fix(rework): anchor rework-pr prompt to current branch tip + configurable token cap (§56)

docs/BACKLOG/open.md

@@ -347,3 +347,86 @@ Checks:
   symbols) and confirm the filtered finding count = 0
 
 ---
+
+## 56. Fix the `--rework-pr` workflow in `solve_issues.py`
+
+Labels: `kind/bug`, `theme/solver`, `area/cost-cap`, `priority/2`
+
+Priority: `2`
+
+The `--rework-pr` workflow in `solve_issues.py` is broken for
+follow-up-edits. Measured across this session (2026-06-24/25):
+
+- **Run 1** (PR #425, `opencode/deepseek-v4-flash-free` via
+  `--model opencode`): OpenRouter 400 Bad Request. opencode-serve
+  1.14.28 rejects the slug in this code path (the same model
+  worked in the normal solve path for the same PR's first push).
+- **Run 2** (PR #425, `mistral/mistral-large-latest` via
+  `--model openrouter_direct`): same 400. Not the model — the
+  request shape.
+- **Run 3** (PR #425, `openai/gpt-4o-mini` via
+  `--model openrouter_direct`): no 400, model produced valid
+  patch JSON, but output truncated mid-JSON by the 4096-token
+  output cap. Worker reports `status: no_patches`.
+- **Run 4** (PR #425, `openai/gpt-4o` via
+  `--model openrouter_direct`): no 400, full patch JSON produced,
+  but the worker-patcher (likely `git apply`) rejected it 25x with
+  "Keine gueltigen Patches in der Eingabe". Most likely cause:
+  the model wrote a diff against the pre-PR-433 code, but the
+  worker-patcher applies against the current branch tip (which
+  already has PR #433's commits). Mismatch.
+- **Run 5** (PR #437, `openai/gpt-4o` via
+  `--model openrouter_direct`): same `patches_failed` /
+  `worker_exit_code: 3` pattern. Confirms the failure is not
+  PR-specific.
+
+`/tmp/compare_temperatures.py` (still on disk in this session)
+can reproduce the 400 / no_patches / patches_failed modes
+deterministically.
+
+**Most likely root cause for Run 4/5 (the patches_failed case):**
+the rework-pr prompt gives the model the PR diff as "what to fix"
+but does not anchor the model to "your diff must apply cleanly on
+top of the current branch tip". The model writes a
+rewrite-from-scratch diff and the patcher rejects it. Fix would
+need to change the rework prompt in `solve_issues.py` (out of
+scope for the §54 review_pr.py work).
+
+**Secondary root cause for Runs 1/2 (the OpenRouter 400):**
+unknown — both `deepseek-v4-flash-free` and `mistral-large-latest`
+are listed as valid OpenRouter slugs. Could be a request-shape
+mismatch in `_gh_api`-equivalent call inside `solve_issues.py`,
+or could be a transient OpenRouter API issue. Needs a 3-run
+reproduction in a controlled environment before any fix is
+designed.
+
+Suggested scope:
+- run a deterministic reproduction with the existing
+  `/tmp/compare_temperatures.py` against 3 different PRs to
+  characterise the failure surface
+- read the rework-pr prompt construction in `solve_issues.py`
+  and identify the anchoring gap (model doesn't know about the
+  current branch tip's existing commits)
+- design a fix that:
+  - explicitly passes the current branch tip SHA + the
+    already-existing commits on the PR branch to the model
+  - asks the model for an **incremental** diff against that
+    base, not a full rewrite
+  - verifies each produced patch with `git apply --check`
+    *before* the worker marks the run as successful
+- add unit tests in `tests/test_solve_issues.py` for the
+  rework-pr prompt shape (mock at the openrouter-call boundary)
+
+Touches: `scripts/solve_issues.py`, `tests/test_solve_issues.py`
+
+Checks:
+- `git diff --check`
+- `python -m unittest tests.test_solve_issues -v`
+- `python -m unittest discover -s tests`
+- new unit test that exercises the rework-pr prompt shape with a
+  realistic PR-diff + current-branch-tip context
+- 3-run reproduction against 3 different real PRs from this repo:
+  all 3 should now succeed or fail with a clear, actionable error
+  (not the current opaque `patches_failed` 25x retry loop)
+
+---

prompts/rework_pr.md

@@ -8,8 +8,19 @@ provided that must be addressed by making additional changes on the same branch.
 - **PR #{pr_number}** in `{owner}/{repo}`
 - **Base branch:** `{base_branch}`
 - **Head branch:** `{head_branch}` (the branch you will modify)
+- **Current head commit:** `{head_sha}`
 - **Reviewers:** {reviewer_usernames}
 
+## Current Branch State
+
+The PR branch already contains the following commits, newest first:
+
+{existing_commits_list}
+
+Your patch MUST be a minimal incremental diff that applies cleanly on top of
+commit `{head_sha}`. Do not rewrite files from scratch against older PR or base
+branch content. Reference files by their current post-`{head_sha}` content.
+
 ## PR Diff (current state vs base)
 
 ```
@@ -29,6 +40,7 @@ The following review comments must be addressed:
 3. Preserve the existing branch name — push follow-up commits to `{head_branch}`.
 4. If a reviewer comment is unclear, use your best judgment to resolve it.
 5. After applying changes, verify the code is syntactically correct.
+6. Return only an incremental patch for the current branch tip `{head_sha}`.
 
 ## Constraints
 

scripts/validation/github_client.py

@@ -157,6 +157,18 @@ def get_pull_request(self, repo: str, number: int) -> PullRequestInfo | None:
             merge_commit_sha=(pr.get("merge_commit_sha") or None),
         )
 
+    def get_pull_request_commits(self, repo: str, number: int) -> list[dict[str, Any]]:
+        """Return commit metadata for a pull request, newest commit last."""
+        resp = self.session.get(
+            f"{self.BASE}/repos/{self.owner}/{repo}/pulls/{number}/commits",
+            params={"per_page": 100},
+        )
+        if resp.status_code == 404:
+            return []
+        self._raise_for_status(resp, f"get PR commits: {repo}#{number}")
+        data = resp.json()
+        return data if isinstance(data, list) else []
+
     def get_ci_status(self, repo: str, ref: str) -> CiStatus:
         resp = self.session.get(
             f"{self.BASE}/repos/{self.owner}/{repo}/commits/{ref}/status",

scripts/validation/rework.py

@@ -33,6 +33,7 @@
 _REPO_ROOT = Path(__file__).resolve().parents[2]
 REWORK_PROMPT_PATH = _REPO_ROOT / "prompts" / "rework_pr.md"
 REWORK_COMMIT_MESSAGE_PREFIX = "rework: apply review feedback"
+DEFAULT_REWORK_MAX_TOKENS = 16384
 
 
 def _load_rework_prompt_template() -> str:
@@ -55,6 +56,8 @@ def _build_rework_prompt(
     head_branch: str,
     diff: str,
     review_threads: list[ReviewThread],
+    head_sha: str = "",
+    existing_commits_list: str = "",
 ) -> str:
     """Build the final rework prompt by filling in the template."""
     reviewer_usernames = ", ".join(
@@ -73,12 +76,44 @@ def _build_rework_prompt(
         repo=repo,
         base_branch=base_branch,
         head_branch=head_branch,
+        head_sha=head_sha or "(unknown)",
+        existing_commits_list=existing_commits_list or "(commit list unavailable)",
         reviewer_usernames=reviewer_usernames,
         diff=diff,
         review_threads=review_text,
     )
 
 
+def _format_pr_commits_for_prompt(commits: list[dict[str, Any]], *, limit: int = 12) -> str:
+    """Format PR commits for the rework prompt, newest first."""
+    if not commits:
+        return "(no existing PR commits returned by GitHub)"
+
+    lines: list[str] = []
+    for item in reversed(commits[-limit:]):
+        sha = str(item.get("sha") or "")[:12] or "unknown"
+        commit = item.get("commit") if isinstance(item.get("commit"), dict) else {}
+        message = str(commit.get("message") or "").splitlines()[0].strip()
+        if not message:
+            message = "(no commit message)"
+        lines.append(f"- {sha} {message}")
+    if len(commits) > limit:
+        lines.append(f"- ... {len(commits) - limit} older commit(s) omitted")
+    return "\n".join(lines)
+
+
+def _rework_max_tokens_from_env(default: int = DEFAULT_REWORK_MAX_TOKENS) -> int:
+    """Return the OpenRouter completion cap for PR rework runs."""
+    raw = os.getenv("OPENROUTER_REWORK_MAX_TOKENS")
+    if not raw:
+        return default
+    try:
+        value = int(raw)
+    except ValueError:
+        return default
+    return value if value > 0 else default
+
+
 def _run_worker_via_subprocess(
     prompt: str,
     repo_dir: str,
@@ -104,14 +139,14 @@ def _run_worker_via_subprocess(
         api_key=openrouter_key,
         model=model,
         request_timeout_seconds=timeout_seconds,
-        use_structured_output=True,
+        use_structured_output=False,
     )
 
     try:
         response_text, usage = worker.generate_with_usage(
-            prompt=prompt,
+            prompt=worker.build_patch_prompt(prompt, structured=False),
             temperature=0.3,
-            max_tokens=8192,
+            max_tokens=_rework_max_tokens_from_env(),
             timeout=timeout_seconds,
         )
     except Exception as exc:
@@ -261,6 +296,12 @@ def run_pr_rework(
     except RuntimeError:
         review_threads = []
 
+    try:
+        existing_commits = client.get_pull_request_commits(repo, pr_number)
+    except RuntimeError:
+        existing_commits = []
+    existing_commits_list = _format_pr_commits_for_prompt(existing_commits)
+
     # 3. Build prompt
     try:
         template = _load_rework_prompt_template()
@@ -285,6 +326,8 @@ def run_pr_rework(
         head_branch=head_branch,
         diff=diff,
         review_threads=review_threads,
+        head_sha=pr_info.head_sha,
+        existing_commits_list=existing_commits_list,
     )
 
     # Dry-run: print metadata and prompt, return early

tests/test_validation/test_github_client.py

@@ -111,6 +111,23 @@ def test_get_pull_request_returns_none_on_404(self):
         pr = self.client.get_pull_request("repo", 999)
         self.assertIsNone(pr)
 
+    def test_get_pull_request_commits_returns_list(self):
+        commits = [
+            {"sha": "abc123", "commit": {"message": "first"}},
+            {"sha": "def456", "commit": {"message": "second"}},
+        ]
+        self.session.get.return_value = self._mock_response(200, commits)
+
+        result = self.client.get_pull_request_commits("repo", 10)
+
+        self.assertEqual(result, commits)
+        self.session.get.assert_called_once()
+        self.assertIn("/pulls/10/commits", self.session.get.call_args.args[0])
+
+    def test_get_pull_request_commits_returns_empty_on_404(self):
+        self.session.get.return_value = self._mock_response(404)
+        self.assertEqual(self.client.get_pull_request_commits("repo", 999), [])
+
     def test_get_ci_status_returns_status(self):
         self.session.get.return_value = self._mock_response(200, {
             "state": "success", "total_count": 3, "statuses": [