Add agent library

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

pkg/agent/system_prompt_test.go

@@ -0,0 +1,121 @@
+// Copyright (c) 2026 Probo Inc <hello@getprobo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package agent
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestBuildSystemPrompt(t *testing.T) {
+	t.Run(
+		"empty data returns empty string",
+		func(t *testing.T) {
+			got := buildSystemPrompt(systemPromptData{})
+			assert.Equal(t, "", got)
+		},
+	)
+
+	t.Run(
+		"instructions only",
+		func(t *testing.T) {
+			got := buildSystemPrompt(systemPromptData{
+				Instructions: "You are a helpful assistant.",
+			})
+			assert.Equal(t, "You are a helpful assistant.", got)
+		},
+	)
+
+	t.Run(
+		"handoffs only",
+		func(t *testing.T) {
+			got := buildSystemPrompt(systemPromptData{
+				Handoffs: []systemPromptHandoff{
+					{Name: "billing-agent", Description: "Handles billing questions."},
+				},
+			})
+
+			assert.Contains(t, got, "## Handoffs")
+			assert.Contains(t, got, "- billing-agent: Handles billing questions.")
+		},
+	)
+
+	t.Run(
+		"instructions with handoffs",
+		func(t *testing.T) {
+			got := buildSystemPrompt(systemPromptData{
+				Instructions: "You are a triage agent.",
+				Handoffs: []systemPromptHandoff{
+					{Name: "billing-agent", Description: "Handles billing."},
+					{Name: "support-agent", Description: "Handles support."},
+				},
+			})
+
+			assert.True(t, strings.HasPrefix(got, "You are a triage agent."))
+			assert.Contains(t, got, "## Handoffs")
+			assert.Contains(t, got, "- billing-agent: Handles billing.")
+			assert.Contains(t, got, "- support-agent: Handles support.")
+		},
+	)
+
+	t.Run(
+		"handoff without description",
+		func(t *testing.T) {
+			got := buildSystemPrompt(systemPromptData{
+				Handoffs: []systemPromptHandoff{
+					{Name: "silent-agent"},
+				},
+			})
+
+			assert.Contains(t, got, "- silent-agent\n")
+			assert.NotContains(t, got, "- silent-agent:")
+		},
+	)
+
+	t.Run(
+		"multiple handoffs preserve order",
+		func(t *testing.T) {
+			got := buildSystemPrompt(systemPromptData{
+				Handoffs: []systemPromptHandoff{
+					{Name: "alpha"},
+					{Name: "beta"},
+					{Name: "gamma"},
+				},
+			})
+
+			idxAlpha := strings.Index(got, "- alpha")
+			idxBeta := strings.Index(got, "- beta")
+			idxGamma := strings.Index(got, "- gamma")
+
+			assert.Greater(t, idxBeta, idxAlpha)
+			assert.Greater(t, idxGamma, idxBeta)
+		},
+	)
+
+	t.Run(
+		"no trailing newline after last handoff",
+		func(t *testing.T) {
+			got := buildSystemPrompt(systemPromptData{
+				Handoffs: []systemPromptHandoff{
+					{Name: "agent-a", Description: "Does A."},
+				},
+			})
+
+			assert.False(t, strings.HasSuffix(got, "\n\n"), "should not end with double newline")
+		},
+	)
+}