tools: expose a2a_pack source to the inner LLM loop

list_a2a_pack(subdir) / read_a2a_pack(path) let the builder browse the
installed SDK at runtime — the same package the scaffolded agents
import. Path-traversal guarded; reads capped at 64 KiB.

Why: the system prompt only carries an abridged example. When the LLM
needed to recall what RunContext exposes (ctx.workspace API, ctx.ask
vs ctx.collect, runtime.apt_packages…) it would guess. Now it can
read the actual source.

Prompt updated to point at the new tools.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

agent_builder/tools.py

@@ -352,9 +352,91 @@ def build_tools(ctx: ToolContext) -> list[Any]:
             "card": body.get("card") or {},
         })
 
+    @tool
+    def list_a2a_pack(subdir: str = "") -> str:
+        """List ``.py`` files under the installed ``a2a_pack`` package.
+
+        Use this when you need to recall what the SDK actually exposes
+        — what decorators exist, what ``RunContext`` methods are
+        available, how ``WorkspaceClient`` is shaped, what the
+        ``AgentRuntime`` fields are, etc. The agent code you scaffold
+        runs on this exact package, so reading it is the authoritative
+        reference (the system-prompt examples are intentionally
+        abridged).
+
+        ``subdir`` is an optional path segment relative to the package
+        root (``""``, ``"serve"``, ``"mcp"`` …). Returns JSON:
+        ``{root, files: [{path, size}]}``.
+        """
+        try:
+            import a2a_pack
+            import os as _os
+            base = _os.path.dirname(_os.path.abspath(a2a_pack.__file__))
+        except Exception as exc:  # noqa: BLE001
+            return json.dumps({"error": f"a2a_pack not importable: {exc}"})
+        if subdir.startswith("/") or ".." in subdir.split("/"):
+            return json.dumps({"error": "subdir must be relative; '..' disallowed"})
+        root = _os.path.join(base, subdir) if subdir else base
+        if not _os.path.commonpath([base, _os.path.abspath(root)]) == base:
+            return json.dumps({"error": "subdir escapes package root"})
+        if not _os.path.isdir(root):
+            return json.dumps({"error": f"not a directory: {subdir}"})
+        out: list[dict[str, Any]] = []
+        for dirpath, _, filenames in _os.walk(root):
+            for fname in filenames:
+                if not fname.endswith(".py"):
+                    continue
+                full = _os.path.join(dirpath, fname)
+                rel = _os.path.relpath(full, base)
+                try:
+                    size = _os.path.getsize(full)
+                except OSError:
+                    continue
+                out.append({"path": rel, "size": size})
+        out.sort(key=lambda r: r["path"])
+        return json.dumps({"root": base, "files": out})
+
+    @tool
+    def read_a2a_pack(path: str) -> str:
+        """Read a file from the installed ``a2a_pack`` package.
+
+        Path is relative to the package root, e.g. ``agent.py``,
+        ``context.py``, ``runtime.py``, ``workspace.py``, ``card.py``,
+        ``mcp/server.py``. Returns JSON: ``{path, content}``. Capped
+        at 64 KiB; reach for ``list_a2a_pack`` first if you need to
+        chase a deeper file.
+        """
+        try:
+            import a2a_pack
+            import os as _os
+            base = _os.path.dirname(_os.path.abspath(a2a_pack.__file__))
+        except Exception as exc:  # noqa: BLE001
+            return json.dumps({"error": f"a2a_pack not importable: {exc}"})
+        if path.startswith("/") or ".." in path.split("/"):
+            return json.dumps({"error": "path must be relative; '..' disallowed"})
+        full = _os.path.abspath(_os.path.join(base, path))
+        if _os.path.commonpath([base, full]) != base:
+            return json.dumps({"error": "path escapes package root"})
+        if not _os.path.isfile(full):
+            return json.dumps({"error": f"not a file: {path}"})
+        try:
+            with open(full, "r", encoding="utf-8") as fh:
+                content = fh.read(64 * 1024 + 1)
+        except OSError as exc:
+            return json.dumps({"error": str(exc)})
+        truncated = len(content) > 64 * 1024
+        if truncated:
+            content = content[: 64 * 1024]
+        return json.dumps({
+            "path": path,
+            "content": content,
+            "truncated": truncated,
+        })
+
     return [
         list_agent_files, write_agent_file, read_agent_file,
         test_agent_in_sandbox, cp_deploy_tarball, cp_refresh_agent,
+        list_a2a_pack, read_a2a_pack,
     ]