diff --git a/packages/core/src/discovery/cache/__tests__/sessions.test.ts b/packages/core/src/discovery/cache/__tests__/sessions.test.ts
index b6f046e..0200aa2 100644
--- a/packages/core/src/discovery/cache/__tests__/sessions.test.ts
+++ b/packages/core/src/discovery/cache/__tests__/sessions.test.ts
@@ -2,6 +2,7 @@ import { mkdtempSync, rmSync } from "node:fs";
 import { join } from "node:path";
 import { tmpdir } from "node:os";
 import { afterEach, describe, expect, it, vi } from "vitest";
+import { setCoreDiagnostics } from "../../../utils/diagnostics.js";
 import {
   clearCache,
   getCacheInfo,
@@ -22,6 +23,7 @@ vi.mock("node:os", async (importOriginal) => {
 });
 
 afterEach(() => {
+  setCoreDiagnostics(null);
   rmSync(join(testHomeDir, ".cache"), { recursive: true, force: true });
   setSchemaEnsuredPath(null);
 });
@@ -50,6 +52,43 @@ describe("cached sessions", () => {
     ]);
   });
 
+  it("preserves and reports metadata omitted from an incremental change", () => {
+    const events: Array<{ event: string; detail?: Record<string, unknown> }> = [];
+    setCoreDiagnostics({ warn: (event, detail) => events.push({ event, detail }) });
+    saveCachedSessions("codex", [makeSessionHead("one")], {
+      one: { id: "one", sourcePath: "/transcripts/one.jsonl" },
+    });
+
+    saveCachedSessionChanges(
+      "codex",
+      [{ session: makeSessionHead("one", { title: "Updated" }), sortIndex: 0 }],
+      [],
+    );
+
+    const cached = loadCachedSessions("codex");
+    expect(cached?.sessions[0]?.title).toBe("Updated");
+    expect(cached?.meta.one).toEqual({
+      id: "one",
+      sourcePath: "/transcripts/one.jsonl",
+    });
+    expect(
+      withCacheDb(
+        (db) =>
+          (
+            db
+              .prepare("SELECT source_path FROM sessions WHERE agent_name = ? AND session_id = ?")
+              .get("codex", "one") as { source_path?: string }
+          ).source_path,
+      ),
+    ).toBe("/transcripts/one.jsonl");
+    expect(events).toEqual([
+      {
+        event: "cache.session_meta_missing",
+        detail: { agent: "codex", session_id: "one" },
+      },
+    ]);
+  });
+
   it("round-trips parent references through the structured cache", () => {
     const child = makeSessionHead("child", {
       parent_reference: { agentName: "codex", sessionId: "parent" },
diff --git a/packages/core/src/discovery/cache/messages.ts b/packages/core/src/discovery/cache/messages.ts
index 7294d01..6a5bc60 100644
--- a/packages/core/src/discovery/cache/messages.ts
+++ b/packages/core/src/discovery/cache/messages.ts
@@ -151,7 +151,10 @@ export function prepareUpsertSession(db: SQLiteDatabase): SQLiteStatement {
       sort_index = excluded.sort_index,
       slug = excluded.slug,
       title = excluded.title,
-      source_path = excluded.source_path,
+      source_path = CASE
+        WHEN excluded.meta_json IS NULL THEN sessions.source_path
+        ELSE excluded.source_path
+      END,
       directory = excluded.directory,
       parent_agent_name = excluded.parent_agent_name,
       parent_session_id = excluded.parent_session_id,
@@ -175,7 +178,7 @@ export function prepareUpsertSession(db: SQLiteDatabase): SQLiteStatement {
       smart_tags_json = excluded.smart_tags_json,
       smart_tags_source_updated_at = excluded.smart_tags_source_updated_at,
       smart_tags_classifier_revision = excluded.smart_tags_classifier_revision,
-      meta_json = excluded.meta_json,
+      meta_json = COALESCE(excluded.meta_json, sessions.meta_json),
       publication_id = excluded.publication_id
   `);
 }
diff --git a/packages/core/src/discovery/cache/sessions.ts b/packages/core/src/discovery/cache/sessions.ts
index 5ebf930..2c065ba 100644
--- a/packages/core/src/discovery/cache/sessions.ts
+++ b/packages/core/src/discovery/cache/sessions.ts
@@ -6,6 +6,7 @@ import type { SessionCacheMeta } from "../../agents/base.js";
 import type { ReferencedSessionHead, SessionReference } from "../../contract/index.js";
 import { formatSessionReference, normalizeSessionReference } from "../../contract/index.js";
 import type { SessionDetail, SessionHead } from "../../types/index.js";
+import { getCoreDiagnostics } from "../../utils/diagnostics.js";
 import { tableExists, type SQLiteDatabase } from "../../utils/sqlite.js";
 import {
   getCachePath,
@@ -604,6 +605,12 @@ export function writeCachedSessionChanges(
 
   for (const { session, sortIndex } of changes) {
     const sessionMeta = meta[session.id];
+    if (!sessionMeta) {
+      getCoreDiagnostics()?.warn("cache.session_meta_missing", {
+        agent: agentName,
+        session_id: session.id,
+      });
+    }
     const metaJson = sessionMeta ? JSON.stringify(sessionMeta) : null;
     upsertSessionRow(
       upsertSession,