Check Engine 0.9.2

Check Engine tells you what's broken in a modpack, in plain English. It never changes gameplay.

Check a pack without starting the game (easiest)

1. Download CheckEngine-v0.9.2.zip below and unzip it anywhere.
2. Drag your modpack folder (for example a CurseForge instance folder) onto Check Pack.bat.
3. Read the report. It lists duplicate mods, missing dependencies, version mismatches and client-only mods that would break a server.

It uses the Java that CurseForge or the Minecraft launcher already installed, so there's nothing else to set up. Keep Check Pack.bat and checkengine-cli-0.9.2.jar in the same folder.

The checker can also explain a crash, build a server copy of a pack without the client-only mods, and compare a pack with a server to find out why players can't join:

java -jar checkengine-cli-0.9.2.jar scan <pack folder> [--server]
java -jar checkengine-cli-0.9.2.jar crash <pack folder or crash-*.txt>
java -jar checkengine-cli-0.9.2.jar serverpack <pack folder> [--out <new folder>] [--zip]
java -jar checkengine-cli-0.9.2.jar compare <pack folder> <server folder or server pack .zip>
java -jar checkengine-cli-0.9.2.jar export <pack folder>

Or use it as a mod

Put the jar for your version in the pack's mods folder:

• checkengine-neoforge-26.1-0.9.2.jar for NeoForge 26.1.2 and newer 26.1.x
• checkengine-neoforge-1.21.1-0.9.2.jar for NeoForge 1.21.1
• checkengine-forge-1.20.1-0.9.2.jar for Forge 1.20.1

Problems show on a screen before the title screen and are saved to checkengine/boot-report.txt. Server ops can run /checkengine scan to find quests, recipes and loot tables that point at mods that aren't installed, and /checkengine startup to see how long the last startup took and which mods were slowest.

What's new in 0.9.2

• Important fix: in big packs, Check Engine could make another mod crash while the game was starting (seen as "Duplicate client extensions registration for Block{create:belt}" with Create). The startup timer now waits until every mod is set up before measuring them. Please update.

What's new in 0.9.1

• Fixed the pre-launch window: panels now fill its width, and arrows no longer show as a box.

What's new in 0.9.0

Your 400-mod pack is broken? Find out in seconds, not after Minecraft finishes loading.

• Pre-launch check: if the pack can't start, a small Check Engine window opens right after you press Play, before the loading screen: what's wrong, what changed since it last worked, and the fix. Quit closes the game immediately, Load anyway carries on, Show full error opens the full report.
• "Same crash again" warning: if the last launch crashed and no mods changed since, the window warns you before you sit through the load again.
• It only says "Startup will fail" when the loader's own rules say so. Turn the window off with launch_popup=false in checkengine/settings.properties.
• On NeoForge 26.1, NeoForge 1.21.1 and Forge 1.20.1. Servers just log it.

What's new in 0.8.0

Every feature now works on all three versions: NeoForge 26.1, NeoForge 1.21.1 and Forge 1.20.1.

• NeoForge 26.1 and Forge 1.20.1 now explain why a pack won't start right on the loader's error screen (like 1.21.1 since 0.3.0): the root problem, everything it stops, what changed since the pack last worked, and the fix. Check Engine uses each loader's own rules, so it only speaks up when the loader is really about to stop.
• Crash Doctor ("Your last game crashed: here's why, and the fix") and the help file (/checkengine export) on 26.1 and Forge 1.20.1 too.
• 26.1: clues under any loading error (duplicate mods, broken downloads...), and Crash Doctor understands 26.1's crash reports.
• Still one jar to install for each version.

What's new in 0.7.0

• Help file: one zip with Check Engine's reports, the mod list, the latest log and the newest crash report, ready to drag into Discord or a GitHub issue. Your Windows user name, player name and IP addresses are blanked out first. Make it from Check Pack.bat (it asks at the end), with /checkengine export in game (1.21.1), or with the "Make help file" button on the Crash Doctor screen.

What's new in 0.6.0

• Crash Doctor (NeoForge 1.21.1): the launch after a crash opens with "Your last game crashed": what happened in plain English, the fix, and what changed since the pack last worked. Each crash is explained once, and old crash reports from before Check Engine was installed are ignored. Also saved to checkengine/last-crash.txt. On servers it goes to the log.
• Crash reports where a mod fails while loading now name the mod and its error ("X crashed while loading") instead of a generic message.

What's new in 0.5.0

• "It worked yesterday": what changed? After every launch that works, Check Engine remembers the pack's mods (checkengine/last-good-launch.properties). When the pack breaks later, it lists what was added, removed or updated since then, and links it to the problem: "create is missing. Changed since it last worked: you removed Create 6.0.4." Shown on NeoForge's error screen (1.21.1), under any loading crash, in the in-game report and in Check Pack.bat.

What's new in 0.4.0

• Root causes, not error lists. One problem per cause ("Create is missing"), with every mod it stops from loading, including mods that only need those mods. The biggest problem comes first.
• The real reason behind a "missing" mod: if it's actually there as the Fabric build, a build for another Minecraft version, a broken download or a switched-off .disabled file, Check Engine says so (with how sure it is) and how to fix it.
• Clearer version problems: "Create is too old: update to 6.0 or newer", "too new", or "no single version works for these mods". Mods made for another Minecraft version are grouped into one problem.
• Pack health at the top of every report ("394 jars: 392 OK, 2 with warnings, 0 can't load"), and a FIX THESE FIRST list.
• Works on NeoForge's error screen (1.21.1), in the in-game reports (all versions) and in Check Pack.bat.

What's new in 0.3.0

• NeoForge 1.21.1: Check Engine now explains why a pack won't start, right on NeoForge's error screen. It runs just before NeoForge checks dependencies, so even when the game refuses to load you see what's wrong and how to fix it, grouped ("3 mods need Create, but it isn't installed"). It uses NeoForge's own rules, so it only speaks up when NeoForge is really about to stop. Also saved to checkengine/early-report.txt.
• Clues when loading fails: if any mod breaks loading, Check Engine's warnings (duplicate mods, broken downloads...) show under the error with their fix. When the pack loads fine you just get the usual popup.
• Still one jar to install. 26.1 and Forge 1.20.1 get the same early check in a later version.

What's new in 0.2.0

• Minecraft 26.1 support (NeoForge 26.1.2+): startup check, warning screen, /checkengine scan and the join mismatch screen.
• /checkengine scan understands the newer recipe format (1.21.2+), where ingredients are plain item names.
• New /checkengine startup: how long the last startup took and the slowest mods, also saved to checkengine/startup-report.txt. The checker shows the same numbers in its report.

What's new in 0.1.4

• New logo: the Check Engine block, with its warning light and green check, also shown in the Mods menu.

What's new in 0.1.3

• Check Engine now has its icon and a link to its GitHub page in the Mods menu.
• GitHub links point at the renamed repo (Uanme37/CheckEngine).

What's new in 0.1.2

• New name: Check Engine (it was Pack Doctor; another mod already had that name). The command is now /checkengine scan and reports go to the checkengine folder.

What's new in 0.1.1

Fixes found by testing on 29 real modpacks:

• The mod now loads on any NeoForge 21.1. 0.1.0 needed 21.1.252 or newer, so it refused to start in packs like FTB Skies 2, ATM10 and Better MC.
• No more false "wrong NeoForge version" errors for mods made for 1.21 (NeoForge 1.21.1 loads them anyway).
• Respects config/fml.toml dependency overrides, and knows Monocle lets Iris run on Embeddium.
• /checkengine scan no longer warns about recipes a mod ships for optional mods you don't have (Farming for Blockheads has hundreds); they're one note now.
• scan --server gives server memory advice instead of CurseForge steps.

Check Engine 0.9.1

Check Engine tells you what's broken in a modpack, in plain English. It never changes gameplay.

Check a pack without starting the game (easiest)

1. Download CheckEngine-v0.9.1.zip below and unzip it anywhere.
2. Drag your modpack folder (for example a CurseForge instance folder) onto Check Pack.bat.
3. Read the report. It lists duplicate mods, missing dependencies, version mismatches and client-only mods that would break a server.

It uses the Java that CurseForge or the Minecraft launcher already installed, so there's nothing else to set up. Keep Check Pack.bat and checkengine-cli-0.9.1.jar in the same folder.

The checker can also explain a crash, build a server copy of a pack without the client-only mods, and compare a pack with a server to find out why players can't join:

java -jar checkengine-cli-0.9.1.jar scan <pack folder> [--server]
java -jar checkengine-cli-0.9.1.jar crash <pack folder or crash-*.txt>
java -jar checkengine-cli-0.9.1.jar serverpack <pack folder> [--out <new folder>] [--zip]
java -jar checkengine-cli-0.9.1.jar compare <pack folder> <server folder or server pack .zip>
java -jar checkengine-cli-0.9.1.jar export <pack folder>

Or use it as a mod

Put the jar for your version in the pack's mods folder:

• checkengine-neoforge-26.1-0.9.1.jar for NeoForge 26.1.2 and newer 26.1.x
• checkengine-neoforge-1.21.1-0.9.1.jar for NeoForge 1.21.1
• checkengine-forge-1.20.1-0.9.1.jar for Forge 1.20.1

Problems show on a screen before the title screen and are saved to checkengine/boot-report.txt. Server ops can run /checkengine scan to find quests, recipes and loot tables that point at mods that aren't installed, and /checkengine startup to see how long the last startup took and which mods were slowest.

What's new in 0.9.0

Your 400-mod pack is broken? Find out in seconds, not after Minecraft finishes loading.

• Pre-launch check: if the pack can't start, a small Check Engine window opens right after you press Play, before the loading screen: what's wrong, what changed since it last worked, and the fix. Quit closes the game immediately, Load anyway carries on, Show full error opens the full report.
• "Same crash again" warning: if the last launch crashed and no mods changed since, the window warns you before you sit through the load again.
• It only says "Startup will fail" when the loader's own rules say so. Turn the window off with launch_popup=false in checkengine/settings.properties.
• On NeoForge 26.1, NeoForge 1.21.1 and Forge 1.20.1. Servers just log it.

What's new in 0.8.0

Every feature now works on all three versions: NeoForge 26.1, NeoForge 1.21.1 and Forge 1.20.1.

• NeoForge 26.1 and Forge 1.20.1 now explain why a pack won't start right on the loader's error screen (like 1.21.1 since 0.3.0): the root problem, everything it stops, what changed since the pack last worked, and the fix. Check Engine uses each loader's own rules, so it only speaks up when the loader is really about to stop.
• Crash Doctor ("Your last game crashed: here's why, and the fix") and the help file (/checkengine export) on 26.1 and Forge 1.20.1 too.
• 26.1: clues under any loading error (duplicate mods, broken downloads...), and Crash Doctor understands 26.1's crash reports.
• Still one jar to install for each version.

What's new in 0.7.0

• Help file: one zip with Check Engine's reports, the mod list, the latest log and the newest crash report, ready to drag into Discord or a GitHub issue. Your Windows user name, player name and IP addresses are blanked out first. Make it from Check Pack.bat (it asks at the end), with /checkengine export in game (1.21.1), or with the "Make help file" button on the Crash Doctor screen.

What's new in 0.6.0

• Crash Doctor (NeoForge 1.21.1): the launch after a crash opens with "Your last game crashed": what happened in plain English, the fix, and what changed since the pack last worked. Each crash is explained once, and old crash reports from before Check Engine was installed are ignored. Also saved to checkengine/last-crash.txt. On servers it goes to the log.
• Crash reports where a mod fails while loading now name the mod and its error ("X crashed while loading") instead of a generic message.

What's new in 0.5.0

• "It worked yesterday": what changed? After every launch that works, Check Engine remembers the pack's mods (checkengine/last-good-launch.properties). When the pack breaks later, it lists what was added, removed or updated since then, and links it to the problem: "create is missing. Changed since it last worked: you removed Create 6.0.4." Shown on NeoForge's error screen (1.21.1), under any loading crash, in the in-game report and in Check Pack.bat.

What's new in 0.4.0

• Root causes, not error lists. One problem per cause ("Create is missing"), with every mod it stops from loading, including mods that only need those mods. The biggest problem comes first.
• The real reason behind a "missing" mod: if it's actually there as the Fabric build, a build for another Minecraft version, a broken download or a switched-off .disabled file, Check Engine says so (with how sure it is) and how to fix it.
• Clearer version problems: "Create is too old: update to 6.0 or newer", "too new", or "no single version works for these mods". Mods made for another Minecraft version are grouped into one problem.
• Pack health at the top of every report ("394 jars: 392 OK, 2 with warnings, 0 can't load"), and a FIX THESE FIRST list.
• Works on NeoForge's error screen (1.21.1), in the in-game reports (all versions) and in Check Pack.bat.

What's new in 0.3.0

• NeoForge 1.21.1: Check Engine now explains why a pack won't start, right on NeoForge's error screen. It runs just before NeoForge checks dependencies, so even when the game refuses to load you see what's wrong and how to fix it, grouped ("3 mods need Create, but it isn't installed"). It uses NeoForge's own rules, so it only speaks up when NeoForge is really about to stop. Also saved to checkengine/early-report.txt.
• Clues when loading fails: if any mod breaks loading, Check Engine's warnings (duplicate mods, broken downloads...) show under the error with their fix. When the pack loads fine you just get the usual popup.
• Still one jar to install. 26.1 and Forge 1.20.1 get the same early check in a later version.

What's new in 0.2.0

• Minecraft 26.1 support (NeoForge 26.1.2+): startup check, warning screen, /checkengine scan and the join mismatch screen.
• /checkengine scan understands the newer recipe format (1.21.2+), where ingredients are plain item names.
• New /checkengine startup: how long the last startup took and the slowest mods, also saved to checkengine/startup-report.txt. The checker shows the same numbers in its report.

What's new in 0.1.4

• New logo: the Check Engine block, with its warning light and green check, also shown in the Mods menu.

What's new in 0.1.3

• Check Engine now has its icon and a link to its GitHub page in the Mods menu.
• GitHub links point at the renamed repo (Uanme37/CheckEngine).

What's new in 0.1.2

• New name: Check Engine (it was Pack Doctor; another mod already had that name). The command is now /checkengine scan and reports go to the checkengine folder.

What's new in 0.1.1

Fixes found by testing on 29 real modpacks:

• The mod now loads on any NeoForge 21.1. 0.1.0 needed 21.1.252 or newer, so it refused to start in packs like FTB Skies 2, ATM10 and Better MC.
• No more false "wrong NeoForge version" errors for mods made for 1.21 (NeoForge 1.21.1 loads them anyway).
• Respects config/fml.toml dependency overrides, and knows Monocle lets Iris run on Embeddium.
• /checkengine scan no longer warns about recipes a mod ships for optional mods you don't have (Farming for Blockheads has hundreds); they're one note now.
• scan --server gives server memory advice instead of CurseForge steps.

Check Engine 0.9.0

Check Engine tells you what's broken in a modpack, in plain English. It never changes gameplay.

Check a pack without starting the game (easiest)

1. Download CheckEngine-v0.9.0.zip below and unzip it anywhere.
2. Drag your modpack folder (for example a CurseForge instance folder) onto Check Pack.bat.
3. Read the report. It lists duplicate mods, missing dependencies, version mismatches and client-only mods that would break a server.

It uses the Java that CurseForge or the Minecraft launcher already installed, so there's nothing else to set up. Keep Check Pack.bat and checkengine-cli-0.9.0.jar in the same folder.

The checker can also explain a crash, build a server copy of a pack without the client-only mods, and compare a pack with a server to find out why players can't join:

java -jar checkengine-cli-0.9.0.jar scan <pack folder> [--server]
java -jar checkengine-cli-0.9.0.jar crash <pack folder or crash-*.txt>
java -jar checkengine-cli-0.9.0.jar serverpack <pack folder> [--out <new folder>] [--zip]
java -jar checkengine-cli-0.9.0.jar compare <pack folder> <server folder or server pack .zip>
java -jar checkengine-cli-0.9.0.jar export <pack folder>

Or use it as a mod

Put the jar for your version in the pack's mods folder:

• checkengine-neoforge-26.1-0.9.0.jar for NeoForge 26.1.2 and newer 26.1.x
• checkengine-neoforge-1.21.1-0.9.0.jar for NeoForge 1.21.1
• checkengine-forge-1.20.1-0.9.0.jar for Forge 1.20.1

Problems show on a screen before the title screen and are saved to checkengine/boot-report.txt. Server ops can run /checkengine scan to find quests, recipes and loot tables that point at mods that aren't installed, and /checkengine startup to see how long the last startup took and which mods were slowest.

What's new in 0.9.0

Your 400-mod pack is broken? Find out in seconds, not after Minecraft finishes loading.

• Pre-launch check: if the pack can't start, a small Check Engine window opens right after you press Play, before the loading screen: what's wrong, what changed since it last worked, and the fix. Quit closes the game immediately, Load anyway carries on, Show full error opens the full report.
• "Same crash again" warning: if the last launch crashed and no mods changed since, the window warns you before you sit through the load again.
• It only says "Startup will fail" when the loader's own rules say so. Turn the window off with launch_popup=false in checkengine/settings.properties.
• On NeoForge 26.1, NeoForge 1.21.1 and Forge 1.20.1. Servers just log it.

What's new in 0.8.0

Every feature now works on all three versions: NeoForge 26.1, NeoForge 1.21.1 and Forge 1.20.1.

• NeoForge 26.1 and Forge 1.20.1 now explain why a pack won't start right on the loader's error screen (like 1.21.1 since 0.3.0): the root problem, everything it stops, what changed since the pack last worked, and the fix. Check Engine uses each loader's own rules, so it only speaks up when the loader is really about to stop.
• Crash Doctor ("Your last game crashed: here's why, and the fix") and the help file (/checkengine export) on 26.1 and Forge 1.20.1 too.
• 26.1: clues under any loading error (duplicate mods, broken downloads...), and Crash Doctor understands 26.1's crash reports.
• Still one jar to install for each version.

What's new in 0.7.0

• Help file: one zip with Check Engine's reports, the mod list, the latest log and the newest crash report, ready to drag into Discord or a GitHub issue. Your Windows user name, player name and IP addresses are blanked out first. Make it from Check Pack.bat (it asks at the end), with /checkengine export in game (1.21.1), or with the "Make help file" button on the Crash Doctor screen.

What's new in 0.6.0

• Crash Doctor (NeoForge 1.21.1): the launch after a crash opens with "Your last game crashed": what happened in plain English, the fix, and what changed since the pack last worked. Each crash is explained once, and old crash reports from before Check Engine was installed are ignored. Also saved to checkengine/last-crash.txt. On servers it goes to the log.
• Crash reports where a mod fails while loading now name the mod and its error ("X crashed while loading") instead of a generic message.

What's new in 0.5.0

• "It worked yesterday": what changed? After every launch that works, Check Engine remembers the pack's mods (checkengine/last-good-launch.properties). When the pack breaks later, it lists what was added, removed or updated since then, and links it to the problem: "create is missing. Changed since it last worked: you removed Create 6.0.4." Shown on NeoForge's error screen (1.21.1), under any loading crash, in the in-game report and in Check Pack.bat.

What's new in 0.4.0

• Root causes, not error lists. One problem per cause ("Create is missing"), with every mod it stops from loading, including mods that only need those mods. The biggest problem comes first.
• The real reason behind a "missing" mod: if it's actually there as the Fabric build, a build for another Minecraft version, a broken download or a switched-off .disabled file, Check Engine says so (with how sure it is) and how to fix it.
• Clearer version problems: "Create is too old: update to 6.0 or newer", "too new", or "no single version works for these mods". Mods made for another Minecraft version are grouped into one problem.
• Pack health at the top of every report ("394 jars: 392 OK, 2 with warnings, 0 can't load"), and a FIX THESE FIRST list.
• Works on NeoForge's error screen (1.21.1), in the in-game reports (all versions) and in Check Pack.bat.

What's new in 0.3.0

• NeoForge 1.21.1: Check Engine now explains why a pack won't start, right on NeoForge's error screen. It runs just before NeoForge checks dependencies, so even when the game refuses to load you see what's wrong and how to fix it, grouped ("3 mods need Create, but it isn't installed"). It uses NeoForge's own rules, so it only speaks up when NeoForge is really about to stop. Also saved to checkengine/early-report.txt.
• Clues when loading fails: if any mod breaks loading, Check Engine's warnings (duplicate mods, broken downloads...) show under the error with their fix. When the pack loads fine you just get the usual popup.
• Still one jar to install. 26.1 and Forge 1.20.1 get the same early check in a later version.

What's new in 0.2.0

• Minecraft 26.1 support (NeoForge 26.1.2+): startup check, warning screen, /checkengine scan and the join mismatch screen.
• /checkengine scan understands the newer recipe format (1.21.2+), where ingredients are plain item names.
• New /checkengine startup: how long the last startup took and the slowest mods, also saved to checkengine/startup-report.txt. The checker shows the same numbers in its report.

What's new in 0.1.4

• New logo: the Check Engine block, with its warning light and green check, also shown in the Mods menu.

What's new in 0.1.3

• Check Engine now has its icon and a link to its GitHub page in the Mods menu.
• GitHub links point at the renamed repo (Uanme37/CheckEngine).

What's new in 0.1.2

• New name: Check Engine (it was Pack Doctor; another mod already had that name). The command is now /checkengine scan and reports go to the checkengine folder.

What's new in 0.1.1

Fixes found by testing on 29 real modpacks:

• The mod now loads on any NeoForge 21.1. 0.1.0 needed 21.1.252 or newer, so it refused to start in packs like FTB Skies 2, ATM10 and Better MC.
• No more false "wrong NeoForge version" errors for mods made for 1.21 (NeoForge 1.21.1 loads them anyway).
• Respects config/fml.toml dependency overrides, and knows Monocle lets Iris run on Embeddium.
• /checkengine scan no longer warns about recipes a mod ships for optional mods you don't have (Farming for Blockheads has hundreds); they're one note now.
• scan --server gives server memory advice instead of CurseForge steps.

Check Engine 0.8.0

Check Engine tells you what's broken in a modpack, in plain English. It never changes gameplay.

Check a pack without starting the game (easiest)

1. Download CheckEngine-v0.8.0.zip below and unzip it anywhere.
2. Drag your modpack folder (for example a CurseForge instance folder) onto Check Pack.bat.
3. Read the report. It lists duplicate mods, missing dependencies, version mismatches and client-only mods that would break a server.

It uses the Java that CurseForge or the Minecraft launcher already installed, so there's nothing else to set up. Keep Check Pack.bat and checkengine-cli-0.8.0.jar in the same folder.

The checker can also explain a crash, build a server copy of a pack without the client-only mods, and compare a pack with a server to find out why players can't join:

java -jar checkengine-cli-0.8.0.jar scan <pack folder> [--server]
java -jar checkengine-cli-0.8.0.jar crash <pack folder or crash-*.txt>
java -jar checkengine-cli-0.8.0.jar serverpack <pack folder> [--out <new folder>] [--zip]
java -jar checkengine-cli-0.8.0.jar compare <pack folder> <server folder or server pack .zip>
java -jar checkengine-cli-0.8.0.jar export <pack folder>

Or use it as a mod

Put the jar for your version in the pack's mods folder:

• checkengine-neoforge-26.1-0.8.0.jar for NeoForge 26.1.2 and newer 26.1.x
• checkengine-neoforge-1.21.1-0.8.0.jar for NeoForge 1.21.1
• checkengine-forge-1.20.1-0.8.0.jar for Forge 1.20.1

Problems show on a screen before the title screen and are saved to checkengine/boot-report.txt. Server ops can run /checkengine scan to find quests, recipes and loot tables that point at mods that aren't installed, and /checkengine startup to see how long the last startup took and which mods were slowest.

What's new in 0.8.0

Every feature now works on all three versions: NeoForge 26.1, NeoForge 1.21.1 and Forge 1.20.1.

• NeoForge 26.1 and Forge 1.20.1 now explain why a pack won't start right on the loader's error screen (like 1.21.1 since 0.3.0): the root problem, everything it stops, what changed since the pack last worked, and the fix. Check Engine uses each loader's own rules, so it only speaks up when the loader is really about to stop.
• Crash Doctor ("Your last game crashed: here's why, and the fix") and the help file (/checkengine export) on 26.1 and Forge 1.20.1 too.
• 26.1: clues under any loading error (duplicate mods, broken downloads...), and Crash Doctor understands 26.1's crash reports.
• Still one jar to install for each version.

What's new in 0.7.0

• Help file: one zip with Check Engine's reports, the mod list, the latest log and the newest crash report, ready to drag into Discord or a GitHub issue. Your Windows user name, player name and IP addresses are blanked out first. Make it from Check Pack.bat (it asks at the end), with /checkengine export in game (1.21.1), or with the "Make help file" button on the Crash Doctor screen.

What's new in 0.6.0

• Crash Doctor (NeoForge 1.21.1): the launch after a crash opens with "Your last game crashed": what happened in plain English, the fix, and what changed since the pack last worked. Each crash is explained once, and old crash reports from before Check Engine was installed are ignored. Also saved to checkengine/last-crash.txt. On servers it goes to the log.
• Crash reports where a mod fails while loading now name the mod and its error ("X crashed while loading") instead of a generic message.

What's new in 0.5.0

• "It worked yesterday": what changed? After every launch that works, Check Engine remembers the pack's mods (checkengine/last-good-launch.properties). When the pack breaks later, it lists what was added, removed or updated since then, and links it to the problem: "create is missing. Changed since it last worked: you removed Create 6.0.4." Shown on NeoForge's error screen (1.21.1), under any loading crash, in the in-game report and in Check Pack.bat.

What's new in 0.4.0

• Root causes, not error lists. One problem per cause ("Create is missing"), with every mod it stops from loading, including mods that only need those mods. The biggest problem comes first.
• The real reason behind a "missing" mod: if it's actually there as the Fabric build, a build for another Minecraft version, a broken download or a switched-off .disabled file, Check Engine says so (with how sure it is) and how to fix it.
• Clearer version problems: "Create is too old: update to 6.0 or newer", "too new", or "no single version works for these mods". Mods made for another Minecraft version are grouped into one problem.
• Pack health at the top of every report ("394 jars: 392 OK, 2 with warnings, 0 can't load"), and a FIX THESE FIRST list.
• Works on NeoForge's error screen (1.21.1), in the in-game reports (all versions) and in Check Pack.bat.

What's new in 0.3.0

• NeoForge 1.21.1: Check Engine now explains why a pack won't start, right on NeoForge's error screen. It runs just before NeoForge checks dependencies, so even when the game refuses to load you see what's wrong and how to fix it, grouped ("3 mods need Create, but it isn't installed"). It uses NeoForge's own rules, so it only speaks up when NeoForge is really about to stop. Also saved to checkengine/early-report.txt.
• Clues when loading fails: if any mod breaks loading, Check Engine's warnings (duplicate mods, broken downloads...) show under the error with their fix. When the pack loads fine you just get the usual popup.
• Still one jar to install. 26.1 and Forge 1.20.1 get the same early check in a later version.

What's new in 0.2.0

• Minecraft 26.1 support (NeoForge 26.1.2+): startup check, warning screen, /checkengine scan and the join mismatch screen.
• /checkengine scan understands the newer recipe format (1.21.2+), where ingredients are plain item names.
• New /checkengine startup: how long the last startup took and the slowest mods, also saved to checkengine/startup-report.txt. The checker shows the same numbers in its report.

What's new in 0.1.4

• New logo: the Check Engine block, with its warning light and green check, also shown in the Mods menu.

What's new in 0.1.3

• Check Engine now has its icon and a link to its GitHub page in the Mods menu.
• GitHub links point at the renamed repo (Uanme37/CheckEngine).

What's new in 0.1.2

• New name: Check Engine (it was Pack Doctor; another mod already had that name). The command is now /checkengine scan and reports go to the checkengine folder.

What's new in 0.1.1

Fixes found by testing on 29 real modpacks:

• The mod now loads on any NeoForge 21.1. 0.1.0 needed 21.1.252 or newer, so it refused to start in packs like FTB Skies 2, ATM10 and Better MC.
• No more false "wrong NeoForge version" errors for mods made for 1.21 (NeoForge 1.21.1 loads them anyway).
• Respects config/fml.toml dependency overrides, and knows Monocle lets Iris run on Embeddium.
• /checkengine scan no longer warns about recipes a mod ships for optional mods you don't have (Farming for Blockheads has hundreds); they're one note now.
• scan --server gives server memory advice instead of CurseForge steps.

Check Engine 0.7.0

Check Engine tells you what's broken in a modpack, in plain English. It never changes gameplay.

Check a pack without starting the game (easiest)

1. Download CheckEngine-v0.7.0.zip below and unzip it anywhere.
2. Drag your modpack folder (for example a CurseForge instance folder) onto Check Pack.bat.
3. Read the report. It lists duplicate mods, missing dependencies, version mismatches and client-only mods that would break a server.

It uses the Java that CurseForge or the Minecraft launcher already installed, so there's nothing else to set up. Keep Check Pack.bat and checkengine-cli-0.7.0.jar in the same folder.

The checker can also explain a crash, build a server copy of a pack without the client-only mods, and compare a pack with a server to find out why players can't join:

java -jar checkengine-cli-0.7.0.jar scan <pack folder> [--server]
java -jar checkengine-cli-0.7.0.jar crash <pack folder or crash-*.txt>
java -jar checkengine-cli-0.7.0.jar serverpack <pack folder> [--out <new folder>] [--zip]
java -jar checkengine-cli-0.7.0.jar compare <pack folder> <server folder or server pack .zip>
java -jar checkengine-cli-0.7.0.jar export <pack folder>

Or use it as a mod

Put the jar for your version in the pack's mods folder:

• checkengine-neoforge-26.1-0.7.0.jar for NeoForge 26.1.2 and newer 26.1.x
• checkengine-neoforge-1.21.1-0.7.0.jar for NeoForge 1.21.1
• checkengine-forge-1.20.1-0.7.0.jar for Forge 1.20.1

Problems show on a screen before the title screen and are saved to checkengine/boot-report.txt. Server ops can run /checkengine scan to find quests, recipes and loot tables that point at mods that aren't installed, and /checkengine startup to see how long the last startup took and which mods were slowest.

What's new in 0.7.0

• Help file: one zip with Check Engine's reports, the mod list, the latest log and the newest crash report, ready to drag into Discord or a GitHub issue. Your Windows user name, player name and IP addresses are blanked out first. Make it from Check Pack.bat (it asks at the end), with /checkengine export in game (1.21.1), or with the "Make help file" button on the Crash Doctor screen.

What's new in 0.6.0

• Crash Doctor (NeoForge 1.21.1): the launch after a crash opens with "Your last game crashed": what happened in plain English, the fix, and what changed since the pack last worked. Each crash is explained once, and old crash reports from before Check Engine was installed are ignored. Also saved to checkengine/last-crash.txt. On servers it goes to the log.
• Crash reports where a mod fails while loading now name the mod and its error ("X crashed while loading") instead of a generic message.

What's new in 0.5.0

• "It worked yesterday": what changed? After every launch that works, Check Engine remembers the pack's mods (checkengine/last-good-launch.properties). When the pack breaks later, it lists what was added, removed or updated since then, and links it to the problem: "create is missing. Changed since it last worked: you removed Create 6.0.4." Shown on NeoForge's error screen (1.21.1), under any loading crash, in the in-game report and in Check Pack.bat.

What's new in 0.4.0

• Root causes, not error lists. One problem per cause ("Create is missing"), with every mod it stops from loading, including mods that only need those mods. The biggest problem comes first.
• The real reason behind a "missing" mod: if it's actually there as the Fabric build, a build for another Minecraft version, a broken download or a switched-off .disabled file, Check Engine says so (with how sure it is) and how to fix it.
• Clearer version problems: "Create is too old: update to 6.0 or newer", "too new", or "no single version works for these mods". Mods made for another Minecraft version are grouped into one problem.
• Pack health at the top of every report ("394 jars: 392 OK, 2 with warnings, 0 can't load"), and a FIX THESE FIRST list.
• Works on NeoForge's error screen (1.21.1), in the in-game reports (all versions) and in Check Pack.bat.

What's new in 0.3.0

• NeoForge 1.21.1: Check Engine now explains why a pack won't start, right on NeoForge's error screen. It runs just before NeoForge checks dependencies, so even when the game refuses to load you see what's wrong and how to fix it, grouped ("3 mods need Create, but it isn't installed"). It uses NeoForge's own rules, so it only speaks up when NeoForge is really about to stop. Also saved to checkengine/early-report.txt.
• Clues when loading fails: if any mod breaks loading, Check Engine's warnings (duplicate mods, broken downloads...) show under the error with their fix. When the pack loads fine you just get the usual popup.
• Still one jar to install. 26.1 and Forge 1.20.1 get the same early check in a later version.

What's new in 0.2.0

• Minecraft 26.1 support (NeoForge 26.1.2+): startup check, warning screen, /checkengine scan and the join mismatch screen.
• /checkengine scan understands the newer recipe format (1.21.2+), where ingredients are plain item names.
• New /checkengine startup: how long the last startup took and the slowest mods, also saved to checkengine/startup-report.txt. The checker shows the same numbers in its report.

What's new in 0.1.4

• New logo: the Check Engine block, with its warning light and green check, also shown in the Mods menu.

What's new in 0.1.3

• Check Engine now has its icon and a link to its GitHub page in the Mods menu.
• GitHub links point at the renamed repo (Uanme37/CheckEngine).

What's new in 0.1.2

• New name: Check Engine (it was Pack Doctor; another mod already had that name). The command is now /checkengine scan and reports go to the checkengine folder.

What's new in 0.1.1

Fixes found by testing on 29 real modpacks:

• The mod now loads on any NeoForge 21.1. 0.1.0 needed 21.1.252 or newer, so it refused to start in packs like FTB Skies 2, ATM10 and Better MC.
• No more false "wrong NeoForge version" errors for mods made for 1.21 (NeoForge 1.21.1 loads them anyway).
• Respects config/fml.toml dependency overrides, and knows Monocle lets Iris run on Embeddium.
• /checkengine scan no longer warns about recipes a mod ships for optional mods you don't have (Farming for Blockheads has hundreds); they're one note now.
• scan --server gives server memory advice instead of CurseForge steps.

Check Engine 0.6.0

Check Engine tells you what's broken in a modpack, in plain English. It never changes gameplay.

Check a pack without starting the game (easiest)

1. Download CheckEngine-v0.6.0.zip below and unzip it anywhere.
2. Drag your modpack folder (for example a CurseForge instance folder) onto Check Pack.bat.
3. Read the report. It lists duplicate mods, missing dependencies, version mismatches and client-only mods that would break a server.

It uses the Java that CurseForge or the Minecraft launcher already installed, so there's nothing else to set up. Keep Check Pack.bat and checkengine-cli-0.6.0.jar in the same folder.

The checker can also explain a crash, build a server copy of a pack without the client-only mods, and compare a pack with a server to find out why players can't join:

java -jar checkengine-cli-0.6.0.jar scan <pack folder> [--server]
java -jar checkengine-cli-0.6.0.jar crash <pack folder or crash-*.txt>
java -jar checkengine-cli-0.6.0.jar serverpack <pack folder> [--out <new folder>] [--zip]
java -jar checkengine-cli-0.6.0.jar compare <pack folder> <server folder or server pack .zip>

Or use it as a mod

Put the jar for your version in the pack's mods folder:

• checkengine-neoforge-26.1-0.6.0.jar for NeoForge 26.1.2 and newer 26.1.x
• checkengine-neoforge-1.21.1-0.6.0.jar for NeoForge 1.21.1
• checkengine-forge-1.20.1-0.6.0.jar for Forge 1.20.1

Problems show on a screen before the title screen and are saved to checkengine/boot-report.txt. Server ops can run /checkengine scan to find quests, recipes and loot tables that point at mods that aren't installed, and /checkengine startup to see how long the last startup took and which mods were slowest.

What's new in 0.6.0

• Crash Doctor (NeoForge 1.21.1): the launch after a crash opens with "Your last game crashed": what happened in plain English, the fix, and what changed since the pack last worked. Each crash is explained once, and old crash reports from before Check Engine was installed are ignored. Also saved to checkengine/last-crash.txt. On servers it goes to the log.
• Crash reports where a mod fails while loading now name the mod and its error ("X crashed while loading") instead of a generic message.

What's new in 0.5.0

• "It worked yesterday": what changed? After every launch that works, Check Engine remembers the pack's mods (checkengine/last-good-launch.properties). When the pack breaks later, it lists what was added, removed or updated since then, and links it to the problem: "create is missing. Changed since it last worked: you removed Create 6.0.4." Shown on NeoForge's error screen (1.21.1), under any loading crash, in the in-game report and in Check Pack.bat.

What's new in 0.4.0

• Root causes, not error lists. One problem per cause ("Create is missing"), with every mod it stops from loading, including mods that only need those mods. The biggest problem comes first.
• The real reason behind a "missing" mod: if it's actually there as the Fabric build, a build for another Minecraft version, a broken download or a switched-off .disabled file, Check Engine says so (with how sure it is) and how to fix it.
• Clearer version problems: "Create is too old: update to 6.0 or newer", "too new", or "no single version works for these mods". Mods made for another Minecraft version are grouped into one problem.
• Pack health at the top of every report ("394 jars: 392 OK, 2 with warnings, 0 can't load"), and a FIX THESE FIRST list.
• Works on NeoForge's error screen (1.21.1), in the in-game reports (all versions) and in Check Pack.bat.

What's new in 0.3.0

• NeoForge 1.21.1: Check Engine now explains why a pack won't start, right on NeoForge's error screen. It runs just before NeoForge checks dependencies, so even when the game refuses to load you see what's wrong and how to fix it, grouped ("3 mods need Create, but it isn't installed"). It uses NeoForge's own rules, so it only speaks up when NeoForge is really about to stop. Also saved to checkengine/early-report.txt.
• Clues when loading fails: if any mod breaks loading, Check Engine's warnings (duplicate mods, broken downloads...) show under the error with their fix. When the pack loads fine you just get the usual popup.
• Still one jar to install. 26.1 and Forge 1.20.1 get the same early check in a later version.

What's new in 0.2.0

• Minecraft 26.1 support (NeoForge 26.1.2+): startup check, warning screen, /checkengine scan and the join mismatch screen.
• /checkengine scan understands the newer recipe format (1.21.2+), where ingredients are plain item names.
• New /checkengine startup: how long the last startup took and the slowest mods, also saved to checkengine/startup-report.txt. The checker shows the same numbers in its report.

What's new in 0.1.4

• New logo: the Check Engine block, with its warning light and green check, also shown in the Mods menu.

What's new in 0.1.3

• Check Engine now has its icon and a link to its GitHub page in the Mods menu.
• GitHub links point at the renamed repo (Uanme37/CheckEngine).

What's new in 0.1.2

• New name: Check Engine (it was Pack Doctor; another mod already had that name). The command is now /checkengine scan and reports go to the checkengine folder.

What's new in 0.1.1

Fixes found by testing on 29 real modpacks:

• The mod now loads on any NeoForge 21.1. 0.1.0 needed 21.1.252 or newer, so it refused to start in packs like FTB Skies 2, ATM10 and Better MC.
• No more false "wrong NeoForge version" errors for mods made for 1.21 (NeoForge 1.21.1 loads them anyway).
• Respects config/fml.toml dependency overrides, and knows Monocle lets Iris run on Embeddium.
• /checkengine scan no longer warns about recipes a mod ships for optional mods you don't have (Farming for Blockheads has hundreds); they're one note now.
• scan --server gives server memory advice instead of CurseForge steps.

Check Engine 0.5.0

Check Engine tells you what's broken in a modpack, in plain English. It never changes gameplay.

Check a pack without starting the game (easiest)

1. Download CheckEngine-v0.5.0.zip below and unzip it anywhere.
2. Drag your modpack folder (for example a CurseForge instance folder) onto Check Pack.bat.
3. Read the report. It lists duplicate mods, missing dependencies, version mismatches and client-only mods that would break a server.

It uses the Java that CurseForge or the Minecraft launcher already installed, so there's nothing else to set up. Keep Check Pack.bat and checkengine-cli-0.5.0.jar in the same folder.

The checker can also explain a crash, build a server copy of a pack without the client-only mods, and compare a pack with a server to find out why players can't join:

java -jar checkengine-cli-0.5.0.jar scan <pack folder> [--server]
java -jar checkengine-cli-0.5.0.jar crash <pack folder or crash-*.txt>
java -jar checkengine-cli-0.5.0.jar serverpack <pack folder> [--out <new folder>] [--zip]
java -jar checkengine-cli-0.5.0.jar compare <pack folder> <server folder or server pack .zip>

Or use it as a mod

Put the jar for your version in the pack's mods folder:

• checkengine-neoforge-26.1-0.5.0.jar for NeoForge 26.1.2 and newer 26.1.x
• checkengine-neoforge-1.21.1-0.5.0.jar for NeoForge 1.21.1
• checkengine-forge-1.20.1-0.5.0.jar for Forge 1.20.1

Problems show on a screen before the title screen and are saved to checkengine/boot-report.txt. Server ops can run /checkengine scan to find quests, recipes and loot tables that point at mods that aren't installed, and /checkengine startup to see how long the last startup took and which mods were slowest.

What's new in 0.5.0

• "It worked yesterday": what changed? After every launch that works, Check Engine remembers the pack's mods (checkengine/last-good-launch.properties). When the pack breaks later, it lists what was added, removed or updated since then, and links it to the problem: "create is missing. Changed since it last worked: you removed Create 6.0.4." Shown on NeoForge's error screen (1.21.1), under any loading crash, in the in-game report and in Check Pack.bat.

What's new in 0.4.0

• Root causes, not error lists. One problem per cause ("Create is missing"), with every mod it stops from loading, including mods that only need those mods. The biggest problem comes first.
• The real reason behind a "missing" mod: if it's actually there as the Fabric build, a build for another Minecraft version, a broken download or a switched-off .disabled file, Check Engine says so (with how sure it is) and how to fix it.
• Clearer version problems: "Create is too old: update to 6.0 or newer", "too new", or "no single version works for these mods". Mods made for another Minecraft version are grouped into one problem.
• Pack health at the top of every report ("394 jars: 392 OK, 2 with warnings, 0 can't load"), and a FIX THESE FIRST list.
• Works on NeoForge's error screen (1.21.1), in the in-game reports (all versions) and in Check Pack.bat.

What's new in 0.3.0

• NeoForge 1.21.1: Check Engine now explains why a pack won't start, right on NeoForge's error screen. It runs just before NeoForge checks dependencies, so even when the game refuses to load you see what's wrong and how to fix it, grouped ("3 mods need Create, but it isn't installed"). It uses NeoForge's own rules, so it only speaks up when NeoForge is really about to stop. Also saved to checkengine/early-report.txt.
• Clues when loading fails: if any mod breaks loading, Check Engine's warnings (duplicate mods, broken downloads...) show under the error with their fix. When the pack loads fine you just get the usual popup.
• Still one jar to install. 26.1 and Forge 1.20.1 get the same early check in a later version.

What's new in 0.2.0

• Minecraft 26.1 support (NeoForge 26.1.2+): startup check, warning screen, /checkengine scan and the join mismatch screen.
• /checkengine scan understands the newer recipe format (1.21.2+), where ingredients are plain item names.
• New /checkengine startup: how long the last startup took and the slowest mods, also saved to checkengine/startup-report.txt. The checker shows the same numbers in its report.

What's new in 0.1.4

• New logo: the Check Engine block, with its warning light and green check, also shown in the Mods menu.

What's new in 0.1.3

• Check Engine now has its icon and a link to its GitHub page in the Mods menu.
• GitHub links point at the renamed repo (Uanme37/CheckEngine).

What's new in 0.1.2

• New name: Check Engine (it was Pack Doctor; another mod already had that name). The command is now /checkengine scan and reports go to the checkengine folder.

What's new in 0.1.1

Fixes found by testing on 29 real modpacks:

• The mod now loads on any NeoForge 21.1. 0.1.0 needed 21.1.252 or newer, so it refused to start in packs like FTB Skies 2, ATM10 and Better MC.
• No more false "wrong NeoForge version" errors for mods made for 1.21 (NeoForge 1.21.1 loads them anyway).
• Respects config/fml.toml dependency overrides, and knows Monocle lets Iris run on Embeddium.
• /checkengine scan no longer warns about recipes a mod ships for optional mods you don't have (Farming for Blockheads has hundreds); they're one note now.
• scan --server gives server memory advice instead of CurseForge steps.

Check Engine 0.4.0

Check Engine tells you what's broken in a modpack, in plain English. It never changes gameplay.

Check a pack without starting the game (easiest)

1. Download CheckEngine-v0.4.0.zip below and unzip it anywhere.
2. Drag your modpack folder (for example a CurseForge instance folder) onto Check Pack.bat.
3. Read the report. It lists duplicate mods, missing dependencies, version mismatches and client-only mods that would break a server.

It uses the Java that CurseForge or the Minecraft launcher already installed, so there's nothing else to set up. Keep Check Pack.bat and checkengine-cli-0.4.0.jar in the same folder.

The checker can also explain a crash, build a server copy of a pack without the client-only mods, and compare a pack with a server to find out why players can't join:

java -jar checkengine-cli-0.4.0.jar scan <pack folder> [--server]
java -jar checkengine-cli-0.4.0.jar crash <pack folder or crash-*.txt>
java -jar checkengine-cli-0.4.0.jar serverpack <pack folder> [--out <new folder>] [--zip]
java -jar checkengine-cli-0.4.0.jar compare <pack folder> <server folder or server pack .zip>

Or use it as a mod

Put the jar for your version in the pack's mods folder:

• checkengine-neoforge-26.1-0.4.0.jar for NeoForge 26.1.2 and newer 26.1.x
• checkengine-neoforge-1.21.1-0.4.0.jar for NeoForge 1.21.1
• checkengine-forge-1.20.1-0.4.0.jar for Forge 1.20.1

Problems show on a screen before the title screen and are saved to checkengine/boot-report.txt. Server ops can run /checkengine scan to find quests, recipes and loot tables that point at mods that aren't installed, and /checkengine startup to see how long the last startup took and which mods were slowest.

What's new in 0.4.0

• Root causes, not error lists. One problem per cause ("Create is missing"), with every mod it stops from loading, including mods that only need those mods. The biggest problem comes first.
• The real reason behind a "missing" mod: if it's actually there as the Fabric build, a build for another Minecraft version, a broken download or a switched-off .disabled file, Check Engine says so (with how sure it is) and how to fix it.
• Clearer version problems: "Create is too old: update to 6.0 or newer", "too new", or "no single version works for these mods". Mods made for another Minecraft version are grouped into one problem.
• Pack health at the top of every report ("394 jars: 392 OK, 2 with warnings, 0 can't load"), and a FIX THESE FIRST list.
• Works on NeoForge's error screen (1.21.1), in the in-game reports (all versions) and in Check Pack.bat.

What's new in 0.3.0

• NeoForge 1.21.1: Check Engine now explains why a pack won't start, right on NeoForge's error screen. It runs just before NeoForge checks dependencies, so even when the game refuses to load you see what's wrong and how to fix it, grouped ("3 mods need Create, but it isn't installed"). It uses NeoForge's own rules, so it only speaks up when NeoForge is really about to stop. Also saved to checkengine/early-report.txt.
• Clues when loading fails: if any mod breaks loading, Check Engine's warnings (duplicate mods, broken downloads...) show under the error with their fix. When the pack loads fine you just get the usual popup.
• Still one jar to install. 26.1 and Forge 1.20.1 get the same early check in a later version.

What's new in 0.2.0

• Minecraft 26.1 support (NeoForge 26.1.2+): startup check, warning screen, /checkengine scan and the join mismatch screen.
• /checkengine scan understands the newer recipe format (1.21.2+), where ingredients are plain item names.
• New /checkengine startup: how long the last startup took and the slowest mods, also saved to checkengine/startup-report.txt. The checker shows the same numbers in its report.

What's new in 0.1.4

• New logo: the Check Engine block, with its warning light and green check, also shown in the Mods menu.

What's new in 0.1.3

• Check Engine now has its icon and a link to its GitHub page in the Mods menu.
• GitHub links point at the renamed repo (Uanme37/CheckEngine).

What's new in 0.1.2

• New name: Check Engine (it was Pack Doctor; another mod already had that name). The command is now /checkengine scan and reports go to the checkengine folder.

What's new in 0.1.1

Fixes found by testing on 29 real modpacks:

• The mod now loads on any NeoForge 21.1. 0.1.0 needed 21.1.252 or newer, so it refused to start in packs like FTB Skies 2, ATM10 and Better MC.
• No more false "wrong NeoForge version" errors for mods made for 1.21 (NeoForge 1.21.1 loads them anyway).
• Respects config/fml.toml dependency overrides, and knows Monocle lets Iris run on Embeddium.
• /checkengine scan no longer warns about recipes a mod ships for optional mods you don't have (Farming for Blockheads has hundreds); they're one note now.
• scan --server gives server memory advice instead of CurseForge steps.

Check Engine 0.3.0

Check Engine tells you what's broken in a modpack, in plain English. It never changes gameplay.

Check a pack without starting the game (easiest)

1. Download CheckEngine-v0.3.0.zip below and unzip it anywhere.
2. Drag your modpack folder (for example a CurseForge instance folder) onto Check Pack.bat.
3. Read the report. It lists duplicate mods, missing dependencies, version mismatches and client-only mods that would break a server.

It uses the Java that CurseForge or the Minecraft launcher already installed, so there's nothing else to set up. Keep Check Pack.bat and checkengine-cli-0.3.0.jar in the same folder.

The checker can also explain a crash, build a server copy of a pack without the client-only mods, and compare a pack with a server to find out why players can't join:

java -jar checkengine-cli-0.3.0.jar scan <pack folder> [--server]
java -jar checkengine-cli-0.3.0.jar crash <pack folder or crash-*.txt>
java -jar checkengine-cli-0.3.0.jar serverpack <pack folder> [--out <new folder>] [--zip]
java -jar checkengine-cli-0.3.0.jar compare <pack folder> <server folder or server pack .zip>

Or use it as a mod

Put the jar for your version in the pack's mods folder:

• checkengine-neoforge-26.1-0.3.0.jar for NeoForge 26.1.2 and newer 26.1.x
• checkengine-neoforge-1.21.1-0.3.0.jar for NeoForge 1.21.1
• checkengine-forge-1.20.1-0.3.0.jar for Forge 1.20.1

Problems show on a screen before the title screen and are saved to checkengine/boot-report.txt. Server ops can run /checkengine scan to find quests, recipes and loot tables that point at mods that aren't installed, and /checkengine startup to see how long the last startup took and which mods were slowest.

What's new in 0.3.0

• NeoForge 1.21.1: Check Engine now explains why a pack won't start, right on NeoForge's error screen. It runs just before NeoForge checks dependencies, so even when the game refuses to load you see what's wrong and how to fix it, grouped ("3 mods need Create, but it isn't installed"). It uses NeoForge's own rules, so it only speaks up when NeoForge is really about to stop. Also saved to checkengine/early-report.txt.
• Clues when loading fails: if any mod breaks loading, Check Engine's warnings (duplicate mods, broken downloads...) show under the error with their fix. When the pack loads fine you just get the usual popup.
• Still one jar to install. 26.1 and Forge 1.20.1 get the same early check in a later version.

What's new in 0.2.0

• Minecraft 26.1 support (NeoForge 26.1.2+): startup check, warning screen, /checkengine scan and the join mismatch screen.
• /checkengine scan understands the newer recipe format (1.21.2+), where ingredients are plain item names.
• New /checkengine startup: how long the last startup took and the slowest mods, also saved to checkengine/startup-report.txt. The checker shows the same numbers in its report.

What's new in 0.1.4

• New logo: the Check Engine block, with its warning light and green check, also shown in the Mods menu.

What's new in 0.1.3

• Check Engine now has its icon and a link to its GitHub page in the Mods menu.
• GitHub links point at the renamed repo (Uanme37/CheckEngine).

What's new in 0.1.2

• New name: Check Engine (it was Pack Doctor; another mod already had that name). The command is now /checkengine scan and reports go to the checkengine folder.

What's new in 0.1.1

Fixes found by testing on 29 real modpacks:

• The mod now loads on any NeoForge 21.1. 0.1.0 needed 21.1.252 or newer, so it refused to start in packs like FTB Skies 2, ATM10 and Better MC.
• No more false "wrong NeoForge version" errors for mods made for 1.21 (NeoForge 1.21.1 loads them anyway).
• Respects config/fml.toml dependency overrides, and knows Monocle lets Iris run on Embeddium.
• /checkengine scan no longer warns about recipes a mod ships for optional mods you don't have (Farming for Blockheads has hundreds); they're one note now.
• scan --server gives server memory advice instead of CurseForge steps.

Check Engine 0.2.0

Check Engine tells you what's broken in a modpack, in plain English. It never changes gameplay.

Check a pack without starting the game (easiest)

1. Download CheckEngine-v0.2.0.zip below and unzip it anywhere.
2. Drag your modpack folder (for example a CurseForge instance folder) onto Check Pack.bat.
3. Read the report. It lists duplicate mods, missing dependencies, version mismatches and client-only mods that would break a server.

It uses the Java that CurseForge or the Minecraft launcher already installed, so there's nothing else to set up. Keep Check Pack.bat and checkengine-cli-0.2.0.jar in the same folder.

The checker can also explain a crash, build a server copy of a pack without the client-only mods, and compare a pack with a server to find out why players can't join:

java -jar checkengine-cli-0.2.0.jar scan <pack folder> [--server]
java -jar checkengine-cli-0.2.0.jar crash <pack folder or crash-*.txt>
java -jar checkengine-cli-0.2.0.jar serverpack <pack folder> [--out <new folder>] [--zip]
java -jar checkengine-cli-0.2.0.jar compare <pack folder> <server folder or server pack .zip>

Or use it as a mod

Put the jar for your version in the pack's mods folder:

• checkengine-neoforge-26.1-0.2.0.jar for NeoForge 26.1.2 and newer 26.1.x
• checkengine-neoforge-1.21.1-0.2.0.jar for NeoForge 1.21.1
• checkengine-forge-1.20.1-0.2.0.jar for Forge 1.20.1

Problems show on a screen before the title screen and are saved to checkengine/boot-report.txt. Server ops can run /checkengine scan to find quests, recipes and loot tables that point at mods that aren't installed, and /checkengine startup to see how long the last startup took and which mods were slowest.

What's new in 0.2.0

• Minecraft 26.1 support (NeoForge 26.1.2+): startup check, warning screen, /checkengine scan and the join mismatch screen.
• /checkengine scan understands the newer recipe format (1.21.2+), where ingredients are plain item names.
• New /checkengine startup: how long the last startup took and the slowest mods, also saved to checkengine/startup-report.txt. The checker shows the same numbers in its report.

What's new in 0.1.4

• New logo: the Check Engine block, with its warning light and green check, also shown in the Mods menu.

What's new in 0.1.3

• Check Engine now has its icon and a link to its GitHub page in the Mods menu.
• GitHub links point at the renamed repo (Uanme37/CheckEngine).

What's new in 0.1.2

• New name: Check Engine (it was Pack Doctor; another mod already had that name). The command is now /checkengine scan and reports go to the checkengine folder.

What's new in 0.1.1

Fixes found by testing on 29 real modpacks:

• The mod now loads on any NeoForge 21.1. 0.1.0 needed 21.1.252 or newer, so it refused to start in packs like FTB Skies 2, ATM10 and Better MC.
• No more false "wrong NeoForge version" errors for mods made for 1.21 (NeoForge 1.21.1 loads them anyway).
• Respects config/fml.toml dependency overrides, and knows Monocle lets Iris run on Embeddium.
• /checkengine scan no longer warns about recipes a mod ships for optional mods you don't have (Farming for Blockheads has hundreds); they're one note now.
• scan --server gives server memory advice instead of CurseForge steps.