fix(runner): skip parallel_tool_calls default for WebSearch agents on Anthropic

api/scripts/run_pydantic.py

@@ -1082,15 +1082,38 @@ def build_agent(
     name = spec.get("name")
     if isinstance(name, str) and name.strip():
         kwargs["name"] = name
+    # Provider-adaptive capabilities from `capabilities:` (e.g. WebSearch).
+    # Built before model settings because WebSearch changes which settings are
+    # legal (see below).
+    capabilities = _build_capabilities(spec)
+    has_web_search = any(type(c).__name__ == "WebSearch" for c in capabilities)
+
     # Default to SEQUENTIAL tool calls. parallel_tool_calls=False is a real,
     # API-level limiter (OpenAI parallel_tool_calls / Anthropic
     # disable_parallel_tool_use): the model emits one tool call per turn instead
     # of fanning out many at once, which is what was getting Attio (and other
     # providers) rate-limited. An agent that genuinely needs parallel calls can
     # opt back in with model_settings.parallel_tool_calls: true in its spec.
+    # EXCEPT WebSearch on Anthropic: the current web_search tool runs with
+    # server-side programmatic tool calling, and the API rejects
+    # `tool_choice.disable_parallel_tool_use: true` combined with it (400) as
+    # soon as the agent also has any client/MCP tool. Skip the default there —
+    # pydantic-ai only emits disable_parallel_tool_use when the key is present.
+    anthropic_web_search = (
+        has_web_search and isinstance(model, str) and model.startswith("anthropic:")
+    )
     model_settings = spec.get("model_settings")
     ms = dict(model_settings) if isinstance(model_settings, dict) else {}
-    ms.setdefault("parallel_tool_calls", False)
+    if not anthropic_web_search:
+        ms.setdefault("parallel_tool_calls", False)
+    elif "parallel_tool_calls" in ms:
+        print(
+            "[capabilities] WebSearch on Anthropic is incompatible with the "
+            "parallel_tool_calls setting (programmatic tool calling); "
+            "dropping it for this run",
+            file=sys.stderr,
+        )
+        ms.pop("parallel_tool_calls", None)
     # Anthropic prompt caching. An agentic run re-sends the whole prompt every
     # step; without caching the big static prefix — system instructions + the
     # MCP/Composio tool schemas — is re-billed at full input rate on each of the
@@ -1129,10 +1152,7 @@ def build_agent(
     if tools:
         kwargs["tools"] = tools
 
-    # Provider-adaptive capabilities from `capabilities:` (e.g. WebSearch).
-    # These are model-side abilities (not MCP/sidecar) — pydantic-ai uses the
-    # provider's native implementation where available, with a local fallback.
-    capabilities = _build_capabilities(spec)
+    # Capabilities were built up top (before model settings); attach them here.
     if capabilities:
         kwargs["capabilities"] = capabilities
 

api/tests/test_run_pydantic_build_agent.py

@@ -100,6 +100,43 @@ def test_build_agent_attaches_websearch_capability() -> None:
     assert isinstance(agent, Agent)
 
 
+def test_websearch_on_anthropic_skips_parallel_tool_calls_setting() -> None:
+    # Anthropic's web_search tool uses server-side programmatic tool calling,
+    # which the API rejects in combination with disable_parallel_tool_use
+    # (400 as soon as the agent also has a client/MCP tool). The sequential
+    # default must not apply — and an explicit spec value must be dropped —
+    # for WebSearch agents on Anthropic, while non-WebSearch agents keep it.
+    ws = run_pydantic.build_agent(
+        {
+            "name": "searcher",
+            "model": "anthropic:claude-sonnet-5",
+            "instructions": "Search.",
+            "capabilities": ["WebSearch"],
+        }
+    )
+    assert "parallel_tool_calls" not in ws.model_settings
+
+    ws_explicit = run_pydantic.build_agent(
+        {
+            "name": "searcher-explicit",
+            "model": "anthropic:claude-sonnet-5",
+            "instructions": "Search.",
+            "capabilities": ["WebSearch"],
+            "model_settings": {"parallel_tool_calls": False},
+        }
+    )
+    assert "parallel_tool_calls" not in ws_explicit.model_settings
+
+    plain = run_pydantic.build_agent(
+        {
+            "name": "plain",
+            "model": "anthropic:claude-sonnet-5",
+            "instructions": "Reply.",
+        }
+    )
+    assert plain.model_settings["parallel_tool_calls"] is False
+
+
 def test_build_agent_with_scaledown_enabled(monkeypatch: pytest.MonkeyPatch) -> None:
     # scaledown attaches its compressor as a ProcessHistory capability
     # (pydantic-ai 2.x dropped Agent(history_processors=...)).

web/src/lib/cap-api.ts

@@ -12,11 +12,13 @@ import type { CommitMode } from "@/lib/commit-mode-constants";
 
 // Thin client for the Tembo Coding Agent Platform task API. The task
 // endpoints live under the **/public-api** namespace and authenticate
-// with the workspace's Tembo API key as `Authorization: Bearer` — the
-// bare `/task/create` path hits a different internal auth gate that
-// rejects the public key ("Invalid token"). POSTs a free-text prompt +
-// repo URL to POST /public-api/task/create and returns a task record
-// with an htmlUrl the user can follow; the task is what opens the PR.
+// with the workspace's Tembo API key as `Authorization: Bearer`. POSTs
+// a free-text prompt + repo URL to POST /public-api/session/create and
+// returns a task record with an htmlUrl the user can follow; the task
+// is what opens the PR. CAP renamed the mount from /public-api/task to
+// /public-api/session with no alias (tembo/monorepo#9519, 2026-07-16);
+// the old path falls through to a catch-all that 400s with
+// {"error":{"message":"invalid request path"}}.
 
 const DEFAULT_TEMBO_API_URL = "https://api.tembo.io";
 
@@ -61,7 +63,7 @@ export async function createTemboTask(args: {
     queueRightAway: true,
   };
 
-  const url = `${baseUrl}/public-api/task/create`;
+  const url = `${baseUrl}/public-api/session/create`;
   // Breadcrumb only — never log `body`: it embeds the prompt (run input/output,
   // user data) and would leak to plaintext container logs / aggregators (#44).
   console.log("[cap] POST", url);