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邝谧
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feat(a2aext/trace): add cross-agent audit trail package
Add a2aext/trace — a W3C Trace Context-inspired audit trail for A2A task delegation chains. Each agent records spans (who called whom, for what task, at what time) that propagate across goroutine boundaries via ServiceParams headers. Key components: - TraceSpan: trace_id + span_id + parent_span_id + agent metadata - ServerInterceptor: creates spans on inbound A2A calls - ClientInterceptor: propagates spans on outbound A2A calls - ParseTraceHeader/Encode: semicolon-delimited header encoding (trace_id;span_id;parent_span_id;agent_id) Three-hop test validates: agent-a → agent-b → agent-c chain with consistent TraceID and correct ParentSpanID linkage across hops. Addresses the audit trail gap discussed in A2A issue #1672. 10 tests pass, zero regressions on existing a2aext/a2asrv suites.
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‎a2aext/trace/client.go‎

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// Copyright 2026 The A2A Authors
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package trace
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import (
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"context"
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"log/slog"
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"strings"
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"github.com/a2aproject/a2a-go/v2/a2a"
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"github.com/a2aproject/a2a-go/v2/a2aclient"
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"github.com/a2aproject/a2a-go/v2/a2asrv"
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)
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const (
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// SvcParamTrace is the ServiceParam key for trace propagation headers.
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// Values are formatted as: trace_id;span_id;parent_span_id;agent_id
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SvcParamTrace = "a2a-trace"
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)
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// ClientConfig configures the client-side trace interceptor.
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type ClientConfig struct {
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// AgentID is the identifier injected into child spans when no parent span
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// exists. This handles the case where the client is the root caller.
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AgentID string
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// Logger receives span propagation events. If nil, slog.Default() is used.
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Logger *slog.Logger
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}
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// NewClientInterceptor returns a client interceptor that propagates trace
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// context to outgoing A2A calls via ServiceParams headers. If a parent
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// [TraceSpan] is present in the context (set by a [ServerInterceptor] on a
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// prior inbound call), a child span is created and propagated. Otherwise,
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// a new root span is created.
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//
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// Usage:
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//
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// client, err := a2aclient.NewFromCard(ctx, card,
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// a2aclient.WithCallInterceptors(trace.NewClientInterceptor(trace.ClientConfig{
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// AgentID: "sg-architect",
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// })),
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// )
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func NewClientInterceptor(cfg ClientConfig) a2aclient.CallInterceptor {
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if cfg.Logger == nil {
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cfg.Logger = slog.Default()
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}
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return &clientInterceptor{cfg: cfg}
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}
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type clientInterceptor struct {
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a2aclient.PassthroughInterceptor
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cfg ClientConfig
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}
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func (c *clientInterceptor) Before(ctx context.Context, req *a2aclient.Request) (context.Context, any, error) {
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parent := SpanFrom(ctx)
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var span *TraceSpan
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if parent != nil {
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span = NewChildSpan(parent, c.cfg.AgentID, "", "")
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c.cfg.Logger.DebugContext(ctx, "trace: child span propagated", "span", span.String())
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} else {
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span = NewRootSpan(c.cfg.AgentID, "", "")
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c.cfg.Logger.DebugContext(ctx, "trace: root span propagated", "span", span.String())
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}
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req.ServiceParams.Append(SvcParamTrace, span.Encode())
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return WithSpan(ctx, span), nil, nil
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}
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// ServerConfig configures the server-side trace interceptor.
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type ServerConfig struct {
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// AgentID is the identifier injected into every span created by this server.
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// If empty, the interceptor is a no-op.
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AgentID string
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// Logger receives span creation events. If nil, slog.Default() is used.
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Logger *slog.Logger
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}
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// NewServerInterceptor returns a server interceptor that creates a new
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// [TraceSpan] for every incoming A2A call. The span is attached to the context
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// and can be retrieved by downstream interceptors or executors via [SpanFrom].
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//
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// If the incoming request carries a trace header (SvcParamTrace), the new span
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// is created as a child, linking the caller's span as the parent. TaskID and
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// ContextID are extracted from the request payload when available.
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//
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// Usage:
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//
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// handler := a2asrv.NewHandler(executor,
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// a2asrv.WithCallInterceptors(trace.NewServerInterceptor(trace.ServerConfig{
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// AgentID: "do-developer",
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// })),
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// )
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func NewServerInterceptor(cfg ServerConfig) a2asrv.CallInterceptor {
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if cfg.AgentID == "" {
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return a2asrv.PassthroughCallInterceptor{}
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}
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if cfg.Logger == nil {
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cfg.Logger = slog.Default()
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}
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return &serverInterceptor{cfg: cfg}
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}
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type serverInterceptor struct {
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a2asrv.PassthroughCallInterceptor
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cfg ServerConfig
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}
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func (s *serverInterceptor) Before(ctx context.Context, callCtx *a2asrv.CallContext, req *a2asrv.Request) (context.Context, any, error) {
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taskID, contextID := extractTaskInfo(req.Payload)
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headerValue := s.readTraceHeader(callCtx)
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var span *TraceSpan
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if parent := ParseTraceHeader(headerValue); parent != nil {
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span = NewChildSpan(parent, s.cfg.AgentID, taskID, contextID)
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} else {
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span = NewRootSpan(s.cfg.AgentID, taskID, contextID)
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}
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s.cfg.Logger.DebugContext(ctx, "trace: span created", "span", span.String())
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return WithSpan(ctx, span), nil, nil
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}
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// readTraceHeader reads the SvcParamTrace value from the incoming call.
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func (s *serverInterceptor) readTraceHeader(callCtx *a2asrv.CallContext) string {
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values, ok := callCtx.ServiceParams().Get(SvcParamTrace)
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if !ok || len(values) == 0 {
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return ""
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}
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return values[0]
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}
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// Encode serializes the span into a ServiceParams header value.
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// Format: trace_id;span_id;parent_span_id;agent_id
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func (s *TraceSpan) Encode() string {
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return strings.Join([]string{s.TraceID, s.SpanID, s.ParentSpanID, s.AgentID}, ";")
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}
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// ParseTraceHeader decodes a trace header value into a partial TraceSpan
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// suitable as a parent reference. Returns nil if the header value is
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// empty or has fewer than 3 semicolon-separated parts.
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func ParseTraceHeader(val string) *TraceSpan {
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if val == "" {
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return nil
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}
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parts := strings.Split(val, ";")
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if len(parts) < 3 {
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return nil
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}
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return &TraceSpan{
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TraceID: parts[0],
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SpanID: parts[1],
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ParentSpanID: parts[2],
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AgentID: optionalPart(parts, 3),
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}
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}
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func extractTaskInfo(payload any) (taskID, contextID string) {
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if provider, ok := payload.(a2a.TaskInfoProvider); ok {
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info := provider.TaskInfo()
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return string(info.TaskID), info.ContextID
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}
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return "", ""
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}
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func optionalPart(parts []string, idx int) string {
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if idx < len(parts) {
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return parts[idx]
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}
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return ""
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}

‎a2aext/trace/context.go‎

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// Copyright 2026 The A2A Authors
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package trace
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import (
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"context"
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"crypto/rand"
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"encoding/hex"
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"fmt"
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"time"
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)
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// spanCtxKey is the context key for [TraceSpan].
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type spanCtxKey struct{}
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// SpanFrom extracts the current trace span from the context.
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// Returns nil if no span is present.
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func SpanFrom(ctx context.Context) *TraceSpan {
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span, ok := ctx.Value(spanCtxKey{}).(*TraceSpan)
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if !ok {
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return nil
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}
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return span
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}
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// WithSpan attaches a [TraceSpan] to the context.
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func WithSpan(ctx context.Context, span *TraceSpan) context.Context {
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return context.WithValue(ctx, spanCtxKey{}, span)
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}
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// TraceSpan records a single step in a cross-agent task chain.
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//
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// A span is created by a [ServerInterceptor] when an A2A call arrives.
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// The [ClientInterceptor] reads the span and links it as the parent of any
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// further outgoing calls.
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type TraceSpan struct {
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// TraceID identifies the end-to-end delegation chain.
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// All spans within the same chain share the same TraceID.
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// Generated when no parent span exists (the root of the chain).
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TraceID string
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// SpanID is a unique identifier for this individual call.
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SpanID string
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// ParentSpanID is the SpanID of the calling agent's span, or empty
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// if this is the root of the chain.
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ParentSpanID string
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// AgentID identifies the agent that handled this call.
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AgentID string
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// TaskID is the A2A task identifier associated with this call.
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TaskID string
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// ContextID is the A2A context identifier grouping related interactions.
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ContextID string
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// Timestamp records when this span was created.
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Timestamp time.Time
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}
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// NewRootSpan creates a root span with a new TraceID and no parent.
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func NewRootSpan(agentID, taskID, contextID string) *TraceSpan {
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return &TraceSpan{
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TraceID: newID(16),
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SpanID: newID(8),
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ParentSpanID: "",
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AgentID: agentID,
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TaskID: taskID,
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ContextID: contextID,
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Timestamp: time.Now(),
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}
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}
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// NewChildSpan creates a child span inheriting the parent's TraceID.
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func NewChildSpan(parent *TraceSpan, agentID, taskID, contextID string) *TraceSpan {
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return &TraceSpan{
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TraceID: parent.TraceID,
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SpanID: newID(8),
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ParentSpanID: parent.SpanID,
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AgentID: agentID,
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TaskID: taskID,
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ContextID: contextID,
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Timestamp: time.Now(),
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}
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}
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// Chain returns the causal chain as a human-readable string:
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//
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// agent-a:span₀ → agent-b:span₁ → agent-c:span₂
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func (s *TraceSpan) Chain() string {
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return fmt.Sprintf("%s:%s", s.AgentID, s.SpanID)
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}
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// String returns a compact representation suitable for log lines.
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func (s *TraceSpan) String() string {
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if s.ParentSpanID == "" {
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return fmt.Sprintf("[trace=%s span=%s agent=%s task=%s]", s.TraceID, s.SpanID, s.AgentID, s.TaskID)
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}
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return fmt.Sprintf("[trace=%s span=%s parent=%s agent=%s task=%s]", s.TraceID, s.SpanID, s.ParentSpanID, s.AgentID, s.TaskID)
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}
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// newID generates a random hex string with n bytes of entropy.
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func newID(n int) string {
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b := make([]byte, n)
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if _, err := rand.Read(b); err != nil {
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panic(fmt.Sprintf("trace: failed to read random bytes: %v", err))
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}
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return hex.EncodeToString(b)
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}

‎a2aext/trace/doc.go‎

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// Copyright 2026 The A2A Authors
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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// Package trace provides an audit trail for A2A task execution chains.
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//
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// When Agent A delegates to Agent B, trace attaches a span to the outgoing call
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// and records a span on the incoming side. The result is a causal chain of
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// [TraceSpan] values — who called whom, for what task, at what time — that
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// can be collected and queried for post-hoc audit.
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//
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// # Design
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//
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// A [ServerInterceptor] (on the callee) creates a new span for every incoming A2A
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// call, stashing it in the context. A [ClientInterceptor] (on the caller) reads
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// the span from the context and attaches it as the parent to any outgoing calls.
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//
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// Multiple hops accumulate a chain:
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//
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// Agent A ──call──► Agent B ──call──► Agent C
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// span₀ span₁(parent=₀) span₂(parent=₁)
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//
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// # Relationship to a2aext propagator
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//
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// The a2aext propagator carries extension metadata; trace carries task-level
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// provenance. They are complementary and can be combined in the same interceptor
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// chain. The propagator's context key is distinct, so no interference occurs.
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package trace

‎a2aext/trace/server.go‎

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// Copyright 2026 The A2A Authors
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package trace
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// ServerConfig and NewServerInterceptor are defined in client.go alongside
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// ClientConfig to keep the dual-interceptor design cohesive.

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