Fix Agents.UnmarshalYAML to reject unknown fields

Agents.UnmarshalYAML re-marshals each agent value to bytes and calls
yaml.Unmarshal to parse it into AgentConfig. This plain Unmarshal call
does not enforce strict field validation, so unknown fields like
'instructions' (instead of 'instruction') are silently ignored — even
though Parse() uses yaml.Strict() at the top level.

Switch to yaml.UnmarshalWithOptions with DisallowUnknownField so that
the strictness is propagated into agent blocks.

Fixes #1683

Assisted-By: cagent

Author: simonferquel

pkg/config/latest/types.go

@@ -45,7 +45,7 @@ func (c *Agents) UnmarshalYAML(unmarshal func(any) error) error {
 		}
 
 		var agent AgentConfig
-		if err := yaml.Unmarshal(valueBytes, &agent); err != nil {
+		if err := yaml.UnmarshalWithOptions(valueBytes, &agent, yaml.DisallowUnknownField()); err != nil {
 			return fmt.Errorf("failed to unmarshal agent config for %s: %w", name, err)
 		}
 

pkg/config/latest/types_test.go

@@ -121,6 +121,40 @@ func TestThinkingBudget_MarshalUnmarshal_Zero(t *testing.T) {
 	require.Equal(t, "thinking_budget: 0\n", string(output))
 }
 
+func TestAgents_UnmarshalYAML_RejectsUnknownFields(t *testing.T) {
+	t.Parallel()
+
+	// "instructions" (plural) is not a valid field; the correct field is "instruction" (singular).
+	// Agents.UnmarshalYAML must reject it so that typos don't silently drop config.
+	input := []byte(`version: "5"
+agents:
+  root:
+    model: openai/gpt-4o
+    instructions: "You are a helpful assistant."
+`)
+
+	_, err := Parse(input)
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "instructions")
+}
+
+func TestAgents_UnmarshalYAML_AcceptsValidConfig(t *testing.T) {
+	t.Parallel()
+
+	input := []byte(`version: "5"
+agents:
+  root:
+    model: openai/gpt-4o
+    instruction: "You are a helpful assistant."
+`)
+
+	cfg, err := Parse(input)
+	require.NoError(t, err)
+	require.Len(t, cfg.Agents, 1)
+	require.Equal(t, "root", cfg.Agents[0].Name)
+	require.Equal(t, "You are a helpful assistant.", cfg.Agents[0].Instruction)
+}
+
 func TestRAGStrategyConfig_MarshalUnmarshal_FlattenedParams(t *testing.T) {
 	t.Parallel()
 

pkg/config/v4/types.go

@@ -45,7 +45,7 @@ func (c *Agents) UnmarshalYAML(unmarshal func(any) error) error {
 		}
 
 		var agent AgentConfig
-		if err := yaml.Unmarshal(valueBytes, &agent); err != nil {
+		if err := yaml.UnmarshalWithOptions(valueBytes, &agent, yaml.DisallowUnknownField()); err != nil {
 			return fmt.Errorf("failed to unmarshal agent config for %s: %w", name, err)
 		}
 

pkg/config/v4/types_test.go

@@ -121,6 +121,40 @@ func TestThinkingBudget_MarshalUnmarshal_Zero(t *testing.T) {
 	require.Equal(t, "thinking_budget: 0\n", string(output))
 }
 
+func TestAgents_UnmarshalYAML_RejectsUnknownFields(t *testing.T) {
+	t.Parallel()
+
+	// "instructions" (plural) is not a valid field; the correct field is "instruction" (singular).
+	// Agents.UnmarshalYAML must reject it so that typos don't silently drop config.
+	input := []byte(`version: "4"
+agents:
+  root:
+    model: openai/gpt-4o
+    instructions: "You are a helpful assistant."
+`)
+
+	_, err := Parse(input)
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "instructions")
+}
+
+func TestAgents_UnmarshalYAML_AcceptsValidConfig(t *testing.T) {
+	t.Parallel()
+
+	input := []byte(`version: "4"
+agents:
+  root:
+    model: openai/gpt-4o
+    instruction: "You are a helpful assistant."
+`)
+
+	cfg, err := Parse(input)
+	require.NoError(t, err)
+	require.Len(t, cfg.Agents, 1)
+	require.Equal(t, "root", cfg.Agents[0].Name)
+	require.Equal(t, "You are a helpful assistant.", cfg.Agents[0].Instruction)
+}
+
 func TestRAGStrategyConfig_MarshalUnmarshal_FlattenedParams(t *testing.T) {
 	t.Parallel()