEventPubSub);
+    },
   };
 
   const setSession = (session: ProviderSession): void => {
diff --git a/apps/server/src/orchestration/Layers/RuntimeReceiptBus.ts b/apps/server/src/orchestration/Layers/RuntimeReceiptBus.ts
index 56c526c08b..5c3148c943 100644
--- a/apps/server/src/orchestration/Layers/RuntimeReceiptBus.ts
+++ b/apps/server/src/orchestration/Layers/RuntimeReceiptBus.ts
@@ -1,3 +1,13 @@
+/**
+ * RuntimeReceiptBus layers.
+ *
+ * `RuntimeReceiptBusLive` is the production default and intentionally does not
+ * retain or broadcast receipts. `RuntimeReceiptBusTest` installs the in-memory
+ * PubSub-backed implementation used by integration tests that need to await
+ * checkpoint-reactor milestones precisely.
+ *
+ * @module RuntimeReceiptBus
+ */
 import { Effect, Layer, PubSub, Stream } from "effect";
 
 import {
@@ -6,13 +16,21 @@ import {
   type OrchestrationRuntimeReceipt,
 } from "../Services/RuntimeReceiptBus.ts";
 
-const makeRuntimeReceiptBus = Effect.gen(function* () {
+const makeRuntimeReceiptBus = Effect.succeed({
+  publish: () => Effect.void,
+  streamEventsForTest: Stream.empty,
+} satisfies RuntimeReceiptBusShape);
+
+const makeRuntimeReceiptBusTest = Effect.gen(function* () {
   const pubSub = yield* PubSub.unbounded<OrchestrationRuntimeReceipt>();
 
   return {
     publish: (receipt) => PubSub.publish(pubSub, receipt).pipe(Effect.asVoid),
-    stream: Stream.fromPubSub(pubSub),
+    get streamEventsForTest() {
+      return Stream.fromPubSub(pubSub);
+    },
   } satisfies RuntimeReceiptBusShape;
 });
 
 export const RuntimeReceiptBusLive = Layer.effect(RuntimeReceiptBus, makeRuntimeReceiptBus);
+export const RuntimeReceiptBusTest = Layer.effect(RuntimeReceiptBus, makeRuntimeReceiptBusTest);
diff --git a/apps/server/src/orchestration/Services/RuntimeReceiptBus.ts b/apps/server/src/orchestration/Services/RuntimeReceiptBus.ts
index a08ff91685..bf5c94f3a0 100644
--- a/apps/server/src/orchestration/Services/RuntimeReceiptBus.ts
+++ b/apps/server/src/orchestration/Services/RuntimeReceiptBus.ts
@@ -1,3 +1,19 @@
+/**
+ * RuntimeReceiptBus - Internal checkpoint-reactor synchronization receipts.
+ *
+ * This service exists to expose short-lived orchestration milestones that are
+ * useful in tests and harnesses but are not part of the production runtime
+ * event model. `CheckpointReactor` publishes receipts such as baseline capture,
+ * diff finalization, and turn-processing quiescence so integration tests can
+ * wait for those exact points without inferring them indirectly from persisted
+ * state.
+ *
+ * Production code should only call `publish`. Test code may subscribe via
+ * `streamEventsForTest`, which is intentionally named to make the intended
+ * usage explicit.
+ *
+ * @module RuntimeReceiptBus
+ */
 import { CheckpointRef, IsoDateTime, NonNegativeInt, ThreadId, TurnId } from "@t3tools/contracts";
 import { Schema, ServiceMap } from "effect";
 import type { Effect, Stream } from "effect";
@@ -40,7 +56,7 @@ export type OrchestrationRuntimeReceipt = typeof OrchestrationRuntimeReceipt.Typ
 
 export interface RuntimeReceiptBusShape {
   readonly publish: (receipt: OrchestrationRuntimeReceipt) => Effect.Effect<void>;
-  readonly stream: Stream.Stream<OrchestrationRuntimeReceipt>;
+  readonly streamEventsForTest: Stream.Stream<OrchestrationRuntimeReceipt>;
 }
 
 export class RuntimeReceiptBus extends ServiceMap.Service<
diff --git a/apps/server/src/provider/Layers/ClaudeAdapter.ts b/apps/server/src/provider/Layers/ClaudeAdapter.ts
index d99e2ad203..9f2eeb014e 100644
--- a/apps/server/src/provider/Layers/ClaudeAdapter.ts
+++ b/apps/server/src/provider/Layers/ClaudeAdapter.ts
@@ -3054,7 +3054,9 @@ const makeClaudeAdapter = Effect.fn("makeClaudeAdapter")(function* (
     listSessions,
     hasSession,
     stopAll,
-    streamEvents: Stream.fromQueue(runtimeEventQueue),
+    get streamEvents() {
+      return Stream.fromQueue(runtimeEventQueue);
+    },
   } satisfies ClaudeAdapterShape;
 });
 
diff --git a/apps/server/src/provider/Layers/CodexAdapter.ts b/apps/server/src/provider/Layers/CodexAdapter.ts
index cee6bca6ed..8b9f3b59e7 100644
--- a/apps/server/src/provider/Layers/CodexAdapter.ts
+++ b/apps/server/src/provider/Layers/CodexAdapter.ts
@@ -1631,7 +1631,9 @@ const makeCodexAdapter = Effect.fn("makeCodexAdapter")(function* (
     listSessions,
     hasSession,
     stopAll,
-    streamEvents: Stream.fromQueue(runtimeEventQueue),
+    get streamEvents() {
+      return Stream.fromQueue(runtimeEventQueue);
+    },
   } satisfies CodexAdapterShape;
 });
 
diff --git a/apps/server/src/provider/Layers/EventNdjsonLogger.test.ts b/apps/server/src/provider/Layers/EventNdjsonLogger.test.ts
index 258b50d6f2..9284ffac45 100644
--- a/apps/server/src/provider/Layers/EventNdjsonLogger.test.ts
+++ b/apps/server/src/provider/Layers/EventNdjsonLogger.test.ts
@@ -112,6 +112,49 @@ describe("EventNdjsonLogger", () => {
       }),
   );
 
+  it.effect("serializes concurrent first writes for the same segment", () =>
+    Effect.gen(function* () {
+      const tempDir = fs.mkdtempSync(path.join(os.tmpdir(), "t3-provider-log-"));
+      const basePath = path.join(tempDir, "provider-canonical.ndjson");
+
+      try {
+        const logger = yield* makeEventNdjsonLogger(basePath, {
+          stream: "canonical",
+          batchWindowMs: 0,
+        });
+        assert.notEqual(logger, undefined);
+        if (!logger) {
+          return;
+        }
+
+        yield* Effect.all(
+          [
+            logger.write({ id: "evt-concurrent-1" }, null),
+            logger.write({ id: "evt-concurrent-2" }, null),
+          ],
+          { concurrency: "unbounded" },
+        );
+        yield* logger.close();
+
+        const globalPath = path.join(tempDir, "_global.log");
+        assert.equal(fs.existsSync(globalPath), true);
+        const lines = fs
+          .readFileSync(globalPath, "utf8")
+          .trim()
+          .split("\n")
+          .map((line) => parseLogLine(line));
+
+        assert.equal(lines.length, 2);
+        assert.deepEqual(lines.map((line) => line.payload).toSorted(), [
+          '{"id":"evt-concurrent-1"}',
+          '{"id":"evt-concurrent-2"}',
+        ]);
+      } finally {
+        fs.rmSync(tempDir, { recursive: true, force: true });
+      }
+    }),
+  );
+
   it.effect("rotates per-thread files when max size is exceeded", () =>
     Effect.gen(function* () {
       const tempDir = fs.mkdtempSync(path.join(os.tmpdir(), "t3-provider-log-"));
diff --git a/apps/server/src/provider/Layers/EventNdjsonLogger.ts b/apps/server/src/provider/Layers/EventNdjsonLogger.ts
index a4fd6f235d..2a512efe66 100644
--- a/apps/server/src/provider/Layers/EventNdjsonLogger.ts
+++ b/apps/server/src/provider/Layers/EventNdjsonLogger.ts
@@ -10,7 +10,7 @@ import path from "node:path";
 
 import type { ThreadId } from "@t3tools/contracts";
 import { RotatingFileSink } from "@t3tools/shared/logging";
-import { Effect, Exit, Logger, Scope } from "effect";
+import { Effect, Exit, Logger, Scope, SynchronizedRef } from "effect";
 
 import { toSafeThreadAttachmentSegment } from "../../attachmentStore.ts";
 
@@ -40,6 +40,11 @@ interface ThreadWriter {
   close: () => Effect.Effect<void>;
 }
 
+interface LoggerState {
+  readonly threadWriters: Map<string, ThreadWriter>;
+  readonly failedSegments: Set<string>;
+}
+
 function logWarning(message: string, context: Record<string, unknown>): Effect.Effect<void> {
   return Effect.logWarning(message, context).pipe(Effect.annotateLogs({ scope: LOG_SCOPE }));
 }
@@ -193,34 +198,56 @@ export const makeEventNdjsonLogger = Effect.fn("makeEventNdjsonLogger")(function
     return undefined;
   }
 
-  const threadWriters = new Map<string, ThreadWriter>();
-  const failedSegments = new Set<string>();
+  const stateRef = yield* SynchronizedRef.make<LoggerState>({
+    threadWriters: new Map(),
+    failedSegments: new Set(),
+  });
 
   const resolveThreadWriter = Effect.fn("resolveThreadWriter")(function* (
     threadSegment: string,
   ): Effect.fn.Return<ThreadWriter | undefined> {
-    if (failedSegments.has(threadSegment)) {
-      return undefined;
-    }
-    const existing = threadWriters.get(threadSegment);
-    if (existing) {
-      return existing;
-    }
+    return yield* SynchronizedRef.modifyEffect(stateRef, (state) => {
+      if (state.failedSegments.has(threadSegment)) {
+        return Effect.succeed([undefined, state] as const);
+      }
 
-    const writer = yield* makeThreadWriter({
-      filePath: path.join(path.dirname(filePath), `${threadSegment}.log`),
-      maxBytes,
-      maxFiles,
-      batchWindowMs,
-      streamLabel,
-    });
-    if (!writer) {
-      failedSegments.add(threadSegment);
-      return undefined;
-    }
+      const existing = state.threadWriters.get(threadSegment);
+      if (existing) {
+        return Effect.succeed([existing, state] as const);
+      }
+
+      return makeThreadWriter({
+        filePath: path.join(path.dirname(filePath), `${threadSegment}.log`),
+        maxBytes,
+        maxFiles,
+        batchWindowMs,
+        streamLabel,
+      }).pipe(
+        Effect.map((writer) => {
+          if (!writer) {
+            const nextFailedSegments = new Set(state.failedSegments);
+            nextFailedSegments.add(threadSegment);
+            return [
+              undefined,
+              {
+                ...state,
+                failedSegments: nextFailedSegments,
+              },
+            ] as const;
+          }
 
-    threadWriters.set(threadSegment, writer);
-    return writer;
+          const nextThreadWriters = new Map(state.threadWriters);
+          nextThreadWriters.set(threadSegment, writer);
+          return [
+            writer,
+            {
+              ...state,
+              threadWriters: nextThreadWriters,
+            },
+          ] as const;
+        }),
+      );
+    });
   });
 
   const write = Effect.fn("write")(function* (event: unknown, threadId: ThreadId | null) {
@@ -239,10 +266,21 @@ export const makeEventNdjsonLogger = Effect.fn("makeEventNdjsonLogger")(function
   });
 
   const close = Effect.fn("close")(function* () {
-    for (const writer of threadWriters.values()) {
-      yield* writer.close();
-    }
-    threadWriters.clear();
+    yield* SynchronizedRef.modifyEffect(stateRef, (state) =>
+      Effect.gen(function* () {
+        for (const writer of state.threadWriters.values()) {
+          yield* writer.close();
+        }
+
+        return [
+          undefined,
+          {
+            threadWriters: new Map<string, ThreadWriter>(),
+            failedSegments: new Set<string>(),
+          },
+        ] as const;
+      }),
+    );
   });
 
   return {
diff --git a/apps/server/src/provider/Layers/ProviderRegistry.test.ts b/apps/server/src/provider/Layers/ProviderRegistry.test.ts
index 116c008d67..ca27371b61 100644
--- a/apps/server/src/provider/Layers/ProviderRegistry.test.ts
+++ b/apps/server/src/provider/Layers/ProviderRegistry.test.ts
@@ -115,7 +115,9 @@ function makeMutableServerSettingsService(
           yield* PubSub.publish(changes, next);
           return next;
         }),
-      streamChanges: Stream.fromPubSub(changes),
+      get streamChanges() {
+        return Stream.fromPubSub(changes);
+      },
     } satisfies ServerSettingsShape;
   });
 }
diff --git a/apps/server/src/provider/Layers/ProviderService.test.ts b/apps/server/src/provider/Layers/ProviderService.test.ts
index cd6c81405b..fc3c9cf25c 100644
--- a/apps/server/src/provider/Layers/ProviderService.test.ts
+++ b/apps/server/src/provider/Layers/ProviderService.test.ts
@@ -191,7 +191,9 @@ function makeFakeCodexAdapter(provider: ProviderKind = "codex") {
     readThread,
     rollbackThread,
     stopAll,
-    streamEvents: Stream.fromPubSub(runtimeEventPubSub),
+    get streamEvents() {
+      return Stream.fromPubSub(runtimeEventPubSub);
+    },
   };
 
   const emit = (event: LegacyProviderRuntimeEvent): void => {
diff --git a/apps/server/src/provider/Layers/ProviderService.ts b/apps/server/src/provider/Layers/ProviderService.ts
index da2c293fac..85fe9fbc32 100644
--- a/apps/server/src/provider/Layers/ProviderService.ts
+++ b/apps/server/src/provider/Layers/ProviderService.ts
@@ -22,7 +22,7 @@ import {
   type ProviderRuntimeEvent,
   type ProviderSession,
 } from "@t3tools/contracts";
-import { Effect, Layer, Option, PubSub, Queue, Schema, SchemaIssue, Stream } from "effect";
+import { Effect, Layer, Option, PubSub, Schema, SchemaIssue, Stream } from "effect";
 
 import {
   increment,
@@ -156,7 +156,6 @@ const makeProviderService = Effect.fn("makeProviderService")(function* (
 
   const registry = yield* ProviderAdapterRegistry;
   const directory = yield* ProviderSessionDirectory;
-  const runtimeEventQueue = yield* Queue.unbounded<ProviderRuntimeEvent>();
   const runtimeEventPubSub = yield* PubSub.unbounded<ProviderRuntimeEvent>();
 
   const publishRuntimeEvent = (event: ProviderRuntimeEvent): Effect.Effect<void> =>
@@ -194,15 +193,8 @@ const makeProviderService = Effect.fn("makeProviderService")(function* (
       eventType: event.type,
     }).pipe(Effect.andThen(publishRuntimeEvent(event)));
 
-  const worker = Effect.forever(
-    Queue.take(runtimeEventQueue).pipe(Effect.flatMap(processRuntimeEvent)),
-  );
-  yield* Effect.forkScoped(worker);
-
   yield* Effect.forEach(adapters, (adapter) =>
-    Stream.runForEach(adapter.streamEvents, (event) =>
-      Queue.offer(runtimeEventQueue, event).pipe(Effect.asVoid),
-    ).pipe(Effect.forkScoped),
+    Stream.runForEach(adapter.streamEvents, processRuntimeEvent).pipe(Effect.forkScoped),
   ).pipe(Effect.asVoid);
 
   const recoverSessionForThread = Effect.fn("recoverSessionForThread")(function* (input: {