diff --git a/docs/diff-statistics-extension.md b/docs/diff-statistics-extension.md
new file mode 100644
index 00000000..a493f2b9
--- /dev/null
+++ b/docs/diff-statistics-extension.md
@@ -0,0 +1,90 @@
+# AIR diff statistics extension
+
+Status: Experimental
+
+Agents can attach added and removed line counts to an ACP `diff` content block.
+Clients use these values without comparing the block's texts again.
+The extension applies to any ACP agent, including Codex.
+
+## Wire format
+
+The payload belongs to the individual diff block at `_meta.jetbrains.air.diffStats`.
+
+```json
+{
+  "type": "diff",
+  "path": "/project/file.txt",
+  "oldText": "old\n",
+  "newText": "new\nextra\n",
+  "_meta": {
+    "kind": "update",
+    "jetbrains": {
+      "air": {
+        "version": 1,
+        "diffStats": {
+          "version": 1,
+          "added": 2,
+          "removed": 1
+        }
+      }
+    }
+  }
+}
+```
+
+`jetbrains.air.version` identifies the AIR envelope. Clients accept integer versions of at least 1.
+`diffStats.version` identifies this payload. This specification defines version 1 only.
+Agents preserve other metadata, including `kind`.
+
+| Field | Type | Meaning |
+| --- | --- | --- |
+| `version` | integer | Must equal `1`. |
+| `added` | integer | Number of added lines, between 0 and 2147483647. |
+| `removed` | integer | Number of removed lines, between 0 and 2147483647. |
+
+All three fields are required. Numeric strings are invalid.
+Statistics contain no navigation coordinates. Clients must not compare texts to obtain coordinates when they receive valid counts.
+
+## Count semantics
+
+For updates, counts describe the addition and deletion operations in the supplied patch.
+Context lines and `No newline at end of file` markers do not contribute to counts.
+A replacement contributes both added and removed lines.
+A patch can contain operations that leave the normalized file content unchanged.
+Clients preserve the patch counts instead of recomputing a minimal diff.
+Relocating an exact hunk does not change its counts.
+
+For creation and deletion, count the supplied file content.
+Treat CRLF and CR as line boundaries and do not count an extra line after the final terminator.
+An empty string has zero lines. One line terminator represents one empty line.
+Creation has zero removed lines; deletion has zero added lines.
+
+Each diff block owns its statistics.
+A text revision carries statistics for that revision, or omits the payload.
+Clients invalidate old statistics when the texts change.
+Status-only updates preserve previous statistics.
+Late statistics may replace calculated values for unchanged texts.
+
+## Availability and compatibility
+
+This is optional display metadata. No capability negotiation is required.
+Clients that do not understand it can ignore it and render the standard diff content.
+Agents still send the usual `path`, `oldText`, and `newText` values.
+
+An agent omits statistics when it cannot produce valid counts.
+Clients use their normal comparison when metadata is missing, malformed, or unsupported.
+Unknown fields do not invalidate a valid payload.
+
+The earlier experimental `com.intellij/diffStats` key is not part of this contract.
+AIR ignores that key and uses its normal fallback.
+Existing persisted statistics, including stored navigation lines, remain readable without migration.
+
+## Codex behavior
+
+The existing patch application validates the file change and produces the texts for ACP.
+The statistics calculator then reads only the parsed patch. It receives no file texts.
+It validates hunk sizes and coordinates and counts `+` and `-` operations.
+It does not verify file contents again or locate a navigation line.
+
+Tests: `src/__tests__/DiffStats.test.ts` and
+`src/__tests__/CodexACPAgent/file-change-events.test.ts`.
diff --git a/readme-dev.md b/readme-dev.md
index bc147807..135fb112 100644
--- a/readme-dev.md
+++ b/readme-dev.md
@@ -84,3 +84,8 @@ npm run package:all
 1. Update the `@openai/codex` version in `package.json` (under `dependencies`).
 2. Regenerate Codex types in `src/app-server/`: `npm run generate-types`
 3. Ensure there are no type errors or failed tests: `npm run typecheck` and `npm run test`
+
+### AIR diff statistics
+
+See the [diff statistics specification](docs/diff-statistics-extension.md) for the
+`_meta.jetbrains.air.diffStats` payload and its compatibility rules.
diff --git a/src/AirExtension.ts b/src/AirExtension.ts
index 5ffe2ee2..28af8784 100644
--- a/src/AirExtension.ts
+++ b/src/AirExtension.ts
@@ -12,6 +12,7 @@ export const JETBRAINS_META_KEY = "jetbrains";
 export const AIR_META_KEY = "air";
 export const AIR_EXTENSION_VERSION_KEY = "version";
 export const AIR_EXTENSION_CAPABILITIES_KEY = "capabilities";
+export const AIR_DIFF_STATS_KEY = "diffStats";
 export const AIR_SESSION_FAILURE_KEY = "sessionFailure";
 export const AIR_AGENT_FILE_CHANGE_REPORT_KEY = "agentFileChangeReport";
 export const AIR_NATIVE_SUBAGENT_SESSIONS_KEY = "nativeSubagentSessions";
diff --git a/src/CodexToolCallMapper.ts b/src/CodexToolCallMapper.ts
index d1b4cbb9..d21bbaf4 100644
--- a/src/CodexToolCallMapper.ts
+++ b/src/CodexToolCallMapper.ts
@@ -1,5 +1,7 @@
 import type { ContentBlock, ToolCallContent } from "@agentclientprotocol/sdk";
-import { applyPatch, parsePatch, reversePatch } from "diff";
+import { applyPatch, parsePatch, reversePatch, type StructuredPatch } from "diff";
+import { DiffStatsCalculator } from "./DiffStats";
+import { AIR_DIFF_STATS_KEY, withAirMeta } from "./AirExtension";
 import { readFile } from "node:fs/promises";
 import path from "node:path";
 import type { UpdateSessionEvent } from "./ACPSessionConnection";
@@ -47,6 +49,7 @@ type ContextCompactionItem = ThreadItem & { type: "contextCompaction" };
 type AcpToolCallEvent = Extract<UpdateSessionEvent, { sessionUpdate: "tool_call" }>;
 
 const CONTEXT_COMPACTION_META = createContextCompactionMeta();
+const DIFF_STATS = new DiffStatsCalculator();
 
 function toAcpStatus(status: CodexItemStatus): AcpToolCallStatus {
     switch (status) {
@@ -838,9 +841,7 @@ async function createAddFileContent(change: FileUpdateChange): Promise<ToolCallC
         oldText: null,
         newText: change.diff, // app-server always returns file content instead of diff
         path: change.path,
-        _meta: {
-            kind: "add",
-        },
+        _meta: withAirMeta({ kind: "add" }, AIR_DIFF_STATS_KEY, DIFF_STATS.addedFile(change.diff)),
     };
 }
 
@@ -848,55 +849,44 @@ async function createUpdateFileContent(change: FileUpdateChange): Promise<ToolCa
     if (change.kind.type !== "update") return null;
 
     const unifiedDiff = recoverCorruptedDiff(change.diff);
+    const patches = parsePatch(unifiedDiff);
+    if (patches.length !== 1) return null;
+    const patch = patches[0]!;
     const movePath = change.kind.move_path;
 
     const oldContent = await readFileContent(change.path);
     if (oldContent !== null) {
-        const patchedContent = applyPatch(oldContent, unifiedDiff);
+        const patchedContent = applyPatch(oldContent, patch);
         if (patchedContent === false) {
             // If Codex runs in full access mode, the file might already be patched.
             // we can verify this by checking if the reverted patch applies.
-            const revertedPatch = revertPatch(unifiedDiff);
-            if (revertedPatch) {
-                const revertedContent = applyPatch(oldContent, revertedPatch);
-                if (revertedContent !== false) {
-                    return createUpdateDiffContent(change.path, revertedContent, oldContent);
-                }
+            const revertedContent = applyPatch(oldContent, reversePatch(patch));
+            if (revertedContent !== false) {
+                return createUpdateDiffContent(change.path, revertedContent, oldContent, patch);
             }
             return null;
         }
-        return createUpdateDiffContent(movePath ?? change.path, oldContent, patchedContent);
+        return createUpdateDiffContent(movePath ?? change.path, oldContent, patchedContent, patch);
     }
 
     if (!movePath) return null;
     const newContent = await readFileContent(movePath);
     if (newContent === null) return null;
 
-    const revertedPatch = revertPatch(unifiedDiff);
-    if (!revertedPatch) return null;
-
-    const revertedContent = applyPatch(newContent, revertedPatch);
+    const revertedContent = applyPatch(newContent, reversePatch(patch));
     if (revertedContent === false) return null;
 
-    return createUpdateDiffContent(movePath, revertedContent, newContent);
-}
-
-function revertPatch(unifiedDiff: string) {
-    const [patch] = parsePatch(unifiedDiff);
-    if (!patch) return null;
-
-    return reversePatch(patch);
+    return createUpdateDiffContent(movePath, revertedContent, newContent, patch);
 }
 
-function createUpdateDiffContent(path: string, oldText: string, newText: string): ToolCallContent {
+function createUpdateDiffContent(path: string, oldText: string, newText: string, patch: StructuredPatch): ToolCallContent {
+    const stats = DIFF_STATS.update(patch);
     return {
         type: "diff",
         oldText,
         newText,
         path,
-        _meta: {
-            kind: "update",
-        },
+        _meta: stats ? withAirMeta({ kind: "update" }, AIR_DIFF_STATS_KEY, stats) : { kind: "update" },
     };
 }
 
@@ -906,9 +896,7 @@ async function createDeleteFileContent(change: FileUpdateChange): Promise<ToolCa
         oldText: change.diff, // app-server always returns file content instead of diff
         newText: "",
         path: change.path,
-        _meta: {
-            kind: "delete",
-        }
+        _meta: withAirMeta({ kind: "delete" }, AIR_DIFF_STATS_KEY, DIFF_STATS.deletedFile(change.diff))
     }
 }
 
diff --git a/src/DiffStats.ts b/src/DiffStats.ts
new file mode 100644
index 00000000..267e70ab
--- /dev/null
+++ b/src/DiffStats.ts
@@ -0,0 +1,74 @@
+import type { StructuredPatch } from "diff";
+
+export type DiffStats = {
+    version: 1;
+    added: number;
+    removed: number;
+};
+
+export class DiffStatsCalculator {
+    addedFile(text: string): DiffStats {
+        return { version: 1, added: this.lineCount(text), removed: 0 };
+    }
+
+    deletedFile(text: string): DiffStats {
+        return { version: 1, added: 0, removed: this.lineCount(text) };
+    }
+
+    update(patch: StructuredPatch): DiffStats | null {
+        if (patch.isBinary || patch.hunks.length === 0) return null;
+        let added = 0;
+        let removed = 0;
+        let previousOldEnd = 1;
+        let previousNewEnd = 1;
+        for (const hunk of patch.hunks) {
+            const { oldStart, oldLines, newStart, newLines } = hunk;
+            if (![oldStart, oldLines, newStart, newLines].every(Number.isSafeInteger) ||
+                oldStart < previousOldEnd || newStart < previousNewEnd || oldLines < 0 || newLines < 0 ||
+                newStart - oldStart !== added - removed) return null;
+            let oldConsumed = 0;
+            let newConsumed = 0;
+            let previousWasContent = false;
+            for (const line of hunk.lines) {
+                switch (line[0]) {
+                    case '+':
+                        added++;
+                        newConsumed++;
+                        previousWasContent = true;
+                        break;
+                    case '-':
+                        removed++;
+                        oldConsumed++;
+                        previousWasContent = true;
+                        break;
+                    case ' ':
+                    case undefined:
+                        oldConsumed++;
+                        newConsumed++;
+                        previousWasContent = true;
+                        break;
+                    case '\\':
+                        if (!previousWasContent || line.replace(/\r$/, '') !== '\\ No newline at end of file') return null;
+                        previousWasContent = false;
+                        break;
+                    default:
+                        return null;
+                }
+            }
+            if (oldConsumed !== oldLines || newConsumed !== newLines) return null;
+            previousOldEnd = oldStart + oldLines;
+            previousNewEnd = newStart + newLines;
+        }
+        return { version: 1, added, removed };
+    }
+
+    private lineCount(text: string): number {
+        let count = 0;
+        for (let offset = text.indexOf('\n'); offset >= 0; offset = text.indexOf('\n', offset + 1)) count++;
+        for (let offset = text.indexOf('\r'); offset >= 0; offset = text.indexOf('\r', offset + 1)) {
+            if (text[offset + 1] !== '\n') count++;
+        }
+        if (text.length > 0 && !text.endsWith('\n') && !text.endsWith('\r')) count++;
+        return count;
+    }
+}
diff --git a/src/__tests__/CodexACPAgent/data/file-change-add-multiple-files.json b/src/__tests__/CodexACPAgent/data/file-change-add-multiple-files.json
index 41b58b87..26848a01 100644
--- a/src/__tests__/CodexACPAgent/data/file-change-add-multiple-files.json
+++ b/src/__tests__/CodexACPAgent/data/file-change-add-multiple-files.json
@@ -16,7 +16,17 @@
             "newText": "class FileA\n",
             "path": "/test/project/FileA.kt",
             "_meta": {
-              "kind": "add"
+              "kind": "add",
+              "jetbrains": {
+                "air": {
+                  "version": 1,
+                  "diffStats": {
+                    "version": 1,
+                    "added": 1,
+                    "removed": 0
+                  }
+                }
+              }
             }
           },
           {
@@ -25,7 +35,17 @@
             "newText": "class FileB\n",
             "path": "/test/project/FileB.kt",
             "_meta": {
-              "kind": "add"
+              "kind": "add",
+              "jetbrains": {
+                "air": {
+                  "version": 1,
+                  "diffStats": {
+                    "version": 1,
+                    "added": 1,
+                    "removed": 0
+                  }
+                }
+              }
             }
           }
         ]
diff --git a/src/__tests__/CodexACPAgent/data/file-change-add-new-file.json b/src/__tests__/CodexACPAgent/data/file-change-add-new-file.json
index 4bf59ca9..8ede27a2 100644
--- a/src/__tests__/CodexACPAgent/data/file-change-add-new-file.json
+++ b/src/__tests__/CodexACPAgent/data/file-change-add-new-file.json
@@ -16,7 +16,17 @@
             "newText": "package test.project\n\nclass NewFile {\n    fun hello() = \"Hello\"\n}\n",
             "path": "/test/project/NewFile.kt",
             "_meta": {
-              "kind": "add"
+              "kind": "add",
+              "jetbrains": {
+                "air": {
+                  "version": 1,
+                  "diffStats": {
+                    "version": 1,
+                    "added": 5,
+                    "removed": 0
+                  }
+                }
+              }
             }
           }
         ]
diff --git a/src/__tests__/CodexACPAgent/data/file-change-add-raw-content.json b/src/__tests__/CodexACPAgent/data/file-change-add-raw-content.json
index 31f18f5a..2e009c80 100644
--- a/src/__tests__/CodexACPAgent/data/file-change-add-raw-content.json
+++ b/src/__tests__/CodexACPAgent/data/file-change-add-raw-content.json
@@ -16,7 +16,17 @@
             "newText": "fun main() {\n    println(\"Hello, World!\")\n}\n",
             "path": "/test/project/RawFile.kt",
             "_meta": {
-              "kind": "add"
+              "kind": "add",
+              "jetbrains": {
+                "air": {
+                  "version": 1,
+                  "diffStats": {
+                    "version": 1,
+                    "added": 3,
+                    "removed": 0
+                  }
+                }
+              }
             }
           }
         ]
diff --git a/src/__tests__/CodexACPAgent/data/file-change-delete-file.json b/src/__tests__/CodexACPAgent/data/file-change-delete-file.json
index ea6dd487..63b0cb18 100644
--- a/src/__tests__/CodexACPAgent/data/file-change-delete-file.json
+++ b/src/__tests__/CodexACPAgent/data/file-change-delete-file.json
@@ -16,7 +16,17 @@
             "newText": "",
             "path": "/test/project/OldFile.kt",
             "_meta": {
-              "kind": "delete"
+              "kind": "delete",
+              "jetbrains": {
+                "air": {
+                  "version": 1,
+                  "diffStats": {
+                    "version": 1,
+                    "added": 0,
+                    "removed": 3
+                  }
+                }
+              }
             }
           }
         ]
diff --git a/src/__tests__/CodexACPAgent/data/file-change-delete-raw-content.json b/src/__tests__/CodexACPAgent/data/file-change-delete-raw-content.json
index 60f83da7..ba03757b 100644
--- a/src/__tests__/CodexACPAgent/data/file-change-delete-raw-content.json
+++ b/src/__tests__/CodexACPAgent/data/file-change-delete-raw-content.json
@@ -16,7 +16,17 @@
             "newText": "",
             "path": "/test/project/RawDeleteFile.kt",
             "_meta": {
-              "kind": "delete"
+              "kind": "delete",
+              "jetbrains": {
+                "air": {
+                  "version": 1,
+                  "diffStats": {
+                    "version": 1,
+                    "added": 0,
+                    "removed": 3
+                  }
+                }
+              }
             }
           }
         ]
diff --git a/src/__tests__/CodexACPAgent/data/load-session-history.json b/src/__tests__/CodexACPAgent/data/load-session-history.json
index 69644617..fbdbe046 100644
--- a/src/__tests__/CodexACPAgent/data/load-session-history.json
+++ b/src/__tests__/CodexACPAgent/data/load-session-history.json
@@ -279,7 +279,17 @@
             "newText": "Hello\nWorld\n",
             "path": "/test/project/Added.txt",
             "_meta": {
-              "kind": "add"
+              "kind": "add",
+              "jetbrains": {
+                "air": {
+                  "version": 1,
+                  "diffStats": {
+                    "version": 1,
+                    "added": 2,
+                    "removed": 0
+                  }
+                }
+              }
             }
           }
         ]
diff --git a/src/__tests__/CodexACPAgent/file-change-events.test.ts b/src/__tests__/CodexACPAgent/file-change-events.test.ts
index 795d71e3..426fefd0 100644
--- a/src/__tests__/CodexACPAgent/file-change-events.test.ts
+++ b/src/__tests__/CodexACPAgent/file-change-events.test.ts
@@ -268,6 +268,31 @@ describe('CodexEventHandler - file change events', () => {
         );
     });
 
+    it.each([
+        { name: 'before application', disk: 'old\n', expected: { version: 1, added: 2, removed: 1 } },
+        { name: 'after application', disk: 'new\nextra\n', expected: { version: 1, added: 2, removed: 1 } },
+        { name: 'a relocated hunk', disk: 'prefix\nold\n', expected: { version: 1, added: 2, removed: 1 } },
+    ])('publishes reliable update statistics $name', async ({ disk, expected }) => {
+        mockFileContent('/test/project/OldFile.kt', disk);
+        const event = await createFileChangeUpdate({
+            type: 'fileChange',
+            id: 'stats-update',
+            changes: [{
+                path: '/test/project/OldFile.kt',
+                kind: { type: 'update', move_path: null },
+                diff: '@@ -1 +1,2 @@\n-old\n+new\n+extra\n',
+            }],
+            status: 'completed',
+        });
+        expect(event.sessionUpdate).toBe('tool_call');
+        if (event.sessionUpdate !== 'tool_call') throw new Error('Expected a file-change tool call');
+        expect(event.content).toHaveLength(1);
+        expect(event.content![0]!._meta).toEqual({
+            kind: 'update',
+            ...(expected ? { jetbrains: { air: { version: 1, diffStats: expected } } } : {}),
+        });
+    });
+
     it('should ignore broken unified diffs in update file changes', async () => {
         const fileChange: ThreadItem & { type: 'fileChange' } = {
             type: 'fileChange',
diff --git a/src/__tests__/DiffStats.test.ts b/src/__tests__/DiffStats.test.ts
new file mode 100644
index 00000000..c553c95e
--- /dev/null
+++ b/src/__tests__/DiffStats.test.ts
@@ -0,0 +1,114 @@
+import { parsePatch } from 'diff';
+import { describe, expect, it } from 'vitest';
+import { DiffStatsCalculator } from '../DiffStats';
+
+describe('ACP diff statistics', () => {
+    const calculator = new DiffStatsCalculator();
+
+    it.each([
+        ['', 0],
+        ['line', 1],
+        ['line\n', 1],
+        ['\n', 1],
+        ['\n\n', 2],
+        ['first\n\nlast\n', 3],
+        ['first\r\n\r\nlast\r\n', 3],
+        ['first\r\rlast\r', 3],
+        ['first\r\nsecond\rthird\n', 3],
+    ])('counts added and deleted lines in %j', (text, count) => {
+        expect(calculator.addedFile(text)).toEqual({
+            version: 1, added: count, removed: 0,
+        });
+        expect(calculator.deletedFile(text)).toEqual({
+            version: 1, added: 0, removed: count,
+        });
+    });
+
+    it.each([
+        {
+            name: 'replacement with blank context',
+            patch: '@@ -1,3 +1,4 @@\n first\n \n-old\n+new\n+extra\n',
+            added: 2, removed: 1,
+        },
+        {
+            name: 'deletion at EOF',
+            patch: '@@ -2 +1,0 @@\n-last\n',
+            added: 0, removed: 1,
+        },
+        {
+            name: 'deletion of the entire file',
+            patch: '@@ -1,2 +0,0 @@\n-first\n-last\n',
+            added: 0, removed: 2,
+        },
+        {
+            name: 'insertion after EOF',
+            patch: '@@ -1,0 +2,2 @@\n+second\n+third\n',
+            added: 2, removed: 0,
+        },
+        {
+            name: 'multiple hunks with shifted coordinates',
+            patch: '@@ -2 +2,2 @@\n-two\n+TWO\n+inserted\n@@ -5 +6 @@\n-five\n+FIVE\n',
+            added: 3, removed: 2,
+        },
+        {
+            name: 'missing EOF newline markers do not count as lines',
+            patch: '@@ -1 +1 @@\n-old\n\\ No newline at end of file\n+new\n\\ No newline at end of file\n',
+            added: 1, removed: 1,
+        },
+        {
+            name: 'counts the patch operations for an EOF newline change',
+            patch: '@@ -1 +1 @@\n-same\n\\ No newline at end of file\n+same\n',
+            added: 1, removed: 1,
+        },
+        {
+            name: 'a blank line differs from an empty file',
+            patch: '@@ -0,0 +1 @@\n+\n',
+            added: 1, removed: 0,
+        },
+        {
+            name: 'CRLF patch lines',
+            patch: '@@ -1,2 +1,2 @@\r\n first\r\n-old\r\n+new\r\n',
+            added: 1, removed: 1,
+        },
+        {
+            name: 'the patch counts are retained even when a minimal diff is smaller',
+            patch: '@@ -1,2 +1,2 @@\n-same\n-old\n+same\n+new\n',
+            added: 2, removed: 2,
+        },
+    ])('$name', ({ patch, added, removed }) => {
+        expect(calculator.update(parsePatch(patch)[0]!)).toEqual({ version: 1, added, removed });
+    });
+
+    it.each([
+        { oldStart: -1 },
+        { newStart: NaN },
+        { newStart: 1.5 },
+        { oldLines: 2 },
+        { newLines: 0 },
+        { lines: ['-old', '+new', '?garbage'] },
+        { lines: ['\\ No newline at end of file', '-old', '+new'] },
+        { lines: ['-old', '\\ invalid marker', '+new'] },
+    ])('omits malformed hunk statistics: %j', (change) => {
+        const patch = parsePatch('@@ -1 +1 @@\n-old\n+new\n')[0]!;
+        patch.hunks[0] = { ...patch.hunks[0]!, ...change };
+        expect(calculator.update(patch)).toBeNull();
+    });
+
+    it('counts a patch at large coordinates without requiring file texts', () => {
+        const patch = parsePatch('@@ -1000000 +1000000 @@\n-old\n+new\n')[0]!;
+        expect(calculator.update(patch)).toEqual({ version: 1, added: 1, removed: 1 });
+    });
+
+    it('omits overlapping hunks', () => {
+        const patch = parsePatch('@@ -1 +1 @@\n-old\n+new\n')[0]!;
+        patch.hunks.push({ ...patch.hunks[0]! });
+        expect(calculator.update(patch)).toBeNull();
+    });
+
+    it('omits binary patches and missing hunks', () => {
+        const patch = parsePatch('@@ -1 +1 @@\n-old\n+new\n')[0]!;
+        expect(calculator.update({ ...patch, hunks: [] })).toBeNull();
+        patch.isBinary = true;
+        expect(calculator.update(patch)).toBeNull();
+    });
+});