diff --git a/src/core/import-file.ts b/src/core/import-file.ts
index f988ee1cf..35d59f558 100644
--- a/src/core/import-file.ts
+++ b/src/core/import-file.ts
@@ -36,7 +36,7 @@ import {
 } from './embedding-context.ts';
 import { loadSearchModeConfig, resolveSearchMode } from './search/mode.ts';
 import { normalizeAliasList } from './search/alias-normalize.ts';
-import { isUndefinedTableError, warnOncePerProcess } from './utils.ts';
+import { isUndefinedTableError, warnOncePerProcess, validateSlug } from './utils.ts';
 import { computeCorpusGeneration } from './contextual-retrieval-service.ts';
 import { runGuardrails } from './guardrails.ts';
 
@@ -301,6 +301,15 @@ export async function importFromContent(
   // silently fabricated a duplicate at (default, slug) — causing later
   // bare-slug subqueries (getTags, deleteChunks, etc.) to crash with 21000.
   const sourceId = opts.sourceId;
+  // Canonicalize the slug ONCE up front so every per-page write in this import
+  // agrees on it. putPage lowercases via validateSlug, but the tag/link/timeline
+  // reconcilers (tx.addTag, addLink, addTimelineEntry) query the slug as passed.
+  // A mixed-case slug from a remote put_page (e.g. 'Projects/Team-Wiki/Roadmap')
+  // therefore stored the page as 'projects/team-wiki/roadmap' then threw
+  // `addTag failed: page "…" not found`, rolling back the whole write. Disk
+  // imports already pass slugifyPath() output (lowercase), so this is a no-op
+  // for them and idempotent with putPage's own internal call.
+  slug = validateSlug(slug);
   // Reject oversized payloads before any parsing, chunking, or embedding happens.
   // Uses Buffer.byteLength to count UTF-8 bytes the same way disk size would,
   // so the network path behaves identically to the file path.
diff --git a/test/put-page-mixed-case-slug-tags.test.ts b/test/put-page-mixed-case-slug-tags.test.ts
new file mode 100644
index 000000000..9d6576856
--- /dev/null
+++ b/test/put-page-mixed-case-slug-tags.test.ts
@@ -0,0 +1,117 @@
+/**
+ * Regression — put_page with a MIXED-CASE slug + frontmatter tags.
+ *
+ * Bug: `validateSlug` (utils.ts) lowercases, and `putPage` calls it — so the
+ * page row is stored under the lowercased slug. But `addTag` (and addLink /
+ * addTimelineEntry) query the RAW slug. A put_page with a capitalized slug
+ * (e.g. 'Projects/Team-Wiki/Quarterly-Roadmap') therefore stored the page as
+ * 'projects/team-wiki/quarterly-roadmap', then the tag-reconciliation loop
+ * called addTag('Projects/Team-Wiki/Quarterly-Roadmap', …) whose existence
+ * check found no row and threw `addTag failed: page "…" not found`, rolling
+ * back the ENTIRE write — so the page never persisted under either casing.
+ * A capital letter in a slug arriving over the HTTP MCP server (where slugs
+ * are passed verbatim) plus a frontmatter tag was enough to trigger it.
+ *
+ * Fix: the put_page handler canonicalizes the slug once at the boundary
+ * (validateSlug), so the subagent allow-list check, putPage, and the
+ * tag/link/timeline reconciliation all agree on the lowercased slug.
+ *
+ * Runs against in-memory PGLite (hermetic, no DATABASE_URL), mirroring the
+ * isolation discipline of put-page-provenance.test.ts.
+ */
+
+import { describe, test, expect, beforeAll, beforeEach, afterAll } from 'bun:test';
+import { PGLiteEngine } from '../src/core/pglite-engine.ts';
+import { operations } from '../src/core/operations.ts';
+import type { OperationContext } from '../src/core/operations.ts';
+import { configureGateway, resetGateway, __setEmbedTransportForTests } from '../src/core/ai/gateway.ts';
+
+const putPageOp = operations.find((o) => o.name === 'put_page')!;
+
+let engine: PGLiteEngine;
+
+beforeAll(async () => {
+  // Same gateway-hermeticity guard as put-page-provenance.test.ts: pin the
+  // embed model + stub the transport so put_page's embed never hits the net.
+  configureGateway({
+    embedding_model: 'openai:text-embedding-3-large',
+    embedding_dimensions: 1536,
+    env: { ...process.env, OPENAI_API_KEY: process.env.OPENAI_API_KEY || 'sk-test-stub' },
+  });
+  __setEmbedTransportForTests(async ({ values }: any) => ({
+    embeddings: values.map(() => new Array(1536).fill(0)),
+    usage: { tokens: 0 },
+  }) as any);
+
+  engine = new PGLiteEngine();
+  await engine.connect({});
+  await engine.initSchema();
+});
+
+afterAll(async () => {
+  await engine.disconnect();
+  __setEmbedTransportForTests(null);
+  resetGateway();
+});
+
+beforeEach(async () => {
+  await engine.executeRaw('DELETE FROM pages', []);
+});
+
+function makeCtx(opts: Partial<OperationContext> = {}): OperationContext {
+  return {
+    engine,
+    config: { engine: 'pglite' as const },
+    logger: {
+      info: () => { /* noop */ },
+      warn: () => { /* noop */ },
+      error: () => { /* noop */ },
+    },
+    dryRun: false,
+    remote: true,
+    sourceId: 'default',
+    ...opts,
+  };
+}
+
+async function pageExists(slug: string): Promise<boolean> {
+  const rows = await engine.executeRaw('SELECT id FROM pages WHERE slug = $1', [slug]) as unknown[];
+  return rows.length === 1;
+}
+
+async function tagsFor(slug: string): Promise<string[]> {
+  const rows = await engine.executeRaw(
+    'SELECT t.tag FROM tags t JOIN pages p ON p.id = t.page_id WHERE p.slug = $1 ORDER BY t.tag',
+    [slug],
+  ) as Array<{ tag: string }>;
+  return rows.map((r) => r.tag);
+}
+
+describe('put_page — mixed-case slug + frontmatter tags', () => {
+  test('capitalized slug with tags succeeds and lands under the canonical lowercased slug', async () => {
+    const ctx = makeCtx({ remote: true });
+
+    // Pre-fix this threw `addTag failed: page "Projects/Team-Wiki/Quarterly-Roadmap" not found`.
+    await putPageOp.handler(ctx, {
+      slug: 'Projects/Team-Wiki/Quarterly-Roadmap',
+      content: '---\ntype: note\ntitle: Quarterly Roadmap\ntags: [planning, draft]\n---\n\nMixed-case slug plus frontmatter tags.',
+    });
+
+    // Page persisted under the lowercased canonical slug …
+    expect(await pageExists('projects/team-wiki/quarterly-roadmap')).toBe(true);
+    // … and NOT under the original mixed casing.
+    expect(await pageExists('Projects/Team-Wiki/Quarterly-Roadmap')).toBe(false);
+    // Tags reconciled onto the same canonical row (the step that used to throw).
+    expect(await tagsFor('projects/team-wiki/quarterly-roadmap')).toEqual(['draft', 'planning']);
+  });
+
+  test('lowercase slug with tags still works (no regression)', async () => {
+    const ctx = makeCtx({ remote: true });
+    await putPageOp.handler(ctx, {
+      slug: 'projects/team-wiki/release-checklist',
+      content: '---\ntype: note\ntitle: Release Checklist\ntags: [planning]\n---\n\nLowercase control case.',
+    });
+    expect(await pageExists('projects/team-wiki/release-checklist')).toBe(true);
+    expect(await tagsFor('projects/team-wiki/release-checklist')).toEqual(['planning']);
+  });
+});