fix(web): bound the cache-break snapshot map and use sha256 for its signature

cacheBreakSnapshots was keyed by chatId and never evicted, so with cache-break detection enabled it grew with the cumulative number of distinct chats served. Add a FIFO cap that drops the oldest entry on overflow, and replace the hand-rolled djb2 signature hash with a sha256 slice matching getOAuthScopeHash (observability-only and compared in-process, so determinism is all it needs).

Author: jsourcebot

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+- [EE] Improved Ask Sourcebot prompt caching by splitting static and dynamic prompt sections and advancing cache breakpoints after every agent step instead of only after each message. [#<id>](https://github.com/sourcebot-dev/sourcebot/pull/<id>)
+
 ### Added
 - Added per-step token cost tracking and estimated tool call token usage to Ask Sourcebot chat history. [#1353](https://github.com/sourcebot-dev/sourcebot/pull/1353)
 

packages/web/src/ee/features/chat/promptCaching.ts

@@ -1,3 +1,4 @@
+import { createHash } from "crypto";
 import { ProviderOptions } from "@ai-sdk/provider-utils";
 import { createLogger } from "@sourcebot/shared";
 import { LanguageModelProvider } from "@/features/chat/types";
@@ -105,19 +106,14 @@ interface CacheBreakSnapshot {
     requestCount: number;
 }
 
-// Keyed by chatId. In-memory, best-effort: survives within a server process and
-// is naturally bounded by the number of concurrent chats. Mirrors the in-memory
-// caching pattern used elsewhere (e.g. `anthropicThinkingConfigCache`).
+// Keyed by chatId, in-memory, observability-only. Entries are never removed on chat
+// end, so the map is bounded by a FIFO cap (oldest insertion evicted first, below);
+// otherwise it grows with the cumulative count of distinct chats, not the concurrent count.
+const MAX_CACHE_BREAK_SNAPSHOTS = 10_000;
 const cacheBreakSnapshots = new Map<string, CacheBreakSnapshot>();
 
-// Lightweight, stable, non-cryptographic hash (djb2). Observability only.
-const hashString = (input: string): string => {
-    let hash = 5381;
-    for (let i = 0; i < input.length; i++) {
-        hash = ((hash << 5) + hash + input.charCodeAt(i)) | 0;
-    }
-    return (hash >>> 0).toString(36);
-};
+const hashString = (input: string): string =>
+    createHash('sha256').update(input).digest('hex').slice(0, 16);
 
 /**
  * Records the cache-relevant prefix signature for a chat and logs a warning when
@@ -149,6 +145,14 @@ export const detectPromptCacheBreak = ({
         const requestCount = (prev?.requestCount ?? 0) + 1;
         cacheBreakSnapshots.set(chatId, { signature, requestCount });
 
+        // FIFO eviction: once the map overflows, drop the oldest-inserted entry.
+        if (cacheBreakSnapshots.size > MAX_CACHE_BREAK_SNAPSHOTS) {
+            const oldestKey = cacheBreakSnapshots.keys().next().value;
+            if (oldestKey !== undefined) {
+                cacheBreakSnapshots.delete(oldestKey);
+            }
+        }
+
         if (prev && prev.signature !== signature) {
             logger.warn(
                 `Prompt cache break detected for chat ${chatId} (request #${requestCount}): the ` +