perf(shared): cache context window usage per session

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/plugin/event.ts

@@ -1,4 +1,5 @@
 import type { OhMyOpenCodeConfig } from "../config";
+import type { PluginInput } from "@opencode-ai/plugin";
 import type { PluginContext } from "./types";
 
 import {
@@ -26,6 +27,7 @@ import {
 import { resetMessageCursor } from "../shared";
 import { getAgentConfigKey } from "../shared/agent-display-names";
 import { readConnectedProvidersCache } from "../shared/connected-providers-cache";
+import { invalidateContextWindowUsageCache } from "../shared/dynamic-truncator";
 import { log } from "../shared/logger";
 import { shouldRetryError } from "../shared/model-error-classifier";
 import { buildFallbackChainFromModels } from "../shared/fallback-chain-from-models";
@@ -160,8 +162,7 @@ export function createEventHandler(args: {
           body: { parts: Array<{ type: "text"; text: string }> };
           query: { directory: string };
         }) => Promise<unknown>;
-        // eslint-disable-next-line @typescript-eslint/no-explicit-any
-        summarize: (...args: any[]) => Promise<unknown>;
+        summarize: (...args: unknown[]) => Promise<unknown>;
       };
     };
   };
@@ -475,6 +476,9 @@ export function createEventHandler(args: {
       const sessionID = info?.sessionID as string | undefined;
       const agent = info?.agent as string | undefined;
       const role = info?.role as string | undefined;
+      if (sessionID && info?.finish === true) {
+        invalidateContextWindowUsageCache(pluginContext as PluginInput, sessionID);
+      }
       if (sessionID && role === "user") {
         const isCompactionMessage = agent ? isCompactionAgent(agent) : false;
         if (agent && !isCompactionMessage) {

src/shared/dynamic-truncator.test.ts

@@ -2,7 +2,7 @@
 
 import { describe, expect, it, afterEach } from "bun:test"
 
-import { getContextWindowUsage } from "./dynamic-truncator"
+import { getContextWindowUsage, invalidateContextWindowUsageCache } from "./dynamic-truncator"
 
 const ANTHROPIC_CONTEXT_ENV_KEY = "ANTHROPIC_1M_CONTEXT"
 const VERTEX_CONTEXT_ENV_KEY = "VERTEX_ANTHROPIC_1M_CONTEXT"
@@ -53,6 +53,39 @@ function createContextUsageMockContext(
   }
 }
 
+function createCountingContextUsageMockContext(inputTokens: number) {
+  let messagesCalls = 0
+  return {
+    ctx: {
+      client: {
+        session: {
+          messages: async () => {
+            messagesCalls += 1
+            return {
+              data: [
+                {
+                  info: {
+                    role: "assistant",
+                    providerID: "anthropic",
+                    modelID: "claude-sonnet-4-5",
+                    tokens: {
+                      input: inputTokens,
+                      output: 0,
+                      reasoning: 0,
+                      cache: { read: 0, write: 0 },
+                    },
+                  },
+                },
+              ],
+            }
+          },
+        },
+      },
+    },
+    getMessagesCalls: () => messagesCalls,
+  }
+}
+
 describe("getContextWindowUsage", () => {
   afterEach(() => {
     resetContextLimitEnv()
@@ -125,6 +158,39 @@ describe("getContextWindowUsage", () => {
     expect(usage?.remainingTokens).toBe(82144)
   })
 
+  it("reuses context usage for repeated calls in the same session", async () => {
+    // given
+    delete process.env[ANTHROPIC_CONTEXT_ENV_KEY]
+    delete process.env[VERTEX_CONTEXT_ENV_KEY]
+    const { ctx, getMessagesCalls } = createCountingContextUsageMockContext(100000)
+    const modelCacheState = { anthropicContext1MEnabled: false }
+
+    // when
+    const firstUsage = await getContextWindowUsage(ctx as never, "ses_cached_usage", modelCacheState)
+    const secondUsage = await getContextWindowUsage(ctx as never, "ses_cached_usage", modelCacheState)
+
+    // then
+    expect(firstUsage?.remainingTokens).toBe(100000)
+    expect(secondUsage?.remainingTokens).toBe(100000)
+    expect(getMessagesCalls()).toBe(1)
+  })
+
+  it("refetches context usage after cache invalidation", async () => {
+    // given
+    delete process.env[ANTHROPIC_CONTEXT_ENV_KEY]
+    delete process.env[VERTEX_CONTEXT_ENV_KEY]
+    const { ctx, getMessagesCalls } = createCountingContextUsageMockContext(100000)
+    const modelCacheState = { anthropicContext1MEnabled: false }
+
+    // when
+    await getContextWindowUsage(ctx as never, "ses_invalidated_usage", modelCacheState)
+    invalidateContextWindowUsageCache(ctx as never, "ses_invalidated_usage")
+    await getContextWindowUsage(ctx as never, "ses_invalidated_usage", modelCacheState)
+
+    // then
+    expect(getMessagesCalls()).toBe(2)
+  })
+
   it("returns null for non-anthropic providers without a cached limit", async () => {
     // given
     const ctx = createContextUsageMockContext(180000, {

src/shared/dynamic-truncator.ts

@@ -24,6 +24,66 @@ interface MessageWrapper {
 	info: { role: string } & Partial<AssistantMessageInfo>;
 }
 
+type ContextWindowUsage = {
+	usedTokens: number;
+	remainingTokens: number;
+	usagePercentage: number;
+}
+
+type ContextWindowUsageClient = Pick<PluginInput["client"], "session">
+
+const usageCacheByClient = new WeakMap<object, Map<string, Map<string, Promise<ContextWindowUsage | null>>>>()
+
+function createModelCacheKey(modelCacheState?: ContextLimitModelCacheState): string {
+	if (!modelCacheState) {
+		return "default"
+	}
+
+	const cachedLimits = modelCacheState.modelContextLimitsCache
+		? [...modelCacheState.modelContextLimitsCache.entries()]
+			.sort(([leftKey], [rightKey]) => leftKey.localeCompare(rightKey))
+			.map(([modelKey, limit]) => `${modelKey}:${limit}`)
+			.join(",")
+		: ""
+
+	return `${modelCacheState.anthropicContext1MEnabled ? "1m" : "200k"}|${cachedLimits}`
+}
+
+function getUsageCache(
+	client: ContextWindowUsageClient,
+	modelCacheState?: ContextLimitModelCacheState,
+): Map<string, Promise<ContextWindowUsage | null>> {
+	let cacheByModelState = usageCacheByClient.get(client)
+	if (!cacheByModelState) {
+		cacheByModelState = new Map()
+		usageCacheByClient.set(client, cacheByModelState)
+	}
+
+	const modelCacheKey = createModelCacheKey(modelCacheState)
+	let cache = cacheByModelState.get(modelCacheKey)
+	if (!cache) {
+		cache = new Map()
+		cacheByModelState.set(modelCacheKey, cache)
+	}
+
+	return cache
+}
+
+export function invalidateContextWindowUsageCache(ctx: PluginInput, sessionID?: string): void {
+	const cacheByModelState = usageCacheByClient.get(ctx.client)
+	if (!cacheByModelState) {
+		return
+	}
+
+	for (const cache of cacheByModelState.values()) {
+		if (sessionID) {
+			cache.delete(sessionID)
+		} else {
+			cache.clear()
+		}
+	}
+}
+
 export interface TruncationResult {
 	result: string;
 	truncated: boolean;
@@ -112,11 +172,23 @@ export async function getContextWindowUsage(
 	ctx: PluginInput,
 	sessionID: string,
 	modelCacheState?: ContextLimitModelCacheState,
-): Promise<{
-	usedTokens: number;
-	remainingTokens: number;
-	usagePercentage: number;
-} | null> {
+): Promise<ContextWindowUsage | null> {
+	const cache = getUsageCache(ctx.client, modelCacheState)
+	const cached = cache.get(sessionID)
+	if (cached) {
+		return cached
+	}
+
+	const usagePromise = fetchContextWindowUsage(ctx, sessionID, modelCacheState)
+	cache.set(sessionID, usagePromise)
+	return usagePromise
+}
+
+async function fetchContextWindowUsage(
+	ctx: PluginInput,
+	sessionID: string,
+	modelCacheState?: ContextLimitModelCacheState,
+): Promise<ContextWindowUsage | null> {
 	try {
 		const response = await ctx.client.session.messages({
 			path: { id: sessionID },