diff --git a/workspace/prompt.py b/workspace/prompt.py
index 70cce126..7d93152e 100644
--- a/workspace/prompt.py
+++ b/workspace/prompt.py
@@ -4,6 +4,7 @@ import logging
 import os
 from pathlib import Path
 
+from executor_helpers import get_a2a_instructions, get_hma_instructions
 from skill_loader.loader import LoadedSkill
 from shared_runtime import build_peer_section
 
@@ -68,6 +69,7 @@ def build_system_prompt(
     plugin_prompts: list[str] | None = None,
     parent_context: list[dict] | None = None,
     platform_instructions: str = "",
+    a2a_mcp: bool = True,
 ) -> str:
     """Build the complete system prompt.
 
@@ -154,6 +156,20 @@ def build_system_prompt(
             parts.append(skill.instructions)
             parts.append("")
 
+    # Platform tool instructions: A2A (inter-agent communication) and HMA
+    # (persistent memory). These document how to call delegate_task,
+    # commit_memory, etc — without them, agents see the tools registered
+    # but have no instructions on when/how to use them. Placed between
+    # Skills and Peers so the A2A docs precede the peer list (which is
+    # the data shape the A2A tools operate over).
+    #
+    # a2a_mcp=True: MCP tool variant (claude-code, hermes, langchain,
+    # crewai). a2a_mcp=False: CLI subprocess variant (ollama, custom
+    # runtimes that don't speak MCP). Default True matches the
+    # MCP-capable majority; CLI-only adapters override at the call site.
+    parts.append(get_a2a_instructions(mcp=a2a_mcp))
+    parts.append(get_hma_instructions())
+
     # Add peer capabilities with a single shared renderer.
     peer_section = build_peer_section(peers)
     if peer_section:
diff --git a/workspace/tests/test_prompt.py b/workspace/tests/test_prompt.py
index 133a5d7e..5f868c81 100644
--- a/workspace/tests/test_prompt.py
+++ b/workspace/tests/test_prompt.py
@@ -395,3 +395,77 @@ async def test_get_peer_capabilities_exception():
         result = await get_peer_capabilities("http://platform:8080", "ws-abc")
 
     assert result == []
+
+
+# Regression tests for the A2A + HMA tool-instruction injection. Pre-fix,
+# get_a2a_instructions() and get_hma_instructions() were defined in
+# executor_helpers.py but never called from build_system_prompt — workers
+# saw the platform's delegate_task / commit_memory tools registered but
+# had no documentation telling them how to use them.
+
+def test_a2a_instructions_injected_default_mcp(tmp_path):
+    """build_system_prompt embeds A2A MCP-variant instructions by default."""
+    (tmp_path / "system-prompt.md").write_text("Base.")
+
+    result = build_system_prompt(
+        config_path=str(tmp_path),
+        workspace_id="ws-1",
+        loaded_skills=[],
+        peers=[],
+    )
+
+    assert "## Inter-Agent Communication" in result
+    assert "delegate_task" in result
+    assert "list_peers" in result
+    assert "send_message_to_user" in result
+
+
+def test_a2a_instructions_cli_variant_when_disabled(tmp_path):
+    """a2a_mcp=False emits the CLI subprocess variant for non-MCP runtimes."""
+    (tmp_path / "system-prompt.md").write_text("Base.")
+
+    result = build_system_prompt(
+        config_path=str(tmp_path),
+        workspace_id="ws-1",
+        loaded_skills=[],
+        peers=[],
+        a2a_mcp=False,
+    )
+
+    assert "## Inter-Agent Communication" in result
+    assert "molecule_runtime.a2a_cli" in result
+    # MCP-only details must NOT leak into the CLI variant.
+    assert "send_message_to_user" not in result
+
+
+def test_hma_instructions_injected(tmp_path):
+    """build_system_prompt embeds HMA persistent-memory instructions."""
+    (tmp_path / "system-prompt.md").write_text("Base.")
+
+    result = build_system_prompt(
+        config_path=str(tmp_path),
+        workspace_id="ws-1",
+        loaded_skills=[],
+        peers=[],
+    )
+
+    assert "## Hierarchical Memory (HMA)" in result
+    assert "commit_memory" in result
+    assert "recall_memory" in result
+
+
+def test_tool_instructions_precede_peer_section(tmp_path):
+    """A2A docs must precede the peer list — peer IDs are operands of A2A tools."""
+    (tmp_path / "system-prompt.md").write_text("Base.")
+
+    peers = [{"id": "p1", "name": "Worker", "status": "active", "agent_card": None}]
+    result = build_system_prompt(
+        config_path=str(tmp_path),
+        workspace_id="ws-1",
+        loaded_skills=[],
+        peers=peers,
+    )
+
+    a2a_idx = result.index("## Inter-Agent Communication")
+    peers_idx = result.index("## Your Peers")
+    assert a2a_idx < peers_idx, "A2A instructions must come before the peer list"