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packages/server/src/local/routes/chat-context.ts

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+/**
+ * chat-context.ts — Context window management for the chat route.
+ *
+ * Extracted from chat.ts to keep files under 800 LOC.
+ * Pure functions with no server state dependencies.
+ */
+
+/** Maximum number of conversation messages passed to the agent loop per turn. */
+export const MAX_CONTEXT_MESSAGES = 50;
+
+/**
+ * Apply a sliding window to conversation history.
+ * Returns at most MAX_CONTEXT_MESSAGES messages.
+ * If the full history exceeds the limit, a system message is prepended
+ * informing the agent that earlier context was truncated.
+ */
+export function applyContextWindow(
+  fullHistory: Array<{ role: string; content: string }>,
+  maxMessages: number = MAX_CONTEXT_MESSAGES,
+): Array<{ role: string; content: string }> {
+  if (fullHistory.length <= maxMessages) {
+    return fullHistory;
+  }
+
+  // W3.5: Summarize dropped messages instead of just noting their count
+  const droppedMessages = fullHistory.slice(0, fullHistory.length - maxMessages);
+  const truncatedCount = droppedMessages.length;
+  const summary = summarizeDroppedContext(droppedMessages);
+
+  const truncationNotice: { role: string; content: string } = {
+    role: 'system',
+    content: `[Context summary — ${truncatedCount} earlier messages compressed]\n${summary}`,
+  };
+  return [truncationNotice, ...fullHistory.slice(-maxMessages)];
+}
+
+/** W3.5: Extract key decisions, topics, and requests from dropped messages */
+export function summarizeDroppedContext(messages: Array<{ role: string; content: string }>): string {
+  const decisions: string[] = [];
+  const topics: Set<string> = new Set();
+  const userRequests: string[] = [];
+
+  for (const msg of messages) {
+    const text = msg.content;
+    if (!text || text.length < 10) continue;
+
+    // Extract decisions
+    const decisionPatterns = [/\bdecid/i, /\bagreed\b/i, /\bchose\b/i, /\bselected\b/i, /\bwent with\b/i, /\bfinal call\b/i];
+    if (decisionPatterns.some(p => p.test(text))) {
+      const firstSentence = text.split(/[.!?\n]/)[0]?.trim();
+      if (firstSentence && firstSentence.length > 10 && firstSentence.length < 200) {
+        decisions.push(firstSentence);
+      }
+    }
+
+    // Extract user request summaries (first line of user messages)
+    if (msg.role === 'user') {
+      const firstLine = text.split('\n')[0]?.trim();
+      if (firstLine && firstLine.length > 15 && firstLine.length < 150) {
+        userRequests.push(firstLine);
+      }
+    }
+  }
+
+  const lines: string[] = [];
+  if (decisions.length > 0) {
+    lines.push('Decisions made: ' + decisions.slice(0, 5).join(' | '));
+  }
+  if (userRequests.length > 0) {
+    // Show first and last few requests to convey conversation arc
+    const shown = userRequests.length <= 4
+      ? userRequests
+      : [...userRequests.slice(0, 2), '...', ...userRequests.slice(-2)];
+    lines.push('Topics discussed: ' + shown.join(' → '));
+  }
+  if (lines.length === 0) {
+    lines.push(`${messages.length} messages covering earlier conversation context.`);
+  }
+  return lines.join('\n');
+}
+
+/**
+ * Build the skill-awareness section of the system prompt.
+ * Exported for testability.
+ */
+export function buildSkillPromptSection(skills: Array<{ name: string; content: string }>): string {
+  if (skills.length === 0) return '';
+  let section = '\n\n# Active Skills\n\n';
+  section += 'You have specialized skills loaded. **When a user request matches a loaded skill, follow that skill\'s instructions** instead of generic behavior. Skills represent curated, high-quality workflows.\n\n';
+  section += '## Skill-Aware Routing\n';
+  section += 'Before responding to any substantial user request:\n';
+  section += '1. Check if any loaded skill matches the request (catch-up → catch-up skill, draft → draft-memo skill, etc.)\n';
+  section += '2. If a skill matches, follow its structured workflow — it produces better output than ad-hoc responses\n';
+  section += '3. If no skill matches but one could help, mention it: "I have a [skill-name] skill that could help with this"\n';
+  section += '4. Use suggest_skill to find relevant skills when unsure\n\n';
+  section += `## Loaded Skills (${skills.length})\n`;
+  for (const skill of skills) {
+    section += `\n### ${skill.name}\n${skill.content}\n`;
+  }
+  return section;
+}