Keep document bodies out of the cacheable system prompt

buildDocumentsSection was inlining up to 5 document bodies at 2000
chars each into the system prompt. Any edit to any of the listed
documents between wakes invalidated the Anthropic prompt cache — the
prefix that buildTaskPrompt spent real effort to keep stable.

Emit metadata only (id, type, title, author, status, version,
updatedAt) and teach agents to GET the single-document endpoint when
they actually need the body. The single-doc route did not exist, so
add it; update the API doc block in the system prompt accordingly.

src/main/api-server/routes-files.ts

@@ -118,6 +118,24 @@ export function registerFileRoutes(server: AgentApiServer) {
     server.json(res, 200, docs);
   });
 
+  server.addRoute("GET", "/api/companies/:companyId/documents/:documentId", (_req, res, params, token) => {
+    if (params.companyId !== token.companyId) {
+      server.json(res, 403, { error: "Company mismatch" });
+      return;
+    }
+    const documentId = params.documentId;
+    if (!documentId) {
+      server.json(res, 400, { error: "Missing documentId" });
+      return;
+    }
+    const doc = db.getDocumentForUpdate(documentId, token.companyId);
+    if (!doc) {
+      server.json(res, 404, { error: "Document not found in this company" });
+      return;
+    }
+    server.json(res, 200, doc);
+  });
+
   server.addRoute("PATCH", "/api/companies/:companyId/documents/:documentId", (_req, res, params, token, body) => {
     if (params.companyId !== token.companyId) {
       server.json(res, 403, { error: "Company mismatch" });

src/main/prompt-builder/api-docs/files.en.md

@@ -18,6 +18,12 @@ Body: {
 ### List Company Documents
 GET /api/companies/{{companyId}}/documents
 
+### Fetch a Single Document (by id)
+GET /api/companies/{{companyId}}/documents/{documentId}
+Returns the full document record including `content`. The system prompt
+only lists document metadata to keep the cacheable prefix stable — call
+this endpoint when you need the actual text.
+
 ### Update Document (with conflict detection)
 PATCH /api/companies/{{companyId}}/documents/{documentId}
 Body: {

src/main/prompt-builder/context-injection.ts

@@ -146,23 +146,20 @@ interface DocumentsSectionCtx {
 export function buildDocumentsSection(ctx: DocumentsSectionCtx): string | null {
   if (ctx.recentDocuments.length === 0) return null;
 
-  const docLines = ctx.recentDocuments.slice(0, 5).map((d) => {
+  // Intentionally emit metadata only. Inlining document content (even
+  // truncated to 2000 chars × 5 documents) invalidates the Anthropic
+  // prompt cache on every wake where any listed document is edited —
+  // the exact opposite of what buildTaskPrompt's cache-aware layout
+  // promises. Agents fetch the body with
+  //   GET /api/companies/{companyId}/documents/{id}
+  // when they actually need it.
+  const docLines = ctx.recentDocuments.slice(0, 10).map((d) => {
     const author = d.authorAgentId ? (ctx.allAgents.find(a => a.id === d.authorAgentId)?.name ?? "Unknown") : "System";
-    const header = `- [${d.type}] ${d.title} — by ${author}, status: ${d.status}, updated: ${d.updatedAt}`;
-    if (d.content && d.content.length > 0) {
-      const contentLimit = 2000;
-      const excerpt = d.content.length > contentLimit
-        ? d.content.slice(0, contentLimit) + "..."
-        : d.content;
-      return `${header}\n  Content:\n  ${excerpt.split("\n").join("\n  ")}`;
-    }
-    return header;
+    return `- [${d.id}] ${d.type}: "${d.title}" — by ${author}, status: ${d.status}, v${d.version}, updated ${d.updatedAt}`;
   });
 
-  return `# Recent Documents
-${docLines.join("\n\n")}
-
-Reference these documents when relevant to your current work. Create new documents for important work artifacts.
+  return `# Recent Documents (metadata only — fetch content via GET /api/companies/{companyId}/documents/{id})
+${docLines.join("\n")}
 `;
 }