diff --git a/internal/sessions/exec_prompt_order_test.go b/internal/sessions/exec_prompt_order_test.go
new file mode 100644
index 000000000..51d169c51
--- /dev/null
+++ b/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
+}
diff --git a/internal/sessions/exec_session.go b/internal/sessions/exec_session.go
index b86b364b5..2858b3787 100644
--- a/internal/sessions/exec_session.go
+++ b/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)
 			}
 		}