diff --git a/CHANGELOG.md b/CHANGELOG.md
index e7559656f7..3ec9ce6750 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -172,6 +172,7 @@ and adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 #### MCP / indexing
 
 - On Windows, the shared MCP daemon now waits longer for another program — an antivirus scan, an indexer, or another session reading its lock file — to let go of that file, so it starts instead of leaving the session to fall back to a slower in-process server. (#1773)
+- Repeated `codegraph_explore` calls from a running MCP server no longer re-rank the whole graph each time to find the project's core file; the answer is reused until the index changes. (#1864)
 - File watching no longer drops the full re-scan a removed directory asks for when that sync fails, so the deleted files leave the index instead of lingering. (#1964)
 - Daemon startup and cleanup now preserve live legacy PID-only locks while still reclaiming dead or identity-disproved records, preventing two writers from serving the same project.
 - Incremental sync now keeps edge rebinding crash-safe: replacing a resolved edge with its recovery reference commits atomically, so an interruption cannot permanently remove the relationship.
diff --git a/__tests__/dominant-file-cache.test.ts b/__tests__/dominant-file-cache.test.ts
new file mode 100644
index 0000000000..7e4f3a1f35
--- /dev/null
+++ b/__tests__/dominant-file-cache.test.ts
@@ -0,0 +1,118 @@
+/**
+ * getDominantFile() is computed once per database state (#1864).
+ *
+ * The dominant-file heuristic (context ranking's core-directory boost) is a
+ * whole-graph aggregation whose answer does not depend on the query, yet it
+ * ran on every generic explore — seconds per call on a large index. It is now
+ * memoized against a database change stamp. These tests pin both halves:
+ * repeated explores reuse the answer, and any write — by this connection or
+ * by another process's sync — makes the next call see the new graph.
+ */
+
+import { describe, it, expect, afterEach, vi } from 'vitest';
+import * as fs from 'fs';
+import * as path from 'path';
+import * as os from 'os';
+import CodeGraph from '../src';
+import type { QueryBuilder } from '../src/db/queries';
+
+/** A file whose functions call each other in a chain: `n` in-file call edges. */
+function chain(prefix: string, n: number): string {
+  const lines: string[] = [];
+  for (let i = 0; i < n; i++) {
+    const next = i + 1 < n ? `${prefix}${i + 1}();` : '';
+    lines.push(`export function ${prefix}${i}(): void { ${next} }`);
+  }
+  return lines.join('\n') + '\n';
+}
+
+function queriesOf(cg: CodeGraph): QueryBuilder {
+  return (cg as unknown as { queries: QueryBuilder }).queries;
+}
+
+function spyCompute(cg: CodeGraph) {
+  return vi.spyOn(queriesOf(cg) as unknown as { computeDominantFile: () => unknown }, 'computeDominantFile');
+}
+
+describe('dominant file — computed once per index state (#1864)', () => {
+  let dir: string;
+  const open: CodeGraph[] = [];
+
+  afterEach(() => {
+    for (const cg of open.splice(0)) cg.close();
+    vi.restoreAllMocks();
+    if (dir) fs.rmSync(dir, { recursive: true, force: true });
+  });
+
+  async function setup(): Promise<CodeGraph> {
+    dir = fs.mkdtempSync(path.join(os.tmpdir(), 'cg-dominant-'));
+    fs.mkdirSync(path.join(dir, 'core'));
+    fs.mkdirSync(path.join(dir, 'ext'));
+    fs.writeFileSync(path.join(dir, 'core', 'engine.ts'), chain('engineStep', 40));
+    fs.writeFileSync(path.join(dir, 'ext', 'plugin.ts'), chain('pluginStep', 5));
+    const cg = await CodeGraph.init(dir, { index: true });
+    open.push(cg);
+    return cg;
+  }
+
+  it('reuses the answer across explores while the database is unchanged', async () => {
+    const cg = await setup();
+    const compute = spyCompute(cg);
+
+    for (const q of ['engine step', 'plugin step', 'how does the engine run']) {
+      await cg.findRelevantContext(q);
+    }
+    expect(compute).toHaveBeenCalledTimes(1);
+    expect(queriesOf(cg).getDominantFile()?.filePath).toBe('core/engine.ts');
+    expect(compute).toHaveBeenCalledTimes(1);
+  });
+
+  it('sees a new dominant file after a sync changes the graph', async () => {
+    const cg = await setup();
+    expect(queriesOf(cg).getDominantFile()?.filePath).toBe('core/engine.ts');
+
+    fs.writeFileSync(path.join(dir, 'ext', 'plugin.ts'), chain('pluginStep', 120));
+    await cg.sync();
+
+    const compute = spyCompute(cg);
+    expect(queriesOf(cg).getDominantFile()?.filePath).toBe('ext/plugin.ts');
+    expect(compute).toHaveBeenCalledTimes(1);
+  });
+
+  it('sees a sync made through another connection (another process)', async () => {
+    const writer = await setup();
+    const reader = await CodeGraph.open(dir);
+    open.push(reader);
+    expect(queriesOf(reader).getDominantFile()?.filePath).toBe('core/engine.ts');
+
+    fs.writeFileSync(path.join(dir, 'ext', 'plugin.ts'), chain('pluginStep', 120));
+    await writer.sync();
+
+    expect(queriesOf(reader).getDominantFile()?.filePath).toBe('ext/plugin.ts');
+  });
+
+  it('does not keep a result read inside a transaction that is rolled back', async () => {
+    const cg = await setup();
+    const q = queriesOf(cg);
+    const before = q.getDominantFile();
+    const db = (cg as any).db.getDb();
+    db.exec('BEGIN');
+    try {
+      db.exec('DELETE FROM edges');
+      expect(q.getDominantFile()).toBeNull();
+    } finally {
+      db.exec('ROLLBACK');
+    }
+    expect(q.getDominantFile()).toEqual(before);
+  });
+
+  it.runIf(process.platform !== 'win32')('sees a database rebuilt and reopened under it', async () => {
+    const reader = await setup();
+    expect(queriesOf(reader).getDominantFile()?.filePath).toBe('core/engine.ts');
+    fs.writeFileSync(path.join(dir, 'ext', 'plugin.ts'), chain('pluginStep', 120));
+    const rebuilt = await CodeGraph.recreate(dir);
+    try { await rebuilt.indexAll(); } finally { rebuilt.close(); }
+    expect(reader.reopenIfReplaced()).toBe(true);
+    expect(queriesOf(reader).getDominantFile()?.filePath).toBe('ext/plugin.ts');
+  });
+});
diff --git a/src/db/queries.ts b/src/db/queries.ts
index 14a6643412..d4a60156e8 100644
--- a/src/db/queries.ts
+++ b/src/db/queries.ts
@@ -251,6 +251,13 @@ export class QueryBuilder {
   private nodeCache: Map<string, Node> = new Map();
   private readonly maxCacheSize = 1000;
 
+  // getDominantFile()'s answer, tagged with the database change stamp it was
+  // computed under (see getChangeStamp). Query-independent, so one value
+  // serves every explore until the database changes (#1864).
+  private dominantFileMemo:
+    | { stamp: string; value: { filePath: string; edgeCount: number; nextEdgeCount: number } | null }
+    | undefined;
+
   // Prepared statements (lazily initialized)
   private stmts: {
     insertNode?: SqliteStatement;
@@ -290,6 +297,7 @@ export class QueryBuilder {
     getAllFilePaths?: SqliteStatement;
     getAllNodeNames?: SqliteStatement;
     getDominantFile?: SqliteStatement;
+    getChangeStamp?: SqliteStatement;
     getTopRouteFile?: SqliteStatement;
     getRoutingManifest?: SqliteStatement;
     insertNameSegment?: SqliteStatement;
@@ -995,8 +1003,49 @@ export class QueryBuilder {
    * Excludes test/spec files from candidacy via path-pattern. The agent's
    * typical question is "how does X work", not "how is X tested", so
    * boosting a test file's directory would be a misfire.
+   *
+   * The answer depends only on the graph, never on the query, and the
+   * aggregation behind it scans every edge — seconds on a large index, paid
+   * by every generic explore (#1864). So it is memoized per database change
+   * stamp: recomputed only after something wrote to the database.
    */
   getDominantFile(): { filePath: string; edgeCount: number; nextEdgeCount: number } | null {
+    // total_changes() counts writes that are later rolled back, so a result
+    // read inside a transaction could outlive a ROLLBACK under an unchanged
+    // stamp. Never keep one; `undefined` (a runtime without the getter) is
+    // treated the same way.
+    if (this.db.inTransaction !== false) {
+      this.dominantFileMemo = undefined;
+      return this.computeDominantFile();
+    }
+    const stamp = this.getChangeStamp();
+    if (this.dominantFileMemo?.stamp === stamp) return this.dominantFileMemo.value;
+    const value = this.computeDominantFile();
+    this.dominantFileMemo = { stamp, value };
+    return value;
+  }
+
+  /**
+   * A value that differs whenever the database content may have changed
+   * since the last call, whoever changed it: `total_changes()` counts the
+   * rows this connection inserted, updated or deleted, and
+   * `PRAGMA data_version` moves when any OTHER connection — another process's
+   * sync, a CLI `codegraph index` beside a running MCP server — commits. Both
+   * are O(1), so no write path has to remember to invalidate anything.
+   * Coarse on purpose: any write, not just one to nodes/edges, forces a
+   * recompute, which only costs time, never a stale answer.
+   */
+  private getChangeStamp(): string {
+    if (!this.stmts.getChangeStamp) {
+      this.stmts.getChangeStamp = this.db.prepare(
+        'SELECT total_changes() AS changes, (SELECT data_version FROM pragma_data_version) AS version'
+      );
+    }
+    const row = this.stmts.getChangeStamp.get() as { changes: number; version: number };
+    return `${row.changes}:${row.version}`;
+  }
+
+  private computeDominantFile(): { filePath: string; edgeCount: number; nextEdgeCount: number } | null {
     if (!this.stmts.getDominantFile) {
       // Pull top 20 candidates; we then filter out test/generated files
       // in code (regex-grade matching that SQL LIKE can't express). The
diff --git a/src/db/sqlite-adapter.ts b/src/db/sqlite-adapter.ts
index e94b587b95..39b2d8ba0f 100644
--- a/src/db/sqlite-adapter.ts
+++ b/src/db/sqlite-adapter.ts
@@ -32,6 +32,8 @@ export interface SqliteDatabase {
   transaction<T>(fn: (...args: any[]) => T): (...args: any[]) => T;
   close(): void;
   readonly open: boolean;
+  /** Undefined on a runtime without `isTransaction`; callers must then not memoize. */
+  readonly inTransaction?: boolean;
 }
 
 /**
@@ -73,6 +75,10 @@ class NodeSqliteAdapter implements SqliteDatabase {
     return toWslSharedIndexError(err, this._dbPath) ?? err;
   }
 
+  get inTransaction(): boolean | undefined {
+    return this._db.isTransaction;
+  }
+
   get open(): boolean {
     return this._db.isOpen;
   }