diff --git a/src/app/api/insights/chat/route.ts b/src/app/api/insights/chat/route.ts
index e0c5d1693..fde99d741 100644
--- a/src/app/api/insights/chat/route.ts
+++ b/src/app/api/insights/chat/route.ts
@@ -300,11 +300,24 @@ Reply now as the assistant, in ${locale === "de" ? "German" : "English"}.`;
   if (sentinel.malformed) {
     // Graceful degrade: log so ops can spot a provider whose
     // sentinel format has drifted, but pass the prose through
-    // unchanged.
+    // unchanged. v1.4.23 H1 — split the annotation:
+    //   - parse_partial: at least one row parsed AND at least one
+    //     row failed (mixed-format drift on a single reply)
+    //   - parse_failed: the whole block was unusable
+    // Both annotations carry the per-line `reasons` array so an ops
+    // dashboard can attribute the failure cause without re-running
+    // the parser.
+    const reasons = sentinel.malformedEntries.map((entry) => entry.reason);
+    const annotationName =
+      sentinel.keyValues.length > 0 && sentinel.malformedEntries.length > 0
+        ? "coach.keyvalues.parse_partial"
+        : "coach.keyvalues.parse_failed";
     annotate({
-      action: { name: "coach.keyvalues.parse_failed" },
+      action: { name: annotationName },
       meta: {
         kept: sentinel.keyValues.length,
+        malformedCount: sentinel.malformedEntries.length,
+        reasons,
         promptVersion: PROMPT_VERSION,
       },
     });
diff --git a/src/lib/ai/coach/__tests__/keyvalues-parse.test.ts b/src/lib/ai/coach/__tests__/keyvalues-parse.test.ts
index 41f27f7a5..6b1eee0bd 100644
--- a/src/lib/ai/coach/__tests__/keyvalues-parse.test.ts
+++ b/src/lib/ai/coach/__tests__/keyvalues-parse.test.ts
@@ -1,6 +1,10 @@
 import { describe, expect, it } from "vitest";
 
-import { parseKeyValueLine, parseKeyValuesSentinel } from "../keyvalues";
+import {
+  parseKeyValueLine,
+  parseKeyValuesSentinel,
+  tryParseKeyValueLine,
+} from "../keyvalues";
 
 /**
  * v1.4.22 Wave 3 B1+B2 — Coach evidence-block sentinel parser.
@@ -91,6 +95,7 @@ describe("parseKeyValuesSentinel", () => {
       },
     ]);
     expect(out.malformed).toBe(false);
+    expect(out.malformedEntries).toEqual([]);
   });
 
   it("returns prose untouched when no sentinel is present", () => {
@@ -100,6 +105,7 @@ describe("parseKeyValuesSentinel", () => {
     expect(out.prose).toBe("Just a qualitative reply with no numbers cited.");
     expect(out.keyValues).toEqual([]);
     expect(out.malformed).toBe(false);
+    expect(out.malformedEntries).toEqual([]);
   });
 
   it("graceful-degrades when the closing ---END--- is missing", () => {
@@ -115,6 +121,9 @@ describe("parseKeyValuesSentinel", () => {
     expect(out.prose).toBe("Prose still streams cleanly.");
     expect(out.keyValues.length).toBe(1);
     expect(out.malformed).toBe(true);
+    expect(out.malformedEntries).toEqual([
+      { rawLine: "---END---", reason: "no_END_marker" },
+    ]);
   });
 
   it("clips the block to 8 entries (prompt-contract cap)", () => {
@@ -143,7 +152,7 @@ describe("parseKeyValuesSentinel", () => {
     expect(out.malformed).toBe(true);
   });
 
-  it("silently drops malformed lines mixed in with valid ones", () => {
+  it("records malformed lines mixed in with valid ones (v1.4.23 H1)", () => {
     const raw = [
       "Prose.",
       "",
@@ -160,6 +169,65 @@ describe("parseKeyValuesSentinel", () => {
       "avg30 systolic",
       "avg30 mood",
     ]);
+    // Partial malformed surfaces — the good rows survive while the
+    // bad ones land in the entry array with typed reasons.
+    expect(out.malformed).toBe(true);
+    expect(out.malformedEntries).toEqual([
+      { rawLine: "not a valid line", reason: "missing_colon" },
+      { rawLine: ":no-label-here", reason: "missing_colon" },
+    ]);
+  });
+
+  it("flags label_overflow when a row's label exceeds the cap", () => {
+    const longLabel = "x".repeat(120);
+    const raw = [
+      "---KEYVALUES---",
+      "ok label: 138 [mmHg] (last30days)",
+      `${longLabel}: 99 [mmHg]`,
+      "---END---",
+    ].join("\n");
+    const out = parseKeyValuesSentinel(raw);
+    expect(out.keyValues.length).toBe(1);
+    expect(out.keyValues[0].label).toBe("ok label");
+    expect(out.malformed).toBe(true);
+    expect(out.malformedEntries).toEqual([
+      { rawLine: `${longLabel}: 99 [mmHg]`, reason: "label_overflow" },
+    ]);
+  });
+
+  it("flags value_overflow when a row's value exceeds the cap", () => {
+    const longValue = "9".repeat(220);
+    const raw = [
+      "---KEYVALUES---",
+      `avg30 systolic: ${longValue} [mmHg]`,
+      "ok label: 138 [mmHg] (last30days)",
+      "---END---",
+    ].join("\n");
+    const out = parseKeyValuesSentinel(raw);
+    expect(out.keyValues.length).toBe(1);
+    expect(out.keyValues[0].label).toBe("ok label");
+    expect(out.malformed).toBe(true);
+    expect(out.malformedEntries[0]?.reason).toBe("value_overflow");
+  });
+
+  it("flags byte_overflow when the block exceeds the 1 KB cap", () => {
+    const padded = Array.from(
+      { length: 60 },
+      (_, i) => `label-${i}: ${i.toString().padEnd(15, "x")}`,
+    );
+    const raw = ["---KEYVALUES---", ...padded, "---END---"].join("\n");
+    const out = parseKeyValuesSentinel(raw);
+    expect(out.malformed).toBe(true);
+    expect(
+      out.malformedEntries.some((entry) => entry.reason === "byte_overflow"),
+    ).toBe(true);
+  });
+
+  it("tryParseKeyValueLine surfaces missing_colon for separator-less rows", () => {
+    expect(tryParseKeyValueLine("no separator here")).toEqual({
+      ok: false,
+      reason: "missing_colon",
+    });
   });
 
   it("returns malformed when the sentinel block is empty", () => {
@@ -168,6 +236,8 @@ describe("parseKeyValuesSentinel", () => {
     expect(out.keyValues).toEqual([]);
     expect(out.malformed).toBe(true);
     expect(out.prose).toBe("Prose.");
+    // No body lines to record — only the block-level state flips.
+    expect(out.malformedEntries).toEqual([]);
   });
 
   it("returns prose and an empty array for empty input", () => {
@@ -175,5 +245,6 @@ describe("parseKeyValuesSentinel", () => {
     expect(out.prose).toBe("");
     expect(out.keyValues).toEqual([]);
     expect(out.malformed).toBe(false);
+    expect(out.malformedEntries).toEqual([]);
   });
 });
diff --git a/src/lib/ai/coach/keyvalues.ts b/src/lib/ai/coach/keyvalues.ts
index 99cdcf45f..b1d2c1fd2 100644
--- a/src/lib/ai/coach/keyvalues.ts
+++ b/src/lib/ai/coach/keyvalues.ts
@@ -19,11 +19,14 @@
  *   - Hard cap 1 KB on the sentinel block (prompt-injection guard).
  *   - Hard cap 8 lines kept (per the prompt contract).
  *   - Each line is validated against `coachKeyValueSchema`; malformed
- *     lines are dropped silently.
+ *     lines surface in `parsed.malformed[]` with a typed reason
+ *     (v1.4.23 H1 — replaced the silent-drop path).
  *   - When the sentinel pair is malformed (e.g. no closing `---END---`,
- *     unbalanced sentinels), the parser returns the original prose
- *     untouched and emits no keyValues — the caller logs a wide-event
- *     `coach.keyvalues.parse_failed` for ops visibility.
+ *     unbalanced sentinels), the parser still returns whatever rows it
+ *     could parse plus a malformed flag; the caller logs
+ *     `coach.keyvalues.parse_failed` (full block invalid) or
+ *     `coach.keyvalues.parse_partial` (mixed valid + invalid) for ops
+ *     visibility.
  */
 import { coachKeyValueSchema, type CoachKeyValue } from "./types";
 
@@ -31,31 +34,76 @@ import { coachKeyValueSchema, type CoachKeyValue } from "./types";
 const SENTINEL_BYTE_CAP = 1024;
 /** Max key/value rows kept inside the block (prompt-contract value). */
 const SENTINEL_LINE_CAP = 8;
+/** Per-line value cap before we reject the row (defence-in-depth). */
+const VALUE_BYTE_CAP = 200;
+/** Per-line label cap mirroring the prompt-contract value of 40 chars. */
+const LABEL_BYTE_CAP = 80;
 
 const OPEN_SENTINEL = "---KEYVALUES---";
 const CLOSE_SENTINEL = "---END---";
 
+/**
+ * Why a candidate row failed to parse. Surfaces to the caller via
+ * `SentinelParseResult.malformed[]` so ops dashboards can attribute
+ * partial failures (mixed valid + invalid rows) without the silent
+ * drops the v1.4.22 W3 implementation paid.
+ *
+ * - `missing_colon`         — line had no `:` separator.
+ * - `value_overflow`        — value exceeded `VALUE_BYTE_CAP`.
+ * - `label_overflow`        — label exceeded `LABEL_BYTE_CAP`.
+ * - `no_END_marker`         — the closing `---END---` was missing.
+ * - `byte_overflow`         — block exceeded `SENTINEL_BYTE_CAP`.
+ * - `schema_invalid`        — row passed shape but Zod rejected it
+ *                             (e.g. empty value after stripping).
+ */
+export type SentinelMalformedReason =
+  | "missing_colon"
+  | "value_overflow"
+  | "label_overflow"
+  | "no_END_marker"
+  | "byte_overflow"
+  | "schema_invalid";
+
+export interface SentinelMalformedEntry {
+  rawLine: string;
+  reason: SentinelMalformedReason;
+}
+
+/**
+ * Result of parsing one body line. Either returns the `CoachKeyValue`
+ * shape or a typed reason the line failed (so the caller can surface
+ * partial-failure observability in the wide-event annotation).
+ */
+export type LineParseOutcome =
+  | { ok: true; value: CoachKeyValue }
+  | { ok: false; reason: SentinelMalformedReason };
+
 /**
  * Parse one body line of the form
  *   `<label>: <value> [<unit>] (<window>)`
  * into a `CoachKeyValue`. The unit + window are both optional; an
- * input missing either or both still parses cleanly. Returns `null`
- * when the line does not match the expected shape — the caller drops
- * the line rather than rejecting the whole block.
+ * input missing either or both still parses cleanly. Returns
+ * `{ ok: false, reason }` when the line does not match the expected
+ * shape — the caller now records the malformed row rather than
+ * silently dropping it (v1.4.23 H1).
  */
-export function parseKeyValueLine(line: string): CoachKeyValue | null {
+export function tryParseKeyValueLine(line: string): LineParseOutcome {
   const trimmed = line.trim();
-  if (!trimmed) return null;
+  if (!trimmed) return { ok: false, reason: "missing_colon" };
 
   // Find the first `:` — the label cannot itself contain `:`. The
   // remainder may carry the value, an optional `[unit]`, and an
   // optional `(window)`.
   const colon = trimmed.indexOf(":");
-  if (colon < 1) return null;
+  if (colon < 1) return { ok: false, reason: "missing_colon" };
 
   const label = trimmed.slice(0, colon).trim();
   let rest = trimmed.slice(colon + 1).trim();
-  if (!label || !rest) return null;
+  if (!label) return { ok: false, reason: "missing_colon" };
+  if (!rest) return { ok: false, reason: "schema_invalid" };
+  if (label.length > LABEL_BYTE_CAP) {
+    return { ok: false, reason: "label_overflow" };
+  }
 
   let unit: string | undefined;
   let window: string | undefined;
@@ -78,7 +126,10 @@ export function parseKeyValueLine(line: string): CoachKeyValue | null {
   }
 
   const value = rest.trim();
-  if (!value) return null;
+  if (!value) return { ok: false, reason: "schema_invalid" };
+  if (value.length > VALUE_BYTE_CAP) {
+    return { ok: false, reason: "value_overflow" };
+  }
 
   const parsed = coachKeyValueSchema.safeParse({
     label,
@@ -86,8 +137,18 @@ export function parseKeyValueLine(line: string): CoachKeyValue | null {
     ...(unit ? { unit } : {}),
     ...(window ? { window } : {}),
   });
-  if (!parsed.success) return null;
-  return parsed.data;
+  if (!parsed.success) return { ok: false, reason: "schema_invalid" };
+  return { ok: true, value: parsed.data };
+}
+
+/**
+ * Backwards-compatible wrapper around `tryParseKeyValueLine`. Returns
+ * the parsed value on success, `null` on any failure — kept so existing
+ * callers and the public test surface keep working unchanged.
+ */
+export function parseKeyValueLine(line: string): CoachKeyValue | null {
+  const outcome = tryParseKeyValueLine(line);
+  return outcome.ok ? outcome.value : null;
 }
 
 export interface SentinelParseResult {
@@ -101,6 +162,17 @@ export interface SentinelParseResult {
    * Callers log this for ops visibility (graceful degrade).
    */
   malformed: boolean;
+  /**
+   * Per-line diagnostics for rows the parser skipped — populated even
+   * when `keyValues.length > 0` so ops can spot mixed-format drift
+   * (v1.4.23 H1). Empty when every body line parsed cleanly.
+   *
+   * Block-level failures (missing `---END---`, payload truncated to
+   * fit the byte cap) land here as a synthetic entry with `rawLine`
+   * set to the marker / cap descriptor and the corresponding reason
+   * code so the wide-event annotation always carries a typed cause.
+   */
+  malformedEntries: SentinelMalformedEntry[];
 }
 
 /**
@@ -112,12 +184,17 @@ export interface SentinelParseResult {
  */
 export function parseKeyValuesSentinel(raw: string): SentinelParseResult {
   if (!raw) {
-    return { prose: "", keyValues: [], malformed: false };
+    return { prose: "", keyValues: [], malformed: false, malformedEntries: [] };
   }
 
   const openIdx = raw.indexOf(OPEN_SENTINEL);
   if (openIdx === -1) {
-    return { prose: raw, keyValues: [], malformed: false };
+    return {
+      prose: raw,
+      keyValues: [],
+      malformed: false,
+      malformedEntries: [],
+    };
   }
 
   // Everything before the opening marker is prose; trim trailing
@@ -125,6 +202,8 @@ export function parseKeyValuesSentinel(raw: string): SentinelParseResult {
   // bubble doesn't render an extra blank paragraph.
   const prose = raw.slice(0, openIdx).replace(/\s+$/u, "");
 
+  const malformedEntries: SentinelMalformedEntry[] = [];
+
   const afterOpen = raw.slice(openIdx + OPEN_SENTINEL.length);
   // Look for the closing sentinel; if missing, the block is
   // malformed — fall back to the original prose and emit nothing.
@@ -137,6 +216,10 @@ export function parseKeyValuesSentinel(raw: string): SentinelParseResult {
     // the contract wasn't fully met.
     bodyRaw = afterOpen;
     malformedClose = true;
+    malformedEntries.push({
+      rawLine: CLOSE_SENTINEL,
+      reason: "no_END_marker",
+    });
   } else {
     bodyRaw = afterOpen.slice(0, closeRel);
   }
@@ -149,6 +232,10 @@ export function parseKeyValuesSentinel(raw: string): SentinelParseResult {
   if (body.length > SENTINEL_BYTE_CAP) {
     body = body.slice(0, SENTINEL_BYTE_CAP);
     truncated = true;
+    malformedEntries.push({
+      rawLine: `<payload truncated at ${SENTINEL_BYTE_CAP} bytes>`,
+      reason: "byte_overflow",
+    });
   }
 
   const lines = body
@@ -162,19 +249,29 @@ export function parseKeyValuesSentinel(raw: string): SentinelParseResult {
   const kept: CoachKeyValue[] = [];
   for (const line of lines) {
     if (kept.length >= SENTINEL_LINE_CAP) break;
-    const entry = parseKeyValueLine(line);
-    if (entry) kept.push(entry);
+    const outcome = tryParseKeyValueLine(line);
+    if (outcome.ok) {
+      kept.push(outcome.value);
+    } else {
+      malformedEntries.push({ rawLine: line, reason: outcome.reason });
+    }
   }
 
   // Malformed when:
   //   - closing sentinel missing, OR
   //   - block payload was truncated to fit the 1 KB cap, OR
-  //   - the block was present but yielded zero valid rows.
-  const malformed = malformedClose || truncated || kept.length === 0;
+  //   - the block was present but yielded zero valid rows, OR
+  //   - any individual row failed to parse (mixed-format drift).
+  const malformed =
+    malformedClose ||
+    truncated ||
+    kept.length === 0 ||
+    malformedEntries.length > 0;
 
   return {
     prose,
     keyValues: kept,
     malformed,
+    malformedEntries,
   };
 }