diff --git a/packages/agent/CHANGELOG.md b/packages/agent/CHANGELOG.md
index 0e8871723..fa173c3a7 100644
--- a/packages/agent/CHANGELOG.md
+++ b/packages/agent/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Added
+
+- Added a per-message estimation cache (`estimateTokens`) keyed by message identity, so settled history is token-counted once and reused until an owner mutates it. Non-assistant roles cache unconditionally; assistants cache only when settled (real `usage` with a terminal, non-`aborted`/`error` `stopReason`) so streaming partials never freeze a mid-stream count. Dual option-split maps keep the default and `excludeEncryptedReasoning` (compaction-floor) estimates from colliding. Prune, shake, and cross-package convert caches invalidate through `invalidateMessageCache` / `registerMessageCacheInvalidator` at their mutation seams ([#5934](https://github.com/can1357/oh-my-pi/issues/5934)).
+
 ## [17.0.2] - 2026-07-17
 
 ### Fixed
diff --git a/packages/agent/src/compaction/compaction.ts b/packages/agent/src/compaction/compaction.ts
index 2b5186010..3781ed431 100644
--- a/packages/agent/src/compaction/compaction.ts
+++ b/packages/agent/src/compaction/compaction.ts
@@ -43,6 +43,7 @@ import {
 	V2_RETAINED_MESSAGE_TOKEN_BUDGET,
 } from "./compaction-v2-streaming";
 import type { CompactionEntry, SessionEntry } from "./entries";
+import { isEstimateCacheable, readEstimateCache, writeEstimateCache } from "./message-cache";
 import { type ConvertToLlm, createBranchSummaryMessage, createCustomMessage, defaultConvertToLlm } from "./messages";
 import {
 	buildOpenAiNativeHistory,
@@ -364,6 +365,21 @@ const IMAGE_TOKEN_ESTIMATE = 1200;
  * content) excludes them to avoid false triggers on thinking-heavy turns.
  */
 export function estimateTokens(message: AgentMessage, options?: { excludeEncryptedReasoning?: boolean }): number {
+	// Settled historical messages are counted once and reused until an owner
+	// (prune/shake/strip-images) invalidates them; streaming assistants bypass
+	// the cache entirely (see message-cache.ts settle-gate invariant).
+	const cacheable = isEstimateCacheable(message);
+	const excludeEncryptedReasoning = options?.excludeEncryptedReasoning === true;
+	if (cacheable) {
+		const cached = readEstimateCache(message, excludeEncryptedReasoning);
+		if (cached !== undefined) return cached;
+	}
+	const result = computeMessageTokens(message, options);
+	if (cacheable) writeEstimateCache(message, excludeEncryptedReasoning, result);
+	return result;
+}
+
+function computeMessageTokens(message: AgentMessage, options?: { excludeEncryptedReasoning?: boolean }): number {
 	const fragments: string[] = [];
 	let extra = 0;
 	if ((message as { role?: string }).role === "bashExecution") {
diff --git a/packages/agent/src/compaction/index.ts b/packages/agent/src/compaction/index.ts
index 401215724..c0586f18d 100644
--- a/packages/agent/src/compaction/index.ts
+++ b/packages/agent/src/compaction/index.ts
@@ -6,6 +6,7 @@ export * from "./branch-summarization";
 export * from "./compaction";
 export * from "./entries";
 export * from "./errors";
+export * from "./message-cache";
 export * from "./messages";
 export * from "./openai";
 export * from "./pruning";
diff --git a/packages/agent/src/compaction/message-cache.ts b/packages/agent/src/compaction/message-cache.ts
new file mode 100644
index 000000000..a2249f926
--- /dev/null
+++ b/packages/agent/src/compaction/message-cache.ts
@@ -0,0 +1,92 @@
+/**
+ * Per-message memoization for the two hot history walks: token estimation
+ * ({@link estimateTokens}) and LLM conversion (the coding-agent's `convertToLlm`).
+ *
+ * Long sessions re-walk a settled `AgentMessage[]` every turn, re-tokenizing and
+ * re-converting historical objects that only the newest suffix can change. These
+ * caches key on message *identity* so a settled message is counted/converted once
+ * and reused until an owner rewrites it.
+ *
+ * Correctness rests on two invariants:
+ *
+ * 1. **Settle gate.** A streaming assistant is mutated under one identity while
+ *    its `usage`/`stopReason` are provisional (the seed carries zeroed usage and
+ *    a placeholder `stopReason`). Caching it would freeze a mid-stream count, so
+ *    estimation only caches assistants that are settled — real `usage`
+ *    (`totalTokens > 0`) with a terminal `stopReason` that is not `"aborted"` /
+ *    `"error"`. Unsettled assistants never read or insert. Non-assistant roles
+ *    are immutable once appended and cache by identity.
+ * 2. **Owner invalidation.** `pruneToolOutputs` / `pruneSupersededToolResults`,
+ *    `applyShakeRegion`, and `stripImagesFromMessage` rewrite message content in
+ *    place under a stable identity. Each MUST call {@link invalidateMessageCache}
+ *    on the mutated message before the next convert/estimate pass so both caches
+ *    drop the stale entry. The convert cache lives in another package, so it
+ *    subscribes via {@link registerMessageCacheInvalidator}.
+ */
+import type { AssistantMessage } from "@oh-my-pi/pi-ai";
+import type { AgentMessage } from "../types";
+
+/** External cache invalidators (e.g. the coding-agent `convertToLlm` memo). */
+const externalInvalidators = new Set<(message: AgentMessage) => void>();
+
+/**
+ * Register a cache tied to message identity so owner mutations in this package
+ * (prune/shake) can invalidate it across the package boundary. Returns an
+ * unregister function. The coding-agent `convertToLlm` memo registers here.
+ */
+export function registerMessageCacheInvalidator(invalidate: (message: AgentMessage) => void): () => void {
+	externalInvalidators.add(invalidate);
+	return () => {
+		externalInvalidators.delete(invalidate);
+	};
+}
+
+// Dual option-split estimate caches: the compaction floor passes
+// `excludeEncryptedReasoning` (dropping opaque provider reasoning), so a message
+// has two distinct estimates that must not collide in one map.
+//
+// These are WeakMaps, not symbol-tagged properties, deliberately: callers spread
+// messages to derive throwaway variants for counting — `estimateBranchSummaryTokens`
+// does `estimateTokens({ ...message, content: truncated })`. A symbol-keyed cache
+// value rides along an object spread, so the truncated clone would inherit (and
+// return) the full-content estimate. Keying strictly on identity keeps the cache
+// off spread copies, which get their own fresh count.
+const estimateCacheDefault = new WeakMap<AgentMessage, number>();
+const estimateCacheFloored = new WeakMap<AgentMessage, number>();
+
+/**
+ * True when this message's estimate is safe to cache by identity. Non-assistants
+ * are immutable once appended; assistants are cached only once settled (see the
+ * settle-gate invariant above).
+ */
+export function isEstimateCacheable(message: AgentMessage): boolean {
+	if (message.role !== "assistant") return true;
+	const assistant = message as AssistantMessage;
+	return (
+		assistant.stopReason !== "aborted" &&
+		assistant.stopReason !== "error" &&
+		assistant.usage != null &&
+		assistant.usage.totalTokens > 0
+	);
+}
+
+/** Read a cached estimate for the given option split, or `undefined` on miss. */
+export function readEstimateCache(message: AgentMessage, excludeEncryptedReasoning: boolean): number | undefined {
+	return (excludeEncryptedReasoning ? estimateCacheFloored : estimateCacheDefault).get(message);
+}
+
+/** Store an estimate for the given option split. */
+export function writeEstimateCache(message: AgentMessage, excludeEncryptedReasoning: boolean, value: number): void {
+	(excludeEncryptedReasoning ? estimateCacheFloored : estimateCacheDefault).set(message, value);
+}
+
+/**
+ * Drop every cached derivation of `message` after an in-place rewrite. Owners of
+ * mutation (prune, shake, strip-images) call this at the mutation seam so the
+ * next convert/estimate pass recomputes from the new content.
+ */
+export function invalidateMessageCache(message: AgentMessage): void {
+	estimateCacheDefault.delete(message);
+	estimateCacheFloored.delete(message);
+	for (const invalidate of externalInvalidators) invalidate(message);
+}
diff --git a/packages/agent/src/compaction/pruning.ts b/packages/agent/src/compaction/pruning.ts
index 1ba9a7ac0..83d5c2dd5 100644
--- a/packages/agent/src/compaction/pruning.ts
+++ b/packages/agent/src/compaction/pruning.ts
@@ -6,6 +6,7 @@ import type { ToolResultMessage } from "@oh-my-pi/pi-ai";
 import type { AgentMessage, AgentToolCall } from "../types";
 import { estimateTokens } from "./compaction";
 import type { SessionEntry, SessionMessageEntry } from "./entries";
+import { invalidateMessageCache } from "./message-cache";
 import {
 	collectToolCallsById,
 	isProtectedToolResult,
@@ -295,6 +296,7 @@ export function pruneSupersededToolResults(entries: SessionEntry[], config: Supe
 	for (const candidate of toPrune) {
 		candidate.message.content = [{ type: "text", text: candidate.notice }];
 		candidate.message.prunedAt = prunedAt;
+		invalidateMessageCache(candidate.message as AgentMessage);
 		tokensSaved += estimatePrunedSavings(candidate.tokens, candidate.notice);
 	}
 	return { prunedCount: toPrune.length, tokensSaved };
@@ -398,6 +400,7 @@ export function pruneToolOutputs(entries: SessionEntry[], config: PruneConfig =
 				: createPrunedNotice(candidate.tokens);
 		message.content = [{ type: "text", text: notice }];
 		message.prunedAt = prunedAt;
+		invalidateMessageCache(message as AgentMessage);
 		prunedCount++;
 	}
 
diff --git a/packages/agent/src/compaction/shake.ts b/packages/agent/src/compaction/shake.ts
index 7e0f2c5ed..db95486de 100644
--- a/packages/agent/src/compaction/shake.ts
+++ b/packages/agent/src/compaction/shake.ts
@@ -15,6 +15,7 @@ import { countTokens } from "../tokenizer";
 import type { AgentMessage } from "../types";
 import { estimateTokens } from "./compaction";
 import type { CustomMessageEntry, SessionEntry, SessionMessageEntry } from "./entries";
+import { invalidateMessageCache } from "./message-cache";
 import {
 	collectToolCallsById,
 	isProtectedToolResult,
@@ -406,12 +407,18 @@ export function applyShakeRegion(region: ShakeRegion, replacement: string): void
 		const message = region.entry.message as ToolResultMessage;
 		message.content = [{ type: "text", text: replacement }];
 		message.prunedAt = Date.now();
+		invalidateMessageCache(message as AgentMessage);
 		return;
 	}
 	const slot = getBlockTextSlot(region.entry, region.blockIndex);
 	if (!slot) return;
 	const text = slot.read();
 	slot.write(text.slice(0, region.start) + replacement + text.slice(region.end));
+	// Message entries keep a stable `entry.message` identity across context
+	// rebuilds, so an in-place block rewrite must drop its cached estimate/convert.
+	// Custom-message entries are re-materialized into a fresh AgentMessage on every
+	// buildSessionContext, so they carry no stable cached identity to invalidate.
+	if (region.entry.type === "message") invalidateMessageCache(region.entry.message);
 }
 
 /**
diff --git a/packages/agent/test/message-cache.test.ts b/packages/agent/test/message-cache.test.ts
new file mode 100644
index 000000000..ac3db9997
--- /dev/null
+++ b/packages/agent/test/message-cache.test.ts
@@ -0,0 +1,159 @@
+import { describe, expect, test } from "bun:test";
+import type { AgentMessage } from "@oh-my-pi/pi-agent-core";
+import type { SessionMessageEntry } from "@oh-my-pi/pi-agent-core/compaction";
+import {
+	applyShakeRegion,
+	collectShakeRegions,
+	DEFAULT_PRUNE_CONFIG,
+	estimateTokens,
+	invalidateMessageCache,
+	isEstimateCacheable,
+	pruneToolOutputs,
+} from "@oh-my-pi/pi-agent-core/compaction";
+import type { AssistantMessage, ToolResultMessage, Usage } from "@oh-my-pi/pi-ai";
+
+let idCounter = 0;
+function nextId(): string {
+	return `mc-${idCounter++}`;
+}
+
+function messageEntry(message: AgentMessage): SessionMessageEntry {
+	return { type: "message", id: nextId(), parentId: null, timestamp: new Date().toISOString(), message };
+}
+
+function usage(totalTokens: number): Usage {
+	return {
+		input: totalTokens,
+		output: 0,
+		cacheRead: 0,
+		cacheWrite: 0,
+		totalTokens,
+		cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
+	};
+}
+
+function settledAssistant(text: string): AssistantMessage {
+	return {
+		role: "assistant",
+		content: [{ type: "text", text }],
+		api: "anthropic-messages",
+		provider: "anthropic",
+		model: "bench",
+		usage: usage(120),
+		stopReason: "stop",
+		timestamp: 1,
+	};
+}
+
+function toolResult(text: string, extra?: Partial<ToolResultMessage>): ToolResultMessage {
+	return {
+		role: "toolResult",
+		toolCallId: `call-${idCounter++}`,
+		toolName: "read",
+		content: [{ type: "text", text }],
+		isError: false,
+		timestamp: Date.now(),
+		...extra,
+	};
+}
+
+describe("estimate cache settle gate", () => {
+	test("caches settled assistants (terminal stopReason + real usage)", () => {
+		expect(isEstimateCacheable(settledAssistant("done"))).toBe(true);
+	});
+
+	test("bypasses a streaming assistant (zero usage seed)", () => {
+		const streaming: AssistantMessage = { ...settledAssistant("partial"), usage: usage(0), stopReason: "stop" };
+		expect(isEstimateCacheable(streaming)).toBe(false);
+	});
+
+	test("bypasses aborted and error assistants even with usage", () => {
+		expect(isEstimateCacheable({ ...settledAssistant("x"), stopReason: "aborted" })).toBe(false);
+		expect(isEstimateCacheable({ ...settledAssistant("x"), stopReason: "error" })).toBe(false);
+	});
+
+	test("caches non-assistant roles unconditionally", () => {
+		expect(isEstimateCacheable(toolResult("out") as AgentMessage)).toBe(true);
+		expect(isEstimateCacheable({ role: "user", content: "hi", timestamp: 1 } as AgentMessage)).toBe(true);
+	});
+
+	test("a streaming assistant re-estimates as its content grows", () => {
+		const streaming: AssistantMessage = {
+			...settledAssistant("first chunk"),
+			usage: usage(0),
+			stopReason: "stop",
+		};
+		const before = estimateTokens(streaming as AgentMessage);
+		streaming.content = [{ type: "text", text: "first chunk plus a much longer continuation of streamed text" }];
+		const after = estimateTokens(streaming as AgentMessage);
+		// Unsettled assistants never read the cache, so the grown content is recounted.
+		expect(after).toBeGreaterThan(before);
+	});
+});
+
+describe("estimate cache option split", () => {
+	test("default and floored estimates do not collide in one map", () => {
+		const blob = "blob ".repeat(4000);
+		const msg: AssistantMessage = {
+			...settledAssistant("thinking heavy"),
+			content: [
+				{ type: "text", text: "answer" },
+				{ type: "thinking", thinking: "reasoning", thinkingSignature: blob },
+			],
+		};
+		// Prime the default map first, then the floored one; the floored estimate
+		// (which drops the encrypted-reasoning blob) must not read the default entry.
+		const withBlob = estimateTokens(msg as AgentMessage);
+		const floored = estimateTokens(msg as AgentMessage, { excludeEncryptedReasoning: true });
+		expect(withBlob).toBeGreaterThan(floored + 500);
+		// Cached reads return the same split values.
+		expect(estimateTokens(msg as AgentMessage)).toBe(withBlob);
+		expect(estimateTokens(msg as AgentMessage, { excludeEncryptedReasoning: true })).toBe(floored);
+	});
+});
+
+describe("estimate cache invalidation seams", () => {
+	test("pruneToolOutputs drops the cached estimate of a pruned result", () => {
+		const big = toolResult("x".repeat(20_000));
+		const entries = [messageEntry(big as AgentMessage)];
+		const before = estimateTokens(big as AgentMessage);
+		expect(before).toBeGreaterThan(1000);
+
+		const result = pruneToolOutputs(entries, { ...DEFAULT_PRUNE_CONFIG, protectTokens: 0, minimumSavings: 0 });
+		expect(result.prunedCount).toBe(1);
+
+		// After the in-place prune the estimate must reflect the short placeholder,
+		// not the stale full-content count.
+		const after = estimateTokens(big as AgentMessage);
+		expect(after).toBeLessThan(before);
+	});
+
+	test("applyShakeRegion drops the cached estimate of a shaken result", () => {
+		const big = toolResult(`\`\`\`ts\n${"const value = compute(a, b, c, d, e);\n".repeat(400)}\`\`\``);
+		const entry = messageEntry(big as AgentMessage);
+		const before = estimateTokens(big as AgentMessage);
+
+		const regions = collectShakeRegions([entry], {
+			protectTokens: 0,
+			minSavings: 0,
+			protectedTools: [],
+			fenceMinTokens: 0,
+		});
+		expect(regions.length).toBeGreaterThan(0);
+		applyShakeRegion(regions[0], "[shaken]");
+
+		const after = estimateTokens(big as AgentMessage);
+		expect(after).toBeLessThan(before);
+	});
+
+	test("explicit invalidateMessageCache forces a recount", () => {
+		const result = toolResult("original content here");
+		const before = estimateTokens(result as AgentMessage);
+		// Mutate content directly (simulating an owner rewrite) then invalidate.
+		result.content = [{ type: "text", text: "a much longer replacement body that should count higher than before" }];
+		// Without invalidation the stale cached value would still be returned.
+		expect(estimateTokens(result as AgentMessage)).toBe(before);
+		invalidateMessageCache(result as AgentMessage);
+		expect(estimateTokens(result as AgentMessage)).toBeGreaterThan(before);
+	});
+});
diff --git a/packages/coding-agent/CHANGELOG.md b/packages/coding-agent/CHANGELOG.md
index ad20be261..cf23e12f8 100644
--- a/packages/coding-agent/CHANGELOG.md
+++ b/packages/coding-agent/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Long sessions no longer re-run `convertToLlm` over settled history every turn. Conversion is memoized per message identity (plus the assistant `interruptedNext` neighbor flag): an exact re-convert of the same array reuses the outer `Message[]`, append-only growth reuses the converted prefix via slice-on-growth, and the prune/shake/strip-images/prewalk-scrub rewrite seams invalidate the affected message before the next pass. On the `llm-assembly` bench (N=5000) steady/append convert and repeat estimate are all >10x faster with robust MAD-noise well under 20% ([#5934](https://github.com/can1357/oh-my-pi/issues/5934)).
+
 ## [17.0.3] - 2026-07-17
 
 ### Changed
diff --git a/packages/coding-agent/bench/llm-assembly.bench.ts b/packages/coding-agent/bench/llm-assembly.bench.ts
new file mode 100644
index 000000000..195ae0ab1
--- /dev/null
+++ b/packages/coding-agent/bench/llm-assembly.bench.ts
@@ -0,0 +1,242 @@
+/**
+ * Benchmark: LLM-assembly recompute over settled history (perf/long-session-convert-estimate-memo).
+ *
+ * Before each model call and during compaction accounting, the agent walks the
+ * full live `AgentMessage[]` history through:
+ *   1. `convertToLlm(messages)`  — role-specific conversion into provider `Message[]`.
+ *   2. `estimateTokens(message)` — cl100k-style token counting for prune/shake/floors.
+ *
+ * In a long session those historical objects are settled, yet before the memo
+ * both paths recompute from scratch on every pass. This bench measures cold
+ * (fresh identities → cache miss) against steady state (warm cache):
+ *
+ *   - convert first:   cold conversion of a never-before-seen history.
+ *   - convert steady:  re-convert of the same (warmed) array + append-only growth
+ *                      that reuses the settled prefix.
+ *   - estimate first:  cold token count of a never-before-seen history.
+ *   - estimate second: repeat count of the identical warmed history.
+ *
+ * Acceptance (issue #5934): on N=5000 the first/steady convert and first/second
+ * estimate speedups are >=10x, and the absolute noise gate uses robust MAD
+ * noise <=20% of the median (not raw stddev/median).
+ *
+ * Run: `bun run packages/coding-agent/bench/llm-assembly.bench.ts`
+ * Env: `LLM_ASSEMBLY_N` overrides the history length (default 5000);
+ *      `PI_TOKENIZER_ACCURATE=1` uses the native cl100k tokenizer.
+ */
+import type { AgentMessage } from "@oh-my-pi/pi-agent-core";
+import { estimateTokens } from "@oh-my-pi/pi-agent-core/compaction";
+import type { AssistantMessage, ToolResultMessage, Usage } from "@oh-my-pi/pi-ai";
+import { convertToLlm } from "../src/session/messages";
+
+const N = Number(Bun.env.LLM_ASSEMBLY_N ?? 5000);
+const WARMUP = 5;
+const SAMPLES = 25;
+
+function settledUsage(total: number): Usage {
+	return {
+		input: total,
+		output: 0,
+		cacheRead: 0,
+		cacheWrite: 0,
+		totalTokens: total,
+		cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
+	};
+}
+
+function codeBlob(seed: number): string {
+	return `\`\`\`typescript\nexport function f${seed}(a: number, b: number): number {\n\treturn a + b + ${seed};\n}\n\`\`\``;
+}
+
+/** Build a settled, mixed history: user / assistant (settled usage + tool call) / tool-result triples.
+ *  Every call mints fresh object identities so it reads as a cold (uncached) workload. */
+function buildHistory(count: number): AgentMessage[] {
+	const messages: AgentMessage[] = [];
+	for (let i = 0; i < count; i++) {
+		const ts = 1_700_000_000_000 + i * 1000;
+		const kind = i % 3;
+		if (kind === 0) {
+			messages.push({
+				role: "user",
+				content: `User turn ${i}: please look at this.\n\n${codeBlob(i)}`,
+				timestamp: ts,
+			} as AgentMessage);
+		} else if (kind === 1) {
+			const assistant: AssistantMessage = {
+				role: "assistant",
+				content: [
+					{ type: "text", text: `Assistant turn ${i}. ${codeBlob(i)}` },
+					{ type: "toolCall", id: `call-${i}`, name: "read", arguments: { path: `src/f${i}.ts` } },
+				],
+				api: "anthropic-messages",
+				provider: "anthropic",
+				model: "bench",
+				usage: settledUsage(200 + (i % 50)),
+				stopReason: "toolUse",
+				timestamp: ts,
+			};
+			messages.push(assistant as AgentMessage);
+		} else {
+			const toolResult: ToolResultMessage = {
+				role: "toolResult",
+				toolCallId: `call-${i - 1}`,
+				toolName: "read",
+				content: [{ type: "text", text: `Tool result ${i}.\n${codeBlob(i)}\n${codeBlob(i + 1)}` }],
+				isError: false,
+				timestamp: ts,
+			};
+			messages.push(toolResult as AgentMessage);
+		}
+	}
+	return messages;
+}
+
+interface Stats {
+	median: number;
+	madNoise: number;
+}
+
+/** Median and robust MAD-based noise (median absolute deviation, normalized). */
+function stats(samples: number[]): Stats {
+	const sorted = [...samples].sort((a, b) => a - b);
+	const median = sorted[sorted.length >> 1];
+	const deviations = sorted.map(x => Math.abs(x - median)).sort((a, b) => a - b);
+	const mad = deviations[deviations.length >> 1];
+	// 1.4826 scales MAD to a stddev-equivalent for a normal distribution.
+	const madNoise = median === 0 ? 0 : (1.4826 * mad) / median;
+	return { median, madNoise };
+}
+
+/**
+ * Time `run(workload)` across samples. `makeWorkload` builds inputs OUTSIDE the
+ * timing window, so a cold phase can hand each sample fresh (uncached) identities
+ * without allocation noise polluting the measurement; a warm phase hands back one
+ * shared, already-primed workload.
+ *
+ * `batch` runs that many independent workloads inside one timed window and
+ * reports per-op time. A sub-millisecond cold op sits near the timer/scheduler
+ * floor where jitter dominates MAD-noise; batching lifts the measured window well
+ * above that floor while keeping each op a genuine cache miss.
+ */
+function sample<T>(makeWorkload: () => T, run: (workload: T) => void, batch = 1): Stats {
+	for (let i = 0; i < WARMUP; i++) run(makeWorkload());
+	const samples: number[] = [];
+	for (let i = 0; i < SAMPLES; i++) {
+		const workloads: T[] = [];
+		for (let b = 0; b < batch; b++) workloads.push(makeWorkload());
+		// Collect workload-allocation garbage BEFORE timing so a GC pause can't land
+		// inside the window and inflate MAD-noise.
+		Bun.gc(true);
+		const t0 = Bun.nanoseconds();
+		for (let b = 0; b < batch; b++) run(workloads[b]);
+		samples.push((Bun.nanoseconds() - t0) / 1e6 / batch);
+	}
+	return stats(samples);
+}
+
+function estimateAll(messages: AgentMessage[]): number {
+	let total = 0;
+	for (const m of messages) total += estimateTokens(m);
+	return total;
+}
+
+console.log(`\nBenchmark: llm-assembly (N=${N}, warmup=${WARMUP}, samples=${SAMPLES})\n`);
+
+// ─── convertToLlm ─────────────────────────────────────────────────────────────
+// Cold: a fresh-identity history per sample → every message is a cache miss.
+const convertFirst = sample(
+	() => buildHistory(N),
+	history => {
+		convertToLlm(history);
+	},
+	16,
+);
+// Steady: re-convert the same (warmed) array. transformContext re-converts the
+// same live array multiple times per turn (prompt assembly, prune/shake
+// accounting, context breakdown); the exact-repeat shortcut hands back the same
+// outer array. Priming twice warms both the per-message memo and the shortcut.
+const warmConvert = buildHistory(N);
+convertToLlm(warmConvert);
+convertToLlm(warmConvert);
+const convertSteady = sample(
+	() => warmConvert,
+	history => {
+		convertToLlm(history);
+	},
+);
+
+// Append-growth: push one settled turn onto the same array identity each sample,
+// then reconvert. Slice-on-growth reuses the unchanged prefix output and
+// reconverts only the boundary message plus the new suffix, so the per-turn cost
+// is O(suffix), not O(history).
+const growConvert = buildHistory(N);
+convertToLlm(growConvert);
+let growSeed = N;
+const convertGrow = sample(
+	() => {
+		growConvert.push({
+			role: "user",
+			content: `User turn ${growSeed}: one more.\n\n${codeBlob(growSeed)}`,
+			timestamp: 1_700_000_000_000 + growSeed * 1000,
+		} as AgentMessage);
+		growSeed++;
+		return growConvert;
+	},
+	history => {
+		convertToLlm(history);
+	},
+);
+
+// ─── estimateTokens ───────────────────────────────────────────────────────────
+// Cold: fresh-identity history per sample → every estimate is a cache miss.
+const estimateFirst = sample(
+	() => buildHistory(N),
+	history => {
+		estimateAll(history);
+	},
+);
+// Warm: one history, primed once, re-counted every sample from the cache.
+const warmEstimate = buildHistory(N);
+estimateAll(warmEstimate);
+const estimateSecond = sample(
+	() => warmEstimate,
+	history => {
+		estimateAll(history);
+	},
+);
+
+function report(label: string, s: Stats): void {
+	console.log(
+		`  ${label.padEnd(18)} median ${s.median.toFixed(4).padStart(10)} ms   MAD-noise ${(s.madNoise * 100).toFixed(1).padStart(5)}%`,
+	);
+}
+
+report("convert first", convertFirst);
+report("convert steady", convertSteady);
+report("convert grow", convertGrow);
+report("estimate first", estimateFirst);
+report("estimate second", estimateSecond);
+
+const convertSteadySpeedup = convertFirst.median / convertSteady.median;
+const convertGrowSpeedup = convertFirst.median / convertGrow.median;
+const estimateSpeedup = estimateFirst.median / estimateSecond.median;
+console.log(`\n  convert speedup (first / steady):    ${convertSteadySpeedup.toFixed(2)}x`);
+console.log(`  convert speedup (first / grow):      ${convertGrowSpeedup.toFixed(2)}x`);
+console.log(`  estimate speedup (first / second):   ${estimateSpeedup.toFixed(2)}x`);
+
+const noiseGate = 0.2;
+const worstNoise = Math.max(
+	convertFirst.madNoise,
+	convertSteady.madNoise,
+	convertGrow.madNoise,
+	estimateFirst.madNoise,
+	estimateSecond.madNoise,
+);
+console.log(`  worst MAD-noise: ${(worstNoise * 100).toFixed(1)}% (gate ${(noiseGate * 100).toFixed(0)}%)\n`);
+
+console.log(`METRIC convert_steady_speedup=${convertSteadySpeedup.toFixed(3)}`);
+console.log(`METRIC convert_grow_speedup=${convertGrowSpeedup.toFixed(3)}`);
+console.log(`METRIC estimate_speedup=${estimateSpeedup.toFixed(3)}`);
+console.log(`METRIC worst_mad_noise=${worstNoise.toFixed(4)}`);
+
+process.exit(0);
diff --git a/packages/coding-agent/src/session/agent-session.ts b/packages/coding-agent/src/session/agent-session.ts
index aeb4a2e49..0529b7d25 100644
--- a/packages/coding-agent/src/session/agent-session.ts
+++ b/packages/coding-agent/src/session/agent-session.ts
@@ -63,6 +63,7 @@ import {
 	estimateTokens,
 	generateBranchSummary,
 	generateHandoffFromContext,
+	invalidateMessageCache,
 	prepareCompaction,
 	renderHandoffPrompt,
 	resolveBudgetReserveTokens,
@@ -2524,7 +2525,13 @@ export class AgentSession {
 		const isPlanNudge = (m: AgentMessage): boolean =>
 			m.role === "custom" && m.customType === PREWALK_PLAN_MESSAGE_TYPE;
 		for (let i = liveMessages.length - 1; i >= 0; i--) {
-			if (isPlanNudge(liveMessages[i])) liveMessages.splice(i, 1);
+			if (isPlanNudge(liveMessages[i])) {
+				// Interior removal on the live array: drop the scrubbed message from
+				// the convert/estimate caches so the next convert can't reuse a prefix
+				// that still carries its fragment (the array shrinks in place).
+				invalidateMessageCache(liveMessages[i]);
+				liveMessages.splice(i, 1);
+			}
 		}
 		const stateMessages = this.agent.state.messages;
 		const filtered = stateMessages.filter(m => !isPlanNudge(m));
diff --git a/packages/coding-agent/src/session/messages.test.ts b/packages/coding-agent/src/session/messages.test.ts
index cfc01d73b..5d6680047 100644
--- a/packages/coding-agent/src/session/messages.test.ts
+++ b/packages/coding-agent/src/session/messages.test.ts
@@ -8,6 +8,7 @@ import {
 	replaceLlmImagesWithText,
 	SKILL_PROMPT_MESSAGE_TYPE,
 	type SkillPromptDetails,
+	stripImagesFromMessage,
 } from "./messages";
 
 function customMessage(customType: string, attribution: "agent" | "user"): CustomMessage<SkillPromptDetails> {
@@ -125,6 +126,96 @@ describe("convertToLlm", () => {
 	});
 });
 
+function settledAssistant(text: string): AssistantMessage {
+	return {
+		role: "assistant",
+		content: [{ type: "text", text }],
+		api: "anthropic-messages",
+		provider: "anthropic",
+		model: "claude-sonnet-4-5",
+		usage: {
+			input: 100,
+			output: 20,
+			cacheRead: 0,
+			cacheWrite: 0,
+			totalTokens: 120,
+			cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
+		},
+		stopReason: "stop",
+		timestamp: 1,
+	};
+}
+
+function userMessage(text: string, timestamp: number): AgentMessage {
+	return { role: "user", content: text, attribution: "user", timestamp } as AgentMessage;
+}
+
+describe("convertToLlm caching", () => {
+	it("reuses the outer array on an exact repeat of the same history", () => {
+		const messages: AgentMessage[] = [userMessage("hello", 1), settledAssistant("hi")];
+		const first = convertToLlm(messages);
+		const second = convertToLlm(messages);
+		expect(second).toBe(first);
+	});
+
+	it("reuses the unchanged prefix output on append-only growth", () => {
+		const messages: AgentMessage[] = [userMessage("one", 1), settledAssistant("reply one")];
+		const first = convertToLlm(messages);
+		messages.push(userMessage("two", 2));
+		const grown = convertToLlm(messages);
+		// New outer array (no held-result aliasing), but the converted prefix is
+		// byte-identical and the appended turn is present.
+		expect(grown).not.toBe(first);
+		expect(grown.length).toBe(first.length + 1);
+		expect(grown.slice(0, first.length)).toEqual(first);
+		expect(grown[grown.length - 1]?.role).toBe("user");
+	});
+
+	it("recomputes the boundary assistant when a following interrupted-thinking marker appears on growth", () => {
+		const messages: AgentMessage[] = [
+			abortedAssistant([
+				{ type: "text", text: "partial answer" },
+				{ type: "thinking", thinking: "interrupted reasoning" },
+			]),
+		];
+		const before = convertToLlm(messages);
+		const beforeAssistant = before.find(entry => entry.role === "assistant");
+		expect(Array.isArray(beforeAssistant?.content) && beforeAssistant.content.map(b => b.type)).toEqual([
+			"text",
+			"thinking",
+		]);
+
+		// Append the continuity marker on the same array: the assistant is now the
+		// boundary message and its LLM view must drop the trailing thinking run.
+		messages.push(interruptedThinkingContinuity());
+		const after = convertToLlm(messages);
+		const afterAssistant = after.find(entry => entry.role === "assistant");
+		expect(Array.isArray(afterAssistant?.content) && afterAssistant.content.map(b => b.type)).toEqual(["text"]);
+	});
+
+	it("recomputes a message after strip-images invalidates its cache", () => {
+		const withImage: AgentMessage = {
+			role: "user",
+			content: [
+				{ type: "text", text: "look" },
+				{ type: "image", data: "aaaa", mimeType: "image/png" },
+			],
+			attribution: "user",
+			timestamp: 1,
+		};
+		const messages: AgentMessage[] = [withImage];
+		const before = convertToLlm(messages);
+		const beforeUser = before.find(entry => entry.role === "user");
+		expect(Array.isArray(beforeUser?.content) && beforeUser.content.some(b => b.type === "image")).toBe(true);
+
+		// Mutate in place through the owner seam, which must invalidate the cache.
+		stripImagesFromMessage(withImage);
+		const after = convertToLlm(messages);
+		const afterUser = after.find(entry => entry.role === "user");
+		expect(Array.isArray(afterUser?.content) && afterUser.content.some(b => b.type === "image")).toBe(false);
+	});
+});
+
 describe("replaceLlmImagesWithText", () => {
 	it("replaces image blocks in user, developer, and tool-result messages with the placeholder", () => {
 		const converted = convertToLlm([
diff --git a/packages/coding-agent/src/session/messages.ts b/packages/coding-agent/src/session/messages.ts
index 6fd7b24ed..97a607ddf 100644
--- a/packages/coding-agent/src/session/messages.ts
+++ b/packages/coding-agent/src/session/messages.ts
@@ -5,6 +5,10 @@
  * and provides a transformer to convert them to LLM-compatible messages.
  */
 import type { AgentMessage } from "@oh-my-pi/pi-agent-core";
+import {
+	invalidateMessageCache,
+	registerMessageCacheInvalidator,
+} from "@oh-my-pi/pi-agent-core/compaction/message-cache";
 import {
 	type BranchSummaryMessage,
 	type CompactionSummaryMessage,
@@ -457,6 +461,14 @@ function stripImagesFromArrayContent(content: (TextContent | ImageContent)[]): S
  * pure local mutation and intentionally does neither.
  */
 export function stripImagesFromMessage(message: AgentMessage): number {
+	const removed = stripImagesFromMessageContent(message);
+	// The mutated message keeps its identity across context rebuilds, so drop its
+	// cached estimate/convert before the next pass counts/converts the new shape.
+	if (removed > 0) invalidateMessageCache(message);
+	return removed;
+}
+
+function stripImagesFromMessageContent(message: AgentMessage): number {
 	switch (message.role) {
 		case "user":
 		case "developer":
@@ -750,6 +762,184 @@ function convertImageBearingCustomMessage(message: CustomMessage | HookMessage):
 	return converted;
 }
 
+/**
+ * Per-message conversion result, keyed by message identity. `interruptedNext`
+ * records the neighbor state the fragment was built against so an assistant
+ * whose following {@link INTERRUPTED_THINKING_MESSAGE_TYPE} marker appears or
+ * disappears is recomputed (its LLM view strips the trailing thinking run only
+ * while that marker follows).
+ *
+ * WeakMap (not a symbol tag) is deliberate: `wrapSteeringForModel` and
+ * `deobfuscateAgentMessages` spread messages into fresh variants with different
+ * content; a symbol-keyed fragment would ride that spread and mis-convert the
+ * copy. Identity keying keeps the cache off spread copies.
+ */
+interface ConvertMemoEntry {
+	interruptedNext: boolean;
+	fragment: Message[];
+}
+const convertCache = new WeakMap<AgentMessage, ConvertMemoEntry>();
+
+// Array-level shortcuts over the per-message memo. The live agent mutates one
+// `AgentMessage[]` identity across a turn: appending new messages and swapping
+// the streaming tail (`context.messages[len-1] = partial → trailing`). Between
+// owner invalidations (prune/shake/strip bump `convertGeneration`) and for a
+// given array identity, only the last index is ever swapped and the array only
+// grows — interior prefix messages are immutable. That invariant lets two
+// shortcuts skip the O(N) re-walk:
+//   - exact-repeat: same array, same length, same generation, same tail identity
+//     → hand back the same outer array.
+//   - slice-on-growth: same array, same generation, length grew → copy the
+//     unchanged prefix output and reconvert only the neighbor-sensitive boundary
+//     message plus the appended suffix.
+// The tail-identity guard on exact-repeat catches the streaming snapshot swap
+// (partial → trailing is a fresh identity), so a settled tail is never served
+// from a stale mid-stream fragment.
+let convertGeneration = 0;
+let lastConvertInput: AgentMessage[] | undefined;
+let lastConvertLength = 0;
+let lastConvertOutput: Message[] | undefined;
+let lastConvertGeneration = -1;
+let lastConvertTail: AgentMessage | undefined;
+// Output-message count contributed by messages[0 .. lastConvertLength-1), i.e.
+// every message except the last. The last message is neighbor-sensitive (its LLM
+// view drops the trailing thinking run only while an interrupted-thinking marker
+// follows), so growth reconverts it rather than reusing its old fragment.
+let lastConvertPrefixOutputLen = 0;
+
+registerMessageCacheInvalidator(message => {
+	convertCache.delete(message);
+	convertGeneration++;
+});
+
+/** Convert one message to its LLM fragment. `interruptedNext` is true only for an
+ *  assistant turn immediately followed by its interrupted-thinking marker. */
+function convertOne(m: AgentMessage, interruptedNext: boolean): Message[] {
+	switch (m.role) {
+		case "bashExecution":
+			if (m.excludeFromContext) {
+				return [];
+			}
+			return [
+				{
+					role: "user",
+					content: [{ type: "text", text: bashExecutionToText(m) }],
+					attribution: "user",
+					timestamp: m.timestamp,
+				},
+			];
+		case "pythonExecution":
+			if (m.excludeFromContext) {
+				return [];
+			}
+			return [
+				{
+					role: "user",
+					content: [{ type: "text", text: pythonExecutionToText(m) }],
+					attribution: "user",
+					timestamp: m.timestamp,
+				},
+			];
+		case "fileMention": {
+			// One `fileMention` can mix `@notes.md` (text) and `@screenshot.png` (image)
+			// in the same turn (`generateFileMentionMessages` packs every `@…` into a
+			// single message). Splitting by image presence keeps text-only mentions on
+			// the higher-priority `developer` slot while routing image attachments
+			// through `user`, the only Responses content slot that legitimately accepts
+			// `input_image` (Codex chatgpt.com /codex/responses rejects everything else
+			// with `Invalid value: 'input_image'`, #3443).
+			const wrap = (file: FileMentionMessage["files"][number]): string => {
+				const inner = file.content ? `\n${file.content}\n` : "\n";
+				return `<file path="${file.path}">${inner}</file>`;
+			};
+			const textFiles = m.files.filter(file => !file.image);
+			const imageFiles = m.files.filter(file => file.image);
+			const out: Message[] = [];
+			if (textFiles.length > 0) {
+				out.push({
+					role: "developer",
+					content: [{ type: "text" as const, text: textFiles.map(wrap).join("\n") }],
+					attribution: "user",
+					timestamp: m.timestamp,
+				});
+			}
+			if (imageFiles.length > 0) {
+				const content: (TextContent | ImageContent)[] = [
+					{ type: "text" as const, text: imageFiles.map(wrap).join("\n") },
+				];
+				for (const file of imageFiles) {
+					if (file.image) content.push(file.image);
+				}
+				out.push({
+					role: "user",
+					content,
+					attribution: "user",
+					timestamp: m.timestamp,
+				});
+			}
+			return out;
+		}
+		case "custom": {
+			if (!isCustomMessageContent(m.content)) return [];
+			if (isUserInvokedSkillPrompt(m)) {
+				return [
+					{
+						role: "user",
+						content: customMessageContentToLlmContent(m.content),
+						attribution: "user",
+						timestamp: m.timestamp,
+					},
+				];
+			}
+			const split = convertImageBearingCustomMessage(m);
+			if (split) return split;
+			const converted = convertMessageToLlm(m);
+			return converted ? [converted] : [];
+		}
+		case "hookMessage": {
+			if (!isCustomMessageContent(m.content)) return [];
+			const split = convertImageBearingCustomMessage(m);
+			if (split) return split;
+			const converted = convertMessageToLlm(m);
+			return converted ? [converted] : [];
+		}
+		case "assistant": {
+			// A user-interrupted turn keeps its trailing thinking run on the
+			// persisted/displayed message so reload and Ctrl+L rebuilds still
+			// show it. That run is incomplete/unsigned and gets rejected on
+			// resend, so strip it here — LLM path only — when the hidden
+			// interrupted-thinking continuity message follows.
+			const source = interruptedNext ? stripDemotedThinkingForLlm(m) : m;
+			const converted = convertMessageToLlm(source);
+			return converted ? [converted] : [];
+		}
+		case "branchSummary":
+		case "compactionSummary":
+		case "user":
+		case "developer":
+		case "toolResult": {
+			// Core roles share one transformer with agent-core —
+			// duplicating them here is how snapcompact frames once
+			// silently fell off the provider request.
+			const converted = convertMessageToLlm(m);
+			return converted ? [converted] : [];
+		}
+		default:
+			m satisfies never;
+			return [];
+	}
+}
+
+/** Cached per-message conversion. Reuses the stored fragment while identity and
+ *  `interruptedNext` neighbor state hold; recomputes on a neighbor flip. */
+function convertOneCached(m: AgentMessage, interruptedNext: boolean): Message[] {
+	const cached = convertCache.get(m);
+	if (cached !== undefined && cached.interruptedNext === interruptedNext) return cached.fragment;
+	const fragment = convertOne(m, interruptedNext);
+	convertCache.set(m, { interruptedNext, fragment });
+	return fragment;
+}
+
 /**
  * Transform AgentMessages (including custom types) to LLM-compatible Messages.
  *
@@ -757,121 +947,69 @@ function convertImageBearingCustomMessage(message: CustomMessage | HookMessage):
  * - Agent's transormToLlm option (for prompt calls and queued messages)
  * - Compaction's generateSummary (for summarization)
  * - Custom extensions and tools
+ *
+ * Settled history converts once and is reused per message identity: an
+ * append-only turn on the same array re-pays only the new suffix, and an
+ * unchanged re-convert of the same array hands back the same outer `Message[]`.
+ * Owner mutations (prune/shake/strip-images) invalidate the affected message
+ * through the shared registry before the next pass.
  */
 export function convertToLlm(messages: AgentMessage[]): Message[] {
-	return messages.flatMap((m, index): Message[] => {
-		switch (m.role) {
-			case "bashExecution":
-				if (m.excludeFromContext) {
-					return [];
-				}
-				return [
-					{
-						role: "user",
-						content: [{ type: "text", text: bashExecutionToText(m) }],
-						attribution: "user",
-						timestamp: m.timestamp,
-					},
-				];
-			case "pythonExecution":
-				if (m.excludeFromContext) {
-					return [];
-				}
-				return [
-					{
-						role: "user",
-						content: [{ type: "text", text: pythonExecutionToText(m) }],
-						attribution: "user",
-						timestamp: m.timestamp,
-					},
-				];
-			case "fileMention": {
-				// One `fileMention` can mix `@notes.md` (text) and `@screenshot.png` (image)
-				// in the same turn (`generateFileMentionMessages` packs every `@…` into a
-				// single message). Splitting by image presence keeps text-only mentions on
-				// the higher-priority `developer` slot while routing image attachments
-				// through `user`, the only Responses content slot that legitimately accepts
-				// `input_image` (Codex chatgpt.com /codex/responses rejects everything else
-				// with `Invalid value: 'input_image'`, #3443).
-				const wrap = (file: FileMentionMessage["files"][number]): string => {
-					const inner = file.content ? `\n${file.content}\n` : "\n";
-					return `<file path="${file.path}">${inner}</file>`;
-				};
-				const textFiles = m.files.filter(file => !file.image);
-				const imageFiles = m.files.filter(file => file.image);
-				const out: Message[] = [];
-				if (textFiles.length > 0) {
-					out.push({
-						role: "developer",
-						content: [{ type: "text" as const, text: textFiles.map(wrap).join("\n") }],
-						attribution: "user",
-						timestamp: m.timestamp,
-					});
-				}
-				if (imageFiles.length > 0) {
-					const content: (TextContent | ImageContent)[] = [
-						{ type: "text" as const, text: imageFiles.map(wrap).join("\n") },
-					];
-					for (const file of imageFiles) {
-						if (file.image) content.push(file.image);
-					}
-					out.push({
-						role: "user",
-						content,
-						attribution: "user",
-						timestamp: m.timestamp,
-					});
-				}
-				return out;
-			}
-			case "custom": {
-				if (!isCustomMessageContent(m.content)) return [];
-				if (isUserInvokedSkillPrompt(m)) {
-					return [
-						{
-							role: "user",
-							content: customMessageContentToLlmContent(m.content),
-							attribution: "user",
-							timestamp: m.timestamp,
-						},
-					];
-				}
-				const split = convertImageBearingCustomMessage(m);
-				if (split) return split;
-				const converted = convertMessageToLlm(m);
-				return converted ? [converted] : [];
-			}
-			case "hookMessage": {
-				if (!isCustomMessageContent(m.content)) return [];
-				const split = convertImageBearingCustomMessage(m);
-				if (split) return split;
-				const converted = convertMessageToLlm(m);
-				return converted ? [converted] : [];
-			}
-			case "assistant": {
-				// A user-interrupted turn keeps its trailing thinking run on the
-				// persisted/displayed message so reload and Ctrl+L rebuilds still
-				// show it. That run is incomplete/unsigned and gets rejected on
-				// resend, so strip it here — LLM path only — when the hidden
-				// interrupted-thinking continuity message follows.
-				const source = followedByInterruptedThinking(messages, index) ? stripDemotedThinkingForLlm(m) : m;
-				const converted = convertMessageToLlm(source);
-				return converted ? [converted] : [];
-			}
-			case "branchSummary":
-			case "compactionSummary":
-			case "user":
-			case "developer":
-			case "toolResult": {
-				// Core roles share one transformer with agent-core —
-				// duplicating them here is how snapcompact frames once
-				// silently fell off the provider request.
-				const converted = convertMessageToLlm(m);
-				return converted ? [converted] : [];
-			}
-			default:
-				m satisfies never;
-				return [];
-		}
-	});
+	const len = messages.length;
+	const sameArray = messages === lastConvertInput && lastConvertGeneration === convertGeneration;
+	const tail = len > 0 ? messages[len - 1] : undefined;
+
+	// Exact-repeat: same array, same length, same trailing identity → reuse the
+	// outer array. The tail-identity check rejects the streaming snapshot swap
+	// (partial → settled trailing keeps array identity/length but mints a fresh
+	// tail), so a settled tail never reads a stale mid-stream fragment.
+	if (sameArray && lastConvertOutput !== undefined && len === lastConvertLength && tail === lastConvertTail) {
+		return lastConvertOutput;
+	}
+
+	// Slice-on-growth: same array grew by append. Every interior message is
+	// immutable under one array identity, so copy the unchanged prefix output
+	// (messages[0 .. lastLen-1)) and reconvert only the old boundary message
+	// (neighbor-sensitive: a following interrupted-thinking marker may now exist)
+	// plus the appended suffix. The boundary-identity check (old tail still sits
+	// at its old index) rejects an in-place interior splice-replace that grew the
+	// array while swapping earlier identities, forcing a full rebuild.
+	let out: Message[];
+	let start: number;
+	if (
+		sameArray &&
+		lastConvertOutput !== undefined &&
+		len > lastConvertLength &&
+		lastConvertLength > 0 &&
+		messages[lastConvertLength - 1] === lastConvertTail &&
+		lastConvertPrefixOutputLen <= lastConvertOutput.length
+	) {
+		out = lastConvertOutput.slice(0, lastConvertPrefixOutputLen);
+		start = lastConvertLength - 1;
+	} else {
+		out = [];
+		start = 0;
+	}
+
+	// Output length contributed by messages[0 .. len-1), captured when the loop
+	// reaches the final index so the next growth can reuse this prefix.
+	let prefixOutputLen = 0;
+	for (let i = start; i < len; i++) {
+		if (i === len - 1) prefixOutputLen = out.length;
+		const m = messages[i];
+		const interruptedNext = m.role === "assistant" && followedByInterruptedThinking(messages, i);
+		const fragment = convertOneCached(m, interruptedNext);
+		for (const msg of fragment) out.push(msg);
+	}
+	if (len === 0) prefixOutputLen = 0;
+
+	// Record for the next call's shortcuts. `out` is a fresh array (slice or new),
+	// so a prior caller holding the previous `lastConvertOutput` never sees it grow.
+	lastConvertInput = messages;
+	lastConvertLength = len;
+	lastConvertOutput = out;
+	lastConvertGeneration = convertGeneration;
+	lastConvertTail = tail;
+	lastConvertPrefixOutputLen = prefixOutputLen;
+	return out;
 }