Add checkpoint types and stop signal for agent suspension

Introduce Checkpoint, CheckpointStore, SuspendedError, AgentRegistry,
and CompletedCall types. Add cooperative stop signal via context.
Export CompletedCall (was unexported completedCall) so checkpoints
can reference completed tool results. Add JSON tags to ToolResult
and ApprovalResult for checkpoint serialization.

Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>

pkg/agent/approval.go

@@ -29,8 +29,8 @@ type (
 	}
 
 	ApprovalResult struct {
-		Approved bool
-		Message  string
+		Approved bool   `json:"approved"`
+		Message  string `json:"message"`
 	}
 
 	ResumeInput struct {

pkg/agent/checkpoint.go

@@ -0,0 +1,80 @@
+// 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 (
+	"context"
+
+	"go.probo.inc/probo/pkg/llm"
+)
+
+type (
+	CheckpointStatus string
+
+	Checkpoint struct {
+		Version       int              `json:"version"`
+		Status        CheckpointStatus `json:"status"`
+		AgentName     string           `json:"agent_name"`
+		Messages      []llm.Message    `json:"messages"`
+		Usage         llm.Usage        `json:"usage"`
+		Turns         int              `json:"turns"`
+		ToolUsedInRun bool             `json:"tool_used_in_run"`
+
+		// Approval-interrupted checkpoints carry pending tool calls.
+		PendingToolCalls []llm.ToolCall            `json:"pending_tool_calls,omitempty"`
+		PendingApprovals []llm.ToolCall            `json:"pending_approvals,omitempty"`
+		ApprovalInput    map[string]ApprovalResult `json:"approval_input,omitempty"`
+
+		// Nested agent-as-tool suspension: one entry per suspended inner agent.
+		AllToolCalls     []llm.ToolCall         `json:"all_tool_calls,omitempty"`
+		InnerCheckpoints map[string]*Checkpoint `json:"inner_checkpoints,omitempty"`
+		CompletedCalls   []CompletedCall        `json:"completed_calls,omitempty"`
+	}
+
+	CompletedCall struct {
+		ToolCallID string     `json:"tool_call_id"`
+		Result     ToolResult `json:"result"`
+	}
+
+	// CheckpointStore is supervisor-internal. Implementations may use raw
+	// run IDs because public API/service methods perform tenant scoping and
+	// authorization before a run reaches the supervisor.
+	CheckpointStore interface {
+		Save(ctx context.Context, runID string, cp *Checkpoint) error
+		Load(ctx context.Context, runID string) (*Checkpoint, error)
+		Delete(ctx context.Context, runID string) error
+	}
+
+	AgentRegistry interface {
+		Agent(name string) (*Agent, error)
+	}
+
+	SuspendedError struct {
+		RunID      string      // Set when the outer loop has a store+runID (supervisor-managed).
+		Checkpoint *Checkpoint // Set when returning from an inner agent-as-tool (no store).
+	}
+)
+
+const (
+	CheckpointVersion  = 1
+	MaxCheckpointBytes = 10 * 1024 * 1024
+
+	CheckpointStatusSuspended        CheckpointStatus = "suspended"
+	CheckpointStatusAwaitingApproval CheckpointStatus = "awaiting_approval"
+)
+
+func (e *SuspendedError) Error() string {
+	return "agent run suspended"
+}

pkg/agent/errors.go

@@ -59,7 +59,7 @@ type (
 		inner          *InterruptedError
 		toolCallID     string
 		allToolCalls   []llm.ToolCall
-		completedCalls []completedCall
+		completedCalls []CompletedCall
 	}
 
 	outerLoopState struct {
@@ -69,14 +69,9 @@ type (
 		turns          int
 		allToolCalls   []llm.ToolCall
 		toolCallID     string
-		completedCalls []completedCall
+		completedCalls []CompletedCall
 		innerInterrupt *InterruptedError
 	}
-
-	completedCall struct {
-		toolCallID string
-		result     ToolResult
-	}
 )
 
 func (e *MaxTurnsExceededError) Error() string {

pkg/agent/stop.go

@@ -0,0 +1,31 @@
+// 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 "context"
+
+type stopSignalKey struct{}
+
+// WithStopSignal returns a derived context carrying a stop channel.
+// The agent loop checks this channel at each turn boundary.
+func WithStopSignal(ctx context.Context, ch <-chan struct{}) context.Context {
+	return context.WithValue(ctx, stopSignalKey{}, ch)
+}
+
+// stopSignalFrom retrieves the stop channel from the context, or nil.
+func stopSignalFrom(ctx context.Context) <-chan struct{} {
+	ch, _ := ctx.Value(stopSignalKey{}).(<-chan struct{})
+	return ch
+}

pkg/agent/stop_test.go

@@ -0,0 +1,63 @@
+// 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 (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestStopSignal(t *testing.T) {
+	t.Parallel()
+
+	t.Run("absent returns nil", func(t *testing.T) {
+		t.Parallel()
+		assert.Nil(t, stopSignalFrom(context.Background()))
+	})
+
+	t.Run("round trip", func(t *testing.T) {
+		t.Parallel()
+		ch := make(chan struct{})
+		ctx := WithStopSignal(context.Background(), ch)
+		assert.Equal(t, (<-chan struct{})(ch), stopSignalFrom(ctx))
+	})
+
+	t.Run("non-blocking when open", func(t *testing.T) {
+		t.Parallel()
+		ch := make(chan struct{})
+		ctx := WithStopSignal(context.Background(), ch)
+		sig := stopSignalFrom(ctx)
+		select {
+		case <-sig:
+			t.Fatal("should not fire")
+		default:
+		}
+	})
+
+	t.Run("fires when closed", func(t *testing.T) {
+		t.Parallel()
+		ch := make(chan struct{})
+		ctx := WithStopSignal(context.Background(), ch)
+		close(ch)
+		sig := stopSignalFrom(ctx)
+		select {
+		case <-sig:
+		default:
+			t.Fatal("should have fired")
+		}
+	})
+}

pkg/agent/tool.go

@@ -25,8 +25,8 @@ import (
 
 type (
 	ToolResult struct {
-		Content string
-		IsError bool
+		Content string `json:"content"`
+		IsError bool   `json:"is_error"`
 	}
 
 	// ToolDescriptor describes a tool's name and LLM definition.