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diff --git a/src/features/system-prompt/logic/view-model.ts b/src/features/system-prompt/logic/view-model.ts
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+++ b/src/features/system-prompt/logic/view-model.ts
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+import type { SystemPromptVariable } from "@dispatch/transport-contract";
+
+/**
+ * Pure core for the system prompt builder — zero DOM, zero effects, zero Svelte.
+ *
+ * The system prompt is a global template, stored on the backend, resolved once
+ * per conversation at first turn (then persisted for prompt-cache safety). The
+ * frontend builder exposes the template text plus a palette of available
+ * variables; clicking a variable inserts `[type:name]` at the cursor. This module
+ * holds the pure logic: tag construction, insertion into a template, and
+ * grouping of variables. The HTTP edge is injected via the ports defined below.
+ */
+
+// ── Injected ports (composition root adapts the store to these shapes) ─────────
+
+export type SystemPromptLoadResult =
+  | { readonly ok: true; readonly template: string }
+  | { readonly ok: false; readonly error: string };
+
+export type LoadSystemPrompt = () => Promise<SystemPromptLoadResult>;
+
+export type SystemPromptSaveResult =
+  | { readonly ok: true; readonly template: string }
+  | { readonly ok: false; readonly error: string };
+
+export type SaveSystemPrompt = (template: string) => Promise<SystemPromptSaveResult>;
+
+export type SystemPromptVariablesResult =
+  | { readonly ok: true; readonly variables: readonly SystemPromptVariable[] }
+  | { readonly ok: false; readonly error: string };
+
+export type LoadSystemPromptVariables = () => Promise<SystemPromptVariablesResult>;
+
+// ── Template helpers ──────────────────────────────────────────────────────────
+
+/** Build the literal placeholder `[type:name]` for a variable. */
+export function buildTag(type: string, name: string): string {
+  return `[${type}:${name}]`;
+}
+
+/** Build the literal placeholder `[if type:name]` for a conditional block. */
+export function buildIfTag(type: string, name: string): string {
+  return `[if ${type}:${name}]`;
+}
+
+/** Build the literal placeholder `[if !type:name]` for a negated conditional block. */
+export function buildIfNotTag(type: string, name: string): string {
+  return `[if !${type}:${name}]`;
+}
+
+export interface Insertion {
+  /** Template text after insertion. */
+  template: string;
+  /** New cursor position (caret index) after insertion. */
+  cursor: number;
+}
+
+/**
+ * Insert `tag` into `template` at the current selection range. Replaces any
+ * selected text. Returns both the new template and the new cursor position so
+ * the caller can restore the caret after the inserted tag.
+ */
+export function insertTag(
+  template: string,
+  tag: string,
+  selectionStart: number,
+  selectionEnd: number,
+): Insertion {
+  const before = template.slice(0, selectionStart);
+  const after = template.slice(selectionEnd);
+  const next = before + tag + after;
+  return { template: next, cursor: selectionStart + tag.length };
+}
+
+// ── Variable grouping ─────────────────────────────────────────────────────────
+
+export interface VariableGroup {
+  /** Variable type (e.g. `"system"`, `"file"`, `"prompt"`, `"git"`). */
+  readonly type: string;
+  /** Variables of this type. */
+  readonly variables: readonly SystemPromptVariable[];
+}
+
+/**
+ * Group variables by type, preserving the order of first appearance within each
+ * group and the order of first-appearing types.
+ */
+export function groupVariables(
+  variables: readonly SystemPromptVariable[],
+): readonly VariableGroup[] {
+  const order: string[] = [];
+  const map = new Map<string, SystemPromptVariable[]>();
+  for (const v of variables) {
+    if (!map.has(v.type)) {
+      map.set(v.type, []);
+      order.push(v.type);
+    }
+    map.get(v.type)?.push(v);
+  }
+  return order.map((type) => ({ type, variables: map.get(type) ?? [] }));
+}
+
+/** Whether a variable is "dynamic" (any name is valid, e.g. `file:<path>`). */
+export function isDynamicVariable(variable: SystemPromptVariable): boolean {
+  return variable.dynamic === true;
+}