fix: strengthen group chat collaboration guidance

Audit confirmed that single, sequential, concurrent, and handoff are mapped to distinct workflow builders as intended. Group-chat was also using the correct round-robin manager, but the agents only had one-shot role prompts, so the reviewer could behave like a fresh assistant instead of refining the draft in progress. Add explicit runtime guidance for group-chat participants plus clearer built-in Writer/Reviewer instructions and regression coverage.

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

sidecar/src/Eryx.AgentHost/Services/AgentInstructionComposer.cs

@@ -20,6 +20,25 @@ internal static class AgentInstructionComposer
               """
             : string.Empty;
 
+        if (string.Equals(pattern.Mode, "group-chat", StringComparison.OrdinalIgnoreCase))
+        {
+            string groupChatGuidance = agentIndex == 0
+                ? """
+                  You are participating in a collaborative multi-turn group chat under a round-robin manager.
+                  On your first turn, produce the initial draft for the user.
+                  On later turns, refine your earlier draft based on the other agents' feedback instead of restarting from scratch.
+                  Do not greet the user again or reset the conversation once work is underway.
+                  """
+                : """
+                  You are participating in a collaborative multi-turn group chat under a round-robin manager.
+                  Build on the latest draft from the other agents and contribute specific critique or improvements.
+                  Do not restart the conversation, greet the user again, or answer as though no draft exists yet.
+                  Focus on refining the answer already in progress.
+                  """;
+
+            return JoinInstructionBlocks(baseInstructions, workspaceGuidance, groupChatGuidance);
+        }
+
         if (!string.Equals(pattern.Mode, "handoff", StringComparison.OrdinalIgnoreCase))
         {
             return JoinInstructionBlocks(baseInstructions, workspaceGuidance);

sidecar/tests/Eryx.AgentHost.Tests/AgentInstructionComposerTests.cs

@@ -25,6 +25,34 @@ public sealed class AgentInstructionComposerTests
         Assert.Equal("Review the proposal.", instructions);
     }
 
+    [Fact]
+    public void Compose_StrengthensGroupChatCollaborationRoles()
+    {
+        PatternDefinitionDto pattern = new()
+        {
+            Id = "pattern-group-chat",
+            Name = "Group Chat",
+            Mode = "group-chat",
+            Availability = "available",
+        };
+        PatternAgentDefinitionDto writer = CreateAgent(
+            id: "agent-group-writer",
+            name: "Writer",
+            instructions: "Draft an answer.");
+        PatternAgentDefinitionDto reviewer = CreateAgent(
+            id: "agent-group-reviewer",
+            name: "Reviewer",
+            instructions: "Review the draft.");
+
+        string writerInstructions = AgentInstructionComposer.Compose(pattern, writer, agentIndex: 0);
+        string reviewerInstructions = AgentInstructionComposer.Compose(pattern, reviewer, agentIndex: 1);
+
+        Assert.Contains("collaborative multi-turn group chat", writerInstructions, StringComparison.OrdinalIgnoreCase);
+        Assert.Contains("refine your earlier draft", writerInstructions, StringComparison.OrdinalIgnoreCase);
+        Assert.Contains("specific critique or improvements", reviewerInstructions, StringComparison.OrdinalIgnoreCase);
+        Assert.Contains("Do not restart the conversation", reviewerInstructions, StringComparison.OrdinalIgnoreCase);
+    }
+
     [Fact]
     public void Compose_StrengthensHandoffTriageInstructions()
     {

src/shared/domain/pattern.ts

@@ -204,7 +204,8 @@ export function createBuiltinPatterns(timestamp: string): PatternDefinition[] {
           id: 'agent-group-writer',
           name: 'Writer',
           description: 'Produces candidate answers.',
-          instructions: 'You draft a concise, useful answer for the task.',
+          instructions:
+            'You draft a concise, useful answer for the task. On later turns, refine your earlier draft based on peer feedback rather than restarting.',
           model: defaultModels.gpt54,
           reasoningEffort: 'medium',
         },
@@ -212,7 +213,8 @@ export function createBuiltinPatterns(timestamp: string): PatternDefinition[] {
           id: 'agent-group-reviewer',
           name: 'Reviewer',
           description: 'Critiques and refines the answer.',
-          instructions: 'You review the latest answer and improve it when needed.',
+          instructions:
+            'You review the latest draft and offer specific improvements. Focus on critique and refinement instead of restarting the conversation.',
           model: defaultModels.claude,
           reasoningEffort: 'medium',
         },

tests/shared/pattern.test.ts

@@ -90,4 +90,15 @@ describe('pattern validation', () => {
     expect(handoff?.agents[1].instructions).toContain('own the substantive answer');
     expect(handoff?.agents[2].instructions).toContain('own the substantive answer');
   });
+
+  test('group chat builtin instructions frame iterative drafting and review', () => {
+    const groupChat = createBuiltinPatterns(BUILTIN_TIMESTAMP).find(
+      (pattern) => pattern.mode === 'group-chat',
+    );
+
+    expect(groupChat).toBeDefined();
+    expect(groupChat?.agents[0].instructions).toContain('refine your earlier draft');
+    expect(groupChat?.agents[1].instructions).toContain('specific improvements');
+    expect(groupChat?.agents[1].instructions).toContain('instead of restarting the conversation');
+  });
 });