diff --git a/src/models/hollowshaft/index.ts b/src/models/hollowshaft/index.ts
new file mode 100644
index 0000000..06861aa
--- /dev/null
+++ b/src/models/hollowshaft/index.ts
@@ -0,0 +1 @@
+export * from "./schema"
diff --git a/src/models/hollowshaft/parse-model-string.ts b/src/models/hollowshaft/parse-model-string.ts
new file mode 100644
index 0000000..9dcb32e
--- /dev/null
+++ b/src/models/hollowshaft/parse-model-string.ts
@@ -0,0 +1,42 @@
+import type { RawModelprinterParams } from "../../parse-model-string"
+import { splitModelStringTokens } from "../../split-model-string-tokens"
+import { hollowShaftModelDefinitionSchema } from "./schema"
+const aliases: Record<string, string> = {
+  od: "outerDiameter",
+  outerdiameter: "outerDiameter",
+  id: "innerDiameter",
+  innerdiameter: "innerDiameter",
+  l: "length",
+  length: "length",
+  endchamfer: "endChamfer",
+}
+const flags: Record<string, string> = {
+  roundtube: "roundTube",
+}
+export function parseHollowShaftModelParams(raw: RawModelprinterParams) {
+  const [name, ...tokens] = splitModelStringTokens(raw.string)
+  if (raw.fn !== "hollowshaft" || name?.toLowerCase() !== "hollowshaft")
+    throw new Error("Expected hollowshaft without inline values")
+  const props: Record<string, unknown> = { fn: "hollowshaft" }
+  const assign = (property: string, value: unknown) => {
+    if (Object.hasOwn(props, property))
+      throw new Error(`Duplicate hollowshaft property "${property}"`)
+    props[property] = value
+  }
+  for (const token of tokens) {
+    const match = token.match(/^([a-z]+)(.*)$/i)
+    const key = match?.[1]?.toLowerCase()
+    const value = match?.[2]
+    if (!key || value === undefined)
+      throw new Error(`Malformed hollowshaft token "${token}"`)
+    if (Object.hasOwn(flags, key)) {
+      if (value !== "")
+        throw new Error(`hollowshaft flag "${key}" takes no value`)
+      assign(flags[key]!, true)
+    } else if (Object.hasOwn(aliases, key) && value) {
+      const property = aliases[key]!
+      assign(property, property === "pinCount" ? Number(value) : value)
+    } else throw new Error(`Unknown or malformed hollowshaft token "${token}"`)
+  }
+  return hollowShaftModelDefinitionSchema.parse(props)
+}
diff --git a/src/models/hollowshaft/register.ts b/src/models/hollowshaft/register.ts
new file mode 100644
index 0000000..203513b
--- /dev/null
+++ b/src/models/hollowshaft/register.ts
@@ -0,0 +1,11 @@
+import { defineModel, type ModelRegistry } from "../../model-registry"
+import { hollowShaftModelDefinitionSchema } from "./schema"
+import { parseHollowShaftModelParams } from "./parse-model-string"
+export const model = defineModel({
+  name: "hollowshaft",
+  schema: hollowShaftModelDefinitionSchema,
+  parse: parseHollowShaftModelParams,
+})
+export function register(registry: ModelRegistry): void {
+  registry.register(model)
+}
diff --git a/src/models/hollowshaft/schema.ts b/src/models/hollowshaft/schema.ts
new file mode 100644
index 0000000..c9f4501
--- /dev/null
+++ b/src/models/hollowshaft/schema.ts
@@ -0,0 +1,101 @@
+import { z } from "zod"
+import { modelLengthSchema } from "../../model-length-schema"
+
+// Consume the complete decimal length rather than accepting partial numbers.
+const length = z
+  .union([
+    z.number(),
+    z
+      .string()
+      .regex(/^[+]?(?:\d+(?:\.\d*)?|\.\d+)(?:mm|cm|m|in|inch|mil|ft|feet)?$/i)
+      .transform((value) => value.toLowerCase()),
+  ])
+  .pipe(modelLengthSchema)
+const positiveLength = length.refine(
+  (value) => value > 0,
+  "Length must be positive",
+)
+const nonnegativeLength = length.refine(
+  (value) => value >= 0,
+  "Length cannot be negative",
+)
+const shape = {
+  outerDiameter: positiveLength,
+  innerDiameter: positiveLength,
+  length: positiveLength,
+  endChamfer: nonnegativeLength.default(0),
+  roundTube: z.literal(true).default(true),
+}
+function validate(
+  p: z.output<z.ZodObject<typeof shape>>,
+  context: z.RefinementCtx,
+) {
+  const span = Math.max(p.outerDiameter, p.length)
+  if (!Number.isFinite(span ** 3))
+    context.addIssue({
+      code: "custom",
+      message: "Dimensions must have a finite bounding volume",
+    })
+  if (p.innerDiameter >= p.outerDiameter)
+    context.addIssue({
+      code: "custom",
+      path: ["innerDiameter"],
+      message: "Shaft bore must be smaller than the outer diameter",
+    })
+  if (2 * p.endChamfer >= (p.outerDiameter - p.innerDiameter) / 2)
+    context.addIssue({
+      code: "custom",
+      path: ["endChamfer"],
+      message:
+        "Inner and outer end chamfers must leave a positive annular end land",
+    })
+  if (2 * p.endChamfer >= p.length)
+    context.addIssue({
+      code: "custom",
+      path: ["endChamfer"],
+      message: "End chamfers must leave a positive straight shaft length",
+    })
+}
+export const hollowShaftModelPropsSchema = z
+  .object(shape)
+  .strict()
+  .superRefine(validate)
+export const hollowShaftModelDefinitionSchema = z
+  .object({ fn: z.literal("hollowshaft"), ...shape })
+  .strict()
+  .superRefine(validate)
+export type HollowShaftModelPropsInput = z.input<
+  typeof hollowShaftModelPropsSchema
+>
+export type HollowShaftModelProps = z.output<typeof hollowShaftModelPropsSchema>
+export type HollowShaftModelDefinition = z.output<
+  typeof hollowShaftModelDefinitionSchema
+>
+
+/** A concentric round tubular drive shaft with a through bore and 45-degree chamfers on both the inner and outer edges of both ends. The centered shaft axis is +Z and bottom face Z=0. endChamfer is the radial and axial chamfer size; validation preserves a positive annular end land. _roundtube is the value-free flag for the round tubular geometry. All normalized dimensions are millimeters. */
+export function getHollowShaftDimensions(input: HollowShaftModelPropsInput) {
+  const p = hollowShaftModelPropsSchema.parse(input)
+  return {
+    ...p,
+    size: [p.outerDiameter, p.outerDiameter, p.length] as [
+      number,
+      number,
+      number,
+    ],
+    min: [-p.outerDiameter / 2, -p.outerDiameter / 2, 0] as [
+      number,
+      number,
+      number,
+    ],
+    max: [p.outerDiameter / 2, p.outerDiameter / 2, p.length] as [
+      number,
+      number,
+      number,
+    ],
+    bottomZ: 0,
+    topZ: p.length,
+    wallThickness: (p.outerDiameter - p.innerDiameter) / 2,
+    endOuterDiameter: p.outerDiameter - 2 * p.endChamfer,
+    endBoreDiameter: p.innerDiameter + 2 * p.endChamfer,
+  }
+}
diff --git a/tests/hollowshaft-01-contract.test.ts b/tests/hollowshaft-01-contract.test.ts
new file mode 100644
index 0000000..f266e22
--- /dev/null
+++ b/tests/hollowshaft-01-contract.test.ts
@@ -0,0 +1,26 @@
+import { expect, test } from "bun:test"
+import {
+  mp,
+  modelDefinitionSchema,
+  hollowShaftModelPropsSchema,
+  getHollowShaftDimensions,
+} from "../src"
+const source = "hollowshaft_od20mm_id12mm_l200mm_roundtube_endchamfer1mm"
+const props = {
+  outerDiameter: 20,
+  innerDiameter: 12,
+  length: 200,
+  endChamfer: 1,
+  roundTube: true,
+} as const
+test("hollowshaft complete roadmap contract and public schema roundtrip", () => {
+  const expected = { fn: "hollowshaft" as const, ...props }
+  expect(mp.string(source).json()).toEqual(expected)
+  expect(modelDefinitionSchema.parse(expected)).toEqual(expected)
+  expect(hollowShaftModelPropsSchema.parse(props)).toEqual(props)
+  expect(
+    getHollowShaftDimensions(props).size.every(
+      (value) => value > 0 && Number.isFinite(value),
+    ),
+  ).toBe(true)
+})
diff --git a/tests/hollowshaft-02-units.test.ts b/tests/hollowshaft-02-units.test.ts
new file mode 100644
index 0000000..c8b94ab
--- /dev/null
+++ b/tests/hollowshaft-02-units.test.ts
@@ -0,0 +1,27 @@
+import { expect, test } from "bun:test"
+import {
+  mp,
+  modelDefinitionSchema,
+  hollowShaftModelPropsSchema,
+  getHollowShaftDimensions,
+} from "../src"
+const source = "hollowshaft_od20mm_id12mm_l200mm_roundtube_endchamfer1mm"
+const props = {
+  outerDiameter: 20,
+  innerDiameter: 12,
+  length: 200,
+  endChamfer: 1,
+  roundTube: true,
+} as const
+test("hollowshaft accepts full decimal units and case-insensitive strings", () => {
+  expect(mp.string(source.toUpperCase()).json()).toEqual({
+    fn: "hollowshaft",
+    ...props,
+  })
+  expect(
+    hollowShaftModelPropsSchema.parse({ ...props, outerDiameter: "2.0cm" }),
+  ).toEqual(props)
+  expect(
+    hollowShaftModelPropsSchema.parse({ ...props, outerDiameter: "+20mm" }),
+  ).toEqual(props)
+})
diff --git a/tests/hollowshaft-03-invalid-tokens.test.ts b/tests/hollowshaft-03-invalid-tokens.test.ts
new file mode 100644
index 0000000..018d286
--- /dev/null
+++ b/tests/hollowshaft-03-invalid-tokens.test.ts
@@ -0,0 +1,35 @@
+import { expect, test } from "bun:test"
+import {
+  mp,
+  modelDefinitionSchema,
+  hollowShaftModelPropsSchema,
+  getHollowShaftDimensions,
+} from "../src"
+const source = "hollowshaft_od20mm_id12mm_l200mm_roundtube_endchamfer1mm"
+const props = {
+  outerDiameter: 20,
+  innerDiameter: 12,
+  length: 200,
+  endChamfer: 1,
+  roundTube: true,
+} as const
+test("hollowshaft rejects missing, duplicate, unknown and partially parsed tokens", () => {
+  for (const suffix of [
+    "",
+    "typo1mm",
+    "constructor1",
+    "od1mm",
+    "outerdiameter1mm",
+    "od(1mm)",
+    "od1mmjunk",
+  ])
+    expect(() => mp.string(`${source}_${suffix}`).json()).toThrow()
+  expect(() => mp.string("hollowshaft").json()).toThrow()
+  for (const value of [0, -1, Infinity, NaN, 1e308, "1e2", "2mmjunk", "(2mm)"])
+    expect(() =>
+      hollowShaftModelPropsSchema.parse({ ...props, outerDiameter: value }),
+    ).toThrow()
+  expect(() =>
+    hollowShaftModelPropsSchema.parse({ ...props, unknown: true }),
+  ).toThrow()
+})
diff --git a/tests/hollowshaft-04-fit-validation.test.ts b/tests/hollowshaft-04-fit-validation.test.ts
new file mode 100644
index 0000000..c2c1ba4
--- /dev/null
+++ b/tests/hollowshaft-04-fit-validation.test.ts
@@ -0,0 +1,25 @@
+import { expect, test } from "bun:test"
+import {
+  mp,
+  modelDefinitionSchema,
+  hollowShaftModelPropsSchema,
+  getHollowShaftDimensions,
+} from "../src"
+const source = "hollowshaft_od20mm_id12mm_l200mm_roundtube_endchamfer1mm"
+const props = {
+  outerDiameter: 20,
+  innerDiameter: 12,
+  length: 200,
+  endChamfer: 1,
+  roundTube: true,
+} as const
+test("hollowshaft rejects impossible fitting and feature dimensions", () => {
+  for (const invalid of [
+    { innerDiameter: 20 },
+    { endChamfer: 2 },
+    { length: 2 },
+  ])
+    expect(() =>
+      hollowShaftModelPropsSchema.parse({ ...props, ...invalid }),
+    ).toThrow()
+})
diff --git a/tests/hollowshaft-05-flags.test.ts b/tests/hollowshaft-05-flags.test.ts
new file mode 100644
index 0000000..39d6c31
--- /dev/null
+++ b/tests/hollowshaft-05-flags.test.ts
@@ -0,0 +1,24 @@
+import { expect, test } from "bun:test"
+import { mp } from "../src"
+const props = {
+  outerDiameter: 20,
+  innerDiameter: 12,
+  length: 200,
+  endChamfer: 1,
+  roundTube: true,
+} as const
+test("hollowshaft uses value-free flags and rejects removed selector syntax", () => {
+  const base = "hollowshaft_od20mm_id12mm_l200mm_endchamfer1mm"
+  const expected = { fn: "hollowshaft", ...props } as const
+  expect(mp.string(`${base}_roundtube`).json()).toEqual(expected)
+  expect(mp.string(base).json()).toEqual(expected)
+  for (const token of [
+    "roundtube_roundtube",
+    "roundtube(true)",
+    "style(roundtube)",
+    "STYLE(ROUNDTUBE)",
+    "style(unknown)",
+  ]) {
+    expect(() => mp.string(`${base}_${token}`).json()).toThrow()
+  }
+})