fix: retrieve published posts by numeric ID

CHANGELOG.md

@@ -8,6 +8,12 @@ and the git tag history (`v0.1.0`–`v0.5.0`).
 
 ## [Unreleased]
 
+### Fixed
+- `get_post` retrieves published posts using the numeric-ID endpoint and unwraps
+  its `post` response (#125). Previously, numeric IDs were sent to the slug endpoint
+  and returned 404. The upstream `wordcount` field maps to the existing `word_count`
+  output. Missing or mismatched post data fails instead of returning empty success.
+
 ## [1.2.0] - 2026-09-14
 
 ### Added

src/__tests__/get-post.test.ts

@@ -0,0 +1,63 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { Client } from "@modelcontextprotocol/sdk/client/index.js";
+import { InMemoryTransport } from "@modelcontextprotocol/sdk/inMemory.js";
+import { SubstackClient } from "../api/client.js";
+import { createServer } from "../server.js";
+
+const post = {
+  id: 42, title: "Published article", subtitle: null, slug: "published-article",
+  post_date: "2026-09-18T12:00:00Z", audience: "everyone", type: "newsletter",
+  body_html: "<p>Full article body.</p>", wordcount: 4,
+  canonical_url: "https://example.substack.com/p/published-article",
+};
+const envelope = { post, publication: { id: 7, subdomain: "example" }, publicationSettings: { unrelated: true } };
+function mockRead(value: unknown = envelope, status = 200) {
+  const fetchMock = vi.fn(async () => new Response(JSON.stringify(value), { status }));
+  vi.stubGlobal("fetch", fetchMock);
+  return fetchMock;
+}
+afterEach(() => vi.unstubAllGlobals());
+
+describe("published post lookup (#125)", () => {
+  it.each(["https://example.substack.com", "https://newsletter.example.com"])("uses the numeric-ID route and unwraps the post on %s", async origin => {
+    const read = mockRead();
+    const result = await new SubstackClient(origin, "synthetic-token", "1").getPost(42);
+    expect(result).toMatchObject({ id: 42, title: post.title, body_html: post.body_html, word_count: 4 });
+    expect(result).not.toHaveProperty("publicationSettings");
+    expect(result).not.toHaveProperty("post");
+    expect(read).toHaveBeenCalledTimes(1);
+    expect(read).toHaveBeenCalledWith(`${origin}/api/v1/posts/by-id/42`, expect.objectContaining({
+      redirect: "manual", headers: expect.objectContaining({ Cookie: "connect.sid=synthetic-token; substack.sid=synthetic-token;" }),
+    }));
+  });
+  it("preserves an existing word_count, including zero", async () => {
+    mockRead({ post: { ...post, word_count: 0 } });
+    expect((await new SubstackClient("https://example.substack.com", "synthetic-token", "1").getPost(42)).word_count).toBe(0);
+  });
+  it.each([null, {}, { post: null }, { post: { ...post, id: 43 } }])("rejects missing or mismatched post data: %j", async value => {
+    mockRead(value);
+    await expect(new SubstackClient("https://example.substack.com", "synthetic-token", "1").getPost(42)).rejects.toThrow("Unexpected published post response");
+  });
+  it.each([401, 403, 404, 429, 500])("preserves HTTP %i without retries or fallback scans", async status => {
+    const read = mockRead({ error: "Synthetic upstream error" }, status);
+    await expect(new SubstackClient("https://example.substack.com", "synthetic-token", "1").getPost(42)).rejects.toMatchObject({ statusCode: status });
+    expect(read).toHaveBeenCalledTimes(1);
+  });
+  it.each([false, true])("returns the established text and structured MCP output (multi=%s)", async multi => {
+    const read = mockRead();
+    const pubs = [{ key: "first", label: "First", client: new SubstackClient("https://first.example", "synthetic-token", "1") },
+      ...(multi ? [{ key: "second", label: "Second", client: new SubstackClient("https://second.example", "synthetic-token", "2") }] : [])];
+    const server = createServer(pubs), client = new Client({ name: "post-regression", version: "1" });
+    const [ct, st] = InMemoryTransport.createLinkedPair();
+    await Promise.all([client.connect(ct), server.connect(st)]);
+    try {
+      const result = await client.callTool({ name: "get_post", arguments: { post_id: 42, ...(multi ? { publication: "second" } : {}) } });
+      expect(result.isError).not.toBe(true);
+      const expected = { id: 42, title: post.title, subtitle: null, slug: post.slug, post_date: post.post_date,
+        audience: "everyone", word_count: 4, body_html: post.body_html, url: post.canonical_url };
+      expect(result.structuredContent).toEqual(expected);
+      expect(JSON.parse((result.content as { text: string }[])[0].text)).toEqual(expected);
+      expect(read).toHaveBeenCalledWith(`https://${multi ? "second" : "first"}.example/api/v1/posts/by-id/42`, expect.anything());
+    } finally { await client.close(); await server.close(); }
+  });
+});

src/api/client.ts

@@ -300,9 +300,17 @@ export class SubstackClient {
   }
 
   async getPost(id: number): Promise<SubstackPost> {
-    return this.request<SubstackPost>(
-      `${this.publicationUrl}/api/v1/posts/${id}`,
+    // /posts/:slug treats a numeric ID as a slug and returns 404 (#125).
+    // The numeric route returns { post, publication, publicationSettings }.
+    const data = await this.request<{ post?: SubstackPost & { wordcount?: number } }>(
+      `${this.publicationUrl}/api/v1/posts/by-id/${id}`,
     );
+    const post = data?.post;
+    if (!post || post.id !== id) {
+      throw new Error("Unexpected published post response; requested post cannot be verified.");
+    }
+    // This route calls the count `wordcount`; keep the public tool field stable.
+    return { ...post, word_count: post.word_count ?? post.wordcount };
   }
 
   async createDraft(