> ## Documentation Index > Fetch the complete documentation index at: https://docs.cosmon.com/llms.txt > Use this file to discover all available pages before exploring further. # Fixing Errors > Diagnose and fix rebuild errors, broken references, mate conflicts, and other CAD errors. ## Overview Rebuild errors, broken external references, mate conflicts, and corrupted feature trees are routine in CAD work — and routinely time-consuming, because the error message rarely points straight at the cause. An over-defined sketch, a missing mate reference, or a broken link after a file move can each take longer to trace than to fix. Describe what you're seeing — the error message, what changed recently, what you expected instead — and Nexus works through the likely causes the way an experienced user would, then applies the fix on your confirmation. ## What Nexus can help with * **Rebuild errors** — Diagnose failed features: over-defined or under-defined sketches, missing geometry references, invalid feature parameters. * **Broken external references** — Repair dangling links after a file was renamed, moved, or a referenced part changed shape. * **Assembly mate and constraint errors** — Identify which mate is over-constraining or conflicting, and why, in assemblies with many components. * **Corrupted or unstable feature trees** — Isolate which feature in a long history is the actual source of instability, not just the first one flagged. * **Drawing errors after a model change** — Fix out-of-date or broken views, dangling dimensions, and missing annotations after the underlying model changes. * **Import and translation errors** — Diagnose why an imported STEP or IGES file failed to translate cleanly or won't open. ## Example prompts ```text theme={null} Feature "Boss-Extrude3" fails to rebuild with "sketch is over-defined." I haven't touched this sketch recently — the error started after I changed the base plate thickness in an earlier feature. Diagnose the conflicting relation and fix it without changing design intent. ``` ```text theme={null} This assembly has a broken reference: "Could not find external reference 'Bracket_Mount' in Housing_Assy." The part was recently renamed from Bracket_Mount.SLDPRT to Mount_Bracket_v2.SLDPRT and moved to a new folder. Repair the reference to point at the renamed file. ``` ```text theme={null} The assembly shows "Mates in conflict" on three mates involving the Idler_Pulley component. Identify which mates are actually conflicting versus which are just downstream of the real conflict, explain the root cause, and suggest which mate to modify. ``` ```text theme={null} This part has 40 features and rebuild instability appears intermittently — sometimes it rebuilds clean, sometimes three features fail. Find the actual source feature (not just the first one flagged) and explain why it's unstable. ``` ```text theme={null} The Section A-A view on this drawing shows "view out of date" and two dimensions are showing as dangling. The model's bore diameter changed from 20mm to 25mm. Refresh the view and reattach the dangling dimensions to the correct edges. ``` ```text theme={null} C:/Incoming/housing_supplier.step fails to open with "invalid or corrupt geometry data" in the second body. Identify which entities are causing the failure and attempt to recover a valid solid from what can be translated. ``` ## The debugging mindset The most effective error-fixing prompts follow the **describe symptoms** rule from the [Prompting Guide](/getting-started/quickstart#prompting-guide): 1. **State what happened** — the exact error message, the feature or mate it's attached to, and when it started. 2. **State what you expected** — what a correct rebuild or result should look like. 3. **State what changed recently** — the edit, import, or rename that likely triggered it, so Nexus doesn't have to guess at the trigger. Here's an example that does all three: ```text theme={null} Part fails to rebuild at "Cut-Extrude5" with "the sketch plane could not be found." I expected it to rebuild clean — it worked before I suppressed the "Reference_Plane2" feature earlier in the tree. I've already tried un-suppressing that feature — the error goes away, but I need this feature suppressed for the lightweight configuration. Find an alternative sketch plane that doesn't depend on the suppressed feature. ``` ## Tips for error-fixing prompts * **Paste the exact error message** — the precise wording usually names the feature, mate, or entity Nexus needs to locate the problem, rather than a paraphrase. * **Say what changed recently** — a rename, a suppressed feature, an edited dimension. Most CAD errors are triggered by a specific prior change, and naming it narrows the diagnosis immediately. * **Distinguish the trigger from the symptom** — in assemblies and long feature trees, the first error shown often isn't the root cause. Ask Nexus to trace back to the actual source. * **State constraints on the fix** — "don't change the outer profile", "this feature needs to stay suppressed for the lightweight configuration" — so the fix doesn't solve one problem by creating another. * **Ask for the explanation, not just the fix** — understanding why an error happened prevents the same class of error on the next model.