fix: prevent human block overwrite when skills block missing (#7656)

* fix: prevent human block overwrite when skills block missing

**Bug**: When connecting to agents created before skills blocks were
standard, the human block gets overwritten with skills directory content.

**Root cause**: agent_manager.py:1893-1898 had `block = block` (no-op).
When skills block doesn't exist, loop variable ends as last block in
core_memory (often "human"), then updates that wrong block.

**Fix**: Use `matched_block` variable to properly track found block.
Now correctly raises NoResultFound when block label doesn't exist.

**Impact**: Affects pre-December 2025 agents missing skills blocks.

Written by Cameron ◯ Letta Code

"The best error message is the one that never shows up." - Thomas Fuchs

Co-Authored-By: Letta <noreply@letta.com>

* fix: use correct method name in block update test

Change get_block_by_label_async to get_block_with_label_async in test.

Written by Cameron ◯ Letta Code

Co-Authored-By: Letta <noreply@letta.com>

---------

Co-authored-by: Letta <noreply@letta.com>

tests/managers/test_agent_manager_block_update.py

@@ -0,0 +1,73 @@
+import pytest
+
+# Import shared fixtures and constants from conftest
+from conftest import DEFAULT_EMBEDDING_CONFIG
+
+from letta.orm.errors import NoResultFound
+from letta.schemas.agent import CreateAgent
+from letta.schemas.block import Block as PydanticBlock, BlockUpdate
+from letta.schemas.llm_config import LLMConfig
+from letta.server.server import SyncServer
+from letta.services.block_manager import BlockManager
+
+
+@pytest.mark.asyncio
+async def test_modify_nonexistent_block_raises_error(server: SyncServer, default_user):
+    """
+    Test that modifying a non-existent block raises NoResultFound instead of
+    silently updating the wrong block.
+
+    Regression test for bug where `block = block` was a no-op, causing the loop
+    variable to end as the last block in core_memory, which then got incorrectly updated.
+    """
+    # Upsert base tools
+    await server.tool_manager.upsert_base_tools_async(actor=default_user)
+
+    # Create human and persona blocks
+    block_manager = BlockManager()
+    human_block = await block_manager.create_or_update_block_async(
+        PydanticBlock(label="human", value="Test user context", limit=2000), actor=default_user
+    )
+    persona_block = await block_manager.create_or_update_block_async(
+        PydanticBlock(label="persona", value="Test persona context", limit=2000), actor=default_user
+    )
+
+    # Create agent with human and persona blocks (but no "skills" block)
+    create_agent_request = CreateAgent(
+        name="test_block_update_agent",
+        agent_type="memgpt_v2_agent",
+        system="test system",
+        llm_config=LLMConfig.default_config("gpt-4o-mini"),
+        embedding_config=DEFAULT_EMBEDDING_CONFIG,
+        block_ids=[human_block.id, persona_block.id],
+    )
+    agent = await server.agent_manager.create_agent_async(create_agent_request, actor=default_user)
+
+    # Try to update a non-existent block (e.g., "skills")
+    # This should raise NoResultFound, not silently update human block
+    with pytest.raises(NoResultFound, match="No block with label 'skills' found"):
+        await server.agent_manager.modify_block_by_label_async(
+            agent_id=agent.id,
+            block_label="skills",
+            block_update=BlockUpdate(value="Skills directory content that should not overwrite human block"),
+            actor=default_user,
+        )
+
+    # Verify human block wasn't modified
+    retrieved_human_block = await server.agent_manager.get_block_with_label_async(
+        agent_id=agent.id,
+        block_label="human",
+        actor=default_user,
+    )
+    assert retrieved_human_block.value == "Test user context", "Human block should not be modified"
+
+    # Verify persona block wasn't modified
+    retrieved_persona_block = await server.agent_manager.get_block_with_label_async(
+        agent_id=agent.id,
+        block_label="persona",
+        actor=default_user,
+    )
+    assert retrieved_persona_block.value == "Test persona context", "Persona block should not be modified"
+
+    # Clean up
+    await server.agent_manager.delete_agent_async(agent.id, default_user)