diff --git a/apps/sim/lib/billing/core/usage-analytics.ts b/apps/sim/lib/billing/core/usage-analytics.ts
index 60d5ff1a1e9..49c6b4417aa 100644
--- a/apps/sim/lib/billing/core/usage-analytics.ts
+++ b/apps/sim/lib/billing/core/usage-analytics.ts
@@ -8,7 +8,7 @@ import {
 } from '@/lib/billing/core/reporting-period'
 import type { BillingEntity } from '@/lib/billing/core/usage-log'
 import { zonedWallClockToUtc } from '@/lib/core/utils/timezone'
-import { STREAM_TIMEOUT_MS } from '@/lib/mothership/constants'
+import { CHAT_RUN_DEADLINE_MS } from '@/lib/mothership/constants'
 
 /**
  * Pure half of organization usage analytics: window resolution, the ledger scope
@@ -510,15 +510,15 @@ export function usageBucketTimestamps(
  * How long after a stretch of time ends before its ledger rows are final.
  *
  * Rows are stamped when inserted, but a cumulative model charge tops up its row's
- * cost in place for as long as its stream runs — which {@link STREAM_TIMEOUT_MS}
- * caps — plus the retry flushes that follow it. Past the cap and this margin a day or
- * hour can no longer change and is treated as settled.
+ * cost in place for as long as its run lasts — which the worker's run deadline
+ * ({@link CHAT_RUN_DEADLINE_MS}) caps — plus the retry flushes that follow it. Past
+ * the cap and this margin a day or hour can no longer change and is treated as settled.
  *
  * Without a run deadline a Chat turn can top up its row for longer than that, so a
  * settled hour's cached aggregate can under-report that turn's later spend. This is
  * display only: invoices, threshold billing, and the usage gate read live ledger sums.
  */
-export const USAGE_SETTLE_MS = STREAM_TIMEOUT_MS + 2 * 60 * 60 * 1000
+export const USAGE_SETTLE_MS = CHAT_RUN_DEADLINE_MS + 2 * 60 * 60 * 1000
 
 const HOUR_MS = 60 * 60 * 1000
 
diff --git a/apps/sim/lib/core/redis/byte-budget.server.ts b/apps/sim/lib/core/redis/byte-budget.server.ts
index e6124bbd00d..c8554ffb400 100644
--- a/apps/sim/lib/core/redis/byte-budget.server.ts
+++ b/apps/sim/lib/core/redis/byte-budget.server.ts
@@ -12,7 +12,8 @@ import type { Logger } from '@sim/logger'
  * execution's event history is read from a cursor, so the write that would breach
  * the ceiling is refused and the buffer stops growing. The copilot replay ring trims
  * its oldest events by bytes below its ceiling instead, refunding what it drops, so a
- * long run slides rather than refuses; a reader behind the trim gets a replay gap.
+ * long run slides rather than refuses; a reader behind the trim is re-synced from the
+ * worker's run log, and ends with a replay gap only when that log cannot serve it.
  * A live-update feed is bounded differently — see `lib/realtime/event-log.ts`, whose
  * readers already handle a prune by refetching, so it drops oldest-first instead.
  *
diff --git a/apps/sim/lib/mothership/constants.ts b/apps/sim/lib/mothership/constants.ts
index 7f81ab2c245..f5b6a7b95f5 100644
--- a/apps/sim/lib/mothership/constants.ts
+++ b/apps/sim/lib/mothership/constants.ts
@@ -45,8 +45,13 @@ export const CLIENT_TOOL_RESULT_TIMEOUT_MS = 60 * 60 * 1000
 /** Extra slack the resume gate allows past the slowest pending tool's watchdog. */
 export const TOOL_WATCHDOG_RESUME_GRACE_MS = 30_000
 
-/** Timeout for the client-side streaming response handler (60 min). */
-export const STREAM_TIMEOUT_MS = 3_600_000
+/**
+ * The worker's default deadline for one Chat run (60 min).
+ *
+ * Sim does not enforce it: stream legs have no wall clock. It is the base of
+ * `USAGE_SETTLE_MS`, since it bounds how long a run tops up its model charge.
+ */
+export const CHAT_RUN_DEADLINE_MS = 3_600_000
 
 /**
  * How long a workflow tool call waits for a browser to pick it up before the
diff --git a/apps/sim/lib/mothership/request/go/stream.ts b/apps/sim/lib/mothership/request/go/stream.ts
index cd16ae492ad..a7a5ba38b29 100644
--- a/apps/sim/lib/mothership/request/go/stream.ts
+++ b/apps/sim/lib/mothership/request/go/stream.ts
@@ -413,7 +413,7 @@ export async function runStreamLoop(
           context.errors.push(failureMessage)
           logger.error('Received invalid stream event on shared path', {
             reason: parsedEvent.reason,
-            message: parsedEvent.message,
+            detail: parsedEvent.message,
             errors: parsedEvent.errors,
           })
           throw new FatalSseEventError(failureMessage)
@@ -458,7 +458,7 @@ export async function runStreamLoop(
             agentId: streamEvent.scope?.agentId,
             code: errorPayload.code,
             provider: errorPayload.provider,
-            message: errorPayload.message,
+            errorMessage: errorPayload.message,
             error: errorPayload.error,
             displayMessage: errorPayload.displayMessage,
             data: errorPayload.data,
diff --git a/apps/sim/lib/mothership/request/lifecycle/run.test.ts b/apps/sim/lib/mothership/request/lifecycle/run.test.ts
index d9872ecabf2..77feb41971b 100644
--- a/apps/sim/lib/mothership/request/lifecycle/run.test.ts
+++ b/apps/sim/lib/mothership/request/lifecycle/run.test.ts
@@ -2037,6 +2037,57 @@ describe('runCopilotLifecycle', () => {
     }
   )
 
+  it('reports a replay refusal over a reasonless error terminal', async () => {
+    const abortController = new AbortController()
+    const refusal = ownerRefusal()
+    mockRunStreamLoop.mockImplementationOnce(
+      async (_url: string, _init: RequestInit, context: StreamingContext): Promise<void> => {
+        context.completionStatus = MothershipStreamV1CompletionStatus.error
+        abortController.abort(refusal)
+      }
+    )
+
+    const result = await runWithStreamAbort(abortController)
+
+    expect(result).toEqual(
+      expect.objectContaining({
+        success: false,
+        cancelled: false,
+        error: refusal.userMessage,
+        errorCode: REPLAY_BUDGET_EXHAUSTED_CODE,
+      })
+    )
+  })
+
+  it('explains an error terminal that arrives without a reason as an already-ended run', async () => {
+    mockRunStreamLoop.mockImplementationOnce(
+      async (_url: string, _init: RequestInit, context: StreamingContext): Promise<void> => {
+        context.completionStatus = MothershipStreamV1CompletionStatus.error
+      }
+    )
+
+    const result = await runWithStreamAbort(new AbortController())
+
+    expect(result.success).toBe(false)
+    expect(result.cancelled).toBe(false)
+    expect(result.error).toEqual(expect.stringContaining('already ended'))
+  })
+
+  it('keeps a Stop a cancellation when the error terminal carries no reason', async () => {
+    const abortController = new AbortController()
+    mockRunStreamLoop.mockImplementationOnce(
+      async (_url: string, _init: RequestInit, context: StreamingContext): Promise<void> => {
+        context.completionStatus = MothershipStreamV1CompletionStatus.error
+        abortController.abort()
+      }
+    )
+
+    const result = await runWithStreamAbort(abortController)
+
+    expect(result.cancelled).toBe(true)
+    expect(result.error).toBeUndefined()
+  })
+
   it('keeps a Stop a cancellation when a replay refusal follows it', async () => {
     const abortController = new AbortController()
     mockRunStreamLoop.mockImplementationOnce(
@@ -3326,6 +3377,7 @@ describe('runCopilotLifecycle', () => {
     )
 
     expect(result.success).toBe(false)
+    expect(result.error).toBeUndefined()
     expect(result.errors).toEqual(['The provider is overloaded'])
   })
 
diff --git a/apps/sim/lib/mothership/request/lifecycle/run.ts b/apps/sim/lib/mothership/request/lifecycle/run.ts
index b141f7d7e06..7bd939a2a49 100644
--- a/apps/sim/lib/mothership/request/lifecycle/run.ts
+++ b/apps/sim/lib/mothership/request/lifecycle/run.ts
@@ -90,6 +90,12 @@ const logger = createLogger('CopilotLifecycle')
 
 const COPILOT_MODEL_CONTENT_PROJECTION_ERROR = 'Copilot model input could not be safely projected'
 
+/**
+ * Shown when the worker ends a turn with an error terminal but gives no reason. Every surface
+ * (Chat, workflow execute, inbox) reports it, so it carries no surface-specific next step.
+ */
+const ENDED_RUN_MESSAGE = 'This run had already ended before it could continue.'
+
 class CopilotModelContentProjectionError extends Error {
   constructor() {
     super(COPILOT_MODEL_CONTENT_PROJECTION_ERROR)
@@ -573,6 +579,14 @@ export async function runCopilotLifecycle(
         !refusal &&
         !turnWasAborted &&
         (backendFinishedTurn || (!context.completionStatus && context.errors.length === 0))
+      // The worker sends an error terminal with no `error` event only when it replays a run
+      // that already ended (for example at its deadline) to a resume or reattach, because
+      // that replay does not carry the run's stored reason. Say so rather than leave the turn
+      // to a generic failure; a reported reason or a replay refusal always wins.
+      const endedWithoutReason =
+        !turnWasAborted &&
+        context.completionStatus === MothershipStreamV1CompletionStatus.error &&
+        context.errors.length === 0
 
       const result: OrchestratorResult = {
         success: succeeded,
@@ -591,6 +605,7 @@ export async function runCopilotLifecycle(
         toolCalls: buildToolCallSummaries(context),
         chatId: context.chatId,
         requestId: context.requestId,
+        ...(endedWithoutReason ? { error: ENDED_RUN_MESSAGE } : {}),
         ...(refusal ? { error: refusal.userMessage, errorCode: refusal.code } : {}),
         errors: !succeeded && context.errors.length ? context.errors : undefined,
         usage: context.usage,
diff --git a/apps/sim/lib/mothership/request/session/buffer.ts b/apps/sim/lib/mothership/request/session/buffer.ts
index 9e0f1559f80..b990f543262 100644
--- a/apps/sim/lib/mothership/request/session/buffer.ts
+++ b/apps/sim/lib/mothership/request/session/buffer.ts
@@ -487,7 +487,7 @@ export async function readEvents(
       logger.warn('Skipping corrupt outbox entry', {
         streamId,
         reason: parsed.reason,
-        message: parsed.message,
+        detail: parsed.message,
         errors: parsed.errors,
       })
       continue