diff --git a/package-lock.json b/package-lock.json
index bebd58759d..10526dd381 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -100,7 +100,7 @@
 				"style-loader": "4.0.0",
 				"svg-inline-loader": "^0.8.2",
 				"temp": "0.9.4",
-				"terser-webpack-plugin": "5.1.1",
+				"terser-webpack-plugin": "5.3.17",
 				"timers-browserify": "^2.0.12",
 				"ts-loader": "9.5.2",
 				"ts-node": "^10.9.2",
@@ -1347,16 +1347,14 @@
 			}
 		},
 		"node_modules/@jridgewell/gen-mapping": {
-			"version": "0.3.2",
+			"version": "0.3.13",
+			"resolved": "https://registry.npmjs.org/@jridgewell/gen-mapping/-/gen-mapping-0.3.13.tgz",
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 			"dev": true,
 			"license": "MIT",
 			"dependencies": {
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-				"@jridgewell/sourcemap-codec": "^1.4.10",
-				"@jridgewell/trace-mapping": "^0.3.9"
-			},
-			"engines": {
-				"node": ">=6.0.0"
+				"@jridgewell/sourcemap-codec": "^1.5.0",
+				"@jridgewell/trace-mapping": "^0.3.24"
 			}
 		},
 		"node_modules/@jridgewell/resolve-uri": {
@@ -1367,25 +1365,21 @@
 				"node": ">=6.0.0"
 			}
 		},
-		"node_modules/@jridgewell/set-array": {
-			"version": "1.1.2",
-			"dev": true,
-			"license": "MIT",
-			"engines": {
-				"node": ">=6.0.0"
-			}
-		},
 		"node_modules/@jridgewell/source-map": {
-			"version": "0.3.2",
+			"version": "0.3.11",
+			"resolved": "https://registry.npmjs.org/@jridgewell/source-map/-/source-map-0.3.11.tgz",
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 			"dev": true,
 			"license": "MIT",
 			"dependencies": {
-				"@jridgewell/gen-mapping": "^0.3.0",
-				"@jridgewell/trace-mapping": "^0.3.9"
+				"@jridgewell/gen-mapping": "^0.3.5",
+				"@jridgewell/trace-mapping": "^0.3.25"
 			}
 		},
 		"node_modules/@jridgewell/sourcemap-codec": {
-			"version": "1.4.14",
+			"version": "1.5.5",
+			"resolved": "https://registry.npmjs.org/@jridgewell/sourcemap-codec/-/sourcemap-codec-1.5.5.tgz",
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 			"dev": true,
 			"license": "MIT"
 		},
@@ -1406,11 +1400,6 @@
 				"node": ">=6.0.0"
 			}
 		},
-		"node_modules/@jridgewell/trace-mapping/node_modules/@jridgewell/sourcemap-codec": {
-			"version": "1.5.0",
-			"dev": true,
-			"license": "MIT"
-		},
 		"node_modules/@koa/cors": {
 			"version": "5.0.0",
 			"dev": true,
@@ -3090,6 +3079,48 @@
 				"url": "https://github.com/sponsors/epoberezkin"
 			}
 		},
+		"node_modules/ajv-formats": {
+			"version": "2.1.1",
+			"resolved": "https://registry.npmjs.org/ajv-formats/-/ajv-formats-2.1.1.tgz",
+			"integrity": "sha512-Wx0Kx52hxE7C18hkMEggYlEifqWZtYaRgouJor+WMdPnQyEK13vgEWyVNup7SoeeoLMsr4kf5h6dOW11I15MUA==",
+			"dev": true,
+			"license": "MIT",
+			"dependencies": {
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+			},
+			"peerDependencies": {
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+			},
+			"peerDependenciesMeta": {
+				"ajv": {
+					"optional": true
+				}
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+		"node_modules/ajv-formats/node_modules/ajv": {
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+			"resolved": "https://registry.npmjs.org/ajv/-/ajv-8.18.0.tgz",
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+			"dev": true,
+			"license": "MIT",
+			"dependencies": {
+				"fast-deep-equal": "^3.1.3",
+				"fast-uri": "^3.0.1",
+				"json-schema-traverse": "^1.0.0",
+				"require-from-string": "^2.0.2"
+			},
+			"funding": {
+				"type": "github",
+				"url": "https://github.com/sponsors/epoberezkin"
+			}
+		},
+		"node_modules/ajv-formats/node_modules/json-schema-traverse": {
+			"version": "1.0.0",
+			"resolved": "https://registry.npmjs.org/json-schema-traverse/-/json-schema-traverse-1.0.0.tgz",
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+			"dev": true,
+			"license": "MIT"
+		},
 		"node_modules/ajv-keywords": {
 			"version": "3.5.2",
 			"dev": true,
@@ -4871,16 +4902,6 @@
 				"semver": "bin/semver"
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-			"license": "BSD-3-Clause",
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@@ -7467,18 +7488,36 @@
 			}
 		},
 		"node_modules/jest-worker": {
-			"version": "26.6.2",
+			"version": "27.5.1",
+			"resolved": "https://registry.npmjs.org/jest-worker/-/jest-worker-27.5.1.tgz",
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 			"dependencies": {
 				"@types/node": "*",
 				"merge-stream": "^2.0.0",
-				"supports-color": "^7.0.0"
+				"supports-color": "^8.0.0"
 			},
 			"engines": {
 				"node": ">= 10.13.0"
 			}
 		},
+		"node_modules/jest-worker/node_modules/supports-color": {
+			"version": "8.1.1",
+			"resolved": "https://registry.npmjs.org/supports-color/-/supports-color-8.1.1.tgz",
+			"integrity": "sha512-MpUEN2OodtUzxvKQl72cUF7RQ5EiHsGvSsVG0ia9c5RbWGL2CI4C7EpPS8UTBIplnlzZiNuV56w+FuNxy3ty2Q==",
+			"dev": true,
+			"license": "MIT",
+			"dependencies": {
+				"has-flag": "^4.0.0"
+			},
+			"engines": {
+				"node": ">=10"
+			},
+			"funding": {
+				"url": "https://github.com/chalk/supports-color?sponsor=1"
+			}
+		},
 		"node_modules/js-tokens": {
 			"version": "4.0.0",
 			"license": "MIT"
@@ -8465,6 +8504,16 @@
 				"url": "https://github.com/sponsors/isaacs"
 			}
 		},
+		"node_modules/mocha/node_modules/serialize-javascript": {
+			"version": "6.0.1",
+			"resolved": "https://registry.npmjs.org/serialize-javascript/-/serialize-javascript-6.0.1.tgz",
+			"integrity": "sha512-owoXEFjWRllis8/M1Q+Cw5k8ZH40e3zhp/ovX+Xr/vi1qj6QesbyXXViFbpNvWvPNAD62SutwEXavefrLJWj7w==",
+			"dev": true,
+			"license": "BSD-3-Clause",
+			"dependencies": {
+				"randombytes": "^2.1.0"
+			}
+		},
 		"node_modules/mocha/node_modules/supports-color": {
 			"version": "8.1.1",
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@@ -10246,14 +10295,6 @@
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-		"node_modules/serialize-javascript": {
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-			"dev": true,
-			"license": "BSD-3-Clause",
-			"dependencies": {
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 		"node_modules/set-function-length": {
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 			"dev": true,
@@ -10464,6 +10505,16 @@
 				"url": "https://opencollective.com/sinon"
 			}
 		},
+		"node_modules/sinon/node_modules/diff": {
+			"version": "4.0.4",
+			"resolved": "https://registry.npmjs.org/diff/-/diff-4.0.4.tgz",
+			"integrity": "sha512-X07nttJQkwkfKfvTPG/KSnE2OMdcUCao6+eXF3wmnIQRn2aPAHH3VxDbDOdegkd6JbPsXqShpvEOHfAT+nCNwQ==",
+			"dev": true,
+			"license": "BSD-3-Clause",
+			"engines": {
+				"node": ">=0.3.1"
+			}
+		},
 		"node_modules/sisteransi": {
 			"version": "1.0.5",
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@@ -10959,12 +11010,14 @@
 			"license": "ISC"
 		},
 		"node_modules/terser": {
-			"version": "5.14.2",
+			"version": "5.46.1",
+			"resolved": "https://registry.npmjs.org/terser/-/terser-5.46.1.tgz",
+			"integrity": "sha512-vzCjQO/rgUuK9sf8VJZvjqiqiHFaZLnOiimmUuOKODxWL8mm/xua7viT7aqX7dgPY60otQjUotzFMmCB4VdmqQ==",
 			"dev": true,
 			"license": "BSD-2-Clause",
 			"dependencies": {
-				"@jridgewell/source-map": "^0.3.2",
-				"acorn": "^8.5.0",
+				"@jridgewell/source-map": "^0.3.3",
+				"acorn": "^8.15.0",
 				"commander": "^2.20.0",
 				"source-map-support": "~0.5.20"
 			},
@@ -10976,16 +11029,16 @@
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 		},
 		"node_modules/terser-webpack-plugin": {
-			"version": "5.1.1",
+			"version": "5.3.17",
+			"resolved": "https://registry.npmjs.org/terser-webpack-plugin/-/terser-webpack-plugin-5.3.17.tgz",
+			"integrity": "sha512-YR7PtUp6GMU91BgSJmlaX/rS2lGDbAF7D+Wtq7hRO+MiljNmodYvqslzCFiYVAgW+Qoaaia/QUIP4lGXufjdZw==",
 			"dev": true,
 			"license": "MIT",
 			"dependencies": {
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-				"p-limit": "^3.1.0",
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-				"serialize-javascript": "^5.0.1",
-				"source-map": "^0.6.1",
-				"terser": "^5.5.1"
+				"@jridgewell/trace-mapping": "^0.3.25",
+				"jest-worker": "^27.4.5",
+				"schema-utils": "^4.3.0",
+				"terser": "^5.31.1"
 			},
 			"engines": {
 				"node": ">= 10.13.0"
@@ -10996,18 +11049,80 @@
 			},
 			"peerDependencies": {
 				"webpack": "^5.1.0"
+			},
+			"peerDependenciesMeta": {
+				"@swc/core": {
+					"optional": true
+				},
+				"esbuild": {
+					"optional": true
+				},
+				"uglify-js": {
+					"optional": true
+				}
 			}
 		},
-		"node_modules/terser-webpack-plugin/node_modules/serialize-javascript": {
-			"version": "5.0.1",
+		"node_modules/terser-webpack-plugin/node_modules/ajv": {
+			"version": "8.18.0",
+			"resolved": "https://registry.npmjs.org/ajv/-/ajv-8.18.0.tgz",
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 			"dev": true,
-			"license": "BSD-3-Clause",
+			"license": "MIT",
 			"dependencies": {
-				"randombytes": "^2.1.0"
+				"fast-deep-equal": "^3.1.3",
+				"fast-uri": "^3.0.1",
+				"json-schema-traverse": "^1.0.0",
+				"require-from-string": "^2.0.2"
+			},
+			"funding": {
+				"type": "github",
+				"url": "https://github.com/sponsors/epoberezkin"
+			}
+		},
+		"node_modules/terser-webpack-plugin/node_modules/ajv-keywords": {
+			"version": "5.1.0",
+			"resolved": "https://registry.npmjs.org/ajv-keywords/-/ajv-keywords-5.1.0.tgz",
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+			"dev": true,
+			"license": "MIT",
+			"dependencies": {
+				"fast-deep-equal": "^3.1.3"
+			},
+			"peerDependencies": {
+				"ajv": "^8.8.2"
+			}
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+		"node_modules/terser-webpack-plugin/node_modules/json-schema-traverse": {
+			"version": "1.0.0",
+			"resolved": "https://registry.npmjs.org/json-schema-traverse/-/json-schema-traverse-1.0.0.tgz",
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+			"dev": true,
+			"license": "MIT"
+		},
+		"node_modules/terser-webpack-plugin/node_modules/schema-utils": {
+			"version": "4.3.3",
+			"resolved": "https://registry.npmjs.org/schema-utils/-/schema-utils-4.3.3.tgz",
+			"integrity": "sha512-eflK8wEtyOE6+hsaRVPxvUKYCpRgzLqDTb8krvAsRIwOGlHoSgYLgBXoubGgLd2fT41/OUYdb48v4k4WWHQurA==",
+			"dev": true,
+			"license": "MIT",
+			"dependencies": {
+				"@types/json-schema": "^7.0.9",
+				"ajv": "^8.9.0",
+				"ajv-formats": "^2.1.1",
+				"ajv-keywords": "^5.1.0"
+			},
+			"engines": {
+				"node": ">= 10.13.0"
+			},
+			"funding": {
+				"type": "opencollective",
+				"url": "https://opencollective.com/webpack"
 			}
 		},
 		"node_modules/terser/node_modules/source-map-support": {
 			"version": "0.5.21",
+			"resolved": "https://registry.npmjs.org/source-map-support/-/source-map-support-0.5.21.tgz",
+			"integrity": "sha512-uBHU3L3czsIyYXKX88fdrGovxdSCoTGDRZ6SYXtSRxLZUzHg5P/66Ht6uoUlHu9EZod+inXhKo3qQgwXUT/y1w==",
 			"dev": true,
 			"license": "MIT",
 			"dependencies": {
@@ -11325,6 +11440,16 @@
 				}
 			}
 		},
+		"node_modules/ts-node/node_modules/diff": {
+			"version": "4.0.4",
+			"resolved": "https://registry.npmjs.org/diff/-/diff-4.0.4.tgz",
+			"integrity": "sha512-X07nttJQkwkfKfvTPG/KSnE2OMdcUCao6+eXF3wmnIQRn2aPAHH3VxDbDOdegkd6JbPsXqShpvEOHfAT+nCNwQ==",
+			"dev": true,
+			"license": "BSD-3-Clause",
+			"engines": {
+				"node": ">=0.3.1"
+			}
+		},
 		"node_modules/tsconfig-paths": {
 			"version": "3.15.0",
 			"dev": true,
@@ -11808,11 +11933,6 @@
 				"node": ">=6.0.0"
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 		},
-		"node_modules/v8-to-istanbul/node_modules/@jridgewell/sourcemap-codec": {
-			"version": "1.5.0",
-			"dev": true,
-			"license": "MIT"
-		},
 		"node_modules/v8-to-istanbul/node_modules/@jridgewell/trace-mapping": {
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@@ -12125,36 +12245,6 @@
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 			}
 		},
-		"node_modules/webpack/node_modules/@jridgewell/gen-mapping": {
-			"version": "0.3.5",
-			"dev": true,
-			"license": "MIT",
-			"dependencies": {
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-		"node_modules/webpack/node_modules/@jridgewell/set-array": {
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-			"dev": true,
-			"license": "MIT",
-			"engines": {
-				"node": ">=6.0.0"
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-		"node_modules/webpack/node_modules/@jridgewell/source-map": {
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-			"dev": true,
-			"license": "MIT",
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@@ -12172,22 +12262,6 @@
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 			}
 		},
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-			"dev": true,
-			"license": "MIT",
-			"dependencies": {
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 		"node_modules/webpack/node_modules/ajv-keywords": {
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@@ -12207,19 +12281,6 @@
 				"node": ">=0.8.x"
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 		},
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-			"version": "27.4.6",
-			"dev": true,
-			"license": "MIT",
-			"dependencies": {
-				"@types/node": "*",
-				"merge-stream": "^2.0.0",
-				"supports-color": "^8.0.0"
-			},
-			"engines": {
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 		"node_modules/webpack/node_modules/json-schema-traverse": {
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@@ -12243,46 +12304,6 @@
 				"url": "https://opencollective.com/webpack"
 			}
 		},
-		"node_modules/webpack/node_modules/source-map-support": {
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-			"dev": true,
-			"license": "MIT",
-			"dependencies": {
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-				"source-map": "^0.6.0"
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-		"node_modules/webpack/node_modules/supports-color": {
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-			"dev": true,
-			"license": "MIT",
-			"dependencies": {
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-			},
-			"funding": {
-				"url": "https://github.com/chalk/supports-color?sponsor=1"
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-		"node_modules/webpack/node_modules/terser": {
-			"version": "5.46.0",
-			"dev": true,
-			"license": "BSD-2-Clause",
-			"dependencies": {
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-				"acorn": "^8.15.0",
-				"commander": "^2.20.0",
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-			},
-			"bin": {
-				"terser": "bin/terser"
-			},
-			"engines": {
-				"node": ">=10"
-			}
-		},
 		"node_modules/webpack/node_modules/terser-webpack-plugin": {
 			"version": "5.4.0",
 			"resolved": "https://registry.npmjs.org/terser-webpack-plugin/-/terser-webpack-plugin-5.4.0.tgz",
diff --git a/package.json b/package.json
index 5c9d8cfd78..e72d6bcfbd 100644
--- a/package.json
+++ b/package.json
@@ -10,6 +10,9 @@
 	"bugs": {
 		"url": "https://github.com/Microsoft/vscode-pull-request-github/issues"
 	},
+	"engines": {
+		"node": ">=20"
+	},
 	"enabledApiProposals": [
 		"activeComment",
 		"chatContextProvider",
@@ -4127,7 +4130,7 @@
 		"style-loader": "4.0.0",
 		"svg-inline-loader": "^0.8.2",
 		"temp": "0.9.4",
-		"terser-webpack-plugin": "5.1.1",
+		"terser-webpack-plugin": "5.3.17",
 		"timers-browserify": "^2.0.12",
 		"ts-loader": "9.5.2",
 		"ts-node": "^10.9.2",
@@ -4170,6 +4173,13 @@
 		"vscode-tas-client": "^0.1.84",
 		"vsls": "^0.3.967"
 	},
+	"overrides": {
+		"mocha": {
+			"diff": "7.0.0",
+			"serialize-javascript": "6.0.1"
+		},
+		"elliptic": "6.6.1"
+	},
 	"resolutions": {
 		"string_decoder": "^1.3.0"
 	},
diff --git a/src/@types/vscode.proposed.chatSessionsProvider.d.ts b/src/@types/vscode.proposed.chatSessionsProvider.d.ts
index 69679ab35c..3992526ad4 100644
--- a/src/@types/vscode.proposed.chatSessionsProvider.d.ts
+++ b/src/@types/vscode.proposed.chatSessionsProvider.d.ts
@@ -100,7 +100,7 @@ declare module 'vscode' {
 			readonly command?: string;
 		};
 
-		readonly sessionOptions: ReadonlyArray<{ optionId: string; value: string | ChatSessionProviderOptionItem }>;
+		readonly sessionOptions: ReadonlyArray<{ optionId: string; value: ChatSessionProviderOptionItem }>;
 	}
 
 	/**
@@ -108,6 +108,20 @@ declare module 'vscode' {
 	 */
 	export type ChatSessionItemControllerNewItemHandler = (context: ChatSessionItemControllerNewItemHandlerContext, token: CancellationToken) => Thenable<ChatSessionItem>;
 
+	/**
+	 * Extension callback invoked to fork an existing chat session item managed by a {@linkcode ChatSessionItemController}.
+	 *
+	 * The handler should create a new session on the provider's backend and
+	 * return the new {@link ChatSessionItem} representing the forked session.
+	 *
+	 * @param sessionResource The resource of the chat session being forked.
+	 * @param request The request turn that marks the fork point. The forked session includes all turns
+	 * upto this request turn and includes this request turn itself. If undefined, fork the full session.
+	 * @param token A cancellation token.
+	 * @returns The forked session item.
+	 */
+	export type ChatSessionItemControllerForkHandler = (sessionResource: Uri, request: ChatRequestTurn2 | undefined, token: CancellationToken) => Thenable<ChatSessionItem> | ChatSessionItem;
+
 	/**
 	 * Manages chat sessions for a specific chat session type
 	 */
@@ -145,6 +159,14 @@ declare module 'vscode' {
 		 */
 		newChatSessionItemHandler?: ChatSessionItemControllerNewItemHandler;
 
+		/**
+		 * Invoked when an existing chat session is forked.
+		 *
+		 * When both this handler and {@linkcode ChatSession.forkHandler} are registered,
+		 * this handler takes precedence.
+		 */
+		forkHandler?: ChatSessionItemControllerForkHandler;
+
 		/**
 		 * Fired when an item's archived state changes.
 		 */
@@ -374,12 +396,9 @@ declare module 'vscode' {
 		/**
 		 * Options configured for this session as key-value pairs.
 		 * Keys correspond to option group IDs (e.g., 'models', 'subagents').
-		 * Values can be either:
-		 * - A string (the option item ID) for backwards compatibility
-		 * - A ChatSessionProviderOptionItem object to include metadata like locked state
 		 * TODO: Strongly type the keys
 		 */
-		readonly options?: Record<string, string | ChatSessionProviderOptionItem>;
+		readonly options?: Record<string, ChatSessionProviderOptionItem>;
 
 		/**
 		 * Callback invoked by the editor for a currently running response. This allows the session to push items for the
@@ -399,6 +418,22 @@ declare module 'vscode' {
 		// TODO: Revisit this to align with code.
 		// TODO: pass in options?
 		readonly requestHandler: ChatRequestHandler | undefined;
+
+		/**
+		 * Handles a request to fork the session.
+		 *
+		 * The handler should create a new session on the provider's backend and
+		 * return the new {@link ChatSessionItem} representing the forked session.
+		 *
+		 * @deprecated Use {@linkcode ChatSessionItemController.forkHandler} instead. This remains supported for backwards compatibility.
+		 *
+		 * @param sessionResource The resource of the chat session being forked.
+		 * @param request The request turn that marks the fork point. The forked session includes all turns
+		 * until this request turn and includes this request turn itself. If undefined, fork the full session.
+		 * @param token A cancellation token.
+		 * @returns The forked session item.
+		 */
+		readonly forkHandler?: ChatSessionItemControllerForkHandler;
 	}
 
 	/**
@@ -421,7 +456,7 @@ declare module 'vscode' {
 			/**
 			 * The new value assigned to the option. When `undefined`, the option is cleared.
 			 */
-			readonly value: string | ChatSessionProviderOptionItem;
+			readonly value: ChatSessionProviderOptionItem;
 		}>;
 	}
 
@@ -454,7 +489,7 @@ declare module 'vscode' {
 		 * @return The {@link ChatSession chat session} associated with the given URI.
 		 */
 		provideChatSessionContent(resource: Uri, token: CancellationToken, context: {
-			readonly sessionOptions: ReadonlyArray<{ optionId: string; value: string | ChatSessionProviderOptionItem }>;
+			readonly sessionOptions: ReadonlyArray<{ optionId: string; value: ChatSessionProviderOptionItem }>;
 		}): Thenable<ChatSession> | ChatSession;
 
 		/**
@@ -479,7 +514,7 @@ declare module 'vscode' {
 		/**
 		 * The new value assigned to the option. When `undefined`, the option is cleared.
 		 */
-		readonly value: string | undefined;
+		readonly value: ChatSessionProviderOptionItem | undefined;
 	}
 
 	export namespace chat {
@@ -488,10 +523,11 @@ declare module 'vscode' {
 		 *
 		 * @param scheme The uri-scheme to register for. This must be unique.
 		 * @param provider The provider to register.
+		 * @param defaultChatParticipant The default {@link ChatParticipant chat participant} used in sessions provided by this provider.
 		 *
 		 * @returns A disposable that unregisters the provider when disposed.
 		 */
-		export function registerChatSessionContentProvider(scheme: string, provider: ChatSessionContentProvider, chatParticipant: ChatParticipant, capabilities?: ChatSessionCapabilities): Disposable;
+		export function registerChatSessionContentProvider(scheme: string, provider: ChatSessionContentProvider, defaultChatParticipant: ChatParticipant, capabilities?: ChatSessionCapabilities): Disposable;
 	}
 
 	export interface ChatContext {
@@ -508,7 +544,7 @@ declare module 'vscode' {
 		 * The initial option selections for the session, provided with the first request.
 		 * Contains the options the user selected (or defaults) before the session was created.
 		 */
-		readonly initialSessionOptions?: ReadonlyArray<{ optionId: string; value: string | ChatSessionProviderOptionItem }>;
+		readonly initialSessionOptions?: ReadonlyArray<{ optionId: string; value: ChatSessionProviderOptionItem }>;
 	}
 
 	export interface ChatSessionCapabilities {
@@ -631,6 +667,6 @@ declare module 'vscode' {
 		 *
 		 * Keys correspond to option group IDs (e.g., 'models', 'subagents').
 		 */
-		readonly newSessionOptions?: Record<string, string | ChatSessionProviderOptionItem>;
+		readonly newSessionOptions?: Record<string, ChatSessionProviderOptionItem>;
 	}
 }