diff --git a/internal/notify/slack.go b/internal/notify/slack.go
index 14fbdd65..ae6ad00a 100644
--- a/internal/notify/slack.go
+++ b/internal/notify/slack.go
@@ -15,6 +15,7 @@ import (
 	"os"
 	"strings"
 	"time"
+	"unicode"
 
 	"github.com/Smana/runlore/internal/config"
 	"github.com/Smana/runlore/internal/httpx"
@@ -820,7 +821,14 @@ func metadataFields(inv providers.Investigation) []map[string]any {
 	// deploy, which is the first thing they should check.
 	if len(inv.RootCauses) > 0 {
 		if ch := inv.RootCauses[0].ChangeRef; ch != "" {
-			add("What changed", truncate(escapeMrkdwn(ch), 200))
+			// Capped BEFORE escaping, which is the reverse of this file's other two
+			// truncate sites and deliberate. Those cap at 2900 — a last-resort guard on
+			// a pathological payload — where it does not matter that "&" costs 5 of the
+			// budget. This cap fires routinely, so measuring escaped runes would hand a
+			// change_ref describing "1.14.2 -> 1.15.0 && <prod>" a materially smaller
+			// allowance than one without meta characters. Capping the source also means
+			// truncateWords only ever sees real prose, so no cut can sever an entity.
+			add("What changed", escapeMrkdwn(truncateWords(ch, slackWhatChangedRunes)))
 		}
 	}
 	if inv.Occurrences > 1 && inv.Prior == nil {
@@ -1058,6 +1066,56 @@ func truncate(s string, n int) string {
 	return string(r[:n-1]) + "…"
 }
 
+// slackWhatChangedRunes caps the "What changed" metadata field. 200 was tighter
+// than what models actually put in change_ref: it carries a revision range on some
+// cards and a sentence explaining the change on others, and the explanatory form is
+// the more useful of the two. 600 holds it while still leaving the two-column
+// metadata grid readable, and stays well inside the 2000-rune per-field cap add()
+// applies.
+const slackWhatChangedRunes = 600
+
+// truncateWords caps a string to n runes like truncate, but backs the cut off to
+// the last space so the ellipsis never lands inside a word. Use it for prose the
+// model wrote; use truncate for values that are one token (a ref, a URL, a sha)
+// or where the cap is a hard protocol limit.
+//
+// A mid-word cut reads as a rendering fault rather than an intentional elision —
+// live, a change_ref ended "…outside Argo/Fl…", which looks like the card broke.
+// The cap itself is still enforced: a tail with no space in its final half is one
+// long token, and that gets the hard cut rather than losing half the value.
+//
+// Call it on RAW text and escape the result, not the other way round. It counts
+// runes of content, and a cut through escaped text can sever an "&amp;" into a
+// visible "&am" — the same rendering fault, in the one branch the back-off cannot
+// reach, since a long token has no space to back off to.
+//
+// internal/thread.truncateWords is the byte-budget twin of this function, applying
+// the same rule to the KB validator's byte limits (and trimming dangling
+// punctuation, which Slack does not need). Keep the two in step.
+func truncateWords(s string, n int) string {
+	r := []rune(s)
+	if len(r) <= n {
+		return s
+	}
+	if n < 2 {
+		return "" // no room for content and a mark both
+	}
+	// The scan starts at the FIRST DROPPED rune, not the last kept one. That is what
+	// makes the word-boundary case ordinary rather than special: when r[n-1] is
+	// itself a space the prefix already ends on a boundary, and starting there finds
+	// it instead of scanning past a whole word that fitted. Bounded at half the
+	// allowance — a tail with no space in it is one long token (a sha, a URL), and
+	// backing off to a distant space would lose more than the cut does.
+	cut := r[:n-1]
+	for i := n - 1; i > n/2; i-- {
+		if unicode.IsSpace(r[i]) {
+			cut = r[:i]
+			break
+		}
+	}
+	return strings.TrimRightFunc(string(cut), unicode.IsSpace) + "…"
+}
+
 // Multi delivers to several notifiers, best-effort: a failing notifier is logged,
 // not propagated, so one bad sink doesn't block the others.
 type Multi struct {
diff --git a/internal/notify/slack_test.go b/internal/notify/slack_test.go
index d150182b..a75521ef 100644
--- a/internal/notify/slack_test.go
+++ b/internal/notify/slack_test.go
@@ -1838,3 +1838,114 @@ func TestFormatCarriesTheNoRemedyNotice(t *testing.T) {
 		t.Errorf("Format printed the notice over a supplied remedy:\n%s", out)
 	}
 }
+
+// TestTruncateWords is the unit table for the helper itself. The card-level test
+// below drives one realistic fixture through summaryBlocks; this covers the branches
+// that fixture can never reach — the hard cut, the exact-boundary case, a severed
+// entity, and the degenerate caps.
+func TestTruncateWords(t *testing.T) {
+	cases := []struct {
+		name, in string
+		n        int
+		want     string
+	}{
+		{"short enough is returned whole", "hello world", 20, "hello world"},
+		{"exactly n is not truncated", "hello", 5, "hello"},
+		{"backs off to the word boundary", "hello world foo", 12, "hello world…"},
+		// The cut lands on a space, so the prefix is ALREADY word-aligned: backing off
+		// further would drop "bb", a whole word that fitted.
+		{"a cut on the boundary keeps the last whole word", "aaaa bb cc", 8, "aaaa bb…"},
+		// No space in the final half — one long token (a sha, a URL). The cap wins;
+		// losing half the value to reach a distant space would be worse.
+		{"one long token still gets the hard cut", "abcdefghijklmnopqrstuvwxyz", 10, "abcdefghi…"},
+		{"trailing space is trimmed before the ellipsis", "hello world      xx", 13, "hello world…"},
+		{"a cap with no room for content and a mark", "hello", 1, ""},
+		{"n=0 does not panic", "hello", 0, ""},
+		{"n negative does not panic", "hello", -3, ""},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			if got := truncateWords(c.in, c.n); got != c.want {
+				t.Errorf("truncateWords(%q, %d) = %q, want %q", c.in, c.n, got, c.want)
+			}
+		})
+	}
+}
+
+// whatChangedField returns the rendered "What changed" metadata value, without its
+// label. Asserting on the field rather than the whole encoded blob is what keeps the
+// cap assertions honest.
+func whatChangedField(t *testing.T, inv providers.Investigation) string {
+	t.Helper()
+	for _, f := range metadataFields(inv) {
+		txt, _ := f["text"].(string)
+		if after, ok := strings.CutPrefix(txt, "*What changed:*\n"); ok {
+			return after
+		}
+	}
+	t.Fatalf("no What changed field rendered")
+	return ""
+}
+
+// TestSlackWhatChangedTruncatesOnWordBoundary: a long change_ref is cut at a word
+// boundary, never mid-word.
+//
+// Live card, 2026-08-24: the field ended "…consistent with Coder provisioning
+// workspaces outside Argo/Fl…". Two things were wrong at once — the 200-rune cap
+// was tighter than the explanatory text models actually put in change_ref, and the
+// cut landed inside a word, which reads as a rendering fault rather than an
+// intentional elision. The cap is a limit and stays enforced; where it falls is
+// what changed.
+func TestSlackWhatChangedTruncatesOnWordBoundary(t *testing.T) {
+	long := "argocd Application coder-shared sync to bc20dc3a at 09:17:46Z, which recreated the whole " +
+		"shared-0 instance directory and restarted the singleton coder Deployment with a newly mounted " +
+		"kubeconfig ConfigMap plus new environment variables, consistent with Coder provisioning " +
+		"workspaces outside Argo CD rather than through the tracked GitOps path, and additionally " +
+		"retargeted the provisioner daemon concurrency, rewrote the access URL, replaced the instance " +
+		"kustomization, and rolled every workspace template that referenced the previous access URL " +
+		"so that the migrated instance would come up against the new control plane endpoint"
+	inv := providers.Investigation{
+		Title:      "t",
+		RootCauses: []providers.Hypothesis{{Summary: "s", ChangeRef: long}},
+	}
+	got := whatChangedField(t, inv)
+
+	// The cap must be materially larger than the 200 it replaced. Measured on the
+	// FIELD, so lowering the cap fails this — asserting on the whole encoded blocks
+	// blob did not: the header, verdict and footer satisfied it on their own, and it
+	// passed with the cap set to 60.
+	if n := len([]rune(strings.TrimSuffix(got, "…"))); n <= 200 {
+		t.Errorf("the cap is too tight to hold an explanatory change_ref: %d runes\n%s", n, got)
+	}
+	// The cap is still a cap.
+	if n := len([]rune(got)); n > 600 {
+		t.Errorf("the cap is not enforced: %d runes", n)
+	}
+
+	// This fixture reaches the cap exactly ON a space — the case the first revision
+	// got wrong, scanning back past the boundary and dropping "the", a whole word
+	// that fitted. The exact suffix pins the cut point, the word boundary and the
+	// absence of a stray space before the ellipsis in one assertion.
+	if !strings.HasSuffix(got, "against the…") {
+		t.Errorf("a cut landing on a word boundary must keep the last whole word, got tail %q",
+			got[max(0, len(got)-24):])
+	}
+
+	// Shifted so the cap lands INSIDE a word, which is what distinguishes truncateWords
+	// from the plain truncate this field used to call. Without the shift both helpers
+	// return the same string and nothing pins the wiring.
+	inv.RootCauses[0].ChangeRef = "sync: " + long
+	shifted := whatChangedField(t, inv)
+	if !strings.HasSuffix(shifted, "come up…") {
+		t.Errorf("expected a back-off to the word boundary, got tail %q, want a mid-word hard cut to be avoided",
+			shifted[max(0, len(shifted)-24):])
+	}
+
+	// The raised cap must not start truncating the revision-range form, which is the
+	// other shape change_ref carries — and it must still be escaped.
+	const short = "flux: HelmRelease harbor 1.14.2 -> 1.15.0 <prod>"
+	inv.RootCauses[0].ChangeRef = short
+	if got := whatChangedField(t, inv); got != escapeMrkdwn(short) {
+		t.Errorf("a short change_ref must render verbatim and escaped:\ngot  %q\nwant %q", got, escapeMrkdwn(short))
+	}
+}