From ddca3a2f79eeb31ecf9720cbd8d8a124f06a3a9e Mon Sep 17 00:00:00 2001
From: westey <164392973+westey-m@users.noreply.github.com>
Date: Tue, 9 Jun 2026 10:45:45 +0000
Subject: [PATCH 1/4] HarnessAgent: Disable compaction when max tokens not
 provided

---
 .../core/agent_framework/_harness/_agent.py   |  54 ++---
 .../packages/core/agent_framework/_skills.py  |  10 +-
 .../core/tests/core/test_harness_agent.py     |  17 ++
 .../core/tests/core/test_mcp_observability.py |   4 +-
 .../core/tests/core/test_mcp_skills.py        | 190 ++++++++----------
 python/samples/02-agents/harness/README.md    |  14 +-
 6 files changed, 141 insertions(+), 148 deletions(-)

diff --git a/python/packages/core/agent_framework/_harness/_agent.py b/python/packages/core/agent_framework/_harness/_agent.py
index 5896f72141..38a639bf77 100644
--- a/python/packages/core/agent_framework/_harness/_agent.py
+++ b/python/packages/core/agent_framework/_harness/_agent.py
@@ -65,18 +65,14 @@ def _assemble_instructions(
 
 def _assemble_compaction_provider(
     *,
-    disable_compaction: bool,
     max_context_window_tokens: int,
     max_output_tokens: int,
     history_source_id: str,
     before_compaction_strategy: CompactionStrategy | None,
     after_compaction_strategy: CompactionStrategy | None,
     tokenizer: TokenizerProtocol | None,
-) -> CompactionProvider | None:
+) -> CompactionProvider:
     """Build the compaction provider from parameters or defaults."""
-    if disable_compaction:
-        return None
-
     before_strategy = before_compaction_strategy or ContextWindowCompactionStrategy(
         max_context_window_tokens=max_context_window_tokens,
         max_output_tokens=max_output_tokens,
@@ -157,8 +153,8 @@ def create_harness_agent(
     harness_instructions: str | None = None,
     agent_instructions: str | None = None,
     tools: ToolTypes | Callable[..., Any] | Sequence[ToolTypes | Callable[..., Any]] | None = None,
-    max_context_window_tokens: int,
-    max_output_tokens: int,
+    max_context_window_tokens: int | None = None,
+    max_output_tokens: int | None = None,
     history_provider: HistoryProvider | None = None,
     disable_compaction: bool = False,
     before_compaction_strategy: CompactionStrategy | None = None,
@@ -206,8 +202,6 @@ def create_harness_agent(
 
             agent = create_harness_agent(
                 OpenAIChatClient(model="gpt-4o"),
-                max_context_window_tokens=128_000,
-                max_output_tokens=16_384,
             )
             session = agent.create_session()
             response = await agent.run("Plan a weekend trip to Seattle", session=session)
@@ -243,7 +237,10 @@ def create_harness_agent(
             (e.g., "You are a research assistant focused on academic sources.").
         tools: Additional tools to include in the agent's toolset.
         max_context_window_tokens: Maximum tokens the model's context window supports.
+            When None (default), compaction is automatically disabled.
         max_output_tokens: Maximum output tokens per response.
+            When None (default), compaction is automatically disabled and no
+            default max_tokens option is set.
         history_provider: Custom history provider. When None, an InMemoryHistoryProvider is used.
         disable_compaction: When True, skip compaction provider setup.
         before_compaction_strategy: Custom before-run compaction strategy.
@@ -283,29 +280,35 @@ def create_harness_agent(
         A fully configured :class:`~agent_framework.Agent` instance.
 
     Raises:
-        ValueError: If max_context_window_tokens <= 0 or max_output_tokens < 0
-            or max_output_tokens >= max_context_window_tokens.
+        ValueError: If max_context_window_tokens is provided and <= 0, or
+            max_output_tokens is provided and < 0, or max_output_tokens >=
+            max_context_window_tokens when both are provided.
     """
-    if max_context_window_tokens <= 0:
+    if max_context_window_tokens is not None and max_context_window_tokens <= 0:
         raise ValueError("max_context_window_tokens must be positive.")
-    if max_output_tokens < 0:
+    if max_output_tokens is not None and max_output_tokens < 0:
         raise ValueError("max_output_tokens must be non-negative.")
-    if max_output_tokens >= max_context_window_tokens:
+    if (
+        max_context_window_tokens is not None
+        and max_output_tokens is not None
+        and max_output_tokens >= max_context_window_tokens
+    ):
         raise ValueError("max_output_tokens must be less than max_context_window_tokens.")
 
     # Build history provider.
     resolved_history = history_provider or InMemoryHistoryProvider()
 
-    # Build compaction provider.
-    compaction_provider = _assemble_compaction_provider(
-        disable_compaction=disable_compaction,
-        max_context_window_tokens=max_context_window_tokens,
-        max_output_tokens=max_output_tokens,
-        history_source_id=resolved_history.source_id,
-        before_compaction_strategy=before_compaction_strategy,
-        after_compaction_strategy=after_compaction_strategy,
-        tokenizer=tokenizer,
-    )
+    # Build compaction provider (disabled when token params are not provided).
+    compaction_provider: CompactionProvider | None = None
+    if not disable_compaction and max_context_window_tokens is not None and max_output_tokens is not None:
+        compaction_provider = _assemble_compaction_provider(
+            max_context_window_tokens=max_context_window_tokens,
+            max_output_tokens=max_output_tokens,
+            history_source_id=resolved_history.source_id,
+            before_compaction_strategy=before_compaction_strategy,
+            after_compaction_strategy=after_compaction_strategy,
+            tokenizer=tokenizer,
+        )
 
     # Build context providers.
     assembled_providers = _assemble_context_providers(
@@ -347,7 +350,8 @@ def create_harness_agent(
 
     # Build default options dict.
     default_opts: dict[str, Any] = dict(default_options) if default_options else {}
-    default_opts.setdefault("max_tokens", max_output_tokens)
+    if max_output_tokens is not None:
+        default_opts.setdefault("max_tokens", max_output_tokens)
 
     agent = Agent(
         client,
diff --git a/python/packages/core/agent_framework/_skills.py b/python/packages/core/agent_framework/_skills.py
index 97afe66cea..91bdb61914 100644
--- a/python/packages/core/agent_framework/_skills.py
+++ b/python/packages/core/agent_framework/_skills.py
@@ -3516,9 +3516,7 @@ async def get_content(self) -> str:
         result = await self._client.read_resource(_mcp_any_url(self._skill_md_uri))
         text = _mcp_join_text(result)
         if not text:
-            raise ValueError(
-                f"The MCP server returned no text content for SKILL.md resource '{self._skill_md_uri}'."
-            )
+            raise ValueError(f"The MCP server returned no text content for SKILL.md resource '{self._skill_md_uri}'.")
         self._content = text
         return text
 
@@ -3572,11 +3570,7 @@ def _validate_resource_name(name: str) -> str | None:
             or ``None`` if the name is unsafe.
         """
         normalized = name.replace("\\", "/")
-        if (
-            normalized.startswith("/")
-            or "://" in normalized
-            or any(seg == ".." for seg in normalized.split("/"))
-        ):
+        if normalized.startswith("/") or "://" in normalized or any(seg == ".." for seg in normalized.split("/")):
             logger.debug("Rejecting resource name with unsafe path components: %r", name)
             return None
         return normalized
diff --git a/python/packages/core/tests/core/test_harness_agent.py b/python/packages/core/tests/core/test_harness_agent.py
index 58ef3f5f2d..446d9f2fd3 100644
--- a/python/packages/core/tests/core/test_harness_agent.py
+++ b/python/packages/core/tests/core/test_harness_agent.py
@@ -194,6 +194,23 @@ def test_create_harness_agent_returns_full_agent() -> None:
     assert isinstance(agent, FullAgent)
 
 
+def test_create_harness_agent_no_token_params_disables_compaction() -> None:
+    """When token params are omitted, compaction is automatically disabled."""
+    agent = create_harness_agent(
+        client=_FakeChatClient(),  # type: ignore[arg-type]
+    )
+    provider_types = [type(p) for p in agent.context_providers]
+    assert CompactionProvider not in provider_types
+
+
+def test_create_harness_agent_no_token_params_skips_max_tokens_option() -> None:
+    """When max_output_tokens is omitted, max_tokens should not be set in default options."""
+    agent = create_harness_agent(
+        client=_FakeChatClient(),  # type: ignore[arg-type]
+    )
+    assert agent.default_options.get("max_tokens") is None
+
+
 # --- Validation Tests ---
 
 
diff --git a/python/packages/core/tests/core/test_mcp_observability.py b/python/packages/core/tests/core/test_mcp_observability.py
index 226e976120..8b17f3dc88 100644
--- a/python/packages/core/tests/core/test_mcp_observability.py
+++ b/python/packages/core/tests/core/test_mcp_observability.py
@@ -281,9 +281,7 @@ async def test_mcp_prompts_get_creates_client_span(span_exporter: InMemorySpanEx
 async def test_mcp_prompts_get_mcp_error_sets_error_type(span_exporter: InMemorySpanExporter):
     """When session.get_prompt() raises McpError, the span should have error.type and ERROR status."""
     tool = _make_connected_mcp_tool()
-    tool.session.get_prompt = AsyncMock(
-        side_effect=McpError(ErrorData(code=-32602, message="prompt not found"))
-    )
+    tool.session.get_prompt = AsyncMock(side_effect=McpError(ErrorData(code=-32602, message="prompt not found")))
 
     span_exporter.clear()
     with pytest.raises(ToolExecutionException):
diff --git a/python/packages/core/tests/core/test_mcp_skills.py b/python/packages/core/tests/core/test_mcp_skills.py
index 3e7c67662a..74993997d0 100644
--- a/python/packages/core/tests/core/test_mcp_skills.py
+++ b/python/packages/core/tests/core/test_mcp_skills.py
@@ -35,26 +35,22 @@
 Body content here.
 """
 
-SAMPLE_SKILL_INDEX = json.dumps(
-    {
-        "$schema": "https://schemas.agentskills.io/discovery/0.2.0/schema.json",
-        "skills": [
-            {
-                "name": "unit-converter",
-                "type": "skill-md",
-                "description": "Convert between common units.",
-                "url": "skill://unit-converter/SKILL.md",
-            }
-        ],
-    }
-)
+SAMPLE_SKILL_INDEX = json.dumps({
+    "$schema": "https://schemas.agentskills.io/discovery/0.2.0/schema.json",
+    "skills": [
+        {
+            "name": "unit-converter",
+            "type": "skill-md",
+            "description": "Convert between common units.",
+            "url": "skill://unit-converter/SKILL.md",
+        }
+    ],
+})
 
 
 def _make_text_result(text: str, uri: str = "skill://test") -> ReadResourceResult:
     """Create a ReadResourceResult with a single TextResourceContents."""
-    return ReadResourceResult(
-        contents=[TextResourceContents(uri=AnyUrl(uri), text=text, mimeType="text/markdown")]
-    )
+    return ReadResourceResult(contents=[TextResourceContents(uri=AnyUrl(uri), text=text, mimeType="text/markdown")])
 
 
 def _make_blob_result(
@@ -230,12 +226,10 @@ async def test_get_content_raises_on_empty(self) -> None:
 
     @pytest.mark.asyncio
     async def test_get_resource_text(self) -> None:
-        client = _make_client(
-            **{
-                "skill://unit-converter/SKILL.md": _make_text_result(SAMPLE_SKILL_MD),
-                "skill://unit-converter/references/checklist.md": _make_text_result("- check thing 1\n- check thing 2"),
-            }
-        )
+        client = _make_client(**{
+            "skill://unit-converter/SKILL.md": _make_text_result(SAMPLE_SKILL_MD),
+            "skill://unit-converter/references/checklist.md": _make_text_result("- check thing 1\n- check thing 2"),
+        })
         from agent_framework import SkillFrontmatter
 
         fm = SkillFrontmatter(name="unit-converter", description="Convert between common units.")
@@ -249,12 +243,10 @@ async def test_get_resource_text(self) -> None:
     @pytest.mark.asyncio
     async def test_get_resource_binary(self) -> None:
         data = bytes([0x01, 0x02, 0x03, 0x04])
-        client = _make_client(
-            **{
-                "skill://unit-converter/SKILL.md": _make_text_result(SAMPLE_SKILL_MD),
-                "skill://unit-converter/assets/icon.bin": _make_blob_result(data),
-            }
-        )
+        client = _make_client(**{
+            "skill://unit-converter/SKILL.md": _make_text_result(SAMPLE_SKILL_MD),
+            "skill://unit-converter/assets/icon.bin": _make_blob_result(data),
+        })
         from agent_framework import SkillFrontmatter
 
         fm = SkillFrontmatter(name="unit-converter", description="Convert between common units.")
@@ -345,12 +337,10 @@ class TestMCPSkillsSource:
 
     @pytest.mark.asyncio
     async def test_index_based_discovery_returns_skill(self) -> None:
-        client = _make_client(
-            **{
-                "skill://index.json": _make_text_result(SAMPLE_SKILL_INDEX, uri="skill://index.json"),
-                "skill://unit-converter/SKILL.md": _make_text_result(SAMPLE_SKILL_MD),
-            }
-        )
+        client = _make_client(**{
+            "skill://index.json": _make_text_result(SAMPLE_SKILL_INDEX, uri="skill://index.json"),
+            "skill://unit-converter/SKILL.md": _make_text_result(SAMPLE_SKILL_MD),
+        })
         source = MCPSkillsSource(client=client)
         skills = await source.get_skills()
 
@@ -373,9 +363,7 @@ async def test_no_index_returns_empty(self) -> None:
     async def test_does_not_read_skill_md_during_discovery(self) -> None:
         # Index points to a skill, but SKILL.md is not registered on the server.
         # Discovery should succeed because it only reads the index.
-        client = _make_client(
-            **{"skill://index.json": _make_text_result(SAMPLE_SKILL_INDEX, uri="skill://index.json")}
-        )
+        client = _make_client(**{"skill://index.json": _make_text_result(SAMPLE_SKILL_INDEX, uri="skill://index.json")})
         source = MCPSkillsSource(client=client)
         skills = await source.get_skills()
 
@@ -384,19 +372,17 @@ async def test_does_not_read_skill_md_during_discovery(self) -> None:
 
     @pytest.mark.asyncio
     async def test_invalid_name_is_skipped(self) -> None:
-        index_json = json.dumps(
-            {
-                "$schema": "https://schemas.agentskills.io/discovery/0.2.0/schema.json",
-                "skills": [
-                    {
-                        "name": "UnitConverter",  # Invalid: uppercase
-                        "type": "skill-md",
-                        "description": "Convert between common units.",
-                        "url": "skill://UnitConverter/SKILL.md",
-                    }
-                ],
-            }
-        )
+        index_json = json.dumps({
+            "$schema": "https://schemas.agentskills.io/discovery/0.2.0/schema.json",
+            "skills": [
+                {
+                    "name": "UnitConverter",  # Invalid: uppercase
+                    "type": "skill-md",
+                    "description": "Convert between common units.",
+                    "url": "skill://UnitConverter/SKILL.md",
+                }
+            ],
+        })
         client = _make_client(**{"skill://index.json": _make_text_result(index_json, uri="skill://index.json")})
         source = MCPSkillsSource(client=client)
         skills = await source.get_skills()
@@ -404,18 +390,16 @@ async def test_invalid_name_is_skipped(self) -> None:
 
     @pytest.mark.asyncio
     async def test_missing_required_fields_is_skipped(self) -> None:
-        index_json = json.dumps(
-            {
-                "$schema": "https://schemas.agentskills.io/discovery/0.2.0/schema.json",
-                "skills": [
-                    {
-                        "name": "unit-converter",
-                        "type": "skill-md",
-                        # Missing description and url
-                    }
-                ],
-            }
-        )
+        index_json = json.dumps({
+            "$schema": "https://schemas.agentskills.io/discovery/0.2.0/schema.json",
+            "skills": [
+                {
+                    "name": "unit-converter",
+                    "type": "skill-md",
+                    # Missing description and url
+                }
+            ],
+        })
         client = _make_client(**{"skill://index.json": _make_text_result(index_json, uri="skill://index.json")})
         source = MCPSkillsSource(client=client)
         skills = await source.get_skills()
@@ -423,19 +407,17 @@ async def test_missing_required_fields_is_skipped(self) -> None:
 
     @pytest.mark.asyncio
     async def test_unsupported_type_is_skipped(self) -> None:
-        index_json = json.dumps(
-            {
-                "$schema": "https://schemas.agentskills.io/discovery/0.2.0/schema.json",
-                "skills": [
-                    {
-                        "name": "some-skill",
-                        "type": "archive",
-                        "description": "Packaged skill.",
-                        "url": "skill://some-skill.tar.gz",
-                    }
-                ],
-            }
-        )
+        index_json = json.dumps({
+            "$schema": "https://schemas.agentskills.io/discovery/0.2.0/schema.json",
+            "skills": [
+                {
+                    "name": "some-skill",
+                    "type": "archive",
+                    "description": "Packaged skill.",
+                    "url": "skill://some-skill.tar.gz",
+                }
+            ],
+        })
         client = _make_client(**{"skill://index.json": _make_text_result(index_json, uri="skill://index.json")})
         source = MCPSkillsSource(client=client)
         skills = await source.get_skills()
@@ -443,18 +425,16 @@ async def test_unsupported_type_is_skipped(self) -> None:
 
     @pytest.mark.asyncio
     async def test_template_type_is_skipped(self) -> None:
-        index_json = json.dumps(
-            {
-                "$schema": "https://schemas.agentskills.io/discovery/0.2.0/schema.json",
-                "skills": [
-                    {
-                        "type": "mcp-resource-template",
-                        "description": "Per-product documentation skill",
-                        "url": "skill://docs/{product}/SKILL.md",
-                    }
-                ],
-            }
-        )
+        index_json = json.dumps({
+            "$schema": "https://schemas.agentskills.io/discovery/0.2.0/schema.json",
+            "skills": [
+                {
+                    "type": "mcp-resource-template",
+                    "description": "Per-product documentation skill",
+                    "url": "skill://docs/{product}/SKILL.md",
+                }
+            ],
+        })
         client = _make_client(**{"skill://index.json": _make_text_result(index_json, uri="skill://index.json")})
         source = MCPSkillsSource(client=client)
         skills = await source.get_skills()
@@ -462,31 +442,25 @@ async def test_template_type_is_skipped(self) -> None:
 
     @pytest.mark.asyncio
     async def test_empty_index_returns_empty(self) -> None:
-        client = _make_client(
-            **{"skill://index.json": _make_text_result('{"skills": []}', uri="skill://index.json")}
-        )
+        client = _make_client(**{"skill://index.json": _make_text_result('{"skills": []}', uri="skill://index.json")})
         source = MCPSkillsSource(client=client)
         skills = await source.get_skills()
         assert skills == []
 
     @pytest.mark.asyncio
     async def test_malformed_index_json_returns_empty(self) -> None:
-        client = _make_client(
-            **{"skill://index.json": _make_text_result("not valid json", uri="skill://index.json")}
-        )
+        client = _make_client(**{"skill://index.json": _make_text_result("not valid json", uri="skill://index.json")})
         source = MCPSkillsSource(client=client)
         skills = await source.get_skills()
         assert skills == []
 
     @pytest.mark.asyncio
     async def test_sibling_text_resource(self) -> None:
-        client = _make_client(
-            **{
-                "skill://index.json": _make_text_result(SAMPLE_SKILL_INDEX, uri="skill://index.json"),
-                "skill://unit-converter/SKILL.md": _make_text_result(SAMPLE_SKILL_MD),
-                "skill://unit-converter/references/checklist.md": _make_text_result("- check thing 1\n- check thing 2"),
-            }
-        )
+        client = _make_client(**{
+            "skill://index.json": _make_text_result(SAMPLE_SKILL_INDEX, uri="skill://index.json"),
+            "skill://unit-converter/SKILL.md": _make_text_result(SAMPLE_SKILL_MD),
+            "skill://unit-converter/references/checklist.md": _make_text_result("- check thing 1\n- check thing 2"),
+        })
         source = MCPSkillsSource(client=client)
         skill = (await source.get_skills())[0]
         resource = await skill.get_resource("references/checklist.md")
@@ -497,13 +471,11 @@ async def test_sibling_text_resource(self) -> None:
     @pytest.mark.asyncio
     async def test_sibling_binary_resource(self) -> None:
         data = bytes([0x01, 0x02, 0x03, 0x04])
-        client = _make_client(
-            **{
-                "skill://index.json": _make_text_result(SAMPLE_SKILL_INDEX, uri="skill://index.json"),
-                "skill://unit-converter/SKILL.md": _make_text_result(SAMPLE_SKILL_MD),
-                "skill://unit-converter/assets/icon.bin": _make_blob_result(data),
-            }
-        )
+        client = _make_client(**{
+            "skill://index.json": _make_text_result(SAMPLE_SKILL_INDEX, uri="skill://index.json"),
+            "skill://unit-converter/SKILL.md": _make_text_result(SAMPLE_SKILL_MD),
+            "skill://unit-converter/assets/icon.bin": _make_blob_result(data),
+        })
         source = MCPSkillsSource(client=client)
         skill = (await source.get_skills())[0]
         resource = await skill.get_resource("assets/icon.bin")
@@ -649,9 +621,7 @@ async def test_get_resource_generic_mcp_error_propagates(self) -> None:
         from agent_framework import SkillFrontmatter
 
         client = AsyncMock()
-        client.read_resource = AsyncMock(
-            side_effect=McpError(error=ErrorData(code=0, message="Handler error"))
-        )
+        client.read_resource = AsyncMock(side_effect=McpError(error=ErrorData(code=0, message="Handler error")))
         fm = SkillFrontmatter(name="test-skill", description="Test.")
         skill = MCPSkill(frontmatter=fm, skill_md_uri="skill://test/SKILL.md", client=client)
         with pytest.raises(McpError):
diff --git a/python/samples/02-agents/harness/README.md b/python/samples/02-agents/harness/README.md
index 3bf0f09110..15424e1422 100644
--- a/python/samples/02-agents/harness/README.md
+++ b/python/samples/02-agents/harness/README.md
@@ -45,13 +45,23 @@ python samples/02-agents/harness/harness_research.py
 
 ### Minimal Setup
 
-`create_harness_agent` requires only a chat client and token budget parameters:
+`create_harness_agent` requires only a chat client:
 
 ```python
 from agent_framework import create_harness_agent
 from agent_framework.foundry import FoundryChatClient
 from azure.identity import AzureCliCredential
 
+agent = create_harness_agent(
+    client=FoundryChatClient(credential=AzureCliCredential()),
+)
+```
+
+### With Compaction
+
+Provide token budget parameters to enable automatic context-window compaction:
+
+```python
 agent = create_harness_agent(
     client=FoundryChatClient(credential=AzureCliCredential()),
     max_context_window_tokens=128_000,
@@ -59,7 +69,7 @@ agent = create_harness_agent(
 )
 ```
 
-### Customization
+### Further Customization
 
 Disable or customize any feature:
 

From bc43fb39ef15611d4d2595f2c1401d3f32257f79 Mon Sep 17 00:00:00 2001
From: westey <164392973+westey-m@users.noreply.github.com>
Date: Tue, 9 Jun 2026 10:58:38 +0000
Subject: [PATCH 2/4] Fix regression.

---
 .../core/agent_framework/_harness/_agent.py   | 69 ++++++++++++-------
 .../core/tests/core/test_harness_agent.py     | 25 +++++++
 2 files changed, 69 insertions(+), 25 deletions(-)

diff --git a/python/packages/core/agent_framework/_harness/_agent.py b/python/packages/core/agent_framework/_harness/_agent.py
index 38a639bf77..dcbf4459e4 100644
--- a/python/packages/core/agent_framework/_harness/_agent.py
+++ b/python/packages/core/agent_framework/_harness/_agent.py
@@ -65,19 +65,35 @@ def _assemble_instructions(
 
 def _assemble_compaction_provider(
     *,
-    max_context_window_tokens: int,
-    max_output_tokens: int,
+    disable_compaction: bool,
+    max_context_window_tokens: int | None,
+    max_output_tokens: int | None,
     history_source_id: str,
     before_compaction_strategy: CompactionStrategy | None,
     after_compaction_strategy: CompactionStrategy | None,
     tokenizer: TokenizerProtocol | None,
-) -> CompactionProvider:
-    """Build the compaction provider from parameters or defaults."""
-    before_strategy = before_compaction_strategy or ContextWindowCompactionStrategy(
-        max_context_window_tokens=max_context_window_tokens,
-        max_output_tokens=max_output_tokens,
-        tokenizer=tokenizer,
-    )
+) -> CompactionProvider | None:
+    """Build the compaction provider from parameters or defaults.
+
+    Returns None when compaction is explicitly disabled, or when no before-strategy can be
+    resolved (no custom ``before_compaction_strategy`` and no token budget to build the default).
+    """
+    if disable_compaction:
+        return None
+
+    # A user-supplied strategy is used as-is; otherwise fall back to the token-budget-aware
+    # default, which requires the token params.
+    before_strategy = before_compaction_strategy
+    if before_strategy is None and max_context_window_tokens is not None and max_output_tokens is not None:
+        before_strategy = ContextWindowCompactionStrategy(
+            max_context_window_tokens=max_context_window_tokens,
+            max_output_tokens=max_output_tokens,
+            tokenizer=tokenizer,
+        )
+
+    if before_strategy is None:
+        return None
+
     after_strategy = after_compaction_strategy or ToolResultCompactionStrategy(keep_last_tool_call_groups=2)
 
     return CompactionProvider(
@@ -237,14 +253,18 @@ def create_harness_agent(
             (e.g., "You are a research assistant focused on academic sources.").
         tools: Additional tools to include in the agent's toolset.
         max_context_window_tokens: Maximum tokens the model's context window supports.
-            When None (default), compaction is automatically disabled.
+            Used to construct the default token-budget-aware compaction strategy. When None
+            (default) and no ``before_compaction_strategy`` is provided, compaction is
+            automatically disabled.
         max_output_tokens: Maximum output tokens per response.
-            When None (default), compaction is automatically disabled and no
-            default max_tokens option is set.
+            Used to construct the default compaction strategy and sets a default max_tokens
+            chat option. When None (default), no default max_tokens option is set, and if no
+            ``before_compaction_strategy`` is provided, compaction is automatically disabled.
         history_provider: Custom history provider. When None, an InMemoryHistoryProvider is used.
         disable_compaction: When True, skip compaction provider setup.
-        before_compaction_strategy: Custom before-run compaction strategy.
-            Defaults to ContextWindowCompactionStrategy (token-budget aware).
+        before_compaction_strategy: Custom before-run compaction strategy. When provided,
+            compaction runs even if token params are omitted. Defaults to
+            ContextWindowCompactionStrategy (token-budget aware), which requires the token params.
         after_compaction_strategy: Custom after-run compaction strategy.
             Defaults to ToolResultCompactionStrategy.
         tokenizer: Custom tokenizer for compaction strategies.
@@ -298,17 +318,16 @@ def create_harness_agent(
     # Build history provider.
     resolved_history = history_provider or InMemoryHistoryProvider()
 
-    # Build compaction provider (disabled when token params are not provided).
-    compaction_provider: CompactionProvider | None = None
-    if not disable_compaction and max_context_window_tokens is not None and max_output_tokens is not None:
-        compaction_provider = _assemble_compaction_provider(
-            max_context_window_tokens=max_context_window_tokens,
-            max_output_tokens=max_output_tokens,
-            history_source_id=resolved_history.source_id,
-            before_compaction_strategy=before_compaction_strategy,
-            after_compaction_strategy=after_compaction_strategy,
-            tokenizer=tokenizer,
-        )
+    # Build compaction provider.
+    compaction_provider = _assemble_compaction_provider(
+        disable_compaction=disable_compaction,
+        max_context_window_tokens=max_context_window_tokens,
+        max_output_tokens=max_output_tokens,
+        history_source_id=resolved_history.source_id,
+        before_compaction_strategy=before_compaction_strategy,
+        after_compaction_strategy=after_compaction_strategy,
+        tokenizer=tokenizer,
+    )
 
     # Build context providers.
     assembled_providers = _assemble_context_providers(
diff --git a/python/packages/core/tests/core/test_harness_agent.py b/python/packages/core/tests/core/test_harness_agent.py
index 446d9f2fd3..f2cb73d491 100644
--- a/python/packages/core/tests/core/test_harness_agent.py
+++ b/python/packages/core/tests/core/test_harness_agent.py
@@ -211,6 +211,31 @@ def test_create_harness_agent_no_token_params_skips_max_tokens_option() -> None:
     assert agent.default_options.get("max_tokens") is None
 
 
+def test_create_harness_agent_custom_before_strategy_enables_compaction_without_tokens() -> None:
+    """A custom before_compaction_strategy enables compaction even when token params are omitted."""
+    from agent_framework import ToolResultCompactionStrategy
+
+    agent = create_harness_agent(
+        client=_FakeChatClient(),  # type: ignore[arg-type]
+        before_compaction_strategy=ToolResultCompactionStrategy(),
+    )
+    provider_types = [type(p) for p in agent.context_providers]
+    assert CompactionProvider in provider_types
+
+
+def test_create_harness_agent_disable_compaction_overrides_custom_before_strategy() -> None:
+    """disable_compaction=True wins even when a custom before strategy is provided."""
+    from agent_framework import ToolResultCompactionStrategy
+
+    agent = create_harness_agent(
+        client=_FakeChatClient(),  # type: ignore[arg-type]
+        before_compaction_strategy=ToolResultCompactionStrategy(),
+        disable_compaction=True,
+    )
+    provider_types = [type(p) for p in agent.context_providers]
+    assert CompactionProvider not in provider_types
+
+
 # --- Validation Tests ---
 
 

From 6b011b52aedd16a9865c069466bd1d41fcbefe4e Mon Sep 17 00:00:00 2001
From: westey <164392973+westey-m@users.noreply.github.com>
Date: Tue, 9 Jun 2026 11:19:29 +0000
Subject: [PATCH 3/4] Address PR comments

---
 .../core/agent_framework/_harness/_agent.py   | 43 ++++++++++++-------
 .../core/tests/core/test_harness_agent.py     | 15 +++++++
 2 files changed, 42 insertions(+), 16 deletions(-)

diff --git a/python/packages/core/agent_framework/_harness/_agent.py b/python/packages/core/agent_framework/_harness/_agent.py
index dcbf4459e4..5d813f99d6 100644
--- a/python/packages/core/agent_framework/_harness/_agent.py
+++ b/python/packages/core/agent_framework/_harness/_agent.py
@@ -75,14 +75,19 @@ def _assemble_compaction_provider(
 ) -> CompactionProvider | None:
     """Build the compaction provider from parameters or defaults.
 
-    Returns None when compaction is explicitly disabled, or when no before-strategy can be
-    resolved (no custom ``before_compaction_strategy`` and no token budget to build the default).
+    The token-budget defaults (``ContextWindowCompactionStrategy`` for the before phase and
+    ``ToolResultCompactionStrategy`` for the after phase) are only applied when the token
+    params are provided. Caller-supplied strategies are always honored. Either phase may end
+    up ``None``, which ``CompactionProvider`` interprets as "skip that phase".
+
+    Returns None when compaction is explicitly disabled, or when neither phase has a strategy
+    (no custom strategies and no token budget to build the defaults).
     """
     if disable_compaction:
         return None
 
-    # A user-supplied strategy is used as-is; otherwise fall back to the token-budget-aware
-    # default, which requires the token params.
+    # Resolve the before-strategy: custom strategy wins; otherwise fall back to the
+    # token-budget-aware default when token params are available.
     before_strategy = before_compaction_strategy
     if before_strategy is None and max_context_window_tokens is not None and max_output_tokens is not None:
         before_strategy = ContextWindowCompactionStrategy(
@@ -91,10 +96,15 @@ def _assemble_compaction_provider(
             tokenizer=tokenizer,
         )
 
-    if before_strategy is None:
-        return None
+    # Resolve the after-strategy: custom strategy wins; otherwise fall back to the default
+    # when token params are available.
+    after_strategy = after_compaction_strategy
+    if after_strategy is None and max_context_window_tokens is not None and max_output_tokens is not None:
+        after_strategy = ToolResultCompactionStrategy(keep_last_tool_call_groups=2)
 
-    after_strategy = after_compaction_strategy or ToolResultCompactionStrategy(keep_last_tool_call_groups=2)
+    # Nothing to compact in either phase: skip the provider entirely.
+    if before_strategy is None and after_strategy is None:
+        return None
 
     return CompactionProvider(
         before_strategy=before_strategy,
@@ -253,20 +263,21 @@ def create_harness_agent(
             (e.g., "You are a research assistant focused on academic sources.").
         tools: Additional tools to include in the agent's toolset.
         max_context_window_tokens: Maximum tokens the model's context window supports.
-            Used to construct the default token-budget-aware compaction strategy. When None
-            (default) and no ``before_compaction_strategy`` is provided, compaction is
-            automatically disabled.
+            Used to construct the default token-budget-aware compaction strategies. When None
+            (default) and no custom ``before_compaction_strategy`` / ``after_compaction_strategy``
+            is provided, compaction is automatically disabled.
         max_output_tokens: Maximum output tokens per response.
-            Used to construct the default compaction strategy and sets a default max_tokens
-            chat option. When None (default), no default max_tokens option is set, and if no
-            ``before_compaction_strategy`` is provided, compaction is automatically disabled.
+            Used to construct the default compaction strategies and sets a default max_tokens
+            chat option. When None (default), no default max_tokens option is set, and unless a
+            custom compaction strategy is provided, compaction is automatically disabled.
         history_provider: Custom history provider. When None, an InMemoryHistoryProvider is used.
         disable_compaction: When True, skip compaction provider setup.
         before_compaction_strategy: Custom before-run compaction strategy. When provided,
             compaction runs even if token params are omitted. Defaults to
-            ContextWindowCompactionStrategy (token-budget aware), which requires the token params.
-        after_compaction_strategy: Custom after-run compaction strategy.
-            Defaults to ToolResultCompactionStrategy.
+            ContextWindowCompactionStrategy (token-budget aware) when token params are provided.
+        after_compaction_strategy: Custom after-run compaction strategy. When provided,
+            compaction runs even if token params are omitted. Defaults to
+            ToolResultCompactionStrategy when token params are provided.
         tokenizer: Custom tokenizer for compaction strategies.
         disable_todo: When True, skip the TodoProvider.
         todo_provider: Custom TodoProvider instance. Ignored when disable_todo is True.
diff --git a/python/packages/core/tests/core/test_harness_agent.py b/python/packages/core/tests/core/test_harness_agent.py
index f2cb73d491..512546157e 100644
--- a/python/packages/core/tests/core/test_harness_agent.py
+++ b/python/packages/core/tests/core/test_harness_agent.py
@@ -236,6 +236,21 @@ def test_create_harness_agent_disable_compaction_overrides_custom_before_strateg
     assert CompactionProvider not in provider_types
 
 
+def test_create_harness_agent_custom_after_strategy_enables_compaction_without_tokens() -> None:
+    """A custom after_compaction_strategy enables compaction even when token params are omitted."""
+    from agent_framework import ToolResultCompactionStrategy
+
+    agent = create_harness_agent(
+        client=_FakeChatClient(),  # type: ignore[arg-type]
+        after_compaction_strategy=ToolResultCompactionStrategy(),
+    )
+    compaction_providers = [p for p in agent.context_providers if isinstance(p, CompactionProvider)]
+    assert len(compaction_providers) == 1
+    # Before phase is skipped (no token budget, no custom before strategy), after phase is set.
+    assert compaction_providers[0].before_strategy is None
+    assert compaction_providers[0].after_strategy is not None
+
+
 # --- Validation Tests ---
 
 

From cefef16c7a0c36fe9604c482ef5408e865f94dd5 Mon Sep 17 00:00:00 2001
From: westey <164392973+westey-m@users.noreply.github.com>
Date: Tue, 9 Jun 2026 17:11:42 +0000
Subject: [PATCH 4/4] Require max_output_tokens to be positive

Reject max_output_tokens=0 (must be positive), mirroring
max_context_window_tokens. Addresses PR review feedback.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 .../core/agent_framework/_harness/_agent.py     |  6 +++---
 .../core/tests/core/test_harness_agent.py       | 17 +++++++++--------
 2 files changed, 12 insertions(+), 11 deletions(-)

diff --git a/python/packages/core/agent_framework/_harness/_agent.py b/python/packages/core/agent_framework/_harness/_agent.py
index 5d813f99d6..0ae0c73032 100644
--- a/python/packages/core/agent_framework/_harness/_agent.py
+++ b/python/packages/core/agent_framework/_harness/_agent.py
@@ -312,13 +312,13 @@ def create_harness_agent(
 
     Raises:
         ValueError: If max_context_window_tokens is provided and <= 0, or
-            max_output_tokens is provided and < 0, or max_output_tokens >=
+            max_output_tokens is provided and <= 0, or max_output_tokens >=
             max_context_window_tokens when both are provided.
     """
     if max_context_window_tokens is not None and max_context_window_tokens <= 0:
         raise ValueError("max_context_window_tokens must be positive.")
-    if max_output_tokens is not None and max_output_tokens < 0:
-        raise ValueError("max_output_tokens must be non-negative.")
+    if max_output_tokens is not None and max_output_tokens <= 0:
+        raise ValueError("max_output_tokens must be positive.")
     if (
         max_context_window_tokens is not None
         and max_output_tokens is not None
diff --git a/python/packages/core/tests/core/test_harness_agent.py b/python/packages/core/tests/core/test_harness_agent.py
index 512546157e..7da1bdbf36 100644
--- a/python/packages/core/tests/core/test_harness_agent.py
+++ b/python/packages/core/tests/core/test_harness_agent.py
@@ -264,14 +264,15 @@ def test_create_harness_agent_rejects_invalid_context_tokens() -> None:
         )
 
 
-def test_create_harness_agent_rejects_negative_output_tokens() -> None:
-    """max_output_tokens must be non-negative."""
-    with pytest.raises(ValueError, match="max_output_tokens must be non-negative"):
-        create_harness_agent(
-            client=_FakeChatClient(),  # type: ignore[arg-type]
-            max_context_window_tokens=1000,
-            max_output_tokens=-1,
-        )
+def test_create_harness_agent_rejects_non_positive_output_tokens() -> None:
+    """max_output_tokens must be positive when provided."""
+    for invalid_value in (0, -1):
+        with pytest.raises(ValueError, match="max_output_tokens must be positive"):
+            create_harness_agent(
+                client=_FakeChatClient(),  # type: ignore[arg-type]
+                max_context_window_tokens=1000,
+                max_output_tokens=invalid_value,
+            )
 
 
 def test_create_harness_agent_rejects_output_gte_context() -> None: