diff --git a/src/features/team-mode/team-registry/validator.test.ts b/src/features/team-mode/team-registry/validator.test.ts
new file mode 100644
index 000000000..b101ded2c
--- /dev/null
+++ b/src/features/team-mode/team-registry/validator.test.ts
@@ -0,0 +1,166 @@
+/// <reference types="bun-types" />
+
+import { describe, expect, test } from "bun:test"
+
+import { TeamSpecSchema } from "../types"
+
+import type { Member, TeamSpec } from "../types"
+import {
+  TeamSpecValidationError,
+  validateDualSupport,
+  validateMemberEligibility,
+  validateSpec,
+} from "./validator"
+
+const PROMETHEUS_REJECTION_MESSAGE =
+  "Agent 'prometheus' is plan-mode-only; can only write to .sisyphus/*.md (enforced by prometheusMdOnly hook). Cannot write to team mailbox. Use category: 'plan' instead."
+
+function createCategoryMember(name: string): Member {
+  return {
+    kind: "category",
+    name,
+    category: "deep",
+    prompt: `implement the assigned work for ${name}`,
+    backendType: "in-process",
+    isActive: true,
+  }
+}
+
+function createBaseTeamSpec(): TeamSpec {
+  return {
+    version: 1,
+    name: "validator-team",
+    createdAt: 1,
+    leadAgentId: "lead",
+    members: [createCategoryMember("lead"), createCategoryMember("reviewer")],
+  }
+}
+
+describe("team-registry validator", () => {
+  test("rejects members that specify both category and subagent_type", () => {
+    // given
+    const teamSpec = {
+      ...createBaseTeamSpec(),
+      members: [
+        {
+          kind: "category",
+          name: "lead",
+          category: "deep",
+          prompt: "implement the assigned work for lead",
+          subagent_type: "sisyphus",
+        },
+      ],
+    }
+
+    // when
+    const result = TeamSpecSchema.safeParse(teamSpec)
+
+    // then
+    expect(result.success).toBe(false)
+  })
+
+  test("rejects members that omit the kind discriminator", () => {
+    // given
+    const teamSpec = {
+      ...createBaseTeamSpec(),
+      members: [{ name: "lead", category: "deep", prompt: "implement the assigned work for lead" }],
+    }
+
+    // when
+    const result = TeamSpecSchema.safeParse(teamSpec)
+
+    // then
+    expect(result.success).toBe(false)
+  })
+
+  test("rejects prometheus subagent members with the exact plan message", () => {
+    // given
+    const member: Member = {
+      kind: "subagent_type",
+      name: "planner",
+      subagent_type: "prometheus",
+      backendType: "in-process",
+      isActive: true,
+    }
+
+    // when
+    const act = () => validateMemberEligibility(member)
+
+    // then
+    expect(act).toThrow(PROMETHEUS_REJECTION_MESSAGE)
+    expect(act).toThrow(TeamSpecValidationError)
+  })
+
+  test("accepts hephaestus subagent members after the D-36 eligibility change", () => {
+    // given
+    const member: Member = {
+      kind: "subagent_type",
+      name: "craftsman",
+      subagent_type: "hephaestus",
+      backendType: "in-process",
+      isActive: true,
+    }
+
+    // when
+    const act = () => validateMemberEligibility(member)
+
+    // then
+    expect(act).not.toThrow()
+  })
+
+  test("rejects leadAgentId values that do not match a member name", () => {
+    // given
+    const teamSpec = { ...createBaseTeamSpec(), leadAgentId: "ghost" }
+
+    // when
+    const act = () => validateSpec(teamSpec)
+
+    // then
+    expect(act).toThrow("Team 'validator-team' leadAgentId 'ghost' must match exactly one member.name.")
+  })
+
+  test("rejects duplicate member names within a team", () => {
+    // given
+    const duplicateMember = createCategoryMember("lead")
+    const teamSpec = { ...createBaseTeamSpec(), members: [createCategoryMember("lead"), duplicateMember] }
+
+    // when
+    const act = () => validateSpec(teamSpec)
+
+    // then
+    expect(act).toThrow("Member name 'lead' is duplicated within team 'validator-team'. Member names must be unique.")
+  })
+
+  test("rejects teams that exceed the 8-member cap", () => {
+    // given
+    const teamSpec = {
+      ...createBaseTeamSpec(),
+      members: Array.from({ length: 9 }, (_, index) => createCategoryMember(`member-${index}`)),
+      leadAgentId: "member-0",
+    }
+
+    // when
+    const act = () => validateSpec(teamSpec)
+
+    // then
+    expect(act).toThrow("Team 'validator-team' exceeds max 8 members.")
+  })
+
+  test("rejects category prompts that collapse to empty text", () => {
+    // given
+    const member: Member = {
+      kind: "category",
+      name: "lead",
+      category: "deep",
+      prompt: "   ",
+      backendType: "in-process",
+      isActive: true,
+    }
+
+    // when
+    const act = () => validateDualSupport(member)
+
+    // then
+    expect(act).toThrow("Member 'lead' prompt must not be empty after trimming whitespace.")
+  })
+})
diff --git a/src/features/team-mode/team-registry/validator.ts b/src/features/team-mode/team-registry/validator.ts
new file mode 100644
index 000000000..ceea0a1f0
--- /dev/null
+++ b/src/features/team-mode/team-registry/validator.ts
@@ -0,0 +1,106 @@
+import { AGENT_ELIGIBILITY_REGISTRY } from "../types"
+
+import type { Member, TeamSpec } from "../types"
+
+const MAX_TEAM_MEMBERS = 8
+const UNKNOWN_SUBAGENT_MESSAGE =
+  "Unknown subagent_type '<name>'. Available ELIGIBLE agents: sisyphus, atlas, sisyphus-junior, hephaestus (if D-36 applied). Use delegate-task for read-only agents like oracle, librarian, explore, metis, momus, multimodal-looker."
+
+export class TeamSpecValidationError extends Error {
+  constructor(
+    message: string,
+    public readonly code: string,
+    public readonly field?: string,
+    public readonly memberName?: string,
+  ) {
+    super(message)
+    this.name = "TeamSpecValidationError"
+  }
+}
+
+export function validateSpec(spec: TeamSpec): void {
+  if (spec.members.length > MAX_TEAM_MEMBERS) {
+    throw new TeamSpecValidationError(
+      `Team '${spec.name}' exceeds max 8 members.`,
+      "TEAM_MEMBER_LIMIT_EXCEEDED",
+      "members",
+    )
+  }
+
+  const seenMemberNames = new Set<string>()
+  let leadMatchCount = 0
+
+  for (const member of spec.members) {
+    if (seenMemberNames.has(member.name)) {
+      throw new TeamSpecValidationError(
+        `Member name '${member.name}' is duplicated within team '${spec.name}'. Member names must be unique.`,
+        "DUPLICATE_MEMBER_NAME",
+        "members",
+        member.name,
+      )
+    }
+
+    seenMemberNames.add(member.name)
+    validateMemberEligibility(member)
+    validateDualSupport(member)
+
+    if (member.name === spec.leadAgentId) {
+      leadMatchCount += 1
+    }
+  }
+
+  if (leadMatchCount !== 1) {
+    throw new TeamSpecValidationError(
+      `Team '${spec.name}' leadAgentId '${spec.leadAgentId}' must match exactly one member.name.`,
+      "INVALID_LEAD_AGENT_ID",
+      "leadAgentId",
+    )
+  }
+}
+
+export function validateMemberEligibility(member: Member): void {
+  if (member.kind !== "subagent_type") {
+    return
+  }
+
+  const eligibility = AGENT_ELIGIBILITY_REGISTRY[member.subagent_type]
+  if (!eligibility) {
+    throw new TeamSpecValidationError(
+      UNKNOWN_SUBAGENT_MESSAGE.replace("<name>", member.subagent_type),
+      "UNKNOWN_SUBAGENT_TYPE",
+      "subagent_type",
+      member.name,
+    )
+  }
+
+  if (eligibility.verdict === "hard-reject") {
+    throw new TeamSpecValidationError(
+      eligibility.rejectionMessage ?? `Agent '${member.subagent_type}' is not eligible as a team member.`,
+      "INELIGIBLE_AGENT",
+      "subagent_type",
+      member.name,
+    )
+  }
+}
+
+export function validateDualSupport(member: Member): void {
+  const trimmedPrompt = member.prompt?.trim()
+
+  if (trimmedPrompt === "") {
+    throw new TeamSpecValidationError(
+      `Member '${member.name}' prompt must not be empty after trimming whitespace.`,
+      "EMPTY_PROMPT",
+      "prompt",
+      member.name,
+    )
+  }
+
+  if (member.kind === "category" && member.prompt.trim().length < 8) {
+    throw new TeamSpecValidationError(
+      `Member '${member.name}' category prompt must be at least 8 characters long.`,
+      "CATEGORY_PROMPT_TOO_SHORT",
+      "prompt",
+      member.name,
+    )
+  }
+}