diff --git a/src/adapters/google-antigravity-replay.ts b/src/adapters/google-antigravity-replay.ts
index 3d574b54d9..3fd2d66d4e 100644
--- a/src/adapters/google-antigravity-replay.ts
+++ b/src/adapters/google-antigravity-replay.ts
@@ -37,8 +37,8 @@ const MIN_SIGNATURE_LEN = 16;
 const REPLAY_TTL_MS = 60 * 60 * 1000; // 1h
 export const ANTIGRAVITY_REPLAY_MAX_ENTRIES = 10_240;
 const REPLAY_EVICT_BATCH = 128;
-const REPLAY_MAX_CALLS_PER_SESSION = 256;
-export const ANTIGRAVITY_REPLAY_MAX_BYTES_PER_SESSION = 2 * 1024 * 1024;
+const REPLAY_MAX_CALLS_PER_SESSION = 8_192;
+export const ANTIGRAVITY_REPLAY_MAX_BYTES_PER_SESSION = 8 * 1024 * 1024;
 export const ANTIGRAVITY_REPLAY_MAX_TOTAL_BYTES = 64 * 1024 * 1024;
 const REPLAY_MAX_SIGNATURE_BYTES = 64 * 1024;
 /** Fixed 64-hex outer key length, counted once per session entry. */
diff --git a/structure/04_transports-and-sidecars.md b/structure/04_transports-and-sidecars.md
index 699993a97e..bd14b0c014 100644
--- a/structure/04_transports-and-sidecars.md
+++ b/structure/04_transports-and-sidecars.md
@@ -779,6 +779,14 @@ so matching uses the provider-visible tool name.
 - 다른 대안 대신 이 방식을 선택한 이유: Responses ids are not Gemini signatures and previously caused Base64/TYPE_BYTES failures; a second cache duplicates limits; an unscoped cache could send provider-private state across destinations.
 - 장점, 단점 및 영향: Tool loops continue with exact opaque state and bounded memory while cross-transport reuse fails closed. Replay remains process-local, matching the existing Antigravity contract.
 
+[Decision Log: Deep-session replay capacity scaling]
+- 목적과 의도: Prevent premature LRU eviction of early function call signatures in deep conversations (500+ / 1,500+ calls) on Google Antigravity / Vertex without compromising the global memory or disk snapshot bounds.
+- 기존 구현 및 제약 조건: `REPLAY_MAX_CALLS_PER_SESSION` was capped at 256 calls and `REPLAY_MAX_BYTES_PER_SESSION` at 2 MiB, which evicted early historical calls once active sessions exceeded 256 calls, causing upstream Gemini to reject the turn with HTTP 400 (`Function call is missing a thought_signature in functionCall parts`).
+- 검토한 주요 대안: Retain the 256-call cap and rely on client-side re-generation; remove the per-session cap entirely; scale the per-session limits to 8,192 calls and 8 MiB while keeping the unchanged 64 MiB global cap, 10,240 session cap, and 24 MiB snapshot write bound.
+- 선택한 방식: Scale `REPLAY_MAX_CALLS_PER_SESSION` to 8,192 and `REPLAY_MAX_BYTES_PER_SESSION` to 8 MiB. Retain the unchanged 1-hour `REPLAY_TTL_MS`, 10,240 session cap, 64 MiB global memory cap, and 24 MiB snapshot disk cap.
+- 다른 대안 대신 이 방식을 선택한 이유: 8,192 calls per session covers long-running multi-agent tasks and 1,500+ call deep transcripts without memory leaks, while the global 64 MiB cap and snapshot LRU sweep protect against unbounded heap growth.
+- 장점, 단점 및 영향: Deep sessions with up to 8,192 calls reliably restore their historical thought signatures without 400 rejections; global memory limits remain strictly enforced.
+
 ## Google tool-result adjacency repair
 
 Google-family requests serialize a model tool-call turn and its results as one adjacent
diff --git a/tests/google-antigravity-replay.test.ts b/tests/google-antigravity-replay.test.ts
index 8fab813cca..77dac5f309 100644
--- a/tests/google-antigravity-replay.test.ts
+++ b/tests/google-antigravity-replay.test.ts
@@ -298,6 +298,50 @@ describe("antigravity reasoning-replay cache", () => {
     expect(contents.every(c => typeof (c.parts[0] as { thoughtSignature?: string }).thoughtSignature === "string")).toBe(true);
   });
 
+  test("preserves and restores signatures in deep 1500+ call sessions under production default limits", async () => {
+    const totalCalls = 1_500;
+    for (let i = 0; i < totalCalls; i++) {
+      observeAntigravityReplay(MODEL, SESSION, [fcPart("exec", { cmd: `cmd-${i}` }, `sig-call-${i}-${"a".repeat(24)}`)]);
+    }
+    const metricsBefore = antigravityReplayMetrics();
+    expect(metricsBefore.calls).toBe(totalCalls);
+
+    // Both earliest (position 0) and latest (position 1499) calls must restore under default limits:
+    const testContents = [
+      { role: "model", parts: [fcPart("exec", { cmd: "cmd-0" })] },
+      { role: "model", parts: [fcPart("exec", { cmd: `cmd-${totalCalls - 1}` })] },
+    ];
+    applyAntigravityReplay(MODEL, SESSION, testContents);
+    expect((testContents[0].parts[0] as { thoughtSignature?: string }).thoughtSignature).toContain("sig-call-0-");
+    expect((testContents[1].parts[0] as { thoughtSignature?: string }).thoughtSignature).toContain(`sig-call-${totalCalls - 1}-`);
+
+    // Verify survival across durable snapshot flush, reset, and reload:
+    await flushAntigravityReplay();
+    setAntigravityReplayLimitsForTests();
+    const reloadedContents = [
+      { role: "model", parts: [fcPart("exec", { cmd: "cmd-0" })] },
+      { role: "model", parts: [fcPart("exec", { cmd: `cmd-${totalCalls - 1}` })] },
+    ];
+    applyAntigravityReplay(MODEL, SESSION, reloadedContents);
+    expect((reloadedContents[0].parts[0] as { thoughtSignature?: string }).thoughtSignature).toContain("sig-call-0-");
+    expect((reloadedContents[1].parts[0] as { thoughtSignature?: string }).thoughtSignature).toContain(`sig-call-${totalCalls - 1}-`);
+  });
+
+  test("retains session between 2 MiB and 8 MiB without tripping the old 2 MiB cap", () => {
+    // 5000 calls x (64 key bytes + 500 signature bytes) = ~2.8 MiB (exceeds the old 2 MiB session cap):
+    const callCount = 5_000;
+    for (let i = 0; i < callCount; i++) {
+      observeAntigravityReplay(MODEL, SESSION, [fcPart("exec", { index: i }, `sig-${i}-${"s".repeat(500)}`)]);
+    }
+    const metrics = antigravityReplayMetrics();
+    expect(metrics.calls).toBe(callCount);
+    expect(metrics.totalBytes).toBeGreaterThan(2 * 1024 * 1024);
+    expect(metrics.totalBytes).toBeLessThanOrEqual(8 * 1024 * 1024);
+    const contents = [{ role: "model", parts: [fcPart("exec", { index: 0 })] }];
+    applyAntigravityReplay(MODEL, SESSION, contents);
+    expect((contents[0].parts[0] as { thoughtSignature?: string }).thoughtSignature).toContain("sig-0-");
+  });
+
   test("evicts oldest inner call at the exact per-session count boundary", () => {
     setAntigravityReplayLimitsForTests({ maxCallsPerSession: 2 });
     observeAntigravityReplay(MODEL, SESSION, [fcPart("one", {}, "sig-one-aaaaaaaaaaaa")]);