diff --git a/src/tools/estimate-cost.ts b/src/tools/estimate-cost.ts
index 327491a..d6b37cf 100644
--- a/src/tools/estimate-cost.ts
+++ b/src/tools/estimate-cost.ts
@@ -10,7 +10,7 @@ import { findSessionDirs, findSessionFiles } from "../lib/session-parser.js";
 
 // ── Pricing (per 1M tokens) ────────────────────────────────────────────────
 
-const PRICING: Record<string, { input: number; output: number }> = {
+export const PRICING: Record<string, { input: number; output: number }> = {
   "claude-sonnet-4": { input: 3.0, output: 15.0 },
   "claude-opus-4": { input: 15.0, output: 75.0 },
   "claude-haiku-3.5": { input: 0.8, output: 4.0 },
@@ -18,7 +18,7 @@ const PRICING: Record<string, { input: number; output: number }> = {
 
 const DEFAULT_MODEL = "claude-sonnet-4";
 
-const CORRECTION_SIGNALS = /\b(no[,.\s]|wrong|not that|i meant|actually|try again|revert|undo|that's not|not what i)\b/i;
+export const CORRECTION_SIGNALS = /\b(no[,.\s]|wrong|not that|i meant|actually|try again|revert|undo|that's not|not what i)\b/i;
 
 const PREFLIGHT_TOOLS = new Set([
   "preflight_check",
@@ -29,13 +29,13 @@ const PREFLIGHT_TOOLS = new Set([
   "prompt_score",
 ]);
 
-// ── Helpers ─────────────────────────────────────────────────────────────────
+// ── Helpers (exported for testing) ───────────────────────────────────────────
 
-function estimateTokens(text: string): number {
+export function estimateTokens(text: string): number {
   return Math.ceil(text.length / 4);
 }
 
-function extractText(content: unknown): string {
+export function extractText(content: unknown): string {
   if (typeof content === "string") return content;
   if (Array.isArray(content)) {
     return content
@@ -46,25 +46,25 @@ function extractText(content: unknown): string {
   return "";
 }
 
-function extractToolNames(content: unknown): string[] {
+export function extractToolNames(content: unknown): string[] {
   if (!Array.isArray(content)) return [];
   return content
     .filter((b: any) => b.type === "tool_use" && b.name)
     .map((b: any) => b.name as string);
 }
 
-function formatTokens(n: number): string {
+export function formatTokens(n: number): string {
   if (n >= 1_000_000) return `${(n / 1_000_000).toFixed(1)}M`;
   if (n >= 1_000) return `${(n / 1_000).toFixed(1)}k`;
   return String(n);
 }
 
-function formatCost(dollars: number): string {
+export function formatCost(dollars: number): string {
   if (dollars < 0.01) return `<$0.01`;
   return `$${dollars.toFixed(2)}`;
 }
 
-function formatDuration(ms: number): string {
+export function formatDuration(ms: number): string {
   const mins = Math.floor(ms / 60_000);
   if (mins < 60) return `${mins}m`;
   const hours = Math.floor(mins / 60);
diff --git a/tests/tools/estimate-cost.test.ts b/tests/tools/estimate-cost.test.ts
new file mode 100644
index 0000000..8eb9c3f
--- /dev/null
+++ b/tests/tools/estimate-cost.test.ts
@@ -0,0 +1,180 @@
+import { describe, it, expect } from "vitest";
+import {
+  estimateTokens,
+  extractText,
+  extractToolNames,
+  formatTokens,
+  formatCost,
+  formatDuration,
+  PRICING,
+  CORRECTION_SIGNALS,
+} from "../../src/tools/estimate-cost.js";
+
+describe("estimateTokens", () => {
+  it("estimates ~4 chars per token", () => {
+    expect(estimateTokens("abcd")).toBe(1);
+    expect(estimateTokens("abcdefgh")).toBe(2);
+  });
+
+  it("rounds up", () => {
+    expect(estimateTokens("ab")).toBe(1); // 2/4 = 0.5 → ceil = 1
+  });
+
+  it("handles empty string", () => {
+    expect(estimateTokens("")).toBe(0);
+  });
+});
+
+describe("extractText", () => {
+  it("returns string content directly", () => {
+    expect(extractText("hello")).toBe("hello");
+  });
+
+  it("extracts text blocks from array content", () => {
+    const content = [
+      { type: "text", text: "line 1" },
+      { type: "text", text: "line 2" },
+    ];
+    expect(extractText(content)).toBe("line 1\nline 2");
+  });
+
+  it("filters non-text blocks", () => {
+    const content = [
+      { type: "text", text: "hello" },
+      { type: "tool_use", name: "read", input: {} },
+    ];
+    expect(extractText(content)).toBe("hello");
+  });
+
+  it("returns empty for null/undefined/number", () => {
+    expect(extractText(null)).toBe("");
+    expect(extractText(undefined)).toBe("");
+    expect(extractText(42)).toBe("");
+  });
+
+  it("returns empty for empty array", () => {
+    expect(extractText([])).toBe("");
+  });
+});
+
+describe("extractToolNames", () => {
+  it("extracts tool_use names from content blocks", () => {
+    const content = [
+      { type: "text", text: "thinking..." },
+      { type: "tool_use", name: "read", input: { path: "foo.ts" } },
+      { type: "tool_use", name: "write", input: { path: "bar.ts" } },
+    ];
+    expect(extractToolNames(content)).toEqual(["read", "write"]);
+  });
+
+  it("returns empty for non-array", () => {
+    expect(extractToolNames("string")).toEqual([]);
+    expect(extractToolNames(null)).toEqual([]);
+  });
+
+  it("skips tool_use blocks without name", () => {
+    const content = [{ type: "tool_use", input: {} }];
+    expect(extractToolNames(content)).toEqual([]);
+  });
+});
+
+describe("formatTokens", () => {
+  it("formats millions", () => {
+    expect(formatTokens(1_500_000)).toBe("1.5M");
+  });
+
+  it("formats thousands", () => {
+    expect(formatTokens(12_500)).toBe("12.5k");
+  });
+
+  it("formats small numbers as-is", () => {
+    expect(formatTokens(500)).toBe("500");
+  });
+
+  it("handles zero", () => {
+    expect(formatTokens(0)).toBe("0");
+  });
+
+  it("handles boundary at 1000", () => {
+    expect(formatTokens(1000)).toBe("1.0k");
+    expect(formatTokens(999)).toBe("999");
+  });
+});
+
+describe("formatCost", () => {
+  it("formats normal costs", () => {
+    expect(formatCost(1.5)).toBe("$1.50");
+    expect(formatCost(0.05)).toBe("$0.05");
+  });
+
+  it("shows <$0.01 for tiny amounts", () => {
+    expect(formatCost(0.005)).toBe("<$0.01");
+    expect(formatCost(0)).toBe("<$0.01");
+  });
+});
+
+describe("formatDuration", () => {
+  it("formats minutes", () => {
+    expect(formatDuration(5 * 60_000)).toBe("5m");
+  });
+
+  it("formats hours and minutes", () => {
+    expect(formatDuration(90 * 60_000)).toBe("1h 30m");
+  });
+
+  it("handles zero", () => {
+    expect(formatDuration(0)).toBe("0m");
+  });
+});
+
+describe("PRICING", () => {
+  it("has required models", () => {
+    expect(PRICING["claude-sonnet-4"]).toBeDefined();
+    expect(PRICING["claude-opus-4"]).toBeDefined();
+    expect(PRICING["claude-haiku-3.5"]).toBeDefined();
+  });
+
+  it("has positive prices", () => {
+    for (const [, p] of Object.entries(PRICING)) {
+      expect(p.input).toBeGreaterThan(0);
+      expect(p.output).toBeGreaterThan(0);
+      expect(p.output).toBeGreaterThan(p.input); // output always costs more
+    }
+  });
+});
+
+describe("CORRECTION_SIGNALS", () => {
+  const positives = [
+    "no, that's wrong",
+    "Wrong approach",
+    "not that one",
+    "I meant the other file",
+    "actually, use the other method",
+    "try again please",
+    "revert that change",
+    "undo the last edit",
+    "that's not what I wanted",
+    "not what i asked for",
+  ];
+
+  const negatives = [
+    "looks good, ship it",
+    "nice work on that refactor",
+    "add a new function",
+    "read the file",
+    "knowledge base",      // should not match "no" in "knowledge"
+    "annotation",           // should not match "no" in "annotation"
+  ];
+
+  for (const phrase of positives) {
+    it(`matches correction: "${phrase}"`, () => {
+      expect(CORRECTION_SIGNALS.test(phrase)).toBe(true);
+    });
+  }
+
+  for (const phrase of negatives) {
+    it(`does not match non-correction: "${phrase}"`, () => {
+      expect(CORRECTION_SIGNALS.test(phrase)).toBe(false);
+    });
+  }
+});