Microbiota play a key role in gut health of chickens. Its composition and functionality are affected by diet feed ingredients and may be modulated by probiotics (Pro) and postbiotics (Post). This study aimed to assess the effects of 2 diets (standard [SD], challenge [CD]) on the effects of a Lactobacilli-based Pro and Post on broiler caecal microbiota and metabolome. Following a 2×3 randomised block design, day-old Ross male broilers (n=1368) were fed 1 of 2 diets (SD, CD) and 1 of 3 additives (control, Pro, Post) from d1 to 42 with 6 pen replicates per treatment. SD and CD were formulated to contain identical levels of nutrients, with CD formulated to be richer than SD in non-starch polysaccharides. Caecal microbiota composition was assessed by 16S rRNA gene sequencing for 3-4 broilers/pen at d14 and 35, respectively. Variations in α- and β-diversity indexes were analysed by ANOVA and for contrasts. Operational taxonomic unit (OTU) abundances were compared by differential abundance analyses. P-values <0.05 were considered significant. At d14, Observed and Chao1 α-diversity indexes were higher for Pro compared to control in SD, and lower for Post compared to control in CD. At d35, no effects of additives were observed, while the latter indexes were lower for CD compared to SD. At d14, Bray and Jaccard β-diversity indexes showed a diet effect, no additive nor diet x additive effect, whereas at d35 these indexes showed an interaction and a diet effect, but no additive effect. At d14, 2 OTU were differentially abundant (DA) for Pro and Post compared to control for SD, whereas 6 and 17 OTU, respectively, were DA for Pro and Post compared to control for CD. At d35, Pro and Post had no DA OTU compared to control for SD. By contrast, for the CD, Pro and Post had 37 and 44 DA OTU compared to control, respectively. Four and 86 OTU were DA between the SD and CD controls at d14 and 35, respectively. In conclusion, Pro and Post modified broiler caecal microbiota in an age and diet dependent manner. In particular, their effects were more pronounced at d35 in the CD. Pro and Post effects on microbiota are in line with their effects observed for broiler growth, since Pro and Post increased body weight in CD without effects in SD. This highlights the importance of diet composition to assess Pro and Post effects on gut microbiota.