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April 25, 1938 — U.S. Supreme Court

The federal court — whose law governs?

Erie abolished 96 years of federal general common law in a single opinion. 87 years later, Swift v. Tyson still haunts old briefs — and citing York without Hanna misstates the doctrine courts actually apply.

0.8s
Analysis time
87 yr
Precedent age
1
Overruled case found
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01 — Analysis

Real-time citation scanning

MikeCheck reads your diversity jurisdiction brief, extracts every citation, and checks each against live legal databases — including the York-without-Hanna trap that most practitioners miss.

United States District Court
Eastern District of Pennsylvania
Defendant's Brief re: Choice of Law (Diversity Jurisdiction)
Civil Action No. 24-CV-03857
I. INTRODUCTION
Defendant moves to apply state substantive law under Erie Railroad Co. v. Tompkins, 304 U.S. 64 (1938), which requires federal courts sitting in diversity to apply the law of the state in which they sit.
II. LEGAL STANDARD
Under Guaranty Trust Co. v. York, 326 U.S. 99 (1945), a federal court in diversity must apply state law when it would substantially affect the outcome. This standard was further confirmed by Hanna v. Plumer, 380 U.S. 460 (1965) and choice-of-law rules are governed by Klaxon Co. v. Stentor, 313 U.S. 487 (1941).
III. ARGUMENT
Federal courts sitting in diversity must apply state substantive law. See Swift v. Tyson, 41 U.S. 1 (1842) (establishing federal common law authority, later incorporated in diversity practice).
Waiting
0.0s
1 Parsing citations...
2 Querying treatment history...
3 Verifying quote accuracy...
4 Checking diversity jurisdiction basis...
5 Analyzing circuit precedent...
6 Generating risk assessment...
Risk Level
ATTENTION
1 overruled · 1 incomplete citation
1Overruled
1Incomplete
3Good law
02 — Findings

What MikeCheck found

5
Citations analyzed
1
Overruled
1
Incomplete
3
Good law
⚡ 0.8s
⚠ Requires immediate attention
Swift v. Tyson
● Overruled
41 U.S. 1 (1842) · U.S. Supreme Court
⚠ Erie explicitly overruled Swift after 96 years. Federal courts may no longer apply a general federal common law in diversity cases. Citing Swift for any federal common law proposition in a diversity action invites immediate Rule 11 scrutiny.
overruled 96 years of bad law rule 11 risk federal common law foreclosed
99% confidence
WHY THIS MATTERS
Erie overruled Swift after 96 years. No federal general common law survives in diversity cases — federal courts must apply state substantive law as decided by the state's highest court. Citing Swift for any common-law proposition is not merely wrong; it is a bright-line violation that signals to the court that counsel has not done basic research.
RECOMMENDED ACTION
Remove Swift entirely. No substantive legal proposition from Swift survived Erie. If the brief needs federal common law authority, verify the specific federal common law island that survives Erie (e.g., admiralty, interstate water rights) and cite Erie itself for the governing framework, not Swift.
⚡ Incomplete — companion case required
Guaranty Trust Co. v. York
⚡ Incomplete
326 U.S. 99 (1945) · U.S. Supreme Court
⚡ York's "outcome determinative" test has been refined by Hanna v. Plumer (1965). Citing York alone misstates the current two-track Erie analysis — Hanna requires federal courts to follow the FRCP when a direct collision exists, regardless of outcome-determinativeness.
refined by hanna outcome-determinative test incomplete citation two-track framework missed
88% confidence
WHY THIS MATTERS
York's outcome-determinative test is only Track 2 of Hanna v. Plumer's two-track framework. Track 1 (direct FRCP collision) uses a Supremacy Clause analysis: if a valid Federal Rule of Civil Procedure directly controls the issue, it applies regardless of outcome-determinativeness. Citing York alone — without Hanna — misstates the current standard and may cost you a procedural argument that Track 1 would have won.
RECOMMENDED ACTION
Always pair York with Hanna. Identify which track applies first: is there a directly applicable FRCP rule? If yes, apply Track 1 (Hanna) — the federal rule wins if it is a valid exercise of the Rules Enabling Act. Only if there is no direct collision does Track 2 (York's outcome-determinative test) apply.
✓ 3 citations confirmed as good law
✓ Good law
Erie Railroad Co. v. Tompkins
✓ Good Law
304 U.S. 64 (1938) · U.S. Supreme Court
foundational diversity doctrine followed state substantive law governs correctly cited
99% confidence
WHY IT'S SAFE
Erie's core holding has been unquestioned for 88 years: in diversity cases, federal courts apply state substantive law and federal procedural law. No federal general common law exists for state-governed matters. Erie does not require federal courts to follow state procedural rules — only substantive ones — a distinction Hanna v. Plumer developed further.
HOW TO USE IT
Cite Erie for the core substance/procedure distinction and the no-federal-common-law principle. Let Hanna govern the procedural question (which track applies). For choice-of-law issues, add Klaxon — federal courts apply the forum state's conflict-of-laws rules, not a federal standard.
Hanna v. Plumer
✓ Good Law
380 U.S. 460 (1965) · U.S. Supreme Court
two-track erie framework frcp controls direct collision essential erie companion correctly cited
98% confidence
WHY IT'S SAFE
Hanna v. Plumer is the primary authority for procedural Erie questions. Track 1: when a valid FRCP rule directly collides with a state procedural rule, the Federal Rule prevails under the Supremacy Clause (provided it is a valid exercise of the Rules Enabling Act). Track 2: absent a direct collision, apply York's outcome-determinative test. Both tracks remain controlling law.
HOW TO USE IT
When opposing counsel invokes state procedural rules, first check for a directly applicable FRCP rule. If one exists and directly collides, Hanna Track 1 resolves it in favor of the Federal Rule — no outcome-determinativeness analysis needed. If no direct collision, proceed to Track 2 and cite York alongside Hanna.
Klaxon Co. v. Stentor Electric Mfg. Co.
✓ Good Law
313 U.S. 487 (1941) · U.S. Supreme Court
conflict-of-laws forum state choice-of-law rules frequently missed companion correctly cited
97% confidence
WHY IT'S SAFE
Klaxon established that in diversity cases, federal courts must apply the forum state's conflict-of-laws rules — not an independent federal choice-of-law standard. This principle is routinely overlooked, causing counsel to apply the wrong state's substantive law to the merits. Klaxon has been consistently followed and is frequently dispositive in multi-state disputes.
HOW TO USE IT
Cite Klaxon whenever the dispute involves parties or events in multiple states. First determine which state's law governs (using that state's choice-of-law rules), then apply that state's substantive law under Erie. Skipping the Klaxon step and jumping directly to the merits is one of the most common Erie errors in diversity litigation.
03 — Citation Network

The Erie web

87 years of refinement, extension, and fracture — from a single overruling opinion to a two-track framework still contested at the highest level. MikeCheck maps every relationship, including the 96-year-old precedent Erie destroyed.

Anchor (subject)
Refines / Extends / Applies
Outdated (incomplete)
Overruled
Modern fracture
04 — Jurisdiction Intelligence

Three circuits apply it. Two don't. SCOTUS hasn't settled it.

Do state anti-SLAPP statutes apply in federal diversity cases? The most active live Erie circuit split as of 2025 — with multiple cert petitions filed and no SCOTUS resolution. MikeCheck surfaces the split and tells you which forum is most favorable.

7Circuits analyzed
DirectSplit type
MediumSCOTUS review likelihood
"Anti-SLAPP in Diversity"Issue
← State law governs (substantive) Federal rules displace →
9th Cir.
78%
State law
1st Cir.
70%
State law
11th Cir.
65%
State law
D.C. Cir.
Mixed
7th Cir.
60%
Fed. rules
2nd Cir.
68%
Fed. rules
5th Cir.
52%
Developing
SCOTUS Review Probability
Medium
Multiple cert petitions filed 2023–2025; split is acknowledged. Erie is settled, but its application to anti-SLAPP statutes remains unresolved — a live fracture with no Supreme Court resolution in sight.
9th Circuit most favorable for anti-SLAPP defendants · 2nd/7th apply federal rules
05 — Quote Verification

One word. Inverted meaning.

The famous Brandeis holding — "There is no federal general common law" — is only three words from the opposite claim. The brief drops "general," stripping federal common law from admiralty, foreign relations, and government contracts. It also omits the sentence establishing that Erie applies to state court decisions, not just statutes.

✗ As written in the diversity brief
"Except in matters governed by the Federal Constitution or by Acts of Congress, the law to be applied in any case is the law of the State. There is no federal general common law."
Attributed to: Erie Railroad Co. v. Tompkins, 304 U.S. 64, 78 (1938)
✓ Actual Erie text (304 U.S. at 78)
"Except in matters governed by the Federal Constitution or by Acts of Congress, the law to be applied in any case is the law of the State. And whether the law of the State shall be declared by its Legislature in a statute or by its highest court in a decision is not a matter of federal concern. There is no federal general common law."
Erie Railroad Co. v. Tompkins, 304 U.S. 64, 78 (1938)
0%
Similarity score
Two deviations detected: omission of the sentence establishing state common law coverage, and deletion of "general" — which strips federal common law from admiralty, foreign relations, and government contract law.
06 — Treatment History

Eighty-seven years. Three frameworks. One word.

Erie was controversial at birth — Justice Butler's dissent called it a power grab. Hanna v. Plumer (1965) was the turning point: replacing York's uncertain test with a clear two-track framework. The Shady Grove fracture (2010) showed the doctrine is still contested at the highest level.

Positive treatment
Negative treatment
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