diff --git a/server/internal/handler/daemon.go b/server/internal/handler/daemon.go
index e593868339..78775541b2 100644
--- a/server/internal/handler/daemon.go
+++ b/server/internal/handler/daemon.go
@@ -1135,7 +1135,7 @@ func logHeartbeatEndpointSlow(runtimeID, outcome, authPath string, start time.Ti
 // logClaimEndpointSlow emits one structured log when the /tasks/claim endpoint
 // exceeds 500ms, splitting auth / claim / response-build phases so the prod
 // tail can be diagnosed without flooding logs at normal poll rates.
-func logClaimEndpointSlow(runtimeID, outcome string, start time.Time, authMs, claimMs, buildMs int64) {
+func logClaimEndpointSlow(runtimeID, outcome string, start time.Time, authMs, claimMs, buildMs int64, payloadBytes, agentSkillCount, builtinSkillCount, skillPayloadBytes int) {
 	totalMs := time.Since(start).Milliseconds()
 	if totalMs < 500 {
 		return
@@ -1147,6 +1147,10 @@ func logClaimEndpointSlow(runtimeID, outcome string, start time.Time, authMs, cl
 		"auth_ms", authMs,
 		"claim_ms", claimMs,
 		"build_ms", buildMs,
+		"payload_bytes", payloadBytes,
+		"agent_skill_count", agentSkillCount,
+		"builtin_skill_count", builtinSkillCount,
+		"skill_payload_bytes", skillPayloadBytes,
 	)
 }
 
@@ -1159,6 +1163,10 @@ func (h *Handler) ClaimTaskByRuntime(w http.ResponseWriter, r *http.Request) {
 	var (
 		outcome                  = "unauth"
 		authMs, claimMs, buildMs int64
+		payloadBytes             int
+		agentSkillCount          int
+		builtinSkillCount        int
+		skillPayloadBytes        int
 		buildStart               time.Time
 	)
 	defer func() {
@@ -1168,7 +1176,7 @@ func (h *Handler) ClaimTaskByRuntime(w http.ResponseWriter, r *http.Request) {
 		if !buildStart.IsZero() {
 			buildMs = time.Since(buildStart).Milliseconds()
 		}
-		logClaimEndpointSlow(runtimeID, outcome, start, authMs, claimMs, buildMs)
+		logClaimEndpointSlow(runtimeID, outcome, start, authMs, claimMs, buildMs, payloadBytes, agentSkillCount, builtinSkillCount, skillPayloadBytes)
 	}()
 
 	// Verify the caller owns this runtime's workspace. The runtime's
@@ -1194,7 +1202,7 @@ func (h *Handler) ClaimTaskByRuntime(w http.ResponseWriter, r *http.Request) {
 
 	if task == nil {
 		slog.Debug("no task to claim", "runtime_id", runtimeID)
-		writeJSON(w, http.StatusOK, map[string]any{"task": nil})
+		payloadBytes, _ = writeMeasuredJSON(w, http.StatusOK, map[string]any{"task": nil})
 		outcome = "no_task"
 		return
 	}
@@ -1209,7 +1217,10 @@ func (h *Handler) ClaimTaskByRuntime(w http.ResponseWriter, r *http.Request) {
 		// names carry a "multica-" prefix so their on-disk slugs never collide
 		// with a user-authored workspace skill (see writeSkillFiles).
 		skills := h.TaskService.LoadAgentSkills(r.Context(), task.AgentID)
-		skills = append(skills, h.TaskService.BuiltinSkills()...)
+		agentSkillCount = len(skills)
+		builtinSkills := h.TaskService.BuiltinSkills()
+		builtinSkillCount = len(builtinSkills)
+		skills = append(skills, builtinSkills...)
 		var customEnv map[string]string
 		if agent.CustomEnv != nil {
 			if err := json.Unmarshal(agent.CustomEnv, &customEnv); err != nil {
@@ -1800,7 +1811,12 @@ func (h *Handler) ClaimTaskByRuntime(w http.ResponseWriter, r *http.Request) {
 	resp.AuthToken = tokenStr
 
 	slog.Info("task claimed by runtime", "task_id", uuidToString(task.ID), "runtime_id", runtimeID, "agent_id", uuidToString(task.AgentID), "prior_session", resp.PriorSessionID)
-	writeJSON(w, http.StatusOK, map[string]any{"task": resp})
+	if resp.Agent != nil && len(resp.Agent.Skills) > 0 {
+		if skillPayload, err := json.Marshal(resp.Agent.Skills); err == nil {
+			skillPayloadBytes = len(skillPayload)
+		}
+	}
+	payloadBytes, _ = writeMeasuredJSON(w, http.StatusOK, map[string]any{"task": resp})
 }
 
 // trailingUserMessages returns the run of user messages after the last
diff --git a/server/internal/handler/daemon_test.go b/server/internal/handler/daemon_test.go
index cffbc82a9a..d41c746b12 100644
--- a/server/internal/handler/daemon_test.go
+++ b/server/internal/handler/daemon_test.go
@@ -6,6 +6,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"log/slog"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -22,6 +23,29 @@ import (
 	"github.com/multica-ai/multica/server/pkg/protocol"
 )
 
+func TestLogClaimEndpointSlowIncludesPayloadFields(t *testing.T) {
+	var logs bytes.Buffer
+	prev := slog.Default()
+	slog.SetDefault(slog.New(slog.NewTextHandler(&logs, &slog.HandlerOptions{Level: slog.LevelInfo})))
+	t.Cleanup(func() { slog.SetDefault(prev) })
+
+	logClaimEndpointSlow("runtime-1", "claimed", time.Now().Add(-600*time.Millisecond), 10, 20, 30, 4096, 2, 8, 3072)
+
+	got := logs.String()
+	for _, want := range []string{
+		"msg=\"claim_endpoint slow\"",
+		"runtime_id=runtime-1",
+		"payload_bytes=4096",
+		"agent_skill_count=2",
+		"builtin_skill_count=8",
+		"skill_payload_bytes=3072",
+	} {
+		if !strings.Contains(got, want) {
+			t.Fatalf("slow claim log missing %q in %s", want, got)
+		}
+	}
+}
+
 // slowProbeLocalSkillListStore wraps a LocalSkillListStore but blocks inside
 // HasPending until the provided context is cancelled. PopPending delegates
 // to the underlying store. Used to verify that a stalled probe cannot wedge
diff --git a/server/internal/handler/handler.go b/server/internal/handler/handler.go
index dd66ad51ba..a89e627cf7 100644
--- a/server/internal/handler/handler.go
+++ b/server/internal/handler/handler.go
@@ -230,6 +230,24 @@ func writeJSON(w http.ResponseWriter, status int, v any) {
 	json.NewEncoder(w).Encode(v)
 }
 
+// writeMeasuredJSON behaves like writeJSON but returns the encoded body size so
+// callers can record payload bytes in slow-endpoint diagnostics. It measures the
+// uncompressed JSON length and is unrelated to transport compression.
+func writeMeasuredJSON(w http.ResponseWriter, status int, v any) (int, error) {
+	body, err := json.Marshal(v)
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, "failed to encode response")
+		return 0, err
+	}
+	body = append(body, '\n')
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	if _, err := w.Write(body); err != nil {
+		return len(body), err
+	}
+	return len(body), nil
+}
+
 func writeError(w http.ResponseWriter, status int, msg string) {
 	writeJSON(w, status, map[string]string{"error": msg})
 }
diff --git a/server/internal/handler/handler_writejson_test.go b/server/internal/handler/handler_writejson_test.go
new file mode 100644
index 0000000000..268485f983
--- /dev/null
+++ b/server/internal/handler/handler_writejson_test.go
@@ -0,0 +1,99 @@
+package handler
+
+import (
+	"bytes"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+// TestWriteMeasuredJSONByteIdenticalToWriteJSON locks the load-bearing assumption
+// behind the F2 claim-observability patch: swapping writeJSON for writeMeasuredJSON
+// at the /tasks/claim response sites must not change a single byte on the wire.
+//
+// writeJSON encodes via json.NewEncoder(w).Encode (which appends a trailing
+// newline); writeMeasuredJSON marshals via json.Marshal and appends the newline
+// by hand. Both HTML-escape by default, so the emitted bytes must match for every
+// input. This table-driven test fails closed if that invariant ever drifts, so the
+// "no wire-behavior change" claim is provable rather than reasoned.
+func TestWriteMeasuredJSONByteIdenticalToWriteJSON(t *testing.T) {
+	type skill struct {
+		Name        string            `json:"name"`
+		Description string            `json:"description"`
+		Files       map[string]string `json:"files"`
+	}
+	type claimResp struct {
+		ID     string   `json:"id"`
+		Name   string   `json:"name"`
+		Skills []skill  `json:"skills"`
+		Args   []string `json:"args"`
+	}
+
+	cases := []struct {
+		name string
+		v    any
+	}{
+		{"nil", nil},
+		{"no_task", map[string]any{"task": nil}},
+		{"empty_map", map[string]any{}},
+		{"empty_slice", []string{}},
+		{"scalar_string", "plain string"},
+		{"scalar_bool", true},
+		{"numbers", map[string]any{"i": 42, "f": 3.5, "neg": -17, "big": 1234567890123}},
+		{"html_escapable", map[string]any{"s": `a<b> & "c" 'd' <script>`}},
+		{"ampersand_lt_gt", map[string]any{"raw": "1 < 2 && 3 > 2"}},
+		{"unicode_and_separators", map[string]any{"s": "héllo 世界 🚀 

"}},
+		{"nested", map[string]any{"a": []any{1, "two", true, nil}, "b": map[string]any{"c": []int{1, 2, 3}}}},
+		{"large_claim_with_skills", map[string]any{"task": claimResp{
+			ID:   "11111111-2222-3333-4444-555555555555",
+			Name: "agent <CC> & friends",
+			Skills: []skill{
+				{Name: "multica-working-on-issues", Description: "do work <safely> & well", Files: map[string]string{"SKILL.md": "# Title\n<b>x</b> & y"}},
+				{Name: "multica-mentioning", Description: "ping people", Files: map[string]string{"SKILL.md": "line1\nline2"}},
+			},
+			Args: []string{"--flag", "a<b", "c&d"},
+		}}},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			recEnc := httptest.NewRecorder()
+			writeJSON(recEnc, http.StatusOK, tc.v)
+
+			recMeasured := httptest.NewRecorder()
+			n, err := writeMeasuredJSON(recMeasured, http.StatusOK, tc.v)
+			if err != nil {
+				t.Fatalf("writeMeasuredJSON returned error: %v", err)
+			}
+
+			encBody := recEnc.Body.Bytes()
+			measuredBody := recMeasured.Body.Bytes()
+
+			if !bytes.Equal(encBody, measuredBody) {
+				t.Fatalf("wire bytes differ:\n writeJSON         = %q\n writeMeasuredJSON = %q", encBody, measuredBody)
+			}
+			if n != len(measuredBody) {
+				t.Fatalf("reported payload bytes %d != actual body length %d", n, len(measuredBody))
+			}
+			if n != len(encBody) {
+				t.Fatalf("reported payload bytes %d != writeJSON body length %d", n, len(encBody))
+			}
+			if recEnc.Code != recMeasured.Code {
+				t.Fatalf("status code differs: writeJSON=%d writeMeasuredJSON=%d", recEnc.Code, recMeasured.Code)
+			}
+			if got, want := recMeasured.Header().Get("Content-Type"), recEnc.Header().Get("Content-Type"); got != want {
+				t.Fatalf("Content-Type differs: writeMeasuredJSON=%q writeJSON=%q", got, want)
+			}
+		})
+	}
+
+	// Sanity guard: both encoders HTML-escape by default, so a literal '<' rune must
+	// not survive into the body (it is emitted as the escaped form). This documents
+	// the escaping behaviour that makes the byte-identity comparison meaningful,
+	// without depending on the escaped literal appearing in source.
+	rec := httptest.NewRecorder()
+	writeJSON(rec, http.StatusOK, map[string]string{"x": "<&>"})
+	if bytes.ContainsRune(rec.Body.Bytes(), '<') {
+		t.Fatalf("expected '<' to be HTML-escaped out of the body, got %q", rec.Body.String())
+	}
+}