When Seam Tools is installed in Eclipse Luna SR1, Expression Language (EL) content assist in Visual Page Editor stopped working (no suggestion at all, JBoss related or not). This occurs only when xmlns:s="http://jboss.com/products/seam/taglib" tag library is used in a Facelet page.

Eclipse log shows following stacktrace on any such content assist attempt:

org.jboss.tools.common.validation.JBTValidationException: Prefix string too short
at org.jboss.tools.common.validation.AsYouTypeValidatorManager.validate(AsYouTypeValidatorManager.java:187)
at org.jboss.tools.common.validation.AsYouTypeValidatorManager.validateString(AsYouTypeValidatorManager.java:205)
at org.jboss.tools.common.validation.AsYouTypeValidatorManager.validateString(AsYouTypeValidatorManager.java:200)
at org.jboss.tools.common.validation.AsYouTypeValidatorManager.validate(AsYouTypeValidatorManager.java:254)
at org.eclipse.wst.sse.ui.internal.reconcile.validator.ReconcileStepForValidator.validate(ReconcileStepForValidator.java:381)
at org.eclipse.wst.sse.ui.internal.reconcile.validator.ReconcileStepForValidator.reconcileModel(ReconcileStepForValidator.java:259)
at org.eclipse.jface.text.reconciler.AbstractReconcileStep.reconcile(AbstractReconcileStep.java:95)
at org.eclipse.wst.sse.ui.internal.reconcile.validator.ValidatorStrategy.reconcile(ValidatorStrategy.java:269)
at org.eclipse.wst.sse.ui.internal.reconcile.DocumentRegionProcessor.process(DocumentRegionProcessor.java:321)
at org.eclipse.wst.sse.ui.internal.reconcile.StructuredRegionProcessor.process(StructuredRegionProcessor.java:258)
at org.eclipse.wst.sse.ui.internal.reconcile.DirtyRegionProcessor$BackgroundThread.run(DirtyRegionProcessor.java:691)
Caused by: java.lang.IllegalArgumentException: Prefix string too short
at java.io.File.createTempFile(File.java:2001)
at org.jboss.tools.jst.web.kb.taglib.TagLibraryManager.convertUriToFile(TagLibraryManager.java:182)
at org.jboss.tools.jst.web.kb.taglib.TagLibraryManager.getStaticTLD(TagLibraryManager.java:100)
at org.jboss.tools.jst.web.kb.internal.StaticLibraries.getLibraries(StaticLibraries.java:44)
at org.jboss.tools.jst.web.kb.internal.KbProject.getTagLibraries(KbProject.java:135)
at org.jboss.tools.jst.web.kb.taglib.TagLibraryManager.getLibraries(TagLibraryManager.java:57)
at org.jboss.tools.jst.web.kb.PageContextFactory.fillXMLNamespacesForNode(PageContextFactory.java:996)
at org.jboss.tools.jst.web.kb.PageContextFactory.fillContextForNode(PageContextFactory.java:698)
at org.jboss.tools.jst.web.kb.PageContextFactory.fillContextForChildNodes(PageContextFactory.java:687)
at org.jboss.tools.jst.web.kb.PageContextFactory.createPageContext(PageContextFactory.java:431)
at org.jboss.tools.jst.web.kb.PageContextFactory.createPageContext(PageContextFactory.java:236)
at org.jboss.tools.jst.web.kb.PageContextFactory.createPageContext(PageContextFactory.java:199)
at org.jboss.tools.jst.web.kb.PageContextFactory.createPageContext(PageContextFactory.java:184)
at org.jboss.tools.jst.web.kb.internal.validation.ELValidator.validate(ELValidator.java:253)
at org.jboss.tools.common.validation.AsYouTypeValidatorManager.validate(AsYouTypeValidatorManager.java:183)
... 10 more

Digging more deeply into the cause, Eclipse loaded a plugin specified entry for this URI as:

Entry element: URI
Location: taglibs\tld\s.tld in jar file C:\DevTools\IDE\eclipse\eclipse-ee-luna-sr1-64bit\plugins\org.jboss.tools.seam.core_3.6.1.Final-v20141209-0505-B79.jar
URI: jar:file:/C:/DevTools/IDE/eclipse/eclipse-ee-luna-sr1-64bit/plugins/org.jboss.tools.seam.core_3.6.1.Final-v20141209-0505-B79.jar!/taglibs/tld/s.tld
Key type: URI
Key: http://jboss.com/products/seam/taglib

Line 182 of org.jboss.tools.jst.web.kb.taglib.TagLibraryManager shows the "too short" prefix is derived from the name of the file. Here, the culprit prefix is "s" (derived from "s.tld"). Temporary file prefix must be at least 3 characters long (javadoc: http://docs.oracle.com/javase/8/docs/api/java/io/File.html#createTempFile-java.lang.String-java.lang.String-java.io.File-).

I managed to work around this issue temporarily by renaming s.tld and s.taglib.xml in org.jboss.tools.seam.core_3.6.1.Final-v20141209-0505-B79.jar to seam.tld and seam.taglib.tld respectively. Then I changed the entry in JAR's plugin.xml accordingly and started eclipse with "-clean" option for the changes to take effect. Now, content assist runs expectedly.

Either renaming the plugin's TLD file or ensuring the minimum prefix length in code should do the trick.