diff --git a/apps/x/packages/core/src/knowledge/README.md b/apps/x/packages/core/src/knowledge/README.md
new file mode 100644
index 00000000..b548a554
--- /dev/null
+++ b/apps/x/packages/core/src/knowledge/README.md
@@ -0,0 +1,149 @@
+# Knowledge Graph System
+
+This directory contains the knowledge graph building system that processes emails and meeting transcripts to create an Obsidian-style knowledge base.
+
+## Components
+
+### `build_graph.ts`
+Main orchestrator that:
+- Processes source files (emails/transcripts) in batches
+- Runs the `note_creation` agent to extract entities
+- Only processes new or changed files (tracked via state)
+
+### `graph_state.ts`
+State management module that tracks which files have been processed:
+- Uses hybrid mtime + hash approach for change detection
+- Stores state in `~/.rowboat/knowledge_graph_state.json`
+- Provides modular functions for state operations
+
+### `sync_gmail.ts` & `sync_fireflies.ts`
+Sync scripts that:
+- Pull data from Gmail and Fireflies
+- Save as markdown files in their respective directories
+- Trigger knowledge graph build after successful sync
+
+## How It Works
+
+### Change Detection Strategy
+
+The system uses a **hybrid mtime + hash approach**:
+
+1. **Quick check**: Compare file modification time (mtime)
+   - If mtime unchanged → file definitely hasn't changed → skip
+
+2. **Verification**: If mtime changed, compute content hash
+   - If hash unchanged → false positive (mtime changed but content didn't) → skip
+   - If hash changed → file actually changed → process
+
+This is efficient (only hashes potentially changed files) and reliable (confirms actual content changes).
+
+### State File Structure
+
+`~/.rowboat/knowledge_graph_state.json`:
+```json
+{
+  "processedFiles": {
+    "/path/to/file.md": {
+      "mtime": "2026-01-07T10:30:00.000Z",
+      "hash": "a3f5e9d2c8b1...",
+      "lastProcessed": "2026-01-07T10:35:00.000Z"
+    }
+  },
+  "lastBuildTime": "2026-01-07T10:35:00.000Z"
+}
+```
+
+### Processing Flow
+
+1. **Sync runs** (Gmail or Fireflies)
+   - Fetches new/updated data
+   - Saves as markdown files
+   - Calls `buildGraph(SYNC_DIR)`
+
+2. **buildGraph()**
+   - Loads state
+   - Scans source directory for files
+   - Filters to only new/changed files
+   - Processes in batches of 25
+   - Updates state after each successful batch (saves progress incrementally)
+
+3. **Agent processes batch**
+   - Extracts entities (people, orgs, projects, topics)
+   - Creates/updates notes in `~/.rowboat/notes/`
+   - Merges information for entities appearing in multiple files
+
+## Replacing the Change Detection Logic
+
+The state management is modular. To implement a different change detection strategy:
+
+### Option 1: Modify `graph_state.ts`
+
+Replace the functions while keeping the same interface:
+
+```typescript
+// Current: mtime + hash
+export function hasFileChanged(filePath: string, state: GraphState): boolean {
+    // Your custom logic here
+}
+
+export function markFileAsProcessed(filePath: string, state: GraphState): void {
+    // Your custom tracking here
+}
+```
+
+### Option 2: Create a new state module
+
+Create `graph_state_v2.ts` with the same exported interface:
+
+```typescript
+export interface FileState { /* ... */ }
+export interface GraphState { /* ... */ }
+export function loadState(): GraphState { /* ... */ }
+export function saveState(state: GraphState): void { /* ... */ }
+export function getFilesToProcess(sourceDir: string, state: GraphState): string[] { /* ... */ }
+export function markFileAsProcessed(filePath: string, state: GraphState): void { /* ... */ }
+```
+
+Then update the import in `build_graph.ts`:
+```typescript
+import { /* ... */ } from './graph_state_v2.js';
+```
+
+### Option 3: Pass a strategy object
+
+Refactor to accept a change detection strategy:
+
+```typescript
+interface ChangeDetectionStrategy {
+    hasFileChanged(filePath: string, state: GraphState): boolean;
+    markFileAsProcessed(filePath: string, state: GraphState): void;
+}
+
+export async function buildGraph(sourceDir: string, strategy?: ChangeDetectionStrategy) {
+    const detector = strategy || defaultStrategy;
+    // Use detector.hasFileChanged(), etc.
+}
+```
+
+## Resetting State
+
+To force reprocessing of all files:
+
+```typescript
+import { resetGraphState } from './build_graph.js';
+
+resetGraphState(); // Clears the state file
+```
+
+Or manually delete: `~/.rowboat/knowledge_graph_state.json`
+
+## Configuration
+
+### Batch Size
+Change `BATCH_SIZE` in `build_graph.ts` (currently 25 files per batch)
+
+### State File Location
+Change `STATE_FILE` in `graph_state.ts` (currently `~/.rowboat/knowledge_graph_state.json`)
+
+### Hash Algorithm
+Change `crypto.createHash('sha256')` in `graph_state.ts` to use a different algorithm (md5, sha1, etc.)
diff --git a/apps/x/packages/core/src/knowledge/build_graph.ts b/apps/x/packages/core/src/knowledge/build_graph.ts
index 82860bdc..a57fd6a7 100644
--- a/apps/x/packages/core/src/knowledge/build_graph.ts
+++ b/apps/x/packages/core/src/knowledge/build_graph.ts
@@ -3,6 +3,14 @@ import path from 'path';
 import { WorkDir } from '../config/config.js';
 import { createRun, createMessage } from '../runs/runs.js';
 import { bus } from '../runs/bus.js';
+import {
+    loadState,
+    saveState,
+    getFilesToProcess,
+    markFileAsProcessed,
+    resetState,
+    type GraphState,
+} from './graph_state.js';
 
 /**
  * Build obsidian-style knowledge graph by running topic extraction
@@ -13,24 +21,17 @@ const NOTES_OUTPUT_DIR = path.join(WorkDir, 'notes');
 const NOTE_CREATION_AGENT = 'note_creation';
 
 /**
- * Read all markdown files from the specified source directory
+ * Read content for specific files
  */
-async function getContentFiles(sourceDir: string): Promise<{ path: string; content: string }[]> {
-    if (!fs.existsSync(sourceDir)) {
-        console.log(`Knowledge source directory not found: ${sourceDir}`);
-        return [];
-    }
-
+async function readFileContents(filePaths: string[]): Promise<{ path: string; content: string }[]> {
     const files: { path: string; content: string }[] = [];
-    const entries = fs.readdirSync(sourceDir);
 
-    for (const entry of entries) {
-        const fullPath = path.join(sourceDir, entry);
-        const stat = fs.statSync(fullPath);
-
-        if (stat.isFile() && entry.endsWith('.md')) {
-            const content = fs.readFileSync(fullPath, 'utf-8');
-            files.push({ path: fullPath, content });
+    for (const filePath of filePaths) {
+        try {
+            const content = fs.readFileSync(filePath, 'utf-8');
+            files.push({ path: filePath, content });
+        } catch (error) {
+            console.error(`Error reading file ${filePath}:`, error);
         }
     }
 
@@ -92,21 +93,41 @@ async function createNotesFromBatch(files: { path: string; content: string }[],
 
 /**
  * Build the knowledge graph from all content files in the specified source directory
+ * Only processes new or changed files based on state tracking
  */
 export async function buildGraph(sourceDir: string): Promise<void> {
-    const contentFiles = await getContentFiles(sourceDir);
+    console.log(`[buildGraph] Starting build for directory: ${sourceDir}`);
 
-    if (contentFiles.length === 0) {
-        console.log(`No files found in ${sourceDir}`);
+    // Load current state
+    const state = loadState();
+    const previouslyProcessedCount = Object.keys(state.processedFiles).length;
+    console.log(`[buildGraph] State loaded. Previously processed: ${previouslyProcessedCount} files`);
+
+    // Get files that need processing (new or changed)
+    const filesToProcess = getFilesToProcess(sourceDir, state);
+
+    if (filesToProcess.length === 0) {
+        console.log(`[buildGraph] No new or changed files to process in ${path.basename(sourceDir)}`);
         return;
     }
 
-    const BATCH_SIZE = 10; // Process 10 emails per agent run
+    console.log(`[buildGraph] Found ${filesToProcess.length} new/changed files to process in ${path.basename(sourceDir)}`);
+
+    // Read file contents
+    const contentFiles = await readFileContents(filesToProcess);
+
+    if (contentFiles.length === 0) {
+        console.log(`No files could be read from ${sourceDir}`);
+        return;
+    }
+
+    const BATCH_SIZE = 25; // Process 25 files per agent run
     const totalBatches = Math.ceil(contentFiles.length / BATCH_SIZE);
 
-    console.log(`Processing ${contentFiles.length} files from ${path.basename(sourceDir)} in ${totalBatches} batches (${BATCH_SIZE} files per batch)...`);
+    console.log(`Processing ${contentFiles.length} files in ${totalBatches} batches (${BATCH_SIZE} files per batch)...`);
 
     // Process files in batches
+    const processedFiles: string[] = [];
     for (let i = 0; i < contentFiles.length; i += BATCH_SIZE) {
         const batch = contentFiles.slice(i, i + BATCH_SIZE);
         const batchNumber = Math.floor(i / BATCH_SIZE) + 1;
@@ -115,13 +136,27 @@ export async function buildGraph(sourceDir: string): Promise<void> {
             console.log(`Processing batch ${batchNumber}/${totalBatches} (${batch.length} files)...`);
             await createNotesFromBatch(batch, batchNumber);
             console.log(`Batch ${batchNumber}/${totalBatches} complete`);
+
+            // Mark files in this batch as processed
+            for (const file of batch) {
+                markFileAsProcessed(file.path, state);
+                processedFiles.push(file.path);
+            }
+
+            // Save state after each successful batch
+            // This ensures partial progress is saved even if later batches fail
+            saveState(state);
         } catch (error) {
             console.error(`Error processing batch ${batchNumber}:`, error);
-            // Continue with next batch
+            // Continue with next batch (without saving state for failed batch)
         }
     }
 
-    console.log('Knowledge graph build complete');
+    // Update state with last build time and save
+    state.lastBuildTime = new Date().toISOString();
+    saveState(state);
+
+    console.log(`Knowledge graph build complete. Processed ${processedFiles.length} files.`);
 }
 
 /**
@@ -131,3 +166,13 @@ export async function init() {
     const defaultSourceDir = path.join(WorkDir, 'gmail_sync');
     await buildGraph(defaultSourceDir);
 }
+
+/**
+ * Reset the knowledge graph state - forces reprocessing of all files on next run
+ * Useful for debugging or when you want to rebuild everything from scratch
+ */
+export function resetGraphState(): void {
+    console.log('Resetting knowledge graph state...');
+    resetState();
+    console.log('State reset complete. All files will be reprocessed on next build.');
+}
diff --git a/apps/x/packages/core/src/knowledge/graph_state.ts b/apps/x/packages/core/src/knowledge/graph_state.ts
new file mode 100644
index 00000000..ad5ccfe3
--- /dev/null
+++ b/apps/x/packages/core/src/knowledge/graph_state.ts
@@ -0,0 +1,140 @@
+import fs from 'fs';
+import path from 'path';
+import crypto from 'crypto';
+import { WorkDir } from '../config/config.js';
+
+/**
+ * State tracking for knowledge graph processing
+ * Uses mtime + hash hybrid approach to detect file changes
+ */
+
+const STATE_FILE = path.join(WorkDir, 'knowledge_graph_state.json');
+
+export interface FileState {
+    mtime: string; // ISO timestamp of last modification
+    hash: string; // Content hash
+    lastProcessed: string; // ISO timestamp of when it was processed
+}
+
+export interface GraphState {
+    processedFiles: Record<string, FileState>; // filepath -> FileState
+    lastBuildTime: string; // ISO timestamp of last successful build
+}
+
+/**
+ * Load the current state from disk
+ */
+export function loadState(): GraphState {
+    if (fs.existsSync(STATE_FILE)) {
+        try {
+            return JSON.parse(fs.readFileSync(STATE_FILE, 'utf-8'));
+        } catch (error) {
+            console.error('Error loading knowledge graph state:', error);
+        }
+    }
+
+    return {
+        processedFiles: {},
+        lastBuildTime: new Date(0).toISOString(), // epoch
+    };
+}
+
+/**
+ * Save the current state to disk
+ */
+export function saveState(state: GraphState): void {
+    try {
+        fs.writeFileSync(STATE_FILE, JSON.stringify(state, null, 2));
+    } catch (error) {
+        console.error('Error saving knowledge graph state:', error);
+        throw error;
+    }
+}
+
+/**
+ * Compute hash of file content
+ */
+export function computeFileHash(filePath: string): string {
+    const content = fs.readFileSync(filePath, 'utf-8');
+    return crypto.createHash('sha256').update(content).digest('hex');
+}
+
+/**
+ * Check if a file has changed since it was last processed
+ * Uses mtime for quick check, then hash for verification
+ */
+export function hasFileChanged(filePath: string, state: GraphState): boolean {
+    const fileState = state.processedFiles[filePath];
+
+    // New file - never processed
+    if (!fileState) {
+        return true;
+    }
+
+    // Check mtime first (fast)
+    const stats = fs.statSync(filePath);
+    const currentMtime = stats.mtime.toISOString();
+
+    // If mtime is the same, file definitely hasn't changed
+    if (currentMtime === fileState.mtime) {
+        return false;
+    }
+
+    // mtime changed - verify with hash to confirm actual content change
+    const currentHash = computeFileHash(filePath);
+    return currentHash !== fileState.hash;
+}
+
+/**
+ * Update state after processing a file
+ */
+export function markFileAsProcessed(filePath: string, state: GraphState): void {
+    const stats = fs.statSync(filePath);
+    const hash = computeFileHash(filePath);
+
+    state.processedFiles[filePath] = {
+        mtime: stats.mtime.toISOString(),
+        hash: hash,
+        lastProcessed: new Date().toISOString(),
+    };
+}
+
+/**
+ * Get list of files that need processing from a source directory
+ * Returns only new or changed files
+ */
+export function getFilesToProcess(
+    sourceDir: string,
+    state: GraphState
+): string[] {
+    if (!fs.existsSync(sourceDir)) {
+        return [];
+    }
+
+    const filesToProcess: string[] = [];
+    const entries = fs.readdirSync(sourceDir);
+
+    for (const entry of entries) {
+        const fullPath = path.join(sourceDir, entry);
+        const stat = fs.statSync(fullPath);
+
+        if (stat.isFile() && entry.endsWith('.md')) {
+            if (hasFileChanged(fullPath, state)) {
+                filesToProcess.push(fullPath);
+            }
+        }
+    }
+
+    return filesToProcess;
+}
+
+/**
+ * Reset state - useful for reprocessing everything
+ */
+export function resetState(): void {
+    const emptyState: GraphState = {
+        processedFiles: {},
+        lastBuildTime: new Date().toISOString(),
+    };
+    saveState(emptyState);
+}