diff --git a/.github/workflows/scripts/autoloop_scheduler.py b/.github/workflows/scripts/autoloop_scheduler.py
index 5066afe..d29509b 100644
--- a/.github/workflows/scripts/autoloop_scheduler.py
+++ b/.github/workflows/scripts/autoloop_scheduler.py
@@ -41,6 +41,7 @@
 import re
 import sys
 import urllib.error
+import urllib.parse
 import urllib.request
 from datetime import datetime, timedelta, timezone
 
@@ -345,6 +346,87 @@ def _parse_target_metric_from_file(path):
         return None
 
 
+# ---------------------------------------------------------------------------
+# Existing PR lookup (single-PR-per-program invariant)
+# ---------------------------------------------------------------------------
+
+
+def _http_get_json(url, headers, timeout=30):
+    """Open ``url`` and return ``(parsed_body, link_header)``.
+
+    Returns ``(None, None)`` on any HTTP/network error so callers can fall
+    through to the next strategy. Broken out into a module-level helper so
+    tests can monkey-patch it without touching ``urllib`` directly.
+    """
+    try:
+        req = urllib.request.Request(url, headers=headers)
+        with urllib.request.urlopen(req, timeout=timeout) as resp:
+            body = json.loads(resp.read().decode())
+            link_header = resp.headers.get("link") or resp.headers.get("Link")
+            return body, link_header
+    except (urllib.error.URLError, urllib.error.HTTPError, ValueError, OSError):
+        return None, None
+
+
+def find_existing_pr_for_branch(repo, program_name, github_token, http_get_json=_http_get_json):
+    """Look up the open draft PR (if any) for ``autoloop/{program_name}``.
+
+    Returns the PR number, or ``None`` if none is found.
+
+    The single-PR-per-program invariant requires that we never open a second
+    draft PR for the same program. The agent uses the returned ``existing_pr``
+    to decide between ``create-pull-request`` (only if ``None``) and
+    ``push-to-pull-request-branch`` (always preferred when an open PR exists).
+
+    We also tolerate legacy framework-suffixed branch names of the form
+    ``autoloop/{program}-<6-40 hex chars>`` so installations upgrading from
+    before ``preserve-branch-name: true`` was set find their in-flight PR
+    rather than opening a second one.
+    """
+    if not repo or not program_name or not github_token:
+        return None
+    owner = repo.split("/", 1)[0]
+    canonical_branch = "autoloop/{}".format(program_name)
+    headers = {
+        "Authorization": "token {}".format(github_token),
+        "Accept": "application/vnd.github.v3+json",
+    }
+    # Strategy 1: exact canonical branch name via the head= filter.
+    head_q = urllib.parse.quote("{}:{}".format(owner, canonical_branch), safe="")
+    url = "https://api.github.com/repos/{}/pulls?head={}&state=open".format(repo, head_q)
+    body, _ = http_get_json(url, headers)
+    if isinstance(body, list) and body:
+        first = body[0]
+        if isinstance(first, dict) and first.get("number"):
+            return first["number"]
+
+    # Strategy 2: paginate open PRs and match either a legacy framework-suffixed
+    # branch (``autoloop/{name}-<6-40 hex>``) or a ``[Autoloop: {name}]`` title prefix.
+    suffix_regex = re.compile(
+        r"^autoloop/" + re.escape(program_name) + r"(-[0-9a-f]{6,40})?$"
+    )
+    title_prefix = "[Autoloop: {}]".format(program_name)
+    next_url = "https://api.github.com/repos/{}/pulls?state=open&per_page=100".format(repo)
+    while next_url:
+        body, link_header = http_get_json(next_url, headers)
+        if not isinstance(body, list):
+            break
+        for pr in body:
+            if not isinstance(pr, dict):
+                continue
+            head_ref = ""
+            head = pr.get("head") or {}
+            if isinstance(head, dict):
+                head_ref = head.get("ref") or ""
+            if suffix_regex.match(head_ref):
+                return pr.get("number")
+            title = pr.get("title")
+            if isinstance(title, str) and title.startswith(title_prefix):
+                return pr.get("number")
+        next_url = parse_link_header(link_header)
+    return None
+
+
 # ---------------------------------------------------------------------------
 # Selection
 # ---------------------------------------------------------------------------
@@ -441,7 +523,15 @@ def main():
         print("NO_PROGRAMS_FOUND")
         with open(OUTPUT_FILE, "w") as f:
             json.dump(
-                {"due": [], "skipped": [], "unconfigured": [], "no_programs": True}, f
+                {
+                    "due": [],
+                    "skipped": [],
+                    "unconfigured": [],
+                    "no_programs": True,
+                    "head_branch": None,
+                    "existing_pr": None,
+                },
+                f,
             )
         sys.exit(0)
 
@@ -513,6 +603,20 @@ def main():
     if forced_program and selected:
         print("FORCED: running program '{}' (manual dispatch)".format(forced_program))
 
+    # Look up the existing draft PR (if any) for the selected program, so the
+    # agent can enforce the single-PR-per-program invariant: never call
+    # create-pull-request when a PR for autoloop/{name} already exists.
+    # head_branch is always the canonical name (no suffix, no hash).
+    head_branch = None
+    existing_pr = None
+    if selected:
+        head_branch = "autoloop/{}".format(selected)
+        try:
+            existing_pr = find_existing_pr_for_branch(repo, selected, github_token)
+        except Exception as e:  # noqa: BLE001 -- best-effort lookup
+            print("  Warning: existing PR lookup failed for {}: {}".format(selected, e))
+            existing_pr = None
+
     result = {
         "selected": selected,
         "selected_file": selected_file,
@@ -525,6 +629,8 @@ def main():
         "skipped": skipped,
         "unconfigured": unconfigured,
         "no_programs": False,
+        "head_branch": head_branch,
+        "existing_pr": existing_pr,
     }
 
     with open(OUTPUT_FILE, "w") as f:
diff --git a/tests/test_scheduler_e2e.py b/tests/test_scheduler_e2e.py
index c9dc5a8..758cdf6 100644
--- a/tests/test_scheduler_e2e.py
+++ b/tests/test_scheduler_e2e.py
@@ -257,3 +257,24 @@ def test_no_programs_found(self, workdir):
         assert proc.returncode == 0, proc.stderr
         assert out["unconfigured"] == ["example"]
         assert out["selected"] is None
+
+    def test_head_branch_set_when_program_selected(self, workdir):
+        """``head_branch`` is exactly ``autoloop/{name}`` for the selected program."""
+        _write_program(workdir, "coverage")
+        proc, out = _run_scheduler(str(workdir))
+        assert proc.returncode == 0, proc.stderr
+        assert out["selected"] == "coverage"
+        assert out["head_branch"] == "autoloop/coverage"
+        # The bogus DNS repo means the PR API call fails → existing_pr is None.
+        assert out["existing_pr"] is None
+
+    def test_head_branch_null_when_nothing_selected(self, workdir):
+        """When no program is due, ``head_branch`` is ``null`` in the output."""
+        # Empty programs dir → bootstrap creates an unconfigured template, which
+        # does NOT count as selected. So head_branch should be null.
+        shutil.rmtree(workdir / ".autoloop" / "programs")
+        proc, out = _run_scheduler(str(workdir))
+        assert proc.returncode == 0, proc.stderr
+        assert out["selected"] is None
+        assert out["head_branch"] is None
+        assert out["existing_pr"] is None
diff --git a/tests/test_scheduling.py b/tests/test_scheduling.py
index 41dc204..7064391 100644
--- a/tests/test_scheduling.py
+++ b/tests/test_scheduling.py
@@ -770,3 +770,208 @@ def test_lock_creds_before_merge(self):
             f"'Configure Git credentials' (index {cred_idx}) must come before "
             f"merge step (index {merge_idx}). Steps: {steps}"
         )
+
+
+# ---------------------------------------------------------------------------
+# Single-PR-per-program invariant: safe-outputs config + existing_pr lookup
+# (issue: enforce single-PR-per-program invariant)
+# ---------------------------------------------------------------------------
+
+class TestSafeOutputsConfig:
+    """Verify the safe-outputs config that defends the single-PR invariant.
+
+    Without `preserve-branch-name: true`, the gh-aw framework auto-suffixes
+    branch names on every run, breaking the single-long-running-branch model.
+    Without `max: 1` on both create-pull-request and push-to-pull-request-branch,
+    the agent could emit a create+create or create+push pair in the same iteration.
+    """
+
+    def _frontmatter(self):
+        import os
+        wf_path = os.path.join(os.path.dirname(__file__), "..", "workflows", "autoloop.md")
+        with open(wf_path) as f:
+            content = f.read()
+        # Frontmatter is the first --- ... --- block
+        m = re.match(r"^---\s*\n(.*?)\n---\s*\n", content, re.DOTALL)
+        assert m, "Could not find YAML frontmatter in workflows/autoloop.md"
+        return m.group(1)
+
+    def test_create_pr_preserves_branch_name(self):
+        fm = self._frontmatter()
+        # create-pull-request block must contain preserve-branch-name: true
+        m = re.search(r"create-pull-request:\s*\n((?:\s{4}.*\n)+)", fm)
+        assert m, "Could not find create-pull-request block in safe-outputs"
+        block = m.group(1)
+        assert "preserve-branch-name: true" in block, (
+            "create-pull-request must set 'preserve-branch-name: true' to keep "
+            "the canonical branch name autoloop/{program}; otherwise gh-aw "
+            "appends a hex salt and breaks the single-PR invariant.\n"
+            f"Block: {block}"
+        )
+
+    def test_create_pr_max_is_one(self):
+        fm = self._frontmatter()
+        m = re.search(r"create-pull-request:\s*\n((?:\s{4}.*\n)+)", fm)
+        assert m
+        block = m.group(1)
+        assert re.search(r"^\s*max:\s*1\s*$", block, re.MULTILINE), (
+            "create-pull-request must set 'max: 1' — the invariant is one "
+            "safe-output of either create or push per iteration, never two.\n"
+            f"Block: {block}"
+        )
+
+    def test_push_to_pr_max_is_one(self):
+        fm = self._frontmatter()
+        m = re.search(r"push-to-pull-request-branch:\s*\n((?:\s{4}.*\n)+)", fm)
+        assert m, "Could not find push-to-pull-request-branch block in safe-outputs"
+        block = m.group(1)
+        assert re.search(r"^\s*max:\s*1\s*$", block, re.MULTILINE), (
+            "push-to-pull-request-branch must set 'max: 1'.\n"
+            f"Block: {block}"
+        )
+
+
+class TestProseGuidance:
+    """Verify the prose guidance enforcing the single-PR invariant is present."""
+
+    def _content(self):
+        import os
+        wf_path = os.path.join(os.path.dirname(__file__), "..", "workflows", "autoloop.md")
+        with open(wf_path) as f:
+            return f.read()
+
+    def test_branch_name_warning_present(self):
+        c = self._content()
+        assert "Branch Name Must Be Exact" in c, (
+            "Missing the 'Branch Name Must Be Exact' warning that tells the "
+            "agent to never use suffixed branch names."
+        )
+        assert "no suffixes" in c.lower(), "Warning should mention 'no suffixes'"
+
+    def test_common_mistakes_section_present(self):
+        c = self._content()
+        assert "## Common Mistakes to Avoid" in c, (
+            "Missing the 'Common Mistakes to Avoid' section."
+        )
+
+    def test_step5_uses_existing_pr(self):
+        c = self._content()
+        # Step 5 accept flow must reference existing_pr from autoloop.json
+        assert "existing_pr" in c, (
+            "Workflow prose must instruct the agent to consult the "
+            "`existing_pr` field from /tmp/gh-aw/autoloop.json."
+        )
+        assert "head_branch" in c, (
+            "Workflow prose must instruct the agent to use the `head_branch` "
+            "field from /tmp/gh-aw/autoloop.json."
+        )
+
+
+# ---------------------------------------------------------------------------
+# find_existing_pr_for_branch helper — tolerant lookup of the open draft PR
+# ---------------------------------------------------------------------------
+
+
+def _run_find_existing_pr(program, mock_responses):
+    """Invoke ``find_existing_pr_for_branch`` with a stubbed HTTP client.
+
+    ``mock_responses`` is a list of dicts: ``{ url_match, status, body, link }``.
+    The first entry whose ``url_match`` substring is contained in the requested
+    URL wins. The optional ``link`` field is returned as the Link response
+    header (used by pagination via ``parse_link_header``). ``status`` is kept
+    for parity with the previous JS-based stub but only the ``200`` path is
+    exercised — non-200 responses surface as ``(None, None)`` from the real
+    ``_http_get_json``, which the helper here mirrors when ``status != 200``.
+    """
+    def stub(url, headers, timeout=30):
+        for r in mock_responses:
+            if r["url_match"] in url:
+                if r.get("status", 200) != 200:
+                    return None, None
+                return r.get("body"), r.get("link")
+        return None, None
+
+    return autoloop_scheduler.find_existing_pr_for_branch(
+        "owner/repo", program, "TOKEN", http_get_json=stub
+    )
+
+
+class TestFindExistingPRForBranch:
+    """The tolerant PR lookup that closes the single-PR-per-program invariant."""
+
+    def test_returns_null_when_no_pr_exists(self):
+        # Strategy 1 returns []; strategy 2 returns []
+        responses = [
+            {"url_match": "head=owner%3Aautoloop%2Fcoverage", "status": 200, "body": []},
+            {"url_match": "/pulls?state=open", "status": 200, "body": []},
+        ]
+        assert _run_find_existing_pr("coverage", responses) is None
+
+    def test_finds_pr_with_canonical_branch_name(self):
+        responses = [
+            {
+                "url_match": "head=owner%3Aautoloop%2Fcoverage",
+                "status": 200,
+                "body": [{"number": 42, "head": {"ref": "autoloop/coverage"}, "title": "[Autoloop] x"}],
+            },
+        ]
+        assert _run_find_existing_pr("coverage", responses) == 42
+
+    def test_finds_pr_with_legacy_hex_suffix(self):
+        # Strategy 1 finds nothing (the open PR has a suffixed branch name);
+        # Strategy 2 falls back to listing all open PRs and matches the suffix regex.
+        responses = [
+            {"url_match": "head=owner%3Aautoloop%2Fcoverage", "status": 200, "body": []},
+            {
+                "url_match": "/pulls?state=open",
+                "status": 200,
+                "body": [
+                    {"number": 99, "head": {"ref": "autoloop/coverage-8724e9f9"}, "title": "[Autoloop] x"},
+                ],
+            },
+        ]
+        assert _run_find_existing_pr("coverage", responses) == 99
+
+    def test_finds_pr_via_title_prefix_fallback(self):
+        # Branch name doesn't match suffix pattern, but title prefix does
+        responses = [
+            {"url_match": "head=owner%3Aautoloop%2Fcoverage", "status": 200, "body": []},
+            {
+                "url_match": "/pulls?state=open",
+                "status": 200,
+                "body": [
+                    {"number": 7, "head": {"ref": "totally-different-branch"}, "title": "[Autoloop: coverage] iter 3"},
+                ],
+            },
+        ]
+        assert _run_find_existing_pr("coverage", responses) == 7
+
+    def test_does_not_match_unrelated_program(self):
+        # autoloop/coverage-extras is a different program, not a hex suffix
+        responses = [
+            {"url_match": "head=owner%3Aautoloop%2Fcoverage", "status": 200, "body": []},
+            {
+                "url_match": "/pulls?state=open",
+                "status": 200,
+                "body": [
+                    {"number": 11, "head": {"ref": "autoloop/coverage-extras"}, "title": "[Autoloop] other"},
+                ],
+            },
+        ]
+        assert _run_find_existing_pr("coverage", responses) is None
+
+    def test_does_not_match_other_program_with_similar_name(self):
+        # Program name with regex-special-ish characters (underscore is fine, but
+        # we want to make sure the regex is properly anchored to ^...$).
+        responses = [
+            {"url_match": "head=owner%3Aautoloop%2Fsignal_processing", "status": 200, "body": []},
+            {
+                "url_match": "/pulls?state=open",
+                "status": 200,
+                "body": [
+                    # Branch for a different program that happens to share a prefix
+                    {"number": 5, "head": {"ref": "autoloop/signal"}, "title": "[Autoloop] other"},
+                ],
+            },
+        ]
+        assert _run_find_existing_pr("signal_processing", responses) is None
diff --git a/workflows/autoloop.md b/workflows/autoloop.md
index 0d71270..fe5b570 100644
--- a/workflows/autoloop.md
+++ b/workflows/autoloop.md
@@ -44,11 +44,12 @@ safe-outputs:
     title-prefix: "[Autoloop] "
     labels: [automation, autoloop]
     protected-files: fallback-to-issue
-    max: 2
+    preserve-branch-name: true
+    max: 1
   push-to-pull-request-branch:
     target: "*"
     title-prefix: "[Autoloop] "
-    max: 2
+    max: 1
   create-issue:
     title-prefix: "[Autoloop] "
     labels: [automation, autoloop]
@@ -194,6 +195,8 @@ The pre-step has already determined which program to run. Read `/tmp/gh-aw/autol
 - **`skipped`**: Programs not due yet based on their per-program schedule.
 - **`no_programs`**: If `true`, no program files exist at all.
 - **`not_due`**: If `true`, programs exist but none are due for this run.
+- **`head_branch`**: The canonical long-running branch name for the selected program — always exactly `autoloop/{program-name}`, never with a suffix or hash. Use this value verbatim when creating, checking out, or pushing to the branch.
+- **`existing_pr`**: The number of the open draft PR for `autoloop/{program-name}`, or `null` if no PR exists yet. Use this to enforce the single-PR-per-program invariant — see [Step 5a: Push and wait for CI](#step-5a-push-and-wait-for-ci) and [Step 5c: Accept](#step-5c-accept).
 
 If `selected` is not null:
 1. Read the program file from the `selected_file` path.
@@ -297,6 +300,16 @@ Examples:
 - `autoloop/signal_processing`
 - `autoloop/coverage`
 
+> ⚠️ **CRITICAL — Branch Name Must Be Exact**
+>
+> The branch name is ALWAYS exactly `autoloop/{program-name}` — **no suffixes, no hashes, no run IDs, no iteration numbers, no random tokens**. Never create branches like:
+> - ❌ `autoloop/coverage-abc123`
+> - ❌ `autoloop/coverage-iter42-deadbeef`
+> - ❌ `autoloop/coverage-1234567890`
+>
+> **Never let the gh-aw framework auto-generate a branch name.** You must explicitly name the branch when creating it. The pre-step provides the canonical name in the `head_branch` field of `/tmp/gh-aw/autoloop.json` — always use that value verbatim.
+
+
 ### How It Works
 
 1. On the **first accepted iteration**, the branch is created from the default branch.
@@ -419,7 +432,10 @@ The accept path is split into three sub-steps: **5a (push and wait for CI)**, **
    - Commit message subject line: `[Autoloop: {program-name}] Iteration <N>: <short description>`
    - Commit message body (after a blank line): `Run: {run_url}` referencing the GitHub Actions run URL.
 2. Push the commit to the long-running branch.
-3. If a draft PR does not already exist for this branch, create it now (see Step 5c for the title/body format). The PR is needed so that CI runs and so `gh pr checks` has a target.
+3. **Find or create the PR** so CI runs and `gh pr checks` has a target. Follow these steps in order:
+   a. Check `existing_pr` from `/tmp/gh-aw/autoloop.json`. If it is not null, that is the existing draft PR — use it as `$EXISTING_PR` below; **never** call `create-pull-request`.
+   b. If `existing_pr` is null, also check the `PR` field in the state file's **⚙️ Machine State** table as a fallback. Verify it is still open via the GitHub API; if it has been closed or merged, treat it as if no PR exists and proceed to step (c).
+   c. If no PR exists (both sources are null): create one with `create-pull-request`, specifying `branch: autoloop/{program-name}` (the value of `head_branch` from `autoloop.json`) explicitly — do not let the framework auto-generate a branch name. See Step 5c for the title/body format.
 4. Wait for CI on the new HEAD and reduce all check-runs to a single status — `success`, `failure`, or `pending`:
 
    ```bash
@@ -458,10 +474,10 @@ If `status == "failure"`, **fix and retry — do not revert, do not accept**:
 **Only entered when `status == "success"`** from Step 5a (possibly after one or more fix attempts in Step 5b).
 
 1. The commit(s) are already on the long-running branch (pushed in Step 5a / 5b). No further pushing needed.
-2. If a draft PR does not already exist for this branch, create one:
+2. If a draft PR does not already exist for this branch (i.e., `existing_pr` from `autoloop.json` is null AND the state file's `PR` field is null or refers to a closed PR), create one — specify `branch: autoloop/{program-name}` (the value of `head_branch` from `autoloop.json`) explicitly so the framework does not auto-generate a branch name:
    - Title: `[Autoloop: {program-name}]`
    - Body includes: a summary of the program goal, link to the program issue, the current best metric, and AI disclosure: `🤖 *This PR is maintained by Autoloop. Each accepted iteration adds a commit to this branch.*`
-   If a draft PR already exists, update the PR body with the latest metric and a summary of the most recent accepted iteration. Add a comment to the PR summarizing the iteration: what changed, old metric, new metric, improvement delta, the **fix-attempt count** if `> 0`, and a link to the actions run.
+   If a draft PR already exists, use `push-to-pull-request-branch` (never `create-pull-request`). Update the PR body with the latest metric and a summary of the most recent accepted iteration. Add a comment to the PR summarizing the iteration: what changed, old metric, new metric, improvement delta, the **fix-attempt count** if `> 0`, and a link to the actions run.
 4. Ensure the program issue exists (see [Program Issue](#program-issue) below) — for file-based programs that have no program issue yet (`selected_issue` is null in `/tmp/gh-aw/autoloop.json`), create one and record its number in the state file's `Issue` field.
 5. Update the state file `{program-name}.md` in the repo-memory folder:
    - Update the **⚙️ Machine State** table: reset `consecutive_errors` to 0, set `best_metric`, increment `iteration_count`, set `last_run` to current UTC timestamp, append `"accepted"` to `recent_statuses` (keep last 10), set `paused` to false.
@@ -802,3 +818,17 @@ After each iteration, prepend an entry to the **📊 Iteration History** section
 - **Safety.** Never modify files outside the target list. Never modify the evaluation script. Never modify the program definition (except via `/autoloop` command mode).
 - **Read AGENTS.md first**: before starting work, read the repository's `AGENTS.md` file (if present) to understand project-specific conventions.
 - **Build and test**: run any build/test commands before creating PRs.
+
+## Common Mistakes to Avoid
+
+> ❌ **Do NOT create a new branch with a suffix for each iteration.**
+> Correct: `autoloop/coverage`
+> Wrong: `autoloop/coverage-abc123`, `autoloop/coverage-iter42`, `autoloop/coverage-deadbeef1234`
+> Use the `head_branch` field from `/tmp/gh-aw/autoloop.json` — it is always the canonical name. Never let the gh-aw framework auto-generate a branch name.
+
+> ❌ **Do NOT create a new PR if one already exists for `autoloop/{program-name}`.**
+> The pre-step provides `existing_pr` in `/tmp/gh-aw/autoloop.json`. If it is not null, **always** use `push-to-pull-request-branch` — never call `create-pull-request`. Only create a PR when `existing_pr` is null AND the state file's `PR` field is also null (or refers to a closed PR).
+
+> ❌ **Do NOT modify files outside the program's Target list.**
+> The Target section of the program file is the allowlist. Touching anything else (including the evaluation script or the program file itself) is forbidden.
+
diff --git a/workflows/scripts/autoloop_scheduler.py b/workflows/scripts/autoloop_scheduler.py
index 5066afe..d29509b 100644
--- a/workflows/scripts/autoloop_scheduler.py
+++ b/workflows/scripts/autoloop_scheduler.py
@@ -41,6 +41,7 @@
 import re
 import sys
 import urllib.error
+import urllib.parse
 import urllib.request
 from datetime import datetime, timedelta, timezone
 
@@ -345,6 +346,87 @@ def _parse_target_metric_from_file(path):
         return None
 
 
+# ---------------------------------------------------------------------------
+# Existing PR lookup (single-PR-per-program invariant)
+# ---------------------------------------------------------------------------
+
+
+def _http_get_json(url, headers, timeout=30):
+    """Open ``url`` and return ``(parsed_body, link_header)``.
+
+    Returns ``(None, None)`` on any HTTP/network error so callers can fall
+    through to the next strategy. Broken out into a module-level helper so
+    tests can monkey-patch it without touching ``urllib`` directly.
+    """
+    try:
+        req = urllib.request.Request(url, headers=headers)
+        with urllib.request.urlopen(req, timeout=timeout) as resp:
+            body = json.loads(resp.read().decode())
+            link_header = resp.headers.get("link") or resp.headers.get("Link")
+            return body, link_header
+    except (urllib.error.URLError, urllib.error.HTTPError, ValueError, OSError):
+        return None, None
+
+
+def find_existing_pr_for_branch(repo, program_name, github_token, http_get_json=_http_get_json):
+    """Look up the open draft PR (if any) for ``autoloop/{program_name}``.
+
+    Returns the PR number, or ``None`` if none is found.
+
+    The single-PR-per-program invariant requires that we never open a second
+    draft PR for the same program. The agent uses the returned ``existing_pr``
+    to decide between ``create-pull-request`` (only if ``None``) and
+    ``push-to-pull-request-branch`` (always preferred when an open PR exists).
+
+    We also tolerate legacy framework-suffixed branch names of the form
+    ``autoloop/{program}-<6-40 hex chars>`` so installations upgrading from
+    before ``preserve-branch-name: true`` was set find their in-flight PR
+    rather than opening a second one.
+    """
+    if not repo or not program_name or not github_token:
+        return None
+    owner = repo.split("/", 1)[0]
+    canonical_branch = "autoloop/{}".format(program_name)
+    headers = {
+        "Authorization": "token {}".format(github_token),
+        "Accept": "application/vnd.github.v3+json",
+    }
+    # Strategy 1: exact canonical branch name via the head= filter.
+    head_q = urllib.parse.quote("{}:{}".format(owner, canonical_branch), safe="")
+    url = "https://api.github.com/repos/{}/pulls?head={}&state=open".format(repo, head_q)
+    body, _ = http_get_json(url, headers)
+    if isinstance(body, list) and body:
+        first = body[0]
+        if isinstance(first, dict) and first.get("number"):
+            return first["number"]
+
+    # Strategy 2: paginate open PRs and match either a legacy framework-suffixed
+    # branch (``autoloop/{name}-<6-40 hex>``) or a ``[Autoloop: {name}]`` title prefix.
+    suffix_regex = re.compile(
+        r"^autoloop/" + re.escape(program_name) + r"(-[0-9a-f]{6,40})?$"
+    )
+    title_prefix = "[Autoloop: {}]".format(program_name)
+    next_url = "https://api.github.com/repos/{}/pulls?state=open&per_page=100".format(repo)
+    while next_url:
+        body, link_header = http_get_json(next_url, headers)
+        if not isinstance(body, list):
+            break
+        for pr in body:
+            if not isinstance(pr, dict):
+                continue
+            head_ref = ""
+            head = pr.get("head") or {}
+            if isinstance(head, dict):
+                head_ref = head.get("ref") or ""
+            if suffix_regex.match(head_ref):
+                return pr.get("number")
+            title = pr.get("title")
+            if isinstance(title, str) and title.startswith(title_prefix):
+                return pr.get("number")
+        next_url = parse_link_header(link_header)
+    return None
+
+
 # ---------------------------------------------------------------------------
 # Selection
 # ---------------------------------------------------------------------------
@@ -441,7 +523,15 @@ def main():
         print("NO_PROGRAMS_FOUND")
         with open(OUTPUT_FILE, "w") as f:
             json.dump(
-                {"due": [], "skipped": [], "unconfigured": [], "no_programs": True}, f
+                {
+                    "due": [],
+                    "skipped": [],
+                    "unconfigured": [],
+                    "no_programs": True,
+                    "head_branch": None,
+                    "existing_pr": None,
+                },
+                f,
             )
         sys.exit(0)
 
@@ -513,6 +603,20 @@ def main():
     if forced_program and selected:
         print("FORCED: running program '{}' (manual dispatch)".format(forced_program))
 
+    # Look up the existing draft PR (if any) for the selected program, so the
+    # agent can enforce the single-PR-per-program invariant: never call
+    # create-pull-request when a PR for autoloop/{name} already exists.
+    # head_branch is always the canonical name (no suffix, no hash).
+    head_branch = None
+    existing_pr = None
+    if selected:
+        head_branch = "autoloop/{}".format(selected)
+        try:
+            existing_pr = find_existing_pr_for_branch(repo, selected, github_token)
+        except Exception as e:  # noqa: BLE001 -- best-effort lookup
+            print("  Warning: existing PR lookup failed for {}: {}".format(selected, e))
+            existing_pr = None
+
     result = {
         "selected": selected,
         "selected_file": selected_file,
@@ -525,6 +629,8 @@ def main():
         "skipped": skipped,
         "unconfigured": unconfigured,
         "no_programs": False,
+        "head_branch": head_branch,
+        "existing_pr": existing_pr,
     }
 
     with open(OUTPUT_FILE, "w") as f: