Add "why" context and Slack thread link to agent-created PRs

[timgl]

Problem

When the PostHog Slack app opens a PR, reviewers only see the diff — not why the change was requested, with no path back to the conversation. PR descriptions also tended to over-explain every change.

Why: Tim asked for agent-created PRs to carry the context behind the request plus a link to the originating Slack thread, and to keep the description brief.
Slack thread: https://posthog.slack.com/archives/C09G8Q32R6F/p1780952041801889

Changes

Updated the cloud PR-creation prompt in packages/agent/src/server/agent-server.ts so the agent now:

• Adds a brief Why to the PR body (the user's motivation, not a restatement of the diff).
• Links the originating Slack thread when the task started from one (read from task run state slack_thread_url).
• Keeps the PR description brief — summarizes the most important changes instead of enumerating every one.

Applied to both PR-creation paths (repository auto-publish and no-repository clone-and-PR).

How did you test this?

• pnpm --filter agent typecheck and biome check — clean.
• pnpm exec vitest run src/server/agent-server.test.ts — all 57 pass (env matching CI). Added 6 tests covering the Why, embedded/fallback thread link, brevity, and the no-repository path.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 3448fd0.}

[greptile-apps]

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
packages/agent/src/server/agent-server.test.ts:1467-1502
**Duplicate prompt construction across tests 1 and 3**

Both "instructs adding a brief Why…" (line 1467) and "falls back to a conditional thread hint…" (line 1492) call `createServer()` with `POSTHOG_CODE_INTERACTION_ORIGIN = "slack"` and `buildCloudSystemPrompt()` with no arguments — they construct the exact same prompt and then each assert a different substring of it. Per the team's OnceAndOnlyOnce principle, these should be merged into a single test (or a parameterised suite) that asserts all expected strings against the same prompt, rather than building it twice. The team also prefers parameterised tests, which would work well here given the four cases share a setup/teardown skeleton.

_{Reviews (1): Last reviewed commit: "Add "why" context and Slack thread link ..." | Re-trigger Greptile}

[greptile-apps]

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
packages/agent/src/server/agent-server.test.ts:1479-1487
Duplicate prompt construction — no-repository path. Both `"instructs keeping the PR description brief on the no-repository path"` (line 1479) and `"adds Why but omits the thread hint for non-Slack no-repository runs"` (line 1528) call `createServer({ repositoryPath: undefined })` and `buildCloudSystemPrompt()` with no arguments, building the exact same prompt twice. Per OnceAndOnlyOnce, merge them into a single test (or a parameterised row alongside the existing cases) that asserts all expected substrings at once.

```suggestion
      it("instructs keeping the PR description brief on the no-repository path", () => {
        delete process.env.POSTHOG_CODE_INTERACTION_ORIGIN;
        const s = createServer({ repositoryPath: undefined });
        const prompt = (
          s as unknown as TestableServer
        ).buildCloudSystemPrompt();
        expect(prompt).toContain("open a draft pull request");
        expect(prompt).toContain("Keep the PR description brief");
        expect(prompt).toContain("**Why**");
        expect(prompt).not.toContain("Slack thread");
      });
```

_{Reviews (2): Last reviewed commit: "Consolidate PR-body prompt tests to avoi..." | Re-trigger Greptile}

[adboio]

i think we should remove this fallback, assuming the agent won't actually have a way to find the slack URL when it's not provided ?

[timgl]

Good call — removed the fallback in 3448fd0. Now the thread link is only added when a concrete slack_thread_url is injected (read from the task run state); when it is not available we simply omit it rather than telling the agent to link a thread it has no way to locate.