diff --git a/assets/oh-my-opencode.schema.json b/assets/oh-my-opencode.schema.json
index c443ff7e2..052956403 100644
--- a/assets/oh-my-opencode.schema.json
+++ b/assets/oh-my-opencode.schema.json
@@ -2830,6 +2830,9 @@
         },
         "safe_hook_creation": {
           "type": "boolean"
+        },
+        "hashline_edit": {
+          "type": "boolean"
         }
       },
       "additionalProperties": false
diff --git a/src/config/schema.test.ts b/src/config/schema.test.ts
index 2d151ec53..448219d61 100644
--- a/src/config/schema.test.ts
+++ b/src/config/schema.test.ts
@@ -663,6 +663,59 @@ describe("ExperimentalConfigSchema feature flags", () => {
       expect(result.data.safe_hook_creation).toBeUndefined()
     }
   })
+
+  test("accepts hashline_edit as true", () => {
+    //#given
+    const config = { hashline_edit: true }
+
+    //#when
+    const result = ExperimentalConfigSchema.safeParse(config)
+
+    //#then
+    expect(result.success).toBe(true)
+    if (result.success) {
+      expect(result.data.hashline_edit).toBe(true)
+    }
+  })
+
+  test("accepts hashline_edit as false", () => {
+    //#given
+    const config = { hashline_edit: false }
+
+    //#when
+    const result = ExperimentalConfigSchema.safeParse(config)
+
+    //#then
+    expect(result.success).toBe(true)
+    if (result.success) {
+      expect(result.data.hashline_edit).toBe(false)
+    }
+  })
+
+  test("hashline_edit is optional", () => {
+    //#given
+    const config = { safe_hook_creation: true }
+
+    //#when
+    const result = ExperimentalConfigSchema.safeParse(config)
+
+    //#then
+    expect(result.success).toBe(true)
+    if (result.success) {
+      expect(result.data.hashline_edit).toBeUndefined()
+    }
+  })
+
+  test("rejects non-boolean hashline_edit", () => {
+    //#given
+    const config = { hashline_edit: "true" }
+
+    //#when
+    const result = ExperimentalConfigSchema.safeParse(config)
+
+    //#then
+    expect(result.success).toBe(false)
+  })
 })
 
 describe("GitMasterConfigSchema", () => {
diff --git a/src/config/schema/experimental.ts b/src/config/schema/experimental.ts
index 52747aae9..927a13a28 100644
--- a/src/config/schema/experimental.ts
+++ b/src/config/schema/experimental.ts
@@ -15,6 +15,8 @@ export const ExperimentalConfigSchema = z.object({
   plugin_load_timeout_ms: z.number().min(1000).optional(),
   /** Wrap hook creation in try/catch to prevent one failing hook from crashing the plugin (default: true at call site) */
   safe_hook_creation: z.boolean().optional(),
+  /** Enable hashline_edit tool for improved file editing with hash-based line anchors */
+  hashline_edit: z.boolean().optional(),
 })
 
 export type ExperimentalConfig = z.infer<typeof ExperimentalConfigSchema>
diff --git a/src/features/hashline-provider-state.test.ts b/src/features/hashline-provider-state.test.ts
new file mode 100644
index 000000000..d8eb272ff
--- /dev/null
+++ b/src/features/hashline-provider-state.test.ts
@@ -0,0 +1,113 @@
+import { describe, expect, test, beforeEach } from "bun:test"
+import { setProvider, getProvider, clearProvider } from "./hashline-provider-state"
+
+describe("hashline-provider-state", () => {
+  beforeEach(() => {
+    // Clear state before each test
+    clearProvider("test-session-1")
+    clearProvider("test-session-2")
+  })
+
+  describe("setProvider", () => {
+    test("should store provider ID for a session", () => {
+      // given
+      const sessionID = "test-session-1"
+      const providerID = "openai"
+
+      // when
+      setProvider(sessionID, providerID)
+
+      // then
+      expect(getProvider(sessionID)).toBe("openai")
+    })
+
+    test("should overwrite existing provider for same session", () => {
+      // given
+      const sessionID = "test-session-1"
+      setProvider(sessionID, "openai")
+
+      // when
+      setProvider(sessionID, "anthropic")
+
+      // then
+      expect(getProvider(sessionID)).toBe("anthropic")
+    })
+  })
+
+  describe("getProvider", () => {
+    test("should return undefined for non-existent session", () => {
+      // given
+      const sessionID = "non-existent-session"
+
+      // when
+      const result = getProvider(sessionID)
+
+      // then
+      expect(result).toBeUndefined()
+    })
+
+    test("should return stored provider ID", () => {
+      // given
+      const sessionID = "test-session-1"
+      setProvider(sessionID, "anthropic")
+
+      // when
+      const result = getProvider(sessionID)
+
+      // then
+      expect(result).toBe("anthropic")
+    })
+
+    test("should handle multiple sessions independently", () => {
+      // given
+      setProvider("session-1", "openai")
+      setProvider("session-2", "anthropic")
+
+      // when
+      const result1 = getProvider("session-1")
+      const result2 = getProvider("session-2")
+
+      // then
+      expect(result1).toBe("openai")
+      expect(result2).toBe("anthropic")
+    })
+  })
+
+  describe("clearProvider", () => {
+    test("should remove provider for a session", () => {
+      // given
+      const sessionID = "test-session-1"
+      setProvider(sessionID, "openai")
+
+      // when
+      clearProvider(sessionID)
+
+      // then
+      expect(getProvider(sessionID)).toBeUndefined()
+    })
+
+    test("should not affect other sessions", () => {
+      // given
+      setProvider("session-1", "openai")
+      setProvider("session-2", "anthropic")
+
+      // when
+      clearProvider("session-1")
+
+      // then
+      expect(getProvider("session-1")).toBeUndefined()
+      expect(getProvider("session-2")).toBe("anthropic")
+    })
+
+    test("should handle clearing non-existent session gracefully", () => {
+      // given
+      const sessionID = "non-existent"
+
+      // when
+      clearProvider(sessionID)
+
+      // then
+      expect(getProvider(sessionID)).toBeUndefined()
+    })
+  })
+})
diff --git a/src/features/hashline-provider-state.ts b/src/features/hashline-provider-state.ts
new file mode 100644
index 000000000..5c04a730f
--- /dev/null
+++ b/src/features/hashline-provider-state.ts
@@ -0,0 +1,13 @@
+const providerStateMap = new Map<string, string>()
+
+export function setProvider(sessionID: string, providerID: string): void {
+  providerStateMap.set(sessionID, providerID)
+}
+
+export function getProvider(sessionID: string): string | undefined {
+  return providerStateMap.get(sessionID)
+}
+
+export function clearProvider(sessionID: string): void {
+  providerStateMap.delete(sessionID)
+}
diff --git a/src/tools/hashline-edit/constants.ts b/src/tools/hashline-edit/constants.ts
new file mode 100644
index 000000000..17638a49a
--- /dev/null
+++ b/src/tools/hashline-edit/constants.ts
@@ -0,0 +1,30 @@
+export const HASH_DICT = [
+  "00", "01", "02", "03", "04", "05", "06", "07", "08", "09",
+  "0a", "0b", "0c", "0d", "0e", "0f", "10", "11", "12", "13",
+  "14", "15", "16", "17", "18", "19", "1a", "1b", "1c", "1d",
+  "1e", "1f", "20", "21", "22", "23", "24", "25", "26", "27",
+  "28", "29", "2a", "2b", "2c", "2d", "2e", "2f", "30", "31",
+  "32", "33", "34", "35", "36", "37", "38", "39", "3a", "3b",
+  "3c", "3d", "3e", "3f", "40", "41", "42", "43", "44", "45",
+  "46", "47", "48", "49", "4a", "4b", "4c", "4d", "4e", "4f",
+  "50", "51", "52", "53", "54", "55", "56", "57", "58", "59",
+  "5a", "5b", "5c", "5d", "5e", "5f", "60", "61", "62", "63",
+  "64", "65", "66", "67", "68", "69", "6a", "6b", "6c", "6d",
+  "6e", "6f", "70", "71", "72", "73", "74", "75", "76", "77",
+  "78", "79", "7a", "7b", "7c", "7d", "7e", "7f", "80", "81",
+  "82", "83", "84", "85", "86", "87", "88", "89", "8a", "8b",
+  "8c", "8d", "8e", "8f", "90", "91", "92", "93", "94", "95",
+  "96", "97", "98", "99", "9a", "9b", "9c", "9d", "9e", "9f",
+  "a0", "a1", "a2", "a3", "a4", "a5", "a6", "a7", "a8", "a9",
+  "aa", "ab", "ac", "ad", "ae", "af", "b0", "b1", "b2", "b3",
+  "b4", "b5", "b6", "b7", "b8", "b9", "ba", "bb", "bc", "bd",
+  "be", "bf", "c0", "c1", "c2", "c3", "c4", "c5", "c6", "c7",
+  "c8", "c9", "ca", "cb", "cc", "cd", "ce", "cf", "d0", "d1",
+  "d2", "d3", "d4", "d5", "d6", "d7", "d8", "d9", "da", "db",
+  "dc", "dd", "de", "df", "e0", "e1", "e2", "e3", "e4", "e5",
+  "e6", "e7", "e8", "e9", "ea", "eb", "ec", "ed", "ee", "ef",
+  "f0", "f1", "f2", "f3", "f4", "f5", "f6", "f7", "f8", "f9",
+  "fa", "fb", "fc", "fd", "fe", "ff",
+] as const
+
+export const HASHLINE_PATTERN = /^(\d+):([0-9a-f]{2})\|(.*)$/
diff --git a/src/tools/hashline-edit/hash-computation.test.ts b/src/tools/hashline-edit/hash-computation.test.ts
new file mode 100644
index 000000000..8a0790357
--- /dev/null
+++ b/src/tools/hashline-edit/hash-computation.test.ts
@@ -0,0 +1,123 @@
+import { describe, it, expect } from "bun:test"
+import { computeLineHash, formatHashLine, formatHashLines } from "./hash-computation"
+
+describe("computeLineHash", () => {
+  it("returns consistent 2-char hex for same input", () => {
+    //#given
+    const lineNumber = 1
+    const content = "function hello() {"
+
+    //#when
+    const hash1 = computeLineHash(lineNumber, content)
+    const hash2 = computeLineHash(lineNumber, content)
+
+    //#then
+    expect(hash1).toBe(hash2)
+    expect(hash1).toMatch(/^[0-9a-f]{2}$/)
+  })
+
+  it("strips whitespace before hashing", () => {
+    //#given
+    const lineNumber = 1
+    const content1 = "function hello() {"
+    const content2 = "  function hello() {  "
+
+    //#when
+    const hash1 = computeLineHash(lineNumber, content1)
+    const hash2 = computeLineHash(lineNumber, content2)
+
+    //#then
+    expect(hash1).toBe(hash2)
+  })
+
+  it("handles empty lines", () => {
+    //#given
+    const lineNumber = 1
+    const content = ""
+
+    //#when
+    const hash = computeLineHash(lineNumber, content)
+
+    //#then
+    expect(hash).toMatch(/^[0-9a-f]{2}$/)
+  })
+
+  it("returns different hashes for different content", () => {
+    //#given
+    const lineNumber = 1
+    const content1 = "function hello() {"
+    const content2 = "function world() {"
+
+    //#when
+    const hash1 = computeLineHash(lineNumber, content1)
+    const hash2 = computeLineHash(lineNumber, content2)
+
+    //#then
+    expect(hash1).not.toBe(hash2)
+  })
+})
+
+describe("formatHashLine", () => {
+  it("formats line with hash prefix", () => {
+    //#given
+    const lineNumber = 42
+    const content = "function hello() {"
+
+    //#when
+    const result = formatHashLine(lineNumber, content)
+
+    //#then
+    expect(result).toMatch(/^42:[0-9a-f]{2}\|function hello\(\) \{$/)
+  })
+
+  it("preserves content after hash prefix", () => {
+    //#given
+    const lineNumber = 1
+    const content = "const x = 42"
+
+    //#when
+    const result = formatHashLine(lineNumber, content)
+
+    //#then
+    expect(result).toContain("|const x = 42")
+  })
+})
+
+describe("formatHashLines", () => {
+  it("formats all lines with hash prefixes", () => {
+    //#given
+    const content = "function hello() {\n  return 42\n}"
+
+    //#when
+    const result = formatHashLines(content)
+
+    //#then
+    const lines = result.split("\n")
+    expect(lines).toHaveLength(3)
+    expect(lines[0]).toMatch(/^1:[0-9a-f]{2}\|/)
+    expect(lines[1]).toMatch(/^2:[0-9a-f]{2}\|/)
+    expect(lines[2]).toMatch(/^3:[0-9a-f]{2}\|/)
+  })
+
+  it("handles empty file", () => {
+    //#given
+    const content = ""
+
+    //#when
+    const result = formatHashLines(content)
+
+    //#then
+    expect(result).toBe("")
+  })
+
+  it("handles single line", () => {
+    //#given
+    const content = "const x = 42"
+
+    //#when
+    const result = formatHashLines(content)
+
+    //#then
+    expect(result).toMatch(/^1:[0-9a-f]{2}\|const x = 42$/)
+  })
+})
diff --git a/src/tools/hashline-edit/hash-computation.ts b/src/tools/hashline-edit/hash-computation.ts
new file mode 100644
index 000000000..348cf3ea2
--- /dev/null
+++ b/src/tools/hashline-edit/hash-computation.ts
@@ -0,0 +1,19 @@
+import { HASH_DICT } from "./constants"
+
+export function computeLineHash(lineNumber: number, content: string): string {
+  const stripped = content.replace(/\s+/g, "")
+  const hash = Bun.hash.xxHash32(stripped)
+  const index = hash % 256
+  return HASH_DICT[index]
+}
+
+export function formatHashLine(lineNumber: number, content: string): string {
+  const hash = computeLineHash(lineNumber, content)
+  return `${lineNumber}:${hash}|${content}`
+}
+
+export function formatHashLines(content: string): string {
+  if (!content) return ""
+  const lines = content.split("\n")
+  return lines.map((line, index) => formatHashLine(index + 1, line)).join("\n")
+}
diff --git a/src/tools/hashline-edit/index.ts b/src/tools/hashline-edit/index.ts
new file mode 100644
index 000000000..6c12810f2
--- /dev/null
+++ b/src/tools/hashline-edit/index.ts
@@ -0,0 +1,5 @@
+export { computeLineHash, formatHashLine, formatHashLines } from "./hash-computation"
+export { parseLineRef, validateLineRef } from "./validation"
+export type { LineRef } from "./validation"
+export type { SetLine, ReplaceLines, InsertAfter, Replace, HashlineEdit } from "./types"
+export { HASH_DICT, HASHLINE_PATTERN } from "./constants"
diff --git a/src/tools/hashline-edit/types.ts b/src/tools/hashline-edit/types.ts
new file mode 100644
index 000000000..3de342b1a
--- /dev/null
+++ b/src/tools/hashline-edit/types.ts
@@ -0,0 +1,26 @@
+export interface SetLine {
+  type: "set_line"
+  line: string
+  text: string
+}
+
+export interface ReplaceLines {
+  type: "replace_lines"
+  start_line: string
+  end_line: string
+  text: string
+}
+
+export interface InsertAfter {
+  type: "insert_after"
+  line: string
+  text: string
+}
+
+export interface Replace {
+  type: "replace"
+  old_text: string
+  new_text: string
+}
+
+export type HashlineEdit = SetLine | ReplaceLines | InsertAfter | Replace
diff --git a/src/tools/hashline-edit/validation.test.ts b/src/tools/hashline-edit/validation.test.ts
new file mode 100644
index 000000000..b7ac1ab69
--- /dev/null
+++ b/src/tools/hashline-edit/validation.test.ts
@@ -0,0 +1,105 @@
+import { describe, it, expect } from "bun:test"
+import { parseLineRef, validateLineRef } from "./validation"
+
+describe("parseLineRef", () => {
+  it("parses valid line reference", () => {
+    //#given
+    const ref = "42:a3"
+
+    //#when
+    const result = parseLineRef(ref)
+
+    //#then
+    expect(result).toEqual({ line: 42, hash: "a3" })
+  })
+
+  it("parses line reference with different hash", () => {
+    //#given
+    const ref = "1:ff"
+
+    //#when
+    const result = parseLineRef(ref)
+
+    //#then
+    expect(result).toEqual({ line: 1, hash: "ff" })
+  })
+
+  it("throws on invalid format - no colon", () => {
+    //#given
+    const ref = "42a3"
+
+    //#when & #then
+    expect(() => parseLineRef(ref)).toThrow()
+  })
+
+  it("throws on invalid format - non-numeric line", () => {
+    //#given
+    const ref = "abc:a3"
+
+    //#when & #then
+    expect(() => parseLineRef(ref)).toThrow()
+  })
+
+  it("throws on invalid format - invalid hash", () => {
+    //#given
+    const ref = "42:xyz"
+
+    //#when & #then
+    expect(() => parseLineRef(ref)).toThrow()
+  })
+
+  it("throws on empty string", () => {
+    //#given
+    const ref = ""
+
+    //#when & #then
+    expect(() => parseLineRef(ref)).toThrow()
+  })
+})
+
+describe("validateLineRef", () => {
+  it("validates matching hash", () => {
+    //#given
+    const lines = ["function hello() {", "  return 42", "}"]
+    const ref = "1:42"
+
+    //#when & #then
+    expect(() => validateLineRef(lines, ref)).not.toThrow()
+  })
+
+  it("throws on hash mismatch", () => {
+    //#given
+    const lines = ["function hello() {", "  return 42", "}"]
+    const ref = "1:00" // Wrong hash
+
+    //#when & #then
+    expect(() => validateLineRef(lines, ref)).toThrow()
+  })
+
+  it("throws on line out of bounds", () => {
+    //#given
+    const lines = ["function hello() {", "  return 42", "}"]
+    const ref = "99:a3"
+
+    //#when & #then
+    expect(() => validateLineRef(lines, ref)).toThrow()
+  })
+
+  it("throws on invalid line number", () => {
+    //#given
+    const lines = ["function hello() {"]
+    const ref = "0:a3" // Line numbers start at 1
+
+    //#when & #then
+    expect(() => validateLineRef(lines, ref)).toThrow()
+  })
+
+  it("error message includes current hash", () => {
+    //#given
+    const lines = ["function hello() {"]
+    const ref = "1:00"
+
+    //#when & #then
+    expect(() => validateLineRef(lines, ref)).toThrow(/current hash/)
+  })
+})
diff --git a/src/tools/hashline-edit/validation.ts b/src/tools/hashline-edit/validation.ts
new file mode 100644
index 000000000..178f35d8e
--- /dev/null
+++ b/src/tools/hashline-edit/validation.ts
@@ -0,0 +1,39 @@
+import { computeLineHash } from "./hash-computation"
+
+export interface LineRef {
+  line: number
+  hash: string
+}
+
+export function parseLineRef(ref: string): LineRef {
+  const match = ref.match(/^(\d+):([0-9a-f]{2})$/)
+  if (!match) {
+    throw new Error(
+      `Invalid line reference format: "${ref}". Expected format: "LINE:HASH" (e.g., "42:a3")`
+    )
+  }
+  return {
+    line: Number.parseInt(match[1], 10),
+    hash: match[2],
+  }
+}
+
+export function validateLineRef(lines: string[], ref: string): void {
+  const { line, hash } = parseLineRef(ref)
+
+  if (line < 1 || line > lines.length) {
+    throw new Error(
+      `Line number ${line} out of bounds. File has ${lines.length} lines.`
+    )
+  }
+
+  const content = lines[line - 1]
+  const currentHash = computeLineHash(line, content)
+
+  if (currentHash !== hash) {
+    throw new Error(
+      `Hash mismatch at line ${line}. Expected hash: ${hash}, current hash: ${currentHash}. ` +
+        `Line content may have changed. Current content: "${content}"`
+    )
+  }
+}