diff --git a/modules/sessiontag/sessiontag.go b/modules/sessiontag/sessiontag.go
new file mode 100644
index 0000000000..4aaa3262cb
--- /dev/null
+++ b/modules/sessiontag/sessiontag.go
@@ -0,0 +1,38 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+// Package sessiontag carries a per-page-load identifier from the
+// originating HTTP request down to the service- and model-layer SSE
+// publishers. The publishers echo the tag back inside event payloads so
+// the originating browser tab can suppress its own event after it has
+// already applied the optimistic update locally.
+//
+// It is deliberately tiny and dependency-free so any feature that emits
+// Server-Sent Events (project boards, milestones, ...) can share one
+// context key without importing one another.
+package sessiontag
+
+import "context"
+
+// sessionTagCtxKey is the context key under which the X-Session-Tag
+// value from the originating HTTP request is stashed.
+type sessionTagCtxKey struct{}
+
+// WithSessionTag returns ctx decorated with the provided session tag.
+// Web/API middleware reads the X-Session-Tag header and calls this so
+// service- and model-layer publishers can pull the tag back out.
+func WithSessionTag(ctx context.Context, tag string) context.Context {
+	if tag == "" {
+		return ctx
+	}
+	return context.WithValue(ctx, sessionTagCtxKey{}, tag)
+}
+
+// SessionTagFromContext returns the session tag previously stored via
+// WithSessionTag, or "" when none was set.
+func SessionTagFromContext(ctx context.Context) string {
+	if v, ok := ctx.Value(sessionTagCtxKey{}).(string); ok {
+		return v
+	}
+	return ""
+}
diff --git a/routers/common/session_tag.go b/routers/common/session_tag.go
index bd91fc2421..fbbdde2065 100644
--- a/routers/common/session_tag.go
+++ b/routers/common/session_tag.go
@@ -6,7 +6,7 @@ package common
 import (
 	"net/http"
 
-	"code.gitea.io/gitea/services/project_events"
+	"code.gitea.io/gitea/modules/sessiontag"
 )
 
 // SessionTagHeader is the HTTP header browser tabs use to broadcast a
@@ -17,7 +17,7 @@ const SessionTagHeader = "X-Session-Tag"
 
 // SessionTagMiddleware decorates each incoming request's context with
 // the X-Session-Tag header value when present. Service- and model-
-// layer publishers read the value via project_events.SessionTagFromContext.
+// layer publishers read the value via sessiontag.SessionTagFromContext.
 //
 // Empty / missing headers are a no-op.
 func SessionTagMiddleware() func(http.Handler) http.Handler {
@@ -25,7 +25,7 @@ func SessionTagMiddleware() func(http.Handler) http.Handler {
 		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 			tag := r.Header.Get(SessionTagHeader)
 			if tag != "" {
-				ctx := project_events.WithSessionTag(r.Context(), tag)
+				ctx := sessiontag.WithSessionTag(r.Context(), tag)
 				r = r.WithContext(ctx)
 			}
 			next.ServeHTTP(w, r)
diff --git a/services/project_events/events.go b/services/project_events/events.go
index e168f6dcd1..936f32e47e 100644
--- a/services/project_events/events.go
+++ b/services/project_events/events.go
@@ -25,31 +25,20 @@ import (
 	"code.gitea.io/gitea/modules/graceful"
 	"code.gitea.io/gitea/modules/json"
 	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/sessiontag"
 )
 
-// sessionTagCtxKey is the context key under which the X-Session-Tag value
-// from the originating HTTP request is stashed. Publishers read it via
-// SessionTagFromContext to attach to outgoing events so the originating
-// browser tab can suppress its own echo.
-type sessionTagCtxKey struct{}
-
-// WithSessionTag returns ctx decorated with the provided session tag.
-// Web/API middleware reads the X-Session-Tag header and calls this so
-// service- and model-layer publishers can pull the tag back out.
+// WithSessionTag re-exports modules/sessiontag.WithSessionTag so existing
+// callers of project_events keep working after the context-key helper was
+// extracted into its own dependency-free package (shared with
+// milestone_events and any future SSE feature).
 func WithSessionTag(ctx context.Context, tag string) context.Context {
-	if tag == "" {
-		return ctx
-	}
-	return context.WithValue(ctx, sessionTagCtxKey{}, tag)
+	return sessiontag.WithSessionTag(ctx, tag)
 }
 
-// SessionTagFromContext returns the session tag previously stored via
-// WithSessionTag, or "" when none was set.
+// SessionTagFromContext re-exports modules/sessiontag.SessionTagFromContext.
 func SessionTagFromContext(ctx context.Context) string {
-	if v, ok := ctx.Value(sessionTagCtxKey{}).(string); ok {
-		return v
-	}
-	return ""
+	return sessiontag.SessionTagFromContext(ctx)
 }
 
 // Event payload structs ------------------------------------------------------