diff --git a/services/runner/src/engines/sandbox_agent/usage.ts b/services/runner/src/engines/sandbox_agent/usage.ts
index 1d08b3447b..afb17d8e01 100644
--- a/services/runner/src/engines/sandbox_agent/usage.ts
+++ b/services/runner/src/engines/sandbox_agent/usage.ts
@@ -25,7 +25,20 @@ export async function readRunUsage(
   }
 }
 
-/** Combine prompt token counts with stream cost when no Pi usage writeback exists. */
+/**
+ * Combine prompt token counts with stream cost when no Pi usage writeback exists.
+ *
+ * The token total is ONLY ever the harness-reported split. There is no fallback token
+ * source: the ACP stream's `usage_update.used` is the agent's context-window occupancy,
+ * not a count of the tokens this run spent, so it must never become a token total. When
+ * the harness reports no split, this returns no tokens at all (cost alone still counts as
+ * usage) — absent data has to read as absent, because a plausible-looking wrong total
+ * silently poisons every aggregate built on it.
+ *
+ * Cost follows the same rule via omission: an unreported cost leaves the key OFF, because a
+ * substituted `0` would claim the run was measured and free. A reported cost is passed through
+ * as-is, including a genuine `0`.
+ */
 export function mergePromptAndStreamUsage(
   promptResult: any,
   streamUsage: AgentUsage | undefined,
@@ -33,10 +46,16 @@ export function mergePromptAndStreamUsage(
   const promptUsage = promptResult?.usage;
   const inputTokens = promptUsage?.inputTokens ?? streamUsage?.input ?? 0;
   const outputTokens = promptUsage?.outputTokens ?? streamUsage?.output ?? 0;
-  const total = inputTokens + outputTokens || streamUsage?.total || 0;
-  const cost = streamUsage?.cost ?? 0;
-  return total > 0 || cost > 0
-    ? { input: inputTokens, output: outputTokens, total, cost }
+  const total = inputTokens + outputTokens;
+  const cost = streamUsage?.cost;
+  const hasCost = cost != null;
+  return total > 0 || hasCost
+    ? {
+        input: inputTokens,
+        output: outputTokens,
+        total,
+        ...(hasCost ? { cost } : {}),
+      }
     : undefined;
 }
 
diff --git a/services/runner/src/protocol.ts b/services/runner/src/protocol.ts
index f06b25b2c2..6b20457812 100644
--- a/services/runner/src/protocol.ts
+++ b/services/runner/src/protocol.ts
@@ -427,7 +427,13 @@ export interface AgentUsage {
   input: number;
   output: number;
   total: number;
-  cost: number;
+  /**
+   * INVARIANT: absent means the cost is UNKNOWN (the harness reported none); a present `0` is a
+   * measured zero — a free model or a fully cached turn. Consumers read presence as evidence of
+   * a measurement, so a producer must never substitute a zero for an absence: doing so records
+   * an unpriced run as a free one, which every downstream aggregate then believes.
+   */
+  cost?: number;
 }
 
 export interface AgentRunRequest {
diff --git a/services/runner/src/tracing/otel.ts b/services/runner/src/tracing/otel.ts
index ea5f3fcccd..2b173b8cb2 100644
--- a/services/runner/src/tracing/otel.ts
+++ b/services/runner/src/tracing/otel.ts
@@ -1163,7 +1163,9 @@ export function createSandboxAgentOtel(
   }
 
   function stampUsage(span: Span, u: AgentUsage | undefined): void {
-    if (!u) return;
+    // No tokens and no cost is not a measured zero, it is the absence of a measurement:
+    // stamping zeros would assert a run cost nothing and spent nothing.
+    if (!u || (u.total <= 0 && u.cost <= 0)) return;
     span.setAttribute("gen_ai.usage.input_tokens", u.input);
     span.setAttribute("gen_ai.usage.output_tokens", u.output);
     span.setAttribute("gen_ai.usage.prompt_tokens", u.input);
@@ -1446,16 +1448,19 @@ export function createSandboxAgentOtel(
     }
 
     if (kind === "usage_update") {
-      // ACP usage_update carries only `used` (context tokens) and `cost.amount`. The
-      // per-call input/output split is NOT on the stream; it rides on the PromptResponse,
-      // which the sandbox-agent engine reads. Keep total + cost here and leave the split to the caller.
+      // ACP usage_update carries `used` and `cost.amount`. `used` is the agent's CONTEXT-WINDOW
+      // occupancy at this point in the turn, NOT the tokens this run spent, so it is dropped:
+      // reporting it as a total produced runs shaped `input 0 / output 0 / total <context size>`.
+      // The token split is not on the stream at all; it rides on the PromptResponse, which the
+      // sandbox-agent engine reads and hands back through `setUsage`. Cost is the only run total
+      // here, so an update without one carries nothing worth recording.
       const cost = update.cost?.amount;
-      const total = update.used;
+      if (typeof cost !== "number") return;
       usage = {
         input: usage?.input ?? 0,
         output: usage?.output ?? 0,
-        total: typeof total === "number" ? total : usage?.total ?? 0,
-        cost: typeof cost === "number" ? cost : usage?.cost ?? 0,
+        total: usage?.total ?? 0,
+        cost,
       };
       record({ type: "usage", ...usage });
     }
diff --git a/services/runner/tests/unit/otel-usage-context-size.test.ts b/services/runner/tests/unit/otel-usage-context-size.test.ts
new file mode 100644
index 0000000000..b2818a502e
--- /dev/null
+++ b/services/runner/tests/unit/otel-usage-context-size.test.ts
@@ -0,0 +1,174 @@
+/**
+ * Unit tests for how the sandbox-agent ACP tracer treats `usage_update`.
+ *
+ * ACP's `usage_update.used` is the agent's context-window occupancy, not the tokens a run
+ * spent. Reporting it as a token total produced runs shaped `input 0 / output 0 / total
+ * <context size>` with no cost — a number a reader cannot tell apart from a real total. These
+ * tests pin that the context size never reaches a `usage` event, `usage()`, or a span, while
+ * the harness-reported split (delivered through `setUsage`) still does.
+ *
+ * Spans export over OTLP from a module-level provider, so we spy on the OTel API tracer and
+ * capture what each span records (same approach as otel-skills-error.test.ts).
+ *
+ * Run: pnpm test (or: pnpm exec vitest run tests/unit/otel-usage-context-size.test.ts)
+ */
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { trace, type Span } from "@opentelemetry/api";
+
+import { createSandboxAgentOtel } from "../../src/tracing/otel.ts";
+import type { AgentEvent } from "../../src/protocol.ts";
+
+interface FakeSpan {
+  name: string;
+  attributes: Record<string, unknown>;
+}
+
+/** Replace the OTel tracer so every span built records into a captured array. */
+function spyTracer(): FakeSpan[] {
+  const spans: FakeSpan[] = [];
+  const makeSpan = (name: string): Span => {
+    const span: FakeSpan = { name, attributes: {} };
+    spans.push(span);
+    const api = {
+      setAttribute(key: string, value: unknown) {
+        span.attributes[key] = value;
+        return api;
+      },
+      setAttributes(attrs: Record<string, unknown>) {
+        Object.assign(span.attributes, attrs);
+        return api;
+      },
+      recordException() {},
+      setStatus() {
+        return api;
+      },
+      end() {},
+      spanContext() {
+        return {
+          traceId: "0".repeat(32),
+          spanId: "0".repeat(16),
+          traceFlags: 1,
+        };
+      },
+      isRecording: () => true,
+      addEvent: () => api,
+      updateName: () => api,
+    };
+    return api as unknown as Span;
+  };
+  vi.spyOn(trace, "getTracer").mockReturnValue({
+    startSpan: (name: string) => makeSpan(name),
+    startActiveSpan: ((name: string, fn: (s: Span) => unknown) =>
+      fn(makeSpan(name))) as any,
+  } as any);
+  return spans;
+}
+
+const contextSizeUpdate = (used: number, cost?: number) => ({
+  sessionUpdate: "usage_update",
+  used,
+  ...(cost === undefined ? {} : { cost: { amount: cost } }),
+});
+
+const usageKeys = (span: FakeSpan | undefined) =>
+  Object.keys(span?.attributes ?? {}).filter((k) =>
+    k.startsWith("gen_ai.usage."),
+  );
+
+afterEach(() => {
+  vi.restoreAllMocks();
+});
+
+describe("usage_update carries context size, not tokens", () => {
+  it("reports nothing when the stream knows only the context size", () => {
+    const spans = spyTracer();
+    const emitted: AgentEvent[] = [];
+    const otel = createSandboxAgentOtel({
+      harness: "claude",
+      model: "anthropic/claude-haiku",
+      emit: (e) => emitted.push(e),
+    });
+    otel.start({ prompt: "hi" });
+    otel.handleUpdate(contextSizeUpdate(63369));
+
+    expect(otel.usage()).toBeUndefined();
+    expect(emitted.filter((e) => e.type === "usage")).toEqual([]);
+
+    // The engine finds no harness split either, so nothing overrides the stream before finish.
+    otel.setUsage(undefined);
+    otel.finish();
+    expect(usageKeys(spans.find((s) => s.name === "invoke_agent"))).toEqual([]);
+    expect(
+      emitted.some((e) => e.type === "usage"),
+      "no usage event anywhere in the run",
+    ).toBe(false);
+  });
+
+  it("keeps the stream cost, without inventing a token total from the context size", () => {
+    const spans = spyTracer();
+    const otel = createSandboxAgentOtel({
+      harness: "claude",
+      model: "anthropic/claude-haiku",
+    });
+    otel.start({ prompt: "hi" });
+    otel.handleUpdate(contextSizeUpdate(63369, 0.04));
+
+    expect(otel.usage()).toEqual({ input: 0, output: 0, total: 0, cost: 0.04 });
+    otel.finish();
+    const agentSpan = spans.find((s) => s.name === "invoke_agent");
+    expect(agentSpan?.attributes["gen_ai.usage.cost"]).toBe(0.04);
+    expect(agentSpan?.attributes["gen_ai.usage.total_tokens"]).toBe(0);
+  });
+
+  it("emits a token-only usage event with no cost key when the harness priced nothing", () => {
+    const spans = spyTracer();
+    const emitted: AgentEvent[] = [];
+    const otel = createSandboxAgentOtel({
+      harness: "codex",
+      model: "openai-codex/gpt-5.5",
+      emit: (e) => emitted.push(e),
+      emitSpans: true,
+    });
+    otel.start({ prompt: "hi" });
+    // No `usage_update` at all — codex reports a token split but never a cost.
+    otel.setUsage({ input: 12, output: 3, total: 15 });
+    otel.finish();
+
+    const usageEvent = emitted.find((e) => e.type === "usage") as Record<
+      string,
+      unknown
+    >;
+    expect(usageEvent).toEqual({
+      type: "usage",
+      input: 12,
+      output: 3,
+      total: 15,
+    });
+    expect("cost" in usageEvent).toBe(false);
+
+    // Nothing downstream may see a zero: the agent span carries no cost either.
+    const agentSpan = spans.find((s) => s.name === "invoke_agent");
+    expect(agentSpan?.attributes["gen_ai.usage.cost"]).toBeUndefined();
+    const chatSpan = spans.find((s) => s.name.startsWith("chat"));
+    expect(chatSpan?.attributes["gen_ai.usage.total_tokens"]).toBe(15);
+    expect(chatSpan?.attributes["gen_ai.usage.cost"]).toBeUndefined();
+  });
+
+  it("stamps the harness-reported split, which arrives through setUsage", () => {
+    const spans = spyTracer();
+    const otel = createSandboxAgentOtel({
+      harness: "claude",
+      model: "anthropic/claude-haiku",
+    });
+    otel.start({ prompt: "hi" });
+    otel.handleUpdate(contextSizeUpdate(63369, 0.04));
+    otel.setUsage({ input: 12, output: 3, total: 15, cost: 0.04 });
+    otel.finish();
+
+    const agentSpan = spans.find((s) => s.name === "invoke_agent");
+    expect(agentSpan?.attributes["gen_ai.usage.input_tokens"]).toBe(12);
+    expect(agentSpan?.attributes["gen_ai.usage.output_tokens"]).toBe(3);
+    expect(agentSpan?.attributes["gen_ai.usage.total_tokens"]).toBe(15);
+    expect(agentSpan?.attributes["gen_ai.usage.cost"]).toBe(0.04);
+  });
+});
diff --git a/services/runner/tests/unit/sandbox-agent-usage.test.ts b/services/runner/tests/unit/sandbox-agent-usage.test.ts
index a43b7c2bb9..4080c1ccfb 100644
--- a/services/runner/tests/unit/sandbox-agent-usage.test.ts
+++ b/services/runner/tests/unit/sandbox-agent-usage.test.ts
@@ -59,6 +59,75 @@ describe("mergePromptAndStreamUsage", () => {
   it("returns undefined when no usage was reported", () => {
     assert.equal(mergePromptAndStreamUsage({}, undefined), undefined);
   });
+
+  it("reports no usage when only the ACP context size is known", () => {
+    // `usage_update.used` is context-window occupancy, not tokens spent: with no harness
+    // split there is nothing honest to report, so the caller writes no usage attributes.
+    assert.equal(
+      mergePromptAndStreamUsage({ stopReason: "end_turn" }, { input: 0, output: 0, total: 63369 }),
+      undefined,
+    );
+  });
+
+  it("keeps a stream cost even when the harness reported no token split", () => {
+    assert.deepEqual(
+      mergePromptAndStreamUsage({}, { input: 0, output: 0, total: 63369, cost: 0.04 }),
+      { input: 0, output: 0, total: 0, cost: 0.04 },
+    );
+  });
+
+  it("uses the harness split unchanged, never the stream total", () => {
+    assert.deepEqual(
+      mergePromptAndStreamUsage(
+        { usage: { inputTokens: 12, outputTokens: 3 } },
+        { input: 0, output: 0, total: 63369 },
+      ),
+      { input: 12, output: 3, total: 15 },
+    );
+  });
+
+  it("keeps a half-reported split (output only) as the total", () => {
+    assert.deepEqual(
+      mergePromptAndStreamUsage({ usage: { outputTokens: 5 } }, undefined),
+      { input: 0, output: 5, total: 5 },
+    );
+  });
+
+  it("omits cost entirely when the harness reported none", () => {
+    // A substituted `0` would say the run was measured and free. Consumers (the SDK's
+    // `record_usage`, the Vercel projection) read the key's PRESENCE as the measurement, so
+    // an unpriced harness — codex among them — has to leave it off.
+    const merged = mergePromptAndStreamUsage(
+      { usage: { inputTokens: 12, outputTokens: 3 } },
+      undefined,
+    );
+    assert.deepEqual(merged, { input: 12, output: 3, total: 15 });
+    assert.equal("cost" in merged!, false);
+  });
+
+  it("keeps a reported zero cost, which is a measurement", () => {
+    // A free model or a fully cached turn really does cost 0. Now that absence is expressible,
+    // a reported zero must survive as the number it is.
+    const merged = mergePromptAndStreamUsage(
+      { usage: { inputTokens: 12, outputTokens: 3 } },
+      { input: 0, output: 0, total: 0, cost: 0 },
+    );
+    assert.deepEqual(merged, { input: 12, output: 3, total: 15, cost: 0 });
+  });
+
+  it("keeps a reported zero cost even with no token split at all", () => {
+    assert.deepEqual(
+      mergePromptAndStreamUsage({}, { input: 0, output: 0, total: 0, cost: 0 }),
+      { input: 0, output: 0, total: 0, cost: 0 },
+    );
+  });
+
+  it("falls back to already-known stream tokens, which only a prior setUsage can set", () => {
+    assert.deepEqual(
+      mergePromptAndStreamUsage({}, { input: 8, output: 2, total: 10, cost: 0.01 }),
+      { input: 8, output: 2, total: 10, cost: 0.01 },
+    );
+  });
 });
 
 describe("resolveRunUsage", () => {
@@ -79,4 +148,17 @@ describe("resolveRunUsage", () => {
       { input: 3, output: 4, total: 7, cost: 0.03 },
     );
   });
+
+  it("resolves to no usage when neither the writeback nor the harness reported a split", async () => {
+    assert.equal(
+      await resolveRunUsage({
+        sandbox: {},
+        usageOutPath: undefined,
+        isDaytona: false,
+        promptResult: { stopReason: "end_turn" },
+        streamUsage: { input: 0, output: 0, total: 63369 },
+      }),
+      undefined,
+    );
+  });
 });