fix(ai-chat): tick the live token counter between agent steps (#163)

The header token badge (and the "Thinking… · N tokens" line) froze between
agent steps and jumped in chunks instead of ticking smoothly. liveTurnTokens
returned the authoritative server `usage` VERBATIM as soon as it appeared, but
the server only attaches usage at a step boundary and it is cumulative over
COMPLETED steps — so during the next (in-flight) step the figure stayed frozen
at the previous boundary and the running text estimate was ignored.

Combine both sources per component via max: always compute the running estimate
(chars/≈4 over the message's reasoning/text parts, which includes the in-flight
step) and take max(authoritativeBase, estimate). Between boundaries the estimate
ticks the number up; at a boundary the authoritative figure snaps it exact; and
because the server usage is cumulative and we only ever take the max, the counter
is monotonic (never drops). Reasoning/output stay split; the #151 reasoning-only
authoritative count is preserved.

Backward compatible: in every existing test the estimate is <= the authoritative
figure, so max returns the same value. +4 tests for the in-flight-step-exceeds-
base case (output + reasoning), the authoritative-wins case, and monotonicity.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/src/features/ai-chat/utils/count-stream-tokens.test.ts

@@ -117,3 +117,55 @@ describe("liveTurnTokens — authoritative path", () => {
     expect(r).toEqual({ reasoning: 0, output: 1, authoritative: false });
   });
 });
+
+describe("liveTurnTokens — combined authoritative + estimate (#163)", () => {
+  it("ticks the in-flight step above the completed-steps authoritative base", () => {
+    // The authoritative usage is the sum over COMPLETED steps (step 1). The
+    // CURRENT step is streaming and its text is NOT in `usage` yet, but it IS in
+    // the parts -> the running estimate must push the live figure above the base
+    // so the badge keeps growing between step boundaries.
+    const longText = "x".repeat(800); // 800 chars -> 200 est output tokens
+    const r = liveTurnTokens(
+      msg([{ type: "text", text: longText }], {
+        usage: { inputTokens: 500, outputTokens: 40 }, // step-1 base: 40 output
+      }),
+    );
+    // max(authOutput=40, estOutput=200) = 200 -> the counter ticks, not frozen.
+    expect(r.output).toBe(200);
+    expect(r.authoritative).toBe(true);
+  });
+
+  it("ticks reasoning of the in-flight step above the authoritative reasoning base", () => {
+    const longReasoning = "r".repeat(400); // 400 chars -> 100 est reasoning
+    const r = liveTurnTokens(
+      msg([{ type: "reasoning", text: longReasoning }], {
+        usage: { inputTokens: 100, outputTokens: 20, reasoningTokens: 20 },
+      }),
+    );
+    // reasoning: max(20, 100) = 100 ; output: max(max(0,20-20)=0, 0) = 0.
+    expect(r.reasoning).toBe(100);
+    expect(r.output).toBe(0);
+    expect(r.authoritative).toBe(true);
+  });
+
+  it("snaps to the authoritative figure once it exceeds the rough estimate", () => {
+    // Short on-screen text (estimate tiny) but a large authoritative output:
+    // the exact figure wins at the boundary (the counter never under-reports).
+    const r = liveTurnTokens(
+      msg([{ type: "text", text: "abcd" }], {
+        usage: { inputTokens: 10, outputTokens: 5000 },
+      }),
+    );
+    expect(r.output).toBe(5000);
+  });
+
+  it("is monotonic: max never drops below the authoritative base when the estimate is smaller", () => {
+    // Mirrors the legacy 'verbatim' tests: estimate < authoritative -> unchanged.
+    const r = liveTurnTokens(
+      msg([{ type: "text", text: "tiny" }], {
+        usage: { inputTokens: 500, outputTokens: 100, reasoningTokens: 30 },
+      }),
+    );
+    expect(r).toEqual({ reasoning: 30, output: 70, authoritative: true });
+  });
+});

apps/client/src/features/ai-chat/utils/count-stream-tokens.ts

@@ -56,39 +56,58 @@ function metadataUsage(message: UIMessage): AuthoritativeUsage | undefined {
 /**
  * Token split for the given (streaming) assistant message.
  *
- * Prefers AUTHORITATIVE `metadata.usage` when the server has attached it (at a
- * step/turn boundary, incl. `reasoningTokens`) — so the live counter snaps to the
- * provider's exact figures. Until then it returns a running ESTIMATE summed over
- * the message parts: `reasoning` parts feed the reasoning estimate, `text` parts
- * feed the output estimate. Multi-part / multi-step turns accumulate naturally
- * because every part of the turn is summed.
+ * COMBINES the authoritative server usage with the running text estimate so the
+ * counter ticks in real time AND lands exact. The server only attaches
+ * `metadata.usage` at a step/turn boundary (`finish-step`/`finish`) and it is
+ * CUMULATIVE over COMPLETED steps — it does NOT yet include the in-flight step.
+ * So a multi-step turn that returned the authoritative figure verbatim would
+ * FREEZE between boundaries and jump in steps (issue #163).
+ *
+ * Instead we always compute the running ESTIMATE (chars/≈4 over the message's
+ * `reasoning`/`text` parts, which grows on every streamed delta) and take the
+ * per-component MAX of the authoritative base and the estimate:
+ *   - between boundaries the estimate of the in-flight step ticks the number up;
+ *   - at a boundary the authoritative figure snaps it to exact;
+ *   - because the server's usage is cumulative and we only ever take the max, the
+ *     number is MONOTONIC — it never drops.
  *
  * Providers that don't stream reasoning text still surface a reasoning count once
- * the authoritative usage arrives (`usage.reasoningTokens`); on the pure estimate
- * path such a turn simply shows `reasoning: 0` until then.
+ * the authoritative usage arrives (`max(reasoningTokens, 0)`); on the pure
+ * estimate path (no usage yet) such a turn shows `reasoning: 0` until then.
  */
 export function liveTurnTokens(message: UIMessage | undefined): LiveTurnTokens {
   if (!message) return { reasoning: 0, output: 0, authoritative: false };
 
-  const usage = metadataUsage(message);
-  if (usage) {
-    // Authoritative branch: outputTokens already INCLUDES reasoning tokens in the
-    // AI SDK usage shape, so subtract reasoning out for the "answer" figure (never
-    // go negative if a provider reports them inconsistently).
-    const reasoning = usage.reasoningTokens ?? 0;
-    const totalOutput = usage.outputTokens ?? 0;
-    const output = Math.max(0, totalOutput - reasoning);
-    return { reasoning, output, authoritative: true };
-  }
-
-  let reasoning = 0;
-  let output = 0;
+  // Running ESTIMATE over every reasoning/text part — grows on each delta. This
+  // includes the IN-FLIGHT step, which the authoritative usage does not cover yet.
+  let estReasoning = 0;
+  let estOutput = 0;
   for (const part of message.parts ?? []) {
     if (part.type === "reasoning") {
-      reasoning += estimateTokens((part as { text?: string }).text ?? "");
+      estReasoning += estimateTokens((part as { text?: string }).text ?? "");
     } else if (part.type === "text") {
-      output += estimateTokens((part as { text?: string }).text ?? "");
+      estOutput += estimateTokens((part as { text?: string }).text ?? "");
     }
   }
-  return { reasoning, output, authoritative: false };
+
+  const usage = metadataUsage(message);
+  if (!usage) {
+    // No authoritative usage streamed yet: the estimate IS the live figure.
+    return { reasoning: estReasoning, output: estOutput, authoritative: false };
+  }
+
+  // Authoritative sum over COMPLETED steps. `outputTokens` already INCLUDES
+  // reasoning in the AI SDK usage shape, so subtract it out for the "answer"
+  // figure (never go negative if a provider reports them inconsistently).
+  const authReasoning = usage.reasoningTokens ?? 0;
+  const authOutput = Math.max(0, (usage.outputTokens ?? 0) - authReasoning);
+
+  // Per-component max: the in-flight step's estimate ticks above the completed-
+  // steps base between boundaries, and the authoritative figure wins once it
+  // exceeds the (rough) estimate at the next boundary. Monotonic by construction.
+  return {
+    reasoning: Math.max(authReasoning, estReasoning),
+    output: Math.max(authOutput, estOutput),
+    authoritative: true,
+  };
 }