"Run something by someone" and "run something past someone" are not two different expressions with a subtle difference to learn. They're the same idiom, with two prepositions that are fully interchangeable in meaning and register.
This is worth stating plainly, because learners who've internalized that near-synonyms usually carry some contrast go looking for one here -- and there isn't one to find.
Both are equally natural
"Let me run this by you before I send it."
"Let me run this past you before I send it."
A native speaker reaching for either sentence isn't making a meaningful choice. Both are common in speech and informal writing, both work at any seniority level, and neither sounds more casual, more polite, or more correct than the other.
The one thing to actually watch for
The real risk with "past" isn't confusing it with "by" -- it's confusing this idiom with an unrelated meaning of "past": exceeding a deadline, budget, or time limit ("we ran past the deadline"). That sentence has nothing to do with seeking feedback. The idiom only applies when there's an object between "run" and the preposition, referring to something being shared for reaction -- not a limit being exceeded.
Want to learn "Run Something By" in depth?
Lyra Practice teaches advanced non-native professionals the nuance of high-value expressions like this one, then has you practice using them in realistic work scenarios.
Start learning for free →Idiom: "I ran the numbers past finance before sending them."
Unrelated: "We ran past the deadline on this deliverable."
The rule
Treat "run something by someone" and "run something past someone" as freely interchangeable -- pick whichever sounds more natural in the moment, and don't waste effort looking for a distinction. Save your attention for the real trap: "run past" without an object usually means exceeding a limit, not seeking feedback.
Practice scenarios
Practice choosing between run something by and run something past in situations like:
- rewriting the same sentence with either preposition, confirming both work
- distinguishing "ran past the deadline" from "ran the plan past legal"
- checking a draft where either preposition would sound equally natural
Useful practice phrases:
- "Let me run this by/past you before I send it."
- "I ran it by/past legal and they're fine with it."
- "We ran past the deadline" (unrelated meaning -- exceeded a limit)
Same idiom, two prepositions, no difference worth memorizing.
Lyra Practice helps advanced non-native English professionals learn the nuance of high-value workplace expressions and practice using them in realistic scenarios, so their English sounds natural, precise, and senior at work. Try Lyra Practice.