diff --git a/oxlint-plugin-t3code/index.ts b/oxlint-plugin-t3code/index.ts
index f7326312463a0..ede9f4ed948f9 100644
--- a/oxlint-plugin-t3code/index.ts
+++ b/oxlint-plugin-t3code/index.ts
@@ -19,6 +19,7 @@ import noTestInLoop from "./rules/no-test-in-loop.ts";
 import noRpcPermissionBypass from "./rules/no-rpc-permission-bypass.ts";
 import noUnscopedHas from "./rules/no-unscoped-has.ts";
 import preferCatchTags from "./rules/prefer-catch-tags.ts";
+import preferEffectFn from "./rules/prefer-effect-fn.ts";
 import requireCenteredScrollGutter from "./rules/require-centered-scroll-gutter.ts";
 import requireSuppressionReason from "./rules/require-suppression-reason.ts";
 
@@ -46,6 +47,7 @@ export default definePlugin({
     "no-rpc-permission-bypass": noRpcPermissionBypass,
     "no-unscoped-has": noUnscopedHas,
     "prefer-catch-tags": preferCatchTags,
+    "prefer-effect-fn": preferEffectFn,
     "require-centered-scroll-gutter": requireCenteredScrollGutter,
     "require-suppression-reason": requireSuppressionReason,
   },
diff --git a/oxlint-plugin-t3code/rules/prefer-effect-fn.test.ts b/oxlint-plugin-t3code/rules/prefer-effect-fn.test.ts
new file mode 100644
index 0000000000000..ba48fc6ad5273
--- /dev/null
+++ b/oxlint-plugin-t3code/rules/prefer-effect-fn.test.ts
@@ -0,0 +1,202 @@
+import { assert, describe } from "@effect/vitest";
+
+import { createOxlintRuleHarness } from "../test/utils.ts";
+
+const rule = createOxlintRuleHarness("t3code/prefer-effect-fn");
+
+describe("t3code/prefer-effect-fn", () => {
+  rule.valid(
+    "allows inline programs and layer construction",
+    `
+    import * as Effect from "effect/Effect";
+    import * as Layer from "effect/Layer";
+    export const program = Effect.gen(function* () { yield* Effect.void; });
+    export const layer = Layer.effectDiscard(Effect.gen(function* () { yield* Effect.void; }));
+  `,
+  );
+
+  rule.valid(
+    "allows named functions and deliberate untraced functions",
+    `
+    import * as Effect from "effect/Effect";
+    export const read = Effect.fn("Files.read")(function* () { yield* Effect.void; });
+    export const chunk = Effect.fnUntraced(function* () { yield* Effect.void; });
+  `,
+  );
+
+  rule.valid(
+    "allows inline combinator callbacks",
+    `
+    import * as Effect from "effect/Effect";
+    export const program = Effect.void.pipe(Effect.flatMap(() => Effect.gen(function* () {
+      yield* Effect.void;
+    })));
+  `,
+  );
+
+  rule.valid(
+    "allows inline option and error callbacks",
+    `
+    import * as Effect from "effect/Effect";
+    import * as Option from "effect/Option";
+    export const program = Option.match(Option.none(), {
+      onNone: () => Effect.gen(function* () { yield* Effect.void; }),
+      onSome: () => Effect.void,
+    });
+    export const recovered = Effect.fail({ _tag: "Failed" }).pipe(Effect.catchTags({
+      Failed: () => Effect.gen(function* () { yield* Effect.void; }),
+    }));
+  `,
+  );
+
+  rule.valid(
+    "allows reference default-value callbacks",
+    `
+    import * as Context from "effect/Context";
+    import * as Effect from "effect/Effect";
+    export const Default = Context.Reference("Default", {
+      defaultValue: () => Effect.gen(function* () { yield* Effect.void; }),
+    });
+  `,
+  );
+
+  for (const name of ["onSome", "onNone", "onSuccess", "onFailure", "defaultValue"]) {
+    rule.invalid(
+      `reports service members named ${name}`,
+      `
+      import * as Effect from "effect/Effect";
+      export const service = Service.of({
+        ${name}: () => Effect.gen(function* () { yield* Effect.void; }),
+      });
+    `,
+    );
+  }
+
+  rule.valid(
+    "does not mistake other namespaces or shadowed locals for Effect",
+    `
+    import * as Effect from "effect/Effect";
+    const Other = { gen: (body: () => unknown) => body() };
+    export const other = () => Other.gen(function* () {});
+    export function scoped(Effect: typeof Other) {
+      return Effect.gen(function* () {});
+    }
+    export const effect = Effect.void;
+  `,
+  );
+
+  rule.valid(
+    "leaves functions with synchronous setup for manual review",
+    `
+    import * as Effect from "effect/Effect";
+    export const read = () => {
+      const state = new Map();
+      return Effect.gen(function* () { yield* Effect.void; return state; });
+    };
+  `,
+  );
+
+  rule.invalid(
+    "reports a named arrow wrapper",
+    `
+    import * as Effect from "effect/Effect";
+    export const read = (path: string) => Effect.gen(function* () {
+      yield* Effect.void;
+      return path;
+    });
+  `,
+    (output) => {
+      assert.match(output, /Effect\.fn/);
+      assert.match(output, /documented hot paths/);
+    },
+  );
+
+  rule.invalid(
+    "reports a block-bodied wrapper",
+    `
+    import * as Effect from "effect/Effect";
+    export const read = () => { return Effect.gen(function* () { yield* Effect.void; }); };
+  `,
+  );
+
+  rule.invalid(
+    "reports a declaration with pipe transforms",
+    `
+    import * as Effect from "effect/Effect";
+    export function read() {
+      return Effect.gen(function* () { yield* Effect.void; }).pipe(Effect.asVoid);
+    }
+  `,
+  );
+
+  rule.invalid(
+    "reports a function expression",
+    `
+    import * as Effect from "effect/Effect";
+    export const read = function() { return Effect.gen(function* () { yield* Effect.void; }); };
+  `,
+  );
+
+  rule.invalid(
+    "reports service members",
+    `
+    import * as Effect from "effect/Effect";
+    export const service = {
+      read: () => Effect.gen(function* () { yield* Effect.void; }),
+    };
+  `,
+  );
+
+  rule.invalid(
+    "reports class fields",
+    `
+    import * as Effect from "effect/Effect";
+    export class Service {
+      read = () => Effect.gen(function* () { yield* Effect.void; });
+    }
+  `,
+  );
+
+  rule.invalid(
+    "recognizes namespace aliases",
+    `
+    import * as Eff from "effect/Effect";
+    export const read = () => Eff.gen(function* () { yield* Eff.void; });
+  `,
+  );
+
+  rule.invalid(
+    "recognizes bare-package Effect imports",
+    `
+    import { Effect } from "effect";
+    export const read = () => Effect.gen(function* () { yield* Effect.void; });
+  `,
+  );
+
+  rule.invalid(
+    "recognizes a direct gen import",
+    `
+    import { gen, void as noop } from "effect/Effect";
+    export const read = () => gen(function* () { yield* noop; });
+  `,
+  );
+  rule.invalid(
+    "reports class methods",
+    `
+    import * as Effect from "effect/Effect";
+    export class Service {
+      read() { return Effect.gen(function* () { yield* Effect.void; }); }
+    }
+  `,
+  );
+
+  rule.valid(
+    "allows getters that expose an effect value",
+    `
+    import * as Effect from "effect/Effect";
+    export class Service {
+      get read() { return Effect.gen(function* () { yield* Effect.void; }); }
+    }
+  `,
+  );
+});
diff --git a/oxlint-plugin-t3code/rules/prefer-effect-fn.ts b/oxlint-plugin-t3code/rules/prefer-effect-fn.ts
new file mode 100644
index 0000000000000..f72cbb7c2dffc
--- /dev/null
+++ b/oxlint-plugin-t3code/rules/prefer-effect-fn.ts
@@ -0,0 +1,116 @@
+import { defineRule, type ESTree } from "@oxlint/plugins";
+
+export default defineRule({
+  meta: {
+    type: "suggestion",
+    docs: {
+      description: "Use Effect.fn for reusable generator-based Effect functions.",
+    },
+  },
+  create(context) {
+    const isEffectImport = (identifier: ESTree.IdentifierReference, member: string) => {
+      let scope = context.sourceCode.getScope(identifier);
+      while (true) {
+        const variable = scope.set.get(identifier.name);
+        if (variable !== undefined) {
+          return variable.defs.some((def) => {
+            if (def.type !== "ImportBinding" || def.parent?.type !== "ImportDeclaration") {
+              return false;
+            }
+            const source = def.parent.source.value;
+            if (source === "effect/Effect") {
+              if (member === "") {
+                return (
+                  def.node.type === "ImportSpecifier" &&
+                  (def.node.imported.type === "Identifier"
+                    ? def.node.imported.name
+                    : def.node.imported.value) === "gen"
+                );
+              }
+              return def.node.type === "ImportNamespaceSpecifier";
+            }
+            return (
+              source === "effect" &&
+              def.node.type === "ImportSpecifier" &&
+              (def.node.imported.type === "Identifier"
+                ? def.node.imported.name
+                : def.node.imported.value) === "Effect"
+            );
+          });
+        }
+        if (scope.upper === null) return false;
+        scope = scope.upper;
+      }
+    };
+
+    const isGeneratorCall = (expression: ESTree.Expression): boolean => {
+      if (expression.type !== "CallExpression") return false;
+      const callee = expression.callee;
+      if (callee.type === "Identifier") return isEffectImport(callee, "");
+      if (callee.type !== "MemberExpression" || callee.computed) return false;
+      if (callee.property.type !== "Identifier") return false;
+      if (callee.property.name === "pipe") return isGeneratorCall(callee.object);
+      return (
+        callee.property.name === "gen" &&
+        callee.object.type === "Identifier" &&
+        isEffectImport(callee.object, "gen")
+      );
+    };
+
+    const check = (node: ESTree.ArrowFunctionExpression | ESTree.Function) => {
+      if (node.async || node.generator) return;
+      const parent = node.parent;
+      if (node.type !== "FunctionDeclaration") {
+        if (parent.type === "MethodDefinition") {
+          if (parent.kind !== "method") return;
+        } else if (parent.type === "Property") {
+          if (parent.kind !== "init") return;
+          const name =
+            parent.key.type === "Identifier"
+              ? parent.key.name
+              : parent.key.type === "Literal"
+                ? parent.key.value
+                : undefined;
+          const call = parent.parent.parent;
+          if (call?.type === "CallExpression") {
+            let callee = call.callee;
+            while (callee.type === "CallExpression") callee = callee.callee;
+            if (callee.type === "MemberExpression" && callee.property.type === "Identifier") {
+              if (
+                ["catchTags", "match", "matchEffect", "matchCauseEffect"].includes(
+                  callee.property.name,
+                ) ||
+                (name === "defaultValue" && callee.property.name === "Reference")
+              ) {
+                return;
+              }
+            }
+          }
+        } else if (parent.type !== "VariableDeclarator" && parent.type !== "PropertyDefinition") {
+          return;
+        }
+      }
+      const body = node.body;
+      if (body === null) return;
+      let expression: ESTree.Expression | null;
+      if (body.type === "BlockStatement") {
+        if (body.body.length !== 1 || body.body[0]?.type !== "ReturnStatement") return;
+        expression = body.body[0].argument;
+      } else {
+        expression = body;
+      }
+      if (expression === null || !isGeneratorCall(expression)) return;
+      context.report({
+        node,
+        message:
+          'Use Effect.fn("Service.operation") for reusable Effect functions. Reserve Effect.fnUntraced for documented hot paths or an existing span; keep Effect.gen for inline programs.',
+      });
+    };
+
+    return {
+      ArrowFunctionExpression: check,
+      FunctionExpression: check,
+      FunctionDeclaration: check,
+    };
+  },
+});
diff --git a/tsconfig.base.json b/tsconfig.base.json
index 17efedde931b0..07c4224cf8d25 100644
--- a/tsconfig.base.json
+++ b/tsconfig.base.json
@@ -22,7 +22,9 @@
       {
         "name": "@effect/language-service",
         "namespaceImportPackages": ["@effect/platform-node", "effect"],
+        "effectFn": ["span", "inferred-span", "suggested-span"],
         "diagnosticSeverity": {
+          "effectFnOpportunity": "suggestion",
           "anyUnknownInErrorContext": "error",
           "unsafeEffectTypeAssertion": "error",
           "instanceOfSchema": "error",
@@ -59,7 +61,23 @@
         "overrides": [
           {
             "include": ["**/*.test.ts", "**/*.test.tsx"],
-            "options": { "diagnosticSeverity": { "preferSchemaOverJson": "off" } }
+            "options": {
+              "diagnosticSeverity": {
+                "preferSchemaOverJson": "off",
+                "effectFnOpportunity": "off"
+              }
+            }
+          },
+          {
+            "include": [
+              "**/*.spec.ts",
+              "**/*.spec.tsx",
+              "**/*.testkit.ts",
+              "**/*.testkit.tsx",
+              "**/{test,tests,testkit,testing,fixtures,integration}/**",
+              "packages/provider-testing/**"
+            ],
+            "options": { "diagnosticSeverity": { "effectFnOpportunity": "off" } }
           }
         ]
       }
diff --git a/vite.config.ts b/vite.config.ts
index e2b3416d88c89..8866c952a5eaa 100644
--- a/vite.config.ts
+++ b/vite.config.ts
@@ -172,10 +172,19 @@ export default defineConfig({
       "t3code/no-unscoped-has": "error",
       "t3code/namespace-node-imports": "error",
       "t3code/prefer-catch-tags": "error",
+      "t3code/prefer-effect-fn": "error",
       "t3code/require-centered-scroll-gutter": "error",
       "t3code/require-suppression-reason": "error",
     },
     overrides: [
+      {
+        files: [
+          "**/*.{test,spec,testkit}.{ts,tsx}",
+          "**/{test,tests,testkit,testing,fixtures,integration}/**",
+          "packages/provider-testing/**",
+        ],
+        rules: { "t3code/prefer-effect-fn": "off" },
+      },
       {
         files: ["apps/**/src/**", "packages/**/src/**"],
         excludeFiles: DOM_TEST_ENVIRONMENT_ALLOWLIST,