obsidian-hermes-plugin/src/context-bridge.ts

import { App, MarkdownView } from "obsidian";

/**
 * Reads the active Obsidian note and produces structured context for Hermes.
 *
 * Strategy: instead of embedding full note content (wastes context tokens),
 * we send a vault-relative path reference. Hermes reads the file on-demand
 * via mcp_obsidian_get_vault_document — zero context burned, same result.
 *
 * If the user has text selected, we embed that selection directly (it's small
 * and intentional). Otherwise we send a path reference.
 */
export class ContextBridge {
  private app: App;

  constructor(app: App) {
    this.app = app;
  }

  /**
   * Get the active note as a vault-relative reference.
   * Returns null if no markdown editor is active.
   */
  getActiveNoteContext(): {
    vault: string;
    path: string;
    title: string;
  } | null {
    const activeView = this.app.workspace.activeEditor;
    if (!activeView || !activeView.editor) return null;

    // Verify it's a Markdown file (not canvas, PDF, etc.)
    const leaf = this.app.workspace.activeLeaf;
    if (!leaf || !(leaf.view instanceof MarkdownView)) return null;

    const file = leaf.view.file;
    if (!file) return null;

    return {
      vault: this.app.vault.getName(),
      path: file.path,
      title: file.basename,
    };
  }

  /**
   * Snapshot the active note context AND selection in one atomic read.
   * Call this when the chat view gains focus — captures the note state
   * before the user's focus move clears activeEditor / deselects text.
   *
   * Returns null if no markdown editor is active.
   */
  snapshot(): {
    vault: string;
    path: string;
    title: string;
    selection: string | null;
  } | null {
    const noteRef = this.getActiveNoteContext();
    if (!noteRef) return null;
    return {
      ...noteRef,
      selection: this.getSelectionContext(),
    };
  }

  /**
   * Get the currently selected text in the active editor.
   * Returns null if no text is selected.
   */
  getSelectionContext(): string | null {
    const activeView = this.app.workspace.activeEditor;
    if (!activeView || !activeView.editor) return null;

    const selection = activeView.editor.getSelection();
    return selection || null;
  }

  /**
   * Format a note reference as a system message that tells Hermes
   * which MCP tool call to use to read the file on Helm.
   */
  formatContextMessage(ref: {
    vault: string;
    path: string;
    title: string;
  }): { role: string; content: string } {
    return {
      role: "system",
      content: [
        `[Current note: ${ref.vault}/${ref.path}]`,
        `Use mcp_obsidian_get_vault_document(vault_name="${ref.vault}", doc_path="${ref.path}") to read the full contents.`,
      ].join("\n"),
    };
  }

  /**
   * Build the messages array for an API call.
   *
   * - If text is selected in the active note: embed it directly as context.
   * - Otherwise if context injection is enabled: send a path reference
   *   so Hermes can read the file via MCP on Helm.
   * - Always appends the user's chat message.
   */
  buildMessages(
    userInput: string,
    includeContext: boolean
  ): { role: string; content: string }[] {
    const messages: { role: string; content: string }[] = [];

    if (includeContext) {
      // Prefer selected text (small, intentional, works anywhere)
      const selection = this.getSelectionContext();
      if (selection) {
        const ref = this.getActiveNoteContext();
        const source = ref ? `${ref.vault}/${ref.path}` : "current note";
        messages.push({
          role: "system",
          content: `[Selection from ${source}]\n\n${selection}`,
        });
      } else {
        // No selection — send path reference for Hermes to read via MCP
        const ref = this.getActiveNoteContext();
        if (ref) {
          messages.push(this.formatContextMessage(ref));
        }
      }
    }

    messages.push({ role: "user", content: userInput });

    return messages;
  }
}