chore: remove default_cluster_b_url + CLUSTER_B_URL env var (langgraph-demo leftover)

`default_cluster_b_url()` was extracted verbatim from the langgraph-demo
monorepo (per CHANGELOG: 'Initial extraction from the langraph-demo monorepo').
Two problems:

1. The localhost:8082 default presumes a specific deployment shape (the
   original two-cluster demo); it is meaningless to a fresh SDK user.
2. 'cluster B' is internal terminology a user reading the SDK has no way to
   understand without context.

Configuration of where YOUR receiver lives belongs in your application, not
the SDK. `post_handoff` already takes the URL as a positional arg — drop
the convenience helper, drop the env var, rename the arg from
`cluster_b_url` to `receiver_url` for clarity.

Breaking for callers that import `default_cluster_b_url` or pass
`cluster_b_url=...` as a keyword arg. CHANGELOG entry added.

Author: aural-psynapse

CHANGELOG.md

@@ -3,6 +3,8 @@
 ## Unreleased
 
 - `trusted_endpoints`: registered URLs may now contain FastAPI/Express-style path placeholders. `{id}` matches exactly one path segment, `{rest:path}` matches any subtree. Plain URLs without `{` keep exact-match semantics — no migration needed for existing rows. Both `is_trusted_endpoint` and the snapshot tamper-check inside `evaluate_handoff` honor the new syntax. Closes #14.
+- README: new "Getting `PROVABLY_API_KEY` and `PROVABLY_ORG_ID`" subsection walking through sign-up at app.provably.ai → create org → Integrations menu, plus a pointer to provably.ai/docs.
+- **BREAKING:** removed `default_cluster_b_url()` and the `CLUSTER_B_URL` env var — leftovers from the langgraph-demo monorepo extraction with a `localhost:8082` default and opaque "cluster B" naming the SDK has no business assuming. `post_handoff(receiver_url, payload)` (positional arg renamed from `cluster_b_url`) takes the URL directly — supply it from your application's own configuration.
 
 ## 0.2.0
 

README.md

@@ -175,7 +175,6 @@ Full product docs: [provably.ai/docs](https://provably.ai/docs).
 | `PROVABLY_RUST_BE_URL` | `initialize_runtime`, evaluator | yes |
 | `POSTGRES_URL` | intercept storage, trusted endpoints, handoff preprocess | yes |
 | `PROVABLY_APP_UI_URL` | optional UI deep-links | no |
-| `CLUSTER_B_URL` | `default_cluster_b_url()` helper only | no |
 | `PROVABLY_QUERY_RESOLVE_MAX_WAIT_S` | max seconds to wait for a query record to appear (default 15) | no |
 
 `POSTGRES_URL` is a hard dependency today. Three SDK modules open Postgres
@@ -266,11 +265,11 @@ on any non-2xx response.
 interceptor's in-memory state — no manual claim construction needed:
 
 ```python
-from provably import build_handoff_payload, post_handoff, default_cluster_b_url
+from provably import build_handoff_payload, post_handoff
 
 # fetch_and_claim is the raw JSON dict the LLM emitted
 payload = build_handoff_payload(fetch_and_claim, run_id="run-001")
-post_handoff(default_cluster_b_url(), payload)
+post_handoff("https://your-verifier.example.com", payload)
 ```
 
 `claim_contract` generates the system-prompt text that tells an LLM how to
@@ -397,7 +396,7 @@ from provably import (
     HandoffPayload, HandoffClaim, HandoffProofAction, HandoffProofBundle,
     BenchmarkRow, Outcome, VerificationMode,
     # handoff transport
-    post_handoff, default_cluster_b_url,
+    post_handoff,
     # handoff builders
     build_handoff_payload, DEFAULT_HANDOFF_TASK,
     claim_contract, default_instructions, field_descriptions,

docs/architecture.md

@@ -21,7 +21,7 @@ src/provably/
     __init__.py
     client.py             initialize_runtime
     types.py              HandoffPayload v2, HandoffClaim, etc.
-    transport.py          post_handoff, default_cluster_b_url
+    transport.py          post_handoff
     evaluator.py          evaluate_handoff, extract_indexed_from_query_record
     eval_modes.py         the four verification modes
     json_utils.py         canonical_json

docs/handoff.md

@@ -67,9 +67,9 @@ post_handoff(
 There is no retry, no batching, no fallback. Failures bubble up as
 `httpx.HTTPError` / `httpx.HTTPStatusError`.
 
-`default_cluster_b_url()` is a small convenience that returns
-`os.getenv("CLUSTER_B_URL", "http://localhost:8082")` with whitespace and
-trailing-slash trimming. Use it where it helps; ignore it otherwise.
+The `receiver_url` is supplied by the caller — the SDK does not read it from the
+environment or assume any default. Configuration of where YOUR verifier lives
+belongs in your application, not the SDK.
 
 ## Eval comparison modes
 

src/provably/__init__.py

@@ -6,7 +6,7 @@
 from provably.handoff.guide import default_instructions, field_descriptions
 from provably.handoff.outcomes import aggregate_outcome, outcome_from_trace
 from provably.handoff.payload_builder import DEFAULT_HANDOFF_TASK, build_handoff_payload
-from provably.handoff.transport import default_cluster_b_url, post_handoff
+from provably.handoff.transport import post_handoff
 from provably.handoff.types import (
     BenchmarkRow,
     HandoffClaim,
@@ -49,7 +49,6 @@
     "check_claim_endpoints_are_trusted",
     "claim_contract",
     "configure_indexing",
-    "default_cluster_b_url",
     "default_instructions",
     "disable",
     "enable",

src/provably/handoff/transport.py

@@ -1,7 +1,5 @@
 from __future__ import annotations
 
-import os
-
 import httpx
 
 from provably.handoff.types import HandoffPayload
@@ -11,15 +9,21 @@
 
 
 def post_handoff(
-    cluster_b_url: str,
+    receiver_url: str,
     handoff_payload: HandoffPayload,
     *,
     headers: dict[str, str] | None = None,
     timeout_s: float = 120.0,
 ) -> None:
-    base = (cluster_b_url or "").strip().rstrip("/")
+    """POST a serialized ``HandoffPayload`` to ``{receiver_url}/handoffs/receive``.
+
+    The receiver is whatever service runs ``evaluate_handoff`` on the payload — typically
+    a separate verifier in a two-service deployment, but the SDK has no opinion on its
+    location: ``receiver_url`` is supplied by the caller, never read from the environment.
+    """
+    base = (receiver_url or "").strip().rstrip("/")
     if not base:
-        raise ValueError("cluster_b_url is empty — set CLUSTER_B_URL to post handoff")
+        raise ValueError("receiver_url is empty — pass the verifier's base URL to post_handoff")
     url = f"{base}/handoffs/receive"
     body = handoff_payload.model_dump(mode="json")
     hdrs = {"Content-Type": "application/json", **(headers or {})}
@@ -29,7 +33,3 @@ def post_handoff(
     except Exception as e:
         _log.error("post_handoff_failed", url=url, error=str(e))
         raise
-
-
-def default_cluster_b_url() -> str:
-    return (os.getenv("CLUSTER_B_URL") or "http://localhost:8082").strip().rstrip("/")

tests/unit/test_transport.py

@@ -4,20 +4,10 @@
 
 import pytest
 
-from provably.handoff.transport import default_cluster_b_url, post_handoff
+from provably.handoff.transport import post_handoff
 from provably.handoff.types import HandoffPayload
 
 
-def test_default_cluster_b_url_env(monkeypatch: pytest.MonkeyPatch) -> None:
-    monkeypatch.setenv("CLUSTER_B_URL", "http://custom:9999/")
-    assert default_cluster_b_url() == "http://custom:9999"
-
-
-def test_default_cluster_b_url_fallback(monkeypatch: pytest.MonkeyPatch) -> None:
-    monkeypatch.delenv("CLUSTER_B_URL", raising=False)
-    assert default_cluster_b_url() == "http://localhost:8082"
-
-
 def test_post_handoff_empty_url_raises() -> None:
     with pytest.raises(ValueError, match="empty"):
         post_handoff("", HandoffPayload())