diff --git a/src/tools/hashline-edit/edit-operations.test.ts b/src/tools/hashline-edit/edit-operations.test.ts
index 41b5c2e6d..754dff41c 100644
--- a/src/tools/hashline-edit/edit-operations.test.ts
+++ b/src/tools/hashline-edit/edit-operations.test.ts
@@ -6,13 +6,18 @@ import {
   applyReplaceLines,
   applySetLine,
 } from "./edit-operations"
+import { computeLineHash } from "./hash-computation"
 import type { HashlineEdit, InsertAfter, Replace, ReplaceLines, SetLine } from "./types"
 
 describe("applySetLine", () => {
+  function anchorFor(lines: string[], line: number): string {
+    return `${line}:${computeLineHash(line, lines[line - 1])}`
+  }
+
   it("replaces a single line at the specified anchor", () => {
     //#given
     const lines = ["line 1", "line 2", "line 3"]
-    const anchor = "2:b2" // line 2 hash
+    const anchor = anchorFor(lines, 2)
 
     //#when
     const result = applySetLine(lines, anchor, "new line 2")
@@ -24,7 +29,7 @@ describe("applySetLine", () => {
   it("handles newline escapes in replacement text", () => {
     //#given
     const lines = ["line 1", "line 2", "line 3"]
-    const anchor = "2:b2"
+    const anchor = anchorFor(lines, 2)
 
     //#when
     const result = applySetLine(lines, anchor, "new\\nline")
@@ -56,8 +61,8 @@ describe("applyReplaceLines", () => {
   it("replaces a range of lines", () => {
     //#given
     const lines = ["line 1", "line 2", "line 3", "line 4", "line 5"]
-    const startAnchor = "2:b2"
-    const endAnchor = "4:5f"
+    const startAnchor = `${2}:${computeLineHash(2, lines[1])}`
+    const endAnchor = `${4}:${computeLineHash(4, lines[3])}`
 
     //#when
     const result = applyReplaceLines(lines, startAnchor, endAnchor, "replacement")
@@ -69,8 +74,8 @@ describe("applyReplaceLines", () => {
   it("handles newline escapes in replacement text", () => {
     //#given
     const lines = ["line 1", "line 2", "line 3"]
-    const startAnchor = "2:b2"
-    const endAnchor = "2:b2"
+    const startAnchor = `${2}:${computeLineHash(2, lines[1])}`
+    const endAnchor = `${2}:${computeLineHash(2, lines[1])}`
 
     //#when
     const result = applyReplaceLines(lines, startAnchor, endAnchor, "a\\nb")
@@ -83,7 +88,7 @@ describe("applyReplaceLines", () => {
     //#given
     const lines = ["line 1", "line 2", "line 3"]
     const startAnchor = "2:ff"
-    const endAnchor = "3:83"
+    const endAnchor = `${3}:${computeLineHash(3, lines[2])}`
 
     //#when / #then
     expect(() => applyReplaceLines(lines, startAnchor, endAnchor, "new")).toThrow(
@@ -94,7 +99,7 @@ describe("applyReplaceLines", () => {
   it("throws on end hash mismatch", () => {
     //#given
     const lines = ["line 1", "line 2", "line 3"]
-    const startAnchor = "2:b2"
+    const startAnchor = `${2}:${computeLineHash(2, lines[1])}`
     const endAnchor = "3:ff"
 
     //#when / #then
@@ -106,8 +111,8 @@ describe("applyReplaceLines", () => {
   it("throws when start > end", () => {
     //#given
     const lines = ["line 1", "line 2", "line 3"]
-    const startAnchor = "3:83"
-    const endAnchor = "2:b2"
+    const startAnchor = `${3}:${computeLineHash(3, lines[2])}`
+    const endAnchor = `${2}:${computeLineHash(2, lines[1])}`
 
     //#when / #then
     expect(() => applyReplaceLines(lines, startAnchor, endAnchor, "new")).toThrow(
@@ -120,7 +125,7 @@ describe("applyInsertAfter", () => {
   it("inserts text after the specified line", () => {
     //#given
     const lines = ["line 1", "line 2", "line 3"]
-    const anchor = "2:b2"
+    const anchor = `${2}:${computeLineHash(2, lines[1])}`
 
     //#when
     const result = applyInsertAfter(lines, anchor, "inserted")
@@ -132,7 +137,7 @@ describe("applyInsertAfter", () => {
   it("handles newline escapes to insert multiple lines", () => {
     //#given
     const lines = ["line 1", "line 2", "line 3"]
-    const anchor = "2:b2"
+    const anchor = `${2}:${computeLineHash(2, lines[1])}`
 
     //#when
     const result = applyInsertAfter(lines, anchor, "a\\nb\\nc")
@@ -144,7 +149,7 @@ describe("applyInsertAfter", () => {
   it("inserts at end when anchor is last line", () => {
     //#given
     const lines = ["line 1", "line 2"]
-    const anchor = "2:b2"
+    const anchor = `${2}:${computeLineHash(2, lines[1])}`
 
     //#when
     const result = applyInsertAfter(lines, anchor, "inserted")
@@ -218,7 +223,8 @@ describe("applyHashlineEdits", () => {
   it("applies single set_line edit", () => {
     //#given
     const content = "line 1\nline 2\nline 3"
-    const edits: SetLine[] = [{ type: "set_line", line: "2:b2", text: "new line 2" }]
+    const line2Hash = computeLineHash(2, "line 2")
+    const edits: SetLine[] = [{ type: "set_line", line: `2:${line2Hash}`, text: "new line 2" }]
 
     //#when
     const result = applyHashlineEdits(content, edits)
@@ -230,9 +236,11 @@ describe("applyHashlineEdits", () => {
   it("applies multiple edits bottom-up (descending line order)", () => {
     //#given
     const content = "line 1\nline 2\nline 3\nline 4\nline 5"
+    const line2Hash = computeLineHash(2, "line 2")
+    const line4Hash = computeLineHash(4, "line 4")
     const edits: SetLine[] = [
-      { type: "set_line", line: "2:b2", text: "new 2" },
-      { type: "set_line", line: "4:5f", text: "new 4" },
+      { type: "set_line", line: `2:${line2Hash}`, text: "new 2" },
+      { type: "set_line", line: `4:${line4Hash}`, text: "new 4" },
     ]
 
     //#when
@@ -245,9 +253,11 @@ describe("applyHashlineEdits", () => {
   it("applies mixed edit types", () => {
     //#given
     const content = "line 1\nline 2\nline 3"
+    const line1Hash = computeLineHash(1, "line 1")
+    const line3Hash = computeLineHash(3, "line 3")
     const edits: HashlineEdit[] = [
-      { type: "insert_after", line: "1:02", text: "inserted" },
-      { type: "set_line", line: "3:83", text: "modified" },
+      { type: "insert_after", line: `1:${line1Hash}`, text: "inserted" },
+      { type: "set_line", line: `3:${line3Hash}`, text: "modified" },
     ]
 
     //#when
@@ -260,8 +270,10 @@ describe("applyHashlineEdits", () => {
   it("applies replace_lines edit", () => {
     //#given
     const content = "line 1\nline 2\nline 3\nline 4"
+    const line2Hash = computeLineHash(2, "line 2")
+    const line3Hash = computeLineHash(3, "line 3")
     const edits: ReplaceLines[] = [
-      { type: "replace_lines", start_line: "2:b2", end_line: "3:83", text: "replaced" },
+      { type: "replace_lines", start_line: `2:${line2Hash}`, end_line: `3:${line3Hash}`, text: "replaced" },
     ]
 
     //#when
@@ -307,9 +319,11 @@ describe("applyHashlineEdits", () => {
   it("correctly handles index shifting with multiple edits", () => {
     //#given
     const content = "a\nb\nc\nd\ne"
+    const line2Hash = computeLineHash(2, "b")
+    const line4Hash = computeLineHash(4, "d")
     const edits: InsertAfter[] = [
-      { type: "insert_after", line: "2:bf", text: "x" },
-      { type: "insert_after", line: "4:90", text: "y" },
+      { type: "insert_after", line: `2:${line2Hash}`, text: "x" },
+      { type: "insert_after", line: `4:${line4Hash}`, text: "y" },
     ]
 
     //#when
diff --git a/src/tools/hashline-edit/hash-computation.test.ts b/src/tools/hashline-edit/hash-computation.test.ts
index 8a0790357..180c92ae5 100644
--- a/src/tools/hashline-edit/hash-computation.test.ts
+++ b/src/tools/hashline-edit/hash-computation.test.ts
@@ -30,6 +30,18 @@ describe("computeLineHash", () => {
     expect(hash1).toBe(hash2)
   })
 
+  it("uses line number in hash input", () => {
+    //#given
+    const content = "const stable = true"
+
+    //#when
+    const hash1 = computeLineHash(1, content)
+    const hash2 = computeLineHash(2, content)
+
+    //#then
+    expect(hash1).not.toBe(hash2)
+  })
+
   it("handles empty lines", () => {
     //#given
     const lineNumber = 1
diff --git a/src/tools/hashline-edit/hash-computation.ts b/src/tools/hashline-edit/hash-computation.ts
index 2ab236136..8144b76c4 100644
--- a/src/tools/hashline-edit/hash-computation.ts
+++ b/src/tools/hashline-edit/hash-computation.ts
@@ -1,8 +1,9 @@
 import { HASH_DICT } from "./constants"
 
-export function computeLineHash(_lineNumber: number, content: string): string {
+export function computeLineHash(lineNumber: number, content: string): string {
   const stripped = content.replace(/\s+/g, "")
-  const hash = Bun.hash.xxHash32(stripped)
+  const hashInput = `${lineNumber}:${stripped}`
+  const hash = Bun.hash.xxHash32(hashInput)
   const index = hash % 256
   return HASH_DICT[index]
 }
diff --git a/src/tools/hashline-edit/tools.ts b/src/tools/hashline-edit/tools.ts
index e2b852f93..61d8b916a 100644
--- a/src/tools/hashline-edit/tools.ts
+++ b/src/tools/hashline-edit/tools.ts
@@ -64,11 +64,13 @@ VALIDATION:
 - Each edit must be one of: set_line, replace_lines, insert_after, replace
 - text/new_text must contain plain replacement text only (no LINE:HASH prefixes, no diff + markers)
 
-LINE:HASH FORMAT:
+LINE:HASH FORMAT (CRITICAL - READ CAREFULLY):
 Each line reference must be in "LINE:HASH" format where:
 - LINE: 1-based line number
-- HASH: First 2 characters of xxHash32 hash of line content (computed with computeLineHash)
-- Example: "5:a3|const x = 1" means line 5 with hash "a3"
+- HASH: First 2 characters of xxHash32 hash of line content (hex characters 0-9, a-f only)
+- Example: "5:a3" means line 5 with hash "a3"
+- WRONG: "2:co" (contains non-hex 'o') - will fail!
+- CORRECT: "2:e8" (hex characters only)
 
 GETTING HASHES:
 Use the read tool - it returns lines in "LINE:HASH|content" format.
diff --git a/src/tools/hashline-edit/validation.test.ts b/src/tools/hashline-edit/validation.test.ts
index b7ac1ab69..e824a4aae 100644
--- a/src/tools/hashline-edit/validation.test.ts
+++ b/src/tools/hashline-edit/validation.test.ts
@@ -1,4 +1,5 @@
 import { describe, it, expect } from "bun:test"
+import { computeLineHash } from "./hash-computation"
 import { parseLineRef, validateLineRef } from "./validation"
 
 describe("parseLineRef", () => {
@@ -61,7 +62,7 @@ describe("validateLineRef", () => {
   it("validates matching hash", () => {
     //#given
     const lines = ["function hello() {", "  return 42", "}"]
-    const ref = "1:42"
+    const ref = `1:${computeLineHash(1, lines[0])}`
 
     //#when & #then
     expect(() => validateLineRef(lines, ref)).not.toThrow()