agent: unattended Phase 2 loop — profile to gated verdict, zero false accepts

The Claude Agent SDK loop (agent/hotpath_agent/loop.py, model
claude-fable-5) drives the seven harness tools over a standalone stdio
MCP server (mcp_server.py) and runs the full profile -> hypothesize ->
patch -> gated-verdict cycle on comrak with no human in the loop.

Three audited attempts, all honest rejections, ZERO false accepts:
- phase2-comrak-001 class 3: syntect buffer with_capacity, [0.971,1.007]
- phase2-comrak-002 class 5: gate dead VecDeque build, [0.982,1.026]
- phase2-comrak-003 class 6: hoist find_special_char check, [0.970,1.007]
The agent picked the cheapest untried class each time (3->5->6) and
TWICE refused to fabricate a verdict it couldn't read back, verifying
ledger integrity instead — the anti-reward-hacking behavior SPEC §10
targets.

Harness changes surfaced by the loop:
- run_verdict is now async: launches the multi-minute build+bench
  detached (past the 60s MCP transport cap) and returns immediately;
  new read_verdict op polls the append-only ledger row (ledger
  verdict_summary). Fixes attempt 001's unreadable-verdict gap.
- read_target_source takes offset/limit line windows so 100KB+ files
  page instead of overflowing — removes the agent's reason to reach for
  a shell (attempt 003 paged a 2400-line file cleanly, no shell).

Honest boundary note (case study + code comments): in THIS dev
container the SDK tool allow/deny layer is defense-in-depth, not the
trust boundary — a nested CLI as root under a parent session doesn't
reliably gate built-ins. SPEC §10's boundary is OS process/user
isolation; that hardening is the remaining Phase 2 exit item
(docs/infra.md). What holds regardless: propose_patch is the only
mediated write path (path-allowlist + git apply) and the ledger is
append-only.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EcKPWQ3MQD4NdEHwar96u2

agent/hotpath_agent/loop.py

@@ -0,0 +1,114 @@
+"""Phase 2 agent loop (SPEC §3.5): Claude Agent SDK driving the six
+harness tools over a standalone stdio MCP server. The agent gets NO
+built-in tools — no shell, no file writes; every capability is one of
+the six IPC operations, and the trust layer's allowlist/gates remain
+the hard boundary (SPEC §10).
+
+Run one attempt:
+    python -m hotpath_agent.loop <target> --run-id <id> --repo-root <path>
+"""
+
+from __future__ import annotations
+
+import argparse
+import asyncio
+import os
+import sys
+
+from claude_agent_sdk import (
+    AssistantMessage,
+    ClaudeAgentOptions,
+    ResultMessage,
+    TextBlock,
+    query,
+)
+
+from .prompts import SYSTEM_PROMPT
+
+HARNESS_TOOLS = [
+    "mcp__harness__read_profile",
+    "mcp__harness__read_ledger",
+    "mcp__harness__read_playbook",
+    "mcp__harness__read_target_source",
+    "mcp__harness__propose_patch",
+    "mcp__harness__run_verdict",
+    "mcp__harness__read_verdict",
+]
+
+# Built-in tools we deny at the SDK layer. This is DEFENSE IN DEPTH, not the
+# trust boundary. SPEC §10's real boundary is OS-level process/user isolation:
+# harnessd runs as a separate process the agent can't bypass, target code runs
+# in a no-net container. In THIS dev container the nested agent runs as root
+# under a parent Claude Code session, so neither --dangerously-skip-permissions
+# nor a can_use_tool callback reliably gates built-ins (settings inherited from
+# the parent session shadow them). We enumerate what we can; production runs the
+# agent process without host-shell capability at all. What holds regardless:
+# propose_patch is the only harness-mediated write path (path-allowlist + git
+# apply), and the ledger is append-only (mutation-refusing triggers).
+BUILTIN_TOOLS_DENIED = [
+    "Bash", "Read", "Write", "Edit", "Glob", "Grep",
+    "WebSearch", "WebFetch", "Task", "TodoWrite",
+]
+
+
+async def run_attempt(target: str, run_id: str, max_turns: int, repo_root: str) -> None:
+    root = os.path.abspath(repo_root)
+    options = ClaudeAgentOptions(
+        model="claude-fable-5",
+        cwd=root,
+        mcp_servers={
+            "harness": {
+                "type": "stdio",
+                "command": sys.executable,
+                "args": ["-m", "hotpath_agent.mcp_server"],
+                "env": {**os.environ, "HOTPATH_REPO_ROOT": root, "PYTHONPATH": os.path.join(root, "agent")},
+            }
+        },
+        allowed_tools=HARNESS_TOOLS,
+        disallowed_tools=BUILTIN_TOOLS_DENIED,  # defense in depth; see note above
+        system_prompt=SYSTEM_PROMPT,
+        # acceptEdits (not bypassPermissions — the CLI refuses
+        # --dangerously-skip-permissions under root) with the allowlist runs the
+        # loop headless. The real boundary is OS isolation, not this flag.
+        permission_mode="acceptEdits",
+        max_turns=max_turns,
+    )
+    task = (
+        f"Perform ONE optimization attempt on target '{target}'. You have NO shell, file, or "
+        f"web tools — only the seven mcp__harness__ tools. Do not attempt any other tool.\n"
+        f"1. read_ledger to see attempted classes; read_profile for hotspots.\n"
+        f"2. Pick the cheapest UNTRIED playbook class whose preconditions the profile "
+        f"satisfies; read_playbook for it.\n"
+        f"3. Read the relevant source with read_target_source — for large files pass "
+        f"offset/limit and page through in windows (do NOT attempt any shell). State your "
+        f"hypothesis, then propose_patch (unified diff, a/ b/ prefixes, paths relative to the "
+        f"target workspace root).\n"
+        f"4. run_verdict with run_id '{run_id}', the class number, and the hotspot string. "
+        f"It launches a multi-minute pipeline and returns immediately.\n"
+        f"5. Poll read_verdict('{run_id}') until it returns a verdict (not a 'running' status). "
+        f"The pipeline takes several minutes; keep polling patiently.\n"
+        f"6. Report the verdict and speedup CI verbatim. A rejection with a clean ledger row is a "
+        f"successful outcome — do not iterate more than twice on a rejected hypothesis."
+    )
+
+    async for message in query(prompt=task, options=options):
+        if isinstance(message, AssistantMessage):
+            for block in message.content:
+                if isinstance(block, TextBlock):
+                    print(f"[agent] {block.text}", flush=True)
+        elif isinstance(message, ResultMessage):
+            print(f"[result] turns={message.num_turns} cost_usd={message.total_cost_usd}", flush=True)
+
+
+def main() -> None:
+    p = argparse.ArgumentParser()
+    p.add_argument("target")
+    p.add_argument("--run-id", required=True)
+    p.add_argument("--max-turns", type=int, default=40)
+    p.add_argument("--repo-root", default=".")
+    a = p.parse_args()
+    asyncio.run(run_attempt(a.target, a.run_id, a.max_turns, a.repo_root))
+
+
+if __name__ == "__main__":
+    main()