diff --git a/CHANGELOG.md b/CHANGELOG.md
index ed8532ab..1d372be0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,10 @@ parallel copies under `docs/` or `scripts/notes/`. At cut time: rename
 
 ### Agent
 
+- **Context estimate syncs incrementally on append.** `syncFromTurns` keys
+  prefix turns by object identity and estimates only the new suffix. A rewrite,
+  shrink, or middle-turn identity break still fully recomputes so image-aging
+  cannot leave a stale total.
 - **Thinking-only replay no longer collapses into an identical request.** Assistant turns with no text or tool_call (empty content, leftover thinking/citation) are replaced with a stable `[thinking-only turn omitted]` marker so the turn is kept, roles still alternate, and the next `buildRequest` body differs from the previous one.
 
 - **Compaction keeps scored work, not retry loops.** Errored tool results are no
diff --git a/src/agent/context-estimate.test.ts b/src/agent/context-estimate.test.ts
index e196571a..d69b2093 100644
--- a/src/agent/context-estimate.test.ts
+++ b/src/agent/context-estimate.test.ts
@@ -155,4 +155,60 @@ describe("createContextEstimate", () => {
     expect(estimate.tokens).toBe(1);
     expect(estimate.turnCount).toBe(1);
   });
+
+  test("append reuses prefix identities and adds only the new turn", () => {
+    const estimate = createContextEstimate();
+    const first = textTurn("xxxx");
+    const second = textTurn("yyyyyyyy", "assistant");
+    const turns = [first];
+    expect(estimate.syncFromTurns(turns)).toBe(1);
+
+    turns.push(second);
+    expect(estimate.syncFromTurns(turns)).toBe(1 + 2);
+    expect(estimate.tokens).toBe(3);
+    expect(estimate.turnCount).toBe(2);
+  });
+
+  test("second syncFromTurns with the same identities is a no-op", () => {
+    const estimate = createContextEstimate();
+    const first = textTurn("xxxx");
+    const second = textTurn("yyyyyyyy", "assistant");
+    const turns = [first, second];
+    expect(estimate.syncFromTurns(turns)).toBe(3);
+    expect(estimate.syncFromTurns(turns)).toBe(3);
+    expect(estimate.syncFromTurns([first, second])).toBe(3);
+    expect(estimate.tokens).toBe(3);
+    expect(estimate.turnCount).toBe(2);
+  });
+
+  test("rewrite or shrink fully recomputes", () => {
+    const estimate = createContextEstimate();
+    const first = textTurn("xxxx");
+    const second = textTurn("yyyyyyyy", "assistant");
+    expect(estimate.syncFromTurns([first, second])).toBe(3);
+
+    const rewritten = [textTurn("xxxx"), textTurn("yyyyyyyy", "assistant")];
+    expect(estimate.syncFromTurns(rewritten)).toBe(estimateContextTokens(rewritten));
+    expect(estimate.tokens).toBe(3);
+    expect(estimate.turnCount).toBe(2);
+
+    const shrunk = rewritten.slice(0, 1);
+    expect(estimate.syncFromTurns(shrunk)).toBe(estimateContextTokens(shrunk));
+    expect(estimate.tokens).toBe(1);
+    expect(estimate.turnCount).toBe(1);
+  });
+
+  test("same-length middle identity break recomputes even when the last ref matches", () => {
+    const estimate = createContextEstimate();
+    const first = textTurn("aaaa");
+    const middle = textTurn("bbbb");
+    const last = textTurn("cccc");
+    expect(estimate.syncFromTurns([first, middle, last])).toBe(3);
+
+    const replacedMiddle = textTurn("bbbbbbbb");
+    const after = [first, replacedMiddle, last];
+    expect(estimate.syncFromTurns(after)).toBe(estimateContextTokens(after));
+    expect(estimate.tokens).toBe(4);
+    expect(estimate.turnCount).toBe(3);
+  });
 });
diff --git a/src/agent/context-estimate.ts b/src/agent/context-estimate.ts
index 97150c37..ce496f34 100644
--- a/src/agent/context-estimate.ts
+++ b/src/agent/context-estimate.ts
@@ -72,12 +72,18 @@ export function estimateContentBlockTokens(block: ContentBlock): number {
   }
 }
 
+function estimateTurnTokens(turn: ConversationTurn): number {
+  let total = 0;
+  for (const block of turn.content) {
+    total += estimateContentBlockTokens(block);
+  }
+  return total;
+}
+
 export function estimateContextTokens(turns: readonly ConversationTurn[]): number {
   let total = 0;
   for (const turn of turns ?? []) {
-    for (const block of turn.content) {
-      total += estimateContentBlockTokens(block);
-    }
+    total += estimateTurnTokens(turn);
   }
   return total;
 }
@@ -98,20 +104,42 @@ export function estimateOverheadTokens(
   return estimateTokensFromChars(chars);
 }
 
-// Mutable running estimate. Callers re-sync from the full turn list after each
-// append so compaction rewrites and tool results stay accurate without
-// incremental add/subtract bookkeeping. `overheadTokens` is fixed per session
-// (system prompt + tool schemas do not change turn to turn) and is folded into
-// every sync so the total tracks what actually goes out on the wire.
+// Mutable running estimate. Mid-cycle callers keep calling `syncFromTurns` so
+// tool results and image-aging stay visible before the next inference.done.
+// Prefix turns are keyed by object identity (===), not content: an append that
+// keeps every prior ref adds only the suffix; a shrink or any prefix identity
+// break fully recomputes. Length + last-turn alone is not enough — aging can
+// replace a middle turn and leave the last ref in place. Callers may push onto
+// the same array, so the cache snapshots refs rather than holding the array.
 export type ContextEstimate = ReturnType<typeof createContextEstimate>;
 
 export function createContextEstimate(overheadTokens = 0) {
   let tokens = overheadTokens;
   let turnCount = 0;
+  let cachedTurns: ConversationTurn[] = [];
+
+  function prefixRefsMatch(turns: readonly ConversationTurn[]): boolean {
+    for (let i = 0; i < cachedTurns.length; i++) {
+      if (turns[i] !== cachedTurns[i]) return false;
+    }
+    return true;
+  }
 
   function syncFromTurns(turns: readonly ConversationTurn[]): number {
-    tokens = overheadTokens + estimateContextTokens(turns);
+    if (turns.length === cachedTurns.length && prefixRefsMatch(turns)) {
+      return tokens;
+    }
+
+    if (turns.length > cachedTurns.length && prefixRefsMatch(turns)) {
+      for (const turn of turns.slice(cachedTurns.length)) {
+        tokens += estimateTurnTokens(turn);
+      }
+    } else {
+      tokens = overheadTokens + estimateContextTokens(turns);
+    }
+
     turnCount = turns.length;
+    cachedTurns = turns.slice();
     return tokens;
   }