diff --git a/src/app/api/insights/chat/__tests__/route-module-gate.test.ts b/src/app/api/insights/chat/__tests__/route-module-gate.test.ts
index 6c92359fb..7b8e75db2 100644
--- a/src/app/api/insights/chat/__tests__/route-module-gate.test.ts
+++ b/src/app/api/insights/chat/__tests__/route-module-gate.test.ts
@@ -71,9 +71,10 @@ vi.mock("@/lib/ai/coach/types", () => ({
 // thin stub keeps the import graph satisfied without pulling the real schemas.
 vi.mock("@/lib/ai/coach/tools", () => ({
   COACH_TOOL_DEFS: [],
-  MAX_ROUNDS: 2,
+  MAX_ROUNDS: 3,
   buildCoachDataInventory: vi.fn(),
   renderDataInventory: vi.fn(),
+  renderFocusHint: vi.fn(() => ""),
   buildToolModeAddendum: vi.fn(),
   runCoachToolLoop: vi.fn(),
 }));
diff --git a/src/app/api/insights/chat/__tests__/route-tool-mode.test.ts b/src/app/api/insights/chat/__tests__/route-tool-mode.test.ts
index c38816b39..e09dc97a2 100644
--- a/src/app/api/insights/chat/__tests__/route-tool-mode.test.ts
+++ b/src/app/api/insights/chat/__tests__/route-tool-mode.test.ts
@@ -138,31 +138,36 @@ vi.mock("@/lib/ai/coach/snapshot", () => ({
 }));
 
 // Tool-module surface — stub the inventory + loop so we assert routing only.
-const { buildCoachDataInventory, renderDataInventory, runCoachToolLoop } =
-  vi.hoisted(() => ({
-    buildCoachDataInventory: vi.fn(async () => ({
-      entries: [],
-      restMode: false,
-      cycleEnabled: false,
-      window: "last30days",
-    })),
-    renderDataInventory: vi.fn(
-      () => "DATA INVENTORY\n- blood pressure: present",
-    ),
-    runCoachToolLoop: vi.fn(async () => ({
-      result: { content: "Your BP is steady.", tokensUsed: 80, model: "m" },
-      workingProviderType: "anthropic",
-      totalTokens: 80,
-      rounds: 2,
-      toolTrace: [{ name: "get_metric_series", present: true }],
-      toolResults: [],
-    })),
-  }));
-vi.mock("@/lib/ai/coach/tools", () => ({
-  COACH_TOOL_DEFS: [{ name: "get_metric_series" }],
-  MAX_ROUNDS: 2,
+const {
   buildCoachDataInventory,
   renderDataInventory,
+  renderFocusHint,
+  runCoachToolLoop,
+} = vi.hoisted(() => ({
+  buildCoachDataInventory: vi.fn(async () => ({
+    entries: [],
+    restMode: false,
+    cycleEnabled: false,
+    window: "last30days",
+    probeScope: { sources: ["bp", "hrv"], window: "last30days" },
+  })),
+  renderDataInventory: vi.fn(() => "DATA INVENTORY\n- blood pressure: present"),
+  renderFocusHint: vi.fn(() => ""),
+  runCoachToolLoop: vi.fn(async () => ({
+    result: { content: "Your BP is steady.", tokensUsed: 80, model: "m" },
+    workingProviderType: "anthropic",
+    totalTokens: 80,
+    rounds: 2,
+    toolTrace: [{ name: "get_metric_series", present: true }],
+    toolResults: [],
+  })),
+}));
+vi.mock("@/lib/ai/coach/tools", () => ({
+  COACH_TOOL_DEFS: [{ name: "get_metric_series" }],
+  MAX_ROUNDS: 3,
+  buildCoachDataInventory,
+  renderDataInventory,
+  renderFocusHint,
   buildToolModeAddendum: vi.fn(() => "TOOL ADDENDUM"),
   runCoachToolLoop,
 }));
@@ -246,11 +251,11 @@ describe("coach chat — tool-mode routing (F1)", () => {
       { providerType: "anthropic", instance: {} },
     ]);
     await post(chatReq({ message: "How is my BP?" }));
-    // maxTokens (1500) × MAX_ROUNDS (2). The 4th arg is the provider-aware
+    // maxTokens (1500) × MAX_ROUNDS (3). The 4th arg is the provider-aware
     // daily cap (F1) — mocked to the user-plan ceiling for this BYOK chain.
     expect(reserveBudget).toHaveBeenCalledWith(
       "u1",
-      3000,
+      4500,
       "2026-06-21",
       2_000_000,
     );
diff --git a/src/app/api/insights/chat/route.ts b/src/app/api/insights/chat/route.ts
index 59183ba6f..512af8e14 100644
--- a/src/app/api/insights/chat/route.ts
+++ b/src/app/api/insights/chat/route.ts
@@ -575,6 +575,11 @@ Reply now as the assistant, in ${locale === "de" ? "German" : "English"}. Fetch
         temperature: AI_BUDGETS.coach.temperature,
         maxTokens: AI_BUDGETS.coach.maxTokens,
         fallbackWindow: effectiveScope?.window,
+        // v1.21.0 (D5-1) — share the inventory's full-source snapshot across
+        // every tool so the turn builds ONE snapshot, not one per tool. The
+        // probe scope is the exact scope the inventory was built against, so the
+        // per-tool reads land its 60s LRU entry.
+        sharedScope: inventory.probeScope,
         // v1.20.1 — thread the abort signal so a mid-generation disconnect tears
         // down the per-round provider calls instead of paying the full cost.
         signal: request.signal,
diff --git a/src/lib/ai/coach/__tests__/tools-executor.test.ts b/src/lib/ai/coach/__tests__/tools-executor.test.ts
index e4edce158..c3c847a80 100644
--- a/src/lib/ai/coach/__tests__/tools-executor.test.ts
+++ b/src/lib/ai/coach/__tests__/tools-executor.test.ts
@@ -299,3 +299,82 @@ describe("executeCoachTool", () => {
     expect(result.reason).toBe("invalid_arguments");
   });
 });
+
+// v1.21.0 (D5-1) — ONE snapshot per turn. When the route threads the shared
+// full-source scope (the inventory's probe scope) and the tool's window
+// matches, the tool must read under the SHARED scope key so every per-tool read
+// lands the inventory's already-built 60s LRU entry — rather than rebuilding a
+// distinct single-source snapshot per tool.
+describe("executeCoachTool — shared snapshot scope (D5-1)", () => {
+  beforeEach(() => {
+    buildCoachSnapshot.mockReset();
+  });
+
+  const sharedScope = {
+    sources: ["bp", "hrv", "sleep", "glucose"] as never,
+    window: "last30days" as const,
+  };
+
+  it("reads under the shared scope when the tool window matches", async () => {
+    buildCoachSnapshot.mockResolvedValue(
+      snapshot({ bloodPressure: { aggregate: { avgSys30: 128 } } }),
+    );
+    const result = await executeCoachTool({
+      userId: "u1",
+      name: "get_metric_series",
+      rawArguments: JSON.stringify({ metric: "bp" }),
+      fallbackWindow: "last30days",
+      sharedScope,
+    });
+    expect(result.present).toBe(true);
+    // The read used the SHARED scope object, not a narrow single-source scope —
+    // so a second tool with the same shared scope hits the same cache key.
+    expect(buildCoachSnapshot).toHaveBeenCalledWith("u1", sharedScope);
+  });
+
+  it("two tools in a turn read under the SAME scope key (one build)", async () => {
+    buildCoachSnapshot.mockResolvedValue(
+      snapshot({
+        bloodPressure: { aggregate: {} },
+        heartRateVariability: { aggregate: {} },
+      }),
+    );
+    await executeCoachTool({
+      userId: "u1",
+      name: "get_metric_series",
+      rawArguments: JSON.stringify({ metric: "bp" }),
+      fallbackWindow: "last30days",
+      sharedScope,
+    });
+    await executeCoachTool({
+      userId: "u1",
+      name: "get_metric_series",
+      rawArguments: JSON.stringify({ metric: "hrv" }),
+      fallbackWindow: "last30days",
+      sharedScope,
+    });
+    // Both reads used the identical shared scope object → identical cache key.
+    const scopes = buildCoachSnapshot.mock.calls.map((c) => c[1]);
+    expect(scopes).toHaveLength(2);
+    expect(scopes[0]).toBe(sharedScope);
+    expect(scopes[1]).toBe(sharedScope);
+  });
+
+  it("falls back to a narrow scope when the tool overrides the window", async () => {
+    buildCoachSnapshot.mockResolvedValue(
+      snapshot({ bloodPressure: { aggregate: {} } }),
+    );
+    await executeCoachTool({
+      userId: "u1",
+      name: "get_metric_series",
+      // The model overrides the window → a distinct, correct build.
+      rawArguments: JSON.stringify({ metric: "bp", window: "last7days" }),
+      fallbackWindow: "last30days",
+      sharedScope,
+    });
+    expect(buildCoachSnapshot).toHaveBeenCalledWith("u1", {
+      sources: ["bp"],
+      window: "last7days",
+    });
+  });
+});
diff --git a/src/lib/ai/coach/__tests__/tools-loop.test.ts b/src/lib/ai/coach/__tests__/tools-loop.test.ts
index 3c4586224..91cf0fbe6 100644
--- a/src/lib/ai/coach/__tests__/tools-loop.test.ts
+++ b/src/lib/ai/coach/__tests__/tools-loop.test.ts
@@ -7,9 +7,9 @@ import { describe, expect, it, vi, beforeEach } from "vitest";
 
 import type { CoachToolResult } from "@/lib/ai/coach/tools/executor";
 
-const executeCoachTool = vi.fn<() => Promise<CoachToolResult>>();
+const executeCoachTool = vi.fn<(args?: unknown) => Promise<CoachToolResult>>();
 vi.mock("@/lib/ai/coach/tools/executor", () => ({
-  executeCoachTool: () => executeCoachTool(),
+  executeCoachTool: (args: unknown) => executeCoachTool(args),
 }));
 
 // The loop calls the real fallback runner; stub it to a deterministic
@@ -20,7 +20,11 @@ vi.mock("@/lib/ai/provider-runner", () => ({
     runRawCompletionWithFallback(args),
 }));
 
-import { runCoachToolLoop, HARD_CAP } from "@/lib/ai/coach/tools/loop";
+import {
+  runCoachToolLoop,
+  HARD_CAP,
+  MAX_ROUNDS,
+} from "@/lib/ai/coach/tools/loop";
 import { COACH_TOOL_DEFS } from "@/lib/ai/coach/tools/definitions";
 
 function completion(opts: {
@@ -199,3 +203,75 @@ describe("runCoachToolLoop", () => {
     expect(msgs[2].toolCallId).toBe("c1");
   });
 });
+
+// v1.21.0 (D5) — the round cap is 3 (sequential cross-metric "why" chains are
+// no longer starved on a 2-round ceiling), and the absolute ceiling tracks one
+// above so the final allowed round still forces prose.
+describe("round bounds (D5)", () => {
+  it("MAX_ROUNDS is 3 and HARD_CAP tracks one above", () => {
+    expect(MAX_ROUNDS).toBe(3);
+    expect(HARD_CAP).toBe(MAX_ROUNDS + 1);
+  });
+
+  it("allows up to three tool-fetch rounds before forcing prose", async () => {
+    executeCoachTool.mockResolvedValue({ present: false });
+    let toolRounds = 0;
+    runRawCompletionWithFallback.mockImplementation(
+      (args: { params: { toolChoice?: string } }) => {
+        if (args.params.toolChoice === "none") {
+          return Promise.resolve(completion({ content: "Forced final." }));
+        }
+        toolRounds += 1;
+        return Promise.resolve(
+          completion({
+            content: "",
+            finishReason: "tool_calls",
+            toolCalls: [
+              {
+                id: "c",
+                name: "get_metric_series",
+                arguments: '{"metric":"bp"}',
+              },
+            ],
+          }),
+        );
+      },
+    );
+    const out = await runCoachToolLoop(baseArgs);
+    // Three rounds offered tools, the fourth forced prose.
+    expect(toolRounds).toBe(MAX_ROUNDS);
+    expect(out.rounds).toBe(HARD_CAP);
+    expect(out.result.content).toBe("Forced final.");
+  });
+});
+
+// v1.21.0 (D5-1) — the loop threads the turn's shared snapshot scope to every
+// tool so each per-tool read lands one cache key (one snapshot build per turn).
+describe("shared snapshot scope threading (D5-1)", () => {
+  it("passes sharedScope through to executeCoachTool", async () => {
+    executeCoachTool.mockResolvedValue({ present: true, data: { x: 1 } });
+    const shared = {
+      sources: ["bp", "hrv"] as never,
+      window: "last30days" as const,
+    };
+    runRawCompletionWithFallback
+      .mockResolvedValueOnce(
+        completion({
+          content: "",
+          finishReason: "tool_calls",
+          toolCalls: [
+            {
+              id: "c1",
+              name: "get_metric_series",
+              arguments: '{"metric":"bp"}',
+            },
+          ],
+        }),
+      )
+      .mockResolvedValueOnce(completion({ content: "done" }));
+    await runCoachToolLoop({ ...baseArgs, sharedScope: shared });
+    expect(executeCoachTool).toHaveBeenCalledWith(
+      expect.objectContaining({ sharedScope: shared }),
+    );
+  });
+});
diff --git a/src/lib/ai/coach/tools/executor.ts b/src/lib/ai/coach/tools/executor.ts
index 60baa2ba1..1d33e9726 100644
--- a/src/lib/ai/coach/tools/executor.ts
+++ b/src/lib/ai/coach/tools/executor.ts
@@ -20,10 +20,17 @@
  *     mutation or egress surface.
  *
  * The builder result is memoised by the 60s snapshot LRU keyed on
- * `(userId, window, sources)`. Each tool scopes to a different source set, so
- * tool calls for distinct domains do NOT share a cache entry; sharing happens
- * only when the SAME scope recurs (a repeated call, or a turn that re-enters
- * the same domain) within the 60s window.
+ * `(userId, window, sources)`.
+ *
+ * v1.21.0 (D5-1) — ONE snapshot per turn. The route hands every tool the SAME
+ * `sharedScope` it built the DATA INVENTORY against (the full source set + the
+ * conversation window). A full-source build already contains every section, so
+ * each tool reads under that shared scope key and slices its own section out —
+ * landing the 60s LRU hit the inventory primed rather than rebuilding a
+ * distinct single-source snapshot per tool (the D5-1 N-builds-per-turn cost).
+ * A tool that overrides the window to something OTHER than the shared window
+ * still gets a correct, distinct build (the rare case); the common path
+ * collapses to one build.
  */
 import { z } from "zod/v4";
 
@@ -79,18 +86,32 @@ function pickSection(
 }
 
 /**
- * Resolve `args.window` into a scope. Falls back to the conversation's
- * effective window so a tool call that omits `window` matches the user's
- * chosen analysis window rather than the builder default.
+ * Resolve the scope a tool reads its snapshot under.
+ *
+ * v1.21.0 (D5-1) — when the route handed us a `sharedScope` (the full-source
+ * inventory build for this turn) AND the tool's effective window matches the
+ * shared window, read under the SHARED scope key so the per-tool read lands the
+ * 60s LRU hit the inventory already primed (the section it needs is present in
+ * the full-source build). Otherwise — no shared scope, or a window override —
+ * fall back to the tight per-source scope (a correct, distinct build).
+ *
+ * The window default chain is unchanged: `args.window` wins, else the
+ * conversation's `fallbackWindow`, else the builder default.
  */
 function scopeFor(
   sources: CoachScope["sources"],
   window: CoachScopeWindow | undefined,
   fallbackWindow: CoachScopeWindow | undefined,
+  sharedScope: CoachScope | undefined,
 ): CoachScope {
+  const effectiveWindow = window ?? fallbackWindow;
+  if (sharedScope && sharedScope.window === effectiveWindow) {
+    // Same window as the inventory's full-source build → reuse its cache entry.
+    return sharedScope;
+  }
   return {
     sources,
-    window: window ?? fallbackWindow,
+    window: effectiveWindow,
   };
 }
 
@@ -107,8 +128,15 @@ export async function executeCoachTool(args: {
   rawArguments: string;
   /** The conversation's effective window, used when a call omits `window`. */
   fallbackWindow?: CoachScopeWindow;
+  /**
+   * v1.21.0 (D5-1) — the turn's shared full-source snapshot scope (the same one
+   * the DATA INVENTORY was built against). When present and the tool's window
+   * matches, the tool reads under this scope key so it lands the inventory's
+   * already-built cache entry instead of rebuilding a single-source snapshot.
+   */
+  sharedScope?: CoachScope;
 }): Promise<CoachToolResult> {
-  const { userId, name, rawArguments, fallbackWindow } = args;
+  const { userId, name, rawArguments, fallbackWindow, sharedScope } = args;
 
   if (!isCoachToolName(name)) {
     annotate({
@@ -130,7 +158,13 @@ export async function executeCoachTool(args: {
   }
 
   try {
-    const result = await dispatch(name, userId, parsedArgs, fallbackWindow);
+    const result = await dispatch(
+      name,
+      userId,
+      parsedArgs,
+      fallbackWindow,
+      sharedScope,
+    );
     annotate({
       action: { name: "coach.tool.executed" },
       meta: { tool: name, present: result.present },
@@ -155,24 +189,30 @@ async function dispatch(
   userId: string,
   rawArgs: unknown,
   fallbackWindow: CoachScopeWindow | undefined,
+  sharedScope: CoachScope | undefined,
 ): Promise<CoachToolResult> {
   switch (name) {
     case "get_metric_series":
-      return getMetricSeries(userId, rawArgs, fallbackWindow);
+      return getMetricSeries(userId, rawArgs, fallbackWindow, sharedScope);
     case "get_glucose_panel":
-      return getGlucosePanel(userId, rawArgs, fallbackWindow);
+      return getGlucosePanel(userId, rawArgs, fallbackWindow, sharedScope);
     case "get_sleep":
-      return getSleep(userId, rawArgs, fallbackWindow);
+      return getSleep(userId, rawArgs, fallbackWindow, sharedScope);
     case "get_medication_compliance":
-      return getMedicationCompliance(userId, rawArgs, fallbackWindow);
+      return getMedicationCompliance(
+        userId,
+        rawArgs,
+        fallbackWindow,
+        sharedScope,
+      );
     case "get_labs":
-      return getLabs(userId, rawArgs, fallbackWindow);
+      return getLabs(userId, rawArgs, fallbackWindow, sharedScope);
     case "get_illness_recovery":
-      return getIllnessRecovery(userId, fallbackWindow);
+      return getIllnessRecovery(userId, fallbackWindow, sharedScope);
     case "get_workouts":
-      return getWorkouts(userId, rawArgs, fallbackWindow);
+      return getWorkouts(userId, rawArgs, fallbackWindow, sharedScope);
     case "get_cycle":
-      return getCycle(userId, rawArgs);
+      return getCycle(userId, rawArgs, sharedScope);
     case "get_correlations":
       return getCorrelations(userId, rawArgs);
   }
@@ -190,6 +230,7 @@ async function getMetricSeries(
   userId: string,
   rawArgs: unknown,
   fallbackWindow: CoachScopeWindow | undefined,
+  sharedScope: CoachScope | undefined,
 ): Promise<CoachToolResult> {
   const parsed = getMetricSeriesArgsSchema.safeParse(rawArgs);
   if (!parsed.success) return badArgs("get_metric_series", parsed.error);
@@ -217,7 +258,7 @@ async function getMetricSeries(
 
   const snapshot = await buildCoachSnapshot(
     userId,
-    scopeFor([metric], window, fallbackWindow),
+    scopeFor([metric], window, fallbackWindow, sharedScope),
   );
   const section = pickSection(snapshot.sections, sectionKey);
   if (section === undefined) {
@@ -234,12 +275,13 @@ async function getGlucosePanel(
   userId: string,
   rawArgs: unknown,
   fallbackWindow: CoachScopeWindow | undefined,
+  sharedScope: CoachScope | undefined,
 ): Promise<CoachToolResult> {
   const parsed = getGlucosePanelArgsSchema.safeParse(rawArgs);
   if (!parsed.success) return badArgs("get_glucose_panel", parsed.error);
   const snapshot = await buildCoachSnapshot(
     userId,
-    scopeFor(["glucose"], parsed.data.window, fallbackWindow),
+    scopeFor(["glucose"], parsed.data.window, fallbackWindow, sharedScope),
   );
   const section = pickSection(snapshot.sections, "glucose");
   if (section === undefined) return { present: false, reason: "no_data" };
@@ -254,12 +296,13 @@ async function getSleep(
   userId: string,
   rawArgs: unknown,
   fallbackWindow: CoachScopeWindow | undefined,
+  sharedScope: CoachScope | undefined,
 ): Promise<CoachToolResult> {
   const parsed = getSleepArgsSchema.safeParse(rawArgs);
   if (!parsed.success) return badArgs("get_sleep", parsed.error);
   const snapshot = await buildCoachSnapshot(
     userId,
-    scopeFor(["sleep"], parsed.data.window, fallbackWindow),
+    scopeFor(["sleep"], parsed.data.window, fallbackWindow, sharedScope),
   );
   const nights = pickSection(snapshot.sections, "sleep");
   const rhythm = pickSection(snapshot.sections, "sleepRhythm");
@@ -280,6 +323,7 @@ async function getMedicationCompliance(
   userId: string,
   rawArgs: unknown,
   fallbackWindow: CoachScopeWindow | undefined,
+  sharedScope: CoachScope | undefined,
 ): Promise<CoachToolResult> {
   const parsed = getMedicationComplianceArgsSchema.safeParse(rawArgs);
   if (!parsed.success) {
@@ -287,7 +331,7 @@ async function getMedicationCompliance(
   }
   const snapshot = await buildCoachSnapshot(
     userId,
-    scopeFor(["compliance"], parsed.data.window, fallbackWindow),
+    scopeFor(["compliance"], parsed.data.window, fallbackWindow, sharedScope),
   );
   const compliance = pickSection(snapshot.sections, "compliance");
   // GLP-1 context rides the `weeklyContext` block.
@@ -311,22 +355,26 @@ async function getLabs(
   userId: string,
   rawArgs: unknown,
   fallbackWindow: CoachScopeWindow | undefined,
+  sharedScope: CoachScope | undefined,
 ): Promise<CoachToolResult> {
   const parsed = getLabsArgsSchema.safeParse(rawArgs);
   if (!parsed.success) return badArgs("get_labs", parsed.error);
   // Labs ride the snapshot regardless of `sources` (attached unconditionally),
-  // so a minimal scope still surfaces them. Pass an empty source set to keep
-  // the read tight.
+  // so a minimal scope still surfaces them.
   //
   // v1.21.0 (A5-F4) — the labs block itself is a "latest reading per biomarker
   // over the last 12 months" snapshot and is intentionally window-AGNOSTIC
   // (the read cutoff is fixed). We still thread the conversation's window onto
   // the scope so the snapshot's `scope` block reports the right horizon for the
   // turn; it does not move the labs read.
-  const snapshot = await buildCoachSnapshot(userId, {
-    sources: [],
-    window: fallbackWindow,
-  });
+  //
+  // v1.21.0 (D5-1) — prefer the turn's shared full-source scope so this read
+  // lands the inventory's cache entry; fall back to a tight empty-source scope
+  // (labs ride unconditionally) when no shared scope or a window mismatch.
+  const snapshot = await buildCoachSnapshot(
+    userId,
+    scopeFor([], fallbackWindow, fallbackWindow, sharedScope),
+  );
   const labs = pickSection(snapshot.sections, "labs") as
     | { recent?: Array<{ name?: string; analyte?: string }> }
     | undefined;
@@ -350,6 +398,7 @@ async function getLabs(
 async function getIllnessRecovery(
   userId: string,
   fallbackWindow: CoachScopeWindow | undefined,
+  sharedScope: CoachScope | undefined,
 ): Promise<CoachToolResult> {
   // Validate the (empty) args shape for consistency; an empty object always
   // passes.
@@ -361,10 +410,19 @@ async function getIllnessRecovery(
   // v1.21.0 (A5-F4) — honour the conversation's window so the recovery/strain/
   // trajectory composites are computed over the horizon the caller asked for
   // rather than the builder default.
-  const snapshot = await buildCoachSnapshot(userId, {
-    sources: ["hrv", "resting_hr", "vo2_max"],
-    window: fallbackWindow,
-  });
+  //
+  // v1.21.0 (D5-1) — prefer the turn's shared full-source scope (which already
+  // carries hrv/resting_hr/vo2_max + illness + derived) so this read lands the
+  // inventory's cache entry; the narrow scope is the window-mismatch fallback.
+  const snapshot = await buildCoachSnapshot(
+    userId,
+    scopeFor(
+      ["hrv", "resting_hr", "vo2_max"],
+      fallbackWindow,
+      fallbackWindow,
+      sharedScope,
+    ),
+  );
   const illness = pickSection(snapshot.sections, "illness");
   const derived = pickSection(snapshot.sections, "derived");
   const dayStrain = pickSection(snapshot.sections, "dayStrain");
@@ -392,6 +450,7 @@ async function getWorkouts(
   userId: string,
   rawArgs: unknown,
   fallbackWindow: CoachScopeWindow | undefined,
+  sharedScope: CoachScope | undefined,
 ): Promise<CoachToolResult> {
   const parsed = getWorkoutsArgsSchema.safeParse(rawArgs);
   if (!parsed.success) return badArgs("get_workouts", parsed.error);
@@ -399,7 +458,7 @@ async function getWorkouts(
   // user has workout rows in the window. Scope the read to that single source.
   const snapshot = await buildCoachSnapshot(
     userId,
-    scopeFor(["workouts"], parsed.data.window, fallbackWindow),
+    scopeFor(["workouts"], parsed.data.window, fallbackWindow, sharedScope),
   );
   const workouts = pickSection(snapshot.sections, "workouts");
   if (workouts === undefined) return { present: false, reason: "no_data" };
@@ -409,6 +468,7 @@ async function getWorkouts(
 async function getCycle(
   userId: string,
   rawArgs: unknown,
+  sharedScope: CoachScope | undefined,
 ): Promise<CoachToolResult> {
   const parsed = getCycleArgsSchema.safeParse(rawArgs);
   if (!parsed.success) return badArgs("get_cycle", parsed.error);
@@ -416,7 +476,14 @@ async function getCycle(
   // (the per-user toggle AND the operator switch), independent of `sources` —
   // so a minimal scope still surfaces it when the account tracks cycles, and a
   // non-cycle account structurally produces no block (→ present:false).
-  const snapshot = await buildCoachSnapshot(userId, { sources: [] });
+  //
+  // v1.21.0 (D5-1) — prefer the turn's shared full-source scope so this read
+  // lands the inventory's cache entry; the empty-source scope is the fallback
+  // when no shared scope is threaded.
+  const snapshot = await buildCoachSnapshot(
+    userId,
+    sharedScope ?? { sources: [] },
+  );
   const cycle = pickSection(snapshot.sections, "cycle");
   if (cycle === undefined) return { present: false, reason: "no_data" };
   return { present: true, data: cycle };
diff --git a/src/lib/ai/coach/tools/inventory.ts b/src/lib/ai/coach/tools/inventory.ts
index 651dca57b..ac8e147e9 100644
--- a/src/lib/ai/coach/tools/inventory.ts
+++ b/src/lib/ai/coach/tools/inventory.ts
@@ -50,6 +50,14 @@ export interface CoachDataInventory {
   cycleEnabled: boolean;
   /** The window the inventory was built against. */
   window: string;
+  /**
+   * v1.21.0 (D5-1) — the exact full-source scope this inventory's snapshot was
+   * built against. The route threads it to the tool loop so every per-tool read
+   * lands the SAME 60s LRU entry (one snapshot build per turn, not N). Returned
+   * here so the route reuses the identical scope object rather than
+   * re-deriving it and risking a key mismatch.
+   */
+  probeScope: CoachScope;
 }
 
 /**
@@ -188,6 +196,7 @@ export async function buildCoachDataInventory(
     restMode,
     cycleEnabled,
     window: scopeBlock?.window ?? scope?.window ?? "last30days",
+    probeScope,
   };
 }
 
diff --git a/src/lib/ai/coach/tools/loop.ts b/src/lib/ai/coach/tools/loop.ts
index 852813e67..ba7b52be3 100644
--- a/src/lib/ai/coach/tools/loop.ts
+++ b/src/lib/ai/coach/tools/loop.ts
@@ -7,10 +7,11 @@
  * FINAL prose for the visible stream exactly as before.
  *
  * Bounds (the loop can never run away):
- *   - MAX_ROUNDS = 2: one tool-fetch round + the answer round.
- *   - HARD_CAP = 3: an absolute ceiling; on the last allowed round the loop
- *     re-calls with `toolChoice:"none"` so the model is FORCED to produce prose
- *     even if it would otherwise keep asking for tools.
+ *   - MAX_ROUNDS = 3 (v1.21.0 D5): up to three tool-fetch rounds before the
+ *     forced answer, so a sequential cross-metric "why" chain isn't starved.
+ *   - HARD_CAP = MAX_ROUNDS + 1: an absolute ceiling; on the last allowed round
+ *     the loop re-calls with `toolChoice:"none"` so the model is FORCED to
+ *     produce prose even if it would otherwise keep asking for tools.
  *   - Tool calls within a round run in parallel (`Promise.all`).
  *
  * Token accounting: the caller reserves `maxTokens * MAX_ROUNDS` up front and
@@ -27,16 +28,28 @@ import { runRawCompletionWithFallback } from "@/lib/ai/provider-runner";
 import type { ProviderChainResolved } from "@/lib/ai/provider-runner";
 import type { AiMessage, AiToolDef, CompletionResult } from "@/lib/ai/types";
 import type { ProviderHealthLedger } from "@/lib/ai/provider-health-ledger";
-import type { CoachScopeWindow } from "@/lib/ai/coach/types";
+import type { CoachScope, CoachScopeWindow } from "@/lib/ai/coach/types";
 import {
   executeCoachTool,
   type CoachToolResult,
   type CoachToolTrace,
 } from "./executor";
 
-export const MAX_ROUNDS = 2;
-/** Absolute ceiling; the final allowed round forces prose (toolChoice none). */
-export const HARD_CAP = 3;
+/**
+ * v1.21.0 (D5) — MAX_ROUNDS raised 2 → 3 so a sequential cross-metric "why"
+ * chain (recovery low → which driver → pull that metric's series) is not
+ * starved on the 2-round ceiling. The per-round token budget is unchanged; the
+ * caller reserves `maxTokens * MAX_ROUNDS` and reconciles the summed actual
+ * tokens, so the bound scales with this constant and stays explicit. HARD_CAP
+ * tracks one above so the final allowed round still forces prose.
+ */
+export const MAX_ROUNDS = 3;
+/**
+ * Absolute ceiling; the final allowed round forces prose (toolChoice none).
+ * Tracks one above MAX_ROUNDS so rounds 1..MAX_ROUNDS may fetch tools and the
+ * final round is always a forced answer.
+ */
+export const HARD_CAP = MAX_ROUNDS + 1;
 
 export interface CoachToolLoopResult {
   /** The final completion (prose). */
@@ -74,6 +87,12 @@ export async function runCoachToolLoop(args: {
   temperature?: number;
   maxTokens?: number;
   fallbackWindow?: CoachScopeWindow;
+  /**
+   * v1.21.0 (D5-1) — the turn's shared full-source snapshot scope (the same one
+   * the DATA INVENTORY was built against). Threaded to every tool so they read
+   * under one cache key — one snapshot build per turn instead of N.
+   */
+  sharedScope?: CoachScope;
   ledger?: ProviderHealthLedger;
   /** Aborts the per-round provider calls on client disconnect. */
   signal?: AbortSignal;
@@ -86,6 +105,7 @@ export async function runCoachToolLoop(args: {
     temperature,
     maxTokens,
     fallbackWindow,
+    sharedScope,
     ledger,
     signal,
   } = args;
@@ -166,6 +186,7 @@ export async function runCoachToolLoop(args: {
           name: call.name,
           rawArguments: call.arguments,
           fallbackWindow,
+          sharedScope,
         });
         toolTrace.push({ name: call.name, present: toolResult.present });
         // v1.21.0 (P6) — retain the present results' payloads for the post-hoc