diff --git a/packages/runtime/src/__tests__/history-compaction.test.ts b/packages/runtime/src/__tests__/history-compaction.test.ts
index ab11cc35454..86a45ab10e0 100644
--- a/packages/runtime/src/__tests__/history-compaction.test.ts
+++ b/packages/runtime/src/__tests__/history-compaction.test.ts
@@ -339,6 +339,73 @@ describe('plan context compaction', () => {
     ]);
   });
 
+  test('a mid-turn retreat with no safe span reports the summarizer failure', async () => {
+    // The proven boundary is a prior-run reply before the head anchor, so the
+    // retreat has no mid_turn coverage. The summarizer was still called and
+    // refused, so the result must say so rather than look like a pool that was
+    // never summarized.
+    let attempts = 0;
+    const result = await planHistoryCompaction(
+      planInput({
+        orderedEvents: [
+          user('old-user', 'turn-0'),
+          modelOnRun('old-model', 'turn-0', 'run-0', 'accepted by this route'),
+          ...longTurnEvents().slice(2),
+        ],
+        invocations: [runOn('run-0', 'model-a', 'conn-a')],
+        acceptedRoute: ROUTE_A,
+        summarize: () => {
+          attempts += 1;
+          throw new HistoryCompactSummarizerError('input_too_large');
+        },
+      }),
+    );
+
+    assert.equal(attempts, 1);
+    assert.deepEqual(result, {
+      decision: 'fail_open',
+      reason: 'summarizer_failed',
+      diagnosticReason: 'input_too_large',
+    });
+  });
+
+  test('a mid-turn retreat still tries a proven span handoff put after the anchor', async () => {
+    // Handoff replay keeps the same logical turn's predecessor events — and the
+    // predecessor run's record — in the pool, so a reply after the head anchor
+    // can be the proven boundary. The retreat must still try it (#5790 review).
+    const attemptedCoverage: string[][] = [];
+    const plan = await planHistoryCompaction(
+      planInput({
+        orderedEvents: [
+          user('old-user', 'turn-0'),
+          modelOnRun('old-model', 'turn-0', 'run-0', 'old reply'),
+          user('anchor', 'turn-1'),
+          modelOnRun('handoff-reply', 'turn-1', 'run-0', 'source-run reply'),
+          call('call-x', 'cx', 'turn-1'),
+          result('res-x', 'cx', 'turn-1'),
+        ],
+        invocations: [runOn('run-0', 'model-a', 'conn-a')],
+        acceptedRoute: ROUTE_A,
+        summarize: ({ coveredRuntimeEvents }) => {
+          attemptedCoverage.push(coveredRuntimeEvents.map((event) => event.id));
+          throw new HistoryCompactSummarizerError('input_too_large');
+        },
+      }),
+    );
+
+    // First attempt covers through the reserved tail cut; the retreat then
+    // retries the proven prefix ending on the predecessor run's reply.
+    assert.deepEqual(attemptedCoverage, [
+      ['old-user', 'old-model', 'anchor', 'handoff-reply'],
+      ['old-user', 'old-model', 'anchor'],
+    ]);
+    assert.deepEqual(plan, {
+      decision: 'fail_open',
+      reason: 'summarizer_failed',
+      diagnosticReason: 'input_too_large',
+    });
+  });
+
   test('fails open when only another route has ever been accepted', async () => {
     let attempts = 0;
     const result = await planHistoryCompaction(
diff --git a/packages/runtime/src/__tests__/mid-turn-capacity-backend.test.ts b/packages/runtime/src/__tests__/mid-turn-capacity-backend.test.ts
index d507cd84824..add55fad4ba 100644
--- a/packages/runtime/src/__tests__/mid-turn-capacity-backend.test.ts
+++ b/packages/runtime/src/__tests__/mid-turn-capacity-backend.test.ts
@@ -970,10 +970,11 @@ function defineMidTurnSuite(consumer: ConsumerMode): void {
   });
 
   test('sends an over-window request no shaper could rescue, rather than ending the turn', async () => {
-    // No prior turns and a window the first step's usage already exceeds: the
-    // pool is [anchor, one open call/result pair], so no safe completed span
-    // and nothing to compact. Only the provider can say whether that request
-    // fits, so it goes out and the turn runs to its own end.
+    // No prior turns and a window the first step's usage already exceeds: at
+    // that step the pool is [anchor, one completed call/result pair], which
+    // mid-turn coverage cannot fold. Only the provider can say whether that
+    // request fits, so it goes out and the turn runs to its own end. The miss
+    // is per-step — the grown pool may still fold a later request (#5790).
     const fixture = buildFixture({
       contextWindow: 120,
       withoutPriorTurns: true,
@@ -1107,6 +1108,55 @@ function defineMidTurnSuite(consumer: ConsumerMode): void {
     assert.equal(failedOpen[0]?.failOpenReason, 'provider_error');
   });
 
+  test('bounds an input-too-large rejection across later steps in the same turn (#5790 review)', async () => {
+    // No invocation proves a boundary, so the rejection is a plain summarizer
+    // failure and the Turn latches it — a later step never re-dispatches it.
+    const fixture = buildFixture({
+      toolSteps: 3,
+      summarize: () => {
+        throw new HistoryCompactSummarizerError('input_too_large');
+      },
+    });
+
+    await runFixtureTurn(fixture, consumer);
+
+    assert.equal(fixture.summarizerCalls, 1);
+    assert.equal(fixture.recorded.length, 0);
+    const failedOpen = compactionDecisions(fixture).filter(
+      (decision) => decision.decision === 'failedOpen',
+    );
+    assert.equal(failedOpen[0]?.failOpenReason, 'input_too_large');
+  });
+
+  test('a no_safe_completed_span attempt does not suppress the fold once the pool grows (#5790)', async () => {
+    // No prior turns: at the first trigger the pool is only the anchor plus
+    // one completed tool pair, which mid-turn coverage cannot fold (the anchor
+    // alone saves nothing). The attempt fails open WITHOUT calling the
+    // summarizer — a property of the pool at that step, not a failure — so it
+    // must not latch the turn. Once the next step lands a second pair the new
+    // attempt re-reads the grown ledger and folds.
+    const fixture = buildFixture({
+      contextWindow: 150,
+      withoutPriorTurns: true,
+      toolSteps: 2,
+    });
+    await runFixtureTurn(fixture, consumer);
+
+    const complete = fixture.events.find((event) => event.type === 'complete');
+    assert.equal(complete?.type === 'complete' ? complete.stopReason : undefined, 'end_turn');
+    const failedOpen = compactionDecisions(fixture).find(
+      (decision) => decision.decision === 'failedOpen',
+    );
+    assert.equal(failedOpen?.failOpenReason, 'no_safe_completed_span');
+    // The grown pool folded: exactly one summarizer call, one checkpoint.
+    assert.equal(fixture.summarizerCalls, 1);
+    assert.equal(fixture.recorded.length, 1);
+    const thirdPrompt = promptJson(fixture, 2);
+    assert.match(thirdPrompt, /maka_history_compact_checkpoint/);
+    assert.equal(thirdPrompt.includes('RAW_SPAN_ONE_'), false);
+    assert.equal(thirdPrompt.includes('RAW_SPAN_TWO_'), true);
+  });
+
   test('fails closed before provider dispatch when the durable ledger read fails', async () => {
     const fixture = buildFixture();
     // Break the seam after construction: every trigger read now rejects.
@@ -1808,9 +1858,10 @@ describe('the shipped runtime default drives the proactive long-turn journey (is
     assert.equal(fixture.events.find((event) => event.type === 'complete')?.stopReason, 'end_turn');
   });
 
-  test('an unrescuable turn under the shipped default still dispatches', async () => {
-    // No prior turns leaves no safe completed span. The request still goes out
-    // because only the provider can decide whether it fits.
+  test('a first trigger with nothing foldable still dispatches under the shipped default', async () => {
+    // No prior turns leaves the first trigger no safe completed span. The
+    // request still goes out because only the provider can decide whether it
+    // fits — and the per-step miss does not latch the turn (#5790).
     const fixture = buildFixture({
       useRuntimeDefaultPolicy: true,
       contextWindow: 120,
diff --git a/packages/runtime/src/__tests__/overflow-reactive-recovery.test.ts b/packages/runtime/src/__tests__/overflow-reactive-recovery.test.ts
index b1975829832..31a0e812a3a 100644
--- a/packages/runtime/src/__tests__/overflow-reactive-recovery.test.ts
+++ b/packages/runtime/src/__tests__/overflow-reactive-recovery.test.ts
@@ -167,6 +167,8 @@ interface ReactiveFixtureOptions {
   providerNative?: boolean;
   /** Explicit send-level step budget forwarded to the backend. */
   maxSteps?: number;
+  /** Per provider-call reported usage, keyed by 1-based call number. */
+  usageByCall?: Record<number, { input: number; output: number }>;
   /** The FIRST tool step reports an unusable usage object (no token counts). */
   firstStepUsageMissing?: boolean;
   /** Tool-search availability with the deferred `Big` tool. */
@@ -242,6 +244,10 @@ function buildReactiveFixture(options: ReactiveFixtureOptions): ReactiveFixture
     inputTokens: { total: input, noCache: input, cacheRead: 0, cacheWrite: 0 },
     outputTokens: { total: output, text: output, reasoning: 0 },
   });
+  const usageOverride = (call: number): ReturnType<typeof usage> | undefined => {
+    const override = options.usageByCall?.[call];
+    return override ? usage(override.input, override.output) : undefined;
+  };
   const toolCallChunks = (
     call: number,
     toolName: string,
@@ -263,9 +269,10 @@ function buildReactiveFixture(options: ReactiveFixtureOptions): ReactiveFixture
       // An unusable first-step usage: the SDK accepts the object but the
       // adapter's normalization fails closed (undefined), the #972 shape.
       usage:
-        options.firstStepUsageMissing && call === 1
+        usageOverride(call) ??
+        (options.firstStepUsageMissing && call === 1
           ? ({ inputTokens: {}, outputTokens: {} } as ReturnType<typeof usage>)
-          : usage(100, 20),
+          : usage(100, 20)),
     },
   ];
   const doneChunks = (call: number): LanguageModelV4StreamPart[] => [
@@ -276,7 +283,7 @@ function buildReactiveFixture(options: ReactiveFixtureOptions): ReactiveFixture
     {
       type: 'finish',
       finishReason: { unified: 'stop', raw: 'stop' },
-      usage: usage(120, 10),
+      usage: usageOverride(call) ?? usage(120, 10),
     },
   ];
   const streamForCall = (call: number): ReadableStream<LanguageModelV4StreamPart> => {
@@ -1946,6 +1953,36 @@ describe('reactive overflow recovery in the streaming backend', () => {
     assert.equal(fixture.summarizerCalls(), 2);
   });
 
+  test('a no_safe_completed_span fail-open does not block overflow recovery once the pool grows (#5790)', async () => {
+    // Step 1's large usage fires the proactive trigger while the durable pool
+    // is only the anchor plus one tool pair — a miss that never reached the
+    // summarizer yet (before the fix) latched the turn's failure flag. Step 2
+    // lands a second pair and reports a small usage so the proactive trigger
+    // stays quiet, then the third request overflows: recovery must re-read
+    // the grown ledger and fold instead of inheriting the earlier miss.
+    const fixture = buildReactiveFixture({
+      script: ['tool', 'tool', 'overflow', 'done'],
+      withoutPriorTurns: true,
+      declareContextWindow: true,
+      contextWindow: 190,
+      usageByCall: { 1: { input: 1_000, output: 50 }, 2: { input: 10, output: 5 } },
+    });
+    await runTurn(fixture);
+
+    // Recovery folded the grown pool and resent: the turn completes for real.
+    assert.equal(complete(fixture)?.stopReason, 'end_turn');
+    assert.equal(fixture.model.doStreamCalls.length, 4);
+    assert.equal(
+      fixture.events.some((event) => event.type === 'error'),
+      false,
+    );
+    assert.equal(fixture.summarizerCalls(), 1);
+    assert.equal(fixture.recorded.length, 1);
+    // The resent request carries the folded checkpoint, not the raw span.
+    const resent = JSON.stringify(fixture.model.doStreamCalls[3]?.prompt);
+    assert.match(resent, /REACTIVE_SUMMARY_SENTINEL/);
+  });
+
   test('a later step that overflows again folds again after real progress', async () => {
     // The budget is one attempt per logical step, not one per send. The first
     // overflow folds and the retry succeeds; two more tool steps are accepted
diff --git a/packages/runtime/src/ai-sdk-compaction.ts b/packages/runtime/src/ai-sdk-compaction.ts
index 39ca0f7582b..a8c716e33df 100644
--- a/packages/runtime/src/ai-sdk-compaction.ts
+++ b/packages/runtime/src/ai-sdk-compaction.ts
@@ -1110,14 +1110,21 @@ export class AiSdkCompaction {
 
     if (plan.decision === 'fail_open') {
       const diagnosticReason = plan.diagnosticReason ?? plan.reason;
-      // Latch every fail-open reason, not only the malformed ones. The baseline
+      // Latch only a fail-open the summarizer actually produced. The baseline
       // that fired this trigger survives a fail-open, so without the latch the
       // next step evaluates the same condition and dispatches the same doomed
       // summarizer call: a provider that answers slowly and fails (kimi's HTTP
       // 200 with an error body) produced 15 such calls over 47 minutes before
       // one main request (#4634). The latch covers the whole Turn and only
       // fail-opens; a successful fold spends no part of it (#4559).
-      state.summarizerFailure = diagnosticReason;
+      //
+      // no_safe_completed_span is not a summarizer failure — no call was made.
+      // It describes the event pool at this step, and a pool that has since
+      // grown a completed tool pair can present a safe cut, so the next
+      // attempt must re-read the ledger rather than inherit this miss (#5790).
+      if (plan.reason === 'summarizer_failed') {
+        state.summarizerFailure = diagnosticReason;
+      }
       return {
         decision: 'fail',
         diagnosticReason,
@@ -1389,7 +1396,12 @@ export class MidTurnCapacityCompactState {
   flushedSteps = 0;
   /** Exact historical image results omitted after a provider overflow. */
   omittedImageToolResults = new Map<string, HistoricalImageToolResult>();
-  /** Malformed summaries spend one bounded repair budget for this whole Turn. */
+  /**
+   * One bounded retry budget for the whole Turn, spent only by a failure the
+   * summarizer actually produced (#4634). A structural miss like
+   * no_safe_completed_span never reaches it: the pool may grow a safe cut, so
+   * the next attempt re-reads the ledger instead of short-circuiting (#5790).
+   */
   summarizerFailure: string | undefined;
 
   constructor(
diff --git a/packages/runtime/src/history-compaction.ts b/packages/runtime/src/history-compaction.ts
index 6c4a15edc16..b33dd38865b 100644
--- a/packages/runtime/src/history-compaction.ts
+++ b/packages/runtime/src/history-compaction.ts
@@ -189,7 +189,9 @@ export interface PlanHistoryCompactionInput {
    * The invocations behind the ordered events, and the route this fold is
    * dispatched on. Together they name the newest reply this route produced,
    * which is the only span a retreat may target: a rejection of a larger one
-   * says nothing about a span another model accepted.
+   * says nothing about a span another model accepted. Handoff counts too: a
+   * predecessor run's reply inside the same logical turn can sit after the
+   * head anchor and still prove a mid_turn retreat.
    */
   invocations?: readonly RuntimeInvocationRecord[];
   acceptedRoute?: { modelId: string; connectionId?: string };
@@ -291,6 +293,14 @@ export async function planHistoryCompaction(
   if (phase !== 'standalone' && headAnchorIndex < 0) {
     return { decision: 'fail_open', reason: 'no_safe_completed_span' };
   }
+  // After a retreat the summarizer has already been called and refused, so a
+  // smaller window with nothing safe to fold is that failure, not a pool that
+  // never reached the summarizer (#5790 review).
+  let retreated = false;
+  const noSafeSpan = (): PlanHistoryCompactionResult =>
+    retreated
+      ? { decision: 'fail_open', reason: 'summarizer_failed', diagnosticReason: 'input_too_large' }
+      : { decision: 'fail_open', reason: 'no_safe_completed_span' };
   let maxCoveredCount = input.orderedEvents.length;
   while (maxCoveredCount > 0) {
     const boundary = selectSafeCompactionPrefix(input.orderedEvents, {
@@ -312,9 +322,7 @@ export async function planHistoryCompaction(
         : phase === 'mid_turn'
           ? boundary.ok && boundary.coveredCount > headAnchorIndex && boundary.coveredCount >= 2
           : boundary.ok && boundary.coveredCount > 0 && boundary.coveredCount <= headAnchorIndex;
-    if (!boundary.ok || !hasSafeCoverage) {
-      return { decision: 'fail_open', reason: 'no_safe_completed_span' };
-    }
+    if (!boundary.ok || !hasSafeCoverage) return noSafeSpan();
     const coveredRuntimeEvents = input.orderedEvents.slice(0, boundary.coveredCount);
     const tailRuntimeEvents = input.orderedEvents.slice(boundary.coveredCount);
 
@@ -394,6 +402,7 @@ export async function planHistoryCompaction(
             };
           }
           maxCoveredCount = proven;
+          retreated = true;
           continue;
         }
         return {
@@ -461,7 +470,7 @@ export async function planHistoryCompaction(
       estimatedTokensAfter,
     };
   }
-  return { decision: 'fail_open', reason: 'no_safe_completed_span' };
+  return noSafeSpan();
 }
 
 export interface HistoryCompactionPolicy {