diff --git a/src/api/providers/gemini.ts b/src/api/providers/gemini.ts
index a49073ea334..662345e0ed3 100644
--- a/src/api/providers/gemini.ts
+++ b/src/api/providers/gemini.ts
@@ -22,6 +22,7 @@ import { TelemetryService } from "@roo-code/telemetry"
 import type { ApiHandlerOptions } from "../../shared/api"
 
 import { convertAnthropicMessageToGemini } from "../transform/gemini-format"
+import { sanitizeSchemaForGemini } from "../transform/gemini-schema"
 import { t } from "i18next"
 import type { ApiStream, GroundingSource } from "../transform/stream"
 import { getModelParams } from "../transform/model-params"
@@ -137,7 +138,9 @@ export class GeminiHandler extends BaseProvider implements SingleCompletionHandl
 				functionDeclarations: (metadata?.tools ?? []).map((tool) => ({
 					name: (tool as any).function.name,
 					description: (tool as any).function.description,
-					parametersJsonSchema: (tool as any).function.parameters,
+					parametersJsonSchema: sanitizeSchemaForGemini(
+						(tool as any).function.parameters as Record<string, unknown>,
+					),
 				})),
 			},
 		]
diff --git a/src/api/transform/__tests__/gemini-schema.spec.ts b/src/api/transform/__tests__/gemini-schema.spec.ts
new file mode 100644
index 00000000000..c66f4278617
--- /dev/null
+++ b/src/api/transform/__tests__/gemini-schema.spec.ts
@@ -0,0 +1,307 @@
+// npx vitest run src/api/transform/__tests__/gemini-schema.spec.ts
+
+import { sanitizeSchemaForGemini } from "../gemini-schema"
+
+describe("sanitizeSchemaForGemini", () => {
+	it("converts type array with null to single type + nullable", () => {
+		const schema = {
+			type: "object",
+			properties: {
+				cwd: {
+					type: ["string", "null"],
+					description: "Working directory",
+				},
+			},
+			required: ["cwd"],
+		}
+
+		const result = sanitizeSchemaForGemini(schema)
+
+		expect(result).toEqual({
+			type: "object",
+			properties: {
+				cwd: {
+					type: "string",
+					nullable: true,
+					description: "Working directory",
+				},
+			},
+			required: ["cwd"],
+		})
+	})
+
+	it("converts type array with number and null", () => {
+		const schema = {
+			type: "object",
+			properties: {
+				timeout: {
+					type: ["number", "null"],
+					description: "Timeout in seconds",
+				},
+			},
+		}
+
+		const result = sanitizeSchemaForGemini(schema)
+
+		expect(result.properties).toEqual({
+			timeout: {
+				type: "number",
+				nullable: true,
+				description: "Timeout in seconds",
+			},
+		})
+	})
+
+	it("removes additionalProperties", () => {
+		const schema = {
+			type: "object",
+			properties: {
+				command: { type: "string" },
+			},
+			required: ["command"],
+			additionalProperties: false,
+		}
+
+		const result = sanitizeSchemaForGemini(schema)
+
+		expect(result).toEqual({
+			type: "object",
+			properties: {
+				command: { type: "string" },
+			},
+			required: ["command"],
+		})
+		expect(result).not.toHaveProperty("additionalProperties")
+	})
+
+	it("removes nested additionalProperties", () => {
+		const schema = {
+			type: "object",
+			properties: {
+				indentation: {
+					type: "object",
+					properties: {
+						anchor_line: { type: "integer" },
+					},
+					required: [],
+					additionalProperties: false,
+				},
+			},
+			additionalProperties: false,
+		}
+
+		const result = sanitizeSchemaForGemini(schema)
+
+		expect(result).toEqual({
+			type: "object",
+			properties: {
+				indentation: {
+					type: "object",
+					properties: {
+						anchor_line: { type: "integer" },
+					},
+					required: [],
+				},
+			},
+		})
+	})
+
+	it("leaves already-valid schemas unchanged (except additionalProperties)", () => {
+		const schema = {
+			type: "object",
+			properties: {
+				path: { type: "string", description: "File path" },
+				mode: { type: "string", enum: ["slice", "indentation"] },
+			},
+			required: ["path"],
+		}
+
+		const result = sanitizeSchemaForGemini(schema)
+
+		expect(result).toEqual(schema)
+	})
+
+	it("handles the full execute_command schema", () => {
+		const schema = {
+			type: "object",
+			properties: {
+				command: {
+					type: "string",
+					description: "Shell command",
+				},
+				cwd: {
+					type: ["string", "null"],
+					description: "Working directory",
+				},
+				timeout: {
+					type: ["number", "null"],
+					description: "Timeout",
+				},
+			},
+			required: ["command", "cwd", "timeout"],
+			additionalProperties: false,
+		}
+
+		const result = sanitizeSchemaForGemini(schema)
+
+		expect(result).toEqual({
+			type: "object",
+			properties: {
+				command: {
+					type: "string",
+					description: "Shell command",
+				},
+				cwd: {
+					type: "string",
+					nullable: true,
+					description: "Working directory",
+				},
+				timeout: {
+					type: "number",
+					nullable: true,
+					description: "Timeout",
+				},
+			},
+			required: ["command", "cwd", "timeout"],
+		})
+	})
+
+	it("handles type array without null", () => {
+		const schema = {
+			type: "object",
+			properties: {
+				value: {
+					type: ["string", "number"],
+					description: "Mixed type",
+				},
+			},
+		}
+
+		const result = sanitizeSchemaForGemini(schema)
+
+		// Should pick the first non-null type
+		expect((result.properties as Record<string, Record<string, unknown>>).value.type).toBe("string")
+		expect((result.properties as Record<string, Record<string, unknown>>).value).not.toHaveProperty("nullable")
+	})
+
+	it("handles empty or null input", () => {
+		expect(sanitizeSchemaForGemini(null as unknown as Record<string, unknown>)).toBeNull()
+		expect(sanitizeSchemaForGemini(undefined as unknown as Record<string, unknown>)).toBeUndefined()
+	})
+
+	it("handles single string type (no conversion needed)", () => {
+		const schema = {
+			type: "object",
+			properties: {
+				name: { type: "string" },
+			},
+		}
+
+		const result = sanitizeSchemaForGemini(schema)
+
+		expect(result).toEqual(schema)
+	})
+
+	it("sanitizes items in array type schemas", () => {
+		const schema = {
+			type: "object",
+			properties: {
+				items: {
+					type: "array",
+					items: {
+						type: "object",
+						properties: {
+							text: { type: ["string", "null"] },
+						},
+						additionalProperties: false,
+					},
+				},
+			},
+		}
+
+		const result = sanitizeSchemaForGemini(schema)
+
+		expect(result).toEqual({
+			type: "object",
+			properties: {
+				items: {
+					type: "array",
+					items: {
+						type: "object",
+						properties: {
+							text: { type: "string", nullable: true },
+						},
+					},
+				},
+			},
+		})
+	})
+
+	it("sanitizes anyOf/oneOf/allOf entries", () => {
+		const schema = {
+			type: "object",
+			properties: {
+				value: {
+					anyOf: [{ type: "string", additionalProperties: false }, { type: ["number", "null"] }],
+				},
+			},
+		}
+
+		const result = sanitizeSchemaForGemini(schema)
+
+		expect((result.properties as Record<string, Record<string, unknown>>).value).toEqual({
+			anyOf: [{ type: "string" }, { type: "number", nullable: true }],
+		})
+	})
+
+	it("handles the ask_followup_question schema with nested objects", () => {
+		const schema = {
+			type: "object",
+			properties: {
+				question: { type: "string" },
+				follow_up: {
+					type: "array",
+					items: {
+						type: "object",
+						properties: {
+							text: { type: "string" },
+							mode: {
+								type: ["string", "null"],
+								description: "Optional mode",
+							},
+						},
+						required: ["text", "mode"],
+						additionalProperties: false,
+					},
+				},
+			},
+			required: ["question", "follow_up"],
+			additionalProperties: false,
+		}
+
+		const result = sanitizeSchemaForGemini(schema)
+
+		expect(result).toEqual({
+			type: "object",
+			properties: {
+				question: { type: "string" },
+				follow_up: {
+					type: "array",
+					items: {
+						type: "object",
+						properties: {
+							text: { type: "string" },
+							mode: {
+								type: "string",
+								nullable: true,
+								description: "Optional mode",
+							},
+						},
+						required: ["text", "mode"],
+					},
+				},
+			},
+			required: ["question", "follow_up"],
+		})
+	})
+})
diff --git a/src/api/transform/gemini-schema.ts b/src/api/transform/gemini-schema.ts
new file mode 100644
index 00000000000..9c0bc377db4
--- /dev/null
+++ b/src/api/transform/gemini-schema.ts
@@ -0,0 +1,88 @@
+/**
+ * Sanitizes JSON Schema objects for compatibility with the Gemini API's
+ * `parametersJsonSchema` field.
+ *
+ * The Gemini API does not support certain standard JSON Schema features:
+ * - `type` as an array (e.g. `["string", "null"]`) for nullable types
+ * - `additionalProperties` in function parameter schemas
+ *
+ * This function recursively converts these constructs into Gemini-compatible
+ * equivalents:
+ * - `type: ["string", "null"]` becomes `type: "string", nullable: true`
+ * - `additionalProperties` is removed
+ *
+ * @see https://github.com/RooCodeInc/Roo-Code/issues/12202
+ */
+export function sanitizeSchemaForGemini(schema: Record<string, unknown>): Record<string, unknown> {
+	if (!schema || typeof schema !== "object") {
+		return schema
+	}
+
+	const result: Record<string, unknown> = {}
+
+	for (const [key, value] of Object.entries(schema)) {
+		// Remove additionalProperties — Gemini does not support it in
+		// function declaration schemas.
+		if (key === "additionalProperties") {
+			continue
+		}
+
+		// Convert array-type `type` fields to single type + nullable.
+		// e.g. `type: ["string", "null"]` -> `type: "string", nullable: true`
+		if (key === "type" && Array.isArray(value)) {
+			const types = value.filter((t) => t !== "null")
+			const hasNull = value.includes("null")
+
+			if (types.length === 1) {
+				result.type = types[0]
+			} else if (types.length > 1) {
+				// Multiple non-null types — keep the first as a best-effort
+				// fallback. This shouldn't happen in our tool schemas but
+				// handles the edge case defensively.
+				result.type = types[0]
+			} else {
+				// All entries were "null" — unusual, default to string.
+				result.type = "string"
+			}
+
+			if (hasNull) {
+				result.nullable = true
+			}
+			continue
+		}
+
+		// Recursively sanitize nested `properties` objects.
+		if (key === "properties" && typeof value === "object" && value !== null) {
+			const sanitizedProps: Record<string, unknown> = {}
+			for (const [propName, propSchema] of Object.entries(value as Record<string, unknown>)) {
+				if (typeof propSchema === "object" && propSchema !== null) {
+					sanitizedProps[propName] = sanitizeSchemaForGemini(propSchema as Record<string, unknown>)
+				} else {
+					sanitizedProps[propName] = propSchema
+				}
+			}
+			result[key] = sanitizedProps
+			continue
+		}
+
+		// Recursively sanitize `items` for array types.
+		if (key === "items" && typeof value === "object" && value !== null) {
+			result[key] = sanitizeSchemaForGemini(value as Record<string, unknown>)
+			continue
+		}
+
+		// Recursively sanitize `anyOf`, `oneOf`, `allOf` entries.
+		if ((key === "anyOf" || key === "oneOf" || key === "allOf") && Array.isArray(value)) {
+			result[key] = value.map((item) =>
+				typeof item === "object" && item !== null
+					? sanitizeSchemaForGemini(item as Record<string, unknown>)
+					: item,
+			)
+			continue
+		}
+
+		result[key] = value
+	}
+
+	return result
+}