test(sessions): name the expected context block instead of the first render

Comparing later renders against the first only asks whether the renderer is
stable, and a renderer that is stably wrong passes it: any fixed field order
satisfies it, including the unsorted one this change replaces.

It also hid something. The events were tool calls, which
promptContextEvents filters out of the resume context on this branch, so
the block being compared was a single message line and nothing said so. The
test now names the block it expects, on events that survive the filter, and
both failures are visible.

internal/sessions/exec_prompt_order_test.go

@@ -0,0 +1,136 @@
+package sessions
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+)
+
+// ONE SESSION HAS TO RENDER ONE PROMPT.
+//
+// extractText walked a map[string]any directly, and Go randomizes map
+// iteration, so the resume context put the same fields in a different order in
+// every process: `c1 read_file {"path":...}` one run, `{"path":...} c1
+// read_file` the next. That block is part of the user prompt, so the prompt
+// itself changed run to run, which leaves a provider nothing stable to
+// prefix-cache and makes two resumes of one session impossible to diff.
+//
+// PINNED AS AN EXACT STRING, NOT AS TWO RUNS AGREEING. A test that renders the
+// same payload twice in one process and compares the results passes whenever
+// the randomized order happens to repeat, which for a small map is often. The
+// only assertion that cannot pass by luck names the order it expects.
+func TestSummarizePayloadRendersFieldsInKeyOrder(t *testing.T) {
+	payload := map[string]any{
+		"id":        "c1",
+		"name":      "read_file",
+		"arguments": `{"path":"deploy/prod.env"}`,
+		"status":    "ok",
+	}
+
+	// Keys sorted: arguments, id, name, status.
+	const want = `{"path":"deploy/prod.env"} c1 read_file ok`
+	for attempt := range 50 {
+		if got := summarizePayload(payload); got != want {
+			t.Fatalf("attempt %d rendered %q, want %q", attempt, got, want)
+		}
+	}
+}
+
+// The same for a payload that arrives as raw JSON, which is the shape the store
+// hands back, since that decodes into a map and takes the same path.
+func TestSummarizePayloadRendersRawJSONInKeyOrder(t *testing.T) {
+	raw, err := json.Marshal(map[string]any{
+		"zulu":  "last",
+		"alpha": "first",
+		"mike":  "middle",
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	const want = "first middle last"
+	for attempt := range 50 {
+		if got := summarizePayload(json.RawMessage(raw)); got != want {
+			t.Fatalf("attempt %d rendered %q, want %q", attempt, got, want)
+		}
+	}
+}
+
+// The summary short-circuit is untouched: a payload carrying one still answers
+// with it alone, rather than with every field in key order.
+func TestSummarizePayloadStillPrefersASummary(t *testing.T) {
+	payload := map[string]any{
+		"alpha":   "not this",
+		"summary": "this one",
+		"zulu":    "nor this",
+	}
+	if got := summarizePayload(payload); got != "this one" {
+		t.Fatalf("rendered %q, want the summary alone", got)
+	}
+}
+
+// And a list keeps the order it was written in, which is the caller's and not
+// this function's to sort.
+func TestSummarizePayloadKeepsListOrder(t *testing.T) {
+	payload := map[string]any{"items": []any{"third", "first", "second"}}
+	if got := summarizePayload(payload); got != "third first second" {
+		t.Fatalf("rendered %q, want the list in its written order", got)
+	}
+}
+
+// THE WHOLE BLOCK IS STABLE, not just one payload. Rendered through the
+// prompt-context path a resume actually uses, so a future renderer that walks a
+// map of its own is caught here too.
+//
+// THE EXPECTED BLOCK IS WRITTEN OUT, NOT TAKEN FROM THE FIRST RENDER. Comparing
+// later renders against the first only asks whether the renderer is stable, and
+// a renderer that is stably wrong passes: any fixed field order satisfies it,
+// including the one this change replaces. It also hid something. The first
+// version of this test used tool events, which promptContextEvents filters out
+// of the resume context on this branch, so the block being compared was a
+// single line and nobody could tell. Naming the expected block makes both
+// failures visible.
+func TestResumeContextBlockIsIdenticalAcrossRenders(t *testing.T) {
+	events := []Event{
+		{Sequence: 1, Type: EventMessage, Payload: mustOrderPayload(t, map[string]any{"role": "user", "content": "rotate the deploy key"})},
+		{Sequence: 2, Type: EventMessage, Payload: mustOrderPayload(t, map[string]any{"role": "assistant", "content": "reading the env file"})},
+		{Sequence: 3, Type: EventMessage, Payload: mustOrderPayload(t, map[string]any{"role": "user", "content": "and the hooks"})},
+	}
+
+	// Keys sorted, so content comes before role in every line.
+	want := strings.Join([]string{
+		"message: rotate the deploy key user",
+		"message: reading the env file assistant",
+		"message: and the hooks user",
+	}, "\n")
+
+	for attempt := range 50 {
+		if got := renderOrderContext(t, events); got != want {
+			t.Fatalf("attempt %d rendered:\n%s\nwant:\n%s", attempt, got, want)
+		}
+	}
+}
+
+func renderOrderContext(t *testing.T, events []Event) string {
+	t.Helper()
+	lines := []string{}
+	for _, event := range promptContextEvents(events) {
+		var decoded any
+		if len(event.Payload) > 0 {
+			if err := json.Unmarshal(event.Payload, &decoded); err != nil {
+				t.Fatal(err)
+			}
+		}
+		lines = append(lines, string(event.Type)+": "+summarizePayload(decoded))
+	}
+	return strings.Join(lines, "\n")
+}
+
+func mustOrderPayload(t *testing.T, payload map[string]any) json.RawMessage {
+	t.Helper()
+	raw, err := json.Marshal(payload)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return raw
+}

internal/sessions/exec_session.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"log"
+	"sort"
 	"strings"
 	"unicode/utf8"
 )
@@ -267,9 +268,24 @@ func extractText(value any) string {
 		if summary, ok := typed["summary"].(string); ok && strings.TrimSpace(summary) != "" {
 			return summary
 		}
-		parts := []string{}
-		for _, item := range typed {
-			if text := extractText(item); text != "" {
+		// BY KEY, BECAUSE THIS TEXT BECOMES PART OF A PROMPT. Go randomizes map
+		// iteration, so the same session rendered its resume context in a
+		// different field order in every process: one run said
+		// `c1 read_file {"path":...}` and the next `{"path":...} c1 read_file`.
+		// The block goes into the user prompt, so the prompt itself changed run
+		// to run, which gives a provider nothing stable to prefix-cache and
+		// leaves two resumes of one session impossible to diff while debugging.
+		//
+		// Only the ordering is decided here. Which fields survive is the
+		// caller's projection, and a slice keeps the order it was written in.
+		keys := make([]string, 0, len(typed))
+		for key := range typed {
+			keys = append(keys, key)
+		}
+		sort.Strings(keys)
+		parts := make([]string, 0, len(keys))
+		for _, key := range keys {
+			if text := extractText(typed[key]); text != "" {
 				parts = append(parts, text)
 			}
 		}