Python: HarnessAgent: Disable compaction when max tokens not provided (#6410)

* HarnessAgent: Disable compaction when max tokens not provided

* Fix regression.

* Address PR comments

* 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>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

python/packages/core/tests/core/test_harness_agent.py

@@ -194,6 +194,63 @@ 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
+
+
+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
+
+
+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 ---
 
 
@@ -207,14 +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: